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S Burton, B Cohen, S Harrison, S Pather-Elias,

W Stafford, R van Hille & H von Blottnitz

Department of Chemical Engineering

University of Cape Town

WRC Report No. 1732/1/09 ISBN 978-1-77005-849-1 Set No. 978-1-77005-846-0

JULY 2009

ii

DISCLAIMER

This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute

endorsement or recommendation for use.

iii

EEXXEECCUUTTIIVVEE SSUUMMMMAARRYY

The current view of wastewaters is that they generally represent a burden and necessarily

incur energy costs in processing before they can safely be released into the environment. The

opportunity exists to improve the current wastewater treatment processes by applying new

solutions and technologies that can also reduce energy inputs and/or generate energy for

other processes. This study explored the various waste streams and the appropriate

technologies that could be used to generate energy.

A survey of the quality and quantity of wastewaters in South Africa identified the top three

sectors having the greatest potential energy recovery as the formal and informal animal

husbandry sector (cows, pigs and chickens), fruit and beverage industries (distillery, brewery,

winery, fruit juicing and canning) and domestic blackwater (sewage). An estimated 10 000

MWth can be recovered from the wastewaters in the whole of South Africa, representing 7%

of the current Eskom electrical power supply*. However, since most of the waste streams are

widely distributed, the energy from wastewater is best viewed as on-site power.

The most appropriate technologies and their limitations are partly determined by the value of

the required energy product (heat, electricity, combined heat and power or fuel) and the

driving market forces that determine how this energy product can be used with our current

technology. Furthermore, the ease of separation of the energy product from water can be key

to the feasibility of the process (e.g. biogas separates easy from wastewater by natural

partitioning whereas bioethanol requires energy intensive distillation). Anaerobic digestion

(AD) is the most commonly recognised technology and has been applied to wastewaters of

different characteristics at both small and large scales. AD is suitable for use with domestic

sewage (particularly since 40% of South Africans are currently not serviced with waterborne

sewage), as well as in the industrial and agricultural sectors. Bioethanol production by

fermentation is suited to concentrated, high carbohydrate wastewaters and has potential in

the fruit industry where sugar-rich wastewaters are generated in large volumes. Similarly,

combustion and gasification are restricted to applications with concentrated waste streams

(containing <40% water) due to the energy expended in de-watering and are most appropriate

in the treatment of dewatered and solar-dried (or previously stockpiled) wastes. In contrast to

these technologies, the growth of plant biomass for combustion/gasification and algal

biomass for biodiesel production is suited to dilute waste streams. The sequestration of

carbon dioxide and facilitated wastewater-treatment by photosynthetic oxygenation are added

benefits. However, there are no large scale algal biodiesel production processes operating at

present, in spite of published claims that the technology is technically an economically

feasible and the growing incentives due to the recent increase in diesel prices. Microbial fuel

cells (MFC) are an emerging technology that can also operate with dilute waste streams while

producing electricity directly. MFCs are suited to applications in remote/rural sites with no

* Approximately 140 000 MWth or 42 000 MWe (Eskom data tables 2007). Where MWth and MWe refer the thermal

and electrical power in megawatts (106 W), respectively

iv

infrastructure, but the technology is still in early development. A single waste stream or

technology may not be suitable for achieving efficient energy recovery and the integration of

technologies and/or waste streams may be required to realise the maximum energy from

wastewater potential. There are also frequently missed opportunities for reducing energy

needs by the recovery of waste heat. The greatest potentials are realised when an industrial

ecology approach is used to integrate process or waste heat from several industries, with pre-

planning and the formation of industrial parks.

The net energy generated, reduction in pollution (wastewater treatment), and water

reclamation are the main costs and benefits considered in assessing the feasibility of an

energy from wastewater project. However, additional benefits such as certified emission

reductions (CERs), fertiliser production, or the production of other secondary products could

tip the balance of economic feasibility. For the implementation of energy from wastewater

technologies, essential services (WWTP operation, schools, and hospitals) and the needs of

communities not serviced by sewage and electrical infrastructure should preferentially be

targeted.

Several risks, barriers and drivers to developing an energy from wastewater project were

identified. There is a general lack of research capacity and skills, and a greater need for

research collaboration and information-sharing between research groups, government

agencies and municipal practitioners. There is also no incentive for the generation of clean,

renewable energy such as feed-in tariffs, green energy tariffs or peak tariffs.

The use of wastewater as a renewable energy resource can improve energy security while

reducing the environmental burdens of waste disposal. Energy from wastewater therefore

facilitates the integration of water, waste and energy management within a model of

sustainable development.

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CONTENTS: Page

1. TECHNICAL REPORT: Appropriate energy from wastewater technologies and a review of literature and national and international practice 1.1 Anaerobic Digestion (AD) to produce biogas 1

1.1.1 Introduction 1.1.2. Advantages and constraints associated with AD 1.1.3 Application and practice of AD technology

1.1.4. Implementation of AD 1.1.5 Costs and benefits

1.2 Combustion and gasification for the production of steam 20 1.2.1 Introduction 1.2.2 Application of combustion / gasification

1.2.3 Costs and benefits of combustion / gasification 1.3 Utilisation of waste heat 23

1.3.1 Introduction 1.3.2 Implementation of utilisation of waste heat 1.3.3 Costs and benefits in utilisation of waste heat

1.4 Fermentation to produce biomass and secondary products 28 1.4.1 Introduction 1.4.2 Implementation of fermentation of wastewaters 1.4.3 Cost and benefits in fermentation of wastewaters

1.5 Algal biomass for biodiesel production 31 1.5.1 Introduction 1.5.2 Implementation of biodiesel production 1.5.3 Costs and benefits in biodiesel production from wastewaters

1.6 Fermentation for production of bioethanol 36 1.6.1 Introduction 1.6.2 Implementation of bioethanol production 1.6.3 Costs and benefits in bioethanol production from wastewaters

1.7 Microbial fuel cells 40 1.7.1 Introduction 1.7.2 Implementation of MFCs 1.7.3 Costs and benefits in the application of MFCs to wastewaters

1.8 References 45

2. APPENDIX: Surveys, case studies and workshops 53 2.1 Survey of WWTP wastewaters 2.2 Survey of brewery wastewater 2.3 Survey of pulp and paper wastewaters 2.4 Survey of petrochemical wastewaters 2.5 Survey of Animal husbandry wastewaters 2.6 Case study of energy from fruit wastewaters 2.7 Case study of combustion versus gasification of waste stream 2.8 Case study of Cape Flats WWTP: Best practice? 2.9 Case study of the performance of household anaerobic digesters 2.10 Industrial stakeholder workshop

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Acknowledgements:

Certain studies that have been incorporated into this project were undertaken by students at

UCT. These are acknowledged as follows:

Thermal gasification or anaerobic digestion of biomass for use in a distributed

renewable energy network (Sarah Dowling)

Optimising algal growth and lipid production (Melinda Griffiths)

Energy potential of the wastewaters from the paper and pulp and the brewing industry

(Andrew Krige)

Energy potential of wastewaters of the petroleum and animal husbandry industries

(Deleki Zuko and Sehoana Kabelo Albert)

Deriving energy from fruit wastes (Andrew de Beer and Peter Dawson)

Carbon dioxide sequestration and fixation by algae (Nick Langley and Bertus Louw)

Cape Flats WWTP: Best practice? (Loice Badza)

1

1: APPROPRIATE ENERGY FROM WASTEWATER TECHNOLOGIES AND NATIONAL AND

INTERNATIONAL PRACTICE

This report comprises a review of the literature with respect to energy produced from wastewater, and

a review of information available on the practice of energy generation from wastewater occurring in

South Africa and internationally.

This report constitutes the technical section of the final report for this research project, WRC

K5/1732. The report is divided into sections, each dealing with one technology, and providing, with

respect to the respective technology: Introduction on the theory of the technology; information on its

application and the energy potential.

SECTION 1.1 Anaerobic digestion to produce biogas

1.1.1 Introduction

Anaerobic digestion leads to the ultimate gasification of organic carbon containing wastewaters and

slurries to carbon dioxide, methane and hydrogen. It has been successfully used in the wastewater

treatment industry to reduce both the volume and COD of waste sludge. Anaerobic digestion of

wastewaters and sludges is a complex process, involving a number of different microorganisms. A

schematic representation of the process, typically consisting of three metabolic stages, is shown in

the process flow diagram in the Figure 1, below (GATE, 1999).

Figure 1. Schematic showing the stages and microorganisms involved in anaerobic digestion for biogas production.

Initially, particulate organic matter such as cellulose, hemicellulose, pectin and lignin are solubilised in

a hydrolysis step catalysed by extracellular enzymes. The soluble products of this stage are converted

2

into organic acids, alcohol, hydrogen and carbon dioxide by acidogenic microorganisms. The simple

organic substrates are further metabolised by acetogenic microorganisms to produce acetate, the

primary organic substrate for methanogenesis, hydrogen and carbon dioxide. Aside from acetate, CO2

and hydrogen, methanogenic microorganisms may also directly ferment other substrates, of which

formic acid and methanol are the most important (Bouallagui et al., 2007).

The AD digester can be designed as a cylindrical, cube shaped, or egg-shaped vessel and is

composed of a waterproof material with an inlet into which the fuel is introduced, normally in slurry

form. In small to medium scale facilities, the gas holder is typically an airtight steel container which

covers the top of the digester, excluding air and collecting the gas produced (Amigum and Von

Blottnitz, 2007). For industrial scale plants or ponding digesters the gas storage component is

appropriately designed for the application. The simplest reactor type consists of a single digester,

operated as a fed-batch or continuously stirred tank reactor. All the processes associated with

anaerobic digestion occur within the single unit, which may lead to process inefficiency, given the

differences in metabolism of the microbes responsible for the different phases of digestion. The typical

influent substrate concentration is in the range of 3 to 8% total solids (Gunaseelan, 1997), although

the solids loading can be as high as 25%. Retention times within these reactors are typically in excess

of six days. To overcome the problem of biomass washout while reducing the residence time, a

number of reactor types have been developed including anaerobic filter or fluidised bed systems

(where biomass is encouraged to attach to solid support material) and the upflow anaerobic sludge

blanket (UASB) reactor (certain microorganisms induce floc formation thereby retaining biomass in

the absence of a solid support). The use of two-stage digesters, that physically separate the

hydrolysis, acidogensis and acetogenesis stage form the methanogenic stage (Ghosh et al., 1975;

Gunaseelan, 1997; Niishio et al., 2007), often result in significantly reduced total retention times and

increased biogas production as the conditions for both sets of organisms can be independently

optimised.

There are several sources of waste with different characteristics: municipal (sewage), agriculture

(waste from animals, crop residues, and food processing) and industries (paper and textiles,

petrochemicals). Anaerobic digesters have been successfully employed for the generation of energy

from a wide range of these wastes. The table below illustrates the range of substrates that have been

tested at laboratory or pilot scale. Typical biogas yields from the digestion of primary sewage sludge in

an egg-shaped digester, common on sewage treatment plants, range from 0.4-0.9 l/g organic dry

substrate.

The yields are typically expressed in terms of gas production per unit of COD or volatile solid (VS),

and are in the order of 0.2 to 0.45 m3/kg VS (Gunaseelan, 1997). The energy yield from methane is

calculated based from a value of 890.3 kJ per mole of methane (or 11.04 kWh/m3 of methane) (Duerr

et al., 2007).

3

Table 1: Methane and hydrogen production potential from agricultural wastes utilising a range of anaerobic digestion

technologies

Feed Temp. (°C)

HRT (days) Organic loading rate

Potential energy recovery

Reference

Bread waste (100 g/L) + SS (10% w/v)

55 - 29 g-wet wt/L.d 145 m3 H2/d (214 kwh)

514 m3 CH4/d

Nishio & Nakashimada, 2007

Beer waste (pressed filtrate from lauter tun)

50 (H2) 37 (CH4)

- - 2.2 l H2/L (24 kJ/L) 2.2 l CH4/L (79

kJ/L)

Nishio & Nakashimada, 2007

Fruit and vegetable waste 47 1.06 g VS/L.d 0.16 l CH4/g VS Bouallagui et al., 2005





Fruit and vegetable waste 2.5 6.8 g VS/L.d 0.35 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 7 + 10 4.4 g VS/L.d 0.34 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 2 + 2.3 5.65 g VS/L.d 0.42 l CH4/g VS Bouallagui et al., 2005

Brewery slurry 55 26 3.23 g COD/L.d 0.37 l CH4/g COD Zupancic et al., 2007

Brewery slurry 55 17.5 5.40 g COD/L.d 0.38 l CH4/g COD Zupancic et al., 2007

Brewery slurry 55 16 6.27 g COD/L.d 0.42 l CH4/g COD Zupancic et al., 2007

Brewery slurry 55 13.5 8.57 g COD/L.d 0.37 l CH4/g COD Zupancic et al., 2007

Artificial garbage slurry 55 0.5 + 0.54 75 g/L.d 199 mmol H2/L.d 442 mmol CH4/L.d

Ueno et al., 2007

Fruit and veg waste + manure

35 21 3.19 g VS/L.d 0.45 l CH4/g VS Callaghan et al., 2002

Barley waste + SS 37 - - 0.025 l CH4/g VS Neves et al., 2006

Hydrolysed barley waste + SS

37 - - 0.22 l CH4/g VS Neves et al., 2006

Cheese whey 37 1 + 4 19.78 g COD/L.d 0.30 l CH4/g COD Saddoud et al., 2007

Olive mill water + solids 55 36 3.62 g COD/L.d 46.2 l CH4/L OMW.d

Fezzani & Ben Cheikh, 2007





Distillery effluent 35 14 2.74 kg/Cu m 4.44 l CH4/L Banerjee & Biswas, 2004

Distillery effluent 55 14 2.74 kg/Cu m 5.40 l CH4/L Banerjee & Biswas, 2004

Molasses 35 7 7 g COD/L 0.23 l CH4/g COD Jimènez et al., 2004

Fermented molasses 35 3 7 g COD/L 0.31 l CH4/g COD Jimènez et al., 2004

Algal sludge 35 10 4 g VS/L.day 0.14 l CH4/g VS Yen & Brune, 2007

Algal sludge + waste- paper

35 10 4 g VS/L.day 0.29 l CH4/g VS Yen & Brune, 2007

Palm oil mill effluent

- 7

40 g COD/L

0.24 l H2/g COD

Vijayaraghavan et al.,

2006 Jackfruit peel - 12 33 g VS/L 0.40 l H2/g VS Vijayaraghavan et al.,

2006

4

1.1.2 Advantages and constraints associated with AD

Anaerobic digestion offers a number of significant advantages over competing technologies for the

treatment of carbonaceous municipal, industrial and agricultural effluents. These include low energy

requirements, reduced sludge production and the economic recovery of energy. Thermophilic1

anaerobic digestion has the additional benefits of increased waste stabilisation and sludge

dewatering. This is particularly relevant to the treatment of sewage sludge, where the elevated

temperature leads to increased destruction of viral and bacterial pathogens (Chen et al., 2007 and the

study comparing combustion with AD and enhanced AD summarised in the K5/1732 Essence report),

but may incur large energy input costs.

In terms of constraints on the feasibility of anaerobic digestion technology, inhibition of the component

microbial processes and the resulting decrease in process efficiency can be a major concern.

Common reasons for poor performance are mixing problems, or the inability to maintain the correct

balance between acid forming and methane forming microbial communities. These two microbial

groups differ widely in their physiology, nutritional requirements, growth kinetics and susceptibility to

environmental perturbations (Chen et al., 2007). Inhibitory substances may be present at high

concentrations in wastewaters and sludges, or may be formed during the initial phases of anaerobic

digestion. Inhibition is typically identified by a decrease in steady state methane production and an

accumulation of organic acids. The most significant inhibitors are discussed briefly below.

Ammonia. Ammonia is produced by the biological degradation of nitrogen-containing compounds,

most commonly urea and proteins (Kayhanian, 1999). Its aqueous speciation is pH dependent, with

the ammonium ion (NH4+) dominating at low pH and free ammonia (NH3) under alkaline conditions.

The free form is freely membrane permeable and has been suggested to be the primary source of

inhibition, resulting in a proton imbalance or potassium deficiency (Gallert et al., 1998). Of the

organisms involved in anaerobic digestion, the methanogens are most susceptible to inhibition by

ammonia (Kayhanian, 1994). The extent of inhibition is mediated by a number of factors, such as pH,

temperature, the presence of antagonistic cations (Na+, Ca2+, Mg2+) and acclimation, to the extent that

literature values for 50% reduction in methane production vary from 1.7-14.0 g/L total ammonia

nitrogen (Chen et al., 2007).

Sulphide. Sulphate is a common constituent of many industrial and agricultural wastewaters. Under

anaerobic conditions it is reduced to sulphide by sulphate reducing bacteria (SRB) (Hilton and

Oleszkiewicz, 1988). Inhibition occurs at two levels, initially through competition for substrate and

secondarily as a result of sulphide toxicity. SRB are not able to utilise complex organic carbon sources

so do not compete with hydrolytic or acidogenic microbes. Competition between SRB and acetogenic

and methanogenic bacteria has been observed, but results are often contradictory. The extent of

1 Thermophilic microorganisms grow optimally at temperatures above 40 °C. Mesophilic microorganisms grow optimally at

temperatures between 25 and 40 °C; and psychrophilic microorganisms grow optimally below 5 °C.

5

competition appears related to the initial microbial concentrations and the COD/SO42- ratio, with SRB

dominating at COD/SO42- ratios below 1.7, MPB above 2.7 and active competition occurring between

the two (Choi and Rim, 1991). Temperature has also been shown to have an effect with SRB more

likely to dominate at 37°C and MPB at 55°C (Colleran and Pender, 2002). With respect to sulphide

toxicity there are considerable discrepancies in the published literature with respect to the nature of

sulphide toxicity and the relative toxicity of dissociated and undissociated sulphide. Mechanistically,

sulphide inhibits cellular activity by denaturing native proteins, interfering with coenzyme sulphide

linkages and affecting assimilatory sulphide metabolism (Speece 1983; Vogels et al., 1988).

Unionised sulphide appears to play the dominant roles between pH 6.4 and 7.2, with total sulphide

concentrations becoming more important above pH 7.8. Fermentative microbes are less susceptible

to sulphide toxicity than acetogenic or SRB, with the MPB being the most susceptible (McCartnery

and Oleszkiewicz, 1991). For MPB the published IC50 values range between 50 and 250 mg/L,

depending on pH (Parkin et al., 1983; Oleszkiewicz et al., 1989; McCartnery and Oleszkiewicz, 1993).

Sulphide removal and acclimation of MPB have been shown to improve AD performance.

Light metal ions. The effect of aluminium, calcium, magnesium, potassium and sodium on

methanogenesis has been extensively studied (Chen et al., 2007). These ions may be present in the

wastewater, be released by digestion of organic matter or be added for pH control. In all cases

moderate amounts of the ions are required for growth, but excessive amounts are inhibitory.

Excessive amounts of Ca and Mg affect the system primarily through the precipitation of carbonate

and phosphate, resulting in the loss of essential nutrients and buffering capacity as well as scaling of

the microbes (Keenan et al., 1993; Van Langerak et al., 1998). Potassium and sodium are more

acutely toxic, given their role in maintaining membrane potential among other functions (Jarrell et al.,

1984; Soto et al., 1991). The IC50 values reported in literature differ significantly due to matrix

composition and degree of acclimation, but range between 0.15-0.74 m for potassium (Mouneimne et

al., 2003) and 5.6-53 g/L for sodium (Soto et al., 1993; Kim et al., 2000; Vallero et al., 2003a, b).

Heavy metal ions. Heavy metals may be present in significant concentrations in municipal sewage

and sludge, as well as a number of industrial effluents. While these elements form essential

components of enzymes that drive many anaerobic reactions they become acutely toxic at elevated

concentrations (Sterrit and Lester, 1980). Significantly, they are not biodegradable and although

present below toxic concentrations may be accumulated to inhibitory levels within the cells over time.

The toxicity of heavy metals is affected by a number of factors such as form of the metal, pH and

redox potential, but once again the methanogenic microbes are more susceptible than acetogens

(Zayed and Winter, 2000). Published IC50 values vary considerably and appear particularly affected by

solids content, which has a mitigating effect. However, the relative sensitivity of acidogenesis and

methanogensis to heavy metals is Cu>Zn>Cr>Cd>Ni>Pb and Cd>Cu>Cr>Zn>Pb>Ni respectively (Lin,

1992; 1993).

6

Organics. A wide range of organic compounds have been found to be inhibitory to anaerobic

processes. These are predominantly non-polar compounds which accumulate in bacterial

membranes, resulting in swelling of the membranes, leakage of cellular components, disruption of

ionic gradients and eventually cell lysis (Sikkema et al., 1994; Chen et al., 2007). The inhibition

concentrations vary for specific compounds and are affected by the concentration of the compound,

biomass concentration, exposure time, cell age, acclimation and temperature. Some of the major

classes of inhibitory organic compounds are chlorophenols, halogenated aliphatics, N-substituted

aromiatics, long chain fatty acids and lignin and lignin related compounds (Chen et al., 2007). A

number of these groups form important fractions of wastewaters and can have a significant impact on

efficiency of biogas formation.

1.1.3 Application and practice of AD technology The application of anaerobic digestion technology appears to fall into two distinct categories, based

largely on the development status of the location. In developing countries, particularly in Africa and

Asia, the technology is applied at small or institutional scale and the biogas generated is used

primarily for heating or cooking, particularly in areas not serviced by the electrical grid. In these

situations the level of technology employed is kept low, such that the plant is robust and does not

require careful monitoring or technically skilled maintenance staff. Ideally the construction of such

units can be achieved by relatively unskilled labour.

In developed countries the installations tend to larger and employ more sophisticated technology. The

drivers for implementing such technology are not the provision of basic services to isolated

communities, but rather generation of renewable, environmentally benign energy. Developed

countries, particularly in Europe, often have an existing natural gas infrastructure for heating,

electricity generation and motor vehicle fuel. For boilers and stationary combined heat and power

(CHP) engines the biogas can be utilised with little or no modification. However, prior to utilisation as

motor vehicle fuel or incorporation into the natural gas grid the biogas needs to be purified to a high (>

95%) methane content. This process is uncommon in developing countries.

Based on the distinctly different technologies and drivers, it is not possible to apply uniform criteria to

assess changes in cost and efficiency with changes in scale. However, a recent study by Amigun and

Von Blottnitz (2007) investigated scale economies of 21 biogas installations across the African

continent. The installations varied in capacity from four to 100 m3. Plant capital costs were converted

from local currency to US$ equivalents and a statistical assessment performed, correlating the plant

capacity to capital cost. The relationship between plant capacity and installation cost is not

proportional and has been represented by the empirical expression:

C = kQn

where k is the dimensioned proportionality factor, C is the cost, Q the capacity and the exponent n the

cost capacity factor. For chemical plants an n value of 0.6 is typical, although this has been shown to

increase to 0.85 where the process involves significant solids handling. The study found a statistically

7

significant cost capacity factor of 1.20 for small scale biogas installations (Amigun and Von Blottnitz,

2007). This is a significant finding and implies that rather than reducing capital costs (6/10 rule) scale

up of small scale biogas installations significantly increases capital investment costs. The outcome of

this study contrasts with a similar study performed in India by Singh and Sooch (2004). They

investigated three different models of small scale biogas plants and concluded that installation and

operating cost increased proportionally with capacity, implying an n value of 1. However, the capacity

of the plants used in their study did not exceed 6 m3. In addition, each model was assessed

individually as the construction costs varied. Despite these discrepancies, both studies indicate that

small scale biogas installation deviated significantly from the 6/10 rule.

1.1.4 Implementation of AD

The application of biogas, generated by the anaerobic digestion of organic material, for heating,

cooking and energy generation has been widely employed for many years. China began a program of

mass adoption of household biogas in 1975 and within a few years units were being constructed at a

rate of 1.6 million per year. The units were poorly designed and of low quality and by the 1980s many

of these were no longer in use. By 1992, only 5 million units remained operational and many of these

had been redesigned due to leakage. This pattern of rapid introduction and failure of biogas units was

repeated in India, Nepal, Vietnam and Sri Lanka. However, as technology and research improved the

units became cheaper and more robust, particularly following the development of polyethylene

digesters (Ho, 2005). There are currently over 2 million family sized units in operation in India, and

over 200 000 families a year are switching from the traditional fireplace to biogas for cooking and

heating (Singh and Sooch, 2004; Ho, 2005).

Biogas utilisation in Nepal is increasing rapidly and the country has recently overtaken India and

China as the nation with the highest number of biogas plants per capita. Its program is regarded as a

model for successful application of alternative energy in the developing world. There are currently

over 120 000 biogas plants in Nepal, with the Biogas Support Program (BSP) targeting in excess of

300 000 plants by 2009 (Acharya et al., 2005). The BSP is facilitating this by ensuring uniform plant

design and quality, continued R & D and importantly through subsidisation (up to 50% of capital cost)

and stimulation of financial support mechanisms (Acharya et al., 2005).

The production of biogas using AD in a decentralised WWTP in South Africa is explored further as a

case study in the current project. (This will be described in the final report).

In developed countries, on-farm and centralised rural biogas facilities are still significant, but the

majority of biogas generation is performed at a larger scale and is driven by environmental and

economic concerns and the burgeoning carbon economy. Many of the applications of the biogas

generated in this manner require improvement of the biogas quality. The composition of biogas, the

applications and the techniques employed for upgrading biogas in the developed world are discussed

below. The biogas produced by anaerobic digestion is composed primarily of methane (CH4) with

8

significant amounts of carbon dioxide and nitrogen and a number of constituents at trace

concentrations. The typical biogas composition is shown in Table 2:

Table 2: Typical biogas composition

Component Typical range (%)

Methane (CH4) 50-80

Carbon dioxide (CO2) 25-50

Nitrogen (N2) 0-10

Hydrogen sulphide (H2S) 0-3

Hydrogen (H2) 0-1

Oxygen (O2) 0-2

Ammonia (NH3) 0-3

Siloxanes Trace

Methane, also known as natural gas, has a heat of combustion of 802 kJ.mol-1. This value is the

lowest of the hydrocarbon fuels, but on a per unit mass basis methane is more attractive than

complex hydrocarbons. Methane is considered to have an energy content of 39 MJ.m-3. Based on the

typical range of methane concentrations in biogas, a heating value of 18.6-26.04 MJ.m-3 is standard

(Amigun and Von Blottnitz, 2007). The characteristics of biogas are somewhere between natural gas

and town gas (derived from cracking of cokes). The Wobbe index is a measure of the

interchangeability of fuel gases and is defined as the higher heating value over the square root of the

specific gravity (Table 3).

Table 3: Characteristics of different fuel gases

Parameter Unit Natural gas Town gas Biogas

(70% CH4, 30% CO2)

Calorific value (lower) MJ.m-3 36.14 16.1 21.48

Density kg.m-3 0.82 0.51 1.21

Wobbe index (lower) MJ.m-3 39.9 22.5 19.5

Max. ignition value m.s-1 0.39 0.70 0.25

Theor. Air requirement m3 air/m3 gas 9.53 3.83 5.71

Max CO2-conc in stack gas vol % 11.9 13.1 17.8

Dew point °C 59 60 60-160

Biogas can be applied to all applications designed for natural gas, although for certain applications

the biogas needs to be upgraded to remove non-methane components. Table 4 indicates the gaseous

components which need to be removed in order to upgrade biogas for particular applications.

9

Table 4: Gaseous components requiring removal prior to application of biogas

Application H2S CO2 H2O

Gas heater (boiler) < 1000 ppm no no

Kitchen stove yes no no

Stationary engine (CHP) < 1000 ppm no no condensation

Vehicle fuel yes recommended yes

Natural gas grid yes yes yes

Gas boilers do not require high quality gas, although it is recommended that H2S be removed to below

1000 ppm in order to maintain the dew point around 150°C. Where significant amounts of H2S are

present, the condensate will contain sulphurous acid which can corrode chimneys and heat

exchangers, particularly if they are composed of mild materials. Biogas has been used as a fuel for

combined heat and power (CHP) engines for many years, typically at sewage works, landfill sites and

biogas installations. Engine sizes range from 45 kW to several MW depending on the size of the

installation. The gas quality requirements are similar to those for a gas boiler, with the exception that

H2S concentration should be reduced to guarantee a reasonable operating life for the engine. Petrol

engines are more susceptible to H2S corrosion than diesel engines. As a result diesel engines are

almost exclusively used for large scale (> 60 kWel) applications.

The development of modern gas turbines for electricity generation is a significant new development

as these robust engines have comparable efficiency to internal combustion engines. The turbines

facilitate heat recovery in the form of steam. Efficient small-scale gas turbines have not been

developed, with the majority of engines being larger than 800 kWel.

The operation of motor vehicles on natural gas is a growing worldwide phenomenon. There are

currently in excess of 1 million gas powered vehicles worldwide. Biogas has been used to power

these vehicles, but the gas quality demands are high. Therefore, purification of the biogas is required

to increase the calorific value, remove corrosive components (H2S, NH3 and H2O) and particulates,

specifically siloxanes (Dewil et al., 2006). The upgraded biogas has a methane content in excess of

95%. A number of European studies have confirmed that upgraded biogas rated as the cleanest fuel

with respect to impact on the environment, climate change and human health.

A wide variety of technologies exist for the upgrading of biogas to vehicle fuel standards. These are

summarised in Table 5, below.

10

Table 5: Summary of current technologies for upgrading biogas

Component removed Technology Notes Carbon dioxide Water scrubbing Counter-current packed bed, can remove some H2S. Potable water not

needed. Polyethylene glycol

scrubbing (Selexol)

Similar to water scrubbing, but more efficient – reduced solvent demand and pumping costs. Solvent stripped with steam or inert gas to minimise S0 formation.

Carbon molecular sieves

Loose adsorption of molecules in pores – selectivity by pore size and gas pressure differences. Typically 4 units in series. Removes CO2 and water vapour.

Membranes systems

Two main technologies: High pressure (gas-gas). Operated at 36 bar. Good removal of CO2, H2S. Less efficient N2 removal. Membrane life ± 3 yrs. Hollow fibre membranes allow compact units. Low pressure (gas-liquid). Uses microporous hydrophopic membrane at 1 bar. Liquid adsorbent NaOH or amine. Efficient removal of H2S and CO2, both can be recovered as saleable products (S0 and industrial CO2)

Hydrogen sulphide Biodesulphurisation Typically done at digester stage. Use Acidithiobacillus sp. to form S0 and some SO4

2-. Stoichiomteric O2 dosing critical. Biological filters Combined with water scrubbing in large applications. Provides increased

surface area for reaction. 4-6% air added to biogas to facilitate oxidation. FeCl3 dosing to

digester slurry Precipitation of insoluble iron sulphide. Reduces high H2S levels, but further reduction required for vehicle fuel quality.

Iron oxide H2S reacts with iron oxides or hydroxides to form iron sulphides. Endothermic reaction so T°C > 12°C required. Iron oxide regenerated by reoxidation, yielding S0. Regeneration in separate bed. Iron oxide surface area increased by coating wood chips or using pellets.

Impregnated activated carbon

In association with CO2 removal by molecular sieves. H2S oxidation catalysed by impregnating AC with potassium iodide. Best operation at 7-8 bar, 50-70°C.

Solvent scrubbing As described above for CO2, using water, Selexol or NaOH. Siloxanes Solvent extraction Siloxanes (organic Si compounds) are oxidised to SiO2 in combustion

engines. Can cause major damage and significantly reduce engine life. Removed by absorption into specific hydrocarbon mixture.

Oxygen and nitrogen Membranes or molecular sieves

Introduced form air through inefficient harvesting of biogas. High O2 increases explosion risk. Removal is expensive – prevention by efficient biogas collection advocated.

Biogas utilisation in EU countries is extensive, as can be seen Table 6. However, the majority is still

derived from landfill sites rather than liquid wastes or sewage sludge. A relatively small portion of the

gas is currently used in electricity generation.

11

Table 6: European Union energy from biogas data for 2006. Data includes substrate source for anaerobic digestion

and the proportion of the energy applied to electricity generation (adapted from Biogas barometer, 2007).

Country Total biogas % for electricity Biogas per AD substrate source (GWh)

(GWh) Landfill Sludge Other

Germany 22 370 33 6 670 4 300 11 400

UK 19 720 25 17 620 2 100 0

Italy 4 110 30 3 610 10 490

Spain 3 890 17 2 930 660 300

France 2 640 19 1 720 870 50

Netherlands 1 380 21 450 590 340

Austria 1 370 30 130 40 1 200

Denmark 1 100 26 170 270 660

Poland 1 090 26 320 770 0

Belgium 970 22 590 290 90

Greece 810 71 630 180 0

Finland 740 3 590 150 0

Czech Republic 700 25 300 360 40

Ireland 400 27 290 60 50

Sweden 390 14 130 250 10

Hungary 120 18 0 90 40

Portugal 110 30 0 0 110

Luxembourg 100 33 0 0 100

Slovenia 100 32 80 10 10

Slovakia 60 7 0 50 10

Estonia 10 70 10 0 0

Malta 0 0 0 0 0

In many European countries where intensive agriculture, particularly animal farming, is practiced

eutrophication of aquatic environments due to influx of animal waste became a significant

environmental issue. In Sweden, during the 1990s, there was a move towards regional biogas plants

to deal with this issue. Two examples are in operation in the cities of Laholm and Linköping. The

inputs and outputs to these plants are shown below:

Table 7: Input and output data from sample Swedish biogas plants, 2004 data (IEA Bioenergy, 2007)

Input (tonnes/year) Laholm plant Linköping plant

Manure from pigs and cattle 28 000 2 000

Abattoir waste 10 000 30 000

Industrial organic waste 3 000 6 000

Household waste 1 000 250

Other 6 000 7 000

Total 48 000 45 000

Output (tonnes/year)

Biofertiliser for farming 28 000 52 000

Other 15 000 0

Total 43 000 52 0001

Both plants are operated as conventional stirred tank reactors (2 250 m3 and 3 700 m3 respectively)

with residence times of 25-30 days. In both cases the biogas is upgraded prior to use. In the case of

12

Laholm the upgraded biogas is supplemented with 5-10% propane to increase the Wobbe number to

that of natural gas. A portion (25% annual average) is directed to the district heating plant, while the

remainder (18 000 MWh/year) is added to the natural gas grid. The Linköping plant produces an

equivalent of 48 000 MWh/year of biogas, over 95% of which is upgraded to vehicle fuel quality. Since

2005 all public transport vehicles in the city (> 60 buses) have been converted to run on biomethane

and in 2005 the first biomethane powered train was unveiled. This has made it possible to reduce CO2

emissions from urban transport by over 9 000 tonnes per year, while reducing local emissions of dust,

sulphur and nitrous oxides (IEA Bioenergy, 2007).

Management of bioenergy facilities is an important factor in ensuring efficient energy recovery. A five

and a half year study on a bioelectricity plant in Greece, generating energy from biogas derived from

sewage sludge digestion, highlighted this issue. Technical issues and the fact that plant operation was

synchronised with the local supply network resulted in the unit only covering about 16% of the energy

requirements of the facility, despite having the capacity to cover almost 40%. Even at a highly

subsidised electricity price the payback on capital costs has been estimated at 16 years, based on

outputs of the last 2 000 days (Tsagarakis and Papadogiannis, 2006).

In South Africa the first biogas to electricity plant, funded by carbon credits generated under the Clean

Development Mechanism (CDM) of the Kyoto Protocol, was launched in October 2007. The plant is

situated near Mossel Bay and utilises process wastewater generated during the operation of

PetroSA’s gas to liquid plant at Duinzicht as the substrate for anaerobic digestion. Historically, the

biogas generated was flared and has led to an estimated loss of at least 1 300 GWh of gross heat

value since the commissioning of the plant. The plant was developed and financed by MethCap, a

company owned by the international environmental and engineering firm WSP, and will make use of

three GE Jenbacher biogas generator sets, each with an output of 1.4 MW, giving the plant a total

output of 4.2 MW. The plant is expected to generate 33 000 certified emissions reductions (CEMs)

annually.

1.1.5 Costs and Benefits

Capital costs for the construction of small scale (6 m3) biogas units have been shown to vary from

around US$300 to US$900, depending on location, plant design and construction material (Amigun

and Von Blottnitz, 2007; Singh and Sooch, 2004). The cost capacity factor governing increase in

construction cost appears to be regionally dependent, with a value of 1.2 being calculated in Africa,

but only 1.0 in Southeast Asia. Annual operating costs for such digesters are in the range of US$230,

with the significant majority being associated with the acquisition of substrate manure. This is

consistent with European studies (IEA Bioenergy, 2007) which highlight acquisition and transport of

substrate as the major operating expense.

The anaerobic digestion of organic wastes has a number of associated benefits, in addition to the

treatment of a waste source and the generation of energy. Methane is a significant greenhouse gas

13

with a global warming impact considerably greater than that of carbon dioxide according to the IPCC

(Ishikawa et al., 2006). Waste organic material degrades naturally, often under anaerobic conditions,

with the associated release of methane. The controlled digestion of this material with the collection

and utilisation of the biogas significantly reduces uncontrolled methane discharge with the associated

reduction of the greenhouse effect.

Upon completion of the anaerobic digestion process a number of by-products remain. The liquid

effluent resulting from the process typically requires an additional aerobic treatment prior to discharge,

in order to reduce COD and BOD to acceptable levels. In addition to the wastewater a solid residue,

termed digestate, remains. The composition of the digestate depends on the initial feedstock and

influences its potential applications (Singh and Sooch, 2004; Ho, 2005).The typical composition is

predominantly lignin, cellulose, biomass sludge and inorganic components, including ammonia,

nitrates and phosphates. In the absence of pathogens or potentially toxic elements the digestate

makes an ideal biofertiliser, often after a period of aerobic treatment to degrade lignin and oxidise

ammonia to nitrate. If the digestate is not suitable for fertiliser it can be incorporated into building

materials such as bricks or fibreboard (IEA Bioenergy, 2007).

As noted above, biogas is a particularly clean burning fuel. When used as a cooking and heating fuel

in rural households this results in associated health benefits, largely due to the reduction in particulate

material. A survey conducted by the BSP in Nepal indicated a significant reduction in occurrences of

respiratory problems, asthma, eye infections and chronic headaches among respondents who had

switched to biogas from dirtier fuels. Additional benefits were derived from no longer having to collect

and carry firewood (Acharya et al., 2005).

Table 8: Total suspended particle concentrations in flue gas of common cooking fuels (Acharya et al., 2005).

Fuel Total suspended particles (mg/m3)

Biogas 0.25

LPG 0.32

Kerosene 0.48

Crop residue 5.74

An additional monetary benefit derived from the combustion of biogas exists in the form of carbon

credits. For example, each of the functioning digesters in Nepal prevents five tonnes of carbon dioxide

equivalents from being pumped into the atmosphere annually. These “credits” can be sold to

developed countries to offset their excessive emissions and are worth in excess of US$ 5 million.

Despite the encouraging models, current anaerobic digestion technology is not yet sufficiently efficient

to compete with fossil fuel technologies on a purely economic level. For this to occur significant

improvements are required in process technology, or additional benefits such as waste reduction or

greenhouse gas mitigation need to be taken into consideration.

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1.2 Combustion and gasification for the production of steam

1.2.1 Introduction

Combustion: The direct combustion of biomass in the presence of excess air results in the formation of hot flue

gases that are typically used to produce steam to drive the electric turbine, with an efficiency of 33 to

45% (Bain and Craig, 1988). Two approaches for combustion are presented: mono-incineration,

where the sewage or biomass sludge is the only energy source and co-combustion, where these are

combusted in the presence of conventional fuels such as coal. In the combustion of sludges, the first

step focuses on dewatering and thermal decomposition with release of volatiles (devolatilisation or

pyrolysis) resulting in a gaseous stream containing H2 (2 to 5%), CO2 (7 to 24%), CO (28 to 66%) and

hydrocarbons (16 to 33%). As the operating temperature increases, the CO and H2 components

increase, and are subsequently oxidised in the second stage to carbon dioxide and water (Werther

and Ogada, 1999).

Gasification: Gasification is a thermal process yielding a combustible gas (or “producer gas” or “fuel gas”) as a

product. Gasification occurs in an atmosphere of sub-stoichiometric oxygen. Biomass particles are

heated up and release their volatile components in a process termed pyrolysis or devolatilisation and

when the oxygen concentration is limited, syngas rich in CO and H2 is produced.

Either sequentially or simultaneously, depending on the heating rate and the supply of oxygen, the

carbon skeleton is consumed via combustion and gasification reactions leaving residual ash and tar.

The reactions involved and the heat enthalpy (∆H0,rxn) are shown in the reaction scheme below (2):

Figure 2. Reaction scheme for the complete combustion of biomass.

The gas produced is characterised as having a low (5 MJ/Nm3) to medium (15 MJ/Nm3) energy

content, depending on whether air or steam is used as the gasifying agent (Scott, 2004; Prins et al.,

2007; McKendry, 2002). Bubbling bed or fluidised-bed gasifiers and fixed bed downdraft gasifiers are

two types which have found application in small scale production. They display good mixing properties

15

and can effectively crack tars if the bed is catalytically active (Min, 2005).

Table 9: Estimation of biomass resources in SA for conversion to energy. The amounts of biomass, coal fines or

sewage required for a medium size power plant (2 mW electric output or 5 mW heat output) is tabulated.

a) Total biomass available = agriculture residues + forestry residues + 5% of available land dedicated to energy crops (invasive

plant excluded)

b) Total RSA coal production for 2003: 238 million tonnes (www.bullion.org.za).

c) Waste: 0.1 kg/person/day dry weight. South Africa population: 48.5 million.

d) Higher heating values or HHV does not account for the formation of water vapour in the flue gas stream. Hence the need for

adjusting the calc according to the moisture content of the feedstock

e) Adapted from reference (Kavalov and Peteves, 2005)

f) Coal fines moisture content as fired: 64%; Coal fines ash content: 47%; Coal fines HHV: 16 mJ/kg

g) Assuming a thermal efficiency of 40%

1.2.2 Application of combustion / gasification

There are limited examples of this application in South Africa (some application has been reported for

the fruit industry e.g. juicers in Ceres and Grabouw, and for bagasse in the sugar industry). It is widely

used in Europe in order to meet the regulations on material sent to landfill (Lurgi and Philip Conoco

are main designers and suppliers of incinerators). BRI Energy (USA) has developed a unique process

in which a range of waste materials undergo thermal gasification at high temperature into CO, CO2

and H2 (http://techmonitor.net/techmon/05jul_aug/nce/nce_waste.htm).

1.2.3 Costs and benefits of combustion / gasification

When considering the potential for combustion or gasification of biomass from wastewater, it is

essential to consider the potential operational problems of this approach. Specifically, moisture

content, tar formation, mineral content and bed agglomeration are highlighted. The feedstock to a

gasifier has to be relatively dry, with a moisture content of 50 to 60%. Several gasifier designs have

been proposed to include a dryer upstream of the combustor, but there is a clear trade off between

the amount of energy available in the feedstock to the amount of energy expended on drying. Tar

formation lowers the overall conversion of biomass to gas. To convert tars, either in-bed technology or

downstream reforming is required. Primary tars can be thermally cracked at 800°C, while secondary

16

tars can be cracked at 1000°C. In addition, the ash that remains may find application in the brickwork

industry or may need to be disposed of into the environment. There are several negative

environmental effects as a result of the gaseous and solid phase pollutants, (heavy metals, dioxin,

furans and N2O and NOx). However, evidence suggests that the heavy metals, dioxin and furan

emissions can be controlled and that NOx emissions remain below 5% while N2O and CO emissions

are prevented by high temperature and methods such as Activated Carbon Facilitated Oxidation (AC-

FOX) (Brain, 2007).

17

1.3 Utilisation of waste heat

1.3.1 Background

Many processes give rise to liquid streams with elevated temperatures. Depending on the source and

fate of these streams (i.e. waste or product streams), the need may exist for reducing the temperature

of the streams prior to discharge or utilisation. This cooling can be achieved either by dedicated

cooling systems or through heat exchange with other streams. The use of the energy inherent in

these streams in other applications which require heat (rather than cooling or discharging without

energy recovery) has the potential to offset energy demand for other heating applications from other

(non-renewable) resources. The process industry has had decades of experience with heat

integration and the literature, analytical tools and practice are well established in this area. The main

motivation has been financial, but there are also growing incentives to improve energy efficiency to

lower greenhouse gas emissions. Therefore, neither on-site process heat recovery nor energy

recovery from hot gaseous streams are considered in this review. There are opportunities to recover

waste heat from processes where onsite recovery is not already widely practiced. The main

motivations for exploring such opportunities are the potential financial savings, and environmental and

sustainability considerations associated with the supply of energy from fossil fuels.

Here we discuss the heat contained in wastewaters which are to be discharged from industrial

processes. This represents one element of overall process heat integration where waste process

heat, including that contained in process streams which are at a certain temperature and require

cooling prior to being fed into another part of the process, is used elsewhere in the process. The

distinction is made in the context of the current study which is focused on energy from wastewater, in

reality this cannot be considered in isolation. An understanding of fundamental thermodynamics

suggests that the recovery of waste heat can be implemented at any scale. Providing there is a heat

gradient between the two streams (i.e. one is hotter than the other), and that they are separated by a

good heat conductor, energy will flow from the hot stream to the cooler stream. It is noted that if the

temperature difference between the two streams is not great enough the heat transfer will be

ineffective. In addition, the heat capacities of the liquid affect their suitability for heat exchange.

Specific heat capacity (expressed in J/g.K) measures the number of joules of energy required to

change the temperature of one gram of the substance by one Kelvin.

The range of applications for waste heat are a function of the quality of the heat – high grade heat has

a wider range of industrial applications than lower grade heat, although the latter can be used

applications such as pre-heating of feed streams. Thus finding the correct match between available

energy supply and energy requirements is essential.

Given that heat exchange is usually conducted over an equipment boundary (such as a heat

exchanger), the chemical composition of the wastewater is relatively unimportant, providing the

construction materials of the heat exchanger are suitably matched to the wastewater composition.

18

The chemical composition will, however, affect the specific heat capacity of the waste stream and this

needs to be matched with the rate of flow of wastewater so that optimal heat transfer down the

temperature gradient can occur. This application may, however, be inappropriate for safety reasons –

when a wastewater stream is used directly in space heating there may be risks of circulating

hazardous wastewaters through offices and warehouses.

The theory surrounding heat integration is well established, with various optimisation tools such as

pinch analysis being available to design energy recovery systems to maximise the amount of energy

recovered. Integrated heat recovery usually simultaneously considers recovery from both liquid and

gaseous streams. Heat recovery may also be integrated with other system optimisations. For example

a WRC funded study by Fraser et al (2006) explored simultaneous optimisation of recovery of heat

and water in industrial processes using thermal pinch analysis and water pinch analysis. One of the

key findings from the study was the significant benefits in doing heat exchanger network design early

in the design process.

Despite the theory being relatively well established, it is identified that the practice of on-site heat

integration in industry is not as extensive as would be expected. Apart from the impact of fundamental

stream properties, and the requirement for finding matching heat streams, other factors which may

limit the realisation of opportunities for heat integration include ensuring sufficient energy availability

on a site, the fact that heat cannot be transported over long distances, and the availability of suitably

qualified personnel (preferably with postgraduate experience in the field) for opportunity identification,

communication, design and implementation. Anecdotal evidence suggests that the latter is a

significant barrier to implementation of opportunities (PI, 1999).

In addition to on-site waste heat recovery, with the emergence of the meta-discipline of Industrial

Ecology in recent years there has been a regional sharing of process heat, outside of specific plant

boundaries. These applications suffer the similar limitations to implementation described from on-site

usage. Some examples are, however, presented below.

1.3.2 Implementation of utilisation of waste heat

Although a priori planning for recovery of process heat is preferred, retrofitting of reticulation for use of

process heat is possible, depending on the particular application and plant configuration. Furthermore,

the financial viability of retrofitting heat integration is increased if an additional savings such as water

usage is simultaneously realised. Thus, if retrofitting is desired, its feasibility needs to be evaluated on

a case-by-case basis.

The level of monitoring and maintenance for applications of process heat are typically fairly low once

they have been optimised, although once again this will depend on the application. Many industrial

processes need a consistent temperature for operation, hence monitoring is required to ensure that

this is maintained by the supplied heat. At the same time, however, the heat source is likely to be

19

consistent (if it originates from another steady state process), and hence the need for constant re-

optimisation of the system is likely to be limited.

A wide variety of examples of overall process heat integration can be found from around the world,

although as mentioned previously experience suggests that these are not exploited as extensively as

they could be. It is further noted that within the context of this study it is difficult to isolate examples of

energy recovery from wastewater rather than process streams. Furthermore, isolation of case studies

which focus on liquid process heat streams only is difficult as total plant heat optimisation focuses on

a combination of liquid and gaseous streams or gaseous streams alone.

Natural Resources Canada (2004) identifies various projects which may be suitable for application of

process integration, and classifies them on the basis of payback periods. Some examples of these are

shown in Table 10.

Table 10: Example projects suitable for heat integration (Natural Resources Canada, 2004)

Quick Wins Short-to medium-payback projects, typically with a one-to three-year payback

Long-term payback projects, typically with a three-to six-year payback:

Operational improvements to refrigeration systems, e.g. adjusting compressor pressure to ambient temperature;

Hot water management improvements, e.g. balancing hot water supplies with demands;

Enhanced evaporator performance, e.g. improved non-condensable venting.

Instrumentation modifications, e.g. improved control loops;

Optimisation of refrigeration systems, e.g. increasing refrigerant pressures within operational constraints;

Evaporator feed preheat by improving heat recovery;

Rearranging existing heat exchangers to make better use of them, and addition of new heat exchangers;

Pasteuriser heat recovery, e.g. use of heat storage;

Recovering steam condensate to boiler house;

CIP water preheat by heat recovery;

Indirect heat recovery via a hot water loop;

Kettle vapour heat recovery; Cogeneration (CHP) via steam

turbine, gas turbine, or gas/diesel engine;

Boiler economiser (payback will be lower for large steam boilers);

Mechanical vapour recompression (heat pumps);

Increased number of evaporator effects.

The success of integrated applications for waste heat utilization has been varied since it depends

upon fostering collaborations for sharing energy and other resources. A good example of regional,

integrated waste heat utilisation is the Asnæs power station in Kalundborg, Denmark which was not

included in planning during construction of the power station (http://www.symbiosis. dk/, 2007). Asnæs

is Denmark’s largest power station with an installed capacity of 1 037 MW. The excess heat from the

power station provides process steam for the local refinery, pharmaceutical and enzyme

manufacturing factories, and supplies heating for over four thousand households in the town. In

addition to energy integration, other benefits include reduced water usage, waste recovery, and

additional employment creation. At the Bruce Energy Centre in Canada some forward planning to

integrate energy usage was undertaken. The Centre is situated adjacent to a nuclear Ontario Power

station enabling a supply of steam and electricity at a low cost. The electricity and steam is used by

an alcohol distillation operation, a company which dehydrates crops to produce nutrient rich feeds for

livestock, a food manufacturer which concentrates fruit and vegetables, a plastic manufacturer and for

heating a greenhouse.

20

A number of further examples of the use of waste heat for district household heating in colder climates

can be found. The city of Götheborg (Sweden) uses process heat from industries to supply a

proportion of the district heating system. The heat is from waste heat from oil refineries in the vicinity

of the city, and from a waste-fired CHP plant. Heat is also recovered via heat pumps that utilise

sewage water (Holmgren, 2007). Heat integration is also feasible for low-level industrial waste heat as

shown by a study in Delft (Netherlands). The heat generated by a pharmaceutical plant in the North of

the city at a temperature of between 25 and 35°C is upgraded by heat pumps before being taken to

the central heating grid (Ajah et al., 2007). Similarly, low grade waste heat from power stations and

industrial applications has been used for water heating in aquaculture applications. The Asnæs trout

fish farm (Denmark) and the “Happy Shrimp” company (Netherlands) make use of waste heat from a

power plant to heat aquaculture ponds. Similar applications are seen the Czech Republic, Canada,

France, the US and others.

Other examples of applications include:

Various Exxon Mobil’s refineries have achieved significant savings through heat integration

(Exxon Mobil, 2007),

Through heat integration, a pulp and paperboard mill in Canada achieved savings on

purchased thermal energy of 15% and reduced the temperature of effluents by 3°C, with a

simple payback period of less than 10 months for most projects (Natural Resources Canada,

2007).

The city of Vancouver has established heat recovery from municipal sewage. Raw sewage is

passed through a heat exchanger where thermal energy is captured. Electrically-powered

heat pumps are used to boost temperatures from the 10 to 20o C sewage temperatures, to

the 65 to 90o C range for the hot water distribution system. The heat pump will produce

roughly three units of heat energy for every one unit of electricity consumed — an efficiency

rating of 300 per cent. Southeast False Creek will be the first sewage wastewater heat

recovery plant in North America, and could set a precedent for the development of this

technology in other locations within Vancouver, and in other urban areas.

(http://vancouver.ca/sustainability/documents/sefc-factsheetheatplant-back.pdf)

Heavy industry in South Africa lends itself favourably to heat integration within and between industrial

plants. However, the limitations presented above, particularly those surrounding human resource

capacity and integrated (pre-) planning have played a role in not realising further opportunities thus

far. It is further identified that the potential for use of low grade heat for district heating is limited due to

the milder weather than in many parts of Europe where such opportunities are realised.

1.3.3 Assessment of costs and benefits in utilisation of waste heat

The primary costs associated with heat integration relate to any additional piping, pumping, insulation

and equipment requirements for heat exchange. On a process plant these are likely lower than when

sharing of process heat is outside the plant boundaries. At the same time, use of process heat can

21

allow for the downsizing or elimination of the need for other energy sources such as boilers. The

financial benefit and payback periods on investment will vary widely depending on the situation. In

general a retrofit will be more expensive than incorporating integration into the original project design.

Operating expenditure will be relatively low, and will be limited to pumping costs and general

maintenance.

Unlike many of the other technologies presented in this report, use of process heat achieves no

wastewater treatment benefits (apart from removing the need for cooling prior to discharge which may

otherwise be required). Furthermore, there is no added benefit associated with the production of

secondary products or wastes which may require management elsewhere. Wastewater containing

pollutants from which heat energy is recovered will still require treatment prior to discharge.

22

1.4 Fermentation to produce biomass and secondary products

1.4.1 Background on fermentation of wastewater

Wastewater streams often contain organic compounds which may be useable as carbon and nutrient

sources for growth of microorganisms. Such compounds would be referred to as fermentable

substrates, and these fermentations can generate fuels such as bioethanol and biogas (see sections

1.1 and 1.6) directly, but also:

Serve as pre-treatments for energy from wastewater technologies (such as enhancing

bioethanol or biogas production from certain wastewaters).

Provide a nutrient medium to support the growth of biomass (microorganisms, algae or plant).

This biomass can subsequently be combusted or used in bioethanol, biogas or biodiesel

production technologies.

The applications of fermentation include treating wastewaters from olive-mills dairies, breweries, wine

distilleries, whiskey distilleries, slaughter-houses, potato waste, and the tomato canning industry.

Examples of South African wastewaters containing potentially fermentable substrates with estimates

of volume and load are shown in Table 11, below.

Table 11: Examples of South African wastewaters containing fermentable substrates Wastewater COD (g/L) VOLUME

(ML per year) Load

(Mg/ year) Sewage 0.8 -1.2

Av= 0.86 2 766 400 2 379 104

Dairy* 1.5-9.2 Av= 5.3

6346 33 637

Red meat and poultry abattoirs

11-21 Av= 16

11 000-31 000 336 000

Olive production 55 – 201 Av= 100

89 8 900

Fruit processing 5-15 Av=10

14 000 140 000

Distilleries: -Grain and grape

-Sugar cane molasses

25 –45 Av=30

Grain: 63 Grape: 342

12 150

35 3500-4000 131 250 Winery 6 1000 6 000

Brewery 3 28 000 23 533 Textile Industry 0.1 -2.5

Av=1 25 000 25 000

Pulp and Paper 0.7 80 000 56 000 Petrochemical waste 0.2-0.9

Av= 0.7 1140 (crude);

3048 (synthetic); 2 to 11(re-refinery)

2 939

*Only the formal dairy industry sector considered here. Other animal husbandry sectors (cattle for beef, pigs and chicken are

not shown (case studies in the Appendix for further details).

Important issues which need to be considered in developing energy-yielding bioconversions from

wastewaters include:

The volume, variability and/or characteristics of the wastewater from any one industrial

activity may be too low to provide suitable media for the fermentation. However, combining or

integrating of wastes may be feasible or necessary.

23

The concentration of carbonaceous components may be very low and need de-watering in

order for efficient fermentation to be achieved. This dewatering process will have its own

energy input requirement.

Many industrial wastes may contain other components which are inhibitory to microbial

growth. These may need to be removed by, for example, solvent extraction or precipitation

before the waster is used for fermentation. This extraction may yield economically valuable

secondary products (as in the case of olive wastewater).

1.4.2 Implementation of fermentation of wastewaters

The basic fermentation technologies and knowledge base are well established. The requirements for

sterility, cooling/ heat transfer and extent of aeration are important issues in the scale-up of

operations. Wastes would be introduced into the bioreactor or separation/extraction equipment and

products would undergo downstream processing that is separate from the main plant and therefore

there should be little problem with retrofitting into existing infrastructure. Bioconversion is a well-

established laboratory technology that generally requires constant monitoring of several parameters

for the control and optimisation for a particular waste stream. Many parameters (e.g. mixing, pH, dO2,

nutrient levels) can readily be automated and controlled, but in some cases may require skilled

operators.

The most abundant agricultural wastes are rich in complex polymeric molecules such as cellulose that

require significant energy for breakdown. In nature, several hydrolytic and oxidative enzymes

produced by a variety of fungi and bacteria work in synergy to degrade cellulose, hemicellulose and

lignin (Gosh and Gosh, 1992). Fermentations can provide biological methods of pre-treatment where

the addition of water may be required for processing of the waste. The pre-treatment of corn stover

with fungi such as Cyathus stercoreus and Phanerochaete chrysosporium has been shown to

effectively improve the cellulose digestibility three- to five-fold (Keller et al 2003), thereby increasing

the yields in the subsequent ethanol fermentation. The fungus Trichoderma reesei was shown to

improve the acid hydrolysis of cotton waste with a subsequent increase in the yield from ethanol

fermentation (Rajesh et al., 2008). Similarly, the biological conversion of wheat and rice straw to

ethanol has been show to be feasible with a pre-treatment of Aspergillus niger or Aspergillus awamori

followed by standard Saccharomyces cerrevisaeia fermentation to yield 2.5 g/L ethanol (Seema et al.,

2007).

The two-stage or multi-stage anaerobic digester systems also consist of fermentation pre-treatments

(see section 1.1). The different digestion vessels are optimised to bring maximum control over the

microbial communities so that the initial stages of hydrolysis, acetogenesis and acidogenesis occur in

separate bioreactor(s) to the methanogenesis that produces methane-rich biogas. The two stage

system has been shown to increase efficiency by reducing the residence time while increasing the

yields of biogas attained for a range of organic substrates (Schäfer et al., 1999; Gunaseelan, 1997).

This has been successfully demonstrated for municipal waste and represents 10% of the anaerobic

24

treatment capacity in Europe (de Baere 2000; Pavan et al., 2000). The two-stage or multi-stage

digestion systems have particular application to wastewaters rich in complex polysaccharides, lipids,

and proteins that are inaccessible for other energy from wastewater technologies such as bioethanol

production (Zeeman and Sanders 2001, Ahring 2001). For example, the biogas yields from cellulose-

rich wastes can be greatly improved through pre-treatment using a two–stage system (Gijzen et al.,

1998) and this has been successfully been applied to the water hyacinth, Eichhornia crassipes, that

often invades watercourses in South Africa (Kivaisi and Mtila, 1997). Similarly, the two stage system

has been shown to be effective for biogas production from the lipid and protein rich wastewaters of

slaughterhouses and fish processing factories (Saddoud and Sayadi 2007; Ahring 2001).

Fermentations have also been used for the production of other valuable secondary products. These

include the cultivation of biomass for animal feed (Ugwuanyi et al., 2007), growth of the valuable

biocontrol fungi, Trichoderma viride, (Verma et al., 2006), and the production of valuable secondary

metabolites such as antibiotics and carotenoids (Ho, 2005; and http://www.ctre.iastate.edu/research).

1.4.3 Assessment of cost and benefits in fermentation of wastewaters

An assessment of the costs and benefits is complex and specific to the waste stream and context.

Costs include the capital costs of the bioreactor and associated equipment, operational costs for

trained staff to maintain and optimise cultures, mixing, aeration (where required), sterilisation and

separation costs. The benefits include water bioremediation through removal of COD (waste disposal

reduction) the energy generated from wastewater and the value of any additional secondary products.

Fermentations can be used to directly produce energy products such as bioethanol and biogas

(discussed elsewhere section 1.1 and 1.2 and 1.6), as a pre-treatment step, or to generate biomass

from the nutrients in the waste stream. The generated biomass may later be used as an energy

resource for a secondary energy generating technology (i.e. combustion/gasification, bioethanol,

biogas or biodiesel production) and/or it may contain valuable secondary products (e.g. plant

fertilisers, animal feeds or neutraceuticals). The feasibility will depend on integration an assessment

of these complex factors, but will primarily depend on the characteristics of the feedstock and its

supply. In the chain of collection and transport, wastes often become complex wastewaters simply

because of dilution; thereby rendering the wastewaters unsuitable for generating energy using a

number technologies. Where this cannot be avoided, the use of dilute wastestreams to grow biomass

or the combining of waste streams may enable an energy from wastewater process to become

feasible.

25

1.5 Algal biomass for the production of biodiesel

1.5.1 Background on production of biodiesel from wastewaters

In order to utilise the dissolved solutes within wastewater for energy generation, it is valuable to

convert these to an energy rich raw material that can be removed from the dilute wastewater

environment through a physical separation of phases (i.e. to create the energy raw material in a

phase other than the liquid phase.) One approach is the formation of biomass for subsequent

processing. An extension of this approach is the accumulation of hydrocarbons of low oxidation state

as oils in the biomass, typically using algae. In this environment, the wastewater body provides

nutrients to support algal growth. The algae synthesise cellular components, including oils, using

energy from sunlight (photosynthesis). Additionally, the major carbon source,CO2, can be provided

from an industrial off-gas. This approach has been successfully demonstrated by using Botryococcus

braunii to produce hydrocarbons from a secondary sewage effluent (Banerjee, 2007). Due to their

simple cellular structure, microalgae have higher rates of growth, photosynthesis and productivity than

conventional crops. The cultivated algae could be used in several technologies such as anaerobic

digestion or the extraction of oil for biodiesel production.

Certain species of algae can produce large quantities of vegetable oil, up to 80% dry weight, as a

storage product (Becker 1994). Hence, algae can be up to 23 times more productive per unit area

than the best oil-seed crop (Table 12). Other advantages are that algae can be grown on wastewaters

(saline and other) and consume less fresh water than conventional crops since many species can

tolerate brackish or saline waters (Tsukahara and Sawayama 2005).

The conversion of the oil through transesterification is shown below (Figure 3). The transesterification

reaction converts triglycerides (oil) to alkyl esters (biodiesel) by the addition of an alcohol such as

methanol in the presence of a catalyst. Glycerol is a by-product (Harding, 2007).

Figure 3. Transesterification of oil to produce biodiesel

26

Most algal species with high lipid contents considered for biodiesel production are usually either

green algae (Chlorophyceae) or diatoms (Bacillariophyceae) (http://www1.eere.energy.

gov/biomass/pdfs/biodiesel_from_algae.pdf.). The main components of algae are proteins (typical 40

to 60%), carbohydrates (typically 15 to 30 %), nucleic acids (<5%) and lipids. Maximum reported lipid

contents for algal species noted for oil production are from 20 to 80% (as % dry weight). The type of

lipid varies with species, but generally the hydrocarbon chain is in the C16 to C20 range which is

suitable for making biodiesel (Ma and Hanna, 1999; Becker, 1994).

Algae have typically been grown in open raceway ponds. This has been by far the most common

method of algae production as such structures are fairly easy and cheap to construct or existing water

bodies may be used. The 5000 to 6000t per annum dry biomass of algae produced worldwide is

generated mainly in open systems. The most effective type of open system used is the raceway, an

oblong looped pond mixed by a paddlewheel. These raceways usually have water depths of 15 to

20cm with biomass concentrations of up to 1 g/L and productivities of 50 to 100 mg/L/day. The main

advantages of open systems are their low cost, use of natural illumination and lack of oxygenation.

Disadvantages include problems with pollution and contamination, evaporation of water and

susceptibility to environmental conditions such as temperature and dilution by rain (Ma et al., 1999;

Pulz, 2001). In contrast, for closed photobioreactor systems, nearly all parameters can be controlled

by the operator. However, photobioreactors are generally much more costly to build and operate than

open systems and are constrained by the need to achieve optimal light penetration. They often utilise

artificial lighting, which decreases the sustainability of the system, but they can achieve higher

productivities – 800 to 1300 mg/L/day (Pulz, 2001).

Algal growth rates and productivities at laboratory scale are very difficult to replicate on a larger-scale

as the area receiving sufficient sunlight becomes limiting at large scale. However, once pilot scale is

achieved, most algal ponds/reactors (e.g. raceways) are modular in design, so scaling up of algal

growth does not change the cost or efficiency significantly. For harvesting, the concentration of algal

biomass is an important determinant. Certain unit operations (straining, filtration, flocculation) become

more economical at larger scale. Centrifugation, in particular, benefits from increased scale since it

requires large capital investment. Processing of harvested algal cells would benefit from economy of

scale (particularly usage time of equipment), but small-scale oil extraction and biodiesel production

are also feasible as the equipment is relatively simple and can be modularised. Large-scale

Table 12: Average productivities of common oil crops compared to algae

27

production may make it economical to purify the glycerol by-product of biodiesel production.

A variety of waste streams can be used for algal oil production, but must have sufficient

concentrations of inorganic nitrogen and phosphate to enable the support of algal growth. The optimal

range of nutrients to support algal growth is 0.2 to1.0 g/L nitrogen (as ammonia or nitrate) and 0.05 to

20 mg/L phosphate (Banerjee et al., 2002; Tsukahara and Sawayama, 2005). Organic carbon often

promotes growth of algae, but high levels of organic carbon can be inhibitory to growth and promote

the growth of heterotrophic bacteria. Sewage wastewaters are a particularly good medium and algae

help to remove the COD, nitrogen and phosphorous load, as well as many odours. Further, the

photosynthetic algae generate enough oxygen to allow bacterial oxidation of organic waste. This

releases nutrients (CO2, ammonia, phosphate) for conversion into additional algal biomass

(Benemann et al., 1977). It should be noted that nitrogen limitation is often used to increase the oil

yield of a variety of species. This is only possible in a high-N content waste stream if algal culture

continues until the nitrate, ammonia and urea are depleted and then nitrogen limitation occurs. In

secondary sewage treatment, it took 9 days for B braunii to reduce nitrate levels to below 0.01 mg/L

(Tsukahara and Sawayama, 2005). Studies at UCT are underway to relate nitrogen availability to lipid

productivity.

Benemann et al. (1977) reported that almost all sewage and wastewater oxidation ponds are of the

‘facultative’ type. These are 1 to 3 m deep and arranged in cells of up to 50ha, mixed only by wind

action and recirculation of effluent. The bottom of these ponds is anoxic, resulting in fermentation of

settled sludge and algae. This design is optimal for waste-treatment but not algal growth. To maximise

algal biomass, ponds must be shallow (30-50cm) and have short retention times (2-4 days in

summer) and be mechanically mixed. These are called ‘high-rate’ algal ponds and can produce over

100 mg/L algal dry weight. This requires a retrofitting of ponds for algal growth for either biomass

(linked to combustion or fermentation) or lipid production.

Harvesting involves concentrating the algae from their dilute suspension, which is a considerable

challenge (Benemann et al., 1977). Settling or chemical flocculation can be economical, but can also

take considerable pond-time (settling can take 2-3weeks). Oil extraction techniques are similar to

those used in the food and vegetable oil industry, but require a purpose-built extraction plant close to

the algal ponds, for integrated operation. Transesterification requires unique equipment, but does not

have to be on-site and extracted oil may be transport to a central transesterification facility.

The components are relatively simple and consist of ponds, pumps and some form of harvesting and

oil extraction process that can be operated by trained, but not highly skilled personnel.

Transesterification is fairly simple but strict production standards are required in order to meet

specifications. Further, chemical handling requires a level of skill, but this operation can be

centralised.

28

1.5.2 Implementation of biodiesel production

No large-scale production of biodiesel from algal oil has been reported. Biodiesel has been made

from algal oil at the lab scale and it has been shown to be technically feasible (Miao and Wu, 2006)

and there are several examples.

Piggery wastewater in rural Korea used to grow Botryococcus braunii for hydrocarbons (An et

al., 2003). The growth rate of B braunii in secondarily treated sewage was 0.35 g/L/week and

algal concentration could be maintained at 400 mg/L after 11 days. (Tsukahara and

Sawayama, 2005)

A group of 7000 gallon ponds and one 1.6 acre pond in California showed that algae could

probably be produced for anaerobic digestion at less than $0.01/pound (1960 $) in open,

shallow, sewage-fertilised ponds with recycle of digester residue and water and the recovery

of heat and CO2 from the power plant for use in the growth process (Oswald and Golueke,

1955). Used algal species found naturally in sewage water (mostly green algae such as

Chlorella and Scenedesmus (together constitute 95-99% of wild sewage algal population in

California), Chlorogonium, Chlamydomonas, Euglena and Microactinium). Chlorella

pyrenoidosa could be maintained at 250 mg/L. A 2-3 day retention time was optimal and a

mixing velocity of at least 1 foot/sec required.

Solix biofuels in Colorado (http://www.solixbiofuels.com/) builds and tests photobioreacors

and a second generation prototype has been launched.

Some parts of South Africa (e.g. in the Upington area, N Cape and near Messina, Limpopo) have

ideal conditions for growth of high-oil content algae: long sunlight hours during summer, relatively high

temperatures. South Africa also has more open land (compared to Europe), and a relatively large

demand for transport fuels such as diesel (8.7 billion litres used in 2006). Several saline waste-water

streams relating to the mining and desalination industries could also potentially be used by salt-

tolerant algal species, supplemented by seawater in coastal areas.

South Africa does not have significant large-scale algal farming experience. The biofuels industry in

South Africa is also still in the formative stage. There are a number of fine chemicals and nutraceutical

algal production, including NCSA, Cape Carotene, BioDelta.

The most recent development is a New Zealand company called Aquaflow Bionomic producing

biodiesel from wild algae grown in sewage ponds. This is believed to be the world's first commercial

production of bio-diesel from "wild" algae. The company expects to produce biodiesel at the rate of at

least one million litres of the fuel each year. They propose to harvest algae directly from the settling or

stabilisation ponds of standard WWTP and other nutrient-rich wastewaters (http://www.nzherald.co.nz/

and http://www.aquaflowgroup.com/)

29

1.5.3 Costs and benefits of biodiesel production from wastewaters

The costs vary widely depending on whether open ponds or closed bioreactors are used, the source

of the water, power used, nutrients and CO2 and the use of wastewater, flue gas, recycled heat and

power. There have been multiple pilot-scale trials of algal growth for biodiesel. The Aquatic Species

Programme (funded from 1978-1996) was a DOE program focussed on production of biodiesel from

high-lipid content algae in open ponds, using waste CO2 from flue gas. Great advances were made in

algal culture engineering, but the process, although technically feasible, was found to be

uneconomical at the time due to high cost of algae production. This technology has not yet been

successfully implemented anywhere on a large scale, mostly due to low algal yields making the

process uneconomical. The main factors were (1996):

* Low cost of conventional fuel (diesel prices were hovering around $1.10 per gallon).

* No monetary value for carbon mitigation capability of biodiesel.

* Higher than expected cost of the production system.

* Lower than expected productivity of outdoor open pond system.

A decade later, the world is different. Diesel is selling at or above the $2.50 mark; while the cost of

petroleum oil continues to increase. There are also new incentive s from the Clean development

Mechanism and Certified Emission Reductions (‘carbon credits’) for algal biodiesel. A robust market

for renewable biofuels is emerging.

30

1.6 Fermentation for production of bioethanol

1.6.1 Background on bioethanol production from wastewaters

The production of bioethanol as a renewable liquid fuel is well established. Bioethanol can either be

used on its own or blended with conventional liquid fuels to form either Gasohol or Diesohol (as an

alternative to butanol). Factors driving its position as a biofuel include ease of transport as it is a

liquid, a heating value of about 67% of gasoline, ability to blend it directly to comprise some 10% of

gasoline with no modification of engines required and ability to enhance the octane rating (Bailey and

Ollis, 1986). While most development of this fuel has focused on its production from agricultural

materials, potential exists to utilise the organic fractions in wastewater and wastewater sludges for

conversion to ethanol (Bailey and Ollis, 1986).

Typically, bioethanol is formed by fermentation of simple sugars such as glucose and fructose to

ethanol under anaerobic conditions. Alternatively bioethanol may be formed by fermentation of the

monosaccharide to acetic acid and subsequent enzymic conversion to ethanol (Shuler and Kargi,

(2002; Nielsen and Villadsen, 1994). The typical batch industrial process has a fermentation time of

approximately 50 hours at 20 to 30 °C. A yield of ethanol from sugar is approximately 90% of the

theoretical value, at an ethanol concentration of 10 to 16% (v/v) (Bailey and Ollis 1986).

Specific drawbacks of the conventional ethanol fermentation process include the dominant

contribution of raw materials to cost and environmental impact. The slow reaction rates result in large

fermenter volumes that contribute to capital cost, water usage and to the subsequent volume of

wastewater generated, all of which influence production cost. It is estimated that 3 to 10 litres of

wastewater are generated per litre of ethanol produced. Indeed this wastewater becomes a further

resource for energy generation. The high water content (typical ethanol concentrations achieved are

10 to 16 % (v/v)) implies that subsequent distillation is energy intensive. Conventional ethanol plants

may expend more than 30% of the bioethanol heating value in distillation. Key considerations for

efficient ethanol production include the use of continuous processes with cell retention or recycle,

allowing productivities to be increased 10 to 20 fold over batch processes (comparative operating

costs are estimated to decrease by 1.6 to 1.7 fold) (Bailley et al., 1986). Further, the use of bacteria

such as Zymomonas mobilis or other novel species enable enhanced productivity through increased

glucose uptake rates and ethanol formation rates. The use of thermophilic microorganisms would

further enable increased production rates and allow partial distillation of the ethanol during the

fermentation.

1.6.2 Implementation of bioethanol production

Various examples of the use of wastewater and wastewater sludges for the production of bioethanol

have been reported. The VTT Technical Research Centre of Finland has developed technology for the

distributed production of ethanol by fermentation of food processing wastewaters. This technology

enables production even at a small scale and is estimated to have potential to meet 2% of the total

31

volume of petrol sold in Finland and is currently being commercialised by St1 Biofuels Oy

(http://www.vtt fi/?lang=en.62).

In the FlexFuel project, developed by the University of Southern Denmark, the core biogas plant has

been expanded to include a pre-treatment plant for household waste, a pre-treatment plant for steam

explosion for treating solid waste to release monomeric units, and a unit for fermentation to ethanol

and its subsequent distillation (http://websrv5.sdu.dk/bio/flexfuel.). The overall process uses a wide

ranging combination of raw and waste materials and allows recovery of both bioethanol and biogas

for processing to combined heat and power, as well as N- and P-rich products for agriculture

(Figure 4).

Figure 4. Illustration of varied feedstocks to the ethanol – biogas process (University of Southern Denmark).

There are several reports on the increased ethanol production following the processing of wastewater

from ethanol fermentations. CSR Distilleries in Australia implemented a continuous fermentation of the

distillery dunder, previously discarded to sea, resulting in the further production of ethanol, a reduction

in water consumption by 70%, and the production of a biodunder secondary-product and is sold as a

potassium rich agricultural fertiliser (http://www.p2pays.org/ref/04/03358.htm.; Rao et al., 2007).

Ethanol and hydrogen (H2) can also be produced by fermentation of the glycerol containing

wastewaters from the biodiesel production process. Despite a high salt content of the wastewaters,

32

productivities of 30 to 65 mmol/L/h and yields 0.80 to 0.85 mol ethanol per mol glycerol were been

achieved using the bacteria Enterobacter aerogenes (Ito et al., 2005).

BRI Energy (USA) has developed a unique process in which a range of waste materials undergo

thermal gasification at high temperature into CO, CO2 and H2

(http://techmonitor.net/techmon/05jul_aug/nce/nce_waste.htm.). The CO is then transformed into

ethanol by the acetogenic bacterium Clostridium ljungdahlii and the ethanol distilled. Typical yields are

340 litres per tonne bio-solids (municipal solid waste, biomass waste, animal wastes, etc.), increasing

to 680 litres per tonne as the oxidation state of the bio-solids decreases (used tyres, hydrocarbons).

1.6.3 Costs and benefits in bioethanol production from wastewaters

While the potential of ethanol production from wastewaters and wastewater sludges has been clearly

demonstrated through the selection of examples given above, a number of potential drawbacks must

be addressed in the implementing of this technology (Von Blottnitz and Curran, 2007). Firstly, the

ethanol fermentation is a very dilute process using large quantities of wastewater and large reactor

volumes. The process intensification is a key factor and may require concentration of the waste

organics where these occur as dilute solutes, suggesting a capital cost and energy penalty. Where

sterile or hygienic processes necessitate the use of heat treatment, the energy requirement is a

function of volume. Further, the ethanol product needs to be continuously extracted to prevent

inhibition of the process and requires energy intensive conventional distillation. The typical ethanol

producing microorganisms do not utilise a broad range of organic materials, hence enzymatic pre-

treatments or microorganisms with a broad substrate range are sought. Finally, co-production of

ethanol and H2 is of increasing interest, allowing effective utilisation of both the carbon and H2

fractions, compared to the alternative production of H2 and CO2.

The production of ethanol is currently limited to the use of carbohydrates (typically hexose

monosaccharides such as glucose and fructose) that are used in fermentations with yeasts

(Saccharomyces spp.) and, more recently, bacteria (Zymomonas spp). These wastewaters are

readily available in the fruit processing industry, which are therefore appropriate sites for generating

energy from wastewaters using bioethanol fermentations. However, the majority of agricultural

processing wastes consist of lignocellulose (combination of cellulose, hemicelulose and lignin) and

the required pre-treatment is a major challenge to the development of cost effective cellulose to

ethanol technology (Mosier et al., 2005). Pre-treatment can be carried out in several ways including

mechanical (Cadoche and Lopez) 1989), steam explosion (Gregg and Saddler, 1996) ammonia fibre

explosion (Kim et al., 2003), acid or alkaline pre-treatment (Damaso et al., 2004; Kuhad et al., 1997)

and biological treatment (Keller, et al., 2003). A number of thermochemical pre-treatment methods

have been developed to improve digestibility (Wyman et al., 2005), and these can also serve as a

source of process heat energy that can be recovered (Sheehan et al., 2003). The pre-treatment may

also involve the selective removal of compounds that are inhibitory to the downstream fermentation

processes.

33

All biomass for bioethanol goes through several steps that require resources (including energy inputs)

and infrastructure: the biomass needs to be grown, collected, dried, fermented, and the biofuel

combusted. The energy balance (total net energy gain; which is the energy output from combustion

the resulting ethanol fuel minus the energy input required to obtain and process the biomass) of

bioethanol from crops is thought to be marginally net positive with current farming practices (e.g.

energy balance of 1.3 for corn in the USA while and up to 8 for sugarcane in Brazil,

http://ngm.nationalgeographic.com). For the traditional corn (maize) fermentation up to 67 % of the

energy inputs are for the fermentation and distillation process, while the remaining 23% are energy

costs associated with the biomass production (agricultural crop) (Chandel et al., 2007 and

http://www.carbohydrateeconomy.org/). The use of wastewaters for bioethanol production reduces

the significant biomass production costs as well as the economic and social conflicts of food crops

being used for fuel production. There are also several developments that have optimised bioethanol

fermentations for improved yields and efficient ethanol distillation (Oh et al., 2000; Iraj et al., 2002). In

particular, membrane distillation (molecular sieve technology) has been shown to be the most efficient

and cost effective option among the available distillation processes (Binat and Simandl 1999 and

http://spie.org/x14497.xml) and it can significantly reduce the energy inputs involved in traditional

ethanol distillation.

34

1.7 Microbial fuel cells

1.7.1 Background on microbial fuel cells

Bacteria can be used to generate electricity that can be harvested in microbial fuel cells (MFCs)

(Davis and Higson, 2007). In a MFC, bacteria that oxidise a substrate are kept physically separated

from the electron acceptor by a proton exchange membrane. Electrons pass from the bacteria to the

anode in the same chamber and then via a circuit to the cathode where they combine with protons

and oxygen to form water. The difference in the potential coupled to electron flow produces electricity,

Figure 5 (below).

Figure 5. Diagram of a dual chamber microbial fuel cell (MFC). Organic material in wastewater entering the anode

chamber, is oxidised by microorganisms. The electrons (e-) can be transferred to the anode and via a wire to the

cathode where oxygen is reduced to water. A proton exchange membrane allows protons (H+) to flow between the

chambers. The flow of electrons (e-) is electrical current (electricity).

A number of studies have been conducted on MFCs operated with both pure cultures (e.g.

Shewanella putrefaciens, Pseudomonas aeruginosa, Geobacter sp., Rhodoferax ferrireducens and

the thermophilic Bacillus licheniformis or Bacillus thermoglucosidasius) (Kim et al., 2002; Rabaey and

Verstraete, 2005); and mixed cultures that were enriched either from sediment or activated sludge

from wastewater treatment plants (Rabaey et al., 2005; Lowy, 2006; Logan and Regan, 2006). MFCs

with mixed bacterial cultures have some important advantages compared to MFCs operated with pure

cultures, including a higher resistance to process disturbances, greater substrate versatility and a

higher power output (Rabaey et al., 2005). MFCs have recently been modified for hydrogen

production (Ditzig et al., 2007 and Liu et al., 2005). This bioelectrochemically assisted microbial

reactor, BEAMR, results in the production hydrogen at the cathode with high yields and a limited

surplus energy investment.

35

1.7.2 Implementation of MFCs

No large-scale MFC have been reported. Several aspects needed for an efficient MFC are hampering scale up:

- The influent needs to reach the whole anode matrix sufficiently

- Protons need rapid diffusion towards the membrane

- Sufficient electrical contact needs to be established between bacteria in suspension and the

anode

- Sufficient voltage needs to be reached over the MFC to have a useful power

- Instatement of an aeration device should be avoided

To overcome these bottlenecks, several techniques can be copied from conventional fuel cells, such

as the use of fuel cell stacks, constructed from several plate shaped fuel cells (Larminie and Dicks

2000). MFCs can enhance the growth of bioelectrochemically active microbes during wastewater

treatment thus they have good operational stabilities. Continuous flow and single-compartment MFCs

and membrane-less MFCs are favored for wastewater treatment due to concerns in scale-up (Jang et

al., 2004; Moon et al., 2005; He et al., 2005). No large scale/ commercial plants are in operation.

Wastewaters of very different characteristics can be used. Sanitary wastes, food processing

wastewater, dairy manure, swine wastewater and corn stover are all suitable biomass sources for

MFCs because they are rich in organic matters (Suzuki et al., 1978; Liu et al., 2004; Oh and Logan,

2005; Min et al., 2005b; Zuo et al., 2006). Up to 80% of the COD can be removed in some cases (Liu

et al.,2004; Min et al., 2005b) and a Coulombic efficiency as high as 80% has been reported (Kim et

al., 2005). This technology can use bacteria already present in wastewater as catalysts to generate

electricity while simultaneously treating wastewater (Lui et al., 2004; Min and Logan, 2004).

The technology can be retrofitted into existing architecture. For example it may replace or supplement

a stage in the WWTP and provide additional source of power without conversion (i.e. produces

electricity directly). They can be employed in remote locations lacking electrical infrastructure or

critical applications that should not rely on external power inputs (i.e. biosensors). The design of the

equipment is still in development and may require skilled technical staff for design, installation and

initial support for the user. The equipment can be constructed with basic skills and understanding of

electronics. The systems are largely in the development stage and therefore may require considerable

optimisation. Although many operational difficulties have been to be overcome before full scale up and

implementation, a limited degree of monitoring is necessary. As long as there is constancy in the

supply and characteristics of the wastewater, the systems can operate robustly and efficiently with

very little operations and maintenance coats.

There are no commercialised applications of MFCs for energy production from wastewaters. There

are a myriad of small electronic remote devices that have favoured the development of MFCs

(Wilkinson, 2000). The robot, EcoBot-II, solely powers itself by MFCs to perform some behavior

including motion, sensing, computing and communication (Leropoulos et al., 2003b; Leropoulos et al.,

36

2004; Melhuish et al., 2006). Additional applications of MFCs are in biosensor and remote sensor

development (Liu et al., 2003); such as BOD sensors (Chang et al., 2004). MFCs are especially

suitable for powering small telemetry systems and wireless sensors that have only low power

requirements to transmit signals such as temperature to receivers in remote locations (Leropoulos et

al., 2005c; Shantaram et al., 2005). MFCs themselves can serve as distributed power systems for

local uses, especially in underdeveloped regions of the world. Locally supplied biomass can be used

to provide renewable power for local consumption. The MFC technology is particularly favoured for

sustainable long-term power applications. The technology is suited to developing countries with a

large proportion of the population not having access to electrical and water-borne/sanitation services,

since MFCs can generate electricity directly and in remote locations without significant costs on

infrastructure.

1.7.3 Costs and benefits in application of MFCs

Capex is the greatest costs with the electrodes and construction of the MFC chambers being

significant costs. The majority of cost in a MFC is attributed to the platinum catalysed carbon paper

used as the cathode component. The electrode costs have been limiting, but there have been several

improvements in design and are continual advancements in this area such as membrane-less MFCs

(Ghangrekar and Shinde, 2007) and alternative, cheaper electrodes (graphite, carbon felt, foam or

packed-granules, and platinum mesh) (Logan and Regan 2006).

To examine the potential for electricity generation, consider a EU food processing plant producing

7500 kg/day COD of effluent wastewater (Logan and Regan 2006). This represents a potential for 950

kW of power or 330 kW assuming 30% efficiency. A MFC (power output 1 kW/m3) with a volume of

350 m3 is therefore needed; costing approximately 2.6 m Euros (Rabaey and Verstraete, 2005) The

produced energy, calculated on the basis of 0.1 Euros per kWh, is worth about 0.3 M Euros per year,

providing a ten year payback without other cost-benefit considerations (i.e. inflation, operation,

maintenance costs, environmental offsets and compliance with local legislation for wastewater

discharge).

MFCs can purify waste to reduce the COD from 40-80% of the input material. The direct conversion

into electricity (coulombic efficiency) is typically more than 90%. There is little information on the

survival of pathogens following MFCs, but one would predict that the growth of a specific bacterial

consortium in the MFC will limit the presence of human enterobacterial pathogens (Salmonella, E.

coli, Ascaris, etc.). More research in this area is needed. There are benefits that need to be taken into

consideration such as the carbon dioxide mitigation due to both the reduction in electricity (coal-

generated) and also the reduced methane emission that would occur if the wastewater was treated by

conventional means. In terms of benefits, the direct conversion to electricity is a distinct advantage

compared to other technologies that generate heat which must then be converted to electricity (at

typical efficiency of 25%).

37

Some sludge will be generated by microbial biomass and non-biodegradable solids in the MFC. The

volume of microbial biomass generated is in between that of aerobic and anaerobic processes. The

sludge may prove useful as a fertiliser (as has sludge from aerobic and anaerobic digesters), or may

provide a biomass resource for energy generation by combustion or gasification. Similarly, effluent

wastewater from the MFC will still contain organic load (COD and nutrient) and may require further

purification, potentially with concomitant power generation via methane, or find application an

agricultural fertiliser.

MFCs are capable of converting the chemical energy stored in the chemical compounds in a biomass

to electrical energy with the aid of microorganisms. The chemical energy from the oxidisation of fuel

molecules is converted directly into electricity instead of heat, the Carnot cycle with a limited thermal

efficiency is avoided and theoretically higher conversion efficiency can be achieved. Efficiencies of 80-

97% have been reported (Chaudhury and Lovley, 2003) (Rabaey et al., 2003) (Rosenbaum et al.,

2006). However, MFC power generation is still very low (Tender et al., 2002; Delong and Chandler,

2002), that is the rate of electron abstraction is very low. One feasible way to solve this problem is to

store the electricity in capacitors or rechargeable devices and then distribute the electricity to end-

users (Leropoulos et al., 2003a). The energy output is of the order 0.18 to 18 kW/m3 of the MFC and

the power output for the removed COD is 200-300 mW/g COD removed with loading rate of 0.574 kg

COD/m3 per day (Mohan et al., 2007).

The distinct advantages of MFCs are:

Able to utilise a variety of substrates at low concentrations

Direct conversion of substrate energy to electricity enabling high conversion efficiency.

Efficient operation at ambient (and below 200C) temperatures.

Elimination of gas treatment requirements because the off-gases of MFCs are mostly carbon

dioxide

Do not need energy input for aeration

Can be employed in remote locations lacking electrical infrastructure.

Furthermore, MFCs have recently been modified for hydrogen production instead of electricity

production (Ditzig et al., 2007). This bioelectrochemically assisted microbial reactor, BEAMR, results

in the production hydrogen at the cathode with high yields and a limited surplus energy investment.

This may be a more favourable route so that the energy generated can be stored and supply a

growing hydrogen economy.

The current high material costs mean that feasibility expenditure (Capex) for energy recovery with a

MFC is estimated to be at a level 10 times that of an anaerobic digester (Rabaey et al., 2005).

However, waste-driven MFCs produce less excess biomass than aerobic wastewater treatment

facilities (Rabaey et al., 2005). Since the sludge treatment cost of wastewater treatment facilities can

be considerable, this can have important implications on the economic feasibility of the process.

38

Furthermore, MFCs are a sustainable platform technology applicable in diverse fields without

substantial modification, because they can use a wide array of electron donors to efficiently generate

energy even at low-moderate temperatures, with low concentrations of electron donors in the

wastewater. The application of MFCs is therefore currently still in development and limited mainly by

material cost of the electrodes and the proton exchange membrane, but there are continual

advancements in this area such as membrane-less MFCs (Ghangrekar and Shinde, 2007) and

alternative, cheaper electrodes (graphite, carbon felt, foam or packed-granules, and platinum mesh)

(Logan et al., 2006). Therefore, current obstacles are the R&D and that should be pursued to develop

this technology sufficiently to implement engineering scale trials in South Africa.

39

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46

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47

2: APPENDIX: SURVEYS, CASE STUDIES AND WORKSHOPS

The following are the findings from surveys, case studies and workshops of the energy from

wastewater project:

2.1 Municipal wastewater treatment plants (WWTP)

The characteristics of the wastewater discharges will vary from location to location depending upon

the population and industrial sector served, land uses, groundwater levels, and degree of separation

between storm water and domestic wastes. The domestic wastewater includes both the blackwater

(faeces and urine from toilets) and greywater (washing and food preparation). The industrial loads will

have characteristics depending upon their nature and will contribute differently to the individual

WWTP.

Wastewater Energy potentials at WWTP

The potential for energy can be calculated for the entire population as well as that currently serviced

by municipal WWTP infrastructure at present.

(1) Total potentials. It is estimated that human activities generate 200 L/day wastewater per person

(http://www.dwaf.gov.za/dir_ws/content/lids/PDF/Q&A.pdf.). Since the population of South Africa is

48.5 million then 9700412600 L per day is generated. With the COD of 0.860 g/L (see typical COD of

Cape Town WWTP, below) this amounts to 8342354836 g per day. As 1 day= 86400s, this is 96555

g/s or 96.555 kg/s. Since the energy content is 15 mJ/kg this amounts to 1488 MJ/s or 1488 MW. This

includes the domestic blackwater and greywater loads.

(2) Potential from current municipal WWTP:

7600 mL/day at WWTP. With a COD of 0.860 g/L this amounts to 6536000000 g per day. As 1 day=

86400s, then 75.648 kg/s is produced. Since the energy content is 15 mJ/kg this amounts to 1134

MJ/s or 1134 MW.

If the WWTP are operating at 75% of their capacity the amount is 850 MW.

Note: The current municipal WWTP service approx. 60% of the population presently (see 4)

(3) Total domestic blackwater load

48 502 063 people generating 100 g (dry weight) of faeces per day with an energy of 15 MJ/kg. This

represents 48 502 06 kg per day or 56.136 kg/s. Since energy value is 15 MJ/kg, this represents 842

MJ/s or 842 MW

(4) Total captured domestic blackwater (human faeces component): the serviced population:

Currently 60.4% of the population (48 502 063) have flush toilets

(http://www.statssa.gov.za/community_new/content.asp Community survey 2007) that the WWTP can

capture and this therefore amounts to 29 295 246 persons. Therefore 29 295 246 people with flush

48

toilets generating 100 g (dry weight) of faeces per day. This represents 29 295 25 kg per day or

33.907 kg/s. Since energy value is 15 MJ/kg, this represents 509 MJ/s or 509 MW.

Table 2.1.1: The capacity of WWTP in South Africa is shown below:

Plant size category (Mℓ/day)

Estimated number of plants (#)1

Median wastewater flow treated (Mℓ/day)2

Total volume of wastewater treated (Mℓ/day)

< 0.5 488 0.25 122 0.5 – 2 108 1.25 134 2 – 10 208 6 1248 10-25 93 17.5 1625

25-100 71 62.5 4460 Estimated total volume of wastewater treated in South Africa 7589 1 Estimated number of plants obtained from the DWAF data base corrected for the plant category sizes used in this report. 2 The actual plant sizes in each plant size category was available.

Data: Golder and Associates study of WWTP commissioned by the WRC

The Western Cape’s spread of wastewater treatment plant sizes is similar to the national

situation; small (0.5 to 2.5 mL/day) and micro size plants (<0.5 mL/day) constitute the majority of

WWTPs in South Africa. Activated sludge process technology appears to be the dominant

treatment process.

Fig. 2.1.1: Spread of WWTP sizes in Western Cape and North West Province

City WWTP

The WWTP in Cape Town are shown in the map below (Fig 2.2). The WWTP are distributed

throughout Cape Town but the density does not always reflect the area served or population density

(e.g. Cape Flats serves an exceptionally large area). More detailed data for the Cape Town WWTP is

shown below.

Western Cape (73)

North West (69)

20-100 Ml/dayLarge plants

<0.5 Ml/day Micro plants

1-5 Ml/daySmall plants

2.5-7.5 Ml/dayMedium plants

5-20 Ml/dayMedium plants

7.5-25 Ml/dayMedium plants

Micro plants <0.5 Ml/day

Small plants 0.5-2.5 Ml/day

Small plants 1-5 Ml/day

Medium plants 2.5-7.5 Ml/day

Medium plants 5-20 Ml/day

Medium plants 7.5-25 Ml/day

Large plants 20-100 Ml/day

49

Fig. 2.1.2: WWTPs in Western Cape

50

Was

tew

ater

tr

eatm

ent

p

lan

t

Infl

uen

t

CO

D

(mg

/L)

[x]

Vo

lum

es

(ML

/mo

nth

) [y

]

Lo

ad

Kg

/mo

nth

([

x].[

y])1

03

CO

D v

alu

es f

or

Cap

e T

ow

n W

aste

wat

er

trea

tmen

t p

lan

ts

050

01,

000

1,50

02,

000

2,50

03,

000

3,50

04,

000

4,50

05,

000 Ath

lone

Bellvi

lle -

DA

Bellvi

lle -

Orb

al

Borch

erds

Qua

rryCam

ps B

ayCap

e Fla

ts

Gor

dons B

ayG

reen

Point Hou

t Bay

Kliphe

uwel

Kraai

font

ein

Llan

dudn

oM

acas

sar

Mel

kboss

trand

Mille

rs P

oint

Mitc

hells

Pla

inO

udek

raal Parow Pot

sdam

Scotts

dene

Simon

s Tow

n

Wesfl

eur D

omes

tic

Wesfl

eur I

ndus

trial

Wild

evoel

vlei

Zandv

liet

WW

TP

COD (mg/L)

Ath

lone

78

9 2

751.

4

2170

5390

00

Be

llvill

e –

DA

1

245

1

662.

3

2069

1900

00

Be

llvill

e –

Orb

al

(818

) nd

-

Bor

cher

ds

Qua

rry

1 64

9

917.

3

1512

1330

00

Cam

ps B

ay

745

63.9

47

6055

000

Cap

e F

lats

88

1 4

419.

8

3893

8440

00

Gor

don

s B

ay

663

80.7

53

5040

00

Gre

en

Poi

nt

614

803.

0

4930

4200

0

Hou

t Ba

y 1

207

12

1.4

14

6530

000

Klip

heuw

el

(1 3

10)

nd

-

Kra

aifo

ntei

n

734

305.

4

2238

7000

0

Llan

dudn

o

473

6.6

3122

000

Mac

assa

r 80

2 96

4.4

77

3449

000

Mel

kbos

stra

nd

586

290.

3

1701

1600

0

Mill

ers

Poi

nt

278

nd

-

Mitc

hells

Pla

in

1 32

9

779.

0

1035

2910

00

Oud

ekra

al

(141

) nd

-

Par

ow

83

4 23

3.3

19

4572

000

Pot

sdam

1

073

83

0.1

89

0697

000

Sco

ttsde

ne

681

414.

1

2819

3400

0

Sim

ons

Tow

n

599

92.2

55

2280

00

Wes

fleur

D

omes

tic

859

155.

3

1334

0300

0

Wes

fleur

In

dust

rial

1 14

9

152.

8

1755

6700

0

Wild

evoë

lvle

i 78

4 26

0.2

20

3997

000

Zan

dvlie

t 71

6 1

219.

8

8733

7700

0

A

v.

CO

D

=83

7

mg/

L To

tal [

v] =

16

523

Σ

[x].

[y]=

15

8294

600

0

Tabl

e 2.

1.2:

CO

D v

alue

s fo

r C

ape

Tow

n W

WT

P’s

51

Fig.2.1.3: Contribution of wastewater from industry in Cape Town

Load (kg/month)

ANODISING

BAKERY

BEVERAGES

BOTTLINGBUILDING

CHEMICALS

CONSTRUCTION

CONTAINER

COSMETICS

DAIRY

ENGINEERINGENZIMES

FOOD

FOOD/FISH

FOOD/SPICE

FUEL

GALVANISING

GASLABORATORY

LAUNDRY

MEAT

METAL

MOTOR

PACKAGING

PAINT

PAPERPHARMACEUT

PLASTIC

PLATING

PRINTING

RADIATOR

RAGS

TANNERYTEXTILE

TISSUE

TRANSPORT

VEH/WASH

WINERY

0 5000000 10000000 15000000 20000000

0 500000 1000000

ANODISINGBAKERY

BEVERAGESBOTTLINGBUILDING

CHEM ICALS

CONSTRUCTIONCONTAINERCOSM ETICS

DAIRYENGINEERING

ENZIM ESFOOD

FOOD/FISHFOOD/SPICE

FUELGALVANISING

GASLABORATORY

LAUNDRYM EAT

M ETALM OTOR

PACKAGINGPAINT

PAPERPHARM ACEUT

PLASTICPLATING

PRINTINGRADIATOR

RAGS

TANNERYTEXTILETISSUE

TRANSPORTVEH/WASH

WINERY

52

The characteristics of wastewater (more detailed analysis of nutrients) for two Cape Town WWTPs

are shown in the table below (data 2007).

Table 2.1.3: Chemical characteristics of the wastewater in some Cape Town WWTPs

CAPE FLATS 2/8 2/15 2/22 3/1 3/8 3/15 3/22 3/29 4/5 4/12 4/19 4/26

COD mg/L 1,266 937 835 1,099 1,326 788 817 547 2,629 846 827 1,393

SS mg/L 920 595 455 705 766 430 463 368 1,840 480 263 880

N mg/L 103.2 73.5 66 90 94.6 77 76.1 56.6 173.4 84.4 79.1 103.9

P mg/L 27.3 17.3 16.3 22.3 26.9 15.8 20 12.8 61.7 15.9 17.7 28.6

ATHLONE 2/8 2/15 2/22 3/1 3/8 3/15 3/22 3/29 4/5 4/12 4/19 4/26

COD mg/L 915 890 751 866 863 873 667 810 990 901 855 921

SS mg/L 392 336 300 318 290 312 254 308 357 370 420 280

N mg/L 61.4 59.3 55.3 61.2 55.6 57.2 53.1 52.4 59.2 60.9 72.4 55.2

P mg/L 10.9 18.5 9.3 10.7 9.4 10.8 9.6 10.8 16.5 11.7 11.6 8.6 COD: Chem Oxygen Demand. SS: Suspended Solids. N: Total Kjeldahl Nitrogen. P: Total Phosphorus

From this data it can be seen that the loads are relatively constant for the majority of plants although

some exceptional intermittent high loads can be seen. This may be due to seasonal or unexpected

loads or operational failure (power failure or other) and seasonal rainfall (resulting in water ingress

into the sewage system). The current loads to the Cape Town municipal WWTP have a typical

average COD of 860 mg/L and a total volume of 16523 ML/month or 551 ML/day. The approximate

values for suspended solids 400 mg/L, total nitrogen of 50-100 mg/L and total phosphorous of 10-30

mg/L. The loads are not evenly distributed at the various WWTP (presumably reflects capacity and

locality/wastewaters served). For, example that of Borcherds quarry is twice that of Athlone. Further

analysis of the characteristics of the wastewater at municipal WWTP is also shown for two WWTP in

Cape Town Cape Flats and Athlone.

It is obvious that the food, beverages and textile industries (in order of deceasing contribution)

contribute the greatest loads to Cape Town WWTPs. Of the remaining minor contributors, dairy and

fish food followed by paper and packaging and the meat industries dominate. The total load for

industry is 35139581 kg/month and the Cape Town WWTPs currently treats 1582946000 kg/month

(see above). Therefore, the industry contributes 22% of the total load to the Cape Town WWTP. Note

that the actual total industry wastewaters are considerable larger since industries may carry out

primary treatment on-site before discharging to the WWTP or they may discharge to land, rivers or

sea.

The industrial loads are difficult to capture since many are received as mixed streams. In Cape Town

only Westefleur operate separate streams for industry and domestic wastewater. There may also be

fluctuations in the characteristics, as can be seen with Cape Flats in the fluctuations in COD nitrate,

and phosphorous. This may be due to performance or fluctuation in the wastewaters received since

many industries may generate seasonal waste streams. In contrast, the characteristics of Athlone are

more constant.

53

In the case of Durban Municipality, the loads and characteristics are fairly similar to Cape Town, but

there are several exceptions where industrial activity results in loads and characteristics very different

from that of domestic sewage. The data on distribution of wastewater (industrial as a % of the total

WWTP) show that wastewaters have selected characteristics due to their locality and degree of

industrial loads. For example, Hammarsdale which is 95% chicken and textile with COD values 4

times that of the typical domestic WWTP (Table below). Many of the industries in Durban do not

discharge to municipality WWTP but directly to sea with very high loads: COD 500-140 000 mg/L (e.g.

Frametex, Sappi, CG Smith, Drum services). Other industries such as SAB have on-site treatment

(AD) and then discharge to the municipal WWTP. The energy potential opportunities are therefore a

lot greater (and the environmental burdens not taken into consideration) and the application of a

universally efficient method/process for energy recovery from city municipal wastewaters with the

present infrastructure will not be easily attainable. The municipal WWTP are currently being

interrogated through a survey.

54

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

CENTRAL

CRAIGIE

BURN

KING

SBURGH

AMANZIM

TOTIIS

IPIN

GO

UMKO

MAAS

KWAM

ASHUNO

RTHERN

NEW G

ERMANY

PHOENIX

VERULAM

UMDLO

TI

UMHLA

NGA

DASSENHOEK

KWANDENG

EZIUM

BILO

MPUM

ALANG

A

HAMM

ARSDALE

HILLC

REST

UMHLA

TUZANA

Lo

ad

(kg

/mo

nth)

WO

RK

S

% I

nd

us

tria

l C

OD

(A

VG

) m

g/L

M

L/m

on

th

CE

NT

RA

L

12%

76

8.53

57

61.4

61

CR

AIG

IE B

UR

N

76

9.19

32

8.14

69

KIN

GS

BU

RG

H

62

3.36

18

96.1

61

AM

AN

ZIM

TO

TI

50%

(b

rew

ery

, che

mic

als)

82

2.08

45

2.89

92

ISIP

ING

O

61

8.51

13

3.35

15

UM

KO

MA

AS

660.

40

737.

1186

K

WA

MA

SH

U

30%

52

8.12

35

6.20

91

NO

RT

HE

RN

30

%

561.

95

1569

.088

N

EW

GE

RM

AN

Y

30%

(te

xtile

) 95

1.04

43

.712

4

PH

OE

NIX

5%

64

8.98

19

83.1

38

VE

RU

LAM

40

% (

text

ile, f

ood,

pap

er)

829.

83

48.9

282

U

MD

LOT

I

410.

92

39.7

086

U

MH

LAN

GA

671.

80

25.3

2393

D

AS

SE

NH

OE

K

68

3.18

17

.100

69

KW

AN

DE

NG

EZ

I

926.

44

231.

4788

U

MB

ILO

837.

42

433.

2646

M

PU

MA

LAN

GA

782.

58

573.

8493

H

AM

MA

RS

DA

LE

95

% (

text

ile +

chi

cken

) 28

06.6

7

265.

0754

H

ILLC

RE

ST

827.

06

31.0

845

U

MH

LAT

UZ

AN

A

50%

(te

xtile

) 73

1.72

18

6.63

69

Fig

. 2.1

.4: C

OD

val

ues

(load

) fo

r D

urba

n M

unic

ipal

ity W

WT

P

55

Tow

n W

WT

P

Sev

eral

sm

alle

r W

WT

P a

re o

pera

ting

on c

apac

ities

of <

5 M

L/da

y (<

150

Ml/m

onth

). A

n ex

ampl

e is

sho

wn

for

Sal

dana

mun

icip

ality

, be

low

.

Tabl

e 2.

1.4:

Cha

ract

eris

tics

of w

aste

wat

er a

t WW

TP

s in

Sal

danh

a, W

este

rn C

ape

WW

TP

P

op

ula

tio

n

Pri

mar

y m

eth

od

A

era

tio

n

Kw

h/d

Cap

acit

y V

olu

me

m3/d

Ac

tual

vo

lum

e

m3/d

Ac

tual

C

OD

K

g/d

Ave

rag

e C

OD

m

g/L

TK

NN

M

g/L

N

H3

Mg

/L

TK

N/C

OD

p

H

%C

OD

re

mo

val

Fin

al e

fflu

ent

Co

lifo

rm/1

00

mL

Sal

dana

14

840

Act

ivat

ed

Slu

dge

1366

50

00

2324

14

84

688

83.7

51

.83

0.12

7.

6 87

10

0

Hop

efie

ld

2830

A

ctiv

ated

S

ludg

e 10

56

904

285

283

956

95.4

41

.5

0.10

8.

1 93

ni

l

She

lly P

oint

?

Seq

uenc

e ba

tch

reac

tor

226

200

129

45.9

35

6 45

.3

23.5

0.

13

7.4

76

>18

00

St.

Hel

ena

71

60

Act

ivat

ed

Slu

dge

659

1256

10

96

716

780

83.5

56

.4

0.11

7.

6 94

20

0

Lang

ebaa

n 90

10

Act

ivat

ed

Slu

dge

1440

14

00

1511

90

1 67

1 70

.3

45.9

0.

10

7.0

88

Nil

Pat

erno

ster

?

Pon

ding

- 22

5 17

5 15

4 45

5 14

.8

59

0.09

8.

1 52

>

1800

Dat

a: A

bott

& A

bott

Fro

m th

is d

ata

it ca

n be

sum

mar

ised

that

:

1.

Was

tew

ater

s w

ith C

OD

val

ues

of 7

00-8

00 m

g/L,

pH

7-8

. T

he t

otal

nitr

ogen

is

50 t

o100

mg/

L; w

ith a

ppro

xim

atel

y ha

lf th

e to

tal

nitr

ogen

pre

sent

as

amm

onia

. T

he t

otal

nitr

ogen

/CO

D i

s 0.

1 an

d th

e to

tal

phos

phor

ous

is a

ppro

x 10

to

20 m

g/L.

How

ever

dep

endi

ng u

pon

the

loca

lity

and

deg

ree

of

indu

stry

in a

giv

en a

rea

the

load

s an

d ch

arac

teris

tics

can

vary

wid

ely.

2.

Sin

ce a

ppro

x 40

% o

f th

e po

pula

tion

do n

ot h

ave

flush

toi

lets

and

man

y in

dust

ries

trea

t-on

site

and

do

not

disc

harg

e th

eir

load

s to

the

mun

icip

al

WW

TP,

the

pote

ntia

l for

ene

rgy

gene

ratio

n is

a lo

t gre

ater

that

that

pre

dict

ed fr

om th

e cu

rren

t WW

TP

ope

ratio

n an

d lo

adin

g ca

paci

ty.

3.

Act

ivat

ed s

ludg

e ap

pear

s to

be

pref

erre

d m

etho

ds f

or m

unic

ipal

WW

TP,

with

con

sequ

ent

larg

e en

ergy

inp

uts

for

aera

tion

(dat

a fr

om S

alda

na

mun

icip

ality

indi

cate

0.6

-3.7

kW

h/m

3 was

tew

ater

trea

ted)

.

4.

In m

any

case

s th

e W

WT

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56

Peri-urban/Rural WWTP and decentralised wastewater treatment systems (DEWATS)

In peri-urban and rural settings wastewater is often treated locally. This may range from the

bucket system, French drains, septic tanks, pit-latrines and no toilet at all. In 2007 a little more

than 60% of households in South Africa had access to a flush toilet. Gauteng, Free State,

North West and Western Cape were the only provinces where more than 50.0% of

households that own a flush toilet. Free State had the highest number of households still

using the bucket system. More than half of households in Limpopo (56.3%) used a pit latrine

without ventilation, and 25.2% of all households in the Eastern Cape had no toilet at all

(http://www.statssa.gov.za/community_new/content.asp)

Table 2.1.5: Percentage of sewage systems in provinces in South Africa

There are many reasons for the failure to implement water-borne sewage (flush toilets) and

centralised WWTP, but a major factor is the infrastructural costs the cost of implementing

WWTP for peri-urban and rural households – the lower density of the population means high

cost of piping. There are therefore huge opportunities to be gained in decentralised approach

that brings local benefits – decentralised wastewater treatment system (DEWATS). The

application of AD systems in these rural or remote settings where the human and animal

faeces are captured and harnessed in an AD digester for the production of biogas fuel for

cooking and heating. The benefits also include water conservation, reduced fertiliser inputs

since the partially purified wastewaters can be used for irrigation (in compliance with

reductions limits of COD and coliforms as determined by Water Act and DWAF). These are

explored in the case study of the performance of household anaerobic digesters.

57

2.2 : Brewing Industry

Introduction and Data Sources

This case study explored the potential for energy recovery from wastewaters generated by

the brewing industry in South Africa. The sector is considered in terms of formal and informal

operations. SAB produces approximately 98% of the clear beer brewed in South Africa (Cole

2007). However, clear beer constitutes only about 60% of total beer production – the

remainder is comprised of sorghum beer, where, informal, small-scale production dominates

(although some industrial scale installations do exist (such as those of United National

Breweries). The wastewaters of this informal production are not deemed to be readily

quantifiable. Therefore it was decided to define the wastewaters produced by the operations

of SAB and scale up accordingly to provide a value for industry as a whole.

To obtain the desired data from SAB’s seven breweries, a detailed characterisation of the

various wastewaters was requested. However, similar to the requests originally submitted to

the pulp and paper industry (Sec. 2.1.1) these were met with reluctance. Therefore, the

necessary data on the volumes and carbonaceous content of combined wastewater streams

was provided by Cole (2007). Supporting information was obtained from SAB (2006) and from

the company website. The work of Cohen (2006) provided data collected from SAB’s

Newlands brewery.

Results of the Study The technologies and wastewater characteristics at each of the seven SAB breweries are

fairly uniform. Approximately 3.2 L of combined wastewater (from packaging, the cellars and

the brew house), with a COD concentration of about 3000 mg/L prior to any on-site treatment,

is generated per L of beer produced (Cole 2007). A total beer production of 2.605 × 107 hL

(SAB 2006), together with knowledge of the relative capacities of the breweries, then allows

the calculation of the individual beer and wastewater volumes presented in Table 1. Untreated

brewery wastewater typically has a temperature in the region of 40°C and a pH of around 5.5

(Cohen 2006).

Current production and wastewater treatment figures

Five of the SAB breweries include their own wastewater treatment plants where the majority

of the COD is removed prior to disposal to sewer. Alrode, Rosslyn, Prospecton, Newlands and

Ibhayi each have an anaerobic digestion unit, whilst the wastewater at Ibhayi undergoes

further aerobic treatment before passing through sand and carbon filters. Information on the

COD removal performance of the anaerobic digesters was gained from Cole (2007) and is

shown in Table 1. Methane production figures are based on a yield of 0.35 m3 CH4 per kg

COD digested (McCarty 1964), a ratio which corresponds well with that estimated by Cohen

58

(2006) in his study of SAB’s Newlands brewery. The energy associated with this methane is

simply calculated using a heat energy value for methane of 39.74 MJ.m-3. The methane-rich

biogas captured from the anaerobic digesters is currently flared. However, it is the company’s

intention to explore opportunities of utilising this renewable energy source in the generation of

process heat and/or electricity across its breweries (Cole 2007). In this regard, Cohen (2006)

notes that, although fluctuations in biogas flow and composition may present technological

design challenges, the overall efficiency of fuel use in cogeneration (combined heat and

power) can be as high as 85%.

Table 2.2.1 Annual combined wastewater and wastewater treatment data for SAB’s breweries

Brewery Beer

x103 (hL)

Wastewater

(Ml)

COD

(mg/L)

COD post-AD

(mg/L)

COD removed

(tonne)

CH4

x103(m3)

Energy

(GJ)

Alrode 6,885 2,203 3,000 400 5,728 2,005 79,673

Rosslyn 5,972 1,911 3,000 1,500 2,867 1,003 39,873

Prospecton 4,396 1,407 3,000 400 3,658 1,280 50,875

Newlands 3,733 1,194 3,000 400 3,106 1,087 43,196

Ibhayi 1,991 637 3,000 400 1,656 580 23,038

Chamdor 1,825 584 3,000 n/a n/a n/a n/a

Polokwane 1,244 398 3,000 n/a n/a n/a n/a

TOTAL 26,046 8,334 17,015 5,955 236,655

Total anaerobic digestion potential

The study of Cohen (2006) found that a relatively constant COD reduction rate of about 90%

was achievable through the anaerobic digestion of SAB’s brewery wastewaters. This results

in the total potential figures for energy recovery presented in Table 2. To provide some

perspective, the potential thermal energy of this methane production, at each of the

breweries, amounts to around 7.6% of the site’s total energy requirements (157.4 MJ.hL-1

(SAB 2006)). Efficiency of conversion from thermal to electrical energy is conservatively

estimated at 25%.

59

Table 2.2.2 Total potential figures for energy recovery from SAB’s wastewaters using anaerobic digestion

COD removed

Per year

(tonne)

CH4 Produced per year

x103 (m3)

Energy produced

per year. (GJ)

Thermal power

(MW)

Breweries

Alrode 5,948 2,082 82,737 2.63

Rosslyn 5,160 1,806 71,772 2.28

Prospecton 3,798 1,329 52,832 1.68

Newlands 3,225 1,129 44,857 1.42

Ibhayi 1,720 602 23,924 0.76

Chamdor 1,577 552 21,930 0.70

Polokwane 1,075 376 14,952 0.48

Sub-total 22,504 7,876 313,005 9.92

Maltings plants

(combined) 1,030 360 14,321 0.46

Total 23,533 8,237 327,325 10.38

SAB’s two malting plants at Caledon and Alrode have been included in the analysis of Table

4. They have a combined capacity of about 220 000 tonnes per annum and have a specific

wastewater production of approximately 2.6 kL per tonne (SAB 2006) at a COD concentration

of just less than 2000 mg/L (Cole 2007).

Scale-up to entire sector

Taking into consideration the proportion of South Africa’s total beer production for which SAB

is responsible (ca. 60%) , the data in Table 2 can be broadly extrapolated to yield a total

theoretical potential of about 17.3 MW of thermal power recoverable from the wastewaters of

the brewing industry sector as a whole. However, due to the informal nature of a large

proportion of the extrapolated production and the resultant uncertainties surrounding the

comparability of the nature and quantity of wastewaters, the accuracy of this total figure is

unclear. Similarly, the extent to which these “informal” wastewaters are accessible for energy

recovery is of concern.

It should also be noted that, although anaerobic treatment of brewery wastewater is a proven,

well-established process, with many systems operational worldwide, alternative technologies

incorporating energy recovery, such as microbial fuel cells (Associated Press 2007) and

bioethanol fermentation (Rao et al., 2007), may also be considered.

60

Concluding remarks – Brewing Sector The wastewaters of the brewing sector are generally suitable for biological energy recovery.

However, the sector is dominated by multinational organisations with sufficient resources and

motivation to explore options for energy recovery from their wastewaters themselves, on a

rigorous, site-specific basis. The impression was gained that such investigations have been,

and continue to be, conducted and that viable technologies will be implemented. Furthermore,

biological treatment of a large proportion of the industry’s wastewaters is already currently

performed, albeit not necessarily incorporating energy recovery.

61

2.3 Survey of Paper and Pulp wastewaters

Introduction and Data Sources

Initial investigations revealed that the South African pulp and paper industry is dominated by

Sappi and Mondi who account for 100% of virgin pulp production and about 88% of all paper

and board production (Chamberlain et al., 2005). Furthermore, only a limited number of

installations exist within the sector. As a result, it was initially decided to compile detailed data

for each of the principal installations, as opposed to largely aggregated sector data. Water

circuit data detailing separate wastewater flows (their volumes, carbonaceous content,

temperature, pH, concentration of potential inhibitors, etc.) would constitute a valuable future

resource for the project, enabling identification of certain individual sites or even streams of

particular interest for potential intervention and assisting in establishing the most suitable

energy recovery technology.

The various process technologies and product ranges in place at South Africa’s different mills

differ markedly and consequently so too do the wastewater volumes and characteristics. Even

if only aggregated sector results are sought, it is not sufficient to obtain wastewater data from

a single sample installation and apply it across the entire industry. Ideally, mill data from each

of the principal sites is required. For this study, it was felt that the most straightforward means

of obtaining data was to directly approach the relevant personnel in the pulp and paper

industry. Being unable to assist with data themselves, recommended contacts at its member

companies were provided by PAMSA, an industry association representing all South Africa’s

major pulp & paper producers (Hunt 2007). Considering their dominance of the industry and

the resultant importance of their co-operation, Sappi and Mondi were then approached with

requests for detailed information on wastewater volumes and characteristics at each of the

mills, in line with the objective of this study. However, due to the environmental pressures

placed on the industry, such data is considered sensitive and the requests were viewed with

some suspicion. Information on combined wastewaters prior to any on-site treatment was

received for the Mondi-related operations at Richards Bay (flow, COD, pH, temperature)

(Naylor 2007) and those at Felixton, Piet Retief and Springs (flow, COD) (Scheckle 2007).

Anecdotal reports were also provided by Van der Westhuizen (2007) and Scheckle (2007).

Further information was gathered through the open literature. Macdonald (2004) documents

the water consumption figures for each of the Mondi, Sappi, Nampak and Kimberly-Clark

operations in South Africa. These can be used to provide a measure of wastewater flows

through broad application of the assumption that typical mills expel about 85% of water

consumed as wastewater (Macdonald 2007). In the absence of mill data, the first step

towards estimating the desired wastewater information is determining the various pulp and

paper production figures at each installation. These can then be employed in conjunction with

62

literature values of specific (i.e. per unit production) wastewater flow associated with the

production of individual pulp or paper products in order to quantify the combined wastewater.

Similarly, values of specific COD load to wastewater can be used to describe the character of

these wastewaters.

A full breakdown of pulp and paper production at the Mondi-related sites at Felixton, Piet

Retief and Springs was provided by Scheckle (2007). The remaining data had to be sourced

from available literature where, a high degree of product specificity was required. The virgin

pulp was characterised by bleach status (bleached or unbleached), fibre source (wood or

bagasse) and method of pulping (groundwood, thermo-mechanical, neutral sulphite semi-

chemical, kraft, soda, Ca-sulphite or Mg-sulphite). Likewise, paper was specified as coated

fine paper, uncoated fine paper, newsprint, linerboard, fluting, cartonboard, other packaging

paper or tissue. A further distinction necessary was the proportion of these paper production

figures (if any) that had as their fibre source an on-site recycled fibre plant. This data was

ultimately compiled through a synthesis of total industry production figures (PAMSA 2007) and

the information obtained from Macdonald (2004), Chamberlain et al. (2005), company

websites and various online engineering articles. As a result of inconsistencies between

sources (due to, for example, recent expansion or lack of distinction between capacity and

production) and incomplete or aggregated data, this exercise was not straightforward and

some educated guesswork was required in reconciling this information.

Specific wastewater data

The primary source for the data covering specific wastewater flows and specific COD loads to

wastewater was the European Commission report (2001), whilst other important sources

include the Food and Agriculture Organisation of the United Nations (1996), Rintala and

Puhakka (1994), Pokhrel and Viraraghavan (2004) and Thompson et al. (2001). Since it is

common industry practice to perform wastewater treatment prior to release to the

environment, the focus is commonly on regulatory compliance and, as such, data on

untreated wastewater can be difficult to source. Such information was nevertheless obtained

for the production of as many as possible of the different pulp and paper products

manufactured in South Africa. Care is also required in attaching appropriate data to integrated

pulp and paper mills or to non-integrated mills producing only either pulp or paper products –

integrated mills have, for example, opportunity for greater recycling, and the effects of

additional processes/products on wastewaters are not necessarily simply additive.

It is important to recognise that this specific wastewater data acquired is for a combined

stream comprising such effluents as cooking and evaporator condensates, washing losses,

accidental spills and those from wood handling, the bleach plant, stock cleaning and the

63

paper machine. As previously mentioned, it was the original objective to provide a more

detailed survey enumerating and characterising the separate available wastewater streams to

whatever extent possible. Some of these individual streams can have very high COD

concentrations, although these are typically reduced through stripping or extraction of organic

by-products. It is also important to recognise that the pulp and paper industry already

recovers a great deal of energy from its process (as opposed to wastewater) streams and is

one of the largest producers of energy from renewable resources in the country (FSA 2004).

Indeed, the recovery of energy and pulping chemicals through the incineration of the

concentrates resulting from the evaporation of spent cooking liquor is essential to the cost

effectiveness of most pulping operations. The fuel value of the recoverable “black liquor” from

kraft pulp mills, for example, is normally enough to make them at least self-sufficient in terms

of heat and electrical energy.

Results of the Study Annual data for combined wastewater prior to any on-site treatment is presented in Table

2.3.1 for each of the principal mills in South Africa. The “Wastewater” column contains the

data extracted from the work of Macdonald (2004), or, in the case of the Mondi-related

operations at Richards Bay, Felixton, Piet Retief and Springs, data received from the mills

themselves. The “Est wastewater” column contains the values calculated using the mills’ pulp

and paper production figures and the various specific wastewater flow figures gained from the

literature. The ranges of these specific flow values are typically substantial and the selection

of a single value for each of the products was guided to an extent by the “Wastewater”

numbers. These single values tend to fall towards the lower end of the ranges, a result which

is expected since, as large consumers of water operating in a water scarce country, South

African mills do generally by necessity exercise efficient water use strategies (FSA 2004). The

“Est wastewater” figures are included primarily as a gauge of the potential accuracy of the

technique. Whilst the exceptions hint at the difficulties in making generalisations in this

industry, it can be seen that the estimates are accurate to within 15% for twelve out of

seventeen mills.

For the Richards Bay, Felixton, Piet Retief and Springs installations, the COD data presented

in Table1 are those provided by the mills themselves. Otherwise, the values are those gained

using the mills’ pulp and paper production figures and the various ranges of specific COD load

to wastewater sourced from the literature. Considering the vastness of the literature ranges of

COD evident in Table 1, it is anticipated that the mill data would fall within these calculated

ranges. Indeed, this is the case for Richards Bay, Piet Retief and Springs (all towards the

lower end). The notable exception, however, is Felixton, where even the upper end of the

calculated COD range (722-3122 ppm) is less than 14% of the actual value. This certainly

underlines the variability within the industry and the potential errors in this analysis.

64

Table 2.3.1 Annual combined wastewater data (prior to any on-site treatment) for the South African pulp and

paper industry sector

Mill Wastewater

(ML)

Est wastewater

(ML)

COD

(mg/L) pH

Temperature

(°C)

Mondi

Merebank 10264 10085 470-1659 – –

Richards Bay 21361 21300 1399 8.24 44.38

Felixton 1933 2000 22842 – –

Piet Retief 566 1750 6021 – –

Springs 1046 1008 1940 – –

Sappi

Saiccor 33320 32582 615-3073 – –

Stanger 6248 3760 319-1175 – –

Enstra 7586 6227 578-1929 – –

Adamas 506 462 848-3221 – –

Ngodwana 10413 13996 1219-4607 – –

Tugela 15470 6387 358-1305 – –

Cape Kraft 428 408 595-4167 – –

Nampak

Bellville 655 576 733-2443 – –

Kliprivier 506 432 711-2372 – –

Riverview 208 180 721-2404 – –

Rosslyn 298 320 671-4698 – –

Kimberly-Clark

Enstra 803 864 897-2989 – –

New Era

Gayatri – 360 625-4375 – –

Other – 1210 789-3116 – –

It may be expected that the temperature of the combined wastewaters at each of the mills

would be of approximately the same order as that at Richards Bay (Macdonald 2007). The

pH, however, may differ markedly, particularly at Sappi’s Saiccor mill due to the acidic nature

of their pulping operation.

65

1. Anaerobic digestion potential

The application of anaerobic digestion technology for the treatment of wastewaters from the

pulp and paper industry is well-researched (Rintala and Puhakka 1994; Pokhrel and

Viraraghavan 2004; Thompson et al., 2001; Lee Jr et al., 2006). Although wastewaters from

particular sources within a mill may be more degradable than those from others, most

contaminants are readily degraded (European Commission 2001), and a relatively constant

COD removal efficiency of 80% is achievable through anaerobic digestion. Residual COD

levels may however necessitate additional treatment prior to release to the environment

(Thompson et al., 2001).

Assuming an 80% reduction in COD and a methane yield of 0.35 m3 CH4 per kg COD

digested (McCarty 1964), the industry’s total energy recovery potential using anaerobic

digestion can be calculated (Table 2). A heat energy value for methane of 39.74 MJ.m-3 is

used.

66

Table 2.3.2 Total potential figures for energy recovery from South Africa’s pulp & paper industry sector’s

wastewaters using anaerobic digestion

Mill COD removed

(tonne)

CH4

X103(m3)

Energy

(GJ)

Electric power

(MW)

Mondi

Merebank 3862-13620 1352-4767 53712-189446 1.7-6.0

Richards Bay 23914 8370 332621 10.55

Felixton 35329 12365 491397 15.58

Piet Retief 2728 954 37942 1.2

Springs 1623 568 22575 0.72

Sappi

Saiccor 16384-81920 5734-28672 227885-1139425 7.23-36.13

Stanger 1593-5872 557-2055 22152-81680 0.7-2.59

Enstra 3506-11707 1227-4097 48764-162834 1.55-5.16

Adamas 343-1304 120-456 4777-18136 0.15-0.58

Ngodwana 10152-38375 3553-13431 141211-533763 4.48-16.92

Tugela 4437-16145 1553-5651 61711-224563 1.96-7.12

Cape Kraft 204-1427 71-499 2835-19843 0.10-0.63

Nampak

Bellville 384-1280 134-448 5341-17804 0.17-0.56

Kliprivier 288-960 101-336 4006-13353 0.13-0.42

Riverview 120-400 42-140 1669-5564 0.05-0.18

Rosslyn 160-1120 56-392 2225-15578 0.07-0.49

Kimberly-Clark

Enstra 576-1920 202-672 8012-26705 0.26-0.85

New Era

Gayatri 180-1260 63-441 2504-17525 0.08-0.56

Other 764-3016 267-1056 10626-41950 0.34-1.33

Total 106547-243921 37292-85372 1481965-3392703 47.0-107.6

Within the context of this study, the wastewater of Mondi’s Felixton mill is obviously of

particular interest due to its elevated COD concentration and associated potential recoverable

energy. However, the competitiveness of the industry dictates that these wastewaters are

understandably also of interest to Mondi and the implementation of various energy recovery

technologies is currently being investigated (Scheckle 2007). Similarly, Sappi have thoroughly

67

researched the possibility of recovering energy from wastewater streams using an array of

technologies at each of their installations (Van der Westhuizen 2007). Currently none is

economically viable, although the steadily increasing trend of fossil fuel prices demands that

the situation be regularly reassessed.

It is also pertinent to note that many of the plants already have in place primary and

secondary wastewater treatment facilities, from which the effluent is released to the

environment. However, anaerobic treatment is not common, as these systems are relatively

sensitive to disturbances. Available information suggests that no energy recovery is

performed.

Concluding remarks The wastewaters from the pulp and paper sector are theoretically suitable for biological

energy recovery, providing some of the operational considerations surrounding system

robustness are able to be managed. The theoretical potential thermal power from the pulp

and paper sector as a whole, using anaerobic digestion to generate biogas is 45-100 mW.

In conclusion, it is noted that the sector is dominated by multinational organisations with

sufficient resources and motivation to explore options for energy recovery from their

wastewaters themselves, and this on a rigorous, site-specific basis. The impression was

gained that such investigations have been, and continue to be, conducted and that viable

technologies will be implemented. Furthermore, biological treatment of a large proportion of

the industry’s wastewaters is already currently performed, albeit not necessarily incorporating

energy recovery.

68

2.4: Survey of petrochemical refineries

Wastewater Generation in Refineries Petroleum refineries generate wastewater during operations including desalting of crude oil,

steam stripping operations, product fractionators, reflux drum drains and boiler blowdown [1].

The crude oil desalter unit is the major source of contaminated wastewater in the crude oil

refinery with the production rates ranging between 40-90 litres of wastewater per tonne of

crude processed. This wastewater contains phenols, oil, chlorides and organic matter, and

suspended solids are also generated during desalting due to the presence of sediments in

crude oil. Sour wastewater is the aqueous waste coming from crude or resulting from

stripping operations and distillation/fractionation overhead condensates; it contains phenols,

sulphides, ammonia and in some cases oil depending on the position of unit source in the

process [2].

Typically the wastewater streams are mixed, thus giving rise to streams that are more difficult

to treat and invariably lead to higher processing costs. These streams are typically treated at

on-site WWTP together with surface water runoff and domestic wastewaters (sewage) from

the site [3, 4]. Different refineries make use of different process routes and feedstocks, and

hence defining a typical wastewater stream is difficult. Attempts to obtain wastewater

compositions from local refineries were unsuccessful. A range of literature values was thus

collected for use in this study, as shown in the Table below.

Table 2.4.1: Refinery wastewater BOD and COD concentrations in (mg/L)

COD BOD Heavy Metals Oil Phenol Reference

1 300-600 150-250 0.1-100 100-300 20-200 5

2 300-800 150-350 0.1-100 <3000 20-200 2

3 236-490 109-275 - 25-266 6-270 6

It is noted that the compositions presented in the Table above are representative of those

from crude oil refineries. The composition of wastewater from coal-to-liquid and gas-to-liquid

refineries will be expected to differ significantly. This is discussed further below. Oil and

solvents are the main wastewater pollutants in petroleum refining operations which enter the

feedstream [4]. Other contaminants include such as ammonia, sulphur compounds and spent

acids. Heavy metals (copper, zinc, chromium, nickel, lead, mercury, cadmium, selenium,

arsenic, silver and barium) and cyanide are also present which potentially impact on the

treatment processes which may be used for these streams, depending on their concentrations

in the stream.

In terms of wastewater volumes, anything from 0.1 to 5 m3 of wastewater is generated per

tonne of crude oil processed [2, 7].

69

Refineries in South Africa

South Africa has four crude oil refineries, as well as the Sasol and PetroSA coal-to-liquid and

gas-to-liquid operations. The four crude oil refineries, along with their crude processing

capacity, are shown in the Table below. Also shown is the outer limits of the wastewater

volumes generated from these refineries.

Table 2.4.2 – Crude Oil Processing and Wastewater Generation from Refineries in South Africa (2002 Figures)

[8]

Refinery

Crude Processing

Capacity (Barrels/day) Crude (tonne/day)*

m3 of wastewater per

day (0.1 m3/tonne)

m3 of wastewater per

year (5 m3/tonne)

Engen 115 000 15 640 1 564 78 200

Sapref 180 000 24 327 2 432 121 635

Natref 108 000 11 623 1 162 58 115

Caltex 100 000 13 515 1 351 67 575

TOTAL 503 000 65 105 6 510 325 525

* At a density of 0.85 kg/L

The total volume of wastewater generator by the four refineries is thus between 118,816,000

and 2,376,000 ML per year.

Comment on the Energy Recovery Potential from Refinery Wastewater Consideration of the COD and BOD values presented in Table 2.4.1 suggests limited potential

for recovery of energy from wastewater from crude oil refineries. In addition, the presence of

various contaminants have the potential to adversely affect any biological processes which

may be used for energy recovery.

A review was conducted to determine best practice in wastewater treatment from crude oil

refineries around the world, and whether energy recovery from wastewater is practiced. The

primary option for energy recovery is separation of oil out of wastewater streams with high

organic contents and then burning the oil or recycling it back into the process. Recovery

would thus be performed on certain streams, prior to combination with other lower organic

content streams.

The World Bank and International Finance Corporation [7] propose a list of the most common

and appropriate approaches for treating refinery wastewater which include source

segregation and pre-treatment of concentrated wastewater streams. Typical wastewater

treatment steps include: grease traps, skimmers, dissolved air floatation or oil water

separators for separation of oils and floatable solids; filtration for separation of filterable solids;

flow and load equalisation; sedimentation for suspended solids reduction using clarifiers;

70

biological treatment, typically aerobic treatment, for reduction of soluble organic matter

(BOD); chemical or biological nutrient removal for reduction in nitrogen and phosphorus;

chlorination of effluent when disinfection is required; dewatering and disposal of residuals in

designated hazardous waste landfills. Thus none of the technologies presented in the EfWW

study are considered as suitable in this context.

When one considers the coal-to-liquid and gas-to-liquid refineries in South Africa more

potential for energy recovery from wastewater through traditional EfWW technologies is

identified. At PetroSA gas-to-liquids refinery in Mossel Bay an anaerobic digester with a 4.2

MWe (or approximately 12 MW thermal) biogas to electricity plant has been installed to

recover energy from their wastewater stream. It is likely that similar installations can be

considered for the Sasol coal-to-liquid plants. It is noted that the Sasol synfuels production is

approximately three times greater than that of PetroSA. If it is assumed that the energy

recovery potential is proportional to refinery output, the energy potential from Sasol

operations could be as high as 36 MW thermal.

Summary of the Case Study Outcomes In summary, it was concluded that limited potential exists for the recovery of energy from

mixed wastewater streams from crude oil refineries in South Africa due to the dilute streams

encountered. Some potential might exist for stripping oil from the more concentrated streams

and recycling or burning, but that was not considered in the context of this study. The

potential for energy recovery from wastewater has already been demonstrated at a gas-to-

liquid refinery (PetroSA), and is likely to have a similar application at coal-liquids-refineries.

The thermal energy generated from PetroSA is 12 mW and the national potential could be as

high as 48 MW.

71

References

1. UAEU, (2006), United Arab Emirates University, Removal of Phenol from Refinery

Wastewater, accessed online at

http://eclsun.uaeu.ac.ae/edc/archive/edc2006/tasks/task3.htm, March 2008.

2. Pearce, K. and Whyte, D. (2005), Water and Wastewater Management in the Oil Refining

and Re-refining Industry, WRC Report TT 180/05.

3. Al Zarooni M and Elshorbagy W., (2006), Characterization and assessment of Al Ruwais

refinery wastewater, Journal of Hazardous Materials A136, pp. 398–405.

4. Alva-Argaez A., Kokossis A.C., Smith R., (2007), The design of water-using systems in

petroleum refining using a water-pinch decomposition, Chemical Engineering Journal 128, pp.

33–46

5. Lenntech Petrochemical Company, ‘Refinery Process Water’, accessed online at

http://www.lenntech.com/petrochemical.htm, March 2008.

6. Beychok M.R., (1973), Aqueous Wastes from Petroleum and Petrochemical Plants, John

Wiley & Sons, London, pp. 33-112 & 156-266

7. World Bank and International Finance Corporation (2007), Environmental, Health, and

Safety Guidelines for Petroleum Refining, accessed online at

http://www.ifc.org/ifcext/enviro.nsf/Content/EnvironmentalGuidelines, March 2008.

8. SAPIA (2003), Annual Report, Appendix 9, accessed online at

www.sapia.co.za/pubs/2003_ARep/files/Sapia_2003_appendices.pdf, March 2008.

72

2.5: Survey of Animal Feedlots and Processing

Introduction and Approach Animal rearing and processing operations were identified as being of relevance to this study

due to the high organic load wastes which are generated from these operations. The use of

organic wastes from this sector for energy recovery is commonplace around the world, both

on a rural or single household scale (such as in China) and for large scale animal processing

operations (such as in the United States). In the US alone, it is estimated that in 2007 alone,

approximately 215 GWh equivalent of energy was generated from liquid and solid wastes

collected at animal feedlots alone [1].

A combination of organic solid and liquid wastes are typically managed together for energy

recovery, and the combined waste stream available at a single location will likely allow for the

full potential for energy generation from this sector to be realised. Anaerobic digestion, the

energy recovery technology most likely to be used in this sector, will handle the combined

waste stream. The diagram below indicates the relationship between manure type,

characteristics and management options as a function of solids content.

Figure 2.5 – Manure Characteristics that Influence Management Options [2]

Within the context of this study, which has a focus on energy from wastewater, a decision had

to be made as to whether to explore only the liquid/slurry proportion of this stream in terms of

its energy potential, or whether to extend this to include solid animal manure as well, given

that liquid and solid waste streams will likely be managed together. It was decided to first

express energy recovery potential of the wastewater stream on its own where information was

available (to be consistent with the project scope), and then, where relevant, extend this to

include animal manure.

In order to achieve the task of determining the overall potential for energy recovery from this

sector, the following information requirements were identified:

73

1. Composition of wastewaters in terms of organic matter (COD, BOD and/or VS)

2. Total wastewater generation from each of the components of the sector, calculated

through quantification of the number of animals held/processed in each component and the

wastewater generation per animal

3. Solid waste generation per animal (where relevant)

4. Potential for energy recovery from liquid and solid waste streams.

5. Number of different types of animals in South Africa

One of the challenges in obtaining an accurate indication of the potential from this sector is

the variability in information presented in the literature. The following generic observations are

offered:

A number of factors influence the composition and volume of wastewater generated

in the sector. These include differences in farming/processing approaches, animal densities,

location, washing of areas, wastewater management, etc. Differences between waste

management at various sites give order of magnitude differences in composition.

Some literature sources report information on the waste generated by the animals

only (i.e. urine and faeces), others report the compositions of the waste stream after

management (e.g. washing of floors, etc. which has been diluted with wash water, and also

includes waste feed and other solids). This affects both volume and composition data.

The ability to capture waste and wastewater will be vastly different in feedlots, dairies,

etc. and in rural areas.

Even at individual locations the literature identifies variability in wastewater

compositions. At any one location, wastewater compositions there is seasonal variation in

compositions due to changing animal densities (for example due to calving or slaughtering)

and seasonal rainfall, as well as the time of day at which samples are taken to name a few.

Waste and wastewater production and composition of an animal are affected by, for

example if an animal is gestating or lactating.

Information on the number of animals in South Africa was difficult to obtain, with

different sources offering significantly different data. One reason offered for this observation is

that some estimates include rural animals and others do not.

In addition, there is a wide variability in information with respect to the potential for recovery of

organic matter from these streams for energy.

Given the variability in data, the approach taken in this case study was to use a selection of

approaches and data to provide the outer bounds of energy potential in the sector. More

detailed, in depth studies would help to provide more accurate data in this regard.

In the study the following components of the animal feedlots and processing sector were

explored:

74

Feedlots

Dairies

Piggeries

Chicken farming

Red meat abattoirs

Poultry abattoirs

Wastewater Composition The table below presents the range of wastewater concentrations established through the

literature survey for the five sectors identified above.

Table 2.5.1 – Range of COD, BOD, VS and TSS for Wastewaters from Various Animal Feedlots and Processing

Operations (mg/L)

COD BOD VS TSS References

Feedlots No data found on wastewater compositions, combined solid and liquid waste used

Dairies 1500-9200 350-1600 255-830 250-600 5, 6, 7, 8

Piggeries 7141-14600 702-7790 3019-13051 5137-16965 9, 10, 11, 12

Red meat abattoirs 1100-21485 600-15510 3100-5600 13, 14

Poultry abattoirs 2360-11600 600-8700 600-1480 640-1213 15, 16, 17, 18

Although much data was found in the open literature on poultry litter from poultry breeding

facilities, limited data on the wastewater component of this stream in isolation could be found.

The reason offered for this is that breeding facilities are often lined with bedding material that

absorbs liquid and is periodically removed, or alternatively poultry manure falls onto the floor

where it is dried by the ventilation systems and removed as dry solid matter. Where cleaning

is done through flushing with water, the resultant wastewater stream is very dilute [2]. Hence

poultry breeding was considered in terms of energy potential from solid waste only.

75

Table 2.5.2 presents solid waste or manure compositions from animals in feedlots, piggeries

and poultry breeding facilities

Table 2.5.2 – Organic Composition of Selected Solid Wastes (kg/kg wet solids)

BOD VS References

Feedlots – Cattle2 0.02667 0.1-0.106 19, 20

Piggeries 0.076-0.092 20

Poultry3 0.187-0.259 2

Wastewater and Solid Waste Generation per Animal The information provided above provides some guidance on the composition of wastes on a

weight basis. In order to translate this into overall availability of wastes in South Africa,

information on wastewater and solid waste generation per animal is required. Once again, a

wide range of values is found in the literature, a summary of this information appears in Table

2.5.3 and Table 2.5.4.

Table 2.5.3 – Wastewater Generation per Animal

Animal Wastewater production

(l/animal/day)

Reference

Dairies4 (l/animal/day) 11-45.3 2, 21

Piggeries (l/animal/day) 5.4-9 12, 23

Red meat abattoirs (l/animal) 90-1250 24,

Poultry abattoirs (l/animal) 19-38 28

In terms of solid waste generation, the amount of waste generated depends on the type and

the weight of animal – Table 2.5.4

2 Assumed to be the same for dairies 3 Includes broilers and layers 4 Milk room + milking parlor + holding area, including manure washed out

76

Table 2.5.4 – Solid Waste Generation per animal

Animal Wet manure production

(kg/day)

Reference

Cattle5

average

220 kg

300 kg

450 kg

40-50

13.2-18.7

26-27

36-38

19, 20, 22

Pigs 6 1.7-26.2 20, 22

Poultry 0.1 22

Number of Animals Reared and Slaughtered

The table below provides an estimate of the number of animals reared and slaughtered in

South Africa in the sectors studied.

Table 2.5.5 – Animals Reared and Slaughtered Annually

Animal Number (million) Reference year Reference

Cattle (includes beef and dairy cattle) 13.911 2007 32

Dairy cattle7 0.621 1991

Pigs 1.65 2007 32

Chicken farming 5598 2003 33

Cattle, sheep, lambs9 and pigs slaughtered (by

commercial operations)

8.62 2003 33

Chickens slaughtered (by commercial operations) 545 2003 33

The following further information and assumptions are made regarding cattle in estimating

energy recovery potential:

1. The Department of Agriculture [35] estimates that 420 000 cattle are kept in feedlots

where the majority of their waste is likely to be able to be recovered. The remainder would

roam free in the veld in the day, and hence their wastes can only be captured at night when

5 Typically, the manure is 85 % to 90 % water [19] 6 Production by pigs is affected by whether they are lactating, gestating, nursing etc. 7 The reference source suggests that there are 136,716 dairy cattle in SA in the National Milk Recording and Progeny Testing Scheme. They also estimate about 22% of cows participate in the scheme, so there are 621,436 dairy cattle in the 5 main breeds in SA. 8 The reference source suggests that this was the total number of broilers slaughtered and layers in the industry as a

whole. 9 Note that the wastewater from rearing sheep and lambs was excluded from the study as they are typically in the

fields and collection of these wastes is thus impossible. Many will, however, be slaughtered in abattoirs, hence their inclusion here.

77

they are placed in holding pens. It is assumed that half of the solid wastes from these cattle is

recovered, and that no liquid waste is recovered from cattle not in feedlots.

2. It is assumed that the waste from dairy cattle can be recovered at night, as well as

during milking and holding in milking houses. For the remainder of the day they are allowed to

graze freely when waste cannot be recovered.

3. Recovery of biogas from free range cattle (see below) is likely to be lower than that

from wastes from cattle in feedlots. In the absence of better information, however, the

recovery rates are assumed to be similar. This will likely result in an over estimate of this

information.

Recovery of Biogas In this case study recovery of energy via Anaerobic Digestion is considered as for the

foreseeable future, this is the technology most likely to be used. As mentioned above, a

variety of information is available on energy recovery potentials. Typically, energy recovery is

reported on the basis of VS. The Table below presents a selection of the literature findings on

energy recovery potentials as used in this current study.

Table 2.5.6 – Energy Recovery Potential from Wastewaters

Wastewater Stream

Recovery Potential Comment Reference source

Feedlot 0.2-0.3 m³ kg-1 VS of biogas with a methane content of 55 to 75%

Based on “cow slurries” with a VS of 75-85% total solids

26

Dairy Values for feedlots are used.

Piggery (i) 0.25-0.5 m³ kg-1 VS of biogas with a methane content of 70-80%,

(ii) 0.24 m3 CH4/kg COD

(i) VS of 70-80% of total solids

(i) 26

(ii) 30

Poultry farming (i) 0.35-0.6 m³ kg-1 VS of biogas with a methane content of 60-80%

(ii) 0.2 to 0.3 m3 methane per kg VS

(i) VS of 70-80% of total solids

(i) 26

(ii) 31

Red Meat Abattoirs 0.18 m3 methane per kg COD in wastewater

Banks (1994) as referenced in 31

Poultry Abattoir 0.14 to 0.26 m3 methane/kg VS Methane content assumed to be the same as for feedlots.

Based on a review of a wide range of literature provided by 31

Results and Conclusions Using the data as compiled above, the ranges of energy recovery potentials shown in Table

2.5.7 were calculated for the wastewater and solid waste streams.

78

Table 2.5.7 – Energy Recovery Potential from Wastewater, Solid Waste and Mixed Wastes

Stream Potential from Liquid

Wastes (MWth)

Potential from Solid

Wastes (MWth)

Potential from Mixed

Waste Slurry (MWth)

Cattle in Feedlots (liquid +

solid)

79.1-214.5

Rural cattle 1270.9-3444.5

Dairies 0.1-2.3 116.8-118.4 116.9-120.7

Piggeries 2.0-33.2 16-681.3 18.0-714.5

Red meat abattoirs 0.2-48.9

Poultry farms 940.1-2976.1

Poultry abattoirs 0.6-7.0

A number of observations can be made from these results. Firstly, in the context of this

current study, energy recovery from wastewater in isolation from that recovered from solid

waste in the sector (as shown in the first column of the table) is not likely to make a significant

contribution to national or even local energy demand in dairies and poultry abattoirs. Solid

and liquid waste is likely to be managed together in most technologies.

Potential for energy recovery exists in centralized operations, including feedlots, dairies,

piggeries and abattoirs. On-site energy recovery systems on small to medium feedlots, dairies

and piggeries are likely to provide at a minimum onsite energy, and larger operations will have

the potential to export energy.

Although solid waste collected from corralled rural cattle is suggested to represent the most

significant potential for energy recovery in this sector, the practicalities of energy recovery

from this source are significant. This is both in terms of collection (manual collection of the

solid wastes from the kraals would be required in the mornings), and in terms of requirement

for a rollout of small scale digesters close to the source of the wastes. Hence the proportion of

this energy potential which could realistically be recovered would be significantly lower than

that shown in the table.

79

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15. U. Tezel, J.A. Pierson, S.G. Pavlostathis, Effect of polyelectrolytes and quaternary

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19. Queensland Department of Primary Industries and Fisheries, Feedlot Waste

Management 1. Manure Production Data, accessed online at

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27. K. Krich, D. Augenstein, JP Batmale, J. Benemann, B. Rutledge and D. Salour,

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82

2.6 ENERGY FROM THE FRUIT INDUSTRY WASTEWATER

Introduction and background

Energy recovery from fruit wastewater in South Africa has, to date, been relatively poorly

exploited and presents a potential opportunity for a source of renewable energy. The

development of suitable technologies for the conversion of waste to energy in the industrial

fruit processing sector can significantly benefit the industry. The aim of the case study was to

investigate the potential for energy recovery from wastewater streams generated by the

industrial fruit processing sector. The study was based in Cape Town and the main focus of

the project was fruit processing activity in the Western Cape where there are a number of

major fruit production and processing companies.

The study included a review of the fruit processing sector in South Africa focusing on the

Western Cape, and a review of techniques currently used to treat the wastewaters from fruit

processing. Technologies available for treatment were also reviewed and the two most

suitable were selected for experimental investigation. Samples of wastewaters from a

selection of local fruit processing plants were obtained and characterised to provide data for

the determination of energy recovery potential. Finally, technical and economic feasibility

were considered.

Overview of fruit production and processing in the Western Cape

The Western Cape agricultural sector generates about 23% of the total value added to the

agriculture sector of South Africa. The province has ideal conditions for both deciduous and

citrus fruit growing and fruit farming in the province was valued at R2.4 billion in 2005. In

2002/2003 approximately 1.46 million tonnes of deciduous fruit was produced, of which

approximately 20% was processed into fruit juice and about 30% was canned and dried.

(Nationally, the annual production was approximately 1 900 000 tonnes). In 2003/2004 the

Western Cape produced approximately 370 000 tonnes of citrus fruit, of which approximately

20 % was processed into juice. (Annual national production of citrus fruit in 2005/2006 was 2

100 000 tonnes).

The major types of fruit processing in the province are canning, juicing, wine making, and fruit

drying. Approximately 205 000 tonnes of deciduous fruit were processed by the canning

industry in 1999/2000. The typical water consumption for canned fruit processing and fruit

juicing is approximately 7 m3/t and 10.7 m3/t of raw produce, respectively. The wastewater

from these processes usually contains suspended solids, particulate organics, as well as

various cleaning solutions and softening or surface-active additives (e.g., sodium hydroxide,

nitric and phosphoric acids at low concentrations).

Fruit juicing typically yields solid residues to approximately 50 %wt of fruit used, and large

volumes of water (up to 10 m3/tonne raw fruit) are used largely for cleaning. This water

83

contains organics including carbohydrates, and can, of course, be mixed with the solid

residues for energy recovery purposes.

In the wine industry it is estimated that water usage of between 0.7 and 3.8 m3/tonne of

grapes processed (0.8-4.4 L/L of wine produced) is typical. The wastewater from the wine and

spirits industry usually contains a high organic load, suspended solids and dissolved solids.

Current treatment methods

The wastewaters from fruit canning and juicing are usually treated by means of land irrigation,

lagooning or treatment in an activated sludge process. With respect to lagoon technologies,

anaerobic lagoons require an input of approximately 100 g BOD/m3/d (Sigge, 2005) and must

operate at temperatures above 10ºC. The methane generated can be utilised for energy.

Facultative lagoons (stabilisation ponds) can be used to treat wastewaters with BOD levels in

the order of 5 g BOD/m3/d. Activated sludge treatment, may be problematic due high

carbohydrate loadings of fruit wastewaters, and the resulting in high microbial growth and

poor settling (Sigge, 2005; Casey, 1999). Anaerobic digestion (AD) is common and

successful as a treatment method; this has been widely investigated

Land irrigation is feasible for low COD wastewaters, or, after dilution, higher COD

wastewaters. More than 95% of South African wineries currently irrigate their wastewater onto

land by means of sprinkler systems, which, as with cannery wastewater, is an effective means

of lowering the COD of the effluent. Composting and landfill disposal are also common but do

not offer any opportunities for energy recovery.

Estimation of energy potential from fruit processing wastewater

A feasibility study was carried out to assess the potential for energy recovery from wastewater

generated by the industrial fruit processors in the Western Cape. The waste streams

generated from specific fruit processors in the province were characterised and classified,

and the potential for ethanol production by means of fermentation and the production of

biogas by means of anaerobic digestion were investigated using the data obtained from the

waste stream classification as the basis for determining energy recovery. Fruit juicing and

canning operations are usually seasonal, operating for about one third of the year, starting in

February/March. The wastewater streams from the juicing processes typically contain a sugar

content of about 10 g/L at a COD of 15000 mg/L. Fruit pomace from the juicing processes,

containing about 70% moisture, is also generated during production and also provides a

potential source of fermentable carbohydrates. The pomace typically contains 5 to 6 wt%

sugars. The wastewater streams from a typical canning operation have a sugar content of

about 3 g/L at a COD of 4400 mg/L.

An experimental investigation into the feasibility of ethanol production by means of yeast

fermentation showed that approximately 6 L or 4.7 kg of ethanol can be produced per ML of

84

wastewater which is equivalent to 101 MJ of energy per ML of wastewater (de Beer and

Dawson, 2007). The biogas yield from anaerobic digestion (AD) of the wastewater is

approximately 3750 L or 2.5 kg of methane per ML of wastewater which has an energy value

of 115 MJ per ML. The fermentation process converts the sugars present in the wastewater

streams into ethanol, while anaerobic digestion converts a greater fraction of the organic

material in the stream into biogas and thus yields more energy per volume than the

fermentation process. Synthetic apple waste was used as the starting solution for the

fermentations, for purposes of simplification. In comparing fermentation with anaerobic

digestion, substrate solutions with a high sugar (8-9 wt%) and COD (250 000 mg/L)

concentration, and a low sugar (0.8-1 wt%) and COD (15000 mg/L) concentration,

respectively, were used. The dilute fruit waste samples used here were similar to the most

concentrated wastewater streams produced by two local fruit processing plants. In anaerobic

and aerobic fermentations yeast (Saccharomyces cerevisiae) was used and in anaerobic

digestion, sludge was used as the inocculum. From the results it is clear that aerobic

fermentation has a larger COD reducing potential than that of anaerobic fermentation. The

greatest ethanol production (5.8 wt% ethanol) was obtained by the anaerobic fermentation of

the concentrate fruit waste sample. The sludge fermentations performed better than was

expected but still only produced 2.2% ethanol which is considerably less than the yeast

fermentation and not considered viable. It was concluded from the experimental results that

some form of waste stream concentration would have to be performed to increase the sugars

concentration to make ethanol production more viable.

Consideration of feasibility

Although both fermentation and AD technologies are suitable for the recovery of energy from

fruit processing wastewater, the viability of each is very case specific and factors such as

waste COD, sugar concentration, and volumes need to be considered. It is also important to

consider that biofuel production from fruit waste must compete commercially, with not only

other methods of well established bioethanol production, but also producers of regular

petroleum products, which currently operate at large production capacities and typically have

a well established market share. However, further developments and increased interest in

biofuel production could lead to the commercialisation of more large-scale plants, with higher

production capacities. Economies of scale would thus lower capital expenditure and increase

profitability per volume of biofuel produced. The implementation of new technologies, such

membrane concentration of wastewater streams, could also lead to increased economic

feasibility of biofuel production.

85

2.7 Thermal gasification or anaerobic digestion of biomass for use in a distributed

renewable energy network.

This study compares thermal (gasification) and biological (anaerobic digestion) process

routes for producing a combustible gas from biomass for use in a distributed, renewable

energy network. The global hypothesis of the study states that based on the composition and

physical properties of a particular biomass, so one route will be favourable over the other.

Furthermore, the following objectives have been identified. a.) Identify operational problems

or technical barriers in each process b.) Identify threshold limits, based on composition, to

guide a decision maker as which process to consider c.) Develop process designs using the

above algorithm d.) Evaluate the different designs on the basis of energy yield and exergy

The feedstocks which will be evaluated are: corn stover, wood chips, indigenous grasses and

algal or sewerage sludge. The feedstocks provide a range of compositional variation and

some are predisposed to gasification (wood chips), others to anaerobic digestion (the

sludges) and choice of process for the corn stover and grass is not immediately clear.

Gasification occurs in an atmosphere of sub-stoichiometric oxygen requirement. Biomass

particles are heated up and release volatile their volatile components. This is known as

pyrolysis or devolatilisation. Occurring sequentially, or simultaneously, depending on heating

rate, the carbon in the carbon/ash skeleton is consumed via combustion and gasification

reactions leaving the ash behind. The gas that is produced is a low (5 MJ/Nm3) to medium (15

MJ/Nm3) calorific gas depending on whether air or steam is used as a gasifying agent.

Anaerobic digestion is the conversion of organic material to a mixture of mainly methane and

carbon dioxide via the activity of three classes of micro-organisms, viz. aerobic hydrolytic

fungi or anaerobic hydrolytic bacteria, anaerobic acetogenic bacteria and finally anaerobic

methanogenic Archaea. The operational problems in gasification were identified as bed

agglomeration and tar formation. Simple methods for predicting biomass ash fusion

temperature, as has been done for coal gasification, were not found. Tar formation is not

necessarily a function of the biomass composition, but rather depends on reactor design. In-

bed and post-bed tar reforming methods, especially when both used, can successfully crack

tars to produce a gas usable in gas turbines.

The success of anaerobic digestion of lignocellulosic materials is dependant on an effective

pre-treatment. Characteristics of a potential process must improve the formation of sugars or

the ability to subsequently form sugars by enzymatic hydrolysis; avoid the degradation or loss

of carbohydrate; avoid the formation of byproducts inhibitory to the subsequent hydrolysis and

fermentation processes and must be cost-effective. Processes fitting these criteria include:

dilute acid hydrolysis, hot water hydrolysis, steam explosion, Ammonia Fibre Explosion

(AFEX), Ammonia Recycle Percolation (ARP), Soaking in Aqueous Ammonia (SAA) and lime.

86

Lime and air pretreated biomass exhibits up to 93% conversion from cellulose to glucose and

operates at ambient conditions. The only caveat is its slow rate (2 weeks to a month).

The methodology for this project will be as follows:

1. • Identify, from the literature and from first principles, critical limits of operation

2. • Develop “If, then, else” algorithm using these critical limits

3. • Use the algorithm to create a process flowsheet

4. • Assess the flowsheet using exergy analysis

5. • Validate the choice of unit operation experimentally

Some aspects of the decision algorithm have already been investigated, viz. moisture

content, lignin content and ash content. It has been shown that a moisture content higher than

60% precludes the biomass feedstock from gasification as the energy obtained from the fuel

gas is less than the energy consumption of a drier. An ash content of 1 kg alkali/GJ was

identified as a threshold limit for sound gasification operation. Preliminary work on the

threshold limit for lignin content did not yield usable results as the relationship was linear due

to the assumed constant cellulose conversion. Further investigation into how to represent the

effect of increased lignin content on cellulose conversion is needed.

In conclusion, the applicability of gasification and anaerobic digestion are dependent on the

qualities of the feedstock. This report sets the basis for further work to gain a deeper

understanding of the interaction of the various properties of biomass feedstock and ultimately

propose a decision making framework.

87

2.8. The sludge drying process at Cape Flats Treatment Works (CFTW): Is it best

practice?

88

Introduction

The Cape Flats Treatment Works (CFTW), owned and operated by the City of Cape Town, is

the only such works in the country to have installed a sludge drying process. The primary

stabilised sludge from the anaerobic digesters are sintered into pellets at temperatures of

500°C using biogas from the anaerobic digesters as fuel. Since these pellets can be used as

an alternative/additional fuel in the cement industries, this illustrates two instances of energy

capture and use from wastewater.

With the kind support of the Wastewater Department of the City of Cape Town, Ms. Loice

Badza, a final year Chemical Engineering student, undertook her research project on a

technical, environmental and institutional evaluation of the CFTW sludge drying process.

Hypothesis and Methodology

The research was guided by the following hypothesis: The choice of using the biogas for

sludge drying and subsequently using the sludge pellets as a combustion fuel at PPC cement

works represents best practice for the Cape Flats WWTP.

Technical, environmental and institutional evaluations were carried out. A flowsheet analysis

and a material and energy balance completed the technical evaluation. Aspects of the mass

and energy balance investigated include: CH4 production output, sludge production, pellet

production and fuel use. The environmental assessment considered indicators such as GHG

emissions, the cost of dumping waste and electricity and heat generation and .was based on

qualitative data gathering from interviews and quantitative data from the mass and energy

balance. A set of interviews with plant managers at the plant provided information to complete

the institutional evaluation.

Results

The CFTW sludge drying project was commissioned in 1999 and is run in the form of a public

private partnership; with the sludge drying plant built and operated by a joint venture of

Biwater with Murray and Roberts. One of the major reasons for its construction was that the

practice of on-site sludge burial was causing the seepage of nutrients into the adjacent

Zeekoevlei wetlands causing eutrophication. The drying plant uses the Swiss Combi Drying

system (www.swisscombi.com). A block flow diagram of the drying plant as it fits into the

treatment works is shown in Figure 1.

89

a) Technical evaluation

The flow diagram covers CFTW’s sludge handling process only. The solid lines represent

material flow and the dotted lines, heat networks.

Heat exchanger

P-40AnaerobicDigesters

Biogas GasHolder

Stabilized sludge

Dewatering centrifuges

GasboilerB

Primary sludge

DAF sludge

Thickening centrifuges

A

Rotary drier

water

Sludge cake to landfill

Pellets

water

C

Gasburner

ED

Figure 2.8.1 : Simplified diagram of the Energy from wastewater project at CFTW

The heat network system is described below.

A: This is the main use of biogas currently produced. There are three anaerobic digesters,

each were designed to produce 400 m3/h of biogas at 65% methane. Gas flow-meters

commissioned in September 2007 reported gas flow rates of 140 m3/h, and the biannual

methane content of the biogas was reported as 55% methane (June 2007).

B: Because the plant is currently not producing enough biogas, no biogas is used to fuel the

gas burner. However, in the past, this path was viable.

C: For optimum digester performance, pre-heating the sludge is carried in double pipe heat

exchanger, with sludge on the inside and warm water (heated by the boiler) on the outside.

D: Potential for heat recovery from the sludge drying process involves use of warm water (~

70oC) to pre-heat the sludge.

E: Diesel fuel is currently used in the burner. Between October 2006 and September 2007, no

biogas was used for drying and a total of 493 kilolitres of diesel were used.

Figure 2 gives an idea as to how functional the drying plant has been for the period of

October 2006 to September 2007. Sludge from the dewatering centrifuges is about 20%

solids, whilst the pellets are 95% solids, thus every cubic meter of sludge would reduce to

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roughly 0.2 m3 of pellets. It can be seen that during this review period, there was always a

portion of the sludge that could not be dried and that both the overall amount of sludge to be

handled, and the fraction actually dried, show significant variability.

Sludge handling overview

0

200

400

600

800

1000

1200

Oct-06 Nov-06 Dec-06 Jan-07 Feb-07 Mar-07 Apr-07 May-07 Jun-07 Jul-07 Aug-07 Sep-07

time, month

Vo

lum

e, m

3/m

th

sludge cake

Pellets

Figure 2.8.2 : Amounts of dewatered sludge and dried pellets disposed

In order to run a plant optimally and efficiently, measurements of key operating variables are

required. The table below reports what is actually measured at the CFTW related to the

sludge drying operation.

1. Primary sludge Flowmeters working

Compositions measured via sampling analysis.

COD measured is for supernatant return to 2o treatment only

2. DAF sludge No flowmeters present.

Compositions measured via sampling analysis

3. Thickened centrate Flowmeters working; DS and TSS content measured

4. Digester 1 feed Flowmeters working; DS content measured; COD not measured

5. Digester 2 feed Flowmeters working; DS content measured; COD not measured

6. Digester 3 feed Out of commission

7. Biogas to gas holder Flowmeters installed September 2007; methane content measured biannually

8. Dewatered centrate Flowmeters working; DS and TSS measures

9. Mixed dewatered stream No flowmeters

10. Sludge cake Volume estimated using truck maximum capacity

12. Pellets product Volume estimated using truck maximum capacity and maximum bag volume

The COD measured at the plant are the influent (plant load), primary treatment liquid effluent

(to 2o treatment), secondary treatment effluent and final effluent (to maturation ponds).

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b) Environmental evaluation

The sludge drying plant was built to protect the local environment, esp. Zeekoeivlei, from

leachate generated from sludge buried on-site. The operation of the CFTW sludge drying

facility reduces this environmental burden, although there are months in which the drying

plant is not sufficiently operational to dry all of the sludge.

This primary environmental benefit does however require significant energy usage. At best,

this energy is provided from the wastewater treatment process in the form of biogas from the

anaerobic digesters, in the process transforming methane, a potent greenhouse gas, to the

less potent greenhouse gas carbon dioxide. The project was, however, not designed to utilise

all methane gas, leaving a significant portion (400 m3/h) of biogas, to be vented. In recent

practice, however, since insufficient quantities of biogas are currently being produced (140

m3/h), all of this was used for digester preheating. This meant that the drying plant was run on

a fuel known as LO 10, supplied by FFS Refineries, at about 50% of the price of Diesel). As

this is also an energy product recovered from waste materials, the resulting environmental

issues of resource depletion and greenhouse gas emissions are of secondary importance.

c) Institutional evaluation

The Cape Flats Wastewater Treatment Works was commissioned in 1980. The new

technology, sludge dewatering and drying plant, came on line in 1999. Biwater-Murray and

Roberts JV was awarded the tender to build the plant and also to operate and maintain it. The

plant is owned by the City of Cape Town.

The main recipient of the pellets was a large cement manufacturing company, PPC, who used

the pellets as an energy source to replace coal in their combustion kilns. However, the lack of

a formal contract between the CFTW and PPC confirming consistent supply of pellets was a

drawback to both institutions. Since the CFTW was not obliged to give away the pellets to

PPC only, they also supplied farmers in the Philippi area with the pellets to use as fertiliser

(This is not an agreed or contractual practice but more of an ad hoc basis and only if there are

stockpiles on site). PPC’s level of commitment was so high that they were prepared to pay for

guaranteed supply of the fuel pellets, but the income from such secondary industries are not

permitted (as determined by governmental regulations). PPC stopped using pellets to fire their

cement kilns because the quantities were erratic and the volumes insufficient for their needs

and CFTW presently have a contract with Wasteman to remove all sludge produces to the

Vissershok landfill site. The sludge still requires drying to about 95% solids content in

accordance with the Biwater’s contract with the WWTP.

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Conclusions

This investigation of the sludge drying practice at the Cape Flats Treatment Works from an

‘energy from wastewater’ perspective has yielded the following insights:

1. Although the CFTW process can be regarded as a case of energy from wastewater since it

can utilise both the biogas from the anaerobic digesters and the dried sludge pellets as fuel,

the project was not conceived as an energy project but as an environmental protection

project.

2. The energy and climate change aspects of the sludge drying process can still be

significantly improved.

The CFTW was designed to produce 400 m3/h of biogas at 65% methane, but recent

analysis indicated only 140 m3/h at 55% methane (June 2007). This indicates that the system

is operating at only about 30% efficiency.

The sludge drying operation is significantly cheaper when it can harness biogas fuel

from well-functioning digesters – which was not the case for the review period, where almost

half a million litres of diesel fuel were used over a 12 month period.

If the AD system were functioning optimally, the biogas generated could be used to

supply the CFTW with the majority of its energy requirements (2.45 gWh).

3. The opportunity of harnessing the dried sludge as a replacement energy product was lost

because the plant is not permitted to generate revenue from pellet production.

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2.9: Case study of household AD biogas systems

Introduction

The standard, centralised water-borne sewage system for the disposal of domestic

wastewaters is that they are unsustainable and represents lost opportunities in recovering the

energy and water. This system flushes pathogenic bacteria out of the residential area, using

large amounts of water and thereby transfers a concentrated domestic health problem into a

diffuse health problem for the entire settlement and/or region. Energy input is required to treat

the wastewater at the centralised wastewater treatment plant, where the cost of treatment

increases as the volume of wastewater increases. The centralised municipal wastewater

treatment plants are also particularly costly to deploy in rural or peri-urban settings, making

the provision of wastewater treatment services to the South Africans who do not have access

to water-borne services (approx 40% of the population) a huge challenge

History of household AD biogas

There’s evidence that biogas was used to a heat bath water in Assyria during 10 BC, but the

first recorded digester to produce biogas from wastes was built in a leper colony India in

1859. China began a mass adoption of biogas in 1975 under the slogan “biogas for every

household”. Within the first few years, 1.6 million digesters were constructed annually, but

these were of low quality and by 1980, half of all digesters were not in use and the rate of

adoption had slowed. By 1992, only 5 million family sized plants were still operating, many of

them redesigned to avoid leakage. The technology has continued to be developed and

implemented on a large scale and in 2005 China had 17 million digesters with annual

production of 6.5 billion m3 biogas. This supplied approximately 50 million people biogas fuel

for sufficient for their daily household needs. The government aims to reach annual

production of biogas is projected to reach an annual production of 25 billion m3 biogas by

2020. Importantly, biogas provides energy to one quarter of households in rural areas. Similar

developments took place in India and by 2000 more than 2 million biogas plants were built.

However, there were several problems with poor design, lack of maintenance skills and

insufficient capacity to deal with the problems meaning that 1/3 were non-operational.

Lessons have been learnt from these early years the government is aiding a new

development and implementation program. There are also recent success in countries such

as Nepal, Sri Lanka, Philippines and Vietnam.

There is a great opportunity for developing countries since there is a constant available

supply raw material (animal dung and human domestic wastewaters), the technology is

relatively simple and robust, and it requires relatively small investments. The technical viability

of biogas technology has been repeatedly proven in many field tests and pilot projects but

numerous problems arose with mass implementation (Karekezi, 1994) and biogas has been

94

implemented only to a limited extent in most African countries. The reasons for the lack of

development of this technology are that the investment cost of even the smallest of the biogas

units is prohibitive for most rural households of sub-Saharan Africa (e.g., a family size plant

cost US$1667 – beyond the means of rural farmers), immature technical properties of plants

themselves, a lack of government commitment (policy) and dissemination strategy

(inadequate user training and follow-up services), and a limited private sector input because

of low profit incentive (Amigun and Von Blottnitz, 2007).

Benefits of biogas technology.

Individuals judge the profitability of biogas plants primarily from the monetary surplus gained

from utilising biogas and biofertiliser in relation to the cost of the plants. In terms of energy

value, 1 m3 Biogas (Approx. 6 kWh/m3) is equivalent to:

• Diesel, Kerosene (approx. 12 kWh/kg) 0.5 kg

• Wood (approx. 4.5 kWh/kg) 1.3 kg

• Cow dung (approx. 5 kWh/kg dry matter) 1.2 kg

• Plant residues (approx. 4.5 kWh/kg d.m.) 1.3 kg

• Hard coal (approx. 8.5 kWh/kg) 0.7 kg

• City gas (approx. 5.3 kWh/m3) 1.1 m3

• Propane (approx. 25 kWh/m3) 0.24 m3

In practice, 1 m3 of biogas can cook three meals for a family, generate 1.25 kWh of electricity,

or power a 1hp internal combustion motor for 2 hours.

Biofertiliser slurry can be quantified as a monetary benefit, but this depends largely on the

previous practice of the raw material to be digested. In agricultural settings the dung and

fodder residues are often heaped in the open, leading to heavy losses of minerals through

sun and rain.

The reduced disposal costs can be counted as benefits of a biogas system where the

disposal of waste and wastewater is regulated by law and where disposal opportunities exist.

In rural households, human faeces are collected in pit latrines and once the pit latrines are full

(every two years), they are filled with soil and a new pit is dug. Excavation costs and

subsequent disposal costs can be saved and calculated as benefit. Similarly, if a septic tank is

used, the emptying cost can be counted as benefit. In the city or town setting, the reduced

loads discharged to the municipal WWTP (and the reduction in costs incurred) could also be

counted as benefits if the appropriate levies existed.

The main environmental benefits are that a renewable source of energy is captured. The use

of a renewable energy reduces the CO2 emissions through a reduction of the demand for

fossil fuels or firewood. There are also important benefits of methane mitigation since

methane is the second most important greenhouse gas contributing to global warming.

95

Lastly, applying AD biogas technology to the treatment of industrial or municipal wastewater,

can be protect water resources (rivers, sea, ground and drinking water resources). The

effluent wastewater can often be reused after appropriate polishing, (e.g. as process water in

industry or as irrigation water in agriculture) so the costs avoided in using potable water can

be directly translated into benefits.

Technical performance

Several anaerobic digesters (AD) for biogas production have been installed by

(http://www.agama.co.za/). The aim of this study was to assess the performance of three

small (<10 m3) anaerobic digesters that are being used in households in the Western Cape.

Key performance indicators were the biogas produced (flow rate and %methane) and the

reduction in wastewater organic load (COD, nutrients, and coliform bacteria).

Fig. 2.9.1: Structure of a typical Anaerobic Digester

AD at Noordhoek (Cape Town) and Stanford (Overberg) were assessed. The sizes and

loading capacity of the ADs are shown below, together with the predicted biogas production

(m3/day).

Table 2.9.1: Characteristics of ADs

AD digester AD volume

(m3)

Type and description of load Predicted

optimal biogas

production

(m3/day)

Predicted optimal

energy produced

kWh per day *

Noordhoek

(Thor)

8 Domestic (3 persons) greywater and

blackwater + horse manure 2 wheel

barrows

3.0 18

Noordhoek

(Richmond)

6 Domestic greywater and

blackwater(2-6 persons domestic)

2.3 14

Stanford Valley 10 Domestic backwater (6-20 persons) 3.8 23

* The predicted biogas production is 0.38 m3/day/m3 biodigester. Biogas has a heat energy value of approx. 6 kWh/m3

A B

96

Three separate site visits were carried out in order to take samples of wastewater and biogas.

These were 5 November, 11 December 2007 and 25 January 2008. The time interval was

chosen to accommodate the large residence times in the digester (20-40 days) and the

seasonal fluctuations (may affect performance through changes in temperature) as well as to

reliably asses performance over a time period. The experiments assessed the performance of

the AD digester alone within the complete decentralised wastewater treatment system

(DEWATS). Other components of the system for the further treatment of the wastewater

include an anaerobic baffled reactor (ABR), a constructed wetland (plant reed-bed) and an

algal polishing pond. At Stanford Valley, the effluent wastewater from the AD is passed onto a

reed bed and then onto an algal pond before it is discharged into the fields to irrigate pasture.

A more recent installation in the town of Stanford, uses an anaerobic baffled reactor prior to

the plant reed bed, but this system was not assessed since it has only recently been installed

(September 2007).

Methods

A syringe (50 ml sample) of biogas and 200 mL sample of wastewater were taken and

transported back to the laboratory for analysis. The biogas was analysed by isothermal gas

chromatography (GC) at 140°C. A standard of 9.7% methane was used for calibration and the

% methane in the biogas determined from relative peak areas. The biogas flow rate was

estimated in a rudimentary fashion using a simple 1 kPa pressure gauge. Owners were

instructed to place the gauge on the gas outlet after a period of cooking (usually at the end of

the day), measure the pressure (P1) and then measure the pressure again (P2) after a time

period of several hours. The flow rate m3/day was calculate using time interval and

P1V1=P2V2. To determine the number of coliform bacteria in the provided wastewater sample.

The test was carried out as described by McCarthy, Delaney and Grasso and in accordance

with the American Public Health Association methods for testing drinking water and the U. S.

Environmental Protection Agency for testing water using a two-step, delayed incubation,

membrane filtration method. M-Endo Agar LES is used in the standard membrane filtration

technique whereby E. coli bacteria produce a metallic-red colony within 24 hours incubation at

35°C. A sterile 3 mM a membrane filter absorbent pad (Whatmann) was placed inside the

cover of a Petri dish and 2.0 mL Lauryl Tryptose Broth added. The wastewater sample (100

mL) was filtered through a sterile membrane (0.2µm Whatman nitrocellulose) using a vacuum

manifold, placed onto the pad containing Lauryl Tryptose Broth and incubated at 35°C for 2

hours. The membrane was then transferred from the pad to the surface of the M-Endo Agar

LES medium (keeping the side on which the bacteria have been collected facing upward) and

incubated at 35°C for 24 hours. As a positive control approx. 100 E. coli were seeded into 100

mL sterile water and as a negative control 100 mL of sterile water was used: – both were

treated in the same way as the wastewater sample.

Standard colorimetric methods were used for the testing of total nitrogen, total phosphorus,

97

nitrate, nitrite, ammonia and phosphate (AQUANAL reagents, Sigma) and pH.

Results

The data is summarised in the table below. Values are an average with the standard error

(SE) in parenthesis for three determinations from the three separate site visits. Inflow and

outflow refer to samples taken at the inlet riser and outlet riser, respectively (labelled as A and

B in the diagram above).

Table 2.9.2: Characteristics of inflow and outflow of ADs visited

Stanford Valley Noordhoek_Thor Noodhoek_Richmond

Biogas %methane 68 (±4) 61 (±3) 69 (±6)

Pre-and Post-AD: Inflow Outflow Inflow Outflow Inflow Outflow

pH of AD 6.6 (±0.4) 6.9 (±0.2) 5.8 (±0.4) 6.7 (±0.6) 6.9 (±0.1) 7.1 (±0.2)

COD (mg/L) 1834 1252 967 919

Total Nitrogen

Nitrate

Ammonia

Total Phosphorous

Total coliforms per

x105/100 mL

105 (±14) 35 (±9) 311 (±89) 123 (±21) 38 (±18) 27 (±11)

Stanford Valley outflow from algal pond 2x105cfu/ml (±3)

98

AD biogas performance survey

A questionnaire directed to the owner and user of the household biogas system was also

used to survey the performance, acceptability and benefits of the AD biogas systems. Below

are the responses from the three household AD units assessed above as well as the recently

installed system in the Town of Stanford (Peter Byrsshe):

__________________________________________________________________________

STANFORD VALLEY FARM AND CONFERENCE CENTRE: Contact: Steve Castle.

email: [email protected]

Energy from wastewater questionnaire: Household AD biogas:

The following questionnaire is part of the “Energy from Wastewater” feasibility study, carried out at University of Cape Town (funded by the Water Research Council).

OWNER

Why did you decide on a biogas system?

It is with our sustainable energy use goals

Where/How did you hear about system?

Research. AGAMA Energy

How much did the system cost? (ZAR and date)

ZAR 60 250 for the system without the labour costs (supplied by Stanford Valley). Completed 07/2006

What are the loads that you supply to the anaerobic digester (AD)?

Kitchen waste, household black water. Input varies depending on occupancy (20 max).

What are happens to the wastewater outflow?

Polished water currently used to flood irrigate pasture

How much does the biogas system save you on cooking costs? (gas or money saved (ZAR))

Difficult to gauge as REAL volumes produced are unknown.

Any other important benefits you can identify? No

Is the system reliable? Do you have backup fuel?

SSttaannffoorrdd VVaalllleeyy NNoooorrddhhooeekk__TThhoorr NNooooddhhooeekk__RRiicchhmmoonndd

Biogas %methane 66 (±4) 61 (±3) 62 (±6)

Pre-and Post-AD: Inflow Outflow Inflow Outflow Inflow Outflow

pH of AD 6.6 (±0.4) 6.9 (±0.2) 5.8 (±0.4) 6.7 (±0.5) 6.9 (±0.1) 7.1 (±0.2)

COD (mg/L) 1834 (±175) 1012 (±122) 3910 (±401) 2338 (±389) 967 (±96) 719 (165)

Total Nitrogen 45 (±12) 34 (±18) 76 (±11) 64 (±16) 23 (±13) 22 (±9)

Ammonia 27 (±5) 18 (±4) 68 (±4) 61 (±7) 21 (±6) 20 (±4)

Total Phosphorous 22 (±4) 19 (±3) 45 (±5) 44 (±13) 12 (±7) 7 (±4)

E. coli x105/100

mL

105 (±14) 35 (±19) 211 (±99) 83 (±31) 38 (±18) 19 (±11)

99

We find the system reliable. We do have backup LPG gas and electricity.

Why don’t more people have biogas systems?

Perhaps the upfront costs as well as the unit footprint might be a deterrent.

OPERATOR (user of biogas)

How does the biogas burn compared to ordinary LPG gas?

Better SameX Worse

How does the biogas smell compared to ordinary LPG gas?

Better Same WorseX

How much gas or cooking time do you get per day?

Approximately 6 hours per day.

Any other problems or issues with biogas systems?

Lack of accurate measurement tools.

__________________________________________________________________________

STANFORD Village. Contact: Peter Byrsshe.

email:[email protected]



OWNER


Recycle water and sort out problem with French drain as well as capture energy


Eric at Stanford Valley farm


ZAR 45000 for the system – AD, baffled reactor and reed bed. Completed 11/2007


Kitchen waste, household blackwater and greywater. Poo from 2 dogs and kitchen waste from the restaurant they run in town (20 L tub per day)


Pumped to 13 000 L storage tank for garden irrigation


All household cooking for a family of 4. Approximately ZAR 150 per month saved

Any other important benefits you can identify?

Water usage reduced. Increased energy efficiency and reduced carbon footprint reduced. Reduced

burden on the Municipal WWTP and costs incurred (ZAR100 per month to empty a septic tank system or charges of approximately ZAR35 per month if connected to the municipal sewage system – however no

mechanism to recover these costs)


Reliable and supplies all the cooking needs for the family of 4. Backup LPG gas not required


100

The mindset or awareness. This is increasing with the energy crisis. The initial costs mean it is unaffordable for many.



Better SameX Worse


Better SameX Worse




Some maintenance required – occasional stirring. Need cheaper pre-cast units

__________________________________________________________________________

NOORDHOEK – Richmond : Carol Richmond

email:[email protected]



OWNER


To decrease dependence on coal-based energy (electricity)


At a sustainability conference in Pretoria 2005


ZAR35 000 without labour costs Completed 07/2007


Kitchen waste, household black water and greywater. Also plant material from constructed wetland (natural swimming pool )


Goes into the ground. Plan to have a polishing pond and then irrigate with it (garden)


Saves 2 hrs LPG gas per day.


Free energy. Save on fertilizer requirements (effluent and sludge), reduces global warming by reducing methane emissions


Yes reliable and used for cooking needs. We have backup wood and anthracite burning stove, solar water heaters and a solar pump , LPG gas and mainline grid electricity.


High capital outlay. Perception it is labour intensive, smelly and/or still experimental



101

Better SameX Worse


Better Same X Worse




Does not work well with Eurogas stoves. Causes thermocouple spark to malfunction? Works well with simple cast iron stoves (match lit).

__________________________________________________________________________

NOORDHOEK – Thor. Contact Anthea Thor.



OWNER


Energy saving and reducing methane emission


Biophile magazine and AGAMA Energy


Unsure. Completed 2007


Household of 1-3 persons. Kitchen waste, household blackwater and greywater and manure from two horses (approx half a wheelbarrow per day.


Polished water used to irrigate fruit trees. Uses a solar pump


Approx 9 kg per month (ZAR140).


Close the loop of water cycle and fertilizer use


We find the system reliable ad always sufficient


Exactly! especially when that from several houses could be combined. SA not resourceful and uncommon/unheard of technology



Better SameX Worse


Better SameX Worse


Sufficient for cooking requirements

102


Takes about 3 months to establish the biogas production. Needs some maintenance. Only put biodegradable material in the waste stream and no sand

__________________________________________________________________________

Conclusions

The biogas generated has a methane content of 58-72% and the quantity is sufficient for the

household’s family cooking requirements.

The AD is capable of reducing the COD considerably (42-68%), but the levels of other nutrients

(nitrogen and phosphorous) are relatively unaffected. Similarly. the levels of coliform bacteria

is reduced a mere 3-5 fold. The values for pH were not always in the range considered

optimal for AD (pH 6.8-7.5). This may be a reflection of both the acidic waster source and the

loads – the acid nature of the Stanford (Thor) AD in particular may be a result of the heavy

loading with horse manure.

In general, the quality of the AD effluent exceeds the levels permitted by government

regulations for discharge into environment. This wastewater effluent needs further treatment

systems (reed bed algal pond and/or baffled reactor), and these are in place at the three

locations, but this study only assessed the performance of the AD.

The owners and operators of the AD biogas systems were satisfied with the performance and

generated sufficient biogas for their household cooking needs. Two of the owners wished to

further capture energy and are investigating a generator for electricity production or a biogas

boiler for hot water and under-floor heating. The following were identified as issues or

obstacles to widespread adoption of household biogas systems:

Initial capital expenditure is considerable. Estimated ZAR10000 to ZAR 100000 for a

6-10 m3 AD, depending on other components of the DEWATS system.

There is a lack of government incentives for either clean renewable energy or

reduction in the loads discharged to the municipal WWTP.

There are some problems with using the biogas in more sophisticated gas stoves.

The systems work well with the rudimentary cast iron gas units with minor

modification to adjust the air flow.

Many of the owners were unsure if they were loading their systems properly and

optimally and had no measure of biogas production. Additionally, there was some

uncertainty if the final wastewater discharge was safe for irrigation crops for human

consumption and met the standards set by the government. Presently, the owners

have chosen to irrigate either pasture for cattle grazing or ornamental plants and fruit

and nut trees.

Lack of awareness of the need for clean and sustainable energy and the view of

waste as a costly burden are obstacles to widespread uptake of AD biogas systems

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Initial capital expenditure is considerable. Estimated R10 000 to R100 000 for a 6-10

m3 AD, depending on other components of the DEWATS system.

There is a lack of government incentives for either clean renewable energy or

reduction in the loads discharged to the municipal WWTP.

There are some problems with using the biogas in more sophisticated gas stoves.

The systems work well with the rudimentary cast iron gas units with minor

modification to adjust the air flow.

Many of the owners were unsure if they were loading their systems properly and

optimally and had no measure of biogas production. Additionally, there was some

uncertainty if the final wastewater discharge was safe for irrigation crops for human

consumption and met the standards set by the government. Presently, the owners

have chosen to irrigate either pasture for cattle grazing or ornamental plants and fruit

and nut trees.

A lack of awareness of the need for clean and sustainable energy, and the view that

waste is a costly burden are obstacles to the widespread uptake of AD biogas

systems.

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SECTION 2.10 Energy from Wastewater Industrial Stakeholder Workshop

14 March 2008 9:15-12:30

Studio 2, Dept. of Chemical Engineering, University of Cape Town, Rondebosch

Followed by visit to the SAB Brewery in Newlands

Attendees:

Peter King – City of Cape Town

Nokuzola Lujiza – City of Cape Town

Anton Laubscher – Distell

Kevin Sampson – City of Cape Town

Kevin Fawcitt – City of Cape Town

William Soekoe – City of Cape Town

Greg Austin – Agama Energy

Neil Parker – Agama Energy

Melumzi Nontanga – City of Cape Town

Eleonore Schmollgruber

Harro von Blottnitz – UCT

Peter Hoffman – SAB

Johan van den Berg – CDM Africa

Brett Cohen – UCT

Simisha Pather-Elias – UCT

William Stafford – UCT

Sue Harrison – UCT

Stephanie Burton – UCT

Rob van Hille – UCT

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1. Energy from wastewater: Project aims, definitions, international practice, technology

(William Stafford, UCT)

William Stafford introduced the aims of the project, the approach and the outcomes of the

project. A brief review of local and international practice was included.

Q&A:

• What is the profitability of the biodiesel from algae in terms of carbon

sequestration/emissions and financial payback? There have been studies in Europe which

show a return in capital investment of less than 5 years. There have been studies in other

countries, but each country needs to do its own study due to different costs for building,

electricity, transport, etc. In order to determine the carbon sequestration, a full life cycle

assessment needs to be done, and because this takes such a long time, there haven’t been

very many studies. Environmental factors such as the amount of sunlight and land area for

ponding are also an important consideration.

• What is the efficiency of making biofuels because of the energy intensiveness and

there are many steps to producing the final product? Algal biodiesel can produce secondary

products such as carotene which are profitable. The waste material (cake) can be used as

animal feed, fertiliser, etc. There is great value in the algae cake that remains post processing

(waste) and this can tip the balance of economic feasibility. Peter King then suggested using

the Western Tannery Plant, which also grows Spirulina as a nutritional supplement, as a case

study to determine the economic feasibility and the energy efficiency. Nobody knew if the

plant was still operational since Spirulina that was grown on a tannery waste and used as a

health food supplement resulted in human illness associated with pathogens in the Spirulina

product. Similarly, algae grown on wastewater containing a high heavy metal content is

unsuitable for use as an agricultural fertiliser. Wastewater in Cape Town has very low heavy

metal content, and in that respect is suitable for algal cultivation. Johannesburg has high

levels of heavy metals in their wastewater due to more industry and mining.

2. Energy from wastewater – Our Experiences: SAB Newlands Brewery (Peter Hofman,

SAB)

Peter Hofman gave a presentation on their anaerobic digester at the Newlands plant. Due to

new environmental legislation and the rising the costs of discharging the wastewaters to

municipality for treatment SAB decided to treat wastewater on site and plans to harness the

biogas energy.

• SAB has made a commitment to reduce its energy consumption by 15% by 2012 and

increase the energy consumption from renewables by 4% in the next few years.

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• 70% of SAB’s breweries have wastewaster treatment plants with AD biogas and 60%

of the biogas is used for heat (steam generation). SAB has calculated an 8%

reduction in energy usage by utilising t AD biogas.

• The AD at SAB is of the upflow type (UASB) and has a life span of about 5 years

(time before it needs to be emptied and cleaned of accumulating sludge), and the

residence time is about 1.5 days. The max. COD of the influent is about 8000 mg/L

which is reduced to 350 mg/L at the effluent. The flow rate is 3000 m3/day. Biogas

flow rate 3000-6000 m3/day, about 70% methane and 25% CO2. The biogas energy is

127.4 gJ/day or 5.3 gJ/hr output and can be used for combined heat and power. The

appropriate technology selected for utilising the biogas depends on various factors

which include economic viability, maintenance, quality of gas, noise, safety and

sustainability, supply fluctuations, reticulation, future expansion. These technologies

include:

� Hot water generation through water re-circulation

� Boiler de-aerating water heater

� Steam generation using methane

� Absorption chilling for cooling

� Waste heat recovery from turbines

� Hydrogen fuel cell technology – too early to consider

At SAB Newlands, using the biogas for steam generation was most favoured option

due to ease of adaptation, support and maintenance. This system is typically 83%

efficient and produces 4700 m3/day of steam and 27 MJ/m3. However, SAB is

currently contractually tied to steam generation by electrical boilers for the next few

years. When this contract terminates they hope to pursue this option. The recovery of

capital expenditure is estimated to be 3-7 years.

Q&A:

Is SAB under pressure to do secondary treatment of its wastewater? No, they

discharge wastewater at 350 mg/L to the municipal WWTP so they do not pay any

excess for treatment charges. However, recently council charges have increased by

125% on a volume basis!

The electricity price has increased 3 fold in the last 10 years. Will SAB revisit the

economic feasibility of alternate energy and AD when the electricity price increases?

Yes, we constantly re-evaluate the feasibility in relation to fuel, electricity and WWTP

tariffs for treatment. However, the main consideration is the availability of appropriate

technology.

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3. Energy from wastewater – Our Experiences: PetroSA (Johan van den Berg, WSP)

• Johan van den Berg discussed the financial barriers and implications of CDM at

PetroSA’s GTL plant in Mossel Bay.

• Discussed the Kyoto Protocol in brief. Developed countries need to decrease carbon

emissions by 5%. Developed countries need to determine the best practices for doing

this and then transfer this information and technology to developing countries.

• The CDM and costs for attaining certified emissions reductions (CER) are USD 40 k-

80 k excluding the EIA process. Therefore only large operations are considered viable

(require more than 15 000 tonnes of CO2 equivalent emissions reduction per annum).

• There is a growing carbon market or economy – value of carbon market estimated at

US $30 billion in 2006

• PetroSA has invested USD 4 million, and expects 33 000 CERs per annum

• Risks: Too expensive to hedge forex risk on CERs that are euro-denominated and

inflation differentials should see the rate improve in favour of the project

Other barriers include: Time, Licensing and regulations and Commodity price

fluctuations

Q&A:

What is the payback time? Payback on PetroSA’s CDM project is estimated to be 5-7

years.

Do the drivers of the CDM projects need to be from first world countries? No, in this

case WSP was a South African company and was bought over by a European

company

Can a company approach WSP? Can a company in a developing country approach a

company in a developed country to finance a project? Yes, it can be initiated from

both sides

4. Energy from wastewater in SA on industrial scale – An Estimate of Potential (William

Stafford and Brett Cohen, UCT)

Bret Cohen provided a summary of the energy potentials of wastewaters in South Africa and

this set the scene for the small group discussions which were held.

5. Small Group Discussions

The attendees were broken up into three groups and asked to reflect on the following nine

questions:

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1. What are the technology barriers to greater uptake of Energy from wastewater in SA?

2. What are the perception barriers to greater uptake of Energy from wastewater in SA?

3. What are the funding/financial barriers to greater uptake of Energy from wastewater in

SA?

4. What do you perceive as the institutional barriers to greater uptake of Energy from

wastewater in SA?

5. What additional benefits do you see in greater uptake?

6. What advantages would there be for Energy from wastewater to be done centrally or at

point of wastewater generation?

7. Should individual industries be driven to greater uptake, or is it just a “nice to have” and

not core business?

8. Given potentials which were presented here today, relative to national demand, should

Energy from wastewater be a key national focus area for energy recovery, or are there

other places where larger gains can be had?

9. What is required to remove the barriers

The responses to each of these questions are detailed below. One general observation which

was made repeatedly is that the vast majority of the discussions were focussed around AD for

biogas. This in itself demonstrated the need for greater education and the necessity for on the

ground demonstration of some of the other technologies that form the basis of this project.

1. What are the technology barriers to greater uptake of Energy from wastewater

in SA?

• Variability of influent wastewaters – many are cyclical

• Gas holding requirements

• Requirement for integration of several technologies – one is seldom sufficient

• Need to consider that quality of water after energy-directed treatment may not be as

high as if water quality was the main objective

• Need for sulfur removal, and phosphate accumulation

• Small companies not having enough resources to pursue Energy from wastewater –

need cooperation mechanisms

• Lack of skills for design, implementation and operation

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• Technology is too expensive, both in terms of capital outlay and the skills/expertise

required for maintenance. E.g. need to fly in technicians from abroad to service gas

turbines, etc.

• Scalability of new technologies (fuel cells, etc.) not proven

• Problems of service provision at energy generation level – relates to qualified

personal above.

• Already problems with maintenance of existing capacity – need for public/private

cooperation.

• Some of the above issue have resulted in a move away from technologies with the

potential to generate energy (e.g. AD) to those that don’t (in reference to sewage

treatment).

• Much of the equipment needs to be imported and there are long delays in acquisition.

Maintenance (skills and equipment) is lacking.

• Human resource expertise is lacking

• Design is not always suited to local context of a developing country like SA

2. What are the perception barriers to greater uptake of Energy from wastewater in

SA?

• No one wants to fund it

• Risk perceptions

• Perception that its very complex

• Problems working in the public sector

• Perceived as a wealth generation mechanism rather than a benefit for the community

• “What’s in it for me” attitude

• Lack of concern about GCC in SA at present

• Distaste for dealing with wastewater esp. sewage

• Requirement for proven technology which is successful locally, not just in developed

countries – e.g. not enough SA case studies of AD

• Newer technology requires significant capital spend

• Institutional lethargy

• Perception that novel technologies are too expensive (CapEx and OpEx)

• Centralized energy is best

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• SA has enough cheap electricity

• Government will ensure that there is enough power for all

• Cost of WWTP is reflected in rates only

• Focus of wastewater treatment is on effluent quality not energy generation.

• Despite the fact that high WWTP loads coincide with peak energy demand, there is

little focus on utilising locally generated energy.

3. What are the funding/financial barriers to greater uptake of Energy from

wastewater in SA?

• Wastewater is usually very low on the budget priority list – wastewater does not win

votes

• Huge capital expenditure required

• Govt tenders not awarded for more than 3 years, problem when payback time >3

years

• Projects need to show definite financial benefits within a reasonable timescale to

justify funding. Need to payback time to be short

• Not always financially dependent in industry

• Need for cooperation between companies

• Industry needs to be incentivised

• It will take pressure of regulation to force industry to comply – need regulation

• Ratepayers need to see benefit

• SA has more pressing problems that need fixing before Energy from wastewater

• Municipalities stretched financially to maintain existing infrastructure.

• Benefit to city

• Corporate farmers – important barrier

• Push when electricity price increases

• Length of EIA – fast tracking?

• Public private partnership

• CDM and carbon credits difficult, implementation model – CDM = bad model

• Carbon credit market not accessible to smaller scale operations.

• The budget for WWTP is often available but there is lack of capacity for

implementation and spending

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• The renewable energy from wastewater treatment competes with cheap coal-derived

power and there is no feed in tariff or rate for “peak power”

• Funding from DME or DSM/Eskom possible but process extremely slow

• No feed in tariff for renewable energy

• ?MIG funding possible

4. What do you perceive as the institutional barriers to greater uptake of Energy

from wastewater in SA?

• Lack of skills / qualified staff

• Inefficiency in government departments

• Too much red tape and need for streamlining the process of getting permissions

• Slow and difficult to obtain power generation license (NER)

• Length/cost of EIAs for smaller operations and well-established technologies

• Institutional lethargy -> finances

• Municipal management finance act problematic (contract between municipalities and

private enterprise must be limited to <3 years or projects must be put out to tender).

Longer contracts subject to public participation. 5-10 years is required to make

Energy from wastewater successful.

• Municipalities often need go to open tender on projects so good ideas often do not

progress. Therefore, universities and research institutions are better positioned to

make unsolicited approaches.

• No FIT or green energy tariff (3-5 years for WWTP)

• DWAF is “toothless” and does not enforce legislation for discharge (neither does

municipality). Legislation is “high” and compliance “low”

5. What additional benefits do you see in greater uptake?

• Increasing awareness of GCC

• Clean water as well as energy

• Opportunities for re-engineering equipment and processes and development of new

plants

• Opportunities for distributed energy generation

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• Demo projects will overcome perception barriers

• Finance benefit

• CDM credits

• Decentralised electricity production

• Decentralised energy generation leading to improved grid stability.

• Electricity security

• Energy security for individual sites.

• Sanitary/health issues

• Better fertiliser– at a local level

• Food security – at a local level

• Financial benefits from proven technologies.

• Overcoming public and institutional perception barriers.

• Reduction of pressure on primary resources, particularly in remote communities

(wood for cooking)

• Enhanced production of organic fertilisers (although given the current outcry relating

to sludge fertilisation in Philadelphia this may not be sustainable)

• Avoid wastewater discharge tariffs

• Improve quality of environment (rivers and water table) as well as gas emissions

(avoid coal power)

6. What advantages would there be for Energy from wastewater to be done

centrally or at point of wastewater generation?

• Smaller plants are good for cyclical production of wastewater

• Industrial park-type organisation would be useful

• Problem of inconsistent feed to sewage works if industries do not supply their waste

• Potential that wastewater becomes an asset with monetary value rather than a costly

liability

• Centralised systems will provide less visible benefits

• Localized – people get electricity – esp. for agriculture/industry

• Transportation of load from WWTP to another WWTP to be able to generate enough

electricity – > may work for municipality

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• More beneficial to build 1 big unit than many smaller units – but many smaller units

will work better for rural areas

• Onsite treatment reduces impact on municipal WWTPs.

• In some cases economy of scale may necessitate centralised energy generation

• De-centralization would reduce pumping/transport costs of disposal and increase

efficiency

• possibility of pre-planning and industrial ecology more possible (govt. limited by

legislative hurdles)

• Mixed waste streams a problem at municipal WWTP (varying and unknown

characteristics)

• On-site industrial treatment means that the govt. legislation is avoided (i.e..

secondary industry possible). Industry is also more nimble and efficient at

implementation for servicing and maintenance.

7. Should individual industries be driven to greater uptake, or is it just a “nice to have”

and not core business?

• Industries need to be incentivised

• Freeing up of COD capacity

• Visible brownie points

• Incentives for secondary treatment

• Penalize

• Policies: Too long, need to be incentives rather than penalise

• Needs to be a carrot and stick approach including both incentives for compliant

industries and significant penalties for non-compliance. New DWAF costing system

for penalties may partially address this

• Industry should be driven to greater uptake. Partly through enforcement of legislation

(DWAF) Need to comply with legislation for discharge (Water Act) as well as protect

the environment at large: “the polluter pays” (NEMA).

• The local legislation is inconsistent since each municipality has by-laws for discharge

tariffs – some industries will move to an area with more favourable by-laws

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8. Given the potentials which were presented here today, relative to national demand,

should Energy from wastewater be a key national focus area for energy recovery, or

are there other places where larger gains can be had?

• Still need clean water as a priority

• Should be major focus but info needs to be disseminated

• Will we refer to Energy water instead of wastewater in the future?

• If energy is the driver (and not wastewater treatment), the technologies and

processes will be different

• Rural areas – sludge easy disposal

• Need to look at co-benefits

• There are easier ways to generate energy (solar, wind)?

• The wastewater treatment plant needs to be adapted for energy generation (focus

always on water quality)

• Energy from wastewater will not supply all of SA energy needs but could fill the

current “missing demand” and make waste treatment processes sustainable

9. What is required to remove the barriers?

• International investment

• Large funding agencies (DST, DME, etc.) need to make seed funding available for

these projects.

• Incentives

• Right technologies available and awareness of them

• New national mindset

• Effective legislation

• Need for different departments to come together for the common goal – is this

feasible?

• Demonstration plants are needed

• Operation of successful demonstration plants to overcome negative perceptions.

• Eliminate replication of research and development efforts

• Major enhancement of skills is required at all levels – more engineers and qualified

plant operators