Bioresource Technology 102 (2011) 4076–4082

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Bioresource Technology

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The effect of acid pretreatment on the anaerobic digestion and dewateringof waste activated sludge

D.C. Devlin ⇑, S.R.R. Esteves, R.M. Dinsdale, A.J. GuwyUniversity of Glamorgan, Sustainable Environment Research Centre, Upper Glyntaff, Pontypridd CF37 4AT, Wales, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 September 2010Received in revised form 7 December 2010Accepted 8 December 2010Available online 21 December 2010

Keywords:BiosolidsMethane yieldOxitopPretreatmentWaste activated sludge

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.12.043

⇑ Corresponding author. Tel.: +44 0 1443 482227.E-mail address: dcdevlin@glam.ac.uk (D.C. Devlin)

Waste activated sludge (WAS) is difficult to degrade in anaerobic digestion systems and pretreatmentshave been shown to speed up the hydrolysis stage. Here the effects of acid pretreatment (pH 6–1) usingHCl on subsequent digestion and dewatering of WAS have been investigated. Optimisation of acid dosingwas performed considering digestibility benefits and level of acid required. Pretreatment to pH 2 wasconcluded to be the most effective. In batch digestion this yielded the same biogas after 13 days as com-pared to untreated WAS at 21 days digestion. In semi-continuous digestion experiments (12 day hydrau-lic retention time at 35 �C) it resulted in a 14.3% increase in methane yield compared to untreated WAS,also Salmonella was eradicated in the digestate. Dewatering investigations suggested that the acid pre-treated WAS required 40% less cationic polymer addition to achieve the same cake solid content. A costanalysis was also carried out.

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1. Introduction

Sewage sludge should be adequately stabilised prior to its usevia land application and this can be performed via different treat-ment options. Typically it has been achieved by composting, aero-bic treatment, lime treatment, incineration, chemical stabilisation,landfilling and anaerobic digestion (AD). Treatment of sewagesludge via AD has been carried out widely and technology has re-cently received increased interest in many countries due to therenewable energy it can produce.

In the UK alone 1.3 million tonnes (dry solids) of sewage sludgeare produced every year (Water UK, 2006) and water companiestreat over 60% of this sewage sludge using AD. Rudd et al. (2003)concluded that the most viable method of treatment was AD,which results in a product that can be used in agriculture. Theseauthors based their decision on the ‘best practicable environmentaloption’ for treating sewage sludge, for which they considered fourcategories: technical reliability, cost, environmental sustainabilityand environmental nuisance. Digested sludge, often known asbiosolids, contains many important nutrients and trace elementsvital to plant growth and it is often part of a farm managementplan. The safe sludge matrix is a voluntary code of practice be-tween the UK water industry, the Environmental Agency and theBritish Retail Consortium, which was introduced to reassure thepublic about the use of biosolids in agriculture and lays downtwo categories of sludge treatment defined on the log reduction

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of an indicator organism. The two categories are conventionallytreated sludges and enhanced treated sludges, which should havea 2 log and a 6 log reduction in Escherichia coli, respectively. Alongwith E. coli reduction enhanced treated sludge should be free ofSalmonella.

Treatment via AD results in a reduction of the amount sludgesolids for disposal and also a decrease in odour problems associ-ated with residual putrescible matter and at the same time greenenergy is produced in the form of biogas (Appels et al., 2008). Thisbiogas can be either upgraded to allow it to be directly used astransport fuel or injected into the gas grid or it can be desulphur-ised and dried then utilised in combined heat and power (CHP) sys-tems to provide the Waste Water Treatment Plant (WWTP) withelectricity and heat or export in the case of excess. Since this en-ergy is from a renewable source it also has the added benefit, ina number of countries, of attracting incentives for the energy pro-ducer. Any improvement in AD efficiency will therefore lead to afurther reduction of sludge for transport and disposal. In additionto this it is likely that biogas yield will increase and hence a greateramount of renewable energy produced resulting in better plantenvironmental performance and economies.

It is widely accepted that primary sludge is readily digestiblewhilst secondary sludge or WAS is difficult to digest (Lafitte-Trou-que and Forster, 2002). The physical state of microbial cells presentin WAS makes them an unfavourable substrate for microbial deg-radation as most of the organics are encased within microbial cellmembranes. The cells are protected from osmotic lysis because ofthe semi rigid structure of the cell envelope (Muller et al., 1998).This meant that often high HRTs are necessary (20–30 days) to

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obtain a 30–50% degradation efficiency of the organic dry solids(Appels et al., 2008). This is because the microbial cell walls con-tain glycan strands, which are cross linked by peptide strandsand hence the cross linkage causes the resistance to biodegrada-tion (Weemaes and Verstraete, 1998). Another reason for the lowerdigestibility of WAS is that it contains a significant amount ofextracellular polymeric substances (EPS) (Frølund et al., 1996)which are reported to be only 30–50% biodegradable (Li and Noike,1992) therefore making EPS more bioavailable could lead to animprovement of AD. Anaerobic digestion occurs via several stagesnamely hydrolysis, acidogenesis, acetigenesis and methanogenesis.When WAS is being degraded, the hydrolysis is the rate limitingstep (Li and Noike, 1992). Thus speeding up the hydrolysis stepis expected to increase the rate of the whole digestion processresulting in digesters with shorter retention times, and potentiallyincreased volatile solid (VS) destruction and methane yield.

Cell lysis has been referred as a possible method to releasethe intracellular organics and increase the rate and efficiencyof the digestion process (Pavlostathis and Gossett, 1986). It isalso possible that extracellular polymeric substances becomemore bioavailable (Appels et al., 2008). Sludge disintegrationhas been performed to solubilise and convert slowly biodegrad-able particulate organic materials to low molecular weight read-ily biodegradable compounds (Weemaes and Verstraete, 1998). Aclaimed reduction in the amount of WAS for disposal after beinganaerobically digested can be achieved by using a pretreatmentstage prior to the AD process e.g. Choi et al. (1997). Pretreat-ments can be carried out by employing one or a combinationof a number of methods. The methods can be broadly placedinto one of five categories: chemically e.g. Tanaka et al. (1997),thermally e.g. Bougrier et al. (2006), biologically e.g. Thomaset al. (1993), mechanically e.g. Kopp et al. (1997) and oxidationprocesses e.g. Shang and Hou (2009). All of these have been re-ported to have benefits including a higher biogas yield and im-proved VS destruction, and drawbacks including high capitalcosts and high energy consumption. Chen et al. (2007) investi-gated the effects on carbohydrate and protein solubilisation ofpH treatment between pH 4 and 11 but the study covered onlya limited range of acidification and did not cover the resultanteffects on anaerobic digestion of the pretreated samples or anyfurther assessments. Shang and Hou (2009) investigated the ef-fects of peracetic acid on subsequent AD but this pretreatmentshould be considered an oxidative pretreatment. There has beena lack of research related to the effects of acid pretreatment ofWAS on subsequent anaerobic digestion and dewatering. Fur-thermore, it is uncommon for the effects of a pretreatment tobe studied in terms of digestion, dewatering and pathogen kill.The understanding of all the effects resultant of carrying out apretreatment is nevertheless key if it is to be implemented atfull scale. This whole assessment has not been carried out foran acidification pretreatment and was therefore the objectiveof this study.

Laboratory methods are commonly used in order to assess theefficiency of the AD process. Anaerobic digestibility is defined asthe fraction of compound or compounds that can be converted intobiogas under anaerobic conditions. The greater the reduction in VSduring digestion the more digestible the sludge is said to be (Novakand Park, 2004). The most common method of quantifying degra-dability is by measuring the amount of gas produced. There aretwo main methods of determining gas production. The first is man-ometrically, which involves keeping the volume of gas constantand measuring the pressure increase, secondly are volumetricmethods, which keep the pressure constant and measuring the vol-ume of gas produced. Other less common methods in which digest-ibility can be determined is by measuring the amount of substratedepletion, the formation of intermediates or end products (Guwy,

2004). The Oxitop�-C (WTW, Germany) is a commercially availablemanometric measurement system and its use has previously beenpresented in the literature e.g. Scaglione et al. (2008).

The dewatering of sludge after digestion significantly reducessludge volume, which then leads to a reduction of the cost of trans-portation and reduction in the amount of space required for stor-age. Typical cake solids are 20–40% depending on sludge typesand dewatering method used (Werther and Ogada, 1999). Priorto mechanical dewatering, chemical conditioning is often carriedout to improve the dewaterability by flocculating the sludge parti-cles with flocculants (Feitz et al., 2001). This chemical conditioningis often a significant part of the WWTP running cost and hence anyreduction would be of benefit.

Dewaterability is often assessed by measuring the capillary suc-tion time (CST) and or measuring the specific resistance to filtra-tion (SRF). Chen et al. (2001) noted however that even thoughSRF and CST are well known and widely used methods of gaugingdewaterability they only measure filterability of a sludge, whichdid not correspond to the water content in the dewatered sludge.Chen et al. (2001) justify this by stating that some water in thesludge flocs was bound and it was this water that was difficult towithdraw. Thus, it was possible for activated sludge to be easily fil-terable yet not very dewaterable. Instead of these methods handpresses have been used in R&D studies carried out by a numberof water companies in order to simulate industrial processes i.e.belt presses, as best as possible.

This study investigated the effect of acid pretreatment usinghydrochloric acid on subsequent anaerobic digestion of WAS. Theeffect on carbohydrate, protein and chemical oxygen demand(COD) content was used to help determine the reasons for thechanges in digestibility. Preliminary testing of dewateribility effec-tiveness and pathogen kill was also performed in order to assessthe wider impact of conducting an acid pretreatment on WAS ina WWTP.

2. Methods

2.1. Sewage sludge

Thickened WAS and inoculum sludge was collected from a localWWTP serving a large metropolitan area. WAS sludge age was be-tween 15 and 20 days. The inoculum was collected directly fromthe outlet of a full scale anaerobic digester operating at a 15 dayhydraulic retention time (HRT) at 35 �C. Prior to use, WAS wasstored in a refrigerator between 3 and 7 �C whilst the inoculumwas stored at room temperature. Storage times were no longerthan 4 days for the inoculum and WAS was collected every 7 daysin order to maintain consistency and sample freshness. Prior to sol-ids determination the inoculum was passed through a 1 mm sieveto remove any large debris present. In the semi-continuous diges-tion studies, the WAS was diluted from typically 5.6–6.4% to 5% TSprior to feeding to ensure consistency.

2.2. Sample characterisation

Total and volatile solids were carried out according to (APHA,1998). Carbohydrate analysis was carried out using a phenol sul-phuric assay (DuBois et al., 1956) using D-glucose as a standard.Protein analysis was carried out using a RC-DC protein assay kit(Biorad, UK), which is based on Hartree (1972) using bovine serumalbumin (BSA) as a standard. Lipids were measured as per Blighand Dyer (1959) but using a centrifuge to separate solids afterextraction. COD was measured using an assay kit (Hach Lange,UK) according to the manufacturers’ instructions. Soluble carbohy-drate, protein and COD content were defined as those that passthrough a 0.45 lm syringe filter. All chemical characterisation

Fig. 1. Oxitop batch testing reactor.

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was carried out in triplicate. Gas composition was measured usinga Varian 4900 micro GC fitted with a molecular sieve 5A plot col-umn and a Porapack Q column to enable simultaneous measure-ment of methane, carbon dioxide, nitrogen and oxygen.Bicarbonate alkalinity (BA) was carried out as per Jenkins et al.(1983). Content of volatile fatty acids (VFAs) was determined asper Cruwys et al. (2002). E. coli and Salmonella was carried outusing IDEXX ColilertTM Quantitray 2000.

2.3. Acidification pretreatment

Acidification pretreatment was carried out by adding 37%hydrochloric acid (HCl) stepwise until the required pH wasachieved as measured by an electronic probe (Fisher Scientific,UK). Once the required pH was achieved, the samples were placedinto a refrigerator for 24 h prior to being neutralised i.e. adjusted tothe initial pH prior to acidification (6.8 ± 0.2) with 10 M sodiumhydroxide and were subsequently loading into the reactor. WASwas pretreated at six different pH levels as per Table 1. pH levelsare not exact due to the fact that 37% acid was used and it was veryeasy to over acidify the sludge especially at pH’s near to neutral. A37% acid had to be used despite this drawback, otherwise the solidscontent would have been affected more. The effect of acidificationon the removal of pathogens was also studied. Samples of WASwere acidified to pH 2 using HCl and left acidified for between 1and 24 h. After the time period the samples were neutralised andtested for E. coli and Salmonella.

2.4. Anaerobic digestion

Batch digestion was carried out in triplicate using the Oxitop�-C(WTW, Germany) system (Fig. 1), which had a total nominal vol-ume of 1 l. A sample of 0.4 g of VS of WAS was added to the reactoras well as 100 ml of inoculum. So that volumes were kept constantwithin the reactors all volumes were made up to 150 ml withdeionised (DI) water. The pH was checked to ensure it was be-tween 7.0 and 7.2, which it was on all occasions and therefore noadjustments were made. Once the reactors were loaded the head-space was purged with a stream of nitrogen for 60 s, to remove anyoxygen present, immediately after purging the reactors weresealed with a cap complete with a measuring head that screwsonto the reactor to provide an airtight seal and that also containsa pressure transducer with an upper limit of 330 hPa, to monitorthe pressure within the reactor. The reactors were continuouslystirred during the experiments with a magnetic stirrer.

Over the 21 day digestion period, 360 pressure readings weretaken i.e. once every 84 min. Pressure was released when above300 hPa. Pressure data were converted to volumes of gas usingthe ideal gas law according to Eq. (1)

Va ¼PhVh

Pað1Þ

where V is volume of biogas at 35 �C, P is pressure and subscripts aand h are atmosphere and headspace volume, respectively.

Table 1Acidification pretreatment experiments.

Experiment WAS finalpH

Volume of HCl added(ml/kg wet sludge)

Untreated 6.84 0pH 6 5.46 1.25pH 5 4.80 1.56pH 4 4.11 3.44pH 3 3.17 5.63pH 2 2.13 8.75pH 1 1.30 17.5

The total pressure increase and hence biogas volume measuredby the Oxitop�-C system included the biogas production generatedby the inoculum as well as the WAS test sample. Normalisationwas then performed i.e. the biogas produced from the inoculumwas subtracted.

In attempts to measure biomethane instead of the total biogasusing the Oxitop�-C system, carbon dioxide scrubbers were placedin the headspace of the reactor. Scrubbers assessed were sodiumhydroxide, potassium hydroxide and calcium hydroxide. These tri-als were unsuccessful.

Semi-continuous digestion was carried out using a bench scalecontinuously stirred tank reactor (CSTR) with a working volume of22 l, operating at a HRT of 12 days at 35 �C. Feeding was done man-ually once every 24 h. Online gas flow monitoring was recordedusing a desktop PC fitted with a data acquisition (DAQ) card anda custom monitoring program written using LabVIEW program-ming package (National Instruments, Newbury, UK) via a massflow meter (Cole Parmer, London, UK). pH and BA of the effluentwas recorded once a day along with gas composition. TS, VS andVFA content were analysed five times over a 12 day monitoringperiod after the reactors had been run for 3 complete HRTs(36 days).

2.5. Dewaterability assessment

To determine optimum polymer dose 150 ml of digested sludgewas placed into a 500 ml conical flask. 0.5% (w:v) of a cationic poly-mer (Zetag 78, Ciba) was gradually added whilst gently swirlingthe flask until coagulation occurred, the volume of polymer addedwas noted. For the dewatering assessment 150 ml of digestedsludge was placed into a conical flask and the optimum amountof polymer was added determined by visual inspection. The flaskwas then gently swirled until the sample coagulated. Once coagu-lated, the sample was placed into a hand press complete with filterpaper. Gradual increasing pressure was applied to the cake untilwater stopped flowing, at this point the cake was turned overand pressed again, and this was repeated six times after whichcake solids were determined.

3. Results and discussion

The characteristics of the WAS used in the batch digestion partof this study are shown in Table 2. The biogas production curves

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from the Oxitop�-C batch system are shown in Fig. 3. Table 2shows the breakdown of individual components in the sample.Adding up the individual components a total of 26.4 g/l is obtained,this is only 52% of the total VS. This discrepancy suggests that thecarbohydrate, lipid and protein assays are not detecting all thecomponents, possibly those within the cell wall and the ones with-in the cell content if the cell has not been ruptured.

It is also important to note that the carbohydrate and proteinsresults should be treated as indicators rather than real concentra-tions as they are based on colorimetric methods calibrated withglucose and serum bovine albumin standards. In addition, nucleicacids and humic substances making up part of the organic contentwould not have been measured.

Fig. 2 shows that the acid pretreatment results in the solubilisa-tion of carbohydrates, proteins and COD, the bars indicate onestandard deviation (SD). The results suggest that a degree of cell ly-sis or that EPS solubilisation may have taken place, which then re-sults in an improved biogas production since this solublecarbohydrate will be readily available to bacteria as shown inFig. 3. Other researchers have used only the solubilisation of CODto measure the degree of hydrolysis. Measuring carbohydrate andprotein individually may provide extra insight into the effect ofthe pretreatment process. This study continues to suggest thatthe increase in the soluble components is the reason for the in-crease in the rate of biogas production. The acid pretreatment istherefore contributing to an increase in the rate of hydrolysis,which has been stated to be the rate limiting step for WAS diges-tion. There is still room for further solubilisation as even in exper-iment pH1 the soluble carbohydrate content is only 0.85 g/l from atotal available of 6.2 g/l, but acidification achieves a 4 and a 6 timesincrease of soluble carbohydrates and proteins, respectively.

Along with the organic solubilisation it is also possible that or-ganic acids are being produced and since the pH was low, the

Table 2WAS characteristics without pre-treatment.

Parameter Value

TS 5.65%VS 4.56%pH 6.82Total protein 15.7 g/lSoluble protein 0.05 g/lTotal carbohydrate 6.2 g/lSoluble carbohydrate 0.2 g/lTotal lipid 4.5 g/lTotal COD 7.68 g/l

Fig. 2. Soluble components of the W

methanogens are being inhibited (Yu and Fang, 2002), once neutra-lised and fed into the digester these organic acids were readilyavailable and were consumed very quickly.

The pattern is less obvious in the case of protein, however ageneral trend occurs. Total protein content was 15.7 g/l and in gen-eral the solubilisation of protein increases with the severity of theacid pretreatment employed.

It can be seen that at the end of the 21 day digestion period thebiogas production curves have not completely flattened out sug-gesting that the WAS samples were still being digested, eventhough this is the case several points can be discussed. The resultsof the untreated, pH 6 and pH 5 experiments are not differentshowing that the pretreatment had little or no effect on the biogasyield. Experiments pH 4 and pH 3 showed a slight increase in bio-gas production whilst experiment pH 2 and pH 1 showed an in-crease of 17% and 32%, respectively, by the end of the 21 daytest. Perhaps more interesting is the rate at which the biogas isbeing produced. By day 7 the pH 1 experiment yields 400 ml of bio-gas per g of VS, which is more than the amount untreated sampleproduces in 21 days of digestion. The same yield of biogas isachieved at day 13 for WAS for pH 2.

Both the pH 1 and pH 2 experiments had significant effects onthe batch digestibility of the WAS but it took almost twice theamount of acid to acidify from a pH of 2 down to 1 as it did to acid-ify from a pH of 6.82 down to 2, this is due to the acid’s pKa value.To pretreat WAS at full scale to a pH of 1 would have significantcosts and also could prove to be problematic during digestion asthe amount of alkaline required to neutralise the pretreated WAScould cause digester inhibition e.g. addition of light metal ionssuch as sodium. Thus it was decided to investigate the effect of apretreatment that would result in a WAS of a pH of 2 on a semi-continuous anaerobic reactor system.

The reactors were operated and monitored for 3 HRTs i.e. a totalof 36 days to ensure steady state had been reached. The data pre-sented was gathered after reaching steady state condition. Fig. 4shows an example of daily biogas production for the two reactors.The biogas profiles were quite different in that the acid pretreat-ment resulted in an increased rate of digestion as shown by thehigher peak flow rate. The biogas production rate of the acid reac-tor dropped below that of the untreated reactor after between 11and 16 h, which suggests that the available substrate was reducingfaster than in the untreated reactor. It is highly probable that amore significant increase in digestibility improvement would havebeen observed for the acid pretreatment sample as compared tothe untreated sample if the HRT of operation would have been low-er than the 12 days.

AS sample after pretreatments.

Fig. 3. Biogas generated from batch digestion of untreated WAS and six different acid pH pretreatments.

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Table 3 shows operational data from the semi-continuous reac-tors. Even though the acid pretreated reactor was fed the samesubstrate as the untreated reactor its feed presents a slight increasein feed TS and VS. The increase in TS is due to the sodium hydrox-ide present as part of the pretreatment.

Fig. 4. Typical daily biogas production curves by the semi-continuous reactors.

Table 3Semi-continuous reactor conditions at steady state.

Parameter Units Number of

WAS loading rate g VS/l dayDigester feed TS % 3Digester feed VS % 3Digester feed tCOD g/day 3Digester TS % 5Digester VS % 5VS destruction %Digester tCOD g/day 3tCOD destroyed % 3Biogas production ml/g VS added 12Methane content % 12Biogas yield ml/g VS destroyedMethane yield ml/g VS destroyedBiogas yield ml/g COD addedMethane yield ml/g COD addedBiogas yield ml/g COD destroyedMethane yield ml/g COD destroyedpH 12Bicarbonate alkalinity mg CaCO3/l 12Total VFA mg/l 5

The acid pretreatment may break down the polymers intomonomers or oligomers, which allow an increase in the rate ofdigestion since the hydrolysis step has been partially carried out.Some of the monomers or oligomers produced may have been pre-viously unavailable to bacteria to break down into short chain fattyacids, which results in a higher biogas production. In addition to anincrease in the rate of biogas production, the acid pretreatment atpH 2 resulted in an increased methane production improvement of10.8% when considering the results per VS added and 14.3% ofmethane production improvement if considering per COD added,compared to untreated WAS. Although in the semi-continuousreactors the average biogas improvement is not statistically signif-icant given the spread of results, this increase in biogas productionis similar to that predicted by the Oxitop�-C system after 12 and21 days it predicted a 19.3% and 17.6% increase in biogas produc-tion, respectively. This shows that this batch system could be usedto screen these pretreatments and predict the trends that will oc-cur in a continuous system. Also the system allowed for six treat-ments to be evaluated in just 21 days where a semi-continuoustrial would take over double that time. The batch reactor operationdid not allow for adaptation of the inoculum to pretreated WAS

samples Untreated pH2

Mean SD Mean SD

3.1 0.2 3.2 0.25.0 0.3 5.4 0.43.7 0.1 3.9 0.188.2 15.1 88.2 15.13.8 0.6 4.1 0.22.5 0.1 2.5 0.132.7 0.7 34.4 0.855.6 12.1 53.2 11.836.9 10.2 39.6 11.1231 34 259 1460.9 0.8 60.2 2.6712 84 740 72434 81 446 69178.8 47.1 206.8 49.8108.9 28.7 124.5 30.0484.3 142.1 521.8 121.2294.9 86.5 314.1 73.07.24 0.05 6.96 0.053686 154 2325 12790 6 89 4

Table 4Component details from the semi-continuous reactors.

Component Untreated reactor pH 2 reactor

Feed WAS (g/l) SD Digestate (g/l) SD Feed WAS (g/l) SD Digestate (g/l) SD

Total carbohydrate 3.62 0.48 3.50 0.54 3.62 0.48 3.17 0.41Soluble carbohydrate 0.19 <0.01 0.12 0.01 0.84 <0.01 0.11 0.01Total protein 17.21 3.05 14.12 3.90 17.21 3.05 12.90 3.50Soluble protein 0.52 0.10 0.37 0.05 1.22 <0.01 0.11 0.04Total lipid 5.27 1.10 2.75 0.50 5.27 1.20 2.50 0.60Total COD 48.12 8.21 30.35 4.11 48.12 8.21 29.05 3.94Soluble COD 4.57 0.11 1.17 0.05 5.73 0.08 0.69 0.09

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samples. This however, did not seem to play an important role inthese type of pretreatments.

Table 3 shows that the VS destruction increased slightly from32.7% to 34.4%, although this was not statistically significant whenconsidering the spread of the results. These results were both with-in typical ranges or even higher than found in the literature fordigesters operating on WAS only e.g. Barber (2005) and Choiet al. (1997). It is also important to note that WAS of a high sludgeage (15–20 days) was used in these experiments, this makes theWAS harder to degrade (Dwyer et al., 2008). Biogas and methaneyields are also comparable to other published results. For example,Choi et al. (1997) reported a biogas yield of around 610 ml/g VS de-stroyed and a VS reduction of 24% from a continuous digester run-ning on WAS at a HRT between 20 and 30 days. Using a mechanicalpretreatment Nah et al. (2000) achieved 790–850 ml/g VS de-stroyed with a 30% VS destruction. Although the biogas yield ishigher than the present study, the VS reduction is lower and theauthors stated that no control test was carried out. Also the yieldsin terms of COD added and removed is in line with other research-ers e.g. Bougrier et al. (2006).

Table 4 shows the mean and standard deviation of analysis offive samples taken over 1 HRT at steady state. Acid pretreatmentresults in a higher percentage of soluble carbohydrates, proteinsand also COD. After anaerobic digestion, the amount of non metab-olised total carbohydrate and protein content is reduced whenusing a pH 2 pretreatment where as non metabolised total COD re-mains similar. After digestion the soluble non metabolised carbo-hydrate remained similar in both reactors where as the pH 2treatment reactor had less soluble protein and COD present. Thissuggests that the components that were solubilised were utilisedby the bacteria within the reactor.

Fig. 5 shows the effect of WAS acidification on the E. coli level. Itcan be clearly seen that the acidification leads to a reduction ofE. coli numbers present in the sample from 6.27 log down to 3.48log and therefore after 24 h of pretreatment almost a 3 log reduc-tion is reached. The samples from the pH 2 digester indicated how-ever an E. coli level of 5.44 log which suggests that E. coli is actually

Fig. 5. Effect of acidification on E. coli number in WAS samples.

regrowing or reactivating within the digester. It must be noted thatthe initial inoculum for the experiment was collected from a con-ventional anaerobic digester and also the reactors were not steril-ized prior to the experiment taking place. Although AD (whenoperated at a 12 day HRT and followed by a 14 day storage time)is widely considered to give a 2 log reduction in E. coli a study in1999 showed that it varied from 1.35 to 3.36 log when dealingwith primary sludge only AD (UKWIR, 1999). The current studyshows a 1 log reduction for a 12 day HRT with no storage stagewhich is in line with (Matthews and Assadi, 2003) who found a1.27 log reduction on a full scale plant, these researchers also con-cluded that in a CSTR it would be very unlikely to get an E. colireduction of more than 1.5 log due to the stirred nature, althoughSalmonella can be eradicated.

In addition, Salmonella was not found present in any sample ofacidifications of 1 h or more, it was however present in the blanksample. In the pH 2 pretreated WAS digester Salmonella was notpresent in 2 g of sample. In the control reactor i.e. untreatedWAS, E. coli levels decreased from 6.08 log to 5.01 log showing justover a 1 log reduction, Salmonella was present (in 2 g of sample)both in the feedstock and the digested sludge. This shows that apH 2 pretreatment is effective in the destruction of Salmonella.

Results of the dewatering assessment are shown in Table 5.Whilst the cake solids remained at around 12% in both digesters,the pH 2 pretreatment resulted in a 40% reduction in polymerrequirement as compared to untreated WAS from 67 ml/l downto just over 40 ml/l. The cake solids were low in this study com-pared to other research papers, the method used in the presentstudy was a preliminary method as although actual cake solidsmay not be representative of what may occur at full scale, the com-parative assessment is still relevant. The reduction in polymer de-mand could be due to a lower floc fractal dimension and a smallerproportion of fine particles. The acid could be interfering with thecharge of the sludge floc meaning that large colloidal sludge flocscannot form (Feitz et al., 2001). Acid pretreatment may have re-duced the overall negative charge of the sludge particles allowinga greater effect from the polymer.

Two quotations for acid and alkali chemical costs from commer-cial suppliers for the pH 2 pretreatment were obtained for bulkpurchase (£445/tonne NaOH and £441/m3 HCl) as well as one quo-tation for polymer (£1750/tonne) and a cost analysis was per-formed. This analysis does not consider capital costs related toadditional storage and pumping when implementing such pre-treatment neither the potential reduction of digester size due to

Table 5Cake solids after dewatering and related polymer dosing.

Reactor Cake TS (%) Polymer requirement (ml/l)

Mean SD Mean SD

Untreated 11.8 0.4 67 2pH 2 12.2 0.2 40 1

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a possible decrease of HRT. The financial analysis considers onlyoperational costs (i.e. acid/alkali dosing and reduction of polymeraddition) and revenue for the additional biogas generated. The to-tal cost of the chemicals for the pH 2 pretreatment and then sub-sequent dewatering was £5.28 per tonne of WAS (wet weight)compared to £0.58 for the untreated reactor. Even when consider-ing the case that the biomethane is being injected into the gas gridand hence the £0.04/kWh renewable heat incentive is beingclaimed along with the £0.03/kWh sales revenue (DECC, 2010),the pH 2 pretreatment only yields an extra £0.61p per tonne ofWAS digested, which is insufficient to counteract the additionalchemical costs. The digestate VS and cake solids are similar andtherefore similar transport costs will apply. By examining thedigestion profiles for both the batch and semi-continuous diges-tions there appears to be possibility of an increased WAS through-put as the reactor could be able to run at a lower HRT and stillachieve the same or enhanced methane production and VSdestruction or that digester could be of smaller size, which wouldin turn lead to a decrease in capital as well as operational costs, thisis indicated by the increase flow rate of biogas.

4. Conclusion

A pH 2 pretreatment results in an increase of soluble carbohy-drate, protein and COD in WAS. During batch digestion this resultsin a higher biogas yield. This was also true in semi- continuousreactor operation with an average 14.3% increase in methane yieldcompared to untreated WAS. The rate of gas production was in-creased suggesting higher throughput may be possible. Digestatefrom a pH2 digester has a 40% lower polymer requirement priorto dewatering and is absent from Salmonella. An initial economicevaluation for this pretreatment concluded that at present it wasnot cost effective.

Acknowledgements

The authors would like to acknowledge the financial support ofThames Water Utilities Ltd and the Engineering and Physical Sci-ences Research Council (EPSRC). Also Dwr Cymru for providingsewage sludge.

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