Accepted Manuscripts Recovery of volatile fatty acids (VFA) from complex waste effluents using...

8/9/2019 Accepted Manuscripts Recovery of volatile fatty acids (VFA) from complex waste effluents using membranes Water Science&Technology

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Recovery of volatile fatty acids (VFA) from complex waste effluents

using membranes

M.-P. Zacharof*a,b, R.W. Lovitt*a,b

a Centre for Complex Fluid Processing (CCFP), College of Engineering, Swansea University, Talbot building, Swansea, SA2 8PP, UK

([email protected] [email protected])

b Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Talbot building, S

wansea

University, Swansea, SA2 8PP, UK ([email protected], [email protected])

Abstract:

Waste effluents from anaerobic digesters of agricultural waste were treated with a range of membranes including

microfiltration and nanofiltration to concentrate volatile fatty acids. Microfiltration was applied successfully to produce

sterile, particle free solutions with a VFA concentration of 21.08 mM of acetic acid and 15.81 mM of butyric acid. These,

were further treated using a variety of nanofiltration membranes (NF270, (Dow Chemicals, USA), HL, DL, DK, (Osmonics

, USA), LF10 (Nitto Denko, Japan) achieving retention ratios, up to 75% giving retentates up to 53.94 mM acetate and

28.38 mM butyrate. DK and NF270 membranes were identified as the best candidates for VFA separation and concentration

from these multicomponent effluents, both in terms of retention and permeate flux. When the effluents are adjusted to alkaliconditions highest productivity, retention and flux was achieved at pH 7 at higher pH there was a significant reduction in

flux.

Keywords: acetic acid; butyric acid; effluents;nanofiltration; volatile fatty acids; retention

INTRODUCTION

Volatile fatty acids (VFA) have important uses, as chemical intermediates and are central to the organiccarbon cycling on the planet (Zacharof & Lovitt, 2013). Most of the environmental organic carbon is

metabolised through VFA intermediates either to carbon dioxide (CO2) and methane (CH4)

anaerobically, or CO2and water in oxidative systems (Rittman & McCarty, 2001). Either way, these

acids, especially acetic, are key intracellular and extracellular metabolic intermediates. Consequently, if

carbon could be recovered in the form of VFA, this could represent an alternative, sustainable source of

carbon based chemicals for industrial use, as these can be generated and recovered from organic

degradation processes, such as fermentation and anaerobic digestion. Furthermore, these acids can be

used as a substrate for a number of interesting biotransformations for sustainable production ofchemicals (Popken et al. 2000).

The extensive industrial development of petroleum based fuels and chemicals has significantly

contributed in environmental pollution and climate change, as well as to intensive use of non-renewable

natural resources. Driven by a low carbon economy, dedicated to reduce carbon emissions and develop


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The separation of organics acids from digested or fermented effluents or the discharged waste streams

of these processes is not a straightforward process considering the complex physicochemical nature of

these streams and the concentration of the acids in them. Often these effluents demand extensive pre-treatment to make further processing workable (Masse et al., 2008)

Within this context, membrane filtration can offer a feasible option towards a cost effective

fractionation and recovery of VFA. Nowadays, there is a complete range of membrane filtration

technologies covering microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse

osmosis (RO) which are employed to clarify, fractionate , desalt and concentrate salts and organics

(Bellona et al., 2004). The low molecular mass and the chemical properties of the VFA (Bouchoux et

al., 2005, Choi et al., 2008), makes NF represents an attractive choice. NF employs numerousmechanisms of separation including size (molecular weight), electrical interactions (Donnan effect) and

dependence of pressure (Pronk et al., 2006) while RO can be used to concentrate these valuable

substances.

A diverse range of NF membranes made of different materials, are commercially available. These are

in several membrane arrangements, possessing different pore size, membrane charge; permeability and

productivity in terms of permeate flux and retention of substances. Therefore selecting a suitable

membrane for waste effluent filtration and recovery of acids is not a straightforward process (Han &

Cheryan, 1995). The complex nature of effluents, in terms of viscosity, solids content, and chemistry

can influence the electrical charge of the membrane surface. The choice of the appropriate NF

membrane is therefore an engineering challenge. This can be addressed by testing the membranes

suitability for the selected process at first instance in bench scale.

Consequently, this paper reports on the investigation of the performance of five NF membranes on

enrichment and concentration of VFA, namely acetic and butyric acids from pretreated complex wasteeffluents (agricultural sludge) (Figure 1).These were tested using characterising solutions and treated

digested agricultural sludge to select the most appropriate membrane that would have simultaneously

high permeate flux and acids retention (filtration productivity). The waste effluents were pretreated by

dilution, sedimentation and sieving for the removal of large particulate material such as straw or stones.

Further removal of these particulates and formulation of a sterile effluent suitable to be further

processed using NF, was achieved using MF. The microfiltered effluent was then nanofiltered. The

influence of pH on the separation of VFA was then investigated and the efficiency of NF and as a

separation and concentration method was evaluated in terms of VFA retention and permeates flux.

MATERIALS AND METHODS

Materials

W Effl


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Methods

Microfiltration of agricultural sludge

The pre-treated effluent was then processed through a cross-flow microfiltration unit, employing a

Membralox ceramic filter element (-Al2O3) -with 19 channels of 3.70 mm diameter each and length of

1016 mm- monolith microfiltration (pore size 0.2m, effective membrane area 0.22m) membrane, able

to withstand a pH range between 0 to14, fitted in stainless steel module, commercially available from

Pall (Portsmouth, UK). The filtration unit consisted of a 100 L stainless steel vessel linked via 5 m of 1

inch stainless steel piping arranged in two fluid loops each powered by a centrifugal pumps. A detailed

description of the unit can be found elsewhere (Gerardo et al., 2013). The membrane was characterisedusing tap water.

Analysis of dry matter content and physicochemical characteristics

Total solids (TS, g/L) and total suspended solids (TSS, g/L), were determined according to standard

methods for the examination of water and wastewater published by APHA, AWWA and WPCF 20th

Edition, 1998. Particle size distribution (PSD) of sludge samples was determined by light scattering

technique using Mastersizer 2000 (Malvern, UK), the zeta potential was determined by the Zetasizer(Malvern, UK), the conductivity and salinity of the samples were measured used a conductivity meter

(Russell systems) calibrated with a standard solution of 0.1M of KCl. Acetic and butyric acid were

analysed using head space gas chromatography, VARIAN ProsStar GC-3800 (USA) fitted with flame

ionization detector (FID) , connected with a hydrogen generator (UHP-20H NITROX, Swan Hunter,

UK), with air supplied and helium used as a carrier gas, equipped with a Nukol, fused silica high-

quality coated polymide capillary column 15 m x 0.32 mm I.D., 0.25 m column, using the following

protocol, of a total holding time of 15 minutes, a gas flow rate of 30ml/min and a pressure of 10 psi and an

FID temperature of 220 C as described by Sigma-Aldrich GC Supelko-Nukol columns manual.

Nanofiltration agricultural sludge and synthetic solutions

The membranes used in this study are described in Table 1 with details provided by the manufacturers.

A bench membrane apparatus (stainless steel stirred cell unit, Sterlitech HP4750, Kent, WA, USA) was

used for the filtration of the solutions operated batchwise using high pressure nitrogen gas (Figure 2).

The system was comprised of a stirred cell unit of 200 ml maximum process volume, a magnetic stirrerand a membrane filter with an effective area of 14.6 cm. Each membrane was characterised under a

range of different pressure conditions varying between 0 to 2000 kPa, using sterilised distilled water

(Millipore RiOs) and 10mM phosphate buffer (KH2PO4) (pH 6.5) (Sigma-Aldrich, UK). Permeate flux

was determined over a range of pressures. During the experimental trials with characterising solutions

and waste effluents the cell unit was pressurized by compressed nitrogen up to 1500 kPa the stirrer


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Determination of Permeate Flux

The permeate flux (J, L/m2

h) was defined experimentally for each membrane is calculated accordingthe following formula

=

=

mA

dt

dV

mA

fQ

permeateJ

where Qf is the volumetric flow fate,Am is the membrane area (m2

) , Vis the volume feed (L) and tistime (h) (Marcel , 1996).

Determination of Retention

The retention percentage (R%) of acetic and butyric acid by each membrane is calculated according the

following formula

where Cp,i is the observed concentration (mM) of the acid in the permeate and Cr,iis the observed

concentration (mM) of the acid in retentate (Van der Bruggen et al.1999).

RESULTS AND DISCUSSION

Pretreament and microfiltration of agricultural sludge

The agricultural waste digester effluent was found to contain a significant amount of acetic and butyric

acid (Table 2) however, being rich in solid particulates, it was pre-treated using dilution and

sedimentation described above. The pretreatment enhanced the removal of the larger solids in the

sludge (


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Nanofiltration of characterising solutions and pretreated agricultural waste effluents

The driving force in NF separation is the pressure difference across the membrane, causing a liquid fluxthrough the membrane. In first instance, the mechanism governing the separation function of this process,

can be explained as follows, 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. Prior to the use of the selected NF membranes, for separation and

partial concentration of the acids from the agricultural sludge, they were tested for membrane permeability

using distilled water and phosphate buffer (10mM, pH 6.5) solution, in an effort to analyze the behavior of

the membranes when incorporated in the stirred cell unit. For all membranes, flux values increased linearlywith increasing pressure.1500 kPa where selected as the operating pressure for the experimental trials

achieving high flux (Table 3). Each membrane though gave different flux values for the characterizing

solutions (deionised water and phosphate buffer), with highest flux being achieved by HL and smallest by

LF10. For all the membranes, highest flux was achieved with deionised water than phosphate buffer,

suggesting that the use of ions might enhance the negative electrical charge on the membrane surface,

consequently reducing the flux (Mandale & Jones, 2008).

Several studies have investigated the separation of acids from mixtures using NF, mainly fromsynthetic binary and tertiary solutions of hexoses, salts and acids (Bouchoux et al.2005) or fermented

broths (Gonzalez et al.2008), while limited research has been conducted on anaerobic digestates (Kim

et al. 2005). Digested waste effluents are expected to provide a significant amount of VFA, in this case

being 21.08mM of acetic acid and 15.81 mM butyric acid have been identified.

The pretreated microfiltered effluents (pH 8.25) were filtered using all the five membranes at 1500 kPa

pressure. Highest flux is found in DK membrane (Table 3), while the permeate flux was significantly

lower when compared with the flux achieved during the behaviour characterisation of the membranes

(Table 3). This phenomenon can be attributed to the sludge content of insoluble particulates (Table 2),

colour and negative charge. The alkali pH of the effluents enhanced the electronegativity of the

membrane surface as well as possibly influencing the charge of the acids.

Retentate solutions were enriched showing higher concentration than the starting solutions suggestingthat the membranes do successfully reject the acids. The retention of acids varies between 24.40% to

72.23% (Table 4), with HL having the smallest retention, while highest retention for acetic acid is

achieved with LF10 and for butyric acid with NF270. Among the five membranes, promising results

are achieved with NF270, DK and LF10; however LF10 offers significantly small flux, consisting its

use, ineffective and uneconomical due to the high energy demand to maintain the high pressure

necessary for processing the effluents.

Effect of pH on retention of organic acids


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interactions between the solutes (Figure 3), with LF10 is being the tightest membrane, offering the

smallest flux regardless the pH conditions and could be correlated with higher retention of acids

achieved for both acids when the pH is adjusted above 7.0. (Figure 4 a, b).

Figure 2, shows the effect of pH on the retention of acetate and butyrate using the five different

membranes. As the pH becomes more alkali, the retention of the acids is increasing. For both acids in

all the membranes, at pH 7 and above retention becomes strongly positive varying between 13.36 to

72.23% for acetic acid (Figure 4 a) and 27.67 to 71.36% for butyric acid (Figure 4 b). The varying

positive retention ratios between each membrane when the same conditions are used can be possibly

attributed to the different characteristics each membrane acquires in terms of pore size and material of

fabrication. The variations in the retention can be attributed due to differences in molecular weight aswell as to the different chemical properties of each acid for example its disassociation constant. Several

researchers (Manttari et al.2004, Manttari& Nystrom, 2006) have correlated the disassociation ability

of each acid with its electrical charge in this case becoming highly negative consequently being

repelled by the negatively charged membrane and consequently being better retained (Han & Cheryan,

1995). Taking into account that the concentration of the acids in the solutions might be a decisive factor

on the separation behaviour of the membranes (Bellona et al.2004), if the concentration of the acids in

the feed is higher possibly the retention of the acids would be higher, if the same alkaline treatmentduring filtration is applied.

Although size exclusion was definitely a determining factor on the separation behaviour of the

membranes, retention of organic molecules is not only caused by steric interactions but also by

electrostatic effects including convective transport through the membrane and by adsorption on the

membrane surface and diffusive transport through the membrane matrix. For charged components an

electrostatic interaction takes place between the molecules, described by the Donnan effect, known also

as Donnan exclusion mechanism (van der Bruggen et al., 1999)

These phenomena could be of interest in the biotechnological production of VFA, since commonly, the

acids occur in a relatively dilute mixture of salts, proteins and carbohydrates (Danner & Braun 1999,

Bouchoux et al. 2005, Gonzalez et al.2008). If a concentrating and separating treatment step can be

applied for example UF, acids can be successfully separated from the salts and concentrated in a semi-

purified form with an important reduction in cost.

Interestingly, the phenomenon of negative retention is observed in acidic pH (4.0, 5.5) for butyric acid,for DL membrane. Negative retention is a phenomenon often occurring in nanofiltration especially

when solutions containing salts are filtered such as brines (Mandale & Jones, 2008). In such a system, a

strongly negatively charged solute is better repelled from negatively charged membrane; those divalent

ions will be better retained than monovalent ions. Possibly this phenomenon is occurring in this system

with acetic acid being strongly charged.


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In the current use of anaerobic digestion in the industry, considering the fact that the acids are used as

intermediates to produce methane consequently therefore their concentration might not be as high as if

they were produced biotechnologically, their recovery could be combined with the caption of otheruseful platforms chemicals such as ammonia and phosphate (Zacharof& Lovitt 2012, Gerardo et al.

2013) is also possible. It has been pointed out that farming waste effluents do represent an

environmental hazard as well as a good source virtually in abundance of useful nutrients and metals.

Developing a complete recovery strategy for these substances, with a waste treatment system placedin

situcould be of great benefit for the industry.

NF can be used as a method of isolation and recovery of VFA from complex effluent streams, provided

a pretreatment scheme that will remove coarse particles, so the effluents can be easily filtered. Alkaliconditions enhance the isolation and retention of VFA, with DK and NF270 representing the best

option among the five membranes tested. These findings show potential and could be applied to the

biotechnological production of VFA and their recovery.

Pretreatment and microfiltration of digester effluents consist the digester effluents able to beprocessed through nanofiltration membranes and recover VFA

The use of alkali treatment, especially at points of 8.5 and 9.0 of the digester effluents duringnanofiltration enhanced the retention of the VFA for all the 5 membranes,

Among the membranes tested, LF10 had the highest retention results for acetate and butyratewith of 72.23% and 69.74%, respectively at pH 8.5, followed by NF270 of a 57.23% and

69.74% and DK of a 57.23% and 45.18%.

NF270 and DK have a flux rate of 15.40 and 16.49 L/m2h at pH 8.5 while LF10 has a flux rateof 6.40 L/m

2h, being unsuitable for separation of VFA at this stage. LF10 could be possibly

used to further concentrate VFA after these have been successfully separated by NF270 and

DK.

Acknowledgements

This project was supported by Low Carbon Research Institute (LCRI) project grant title Wales H2

Cymru. The authors would like to thank Dr. Paul M. Williams and Dr. Stephen J. Mandale for their

excellent advice during the experimental trials of this project.

REFERENCES

Bellona C., Drewes J. E., Xu P. & Amy G. 2004 Factors affecting the rejection of organic solutes

during NF/RO treatment-a literature review Water Res.38, 2795-809.

Bouchoux A., Roux-de Balman H. & Lutin F. 2005 Nanofiltration of glucose & sodium lactate

solutions: Variations of retention between single- & mixed-solute solutions J Mem Sci 258 123-32


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Han I. S. & Cheryan M. 1995 Nanofiltration of model acetate solutions.J. Mem. Sci. 107, 107-13.

Kimura K., Iwase T., Kita S. & Watanabe Y. 2009 Influence of residual organic macromoleculesproduced in biological wastewater treatment processes on removal of pharmaceuticals by NF/RO

membranes. Water Res.43, 3751-8.

Kim J.-O., Kim S.-K., Kim R.-K. 2005 Filtration performance of ceramic membrane for the recovery of

volatile fatty acids from liquid organic sludge,Desal, 172, 119-127.

Mandale S. & Jones M. 2008 Interaction of electrolytes & non-electrolytes in nanofiltration . Desal.

219, 262-71.

Manttari M. & Nystrom M. 2006 Negative retention of organic compounds in nanofiltration. Desal.

199, 41-2.

Manttari M., Pekuri T. & Nystrom M. 2004 NF270, a new membrane having promising characteristics

& being suitable for treatment of dilute effluents from the paper industry.J. Mem. Sci.242, 107-16.

Marcel M. 1996 Basic Principles of Membrane Technology, Kluwer, Dordrecht

Masse L., Masse D. I. & Pellerin Y. 2008 The effect of pH on the separation of manure nutrients with

reverse osmosis membranes.J.Mem. Sci. 325, 914-9.

Masse L., Masse D. I., Pellerin Y. & Dubreuil J. 2010 Osmotic pressure & substrate resistance during

the concentration of manure nutrients by reverse osmosis membranesJ.Mem.Sci.348, 28-33.

Popken T., Gotze L. & Gmehling J. 2000 Reaction kinetics & chemical equilibrium of homogeneously

& heterogeneously catalyzed acetic acid esterification with methanol & methyl acetate hydrolysis.Indust. Eng. Chem. Res. 39, 2601-11.

Pronk W., Palmquist H., Biebow M. & Boller M. 2006 Nanofiltration for the separation of

pharmaceuticals from nutrients in source-separated urine. Water Res. 40, 1405-12.

Rittmann B. E. & McCarty P. L. 2001 Environmental Biotechnology, Principles & Applications.

McGraw-Hill, Singapore.

Van der Bruggen B., Braeken L. & Van de Casteele C. 2002a Evaluation of parameters describing flux

decline in nanofiltration of aqueous solutions containing organic compounds.Desal.147, 281-8.

Van der Bruggen B., Schaep J., Wilms D. & Van de Casteele C. 1999 Influence of molecular size,

polarity & charge on the retention of organic molecules by nanofiltration. J.Mem.Sci. 156, 29-41.


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Table 1: Membranes characteristics provided by the manufacturers

Characteristics MembranesManufacturer General Electric -

Osmonics USA

Dow FilmTech USA Nitto Denko Japan

Model HL DL DK NF 270 LF10

Distributors Sterlitech Corporation

http://www.sterlitech.com

Desal Supplies

http://www.desal.co.uk

SOMICON AG WKL

http://www.somicon.com

Material Thin film composite

piperazine based

polyamide microporouspolysulfone

Thin film composite-

Aromatic polyamide

Thin film composite

Polyvinyl alcohol-

aromatic cross linkedpolyamides

Applications Water Softening, Acid Purification, Detergent removal, Heavy metal removal

Geometry Flat Sheet Flat Sheet Flat Sheet

Effective Membrane area (cm2) 14.60 14.60 14.60

Flux rate (L/m

2h) @689 kPa 66.3 52.7 37.4 122.0 11.9

Charge (at neutral pH) Negative

pH 2-10 2-11 3-10 2-10

Ion rejection (%) 98 96 98 97 99.5

MWCO 150-300 150-200


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Table 2:The effect of pretreatement and microfiltration on the physical characteristics and chemical composition

of the anaerobically digested agricultural sludge. The collected samples were diluted 100 times with deionised water

and measured in a 1 cm light path1

Parameters Agricultural Sludge

Untreated

Sludge

Treated

Sludge

Microfiltered

(0.2m)Sludge

Retentate

Microfiltered

(0.2m)Sludge

Permeate

Total Solids (TS, g/L) 15.13 11.99 10.40 5.15

Total Suspended Solids

(TSS, mg/L)

612.50 252.60 258.00 190.00

Conductivity (mS/cm) 9.37 9.11 9.01 8.3

Zeta Potential (mV) -33.25 -30.06 -29.60 -24.2

Sizing (m) 27.17 13.97 13.49 4.93

Optical Density (580nm1) 0.86 0.34 0.27 0.10

Concentration mg/L mmols/L mg/L mmols/L mg/L mmols/L mg/L mmols/L

Acetic Acid 1650.17 27.48 1464.02 24.38 1083.30 18.04 1265.85 21.08

Butyric Acid 1781.58 19.22 1666.16 18.91 1163.93 13.21 1393.02 15.81


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Table 3:The influence of membrane type on permeate flux of deionised water, phosphate buffer solution and

standardised anaerobically digested fluid using a variety of nanofiltration membranes at 1500 kPa operating

pressure.

Permeate Flux (L/m h)

Solutions DeionisedWater

Dihydrogen Orthophosphate

Solution (10mM)

Microfiltered

(0.2m) Sludge

Permeate

pH 7.2 6.5 8.25

Membranes DK 69.61 27.54 16.49

DL 84.04 34.43 14.91

HL 121.43 82.97 14.37NF270 61.66 20.21 15.40

LF10 15.95 06.78 06.00


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Table 4: The effect of membrane type on acetate and butyrate from standardised permeate derived frommicrofiltered digested agricultural sludge (see Table 2) at 1500 kPa. Initial concentration in the feed (pH 8.25) is21.10 mM and 15.81 mM of acetic and butyric acid respectively.

Membranes Acids

Acetic Acid Butyric AcidPermeate

Concentration

(mM)

Retentate

Concentration

(mM)

Retention

(%)

Permeate

Concentration

(mM)

Retentate

Concentration

(mM)

Retention

(%)

DK 17.27 40.38 57.23 10.86 19.81 45.18

DL 14.25 26.49 46.22 13.61 20.76 34.44

HL 20.09 26.57 24.40 8.58 14.28 39.92NF270 14.00 29.56 52.64 8.03 26.54 69.74

LF10 14.98 53.94 72.23 10.74 28.38 62.16


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Table 5: The effect of pH on permeate flux of standardised anaerobically digested fluids using a variety ofnanofiltration membranes. The filtration fluids were derived from microfiltered sludge (see Table 2)

Permeate Flux (L/m2h)

Solution Microfiltered (0.2m) Sludge Permeate

pH 4.0 5.5 7.0 8.5 9.0

Membranes DK 21.48 21.42 17.64 16.49 02.09

DL 18.33 17.92 16.78 14.91 05.06

HL 25.48 22.55 20.04 14.37 11.42

NF270 21.70 20.75 19.05 15.40 03.04LF10 12.09 13.35 05.44 06.00 04.14


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Figure 1:Processing scheme for the recovery of VFA from pretreated complex waste effluents (agricultural sludge)


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Figure 1: Schematic representation of the high pressure stirred cell unit [1]nitrogen cylinder, [2]pressure regulator valve, [3]pressure indicator, [4] stirredcell unit equipped with membrane disc,[5] magnetic stirrer [6], stirring plate [7], permeate collection vessel [8]electronic scale,, [9]personal computer


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Figure 3: Nanofiltration using a variety of membranes of permeates derived from microfiltration of agricultural sludge (see Table 2)


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Figure 4[a, b]:The effect of pH on VFA retention (a) acetic acid (b) butyric acid of a variety of NF membranes using standardised anaerobically digested

fluids. The filtered fluids are permeates derived from microfiltration of agricultural sludge (see Table 2)

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