Anaerobic digestion of food waste for volatile fatty acids (VFAs)

Anaerobic digestion of food waste for volatile fatty acids (VFAs)production with different types of inoculum:Effect ofpHKun Wang,Jun Yin ⇑,Dongsheng Shen,Na LiSchool of Environmental Science and Engineering,Zhejiang Gongshang University,Hangzhou 310012,China Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling,Hangzhou 310012,Chinah i g h l i g h t sInvestigation for high VFA yield under acidic pH was shown.Aerobic and anaerobic activated sludge were inoculated to food waste fermentation for VFA production. The concentration and yield of VFAs were highest at pH 6.0with anaerobic active sludge seeding. Ammonia nitrogen release was linearly related to fermentation time.a r t i c l e i n f o Article history:Received 8January 2014Received in revised form 15March 2014Accepted 18March 2014Available online 27March 2014Keywords:Anaerobic fermentation Food wasteVolatile fatty acid pHInoculuma b s t r a c tFood waste anaerobic fermentation was carried out under acidic conditions using inocula based on aer-obic activated sludge (Inoculum AE)or anaerobic activated sludge (Inoculum AN)for volatile fatty acids (VFAs)production.The results showed that food waste hydrolysis increased obviously when Inoculum AN was used relative to Inoculum AE at any pH investigated.Hydrolysis at pH 4.0and uncontrolled pH was higher than that at other pHs when either inoculum was used.Additionally,VFAs production at pH 6.0was the highest,regardless of the inoculum used.The optimum VFA yields were 0.482g/g VSS removal with Inoculum AE and 0.918g/g VSS removal with Inoculum AN,which were observed after 4d and 20d of fermentation,respectively.VFAs composition analysis showed that butyrate acid was the prevalent acid at pH 6.0,followed by acetate acid and propionic acid.Ó2014Elsevier Ltd.All rights reserved.1.IntroductionVolatile fatty acids (VFAs)including fatty acids from C2to C5(acetic,propionic,butyric,etc.)are potentially renewable carbon sources.VFAs can be used for nutrients removal,production of bio-gas and biodiesel (Fontanille et al.,2012),generation of electricity (Chen et al.,2013c )and synthesis of biosurfactants,bioflocculants and PHAs (Li et al.,2011;Chen et al.,2013a ).Additionally,Srikanth et al.(2009)demonstrated that VFAs are a potential substrate for hydrogen production via photo-fermentation by mixed microbial cultures.VFAs are usually derived from fossil fuels through chem-ical synthesis (Eggeman and Verser,2005);however,anaerobic digestion (fermentation)of biomass has recently gained attention as a cost-effective and environmentally friendly alternative for VFAs production.During the last decade,researchers have focused on waste acti-vated sludge (WAS)fermentation to produce VFAs (Yuan et al.,2006).When compared with WAS,food waste contains higher lev-els of organic materials,such as starches,proteins and lipids;accordingly,it may become a plentiful source of inexpensive or-ganic substrate for fermentative VFAs production as a result of reduction and stabilization of food waste.Moreover,food waste in China accounted for 40%of all MSW in 2011(National Bureau of Statistics of China),and this value appears to be increasing.Ow-ing to stringent environmental regulations,some methods for dis-posal of food waste such as landfill,ocean dumping,incineration,animal feed and fertilizers have been forbidden or become less desirable (Komemoto et al.,2009;Cuéllar and Webber,2010).Anaerobic digestion is preferred as an efficient pathway for the recycling and reduction of food waste.Hydrolysis,acidification,and methanogenesis are usually in-volved in anaerobic digestion,with VFAs produced in the first two steps.To induce the accumulation of higher levels of VFAs,the following strategies can be adopted:(a)improving the hydro-lysis rate to produce more soluble substrates for acidification,(b)/10.1016/j.biortech.2014.03.0880960-8524/Ó2014Elsevier Ltd.All rights reserved.⇑Corresponding author at:School of Environmental Science and Engineering,Zhejiang Gongshang University,Hangzhou 310012,China.Tel.:+8657188071024/7017.E-mail address:jun.yin77@ (J.Yin).promoting the acidogenesis process,and(c)inhibiting the activity of methanogens(Yuan et al.,2006).Hydrolysis is the rate-deter-mining step throughout the fermentation process(Li and Noike, 1992).pH,temperature,C/N ratio and hydraulic retention time (HRT)have been reported to be the key factors controlling the pro-duction of VFAs during fermentation(Chen et al.,2007,2013a;Lee et al.,2014).Chen et al.(2013c)found that the optimal operating conditions for VFAs production during co-fermentation of food waste and WAS were pH8,C/N22,37°C,and a fermentation time of6d.However,the range of the parameters they used to deter-mine this was limited.To improve the production of VFAs,methanogens must beinhibited.To date,many investigations have been conducted to investigate methods of improving VFAs production from WAS by anaerobic digestion,especially under alkaline conditions(Yu et al.,2008).However,a large amount of NaOH must be added into reactors to generate such conditions,and the cost for acid recovery remains a challenge.The inhibition of methanogenesis is one of the primary effects of low pH during anaerobic fermentation(Yuan et al.,2006)because high methanogen activity requires a mini-mum pH of6.5.Previous studies have reported that the optimal pH for effective hydrogenogenesis and inhibited methanogenesis was5.0–6.5(Fang and Liu,2002).Few studies have focused on VFAs production from food waste under acidic conditions.In addition,although type of inoculum is known to be one of the most important factors affecting the evolu-tion of fermentative pathways(De Gioannis et al.,2013).Informa-tion regarding the effects of different inocula on anaerobic acidogenesis is still sparse.Accordingly,this study was conducted to efficiently produce VFAs from food waste by controlling anaero-bic fermentation under acidic conditions while maintaining high hydrolysis and acidification rates without methane production. The effect of inocula on VFAs production was also evaluated,the composition of VFAs was examined,and the mechanism of VFAs production was discussed.2.Methods2.1.Food waste and inoculaFood waste was collected from a canteen on the campus of Zhe-jiang Gongshang University.The food waste,which mainly con-sisted of rice,vegetables and meat,was crushed with an electrical blender after sorting out animal bones and clamshells, after which the resulting slurry was sieved to less than3mm. Two types of inoculum were obtained,anaerobic activated sludge from an up-flow anaerobic sludge bed(UASB)reactor of the Xihu Brewery and aerobic activated sludge from the secondary settling tank of the Qige Wastewater Treatment Plant in Hangzhou.Prior to inoculation,the seeding sludge was re-activated under their respective culture conditions.Before use,the sludge was washed with tap water at least three times to remove organic materials. The characteristics of the food waste and two inocula used in this experiment are shown in Table1.Inoculum AE originated from aerobic activated sludge and Inoculum AN originated from anaero-bic activated sludge.2.2.Batch fermentation experimentsThe fermentation experiments were carried out in brown wide-mouth bottles with working volumes of500mL that were capped with rubber stoppers.Eight batch reactors were divided into two groups of four and inoculated with either aerobic activated sludge or anaerobic activated sludge.The TS content in each reactor was adjusted to7%and the substrate was composed of80%food waste and20%inoculum(dry weight)with an initial VSS content of 32.0±2.1g.The four reactors in each group were operated at pH 4.0,5.0,6.0and pH uncontrolled,respectively,by adding4.5M HCl or NaOH.Experiments were conducted in a greenhouse main-tained at30±2°C and all reactors were mechanically stirred at 120rpm using a magnetic stirrer throughout the experiment.As the optimum oxidation–reduction potential(ORP)condition for methanogen reproduction is underÀ350mV,fermentation tests were carried out under facultative anaerobic conditions to enhance the ORP to inhibit the activities of methanogens.Fermentation tests were carried out for20days.2.3.Specific substrate utilization rateThe glucose degradation rate(GDR)was applied to quantify the activities of the microbial communities.To measure the GDR, 1.5mL of fermented solid was taken from the reactor,after which the sediment was washed with sterile water three times and cen-trifuged at3000rpm for3min to remove the carbohydrates in the suspension.Next,10mL glucose solution(80mg/L)was added and the sediment/suspension was cultivated in a table concentrator at 30±1°C for20h.The glucose concentration in the cultivated supernatant was then detected to calculate the GDR.2.4.Analytical methodsSamples from reactors were immediatelyfiltered through afil-tration membrane with a pore size of0.45l m,after which they were diluted10times with distilled water and used to determine the soluble chemical oxygen demand(SCOD),ammonia nitrogen (NH4+-N),soluble protein,total carbohydrate,and VFAs composi-tions.Afterfiltration,the substrate after was assayed for TSS and VSS.SCOD,TSS,VSS and NH4+-N were analyzed in accordance with the Standard Methods(APHA,1998).Soluble protein was quanti-fied by the Lowry–Folin method using bovine serum albumin (BSA)as the standard(Lowry et al.,1951)and carbohydrate was determined using the phenol–sulfuric method with glucose as the standard(Herbert et al.,1971).Volatile fatty acids(VFAs,C2–C5)including Ac,Pr,n-Bu,iso-Bu,n-Va and iso-Va were deter-mined by gas chromatography using a GC7890-II(Tianmei Co., Shanghai,China)equipped with a3mÂ2mm stainless steel packed columnfilled with GDX-103as the stationary phase and aflame ionization detector(FID).The temperatures of the column, injector and detector were180°C,230°C and250°C,respectively. The conversion factors used for determination of the chemical oxy-gen demand(COD)of soluble organic materials were1.5g-COD/g protein, 1.06g-COD/g carbohydrate, 1.07g-COD/g acetic acid,1.51g-COD/g propionic acid, 1.82g-COD/g butyric acid,and2.04g-COD/g valeric acid(Grady et al.,1999).The quantity of car-bon in VSS,soluble protein and carbohydrate was determined as previously described(Zhang et al.,2009).Lactic acid was analyzed by the para-hydroxydiphenyl colorimetric method using lactic acid as a standard.Briefly,after pre-treating the sample to remove Table1Characteristics of the food waste and inoculums used in the experiment.The data was based on percentage of dry matter except pH and SCOD.Parameter Food waste Inoculum AE Inoculum AN pH 6.1 6.9 6.8TS(%)24.0 6.312.0VSS/TSS(%)96.27270SCOD(g/L)25.2––Total carbohydrate(%)39.5 2.2 1.2Total nitrogen(%) 1.8––Total protein(%)11.0––396K.Wang et al./Bioresource Technology161(2014)395–401interference by carbohydrate and protein,0.5ml supernatant was mixed with25l L4wt.%copper sulfate solution and6ml concen-trated sulfuric acid,after which the mixture was placed in a boiling water bath for5min,then cooled rapidly in an ice bath.Next, 0.05ml0.5wt.%para-hydroxydiphenyl solution was added and the mixture was placed in the boiling water bath for another 90s,and then cooled in the ice bath.Finally,the absorbance of the mixture was measured at565nm.Thefinal experimental data were expressed as the average of duplicate tests.3.Results and discussion3.1.Effect of pH on food waste hydrolysisHydrolysis of food waste can be characterized by the changes in soluble chemical oxygen demand(SCOD)concentrations.During the initial2d,the products of VFAs and methane were relatively low;therefore,the observed SCOD concentrations,which primarily reflected fatty acids,soluble carbohydrates and protein,were re-garded as the main hydrolysates(Zhang et al.,2009).Table2shows the solubilization rates of food waste on the second day under dif-ferent conditions.During thefirst2days,SCOD increased quickly, indicating solubilization of the organic compounds of the solid substrate and an input of dissolved organic compound.As shown in Table2,the hydrolysis of food waste with Inoculum AN was higher than that with Inoculum AE,and lower pH led to increased hydrolysis.One reason for the higher food waste hydrolysis with Inoculum AN was assumed to be the presence of a large amount of facultative and anaerobic microorganisms.High SCOD produc-tion rates were observed at pH4.0and uncontrolled pH,suggesting that acidic conditions could promote the hydrolysis of food waste. Moreover,the pH of each uncontrolled bottle decreased rapidly to 4.0in2days and was still below3.0after10days.3.1.1.Carbohydrate solubilizationDetected soluble carbohydrates and proteins were the result of a net balance between competing rates of release and degradation. Therefore,solubilization rates were calculated according to the VSS removal to characterize the solubilization of carbohydrates and proteins.As shown in Table2,during thefirst2days,fermentation with Inoculum AN produced more carbohydrate than that with Inoculum AE(except at pH6.0).Thesefindings also indicated that Inoculum AE had higher activity during degradation of carbohy-drates.After thefirst2days,soluble carbohydrate concentrations decreased quickly in all reactors,especially at pH5.0and6.0,then remained low after day13(data not shown).Carbohydrate,which is the main organic matter in the fermen-tation substrate,was the preferred substrate of the microbes.The polysaccharides present in food waste werefirst degraded to glu-cose,then to pyruvate(CH3COCOOH).Pyruvate is an important intermediate product that is easily oxidized to acetyl-CoA,which can be further utilized to produce acetate and butyrate by divers-iform enzyme actions(Chen et al.,2013c).In this study,soluble carbohydrates in each reactor decreased rapidly in4days and were consumed by the end of fermentation,which was different from the result of Chen et al.(2007).3.1.2.Protein solubilization and NH4+-N releaseThe time courses of ammonia nitrogen and soluble protein pro-duction in different reactors are presented in Fig.1.As shown in Table2and Fig.1a and b,there was a similar trend during fermen-tation,regardless of whether Inoculum AE or Inoculum AN was used.Specifically,more soluble protein was produced at pH4.0 and uncontrolled pH,demonstrating that acidic conditions can promote the dissolution of protein.However,the concentrations of soluble protein at pH5.0and6.0remained relatively stable and around500–1000mg-COD/L,which were much lower than those at pH4.0and uncontrolled pH,despite somefluctuations in soluble protein at pH5.0and6.0using Inoculum AN before day9(Fig.1a and b).This stability may have been due to the bal-ance between dissolution and consumption of protein.Protein is initially decomposed into amino acids,which are then transformed into ammonia,carbon dioxide and different types of VFAs.A large amount of ammonium from food waste and sludge is released with hydrolysis and acidification,and ammonium nitro-gen is usually used to evaluate the degree of protein degradation. Deamination reactions are mainly completed by Fusiformis den-tium,Mycoplasmas and Streptococcus during anaerobic digestion (Zheng and Feng,1997).Interestingly,as shown in Fig.1c and d, NH4+-N concentrations increased linearly with fermentation time at pH5.0and6.0after4days,regardless of whether Inoculum AE or Inoculum AN was used(Table3).However,at pH4.0and uncontrolled pH the NH4+-N concentrations remained low and sta-ble.A possible reason for the low NH4+-N produced was the toxicity of acidic conditions decreasing the activities of microbial hydro-lytic enzymes such as protease and peptidase.The linear equations (p<0.01)for pH5.0and6.0were determined for x2[5,20],where x is the fermentation time(d),y is the NH4+-N concentration(mg/L) and the k-value represents the NH4+-N release rate(mg/(L d)).The NH4+-N release rate was as follows:Inoculum AN-pH6.0>Inocu-lum AE-pH6.0>Inoculum AE-pH5.0>Inoculum AN-pH5.0.The results revealed that the fermentation process of protein could be divided into two stages under acidic conditions,the dissolu-tion-dominated stage(within4days)and the transformation-dominated stage(after4days).Accordingly,it is assumed that car-bohydrates are utilized prior to proteins during anaerobic fermen-tation,which can also be seen in Table2.3.2.Effect of pH on total VFAs production with different types of inoculaFig.2shows the total VFAs production during fermentation using Inoculum AE and Inoculum AN.The VFAs concentrations at pH5.0and6.0were much higher than those at other pH condi-tions,regardless of which inoculum was used.Moreover,a higher concentration of VFAs was achieved at pH6.0,although with dif-ferent fermentation time.The highest concentration of VFAs wasTable2Hydrolysis of food waste with Inoculum AE and AN at different pHs at fermentation time2d.Designated pH Inoculum AE Inoculum ANgSCOD produced/ gVSS reduction gCOD S-carbohydrate/gVSS reductiongCOD S-protein/gVSS reductiongSCOD produced/gVSS reductiongCOD S-carbohydrate/gVSS reductiongCOD S-protein/gVSS reductionpH4.0 2.030.550.11 4.710.630.17pH5.0 1.07–0.04 1.140.160.03pH6.0 1.52–0.03 1.87–0.04pH uncontrolled 1.25–0.09 2.400.780.06Note:‘–’means degradation of carbohydrate is more than its dissolution.K.Wang et al./Bioresource Technology161(2014)395–401397obtained at pH6.0during fermentation with Inoculum AN and a fermentation time of20d.As shown in Fig.2,during fermentation at pH4.0and uncon-trolled pH,VFAs concentrations increased with fermentation time, but they were much lower than under other pH conditions.These findings confirmed that acidogenesis could be inhibited when the pH was less than4.0.At pH5.0,increasing the fermentation time did not result in an increase of VFAs production using Inoculum AN.Obvious VFAs consumption was observed owing to microbial metabolism or the participation of VFAs consumers,such as hydro-gen production by the mesophilic acidogenic culture(Karadag and Puhakka,2010).For fermentations using Inoculum AE,the maximum VFAs con-centrations at pH4.0,5.0,6.0and uncontrolled pH were8.07,30.1, 30.8and9.96g-COD/L,respectively,whereas those obtained using Inoculum AN were2.91,34.6,51.3and8.80g-COD/L,respectively. Thesefindings demonstrated that higher VFAs production could be obtained by inoculating anaerobic activated sludge for food waste fermentation.During the initial fermentation stage,the total VFAs concentration at pH5.0and6.0reached more than10and30g-COD/L,respectively.Inoculum AN groups showed slightly higher VFAs production than Inoculum AE.This was probably related to the fact that anaerobic activated sludge had more acidogenic bac-teria and higher microbial activities under anaerobic condition, while methanogens were inhibited under acidic conditions(Wang et al.,2013).The ratio of VFA to SCOD is very important,as it shows the amount of soluble substances converted into VFAs(Jiang et al., 2013).Higher VFA/SCOD ratios were achieved at pH5.0and6.0, regardless of which inoculum was used(data not shown).The maximum VFA/SCOD ratio at pH 6.0using Inoculum AN was 97.3%,followed by73.9%and79.7%for pH5.0and6.0using Inoc-ulum AE,respectively.However,at pH4.0and uncontrolled pH,the ratios of VFA/SCOD were relatively low,indicating that strong acidic conditions inhibited the activity of anaerobic acidogenic bacteria,decreasing the generation of VFAs.In addition to VFAs, there was a large portion of SCOD at pH4.0and uncontrolled pH.A large volume of lactic acid was detected on day20(Fig.3)in the fermentation broth,indicating that lactate-type fermentation occurred with Inoculum AN at pH4.0and uncontrolled pH(lactic acid concentration=18.50g/L and14.62g/L,respectively).These findings are in accordance with the observation of Parawria et al. (2004).However,for Inoculum AE,the lactic acid concentrations at pH4.0and uncontrolled pH were very low.The most probable products of these fermentations were alcohols(Ren et al.,2007); however,alcohol was not quantified in this study owing to limita-tions of the experimental conditions.The VFA yield was calculated based on the volatile suspending solids(VSS)reduction(Table4).Fermentation using Inoculum AN resulted in a higher VFA yield than fermentation using Inoculum AE,except at pH4.0.This difference in VFA yield was especially great at pH6.0,where the VFA yield was918.23mg/g VSS removal for Inoculum AE,which is much higher than that reported in pre-vious studies(Table5).In this study,higher VFAs yields wereTable3Ammonia nitrogen release rate linear equations.Reactor Linear equation R2valueAE-pH5.0y=17.25xÀ55.050.97AE-pH6.0y=20.31xÀ66.820.96AN-pH5.0y=14.19xÀ61.020.98AN-pH6.0y=40.80x+67.300.97398K.Wang et al./Bioresource Technology161(2014)395–401obtained at pH6.0,which was similar to the results reported by Jiang et al.(2013).The optimum hydrolytic enzymes activities were observed at pH6.0,which led to the highest concentration of VFAs(Parawria et al.,2004).The high VFA yield may have occurred because the anaerobic activated sludge used as an inoc-ulum had much higher amounts of hydrolytic and acidogenic bac-teria than the aerobic activated sludge.The high level of carbohydrate in food waste was also likely responsible for the high VFA yield.As shown in Table1,carbohydrate was the major organic matter in raw food waste.When compared with the car-bohydrate concentration,the proteins accounted for a small part of the organic materials.The C/N ratio of22for food waste observed in the present study is the optimal ratio for VFAs pro-duction(Chen et al.,2013c)position of VFAsThe produced VFA composition is important as it can provide useful information regarding the degree of hydrolysis and fermen-tation.The percentage of individual VFAs accounting for total VFAs at different pH conditions within a fermentation time of20d is gi-ven in Fig.4.Acetic and butyric acids were the most prevalent products in all reactors except for the pH4.0and uncontrolled pH reactors when Inoculum AE was used.At pH4.0and uncon-trolled pH,the prevalent acids generated when Inoculum AE was used were acetic and propionic acids,which accounted for90.3% and9.43%and75.4%and14.6%of the total,respectively.However, at pH5.0and6.0,butyric acid was the dominant product,which was in agreement with a previous study that showed that butyric acid fermentation occurred at pH5.0–6.0(Ren et al.,2007).In reac-tors with Inoculum AN,acetic and butyric acids accounted for greater than90%of the VFAs.The total carbohydrate content in the food waste used in this study was about40%,indicating that fermentation of carbohy-drate-enriched substrates should produce a great deal of acetic and butyric acids.Very little valeric acid was found in any reactors except those at pH6.0.During batch fermentation with Inoculum AE,propionic acid accounted for a larger portion of the VFAs when Inoculum AN was used.This phenomenon was mostly caused by the self-fermentation of AE sludge,which is in accordance with the results of previous studies(Chen et al.,2013b).In addition, since VFAs with more than three carbons are easily biodegraded to form acetic acid in anaerobic fermentation systems,there were not many propionic,butyric,or valeric acids accumulated(Yuan et al.,2006).pH can affect the type of VFA produced from acidogenic fermen-tation,particularly acetic,propionic and butyric acids(Bengtsson et al.,2008).The production of propionic acid from dairyTable4Max VFAs yield in each reactor.Reactor VFAs yield(mg/g VSS removal)Time(d)AE-pH4.0206.4520AE-pH5.0445.179AE-pH6.0481.854AE-pH uncontrolled229.0620AN-pH4.0124.3020AN-pH5.0650.7613AN-pH6.0918.2320AN-pH uncontrolled337.5120Table5Comparison of VFAs production from different types of fermentation.FermentationstuffSeeding OptimalconditionVFAs productionperformanceReferencesKitchen waste Anaerobicdigested sludgepH7.0,35°C36.00g/L Zhang et al.(2005)Food waste DewateredsludgepH9.0,35°C25.93g-COD/L Chen et al.(2013a)Kitchen waste Waste activatedsludgepH8.0,37°C692.4mg-COD/gVSChen et al.(2013c)Food waste Anaerobicdigested sludgepH6.024.5–25.5g/L Lim et al.(2008)35°C23.0–24.0g/LFood waste Anaerobicdigested sludgepH6.0,35°C0.471g/g VSS removalJiang et al.(2013)Food waste Anaerobicdigested sludgepH7.0,35°C18.46g/L He et al.(2012)Food waste Anaerobicactivated sludgepH6.0,30°C0.918g/g VSS removalThis studyWaste activatedsludgepH6.0,30°C0.482g/g VSS removalK.Wang et al./Bioresource Technology161(2014)395–401399wastewater is favored at pH4.0–4.5,whereas that of acetic and bu-tyric acids is favored at pH6.0–6.5(Yu and Fang,2002).However, the results of studies conducted to date suggest that the optimal pH for the production of a specific VFA is highly dependent on the type of waste used(Lee et al.,2014).Acetic,propionic and bu-tyric acids can be formed directly from the fermentation of soluble proteins,carbohydrates and lipids(Horiuchi et al.,2002),while iso/ n-valeric acid was mainly produced from proteins degradation (McInerney,1988).3.4.Specific substrate utilization rateThe abundance and activity of acidogenic bacteria determines the accumulation of VFAs.To investigate the microbial activities in the reactors at different pH with time,fermented solids were re-moved from batch reactors to test the specific substrate utilization rates(Table6).On day15,glucose was obviously biodegraded at pH 5.0and pH 6.0,while at pH 6.0the GDRs were 1.88±0.05mg/h mL and 1.83±0.01mg/h mL for Inoculum AE and Inoculum AN,respectively,which were higher than those ob-served at pH 5.0(1.09±0.02mg/h mL for Inoculum AE and 1.24±0.03mg/h mL for Inoculum AN).Thesefindings demon-strated that fermentation at pH6.0had higher microbial activities. At pH5.0,the GDR for all batch tests decreased from day5to day 15.This phenomenon coupled with the VFA yields suggests that,at pH5.0,after the maximum VFAs yield the activity of acidogenesis decreased with time.Moreover,as shown in Table6,the glucose concentration increased after start-up of the batch tests at pH 4.0,which was similar to the results shown in Table2.The most possible reason for this is that acidic conditions could promote hydrolysis of fermented solids(food waste),more carbohydrate (glucose)was provided,and the low pH acidification of substrate by acidogenic bacteria was inhibited.At uncontrolled pH,microbial activity was inhibited,so the GDRs were around zero.3.5.Carbon balance analysisThe carbon balance of the fermentation process was determined based on the experiment results(Fig.5).In general,the carbon in VSS was converted to carbon in the products,including VFAs,sol-uble protein and carbohydrate,lactic acid,alcohol,methane and carbon dioxide,during food waste fermentation.Owing to fermen-tation under facultative anaerobic conditions,the composition of the gas phase was not detected.The optimum conditions for VFAs production with Inoculum AE were pH6.0(fermentation time of 4d)and pH5.0(fermentation time of9d);therefore,a fermenta-tion time of5d was selected.For Inoculum AN,the highest VFAsTable6Glucose degrading rate(GDR)of fermented sediment.Mean values and standard deviation of three replicates.pH Glucose degrading rate(mg/h mL)AE seed(5d)AE seed(15d)AN seed(5d)AN seed(15d)4.0À0.97±0.09À1.67±0.15À0.51±0.06À0.15±0.25.0 1.74±0.04 1.09±0.02 1.44±0.05 1.24±0.036.0 1.80±0.08 1.88±0.05 1.77±0.08 1.83±0.01UncontrolledÀ0.07±0.320.20±0.100.11±0.060.39±0.07400K.Wang et al./Bioresource Technology161(2014)395–401。