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Process performance of high-solids batch anaerobicdigestion of sewage sludgeXiaocong Liaoa, Huan Liab, Yingchao Chenga, Nan Chenc, Chenchen Lia & Yuning Yanga

a Shenzhen Environmental Microbial Application and Risk Control Key laboratory, GraduateSchool at Shenzhen, Tsinghua University, Shenzhen 518055, Peoples' Republic of Chinab Joint Research Center of Urban Resource Recycling Technology of Graduate School atShenzhen, Tsinghua University and Shenzhen Green Eco-Manufacturer High-Tech Co. Ltd.,ShenZhen 518055, Peoples' Republic of Chinac College of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518055,Peoples' Republic of ChinaPublished online: 20 May 2014.

To cite this article: Xiaocong Liao, Huan Li, Yingchao Cheng, Nan Chen, Chenchen Li & Yuning Yang (2014) Processperformance of high-solids batch anaerobic digestion of sewage sludge, Environmental Technology, 35:21, 2652-2659, DOI:10.1080/09593330.2014.916756

To link to this article: http://dx.doi.org/10.1080/09593330.2014.916756

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Environmental Technology, 2014Vol. 35, No. 21, 2652–2659, http://dx.doi.org/10.1080/09593330.2014.916756

Process performance of high-solids batch anaerobic digestion of sewage sludge

Xiaocong Liaoa, Huan Lia,b∗, Yingchao Chenga, Nan Chenc, Chenchen Lia and Yuning Yanga

aShenzhen Environmental Microbial Application and Risk Control Key laboratory, Graduate School at Shenzhen, Tsinghua University,Shenzhen 518055, Peoples’ Republic of China; bJoint Research Center of Urban Resource Recycling Technology of Graduate School at

Shenzhen, Tsinghua University and Shenzhen Green Eco-Manufacturer High-Tech Co. Ltd., ShenZhen 518055, Peoples’ Republic ofChina; cCollege of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518055, Peoples’ Republic of China

(Received 30 October 2013; final version received 15 April 2014 )

The characteristics of high-solids anaerobic digestion (AD) of sewage sludge were investigated by comparison with conven-tional low-solids processes. A series of batch experiments were conducted under mesophilic condition and the initial solidcontents were controlled at four levels of 1.79%, 4.47%, 10.28% and 15.67%. During these experiments, biogas production,organic degradation and intermediate products were monitored. The results verified that high-solids batch AD of sewagesludge was feasible. Compared with the low-solids AD with solid contents of 1.79% or 4.47%, the high-solids processesdecreased the specific biogas yield per gram of sludge volatile solids slightly, achieved the same organic degradation rate ofabout 40% within extended degradation time, but increased the volumetric biogas production rate and the treatment capa-bility of digesters significantly. The blocked mass and energy transfer, the low substrate to inoculum rate and the excessivecumulative free ammonia were the main factors impacting the performance of high-solids batch AD.

Keywords: anaerobic digestion; biogas; sewage sludge

1. IntroductionSewage sludge (sludge for short in the paper) is wastegenerated by wastewater treatment plants (WWTPs). Thegenerated volume of the sludge is predicted to increase con-tinuously in the next decade because more WWTPs willbe put into operation with upgraded effluent. In order toreduce the cost of sludge treatment and transportation, itis the first step to minimize sludge quantity during sludgetreatment processes. Hence, anaerobic digestion (AD) hasbeen widely used as a main process for its ability to reducethe amount of sludge solids, further transform organicmatter into biogas and limit possible odour problem asso-ciated with residual putrescible matter.[1] The capability torecover energy from waste biomass provides AD a morepromising prospective.

Conventional anaerobic digesters commonly deal withthe sludge with total solids (TS) content of 2–5% becausemixing, heat transfer and pumping all become inefficientand expensive as a result of high viscosity of the sludgeat greater TS.[2] However, low-solids AD of sludge isnot always feasible in small-scale WWTPs or WWTPsin some undeveloped countries due to poor management,unprofessional operation, economical limitation and inade-quate planning.[3] Moreover, there is commonly not enoughspace for traditional big digesters in many small-scaleWWTPs, especially in highly urbanized areas. In China,

∗Corresponding author. Emails: lihuansz@qq.com, li.huan@sz.tsinghua.edu.cn

there also exist above the limits for the conventional low-solids AD processes. About 80% of the sludge annuallyproduced in China, i.e. 24 million tons of dewatered sludge,has not received necessary treatment and disposal. Sincemost of the sludge has already been dewatered before fur-ther treatment and disposal, high-solids AD would be anattractive option for treating the dewatered sludge. Thisprocess has been recognized to be advantageous over tradi-tional low-solids AD for smaller digester and lower energyrequirement for heating.[4] Thus, the high-solids AD ofsludge has recently attracted special interest.[2,3]

However, high solid concentration has also been con-sidered to be an important factor limiting the stability andefficiency of high-solids sludge AD.[5] Because water maydissolve substrates and aid in the diffusion of substrates tobacterial sites, the water content may not only aid in bacte-rial movement, but is also known to influence the mass andenergy transport in high-solids substrates and the balancebetween volatile fatty acids (VFAs) production by acido-genic bacteria and the conversion of acids to methane bymethanogenic bacteria.[6] High-solids AD process has beenapplied widely to the organic fraction of municipal solidwaste (OFMSW),[7] industrial wastes,[8] yard waste,[9]food wastes [10] and agricultural wastes.[11] However, theknowledge on high-solids AD of sewage sludge is veryrare. Compared with other wastes, sewage sludge with high

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solid content is a plastic semisolid waste, which has ahigher viscosity and lower mass and heat transfer efficiency.Therefore, previous experiences on other wastes cannot bedirectly used to determine the performance of high-solidsAD of sewage sludge.

As far as it is known at present, only few attempts havebeen made to study characteristics of mono-digestion ofhigh-solids sludge.[2,3,5,12,13] However, the TS contentsof the sludge used in these studies were mostly below 12%.Only Duan et al. [3] used sewage sludge with TS of 10%,15% and 20% as the feedstock of high-solids AD. Thesystematic comparison between high-solids and low-solidssludge AD is still lacking. Therefore, the objective of thisstudy is to analyse the performance of high-solids sludgeAD in order to provide an insight to its feasibility. For thispurpose, a series of batch AD experiments with different ini-tial solids concentrations, ranging from 2% to 15%, werecarried out under mesophilic condition. Treatment dura-tion, biogas production, organic degradation and inhibitionfactors were all investigated. Based on these characteris-tics, the improvement methods of high-solids sludge ADare discussed.

2. Materials and methods2.1. Substrates and inoculumsThe dewatered sludge collected from a local full-scaleWWTP was used as the substrate of batch AD experi-ments. In this WWTP, a biological aerated filter process wasapplied. The excess biofilm sludge and the primary sludgewere mixed, thickened, conditioned with polyacrylamideand finally dewatered by centrifugation. The dewateredsludge was collected and stored at 4◦C in a laboratorialrefrigerator before use. The pH of the dewatered sludgewas 7.0–7.5 and the value of C/N was 9. Its TS content andvolatile solids (VS) content are given in Table 1. Duringbatch experiments, the dewatered sludge was first mixedwith the inoculum, and then the mixture was diluted to therequired solid contents with deionized water.

The inoculum used in batch experiments was thedigested sludge discharged from a laboratory-scale semi-continuous anaerobic digester, operated at a temperature of35◦C and the solid retention time of 20 days. The laboratory-scale digester had an effective volume of 6 L. After 300 mLof the digested sludge was discharged, 300 mL of the diluteddewatered sludge with TS 2% was fed into the digesterevery day. This digester had been running over one year,and it was only used to provide the inoculum in this work.The average methanogenic activity of the inoculum wasabout 200–220 mL/(gVS d), and its TS and VS are given inTable 1.

2.2. Batch experimentsBatch AD experiments were carried out in 250-mL jars. Themixture of the dewatered sludge (substrate), the seed sludge(inoculum) and deionized water was prepared according toTable 1. Generally, the inoculum VS to substrate VS ratio(RI/S) of 1.0 was recognized to be optimal with a maximumconversion rate of substrate to methane.[14,15] Neverthe-less, only a small amount of inoculum was used in thisstudy because the solid concentration of the mixture hadto be adjusted using the dewatered sludge. After the mix-ture was added into the jars, the jars were flushed with N2 toremove O2, and then sealed with rubber plugs. The jars wereplaced in a shaking water bath (SHA-C; ZhongDa Instru-ment, China) at a frequency of 60/min to keep the reactiontemperature at 35 ± 2◦C. A polyvinyl chloride connectiontube was properly inserted into a hole pierced at the centreof each jar plug for transferring the biogas to the subsequentmeasurement device. The biogas production was measuredby displacement of a saturated sodium chloride solution, asdescribed by Salam et al.[16] A blank test containing only180 g inoculum indicated that no biogas was produced fromthe inoculum. The endogenous metabolism of the inoculumwas weak, but the activity was recovered when substratewas supplemented. In order to analyse the variations ofdifferent parameters during AD, the same five jars were

Table 1. Characteristics of the mixture of substrate and inoculum.

Parameters G1 G2 G3 G4

Dewatered sludgeTS (%) 20 20 20 20VS/TS (%) 63.3 61.3 65.4 65.4InoculumTS (%) 1.6 1.6 1.6 1.6VS/TS (%) 46.7 46.7 46.7 46.7The mixtureRecipe: dewatered sludge + water + inoculum (g) 13 + 117 + 50 40 + 90 + 50 85 + 45 + 50 130 + 0 + 50TS (%) 1.79 4.47 10.28 15.67VS/TS (%) 59.36 60.02 64.64 64.89pH 7.65 7.71 7.69 7.81Ra

I/S 0.24 0.08 0.04 0.02

aRI/S , the ratio of inoculum VS to substrate VS.

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operated as one group (G1–G4). Among the five jars withthe same solid content, one jar was opened every few daysand the digestate in this jar was collected for analyses.

2.3. Analytical proceduresTS and VS were determined according to standardmethods.[17] The organic degradation was represented bythe reduction rate of VS (VSr , %). It can be calculated by thefollowing equation,[18] assuming that the mass of inorganicfraction is constant during AD

VSr = VS0 − VSt

VS0 × (1 − VSt), (1)

where VSt is the VS of the digestate at a certain time duringAD, g/g TS or %; VS0 is the VS of the initial mixture, g/gTS or %.

In order to analyse soluble parameters during sludgeAD, the digestate taken from the jars was first centrifugedat 5000 rpm (4190g) for 10 min, and then the supernatantwas filtered with membranes with a mesh size of 0.45 μm.The filtrate was used for further analyses. Chemical oxy-gen demand (COD), total ammonia-nitrogen (TAN) andtotal phosphorus (TP) were all measured according to stan-dard methods.[17] Dissolved organic carbon (DOC) andtotal nitrogen (TN) were determined by a TOC analyserassembled with a TN detector (TOC-Vcph; Shimadzu).Free ammonia-nitrogen (FAN) was calculated by the sameequation used by Hansen et al.[19] VFAs were deter-mined with the colorimetric method,[20] and the pH wasdetermined by a pH meter (pH 6+, Eutech).

3. Results and discussion3.1. Biogas production during high-solids ADDuring both low-solids (G1 and G2) and high-solids (G3and G4) sludge AD, the biogas yield was converted as thebiogas yield per gram of VS added by dividing the biogasyield by VS added, and the data are presented in Figure 1.

Figure 1. Cumulative biogas yield (mL/g VSadded) during ADof sludge with different solid contents.

The results showed that the sludge with solid contents rang-ing from 1.79% to 15.67% can be digested successfully, andhigh-solids sludge AD is possible under mesophilic condi-tion. The decrease in the cumulative biogas production wasvery limited when the solid content was increased. More-over, there were no obvious lag phases during the batchexperiments from G1 to G4, which indicated the rapid accli-mation of anaerobic microorganisms to the environmentwith low or high solid concentration.

In spite of these satisfactory performances, the timerequired to complete digestion extended from 16 days to46 days when sludge TS increased from 1.79% to 15.67%(Table 2). The decrease in digestion efficiency would haveresulted from the block of energy and mass transfer dur-ing high-solids AD. During high-solids AD without enoughagitation, convective transfer is negligible and diffusivetransfer predominates and controls the mobility of solu-ble substrates within the digestion media. Due to the hugeincrease in viscosity within high solids content range, theeffective diffusion coefficient decreased drastically withnumerical values 50 to 185 times smaller than the referencevalue in water when the TS content was increased to 8%

Table 2. Average performance of low and high-solids sludge AD.

G1 G2 G3 G4

Performance TS = 1.79% TS = 4.47% TS = 10.28% TS = 15.67%

T (d)a 16 22 38 46Final VSr (%) 35.3 32.2 40.7 36.7Ya (mL/g VSadded)b 138 121 121 115Yr (mL/g VSremoved)c 336 381 277 291Yv [mL/(L·d)]d 134 231 259 295TC [g/(L·d)]e 1.1 2.0 2.7 3.4

aT , degradation time.bYa, cumulative biogas yield (ml) per gram of added VS.cYr , cumulative biogas yield (ml) per gram of removed VS.dYv, specific volumetric biogas production rate, which was the slope of the linear part of the biogas production curve.e TC, treatment capability of digesters, which was calculated by TS/T × 1000.

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Figure 2. SCOD, DOC, TN and TP in the supernatant during AD of sludge with different solid contents.

and 25%, respectively.[21] Even during semi-continuousAD with effective stirring, longer degradation time was stillnecessary for the fed sludge with a higher TS to reach thesame level of organic degradation.[3] Besides the block ofheat and mass transfer, the low ratio of inoculum to substratemay be another factor impacting the efficiency of biogasproduction during high-solids AD. Lopes et al. [22] veri-fied that less inoculum prolonged the degradation time ofOFMSW when using bovine rumen fluid inoculum at RI/Svalues of 0.17, 0.11 and 0.05. Nevertheless, the degradationtime extended only threefold from G1 to G4 when the RI/Sdecreased from 0.24 to 0.02.

Along with the increased sludge solid concentration,the reduction rate of sludge VS increased from 32–35%to 37–41%, but the biogas yield based on removed VSdeclined in the experiments of G3 and G4 (Table 2). Atthe end of G3 and G4, the residual soluble organic matter,derived from the degradation of VS and represented by sol-uble COD (SCOD) and DOC were not converted furtherto biogas (Figure 2). Thus, the biogas yield per gram ofremoved VS was lower than those in G1 and G2. The max-imum biogas production based on removed VS occurredwhen the sludge TS was about 5%, which is also the solidconcentration frequently used in practice. It was noted thatthese biogas yields, even in experiment G1, are relativelylower than the values of 580 mL CH4/g VSadded obtainedfrom sludge AD with thermal pretreatment [2] or of 176 mLCH4/g VSadded obtained from a semi-continuous digesterwith sludge retention time of 16 days.[10] The low specific

biogas production was possibly attributed to the low RI/Sused in this study, which commonly leads to the increase inVFAs in the digestate.[23] It was found that the inhibitioneffect resulted in a low biogas yield when the concentrationof propionic acid reached 900 mg/L.[24] In fact, a simi-lar biogas yield of 145 mL/g VSadded was reported undera low RI/S of 0.25; moreover, the biogas yield increasedsignificantly to 383 mL/g VSadded when RI/S increased to1.0.[25] Similar variation was also found for thermophilicbatch AD of food waste, and the biogas yield decreased byabout 50% when RI/S changed from 0.625 to 0.2.[26] Thus,the biogas productivity of high-solids AD should be close tothat of traditional AD if the negative influence of low RI/Sis eliminated in continuous or semi-continuous digesters.

Contrary to the decrease in the cumulative biogas yieldbased on added or removed VS, the specific volumetricbiogas production rate was enhanced dramatically with theincreasing solid concentration (Table 2). Due to the lowRI/S , the specific volumetric biogas production rate was stilllower than the values obtained from semi-continuous high-solids sludge AD.[10] However, compared with G1, thevolumetric biogas yield (mL/L) of G4 increased 8.6 times.Even considering the extended treatment duration, the spe-cific volumetric biogas production rate of G4 still increased2.2 times. Correspondingly, the treatment capability of G4enlarged 3.1 times. This means that high-solids AD can treatmore sludge in the same digester volume and in the sametreatment duration. Thus, the volume of the digester can bereduced by two-thirds when using high-solids AD instead

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of traditional low-solids AD. Moreover, the digesters canalso decrease its heat dissipation due to its reduction in size.

3.2. Organic degradation during high-solids ADThe characteristics of sludge supernatant were recordedduring these batch experiments (Figure 2). At the initialstage, the rapid increase in SCOD and DOC indicated therelease and solubilization of macromolecular organic sub-stances from sludge solid phase. The soluble nitrogen (TN)also increased along with the hydrolysis and acidificationof protein. After the initial stage, the dissolved carbonicorganic substances were transformed into biogas, leadingto a gradual decrease in DOC and SCOD. Because the sol-uble nitrogen cannot be utilized for biogas production, itsconcentration was increased slowly until the end of diges-tion. The concentration of soluble phosphorus fluctuated atthe initial stage, but subsequently increased because of therelease of accumulated phosphorus inside bacteria. Thesevariations in sludge supernatant during high-solids AD weresimilar with those during low-solids AD, which apparentlysuggested that the two types of AD experienced a similarprocess of organic matter degradation.

According to Figure 2, the concentrations of COD, TNand TP in the supernatant of digested sludge were almostdirectly proportional to the initial solid content. Thus, thedigested sludge derived from high-solids AD would pro-duce wastewater with high nitrogen and phosphorus contentafter subsequent dewatering processes. However, since thewastewater volume was proportionally reduced, the totalamount of COD, TN and TP in the supernatant of digestedsludge derived from high-solids AD was still close to thatfrom low-solids AD.

During the AD experiments, the degradation of sludgeorganic matter was represented with the variation of diges-tate VS (Figure 3). The degradation process can be dividedinto two stages: an initial rapid period and a subsequent slowperiod. When the initial sludge TS was 1.79%, the slowdegradation period was short and most of the degradableorganic substances were removed in the rapid degradationperiod. When the initial sludge TS increased to 4.47%,10.28% or 15.67%, the slow degradation period was pro-longed, and half of the degradable organic substances wereremoved in this period. Moreover, the slow degradationperiod extended with the increasing sludge TS. Thus, apseudo-first order kinetic model describing organic reduc-tion as a function of digestion time was used to predictsludge digestion performance with different initial solidcontents. The model can be described by the followingequation:

lnC0

C= k · t, (2)

where C0 is the initial concentration of organic substrate,mg/L; C is the concentration of organic substrate at time

Figure 3. Variation of VS reduction rate (VSr) during AD ofsludge with different solid contents.

t (day), mg/L and k the coefficient of organic degradation,d−1.

The organic substrate can be represented by VS. Thus,Equation (2) can be converted to the following equationaccording to the definition of VSr

ln1

1 − VSr= k · t. (3)

Using the above model, the coefficients of VS degrada-tion can be calculated out (Table 3). It can be found thatthe coefficients decreased sharply along with the increasingsludge TS. This was in accordance with the performanceof biogas production. The deterioration of organic degra-dation efficiency due to the high substrate concentrationwas also found during anaerobic fermentation of OFMSW[27] and sunflower oil cake.[28] Sewage sludge is mainlycomposed of carbohydrate, protein and lipid, and the over-all performance of organic degradation was dependent onthe degradability of each organic component. Hence, the kvalues of VS degradation were close to the values of 0.035d−1 for carbohydrate, 0.016 d−1 for protein and 0.013 d−1

for fat (on the basis of mass other than COD) during sludgeAD with RI/S of 0.25.[29] It is noted that due to the lowRI/S , the performance of organic degradation here was stilllower than the other reported values of 0.06 d for sludgewith RI/S of 0.06 [30] and 2.38 d−1 for carbohydrates, 4.42d−1 for proteins and 1.49 d−1 for lipids with RI/S of 2.0.[31]It was also reported that the values of k decreased by 62.5%

Table 3. The coefficients of organic degradation during sludgeAD with different initial solid contents.

Substrates k (d−1) R2

G1 (TS = 1.79%) 0.037 0.831G2 (TS = 4.47%) 0.013 0.855G3 (TS = 10.28%) 0.012 0.945G4 (TS = 15.67%) 0.009 0.931

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Figure 4. TAN, FAN, VFA and pH during AD of sludge with different solid contents.

when RI/S decreased from 10 to 0.5 during batch AD ofsecondary sludge.[30] Both the low RI/S and the blockedmass transfer possibly resulted in the build up of some inter-mediate products, whose inhibition effects may be anotherfactor impacting the efficiency of organic degradation dur-ing high-solids AD. Thus, further analyses were carried outin order to verify this hypothesis.

3.3. Effect of high solids content on system stabilityThe stability and inhibition of high-solids AD were analysedby monitoring the variations of VFAs, TAN, FAN and pH.Their average data are shown in Figure 4. Free ammoniais toxic to microorganisms because it can pass throughthe cell membrane and enter cells, causing proton imbal-ance and potassium deficiency.[32,33] The toxicity ofVFAs is derived from its undissociated form. They canflow freely through the cell membrane, dissociate insidescells and hence cause a pH decline and a disruption ofhomoeostasis.[34] At the initial stage of AD, the sharpincrease in TAN and VFAs showed that acidogenic bac-teria were degrading macromolecular organic substancesinto VFAs and other micromolecular intermediate products,including ammonia. However, the variation of FAN wasdiscordant with that of TAN. The concentration of TANincreased rapidly at the initial stage and then increasedslowly, while the concentration of FAN increased graduallytill the end of digestion.

During low-solids AD, the increased VFAs and FANwas not enough to inhibit methanogen, and hence the VFAs

was converted to biogas rapidly without any accumulationduring experiment G1. When the concentration of TANreached 1000–1500 mg/L during experiment G2, VFAaccumulated slightly and its maximum concentration wasabout 2700 mg HAc/L. However, digestion was not dete-riorated because the concentration of FAN was still lowerthan 100 mg/L in the weak alkaline environment with pH7.5–8.0. The influence of free ammonia on biogas pro-duction was exhibited when FAN was close to 500 mg/Lduring experiment G3. The concentration of VFAs reached5900 mg HAc/L at the initial stage. The accumulation ofFAN resulted from the low C/N of substrate and high solidscontent, while the accumulation of VFAs was attributed tothe low RI/S [23] and high solids contents. High concen-tration of substrate produced a large amount of VFAs andammonia in a short time, and these intermediate productswould accumulate when they could not be transformed fur-ther timely. Although these produced VFAs were furtherconverted to biogas continuously, the conversion efficiencyof VFAs was slightly lower than that during experimentG2 according to the slopes of VFAs curves at the secondstage in Figure 4(c). The results were in accordance withthe previous report,[3] which found that the system wasmoderately inhibited but still steady when FAN was 400–600 mg/L. Maximum FAN of 968 mg/L occurred with pH8.5 during experiment G4. Under this condition, high-solidsAD was recognized to be fragile along with an increasein VFAs to 3000–4500 mg/L and an obvious decrease ofbiogas production.[3] However, the degradation of VFAsstill continued in this study, which resulted in a gradual

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decrease in VFAs concentration from 8100 to 1700 mgHAc/L, indicating anaerobic microorganisms were stillactive. Fujishima et al. [12] also reported that anaerobicmicroflora survived and worked in the digester with TAN3100 mg/L. Although anaerobic microorganisms can adaptto high ammonia and VFAs environment during high-solidssludge AD, their performance was restrained to some degreeby the blocked mass and energy transfer and the inhibitiveintermediate products. In fact, the specific biogas yield ofG4 was relatively lower than that of G1 or G2. Moreover,the pH of G4 reached 8.5 at the end of the digestion period,which was a little higher than the common pH range inanaerobic digesters. Therefore, there are chances of systeminstability along with the high FAN of 800–950 mg/L, inspite of the continuous biogas generation and VS reduction.

The pH range from 6.1 to 8.3 was recognized to beacceptable for high-solids sludge AD.[5] In this study, thepH values were almost steady along with the interactionof ammonia and VFAs. During the low-solids AD, the pHwas in the range of 7.5–8.0. During the high-solids AD,the pH increased relatively, ranging from 8.0 to 8.5. Theweak alkaline condition enhanced the formation of freeammonia, which possibly impacted the activity of anaer-obic microorganisms. Hence, a proper adjustment of pHmay be beneficial to high-solids AD of sludge.

4. ConclusionsCompared with low-solids processes, high-solids batch ADof sewage sludge achieved the same organic degradationrate with a longer treatment duration, decreased the specificbiogas yield a little, but increased the volumetric biogasyield and enhanced the treatment capability of digesterssignificantly. The blocked mass and energy transfer andthe low inoculum to substrate ratio were the main factorsimpacting the performance of high-solids batch AD. Thehigh solid concentration and the low inoculum to substrateratio led to the accumulation of VFAs and free ammo-nia in high-solids batch digesters. However, VFAs stilldegraded gradually. High concentrations of free ammo-nia may be the key inhibitor deteriorating high-solids AD.Although high-solids batch AD exhibited some advantages,some measures were still necessary including improvingagitation, increasing inoculum to substrate ratio and pHadjustment.

FundingFinancial support for this project was obtained from the ChinaMajor Science and Technology Program for Water Pollution Con-trol and Treatment [No. 2011ZX07317] and the Fund of ResearchCentre of Urban Resource Recycling Technology of GraduateSchool at Shenzhen, Tsinghua University [No. URRT2013005].

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