M Tech. Seminar Report

On

Energy Recovery from Municipal Solid Waste by Anaerobic

Digestion

Submitted by

Pankaj Kumar Garg

Roll Number - 10318011

Under the guidance of

Professor Anurag Garg

Centre for Environmental Science and Engineering

Indian Institute of Technology Bombay

Mumbai-400 076

4 November 2010

ii

Approval Sheet

This seminar report entitled “Energy Recovery from Municipal Solid Waste by Anaerobic

Digestion” prepared by Pankaj Kumar Garg (10318011) is hereby approved for submission

at Centre for Environmental Science and Engineering (CESE), IIT Bombay

November 4, 2010 Professor Anurag Garg

CESE, IIT Bombay (Guide)

iii

Abstract

Municipal solid waste quantities are increasing globally due to rapid urbanizat ion in

concurrence with industrialization and changing life style. For instance, in India the per

capita waste generation rate is increasing at a rate of 1-1.33% annually. In India, Municipal

solid waste management major disposal method is landfilling. MSW composition in India

contains a large fraction of organic content and high moisture content. These values seem to

be more suitable for biological processes. Anaerobic digestion (AD) is one of the potential

method for treating MSW having such characteristics. The process takes place in the absence

of oxygen and after the reaction biogas and stabilized material (digestate) is produced. The

generated biogas can be used for energy recovery purposes.

The report focuses on the fundamentals of AD technology and utilization routes for the

biogas for energy recovery purpose. Three case studies from other parts of the world a re also

being presented. Potential and current status of AD in India is also mentioned in the report.

Major limitations of the process are also given.

iv

Table of Contents

Approval Sheet .........................................................................................................................ii

Abstract....................................................................................................................................iii

Table of Contents .................................................................................................................... iv

List of Tables ........................................................................................................................... vi

List of figures ..........................................................................................................................vii

Chapter 1

Introduction .............................................................................................................................. 1

1.1. Background ................................................................................................................. 1

1.2. Impacts on the environment of improper MSW disposal ........................................... 1

1.3. Municipal solid waste in India and its potential.......................................................... 2

1.4. Objective of the report................................................................................................. 4

1.5. Organisation of the report ........................................................................................... 4

Chapter 2

Anaerobic Digestion ................................................................................................................. 5

2.1. Introduction ................................................................................................................. 5

2.2. Process in anaerobic digestion .................................................................................... 6

2.3. Anaerobic digestion system ........................................................................................ 7

2.4. Pre and post treatment of MSW ................................................................................ 11

2.5. Some recent developments in AD technology .......................................................... 11

2.6. Energy Recovery options from biogas ...................................................................... 13

2.7. Status of anaerobic digestion in India ....................................................................... 15

2.8. Opportunities and Challenges ................................................................................... 20

Chapter 3

Case Studies ............................................................................................................................ 22

3.1. Rayong waste to energy plant, Thailand ................................................................... 22

v

3.2. Valorga plant at Tilburg ............................................................................................ 28

3.3. BTA plant at Newmarket, Ontario (Canada) ............................................................ 30

Summary................................................................................................................................. 33

References ............................................................................................................................... 34

vi

List of Tables

Table 2.1 Typical Composition of Biogas ............................................................................... 13

Table 2.2 Existing BARC biogas plants in India ..................................................................... 20

Table 3.1 Technical design and installed capacity................................................................... 23

Table 3.2 Quality of Humus..................................................................................................... 26

Table 3.3 Water quality............................................................................................................ 26

Table 3.4 Effect on greenhouse gas reduction after installation of plant ................................. 27

vii

List of figures

Fig. 1.1 General approach for the treatment of heterogeneous MSW ....................................... 3

Fig. 2.1 Process flow of degradation of organic material through anaerobic digestion ............ 5

Fig. 2.2 Material and energy flow............................................................................................ 14

Fig. 2.3 Schematic Diagram of TEAM digester with six acidification reactors and a UASB

unit ........................................................................................................................................... 16

Fig. 2.4 Schematic diagram of SSB process ............................................................................ 18

Fig. 2.5 Schematic description of the small ARTI compact biogas plant................................ 18

Fig. 2.6 Schematic description of the BARC biogas plant ...................................................... 19

Fig. 3.1 Various treatment process employed in Rayong waste to energy plant ..................... 25

Fig. 3.2 Tilburg Plant Mass Balance........................................................................................ 29

1

Chapter 1

Introduction

1.1. Background

Rapid urbanization in concurrence with industrialization and changing lifestyle has led to

increase in municipal solid waste (MSW) at an alarming rate and it has become a global

concern. MSW is any discarded material generated from houses, commercial places,

institutions etc. located in a city or town. It is a heterogeneous mixture of materials (Braber,

1995). This waste contains:

1) Digestible fraction: Biogenic organic matter which is readily degradable i.e. kitchen

wastes, grass cuttings

2) Combustible fraction: Slowly digestible and indigestible organic matter i.e. wood, paper,

cardboard, plastics and other synthetics

3) Inert fraction: Stones, sand, glass, metals, bones and other inorganic material

According to a recent report the total amount of MSW generated globally in 2006 was

estimated to be 2.02 billion tonnes representing a 7% annual increase since 2003 (UNEP,

2009). It will further increase to 37.3% from 2007 to 2011 showing 8% annual increase. It is

also common fact that 30-60% of all the urban solid waste in developing countries remains

uncollected and less than 50% of the population is served (World Bank, 2007). As much as

80% of the collection and transport equipment are either out of service or under repair. In

most of the developing countries, open dumping and burning is a common MSW disposal

method. Developing countries face uphill challenges for proper management of their waste

with most efforts being made to reduce the final volumes and to generate sufficient funds for

waste management (UNEP, 2009). Population explosion and other activities resulted into

increase in land use and use of valuable space for disposing the waste make the problem more

pronounced. There is urgent need for a feasible technology to divert MSW quantities from

landfill sites.

1.2. Impacts on the environment of improper MSW disposal

Significant amount of environmental damage is caused due to unsafe disposal of MSW. The

major pollution problems are related to air, water and soil contamination.

2

Toxic contaminants are released in the air due to incineration of MSW. These emissions

include carcinogenic, endocrine disrupting organic chemicals and heavy metals such as

arsenic, lead, cadmium, mercury and chromium. Approximately 22% of the airborne dioxins

that enter the Great Lakes come from municipal waste incinerators (Jackson, 1999).

As the organic wastes in solid waste landfills decompose, methane (CH4) gas starts

generating. CH4 is a potent greenhouse gas, contributing to climate change. On a per kilo

tonne basis, CH4 is approximately twenty-one times more potent than carbon dioxide as a

greenhouse gas. Almost three quarters of this methane is released into the air, despite the

presence of methane capturing systems at landfills (Jackson, 1999).

Leachate produced due to percolation of rainwater and liquids already in the waste can be

released to either ground or surface waters. Toxic air contaminants released by incinerators

and landfills may eventually fall to the ground with rain drops make the surface water

polluted (Jackson, 1999). An environmentally acceptable waste management practice is

essential if damaging consequences are to be avoided.

1.3. Municipal solid waste in India and its potential

India is a heavily populated country in the world after china with population 1.15 billion (July

2010 est.) and total area of 3.28 million sq km (URL1). It is reported that per capita waste

generation ranges between 0.2 kg and 0.6 kg per day in the Indian cities (Asnani, 2006). The

per capita waste generation rates in India are lower than the low-income countries in other

parts of the world and much lower compared to developed countries. However, around 90

million tonne of waste is generated in 2008 (Sharholy et al., 2008).

The per capita generation rate is estimated to increase at a rate of 1–1.33% annually. The

MSW amount is expected to increase significantly as the country strives to attain an

industrialized nation status till 2020 (Sharholy et al., 2008). The Energy Resources Institute

(TERI) has estimated that waste generation will exceed 260 million tones per year by the year

2047—more than five times waste generated in 1995 (Zia et al., 2002). At present, more than

90% waste is disposed in landfills/open dumping grounds (Sharholy et al., 2008).

In India, the composition and hazardous nature significantly differs from MSW in the western

countries. The composition of MSW contains large organic fraction (40–60%), ash and fine

earth (30–40%) and small fraction of combustible fraction i.e. paper (3–6%) and plastic

(Sharholy et al., 2008). It is reported that the C/N ratio ranges between 20 and 30, and the

3

calorific value ranges between 800 and 1000 kcal/kg (Rao et al., 2010 and Sharholy et al.,

2008). Energy consumption in the developed countries has been more or less stabilized

whereas in developing countries like India it is increasing at a high rate. There are various

technological options available for processing MSW like incineration, gasification, pyrolysis,

composting and anaerobic digestion. These options can also be used in combination to

maximize recycling and resource recovery. A typical approach for treating high moisturized

biodegradable fraction in MSW is shown in Fig. 1.1.

Fig. 1.1 General approach for the treatment of heterogeneous MSW (Visvanathan et al.,

2005)

Recyclables

Municipal solid waste

Manual/ partial mechanized segregation

High moisturized biodegradable fraction

Anaerobic digestion Composting

Inert/ other remains

Energy Recovery

Refused

derived fuels

Compost for Agriculture

and/ Landfill cover soil

Landfill

Landfill gas recovery

Scavengers/waste

pickers

4

In India, Central pollution control board (CPCB) has given Municipal solid wastes

(Management & Handling) Rules, 2000 which is applicable to every municipality responsible

for collection, segregation, storage, transportation, processing and disposal of MSW (CPCB,

2000). The rule consists of four schedules. Schedule I give implementation schedules and

deadlines for submission of proposal to set up waste processing and disposing facilities or

monitoring its performance. Schedule II is related to the management of municipal solid

waste. It gives compliance criteria from collection to disposal of MSW. Schedule III gives

specification for landfill sites as it give guidelines for site selection, facilities at site, and

specifications for land filling, desirable limits for water quality and acceptable level for air

quality. Schedule IV mentions standards for composting, treated leachates and incineration. It

sets standards for disposal of treated leachate, specification for compost quality and emission

standards for incineration.

In the view of composition in India, the government is looking forward to biomethanation

technology (BT) as a secondary source of energy by utilizing industrial, agricultural and

municipal wastes. In India, energy demand from various sectors has risen significantly and

the energy supply with other sources(including renewable and non-renewable energy source)

is not in the pace with the demand which resulted in a deficit of 11,436 MW which is

equivalent to 12.6% of peak demand in 2006 (Rao et al., 2010). The total installed capacity of

bio-energy generation from solid biomass and waste to energy was about 1227 MW till 2007

against a potential of 25,700 MW. The conversion of biomass into energy also results in

significant reduction in volume. The digested sludge can be used as fertilizer for the

agricultural field.

1.4. Objective of the report

The objective of the report is to compile recent information on the options for energy

recovery from the anaerobic digestion of MSW.

1.5. Organisation of the report

This report consists of four chapters. In the first chapter MSW generation and its impact are

mentioned. The second chapter provides information on the various processes involved in

AD and energy recovery potential of biogas (one of the outputs). In third chapter case studies

from different parts of the world are presented. The information present in previous chapters

summarized in chapter 4. Finally reference list is given in which all the references used to

make this report are listed.

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Chapter 2

Anaerobic Digestion

2.1. Introduction

Anaerobic digestion also referred as biomethanation is a multi stage biological process in

which the organic matter decomposes by the action of microorganisms in the oxygen free

environment.

In anaerobic digestion (AD), the mechanism of degradation of organic matter occurs due to

bacterial metabolism. It is a fermentative process in which the organic matter gets

transformed into biogas. The major reactions of the anaerobic digestion are shown in the

Fig. 2.1. Unlike aerobic and anoxic digestion there is no electron transfer in the anaerobic

digestion (no oxidation and reduction). Substrate phosphorylation occurs in the process and

organic matter itself acts as an electron donor and acceptor.

Fig. 2.1 Process flow of degradation of organic material through anaerobic digestion (Li et

al., 2010)

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2.2. Process in anaerobic digestion

Anaerobic digestion of organic fraction involves combined action of four types of anaerobic

microorganisms: hydrolytic, fermentative, acetogenic and methanogenic bacteria

(Veeken and Hamelers, 1999).Organic matter from MSW consist mainly protein,

carbohydrate and lipids. These complex molecules are very large for a cell to ingest and to

utilize for metabolism. Hence these macromolecules are broken into smaller fractions after

adding a water molecule by hydrolytic bacteria. In hydrolytic phase, hydrolytic bacteria

reduce complex particulate compounds to soluble monomeric or dimeric. Conversion

efficiency of the feedstock is determined by hydrolysis which serves as the rate limiting step.

Cellulose found in many municipal wastes, is an example of insoluble compounds that

undergoes enzymatic hydrolysis. Cellulolytic bacteria such as Cellulomonas, Clostridium,

Bacillus, Thermomonospora, Ruminococ-cus, Baceriodes, Erwinia, Acetovibrio,

Microbispora, and Streptomyces can produce cellulases that hydrolyze cellulolytic biomass

(Li et al., 2010).

Fermentative bacteria use the dissolved organic matter formed earlier and converts this into

volatile fatty acids, alcohols, lactic acid and mineral compounds. Most of the fermentative

bacteria are obligate anaerobes though a few are facultative too. The facultative bacteria plays

a very important role by consuming the oxygen present and making the environment

anaerobic.

The fatty acids formed by fermentative bacteria are degraded to acetate and propionate in

anaerobic β oxidation process. It is a cyclic process which releases one acetate unit per cycle

and hydrogen. The end products can be acetate (even number of carbon) or acetate and

propionate (odd number of carbon). In this process, proton is mainly electron acceptor and H2

is released. Since the reaction is thermodynamically endergonic so the partial pressure of H2

should be kept low in the system by making contact between acetogenic and methanogenic

bacteria for the reaction to move in forward direction.

H2 +CO2 CH4

If methanogenic bacteria are not active, then anaerobic β oxidation will not take place and

long chain fatty acids will go on accumulating and thereby pH of the system will reduces.

Methanogenic bacteria are active near neutral pH. In such case, the methanogenic bacteria

will not get conducive environment and hence the system can fail.

7

The product of acetogenesis is converted into final product for methane generation. There are

many bacteria which can convert CO2 and H2 to methane but only a limited number of

microorganisms can convert acetate to methane. 70% of the total methane is formed by

acetate and only 30% is formed by CO2 and H2.

The final step in AD is of two types: hydrogenotrophic methanogenesis and acetotrophic

methanogenesis. In acetotrophic methanogenesis, acetate breaks down into CH4 and

CO2whereas in hydrogenotrophic methanogenesis, H2 and CO2 combines to form methane

and water molecule.

CH3COOH CH4 + CO2

4H2+CO2 CH4 + 2H2O

2.3. Anaerobic digestion system

Different designs and configuration have been developed throughout the world to suit

different solid concentrations and microbial activity. The system used to treat MSW

anaerobically can be classified into following categories:

1) Continuos digestion system

a) Single stage digestion system

b) Multi stage digestion system

2) Batch digestion system

a) Single stage batch digestion system

b) Hybrid batch-UASB digestion system

c) Sequential batch digestion system

These categories can be classified further, based on the total solids (TS) content of the slurry

in the digester. The single stage and the multi stage system can be further categorized as

single stage low solids (SSLS), single stage high solids (SSHS), multi stage low solids

(MSLS) and multi stage high solids (MSHS).

2.3.1. Single stage digestion system

Single stage process have one reactor for both acidogenic and methanogenic phase. These can

be low solids (LS) or high solid (HS) depending on total solids (TS) content in a reactor. This

process can further be classified into two process viz. SSLS and SSHS.

8

Single stage low solids (SSLS) digestion system

This process has been used worldwide for several decades because of its simplicity. The

concentration of solids is kept below 10% (MOUD, 2000) in this process. Continuously

stirred tank reactor (CSTR) is predominantly used to ensure that digestate is continuously and

completely mixed.

Generally a retention time of 14-28 days is provided depending on feedstock and operating

temperature. Some examples of this kind of process are: the Wassa process in Finland, the

EcoTec in Germany, and the SOLCON process at the Disney Resort Complex, Florida.

Major technical problems in this process which results in lower biogas yield are removal of

15-20% volatile solid (VS) in the pre-treatment, periodic removal of the floating scum from

the top layer and heavier fraction from the bottom layer and short-circuiting in the reactor.

Formation of heavy fraction layer at the bottom and floating scum at the top brings non-

homogeneity in the reacting mass and can also damage propellers.

For the solids content to be maintained below 15%, large volume of water is added. This

results in the requirement of large reactor volumes and higher investment costs. Large

amount of energy is also needed. Apart from this, more energy and equipment are required

for de-watering the effluent stream.

Single stage high solids (SSHS) digestion system

This process came into picture after several research studies (Verma, 2002) and it was

established that undiluted waste could give higher biogas yield. TS concentration in this

process is kept between 16-40%. Contrary to the complete mixing prevailing in SSLS, the

SSHS are plug-flow reactors hence require no mechanical device within the reactor. Some of

the examples of SSHS are the DRANCO, Kompogas, and Valorga processes. All three

processes consist of a single stage thermophilic reactor (mesophilic in some Valorga plants)

with retention time of 14-20 days.

In the DRANCO reactor, the feed is introduced from the top and digested matter is extracted

from the bottom. There is no mixing apart from that occurring due to downward plug-flow of

the waste. A part of the extracted matter is reintroduced with the new feed while the rest is

de-watered to produce the compost like product.

The Kompogas process works on similar concept, except the movement takes place in plug

flow mode in a horizontally disposed cylindrical reactor. Mixing is accomplished by the use

9

of an agitator. The process maintains the solids concentration of about 23 % TS. At TS

content lower than 23%, the heavy fraction such as sand and glass can sink and accumulate at

the bottom. Higher TS concentrations may impede the flow of materials.

In the Valorga process, the reactor is a vertical cylindrical reactor divided by a partial vertical

wall in the centre. The feed enters through an inlet near the bottom of the reactor and slowly

moves around the vertical plate until it is discharged through an outlet that is located

diametrically opposite to the inlet. Re-circulated biogas is injected through a network of

injectors at the bottom of the reactor and the rising bubble results in pneumatic mixing of the

slurry. The injectors require regular maintenance, since these are prone to clogging.

The high solids content in these systems requires different handling, mixing and pre-

treatment than those used in the low solids processes. The equipment needed to handle and

transport high solid slurries are more robust and expensive than that required for low solids

system. The high solids systems can handle impurities such as stones, glass or wood that

needs not to be removed as in SSLS.

SSHS processes exhibit higher organic loading rate (OLR), as compared to SSLS. The biogas

yield is usually high in SSHS as heavy fractions or the scum layer is not removed during the

digestion. HS process consumes one tenth of the fresh water used in LS process on per ton

MSW basis (Verma, 2002). Consequently, the volume of wastewater to be discharged is

several- fold less for HS reactors.

2.3.2. Multi-stage digestion system

The multistage AD process was introduced to improve the digestate by separating

methanogenesis stage from hydrolysis, acidogenesis and acetogeneis stages so that each

reaction could be operated close to optimum. Typically two reactors are used, the first for

hydrolysis/liquefaction-acetogenesis and the second for methanogenesis. This process can

further be classified as multi-stage low solids (MMLS) and multi-stage high-solids (MMHS).

Multi-Stage Low Solids (MLSS) digestion system

Some of the MSLS facilities are the Pacques process (Netherlands), the BTA process

(Germany, Canada) and the Biocomp process (Germany). Pacques process uses two reactors

in mesophilic temperature range. The source separated MSW is fed in the first reactor where

hydrolysis occurs. Solids concentration in this reactor is kept around 10% and mixing is done

10

by gas injection. The digestate from the first reactor is de-watered, and the liquid is fed to an

upflow anaerobic sludge blanket (UASB) reactor in which methanogenesis occurs. The

fraction of the digestate from the hydrolysis reactor is re-circulated with the incoming feed to

the first reactor for inoculation. The remaining fraction is used for digestate production. In the

BTA process, the solid content is also maintained at 10% and the reactors are also operated at

mesophilic temperatures. It is very similar to the Pacques process except that the

methanogenic reactor is designed with attached growth (―fixed film reaction‖) to ensure

biomass retention. The effluent from the hydrolysis reactor is de-watered and the liquor is fed

to methanogenic reactor. This reactor receives only the liquid fraction from hydrolysis reactor

to avoid clogging of the attached growth media. Sometimes, the effluent from the

methanogenic reactor is pumped to the hydrolysis reactor to maintain the pH in the range of

6-7. MSLS process faces similar technical problem as stated for SSLS reactors, such as short-

circuiting, foaming, formation of layers of different densities, expensive pre-treatment. In

addition, the MSLS processes are technically more complex and thus require a higher capital

investment.

Multi -stage high solids (MSHS) digestion system

This process is somewhat similar to MSLS process as it consists of a liquefaction/hydrolysis

reactor followed by a methanogenic UASB reactor with attached growth. However hydrolysis

is carried out under high solids and microaerophilic conditions (where limited amount of

oxygen is supplied in anaerobic zone). The aeration in the first stage and the attached growth

reaction in the second stage are provided for complete digestion at retention time of only

seven days.

2.3.3. Batch Reactor

Batch reactors are loaded with feedstock, subjected to reaction, with no in or out flow during

the reaction. The digested material is then discharged and loaded with a new batch. There are

three types of batch systems: single stage batch system, sequential batch system and a hybrid

batch UASB reactor.

The single-stage batch system involves re-circulation of leachate from the top of the same

reactor. The sequential batch process comprises two or more reactors. The leachate from the

first reactor (containing a high level of organic acids) is re-circulated to the second reactor in

which methanogenesis occurs. The leachate from the methanogenic reactor, containing little

or no acid, is combined with pH buffering agents and re-circulated to the first reactor. This

11

guarantees inoculation between the two reactors. The third type of batch process is the hybrid

batch-UASB process, which is very similar to the multi-stage process with two reactors. The

first reactor is simple batch reactor and the second methanogenic reactor is an UASB reactor.

Batch processes offer the advantages of being technically simple, inexpensive and robust.

2.4. Pre and post treatment of MSW

Some kind of treatment methods may be required before and after AD process depending

upon the nature of waste. The methods are explained briefly below:

Pre-treatment

Before anaerobic digestion of organic fraction, it is necessary to pre-treat the waste order to

improve digestibility and to ensure end products quality. In this step, inert and non-

biodegradable materials can be removed. During pre-treatment feedstock can be upgraded for

maintaining C/N ratio and homogenization of waste for digestion. As a general rule, handling

in biogas plant is far easier for source separated MSW. Pre-treatment commonly involves two

steps: sorting and particle size reduction.

Post-Treatment

After anaerobic digestion process, the digested material may require further processing to

produce a refined product with optimum moisture content, particle size and physical structure

(MOUD, 2000). The wet MSW can be spread directly in the farmlands as slurry or it can be

separated in solid or liquid fractions. The solid fraction can be matured to produce compost

like material for about 2-4 weeks and the liquid fraction can be spread on farmland or treated

in wastewater plants. In case of dry MSW digestion process, the digested material is usually

dewatered and matured to compost. The liquid effluent can be used for mixing and

inoculation of the incoming MSW, but there will usually be effluent in excess that has to be

spread on farmland or purified in a wastewater plant to achieve the specified standard for

final disposal.

2.5. Some recent developments in AD technology

González et al. (2010) analysed the codigestion of source collected organic fraction of

municipal solid waste (SC-OFMSW) and fat, oil grease waste from sewage treatment plant

(STP-FOGW). It was reported that codigestion of above waste may enhance valorisation of

STP-FOGW and led to higher biogas yield throughout the anaerobic digestion process.

Mesophilic (37°C) codigestion of STP-FOGW with SC-OFMSW having 15% volatile solids

(VS) was carried out in a 5 L lab scale reactor which showed improvement both in terms of

12

biogas production (72% higher) and methane yield (46%) as compared to anaerobic treatment

of SC-OFMSW. VS and total solids (TS) reduction percentage were stable and was around

65% and 57%, respectively, methane content in biogas was found to be 63%. Anaerobic co

digestion of STP-FOGW and SC-OFMSW is an environmental friendly approach to manage

STP-FOGW and for getting better biogas yield.

Forster-Carneiro et al. (2008) analysed the influence of different type of organic fraction on

AD process under thermophilic conditions (55°C). Food waste (FW), organic fraction of

municipal solid waste (OFMSW) and shredded OFMSW (SH-OFMSW) were used as organic

fractions. All digesters were operated in dry conditions (20% total solids content) and were

inoculated with 30% (in volume) of mesophilic digested sludge. Experimental results showed

difference in the biodegradability and biogas production. After 90 days biodegradation of FW

showed smallest waste biodegradation with VS removal, 32.4% and methane yield of 0.18 L

CH4/g VS. But SH-OFMSW showed higher waste biodegradation with VS removal 73.7%

and lesser methane yield of 0.05 L CH4/g VS. Finally, OFMSW showed the highest VS

removal of 79.5% with a methane yield of 0.08 L CH4/g VS. Hence, the nature of organic

substrate is an important factor affecting the biodegradation process and methane yield.

Erden and Filibeli (2010) assessed the effect of Fenton process on anaerobic sludge

bioprocessing. During experimentation the ratio of 0.067 g Fe (II) per gram H2O2, and 60 g

H2O2/kg dried solids (DS) were applied to biological sludge before anaerobic sludge

digestion. In this study, single stage anaerobic digestion under thermophilic conditions was

compared with two-stage anaerobic digestion (mesophilic digestion prior to thermophilic

digestion). Results showed that the most effective sludge solubilization was observed in the

reactor operated at thermophilic conditions which was fed with fenton processed sludge.

Fenton pre-treatment increased the methane production rates for both single stage

thermophilic digestion and two-stage digestion. Two-stage digestion provides more methane

production compared to the single stage thermophilic digestion. In two stage process total

methane productions with fenton processed sludge was 1.3 times higher than total methane

production in control reactors at the 30th day of operation.

Park et al. (2005) has compared the effects of thermochemical and biological hydrolysis used

as pre-treatment processes for increasing the efficiency of anaerobic digestion of waste

activated sludge. Two different three stage digestion systems showed improved performance,

although thermochemical hydrolysis showed better results than biological hydrolysis in a

13

bench scale operation. After anaerobic digestion with thermochemical pre-treatment, the total

chemical oxygen demand (tCOD) reduced by 88.9%, 77.5% of volatile solids were removed,

with biogas yield was 0.52 m3/kg having methane content of 79.5%.

2.6. Energy Recovery options from biogas

Biogas generated by digesting organic fraction of MSW (OFMSW) can be utilized for

combined heat and power (CHP) production or for transport fuel production as CH4 enriched

biogas (Murphy, 2004). Typical composition of biogas is shown in Table 2.1. The potential

biochemical methane production yield from MSW and water can be as 0.2 m3 / kg of VS

added (Elango et al., 2007). The energy content of typical biogas is between 6.0-6.5 KW/m3

which is equivalent to 0.6 to 0.65 L oil/m3 biogas. The ignition temperature is 650-750 °C

with critical pressure of 75-89 bars. Normal density is 1.2 kg/ m3 (Rao et al., 2010). Fig. 2.2

shows various outputs from a biogas generation plant and options for their utilization

Table 2.1 Typical Composition of Biogas (Igoni et al., 2008)

Constituent Composition

Methane(CH4) 55-75%

Carbon dioxide(CO2) 30-45%

Hydrogen Sulphide(H2S) 1-2%

Nitrogen(N2) 0-1%

Hydrogen(H2) 0-1%

Carbon monoxide(CO) Traces

Oxygen (O2) Traces

There are several ways by which biogas can generate electricity and heat

Combined heat and power generation

Cogeneration is the simultaneous production of two or more forms of energy from a single

fuel source. Combined heat and power (CHP) systems use both the power producing ability

of a fuel and the inevitable waste heat. Some CHP systems aim to produce heat at first place

14

14

Fig

. 2.2

Mat

eri

al an

d e

ner

gy f

low

(P

ösc

hl

et a

l., 2010)

15

rather than electrical power (bottoming cycle). Other CHP systems produce primarily

electrical power and the waste heat is only used to heat process water (topping cycle). A

biogas engine will generate electricity and heat in which the electricity can be fed into the

electricity grid, while the heat may be consumed via district heating networks with typical

transmission losses. Part of the generated heat can be used in the AD process control, and for

sterilization of feedstock.

Fuel Cell Technology

A fuel cell is an electrochemical cell that generates electricity by chemical reaction. Heat and

electricity is produced when hydrogen (supplied to fuel cell) reacts with oxygen (from air). A

fuel reformer attached to the fuel cell can create hydrogen from biogas by an electrochemical

process. Utilization of biogas for high temperature fuel cell can enhance electricity generation

with reduction in green house gases and hazardous. This technology can achieve electricity

efficiency of 50-60% at high temperature of 450 °C (Trogish et al., 2005).

Apart from this, stirling engine, organic rankine cycle (ORC) and microturbine are other

methods that can be used for producing electricity and heat from biogas. Energy demand for

heating the digesters typically ranges between 20% and 25% of total heat component from

CHP generation. Hence some amount of total heat generated can be used for plant processes

and rest can be used for external use.

2.7. Status of anaerobic digestion in India

Anaerobic digestion of organic fraction of municipal solid waste (OFMSW) is commonly and

successfully used in industrial countries. In some developing countries, such as China, India

and Nepal this process is exclusively used in rural areas mainly for the treatment o f animal

feces (Müller, 2007).

Rao et al. (2010) reported that, in India, the compostable matter in the MSW is ranging from

30.84% in very big cities (population > 5 million) to 56.57% in big cities (population 2.0 to

5.0 million). On average, the compostable matter percentage is 42.2% which is considered as

a good amount for anaerobic digestion. C/N ratio varies from 21.13 to 30.94 whereas the

calorific values are varying from 800.70 to 1009.89 Kcal/kg. Studies had been carried on

anaerobic digestion of MSW and it was found that biogas could be generated at a rate of

95m3/t of solid waste. The biogas potential from MSW is estimated as 9.23 Mm3 /day in India.

16

In the recent decade, as a result of Ministry of Non-Conventional Energy Sources (MNES)

programs, there has been much interest in generating power through biomethanation of

MSW. Various institutions and NGO‘s have been involved in the development of

technologies. The development was more focused on low-tech digesters applicable for local

conditions in India (Müller, 2007).There are various low tech anaerobic digesters developed

and implemented in India on pilot scale level. Some of these are mentioned below:

TERI enhanced acidification and methanation (TEAM) digester (Müller, 2007)

The Energy and Resource institute (TERI), New Delhi has developed a high-rate digester for

biomethanation of fibrous and semi-solid organic wastes. TERI enhanced acidification and

methanation (TEAM) process is primarily a two-phase process. The first phase, regarded as

the acidification phase, consists of extracting leachate (COD ≈ 15,000-20,000 mg/L) from the

solid waste present in acidification reactor. In the second phase, known as the methanation

phase, biogas is generated by treating the leachate in an upflow anaerobic sludge blanket

(UASB) reactor. The phase separation provides suitable environment to the microorganisms

in acidification and methanation stages, thus the efficiency of digestion is enhanced. The

schematic diagram of TEAM digester is shown in Fig. 2.3.

Before the process gets started, the organic solid waste is shredded into small pieces and fed

into acidification reactor for 6 days. The waste bed is kept submerged in water.

Fig. 2.3 Schematic Diagram of TEAM digester with six acidification reactors and a UASB

unit (Müller, 2007)

17

Organic acids formed as a result of biodegradation leads to formation of leachate. This

leachate is periodically recirculated through the bed at a predetermined fixed rate to maintain

uniform concentration of microorganisms and nutrients in the bed and to wash off organic

acids formed as a result of further bed degradation. Once COD of the leachate is increased, it

is collected in the leachate collection tank.

Biogas is formed in the methanation phase and is collected. The residue inside the

acidification reactor is dried under the sun and used as manure.

TEAM biogas plant has been installed in TERI campus and at other corporate units like

NTPC India (for household waste management) and Sona Koyo Steering Ltd., Haryana (for

canteen waste management).

Solid-state stratified bed (SSB) reactor (Müller, 2007)

The Centre for Sustainable Technologies has developed a digestion reactor based on solid

stratified bed (SSB) process. In this reactor, an attempt is made to obviate the need for pre-

processing of MSW biomass components for small scale anaerobic digestion process. The

SSB process is divided into two steps based on the key principle that digested biomass itself

can be used as an immobilized bio-film reactor.

Acidogenesis and methanogenesis zones are formed in one container instead of two separate

containers. A small quantity of liquid is recycled daily (manually) to keep the freshly fed

upper layers in acidogenetic stages and transferring VFA produced to methanogens colonized

old biomass in the lower layers.

A bioreactor based on this process is installed and commissioned in February 2005 at

Transport House, Karnataka state road transport corporation (KSRTC) for managing canteen

waste. The plant is designed to handle to 25 kg of canteen waste along with the leaf litter. A

floating dome gasholder is provided at the top for the collection of the biogas.

The feedstock is fed into the plant without crushing or any other pre-processing. Methane

producing bacteria (methanogens) are trapped on partially digested leafy biomass. About 1.5

cubic meters of biogas is produced every day. On the basis of dry matter fed to the

bioreactors, every kilo fed produces between 50 and 80 litres of biogas. At present, the gas is

being used to keep the cooked food warm. Fig. 2.4 shows the schematic diagram of SSB

reactor.

18

Fig. 2.4 Schematic diagram of SSB process (Müller, 2007)

Compact Biogas plant (Müller, 2007)

Appropriate Rural Technology Institute (ARTI), Maharashtra has developed a compact

biogas plant to be used by urban households. Currently about 2500 plants are in use – both in

urban and rural areas in Maharashtra. The standard plant uses two tanks made from high-

density polythene, with volumes of typically 0.75 m3 and 1 m3. The smaller tank is the gas

holder and is inverted over the larger one which holds the decomposing biomass. An inlet is

provided for adding feedstock, and an overflow for removing the digested residue.

Fig. 2.5 Schematic description of the small ARTI compact biogas plant (Müller, 2007)

19

The digestate is mixed with the feedstock and recycled into the plant. A pipe takes the biogas

to the kitchen, where it is used with a biogas stove. The gas holder gradually rises as gas is

produced and sinks down again as the gas is used for cooking. Gas pressure can be increased

by increasing weight on gas holder.

Bhabha atomic research centre (BARC) biogas plant (Müller, 2007)

BARC has developed a biogas plant which is based on a floating dome design. It has two

stage continuous wet systems. The waste is hydrolyzed in the first stage and the biogas is

produced in second stage. The reactor is constructed underground to reduce building cost and

reactor contents flow under gravity by volume displacement.

The biogas plant runs on kitchen waste that is pre-treated (mixed with hot water, heated by

solar heating and pulped) and discontinuously fed to the thermophilic aerobic pre-digester.

The effluent from the pre-digester is fed to the methane reactor where biogas is produced.

The effluent from the methane reactor is collected in open pits that are provided with sand

filters. Once the pit is filled up, the manure is taken out and spread under shade for drying.

The excess water filters out in an underground tank and is reused in the syste m. The

schematic diagram of the plant is shown in Fig. 2.6.Some of the existing biogas plant is given

in Table 2.2

Fig. 2.6 Schematic description of the BARC biogas plant (Müller, 2007)

20

Table 2.2 Existing BARC biogas plants in India (Müller, 2007)

Location Capacity Date of Commissioning Utility of Gas

BARC, Mumbai 1 tpd 2001 Kitchen

Anushaktinagar, Mumbai 5 tpd 2002 Kitchen

BARC Hosptial site 5 tpd 2003 Kitchen

Govandi, Mumbai 5 tpd 2003 Electricity

Deonar,Mumbai 5 tpd 2005 Boiler

INS Kunjali 1 tpd 2004 Kitchen

The situation in India reveals that most of the biogas reactors are not working, although

internationally in technical literature Indian climatic condition are considered to be

favourable. But there are several other issues besides climate like in the last few years many

projects failed only due to their tremendous size. There was a plant based on the advanced

BIMA technology (Lucknow) having a capacity of 300 MT per day that stopped within 6

month of commissioning (Müller, 2007). Likewise a plant in Chennai has been closed as

well. Another problem while implementing biogas plants is the mindset of the people

expecting energy generation from so-called ―low cost systems‖. They underestimate the

robustness of biogas plants required for sustaining operation over 5 to 10 years.

2.8. Opportunities and Challenges

Energy Benifits

1) Net production of energy

2) Reduced CO2 emissions

3) Potential to treat the ―wet‖ fraction of MSW which is not suitable for incineration.

Environmental benefits:

1) Environmental benign waste treatment.

2) Odour reduction.

3) Lower land requirement compared to aerobic composting.

21

4) Volume reduction of waste

Disadvantages

1) Treats only organic fraction of MSW.

2) Information on economic and practical issues is not widely disseminated.

3) Waste water may need to be treated before disposal.

4) Post treatment and post treatment of feedstock is required

22

Chapter 3

Case Studies

3.1. Rayong waste to energy plant, Thailand

The Rayong waste to energy plant is situated in Thailand. The MSW treatment plant was

started in year 1999. It is owned and operated by Rayong Municipality, Thailand. It is the

first anaerobic digestion facility for MSW and cogeneration plant in Thailand and is planned

to serve for 20 years. The plant operation provides front-end treatment (FET) process,

anaerobic and back-end treatment to the receiving waste. Various unit operations in the plant

are shown in Fig.3.1.

This plant uses MSW, food-vegetable and fruit waste (FVFW) and night soil waste (NSW) as

waste materials. The plant is designed for a capacity of 25,500 ton/year. The heat and

electricity produced from the biogas is used for plant operation and surplus electricity is sold

to Provincial electricity authority (PEA). The organic materials are transformed to biogas at

the rate of 80 m3/ton with (methane content about 60%) under 25-31°C ambient conditions.

The overall efficiency of this cogeneration power plant is higher than 89%.

Before anaerobic biological process, the feedstock is pre-treated. The purpose of pre-

treatment is to mix different feedstock, to add water or to remove undesirable materials and to

homogenize slurry for efficient digestion.

The collected MSW is weighed and unloaded on FET process facility of the plant which

comprises receiving platform and waste separation facility equipped with both manual and

mechanical lines of operations.

In the beginning, the hazardous and oversized waste is sorted out and rejected to landfill or

diverted for other treatment. The remaining fraction is subjected to mechanical screening

process where it passes through a bag opener and drum screen to separate organic waste

smaller than 60 mm in size. The magnetic separator removes all ferrous metals and the

residual fraction moves to fragmentizer in order to achieve reduction in size of organic waste.

The slurry thereafter is fed to suspension tank for segregating all heavy fractions or any non

organic fraction, so called ‗bottom rejected‘ and to dilute solid content of the slurry before

23

finally flows to an anaerobic digester. It was suggested that the optimal solid content should

be between 15-20% for optimal yield.

Table 3.1 Technical design and installed capacity (DEE, 2006)

Process Unit Capacity

Waste type

Source separated organic waste t/d 20

Mechanical sorted organic fraction t/d 50

Organic waste

Total Solid % 30

Volatile Solid % 70

Pre-Treatment

Bag Opener t/h 7

Drum Screening t/h 7

Magnetic Separator t/h 3.50

Shredder t/h 8.50

Fragmentizer

1. Volume

2. Low-riddle

3. Solid Content(TS)

t/h 7.50

t/h 16

t/h <=10

t/h 15

Wet separation tank 30m3/ batch Batch/d 4

Anaerobic Digestion

Organic waste throughput t org/d 60

Digester volume m3 2100

Single stage medium solid % 15

Temperature (Mesophilic) °C 30-36

Organic loading rate Kg 6

24

AD of the pre-treated waste is carried out in high organic loading anaerobic digestion

(HLAD) system. The single stage wet-continuous and completely mixed process is performed

in a anaerobic bioreactor (volume ≈ 2100 m3) under mesophilic conditions. The waste solid

content is adjusted to about 15% prior to feeding into the bioreactor. For optimal product

yield, the minimum retention time of 18-20 days is provided in the bioreactor. The mixing of

feed substrate inside the reactor is done by agitation to enhance the activity of anaerobic

digestion under the controlled conditions. The transformation of volatile solid to biogas can

be at least 100 m3/ton of organic waste.

Digested Slurry is removed from the reactor and fed to buffer storage tank where it is mixed

with a polymer to enhance dewatering efficiency of dewatering machine (belt press). The

output from belt press still contains about 50% of water and hence it is further dried in a

thermal dryer to reduce water content to about 35% to produce good quality humus.

Meanwhile, the pathogens are also killed during thermal drying.

VS/m3.d

Solid retention time day 18-20

Optimal pH values pH 6.70-7.40

Volume of dry biogas N

m3/m3.d 2.85

Biogas Yield N m3/t

org 100

Post Treatment Belt Press

Pasteurized rotary drum

m3/h 16

t/h 7.50

Product

Electricity kWh/t

org 230

Compost like material t/t org 0.26

25

Fig. 3.1 Various treatment process employed in Rayong waste to energy plant (Vishwanathan

et al., 2005)

The thermal dryer utilizes the exhausted gas (120-140 °C) from biogas engine or heat from

gas burner to kill all the pathogens. After drying the hygienized humus is bagged and sold

out. The typical characteristics. of humus is tabulated in Table 3.2

Front end Treatment (FET)

process

Receiving floor Bag opener Drum screen Hand sorting

Magnetic separator

Organic fractions Feed Hopper

Fragmentizer

Feed Preparation tank

Bioreactor/digester

Buffer storage tank

Dewatering machine

Anaerobic digestion process

Back end treatment

Thermal dryer Soil Conditioner

Rejects to landfill

and recycled

materials

Biogas

Utilization unit

Gas holder

Gas engine

Electricity

26

Table 3.2 Quality of Humus (DEE, 2006)

Parameter Unit Standard Value Monitored Value

Moisture content % by weight ≤ 35 35

pH - 5.5-8.5 6.46

C/N ratio - ≤20 16

Conductivity dS/m ≤6 2.40

Nitrogen, N % by weight ≥1.0 2.01

Phosphorus % by weight ≥0.5 5.06

Potassium, K % by weight ≥0.5 0.46

Lead, Pb mg/kg ≤500 175.05

Copper, Cu mg/kg ≤500 0.54

Cadmium, Cd mg/kg ≤5 -

Chromium, Cr mg/kg ≤300 0.23

The filtrate from dewatering process is reused in the plant. The rejected water from

dewatering is led into the process water tank where it is treated in activated sludge system.

Various parameter of treated water are shown in the Table 3.3.

Table 3.3 Water quality (DEE, 2006)

Parameter Unit Standard Value Monitored Value

BOD (Biochemical oxygen demand) mg/L 20 6.70

COD (Chemical oxygen demand) mg/L 120 61.4

TDS (Total dissolved solids) mg/L 3000 572

27

Utilization of Biogas

The biogas yield at full designed load is around 2.2 Mm3 /year with 60% methane content,

around 35% carbon dioxide. Biogas produced by the digester is sent to CHP units to produce

electricity and heat recovery. It can produce around 5,062 MWh/year electricity and 3,172

MWh/year heat with 38.7 and 22.7 % efficiencies respectively.

Biogas engines manufactured in Austria are used for a CHP unit of the plant. The engine

serves three purposes:

1) Generation of 625 kWh electricity for the AD plant and sell the remaining to PEA grid

2) Production of hot water for heating of the digester

3) Sterilization of the digestate by exhaust gas

The cooling water of gas engine is utilized in maintaining temperature of digester and about

28% of heat loss from the exhausted gas is compensated by killing pathogens present in the

digestate. The total efficiency of energy utilization is about 89% when electricity generation

and digestate sterilization are combined.

Odour control measures

Odour potentially occurs from several tanks in the system. Hence, the plant (especially the

receiving building) is equipped with ventilation system and the air is treated by ozone before

discharge to atmosphere. To prevent spreading of odour from incoming waste, the negative

pressure is provided inside the building.

Environmental impacts:

The environmental performance of this plant indicates that 2010 tons/year of methane is

recovered and used for electricity generation hence the pressure on fossil fuels is reduced. It

also results in significant reduction of other greenhouse gas emissions as shown in Table 3.4

Table 3.4 Effect on greenhouse gas reduction after installation of plant (DEE, 2006)

Parameter Unit Before operation After operation

Carbon dioxide(CO2) tones/year 4540 96.80

Methane (CH4) tones/year 2010 Neglected

28

Nitrogen dioxide(N2O) tones/year Neglected 9.51

Total Carbon dioxide equivalent tones/year 46750 2856

3.2. Valorga plant at Tilburg

The Valorga full scale plant in Tilburg (the Netherlands) was designed to process 52,000 tons

source separated organic fraction of MSW per year that is collected once a week. It is in

operation since year 1994 and primarily treats ‗Vegetable -Garden –Fruit‘ (VGF) fraction of

MSW. The feed consists of 75% kitchen and garden waste whereas the rest is paper and

cardboard. The plants treats around 80 % of VGF generated per year in Tilburg. The annual

biogas production from the plant is 3.1 MNm3 (80-85 Nm3/ton of input for digestion). The

specific methane yield varies from 220 - 250 Nm3/ton total volatile solids (TVS). During

slack winter period, the waste contains proportionally more food waste that is more readily

biodegradable (within the considered degradation times) than lignocellulosic garden waste

and results in higher methane production The typical mass balance of Tilburg plant is shown

in Fig. 3.2.

The collected waste is subjected to pre-treatment before anaerobic digestion. During pre-

treatment, the waste is screened to remove the bigger particles and finally crushed to reduce

particle size below 80 mm. The pre-treated waste is then subjected to AD process and is

intensively mixed with part of the excess process water and heated by steam injection. The

water added to the waste is calculated to keep TS content of the mixture at around 30%. This

mixture is pumped into the reactors with piston pump. The Valorga digestion process is

carried out in a plug flow reactor in single step. The digestion is carried in two digesters each

having a volume 3,300 m3 under mesophilic temperature condition at 38° C. The two

digesters are operating with the same loading rate. The reactors are vertical cylinders with a

lateral plug-flow type path of the fermenting matter. They contain a vertical median inner

wall on approximately 2/3 of their diameter. The height of the vertical reactor allows to

simply extracting the digested matter by gravity. Mixing is done by pneumatic system.

The digested material produced after anaerobic digestion process extracted from the digesters

is dewatered using screw presses. The liquid sludge from the presses is then sent to a hydro -

cyclone, and a flocculation –filtration unit to remove suspended solids. The filtered effluent is

discharged to the nearby wastewater treatment plant. The solid fraction from the presses and

29

Fig.3.2 Tilburg Plant Mass Balance (URL 2)

band filter are properly mixed and subjected to aerobic conditions for four weeks in a closed

hall to ensure complete stabilization.

The final organic residue is considered as compost like material. It amounts to 25700

tons/year and is reported to be of high quality for agricultural use.

In order to prevent any odour emission, the air from the product treatment units and the

composting hall is purified using biofilter.

Utilization of Biogas

Biogas generated in the digester is refined in an upgrading plant and then supplied to the

municipal network. The biogas originally produced from the digester contains 56% CH4 and

has a calorific value of about 20 MJ/ m3 that is increased to 31.7 MJ/ m3 after refining

process. Gas refining process consists of compressing, cooling, scrubbing, and drying

operations. The refined methane gas is fed to the municipal grid. The Center for Analysis and

Dissemination of Demonstrated Energy Technologies (CADDET) reported that the natural

gas yield was about 50 m3/ ton. The net yield of natural gas, i.e. after providing for heating

and electrical energy for the plant, was 1,360,000 m3 of methane per year, i.e., about 44 m3 of

methane per ton of the processed organic material.

Biogas 4100 tons

3.1MNm3

Final Compost

25 700 tons

Sand

4600 tons

Excess water 11500 tons

Solid Separation Steam, Water 6700 tons

Kitchen and Garden Waste

52 000 tons

Sorting Unit Refuse

2000 tons

Steam

1200 tons Methanisation

30

Environmental and financial benefits

The environmental performance of the Tilburg plant indicates that 1.36 million m3 of

methane per year is recovered and used for electricity generation. This corresponds to 728

tons of carbon in the form of methane (CH4). Considering that one ton of C as methane is

equivalent to 21 tons of C as carbon dioxide, Tilburg operation avoids landfill emissions of

about 15,000 tons of carbon equivalent. The main sources of revenue to this plant are the

―tipping‖ fees paid by the municipalities for waste treatment and the sale of natural gas.

Laclos et al., (1997) suggested that Valorga plant has demonstrated the potential of anaerobic

digestion for the treatment of organic solid waste. Despite the annual variation in waste

quantity and quality, the biological process in the reactor remains well balanced. A key factor

for reliability of the Valorga process is the absence of any mechanical device inside the

reactors, which avoids abrasion and maintenance problems which are of great importance at

full scale. Tilburg facility highlights the technical and economic feasibility of using energy

from waste in the form of biogas to generate electricity.

3.3. BTA plant at Newmarket, Ontario (Canada)

Biotechnische Abfallverwertung GmbH & Co. KG (BTA) was developed in Germany and

applied its plant (via several licensing companies) throughout Western Europe and in select

locations in Canada and Japan. The system is one of the oldest and most successful in terms

of the number of existing operational digesters. Although small units are single-stage, the

majority of the BTA digesters are large (>100,000 MT/yr) (CPEA, 2008) multi-stage and wet

units. A plant located in New Market, Ontario and Canada was started in July 2000 based on

BTA system. It has a capacity to treat 150,000 metric tons of organic waste per year.

The facility receives the mixed waste brought in by collection vehicles that is unloaded on the

tipping floor. Then, the waste is conveyed to a pre-sort station, where oversized,

contaminants as well as recyclables are removed. After the pre-sort station, the material

continues through a trommel screen that separates fine materials (mostly organic), medium

sized materials (mostly containers) and large objects such as newspaper, cardboard, film,

plastic and textiles. The front end of the screen is equipped with a series of knives to open

plastic bags. A manual sorting station segregates plastics (PET and HDPE), glass and textiles.

Also, magnetic and eddy current separators are used to extract ferrous metals and aluminium

cans. The marketable materials are sent for recycling and the non-recyclables are landfilled,

31

while the organic-rich materials is conveyed for wet treatment phase to a 32 m3 capacity

hydropulper which consists of agitator, hydraulic rake and sieve. The feedstock is mixed

with water and processed over a 16 hour period and fractionalization of waste is carried out.

The hydropulper separates the feedstock into three fractions viz. homogenous pulp fraction,

light fraction residual and heavy fraction residual. Hydrocyclone removes the sand and grit

left in the organic suspension after hydropulping. All non-digestible elements captured in the

front end pulping and grit removal steps are typically landfilled.

After the separation of the inert contaminants, the de-gritted pulp is pumped continuously

into the pulp buffer tank for constant hydraulic loading in the digester.

Fig. 3.3 Primary phases and related steps found in typical BTA process facility (URL 3)

The contaminant free heated pulp is fed into the digesters from the pulp buffer tank. The

digester contents are continuously mixed using compressed biogas that is delivered inside the

digester. The retention time of 15 days is provided inside the anaerobic digester.

The digestate is de-watered in a screw press and the filtrate is re-circulated to the hydro

pulping process. The dewatered solids are typically finished into a soil amendment product

using a simple, four week aerobic composting methodology. Bulking materials are added to

32

allow greater porosity and oxygenation during the aerobic finishing step. The 60,000 tons of

compost produced annually are bagged and distributed to retail horticultural outlets.

When the compost material does not meet the prescribed standards, it is used for quarry

restoration and other land rehabilitation projects. Excess liquids are either treated on-site or

discharged to the local sanitary sewer. Air from the building and at source from all the

processing vessels is collected and treated in a bio filter prior to the release to atmosphere.

Utilization of Biogas

The produced biogas is used to provide electrical and thermal energy to run the facility. An

820 kW co-generation generator is installed at Newmarket to utilize the gas. About 5,000

MWh of electricity is produced annually of which the plant uses 2 MWh and the rest is sold

to the local grid for supplying electricity to 3000 homes.

33

Chapter 4

Summary

In this report an overview of energy recovery from municipal solid waste (MSW) using

anaerobic digestion (AD) is presented. At present open dumping is prevailing in most of the

MSW disposal method in developing countries

AD process is one of the MSW treatment options that can help in the diversion of MSW from

landfills. AD is a multi stage biological process in which the organic matter decomposes by

the action of microorganisms in absence of oxygen. All anaerobic system produces biogas

(55-75% of methane and rest carbon dioxide with traces of other gases). From the

composition of MSW in India, this can be suggested that this process can be a feasible option

due to high moisture and organic content.

Various AD systems like single stage, multi stage and batch systems are available. Single

stage is less expensive and simpler than multi stage but has lower biogas yield. In multi stage

system the methanogenic process is separated from other process and each process can be run

close to optimum to increase biogas yield and ensure digestate quality.

In India, several designs of various anaerobic digesters have been developed that include

solid state stratified bed (SSB) digester, TERI enhanced acidification and methanation

(TEAM) digester and compact biogas plant.

To enhance the effectiveness of AD process, MSW needs to be pre-treated by various

mechanical operations. Also, the improvement in outputs (i.e. biogas and digestate) can be

brought by using post-treatment technique

Biogas is a valuable fuel which can be utilized to recover energy. The gas can be used in

combined heat and power unit to generate heat and electricity and it can be fed to local

electricity grid. This gas can also be used as a substitute of natural gas, in fuel cell technology

and compressed gas vehicle fuel (after gas purification).

The three case studies presented in the report show the potential and effectiveness of the

process. However, there are still some problems like quality of digestate, wastewater

treatment and slow rate of digestion that needs to be addressed.

34

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