Accepted Manuscript Complex Effluent Streams as a Potential Source of Volatile Fatty Acids

7/29/2019 Accepted Manuscript Complex Effluent Streams as a Potential Source of Volatile Fatty Acids

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Complex Effluent Streams as a Potential Source of Volatile Fatty Acids

Myrto-Panagiota Zacharof*b,c Robert W. Lovitta,b,c

a Multidisciplinary Nanotechnology Centre (MNC), College of Engineering, Swansea University, Swansea,

Talbot building, SA2 8PP, UK

b Centre for Complex Fluid Processing (CCFP), College of Engineering, Swansea University, Talbot building,

Swansea, SA2 8PP, UK

c Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering,

Talbot building, Swansea University, Swansea, SA2 8PP, UK

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Abstract

The recovery of volatile fatty acids (VFA), from complex effluent streams deriving from numerous sources has

been an area of research interest for more than a century. In the current era, technological and economic

development is widely based on the limited global petroleum resources. Regardless the scarcity faced in coal based

fuels, VFA are still extensively and in most cases solely, synthesised from petroleum. With the constantly risingawareness of the environmental impact the carbon based economy has created, research has been focused in

developing alternative methods of their production. These include fermentation, anaerobic digestion and recovery

from discharged chemical and industrial plants effluents.

During these processes, the hydrolysis of target solid wastes followed by the microbial conversion of them to

biodegradable organic, content results in the production of intermediate VFA, commonly acetate and butyrate.

These, are detected at varying concentrations in the effluent streams and mixed liquors of the reactor systems. Their

concentration is depending on hydraulic, retention and organic loading rates.

Several studies have shown possible environmental and commercial benefits using various techniques for their

separation and recovery. Among these, extensively applied has been reactive extraction. Currently, membrane


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Introduction

Volatile fatty acids (VFA) are heavily involved in the organic carbon cycling on the planet. They are extensively

used in the industry and are typically produced of oil based chemical processing. They are also known as carboxylic

acids due to their carboxylic group or as low molecular weight organic acids due to their small molar mass. They

are commonly applied in the field of food and beverages as acidifiers, but also in the pharmaceutical and chemical

fabrication field [1-4].

Alternative methods of VFA production have been developed, due the global petroleum resources facing scarcity

and the constantly rising awareness of the environmental impact the carbon based economy has created. These

include fermentation, anaerobic digestion and recovery of these acids from industrial waste streams [5, 6].

Fermentation, in other words, the breakdown and re-assembly of biochemicals catalysed by microorganisms,

typically in anaerobic growth conditions, using a wide range of carbon based substrates. These include solid and

liquid wastes derived from agricultural or food sources or other complex effluent streams such as municipal orindustrial wastewater [7, 8].

Through these processes, organic carbon is metabolized through fatty acid intermediates as they are ultimately

metabolized to carbon dioxide (CO2) and methane anaerobically, or CO2 and water in oxidative respiratory

processes [9]. Either way, VFA, especially acetate, are key intracellular and extracellular metabolic intermediates.

It would therefore seem opportune that if carbon could be recovered in the form of VFA, this would represent an

alternative source of sustainable carbon based chemicals for industry that can be generated and recovered from

organic degradation processes, including those linked to fermentation and anaerobic digestion. In addition these

acids can be used as a substrate for a number of interesting biotransformations for sustainable production of

chemicals. Nevertheless, these streams are quite complicated both in chemical composition and in fluid properties

that make the recovery of VFA a technical and economic challenge. Numerous downstream processing methods,

based on the physicochemical characteristics of the substances (Table 1) [11, 12] have been used to try and

overcome this hurdle. Liquid-liquid extraction [13-17], adsorption, distillation, filtration, and electrodialysis, have

been explored for the selective recovery of low molecular weight organic acids from industrial wastewater streams

and fermented broths [18-21].

Th k t h l f ti f i id f t l t t d hi h lit i


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the ability of nanofiltration membranes to selectively separate and retain VFA from aqueous based mixtures.

However, to apply this technology effectively, pretreatment of the mixtures is typically required to achieve

separation from real streams of materials. This review will investigate the process recovery options for fatty acids

recovery and focus on the use membrane processes and their potential and capability for their recovery from waste

streams [25-27].

Volatile fatty acids Costs, Uses and Demand in a Low Carbon Economy

VFA are mostly used in the fields of food and beverages as acidifiers but also in the pharmaceutical and chemical

fabrication field [28-31] (Fig.1).

More recently, renewed interest in making chemicals from biomass, through fermentation has increased. These

processes present a sustainable, carbon neutral, method of chemicals production. Much work is being focused on

third generation biofuels production using lignocellulosic materials in biorefinery concepts [32, 33]. The use of

waste streams thus avoids the problem of using potentially important food carbohydrates as feedstocks.

The wide application these acids have, in combination with the large increase in costs of key petroleum-derived raw

materials and coupled with the escalating increases of energy and transportation costs, have increased the

commercial costs and the value of VFA (Table 2).

Recent use of VFA, especially acetic and butyric acids does include their employment as materials for the

development of biodegradable polymers [polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB), polylactate

(PLA)] of much interest in the current economy as they can be used in the production of plastics replacing their

production of petrochemical feedstock [34-36]. Biodegradable polymers can be applied in a wide range of fields in

todays industrial world. These include their use in the plastics production industry, in textile fabrication, in

medicine and cosmetics especially for fabrication of modules used for prosthetic surgery, in the pharmaceutical

industry as coating agents and in the printing industry [34-38].

Acetic, propionic and lactic acids are widely used in food industry as taste enhancing additives and preservatives, in

the pharmaceutical industry as buffer solutions, in chemical industry for the synthesis of biodegradable polymers

and in the cosmetics industry in moisturizers, skin-lightening or anti-acne agents [39, 40, 41-45].


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hydrocarbons or the carbonisation of methanol [48] or from biological sources [49]. Acetic acid is utilized as an

additive or as a compound in a wide range of products. For example, it is used in the synthesis of acetyl cellulose

and plastics, in the food industry as a food additive under the code E260, as an acidity regulator, and in the printing

and coating field. Another major use of acetic acid is, in the production of vinyl acetate monomer, a compound that

is used for vinyl plastics, adhesives, textile finishes and latex paints and it is closely followed by acetic anhydride

and ester production. Acetic anhydride is used for cellulose acetate, pharmaceuticals and plasticizer production. In

households diluted acetic acid is often used in desalting agent [50].

Propionic acid is used as a preservative for both animal feed and for food for human consumption. When introduced

into animal feed it as is either in the form of ammonium salts or directly as the free acid, though when introduced in

human food especially baked products such as bread, it is introduced either as its sodium (Na +) or its calcium (Ca2+)

salts. It is also used for the fabrication of pesticides and pharmaceuticals Propionic acid is also used as a chemical

intermediate, mainly for the modification of synthetic cellulose fibbers. Its esters are also used as solvents or as

artificial flavourings [51-53].

Prior to the development of petrochemicals based processes VFA (acetic, propionic and butyric acids) were

produced by fermentation (Table 3) either by the microbial oxidation of ethanol or the anaerobic fermentation of

hexose sugars or starch. All these processes occur in aqueous solutions and thus traditionally require energy

intensive separation using distillation. Low energy alternatives had been investigated but these result in more

complex processing and are not that robust. One of the principal challenges is to remove VFA at relative low

concentrations from water and this typically makes it energetically unfeasible to do so.

The microbial fermentation processes were then superseded by the oil based process because they could be carried

out in gaseous phase in the absence of water thus avoiding significant energy cost related to removing water from

the acid products in the distillation process. Fermentative carbon sources include solid and liquid wastes not only

from agricultural or food waste but other complex effluent streams such as municipal or industrial wastewater [7,

41, 42].

Several studies have been conducted, proposing a variety of biotransformation processes using different microbial

cultures as well as different kind of separation techniques for their recovery (Table 4). VFA i.e. acetic, butyric,

propionic, valeric selective recovery can be achieved by various methods that take advantage of their


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waste of various materials such as stalks and husks of cereal crops, grass and silage, hydrolysates of wood and paper

processing, agriculture sludge.

In anaerobic processes the hydrolysis (thermal, catalytic or enzymatic) of solid wastes is followed by the microbial

conversion of carbohydrates to intermediate organic acids. Depending on the microorganisms used several

biochemical systems of conversion of carbohydrates to VFA (Fig.3) can be employed. Volatile fatty acids, are

detected at high concentrations in the effluent streams and mixed liquors of anaerobic membrane reactor systems,

where sudden variations in hydraulic and organic loading rates lead to leakages of microfiltration and ultrafiltration

systems mainly due to their low molecular weight.

Consequently, fermentation effluents need to be treated with a more advanced treatment process such as

nanofiltration and reverse osmosis [61-63], for recovery of VFA. Furthermore, effluents can be recycled and reused

as main feed (Fig.4).

For producing VFA by fermentation, favourable conditions for the growth and maintenance of a healthy populationof acid producing bacteria should be established in order to reach a steady production rate. It might be possible

though for the production rate be relatively small, partially due to the conversion of soluble VFA to gaseous

products [58-61]. However, a major drawback of production of VFA through microbial fermentation is the toxic

effect the produced acids are having on the producing strains, as higher concentrations become inhibitory for growth

[64, 65]. Production systems in industrial scale, are typically involved the fermentation of carbohydrates in

anaerobic conditions and these are produced up to 50-60 g L

-1

concentrations, with their profile being typically amixture of acids, regardless the usage or a pure or a mixed strain culture used [66-68]. At these concentrations batch

or continuous fermentations are highly inhibited [64, 65, 70].

Numerous solutions have been suggested to overcome this problem including changes of operating conditions such

as pH , temperature , agitation , aeration , feed rate, usage of acid tolerant strains with the most prominent and

feasible being the simultaneous separation and recovery(in situ) of the produced acids during the fermentation [58-

61,64]. Several techniques have been proposed, with filtration gaining ground due to the many advantages of its

usage including fractionation of the acids if present in a mixture, scalability, applicability to a wide range of reactor

arrangements and feeds, effectiveness in separation and economy use. Several studies have been conducted

especially combining extraction in anaerobic bioreactors [71, 72].


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In the quest for substitute energy sources, the development of anaerobic digesters for biogas production used to

generate heat and electricity, has been proposed and practiced [136]. Being relatively easy to construct, enhancing

local and national economies by supporting small and medium sized entrepreneurships [137] and relying on a well

know and widely investigated process of fermentation [138], anaerobic digesters are rapidly built throughout

western Europe and United States [139]. In the Western economies the production via anaerobic digestion of biogas

for power generation represents the 1.2% of the annual production of electricity and nearly 10% of renewable

energy, with an installed power close to 1500MW [140].

VFA are the main soluble compounds generated during anaerobic digestion of coagulated sludge (Table 5) [74].

Acetogenesis represents one of the stages towards the production of methane, which is mainly used for the

production of electricity and heat (combined heat and power, CHP) [75, 76]. Identification of the individual acids

formed during the acetogenesis of coagulated raw sludge is of great importance, providing valuable information on

the stability and dynamics of the system. The most abundant component of volatile fatty acids is acetic acid with the

other VFA being produced such as, propionic acid, butyric acid and valeric acid [77].

Current research has mainly been focused on the methane-production phase of the process, its clean-up andutilisation. In consequence, very few studies, have been focused on the acid phase step of the process [18, 81] and

relatively little attention has been paid to the recovery and reuse of fermentation permeates [78, 79].Very few

studies, have been focused on the acid phase step of the process, although it is recognised that methane gas

optimisation may be easy in a two stage process of acidogenisis followed by methanogenesis. In consequence,

relatively little attention has been paid to the recovery and reuse of acidogenic fermentation permeates [18, 81].

The dissolved organic compounds containing volatile fatty acids could be used effectively as a carbon source and

hydrogen donor for biological process production of materials and removal of pollutants or as basic materials for the

synthesis of biodegradable polymers [82, 83] .The generation of acid-phase fermentation products in anaerobic

digestion can be influenced greatly by operational parameters. A very important operational variable, which can be

easily manipulated, is the hydraulic retention time. This parameter can act as a selection parameter for the

acetogenic phase, only though if it encourages the growth of acid formers and concurrently suppress the growth ofmethane producers [84]. Knowledge of the rate-limiting biochemical reaction is crucial when trying to monitor or

predict the behaviour of these systems [85, 86].

In anaerobic treatment process, the utilization of membranes as liquid-solid separation tool is very attractive. A lot


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productivity per unit reactor volume typically generates highly concentrated (up to 20% w/v salt) mixed streams

[83].

Reuse of the components and conventional biodegradation usually requires separation of organics from the salt as

the great majority of the microorganisms used in biotreatment cannot withstand the osmotic stress of high salt

levels. The current alternative treatment solutions, for example incineration, are much more expensive than

biodegradation and the cost of treatment can eventually be undermined the economic viability of the whole process

with progressing tightening of environmental regulations [87, 88].

Examples of these streams the traditional cheese manufacture waste streams , whey as well as the waste streams

deriving from the food, nutritional and alcoholic beverage industry during fermentation.

Benefits of Volatile fatty acids Recovery from Complex Effluent streams

The removal of VFA from wastewater deriving from numerous sources such as chemical plants has been an area of

research interest, attracting great attention [97]. Obvious commercial benefits include the relief of municipaltreatment plants, in terms of volume, the recovery of favourable nutrients and the composition of nutritive

wastewater for biotreatment. The VFA could be both removed from the sludge and reused in the industry, in other

words recycled; avoiding the current practice of sludge which is neutralisation, leading to the loss of a valuable

source [102].

Treatment of effluent streams containing significant amount of VFA, to meet current and future water quality

standards has been recognized. Alkali metal salts of these compounds are entirely soluble in water and insoluble in

hydrocarbon media causing environmental pollution [6]. Therefore, it is important to remove VFA both for

pollution control reasons but also for their use in the industry [118,119].

Downstream Processing of VFA from Complex Effluent Streams

Liquid- Liquid extraction (LLE) -Reactive Extraction

Researchers [6, 12, 18] have reported liquid-liquid extraction processes using several extractants such as

tributylphosphate of acetic, propionic, butyric, valeric and caproic acid from waste streams acid obtained by

fermentation Reactive extraction (RE) or liquid liquid extraction (LLE) is based on the utilisation of a specified


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of solvent. After the extraction of organic acid, solvents are regenerated for recycle in the extraction process. The

cost of separation of these useful products then is significantly reduced [49].

Several physicochemical parameters govern LLE such as the distribution coefficient, degree of extraction, loading

ratio, rate constant of carboxylic acids-extractant reaction, mass transfer characteristics, the properties of the solvent

(extract and diluents ) type of solvent, temperature , pH and acid concentration [28,29].

Considering the separation of VFA, LLE is categorised into three major groups: acid extraction by solvation with

carbon-bonded oxygen bearing extractants (also inert aliphatic and aromatic hydrocarbons and some of their

substituted homolog) [4]; acid extraction by solvation with phosphorus bonded oxygen bearing extractants [89, 90]and acid extraction by proton transfer or by ion pair formation, typically the extractant being high molecular weight

aliphatic amines. LLE efficiency is influenced by the nature of the solvents and diluents, their relative

concentrations, the properties of the VFA to be extracted, the concentration of the acid present in the aqueous

stream, the molar ratio of the solvent to acid and the temperature of the extraction.

The extraction efficiency is high when the organic acid is present in the undissociated (acid) form. This, in turn,

depends on the pH of feed solution with low pH increasing the proportion of undissociated acid in the mixture [89,

90].

LLE has been used to recover VFA from industrial waste streams of the chemical manufacture of cellulose and

vinyl acetate. The most common solvents used were ethers, ketones or alcohols which resulted in relatively high

costs and insufficient distribution coefficients [44, 45].

On the contrary, phosphorus bonded oxygenated extractantants and high-molecular-weight aliphatic amines are ofparticular interest to be used, as they have low solubility in water and distribution coefficients of acetic acid in

amines are high. Tertiary amines, usually preferred over secondary or primary are insoluble in water, not forming

any by-products through side reactions and the distribution coefficient of acetic acid in amines are quite high.

LLE has been used for extraction of acetic and propionic acid during anaerobic fermentation in an effort to reduced

the inhibition effect occurring by the acids to the microbial cell culture. The solvents used in extractive fermentation

processes can adversely affect product formation and cell growth [92].

Research [4] has been done on using tri-n-octylphosphine oxide (TOPO) a solvent with relatively low partition

coefficients for VFA. Cell growth or production of propionic and acetic acids was not hindered, but there was no

enhancement of acid production over the 120 h fermentation, even though extraction maintained acid concentrations


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7), direct acidification with sulphuric acid gave yields of 66% at pH 3.4. Lowering the pH of bipolar electrodialysis

was more effective and gave higher yields of 85%. To maximize yield during LLE or esterification the

concentration of acetic acid, should be as high as possible [6]. LLE is often followed by azeotropic distillation

where the solvent extract stream, containing the acid removed from waste stream and some dissolved water is sent

to distillation column where solvent and water is taken overhead.

Dioctyl adipate (DOA) has also been investigated. In this case with DOA and di-n-decyl-amine no growth was

observed in the medium tested with above 10% w/v of the solvent. In all the tertiary amines, no growth was

observed at above 40%.

Distillation and Evaporation

Volatility based separations (e.g. distillation, evaporation, etc) use differences in vapour pressure of components to

drive the separations. Although distillation and evaporation are particularly important in dewatering, significant

research is unlikely to make a large impact on direct product capture [91].

Despite the fact that acetic acid and water do not form an azeotrope, it is necessary to have a large number ofequilibrium stages and a very high reflux ratio to obtain, for example, glacial grade acid by simple distillation. As an

alternative to fractionation, to reduce energy consumption, azeotropic dehydration, effective for high concentration

of acid, can be employed with addition of another liquid where the entrainer carries the water overhead in the

distillation column, with the mixture being phase separated after condensation and entrainer being returned to the

column [84]. Two phases of distillation were required to get the final product. The first phase comprises of

dewatering the effluent stream and the second in concentrating the organic acids. Further recovery of the organic

acids shall be achieved through evaporation of the remaining components [43].

Esterification

Esterification to recover acetic acid from fermentation broth has been investigated [6]. In this case, the product is an

acetic acid salt either, ammonium acetate or sodium acetate, depending on the alkali used to neutralise the

fermentation broth [91]. Esterification converts acetic acid into an ester by reacting it with an alcohol in the

presence, of an acid catalyst .Then, the ester can be hydrolysed to produce the acid. Esterification depends on the

reaction temperature, the presence of water and the nature of the alcohol used.

Ethanol and butanol have been used for esterification, partially because ethanol is already being produced from

biomass substrates. In addition the esters (ethyl acetate or butyl acetate) do have a market value and could also be


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dissociation of the acetate. At pH 6.5 only 20% of the acetate is undissociated, whereas at pH 2.87 over 80% is

undissociated.

Adsorption

Using a solid adsorbent, VFA acids could be possibly be recovered by adsorption. Limited research has been done

using this method, a study though was found that separation of VFA was achieved by the use of a polymer

adsorbent of pyridine skeletal structure and a cross linked structure [77, 78].This polymer adsorbent showed good

selectivity and high adsorption capacity for volatile fatty acids even in the presence of inorganic salts. The chosen

eluents were aliphatic alcohol, aliphatic ketones and carboxylic esters. The cost though associated with regenerationof commercial adsorbents make adsorption operation very expensive [5].

Precipitation

Various cases has been reported were lactic acid separation and purification has been achieved through

precipitation. Calcium precipitation process has been employed, where the separation and final purification stages

account for up to 50% of the production cost and produces a large quantity of solid waste. Conventional calciumprecipitation is simple and reliable but is expensive and environmentally unfriendly as it consumes calcium oxide

and sulphuric acid, while it produces a large quantity of calcium sulphate sludge as solid waste.

Membrane Processes

A potentially effective treatment, suitable for removal and recovery of organic acids is the use of membranes.

Membranes are semi-permeable barriers across which selective separation can take place. They are driven by

pressure, electrical field or concentration gradients. The separations are based on steric factors, namely size or

molecular weight, shape and charge, and include most forms of filtration including basic membrane processes and

some types of chromatography (i.e. gel permeation). These processes typically function by a differential movement

of exclusion from pores of specified size. Membrane processes are challenged by increasing flux while maintaining

high selectivity. Much on the improvements in separations technologies will develop from new materials or new

modifications of generic materials including resins membranes and solvents [7, 94]. Steric- based separations are

frequently enhanced by the addition of other effects e.g. change of specific affinity interactions, charge interactions

at the molecular level.

Electrically Driven Filtration Process

l d l


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fermentation broths [6]. Several researchers [5, 6, 47] have utilised electrodialysis to separate acetic acid (30-50%

acetate recovery) derived from anaerobic fermentations performed byAcetogenium kivui in liquid media. As in the

streams mostly the salts of the acids are present, application of ED in this system should be judicious as on one hand

the product needs to be obtained in acid solution form and on the other hand to recover and reuse the ions (resulting

in wastefree process) [5].

Pressure Driven Filtration Process

Membrane processes that already have been used for wastewater treatment include ultrafiltration, reverse osmosis,

nanofiltration and pervaporation (Fig.6). The low molecular weight of the organic acids exclude the use ofultrafiltration and microfiltration for recovery though can be used successfully as pre-treatment steps i.e. cell or

particle removal. Especially when recovering the acids from fermented broths or mixed effluent streams, as the

osmotic pressure limitation due to high salt contents render reverse osmosis impractical. Nanofiltration polymeric

membranes though can be effective for treating several types of wastewater streams [5, 62].

There are several beneficial features in membrane processing and include the ability to recover the acids in

relatively concentrated form for reuse or more economical disposal of waste, relatively low pressure operation,

simple scale-up using commercial hollow fibber modules, and ease of in situ regeneration of the polymeric liquid.

The process has shown treatment feasibility for several types of aqueous waste streams. The main problem that

develops is membrane fouling which requires, frequent cleaning of the membrane [98, 99].

Product and process waste streams contain small molecular weight components that can be concentrated, desalted or

in some cases fractionated with membrane filtration technology. Membrane filtration as a standalone process can

offer 50 to 90% removal of the liquid phase if coupled with evaporation [100]. Substantial capital and operating cost

benefits come together with selective separation. This can be derived by use of low energy consumption membrane

technology as a complete stand alone process step in or in conjunction with evaporation enabling product recovery

and water reuse [18]. The cost of treatment of waste streams containing acetic acid using a commercial activated

sludge biological treatment system is between 4 to 10 cents/ kg depending on the concentration and sludge disposal

system [8, 23].

Membrane coupled anaerobic digestion processes have been employed in the wastewater treatment field, as a liquid-

solid separation tool [102]. Most of them have been applied to stabilise the suspended organic materials.

Of all pressure-driven membrane processes, nanofiltration is the best candidate process to deal with the problem as

i h i l fil i l f l i l ll i l l N fil i


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Nanofiltration may have another advantage of exploiting Donnan charge exclusion as most nanofiltration

membranes possess fixed charges. Nanofiltration can be easily scaled up due to the wide availability of commercial

nanofiltration modules and ease of in-situ regeneration of the polymeric liquid [50,102,110].

The benefits of this process include, the ability to process several types of aqueous waste streams, the recovery of

the acids in concentrated form for reuse, an economical disposal of waste, cost effective operational conditions such

as ambient pressure and temperature operation. Cost operation studies have been performed in the recovery of acetic

acid [111] comparing the economical cost of the implementation of a membrane separation process and that of an

evaporative unit suggesting that the cost of a membrane system was found lower than the cost of evaporationequipment [112,113]. Nanofiltration can be used for the recovery of valuable products while reverse osmosis offers

the recovery of high quality water from the process reuse [112].

Pretreatment systems

Processing complex effluent through nanofiltration membranes would almost certainly require pre-treatment with

methods capable of separating large solids from the liquids. Pre-treatment is sought for several reasons including;

treatment requirements by the regulators; treatment objectives such as the removal of colour, total organic carbon

(TOC) and viruses. When the desired flux cannot be achieved with the existing nature of the feed; when the

recovery of the substances is influenced by flux maintenance and clean-in-place frequency creating a less-than-

targeted value due to turbidity, TOC, suspended solids, certain soluble species such as manganese, arsenic and iron

that require oxidation are also parameters enforcing pretreatment. There are also many undesirable elements in the

feed such as polymers or oils [102,111,113,114,124]. Major role in the use of a pretreatment scheme, is played by

the overall lifetime economics of the designed membrane process system. For the economic evaluation several

parameters are taken into account such as the systems capital cost, the membrane replacement costs after a certain

amount of time, the costs of chemicals, the disposal of non-chemical and chemical waste and labour

[102,111,113,114,124].

Nanofiltration and reverse osmosis processes are utilizing semi-permeable non-porous membranes where

backwashing cannot be easily applied. The commonly used low cost, spiral wound configuration is highly

susceptible to particulate fouling requiring extensive pre-treatment using micro- and ultra-filtration multimedia

ranging between 5 to 20 m. Failure to avoid fouling can reduce system productivity, operational problems and

significant reduction of membrane life [102,111,113,114,124].


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oxidants can be used to oxidize metals such iron and manganese for subsequent filtration [102,111,113,114,124].

Most commonly used treatment prior to filtration systems, is the addition of an acid (pH reduction) or a proprietary

scale inhibitor recommended by the membrane manufacturer to prevent the precipitation of sparingly soluble salts

(e.g. calcium carbonate, barium sulfate, strontium sulfate or silica species) on the membrane. However, when using

chemical disinfectants such as oxidants, the membrane material should be taken into consideration especially in the

case of cellulose acetate where extreme pH of the feed water might cause chemical deterioration of the membrane.

As such any strong disinfectant added upstream has to be neutralized by a reducing agent prior to the contact of the

feed with the membranes [102,111,113,114,124].

Filtration fundamentals

The separation of substances through filtration results into the permeate and concentrate or retentate of the total

processing stream. The permeate can be defined as the portion of the processing stream that permeates or crosses

the membrane during processing containing compounds that are small enough to go through the pores of the

membrane. On the other hand the retentate is the portion of the processing stream that cannot cross or is retained by

the membrane during processing containing substances too large to pass through the pores of the membrane,similarly the term concentrate is a portion of the feed stream that is retained by the membrane related to the total

volume of feed [120,121]. The separation efficiency of the membrane is evaluated in terms of rejection or retention

and separations factor, as well as other parameters (Table 6), measuring of how well a membrane retains a solute.

The use of membranes in the industry as a downstream processing option has been proven an attractive cost

effective option. The utilization of membranes though is accompanied by numerous engineering challenges, of

which the most prominent is membrane fouling [122]. Fouling is a complex multifactorial phenomenon, which is

rather difficult to precisely describe. However, the general term used is the deposition of on the membrane surface

of dissolved and undissolved substances forming an undesirable layer causing flux decline.

Fouling can be classified into three major categories a) inorganic fouling due to the deposition on the membrane

surface of colloidal matter, minerals, hardness scales b) biofouling ergo microbial attachment on the membrane

surface followed by growth and multiplication under the presence of sufficient amounts of organic nutrients either

deposited on the membrane or existing in the feed and c) organic fouling caused by humic acid and other derivatives

of nominal organic matter [104,123,124].

When fouling occurs it leads to an increase in production cost, as there is an increase in energy demand, additional

labour for cleaning and maintenance use of chemical agents for cleaning and reduction in membrane life


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membranes enable separation in the molecular range with cut-off values ranging anywhere from 200 to 1000 Dalton

membranes having a smaller pore size, so that smaller organic molecules can be retained. When compared to

reverse osmosis, a lower retention is found for monovalent ions. The separation mechanism used in nanofiltration

can be explained in terms or charge and size effects of the membranes and the molecules interacting with the

membrane and the operating conditions (i.e. pressure) [127].

The driving force in nanofiltration separation is the pressure difference across the membrane, causing a volume flux

through the membrane. Molecules having sizes larger than the pore size of membranes cannot permeate through the

membrane and are consequently rejected or retained, while smaller solutes can permeate through the membranes.The mechanism that governs molecular separation by nanofiltration includes size exclusion. Size exclusion is a

sieving effect that rejects molecules with sizes larger than a pore size of a membrane on the other hand [45]. This

effect results in the removal of uncharged organic species that may be a result of size exclusion or a result of

differences in diffusion rates depending on the molecular size in a non-porous structure [48,127]. For example, of

arsenic removal from drinking water [127].

Uncharged solute transport occurs by convention due to a pressure difference and due to a concentration gradient

across the membrane. A sieving mechanism is responsible for the retention of uncharged solutes. Retention of

organic molecules is not only caused by steric interactions but also by electrostatic effects and by adsorption on the

membrane surface. Other mechanisms include convective transport through the membrane and diffusive transport of

adsorbed compounds through the membrane matrix [61].

For charged components an electrostatic interaction takes place between the molecules, described by the Donnan

effect, known also as Donnan exclusion mechanism. The charge effect, results in removal of mainly divalent and

multivalent ions. [45,127].The Donnan effect derives from the electrical interactions between charged ions in

solution and those on the membrane surface membrane could limit the transport of ions. Generally, the retention of

negatively charged ions by a charged membrane may be greater than if the membrane was uncharged, due to the

Donnan effect. When a charged membrane is put in a salt solution, an equilibrium occurs between the membrane

and the solution. Due to the membranes fixed charge, the ionic concentration in the membrane is not equal to those

in the solution. The counter ion, which has the opposite sign of charge to the fixed membrane charge, concentration

is lower in the membrane phase.


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enough. In other words, a negatively charged membrane has good selectivity toward mono- and divalent anions and

poor selectivity toward mono- and divalent cations [129].

Factors Affecting of the Separation Function in Nanofiltration

It is well known that the properties of the feed solution such as choice of solvent and non-solvent, polymer, and the

polymer concentration,(Fig.7) as well as the assets and structure of the membranes used do substantially affect the

final membrane performance. By changing one or more of these variables the thermodynamic and kinetic process

will fluctuate and thus produce different kinds of membrane structures that ultimately affect the membrane

performance [97,103-107].

Shear rate is one of the phase inversion process parameters that commonly influence the general formation of

membrane thickness, membrane porosity and pore radius. These morphology structures will eventually affect the

membrane separation performance in terms of its selectivity and productivity.

The retention behaviour of an organic molecule is also influenced by the charge effects of the membrane to the

solute. Depending on the nature of the membrane, whether is negatively or positively charged, a higher or lowerretention can be obtained, than expected at the base of molecular weight or on any other size parameter [94]. In

general, the influence of charge is large in membranes with small pores and small in membranes with large pores.

Nanofiltration membranes are usually quite hydrophilic and chemically modified to have a relatively high charge

compared to other membranes [118,130]. The membrane skin acts as the porous strainer and a thicker support layer

is employed underneath. The thin membrane skin for most thin film composite membranes carries a negative

charge. This is to minimize the adsorption of negatively charged foulants present in membrane feed waters and

increase the rejection of dissolved salts. The negative charge on the membrane is mainly caused by sulfonic or

carboxylic acid groups that are deprotonated at neutral pH [102].

The membrane surface charge is usually quantified by zeta potential measurements. pH has an effect upon the

charge of a membrane due to the dissociation of functional groups. The membrane charge depends on the pH of the

solution and on its isoelectric point (IEP). For many commercial nanofiltration membranes, the isoelectric point liesin the pH range between 3 and 6. Thus nanofiltration membranes are usually negatively charged in neutral or

alkaline conditions and positively charged in highly acidic conditions. The IEP of the membrane depends on the

type and concentration of electrolyte. Then similar membranes can show slightly different IEP depending on the

solution composition [45,105,106]. When a nanofiltration membrane is negatively charged solutes with higher


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The separation of ions according to their valence by electric repulsion force (Donnan exclusion) can be described by

equilibrium thermodynamics that allow calculating the chemical potential of the ionic component in the two phases

present, when an ionic solution is in equilibrium with a charged membrane [132].

If the concentration of the substances, in the feed is low and the membrane charge density is high, the Donnan

exclusion effect becomes very important, when increasing though feed concentration this exclusion becomes less

effective. It has been reported that the charge of the pores of the membrane might possibly have a significant effect

on membrane flux and rejection. The apparent pore size of the membrane was found to be significantly reduced at

higher pH values, since charged carboxylic groups gain an extended chain conformation due to electrostatic

repulsion. This expanded conformation reduces the apparent pore size (or pore volume) of the membrane and

thereby causes decreased flux and increased salt rejection [145].

Permeate flux differences have also been attributed to the correlation of the pH to the electroviscous effect. The

electroviscous effect is a physical phenomenon that occurs when an electrolyte solution is pressed through a narrow

capillary or pore with a charged surface. This effect refers to the back-flow of counter-ions and water in the double

layer adjacent to the capillary pore surface due to a streaming potential develops between the capillary ends [128].The electroviscous effect is least pronounced at the pore surface point at zero charge (or isoelectric point), where

double layer effects are negligible. At low pore surface changes, the permeability solution appears to exhibit

reduced viscosity when its flow rate is compared with a flow at a high pore surface charge [122].

Charged functional groups attract ions of the opposite charge inhibiting them from crossing the membrane [133].

Numerous studies have dealt with the influence of pH, chemical potential and osmotic pressure on the rejection of

organic acids in multi-component systems. Most of the research though has been done on the role of membranesurface charge on the rejection of organic acids by nanofiltration membranes in a single-solute solution. A decrease

in ionic rejection is often observed with increase in ion concentrations presumably due to charge shielding which

reduces the charge tends to become more negative as pH increases for many commonly used membranes [88].

There is still though a lack of understanding on the rejection of organic acids in complex wastewaters such as

anaerobic effluents by nanofiltration membranes [134]. Hydrophobic-hydrophilic interactions play a role in the

adsorption of certain compounds, highly polar compounds have been found to interact with membrane surfaces.

Water flux through nanofiltration is dependent upon the ability of water to form hydrogen bonds with the polymer

and solutes that form stronger hydrogen bonds with the membrane can partially disturb water molecules and reduce

flux [63,135]. Other factors that are important structural properties for rejection include steric hindrance, and the


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molecules than for larger molecules, so in general retention increases with molecular size [17]. Organic molecules

are removed by a sieving mechanism, based on the small pore of the membrane [118].

Molecular weight is the most accessible parameter that indicates the size of a molecule. Most studies focus on

molecular weight to obtain information about retention of uncharged molecules of nanofiltration. Organic

molecules, which are considered to be uncharged, are removed by a sieving mechanism, based on the small pore of

the membrane [129].

The membranes are often characterized by the molecular weight cut off (MWCO) which is the molecular weight of

a molecule which is retained for 90%. However this parameter offers only a rough estimate and quantification of theretention characteristics of a membrane for uncharged molecules. Researchers though [47, 73-75,141] have

demonstrated that the MWCO of a nanofiltration membrane is only partially correlated with rejection of the

compounds studied. As a single number, is only capable of providing a rough estimate of the sieving effect. MWCO

does not provide information on the retention for molecules having a molecular weight below the MWCO.

Electrostatic interaction between charged solutes and a porous membrane have been frequently reported to be an

important rejection mechanism.However this parameter offers only a rough estimate of the retention characteristics of a membrane for uncharged

molecules. Electrostatic interaction between charged solutes and a porous membrane have been frequently reported

to be an important rejection mechanism [57]. Moreover, the retention may differ for molecules with other chemical

structures or properties for example charge, polarity. Therefore other parameters had to be found in order to

calculate the retention of organic molecules [142,143].

Another parameter used to describe the rejection characteristics of the membrane is the degree of desalting. The

degree of desalting is commonly reported as the percent rejection of a 500-2000 mg L -1 of sodium chloride or

magnesium sulphate since MWCO is often manufacturer specific, the degree of desalting can be a useful parameter

in estimating the rejection of some compounds, as membranes with the same MWCO have significantly different

desalting degrees [144-146].

Porosity has also been regarded as another useful parameter in previous studies to estimate organic compoundsseparation. Porosity is commonly expressed as pore density (PD) or effective number of pores on nanofiltration

membranes. The membrane porous structure was the dominant parameter in determining the membrane

performance and solute rejection can be explained by membrane porosity parameters such as PD and the effective

number of pores (N) [108,124,127,141]. The pore structure of the skin layer can also contribute to the selectivity of


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This is explained by the Nernst- Planck Equationthat is usedto describe the electrochemical interactions betweenthe solutes in the solution and the nanofiltration membrane surface which can be either positively or negatively

charged (Fig.8). This equation is used to describe the ion transport through the charged membrane.

Separation of Volatile fatty acids using Nanofiltration

The recovery of volatile fatty acids is imposed from the occurring environmental constraints. In this context, a

membrane operation such as nanofiltration appears to be very attractive since the generation of effluents orbyproducts is expected to be significantly reduced [39].

Using synthetic and actual wastewater, nanofiltration showed high rejection rates for the selected organic acids at a

neutral and alkali pH region and at low operation pressure. Nanofiltration membrane revealed considerable

applicability as an advanced wastewater treatment processes for remaining organic acids in wastewaters not

withstanding technical challenges for the practical application remain [62].

Case Studies of Separation of Volatile fatty acids using Nanofiltration

Volatile fatty acids are generally weak acids and are dissociated with regard to the pH in the solution as follows:

+

+

HRpK

HR a where R, H and represent carboxylic group, hydrogen ions and acidity constant,

respectively. As rejection of weak acids and bases in highly pH dependent, their retention in the nanofiltrationprocess will be high in the ionized form. Thus, the organic acid rejection increases significantly at pH levels above

the acidity constant (pKa) but the rejection decreases at pH levels below, when the acids are in neutral form [62].

Rejection tests of volatile fatty acids have been performed using the membranes ES10 (Nitto Denko, Japan) and

NF270 (Dow Filmtech, USA) using solutions of organic acids and volatile fatty acids such as formic , acetic,

propionic, succunic and citric acid at different pH levels. Since the values of the chosen organic acids are below the

pH range of 3-5 an increase of the rejection observed at a pH between 4 and 9 for both membranes and this could be

explained by an increase in the degree of dissociation [62]. For ES10 (Nitto Denko, Japan) membrane, the rejection

of succinic acid (M.E. 118.09 Da) and citric acid (MW: 192.13 Da), was very high (>90%). These acids have a

molecular size very close to the MWCO of the tested membranes.


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acid remained over 90% irrespective, of operating pressure conditions. The level of applied pressure seems to have

little impact on the rejection of charged organic solutes that have a molecular weight larger than or close to the pore

size of employed membranes. On the contrary, the rejection of propionic, acetic and formic acid which have a

molecular weight much smaller than the molecular weight cut-off values of the selected membranes, increased

gradually as the applied pressure increased [63].

The rejection rate of formic acid by the NF270 (Dow Membranes, USA) membrane increased from 17% at 130 kPa

to 55% at 280 kPa. The increase in observed rejection with increasing pressure could be attributed to the increased

solvent flux at a higher pressure [48]. To determine the extent to which the rejection of an organic acid is influencedby feed concentrations, the selected organic acids were prepared at various concentrations such as 50,100, 250 and

500 mg L-1. The rejection by the selected membranes ES10 (Nitto Denko, Japan) and NF270 (Dow Membranes,

USA) was not highly affected as the concentrations increased, with the only exception being formic acid as in

higher concentration it was much smaller. A variety of ions, such as Cl-, SO42-, Na+ and Ca2+, exist in wastewaters,

including the effluent from anaerobic treatment. Research examining the effect of these ions on the rejection of

volatile fatty acids and this has shown the rejection of some organic acids in the nanofiltration process was reducedby hardness addition. This phenomenon can be explained by a decrease in electrostatic interaction between

membrane and solute, resulting from a reduced membrane surface charge due to an increased ionic strength in a

solution.

Theoretically, multivalent co-ions are retained better than monovalent co ions in the nanofiltration process. Despite

the fact that the absolute values, of the salt rejection may vary, the rejection sequence of ions by nanofiltration

membranes follow this pattern >>>>> F3NOClBr3HCO24SO

34PO according to the molecular weight

and the charge of the ions.

Subsequently, the rejection of organic acids existing in an ionized form at pH levels above pK a will be influenced

by the presence of salts in waters. For charged organic compounds with a molecular weight much smaller than the

MWCO value of the nanofiltration membrane, the rejection was influenced strongly by the presence of salts[45].The nanofiltration membrane NF270 (Dow Membranes, USA) shows the highest retention for all compounds.

Retention of organic molecules is not only caused by steric interactions but also by electrostatic effects and by

adsorption on the membrane surface. Other mechanisms include connective transport through the membrane and

diffusive transport of adsorbed compounds through the membrane matrix [61] Most of the micropollutants in alkali


20/49

[45]. During the nanofiltration process with the Desal 5-DK (Osmonics, USA), of the mixed solution of xylose and

acetic acid, the chemical reaction occurring which also would determine the dissociation constant. A tremendous

increase in acetic acid retention was noticed with increasing pH values. While the Stokes diameter of acetic acid is

0.38 nm which is approximately one third of membrane pore size (~1 nm) size exclusion alone was unable to

account for the high retention of acetic acid [45]. It was found, that the retention of acetic acid was negative at all

operation pressures when the solution pH was 2.9. The observed negative retention might be the result of the charge

attraction of acetate and the nanofiltration membrane or intermolecular interactions between acetic acid and xylose.

The majority of acetic acid (98.6%) was found to remain in neutral form. The Desal 5-DK (Osmonics, USA)

membrane used became positively charged at pH 2.9. Therefore the attractive interaction may facilitate the transportof negatively charged acetic acid. It is clear then, that the increased acetic acid retention obtained by the Donnan

exclusion. The highest acetic acid separation factor was 5.4 at pH 2.9 and 24.5 bar. When the pH was greater than

6.9 the acetic acid separation factor was less than one which means that the separation is not favourable [48]. The

separation of acetic acid by Desal 5-DK (Osmonics, USA) membrane can be divided by two groups, pH above 4

and pH below 4. At pH values of 4.9, 6.9 and 9.1 the trend for acetic acid is smaller than that of xylose. This result

shows the significance of charge effect on the separation of xylose retentions, interestingly at pH 9.1 [45]. Theretention of acetic acid was even higher than that of xylose despite that the molecular size of acetic acid is smaller

than that of xylose. This result shows the significance of charge effect on the separation not influenced by

increasingly applied pressure at pH 2.9. It is noted that the acetic acid retention was negative at pH 2.9 meaning that

the permeate was enriched in acetic acid [45]. A strong correlation was found between the xylose retention and the

permeate flux. Xylose retention was 25% at a permeate flux of 6 L m-2 h-1 and increased to 76% at 100 L m-2 h-1. In

contrast, the retention of acetic acid is not dependant on neither permeate flux nor concentration. There was anegligible acetic acid retention ranging from -6.8% to 0.87% at the initial concentration of acetic acid varied from 2

to 10 g L-1 [45]. The results demonstrated that solution acid retention increased from -6.8% to 90% at pH 2.7

because it was in its neutral form over range of the experimental values. Based on these results, separation of acetic

acid from xylose could be achieved by pH control. A high separation factor (>5) was observed when the system was

operated at high pressure and low pH [45].

Other experiments were conducted to investigate the effect of pH on the speciation and rejection of acetic acid,

glutaric acid, and 1, 2- dixydrobenzoic acid. These experiments were conducted with the membranes NF-90 and

NF-200 (Dow Membranes, USA). These organic acids are characterised by values within a pH 3 to 7. The effect of

the increasing pH of the liquid feed, on the rejection of organic acids was tested. This increase resulted in a


21/49

significantly lower by the NF-200 (Dow Membranes, USA) than the NF-90 (Dow Membranes, USA) [118,119]. For

acetic acid, rejection by both the NF-90 (Dow Membranes, USA) and NF-200 (Dow Membranes, USA) remained

relatively constant in a pH range between 7 and 9. Additionally, the rejection of all compounds is approximately thesame between the NF-90 (Dow Membranes, USA) and NF-200 (Dow Membranes, USA) at a pH of 9 even with

small differences in effective membrane surface charge and MWCO changes [118,119]. It has been hypothesized

that although adsorption can result in initial rejection, the adsorbed solute can partition and diffuse across a

membrane and reduce rejection considerably through partitioning in to permeate during long-term operation. After

the feed and permeate concentration stabilised the feed water pH was adjusted to 7 to 550 min of operation

[118,119].

Several studies have reported that increasing the feed water ionic strength especially in the form of divalent ions

(Ca2+ or / and Mg 2+) can decrease the membrane surface charge and subsequently result is a reduced rejection of

inorganic ions. Rejection experiments with target organic acids were repeated in the presence of 1mM calcium and

3 mM calcium added in the form of calcium sulphate to the feed water, of the organic acids tested, only the rejection

of acetic acid by NF-90 (Dow Membranes, USA) was reduced by calcium addition [118,119].Calcium additionreduced the rejection of acetic acid by the NF-200 (Dow Membranes, USA). It has been reported that membranes

with larger pores, like the NF-200 (Dow Membranes, USA) are affected more by inorganic ions than tighter

membranes. Since electrophoretic mobility, zeta potential measurements of the NF-90 (Dow Membranes, USA)

and NF-200 (Dow Membranes, USA) showed that the membranes become less negative in the presence of calcium

ions, the decrease in rejection by the NF-200 (Dow Membranes, USA) for acetic acid can be explained by a

decrease in electrostatic interaction between membrane and solute.

It is hypothesized that, charged organic compounds with a molecular weight close to the MWCO of a membrane are

less affected by decreased electrostatic interactions since steric exclusion also plays a dominant role in the rejection

of these compounds. The removal of charged organic compounds, with a molecular weight smaller than the MWCO

of a membrane, however can be affected by the presence of calcium ions and the reduced negative surface charge of

the membrane [118,119].

Separation of lactic acid from fermented broths using Nanofiltration

The idea of using a membrane, as a removal step, to improve the yield of lactic acid during submerged fermentation


22/49

(Aquious, United Kingdom) membrane, lactate rejection was higher for the former membrane at high pH values.

With membrane NF 70 (Dow Membranes, USA) lactic acid rejection was observed to be slightly lower than to

inorganic salt rejection. Lactic acid rejection was increased from a 35% to a 58%, when transmembrane pressureincreased from 10 to 40 bar, while inorganic salt rejection lied between 45% to 76% in a transmembrane increase of

10 to 40 bar. As a result, the recovery and the demineralization of lactic acid were unsuccessful [40].

For the membranes DK2540C (Osmonics, USA) and AFC80 (Aquious, United Kingdom) as the pH of lactic acid

solution used is 3.67, at pH values between 5.0 and 6.0, lactic acid is found in the dissociated form , whereas at pH

2.7, lactic acid is mainly in undissociated form.

The higher lactate rejection with increasing pH may be due to the electrostatic repulsion between lactate ions and

the membrane surface (both negatively charged) as the membrane charge becomes more negative when the pH

rises. For higher pH values both membranes, DK2540C (Osmonics, USA) and AFC80 (Aquious, United Kingdom)

are negatively charged and thus the repulsion between lactate and the membrane surface, is important due to the

Donnan exclusion effects. For both of the membranes lactic/lactate rejection increases with pH due to the

electrostatic repulsion between the ions and he membrane surface, but rejection of lactate/ lactic acid at the higherpH values increases [131]. Permeate flux was lower at higher concentrations as the osmotic pressure difference

across, the membrane increased with concentration (40 g L-1 and 80 g L-1 lactic acid) permeate flux decreased with

increasing pH [131].

For the membranes DK2540C (Osmonics, USA) and AFC80 (Aquious, United Kingdom), lactic acid rejection

increased with increasing pressure and pH, from 12-35% at pH 3.3 to 36-72% at pH 6.0. An increase in lactate

rejection has been achieved with pH for different nanofiltration and reverse osmosis membranes, made of cellulose

acetate and polyamide. As lactic acid concentration increases, the Donnan exclusion effect becomes less significant

and lactic acid rejection decreases regardless the pH and the type of membrane used. For Desal DK 5, (Osmonics,

USA) lactic acid rejection was low (35-58%) and similar to that of the inorganic salts present in the fermentation

broth (45-76%). The electrostatic effect is a limiting factor in the recovery of lactic acid by means of nanofiltration.

Depending on the charge of the membrane retention can be higher or lower than expected when based of molecular

weight or any other size parameter [131].

The lower lactic acid rejection cannot be explained by the difference in the size of the solute, but electrostatic

repulsion between membrane (Desal DK 5, Osmonics, USA), and anions present in the feeds. At pH 3 the organic


23/49

have an isoelectric point in the pH range of 3-6. So, at pH values higher than the isoelectric point the membrane is

negatively charged, whereas for pH values lower than the isoelectric point is a result of deprotonation of the

carboxylic groups [39].

It has been demonstrated that nanofiltration can successfully remove the divalent ions and any remaining sugars

before and after the conversion step. The separation though of glucose and sodium lactate mixtures was found to be

very poor, because the retention of glucose is decreased in the presence of sodium lactate [39].

Experiments were carried out with mixed solutions containing sodium lactate and salt (NaCl and Na 2SO4) in orderto determine the influence of salts on the retention of neutral and charged solutes. The retention of lactate is mainly

relied on electrostatic repulsions. The addition of a salt like NaCl results in a lower retention due to the screening

effect that makes the electrostatic repulsion weaker. The same phenomenon was noticed with as the permeation of

lactate increases in order to maintain electroneutrality on both sides of the membrane and negative retention can be

achieved. It was then demonstrated, that an increasing salt concentration results in lowering the neutral solute

retention while this decrease depends strongly on the nature of the added salt [39].

This phenomenon can also happen with charged species of which retention is also expected to decrease with

increasing salt concentrations, and it is called the screening effect. The retention of charged species mostly depends

on steric effects and on electrostatic interactions with the membrane. However the effect of the ionic composition

might be different due to the mechanisms governing both kinds of solutes are different.

Conclusions

VFA are substances of great importance and wide use in the industry nowadays. Their use expands in almost every

field of fabrication and recovery of chemical substances and products. As though most of them are currently

produced from petrochemical feedstock, serious efforts have been made to develop alternative, economical methods

of production.

Fermentation and anaerobic digestion of liquid or solid media performed by bacterial cultures have attracted most

attention. Recently other methods have been developed such as recovery of VFA from mixed effluent streams,

deriving either from industrial plants or of municipal, industrial and agricultural wastewater. In all the previous

methods, the separation and concentration of the VFA in other words their recovery is strong a challenge for the


24/49

A new promising method has been applied, nanofiltration. Nanofiltration is a pressure driven separation process,

which is easy to implement and relatively cost-effective. As separation principle it uses a combination of the sieving

mechanism with electrostatic interactions between the membrane surface and the charged molecules [103-107].

One potential drawback of nanofiltration is fouling. The performance of nanofiltration is in many cases, is

negatively influenced by the presence of foulants in the water to be treated. Membrane fouling is caused by

dissolved inorganic or organic components, colloids, bacteria or suspended solids. Fouling by inorganic

components occurs when the solubility of a mineral is exceeded during the filtration process mainly at the end of

operation [103]. Fouling by colloids and non-dissolved compounds can be avoided by an appropriate pre-treatment

such as microfiltration or ultrafiltration. Research has been done for the recovery of VFA using nanofiltration.

Numerous commercially available membranes have been used. These membranes are negatively charged and

fabricated mainly from polyamide. The results although promising, are also contradictory. This can be easily

explained from the fact of different chemical properties of the feed and the different operating conditions. In

addition, limited research has been done to effluents deriving from fermentation broths or wastewater. Research

though has to be conducted to establish the feasibility of nanofiltration as a separation technique suitable for VFA

recovery.

Finally the viability of these separations comes down to costs of operation of membrane systems. There is

considerable scope for low cost separations as demonstrated by the desalination systems and how these have

changed over the past 20 years where processing costs are below 3 kWh per ton water produced. The areas for

improvement include better membranes with high rejection of fatty acids, with high flux and low operating

pressures.

Acknowledgements

The authors would like to thank Dr. Esteves Sandra and Pr. Guwy Alan, Sustainable Environment Research Centre

(SERC), Glamorgan University for their valuable comments to the writing of the research summarised here. The

present work benefited from the input of Dr. Charalambidou Anna, Department of Humanities, School of

Humanities and Social Sciences, European University of Cyprus who provided helpful comments to the writing of

this manuscript. This project was supported by Low Carbon Research Institute (LCRI) project grant title H2

Cymru.

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Accepted Manuscript Complex Effluent Streams as a Potential Source of Volatile Fatty Acids