6评估厌氧消化最佳稀释比

Assessment of optimum dilution ratio for biohydrogen production by anaerobic co-digestion of press mud with sewage and waterB.Radjaram a ,⇑,R.Saravanane b ,1a Department of Civil Engineering,Pondicherry Engineering College,Puducherry 605014,IndiabEnvironmental Engineering Laboratory,Department of Civil Engineering,Pondicherry Engineering College,Puducherry 605014,Indiaa r t i c l e i n f o Article history:Received 21March 2010Received in revised form 16November 2010Accepted 17November 2010Available online 26November 2010Keywords:Press mud Sewage UASBBiohydrogen Co-digestiona b s t r a c tAnaerobic co-digestion of press mud with water or sewage at ratios of 1:7.5,1:10and 1:12.5were per-formed in continuously fed UASB reactors for hydrogen production.At a constant hydraulic retention time of 30h,the specific hydrogen production rate was 187mL/g volatile solids (VS)reduced during max-imum biohydrogen production of 7960mL/day at a 1:10ratio of press mud to sewage.Chemical oxygen demand (COD)and VS reductions of 61%and 59%were noted on peak biohydrogen yield.A pH range of 5–6was suitable at ambient temperature for entire process;a lower pH was inhibitory.Co-digestion of acidic press mud with sewage controlled pH for fermentation.Hence press mud can be exploited for bio-hydrogen production.Ó2010Elsevier Ltd.All rights reserved.1.IntroductionHydrogen is a promising alternative to carbon-based fuels be-cause it is clean,renewable and has a high energy yield of 122kJ/g (Chang and Lin,2004).At present,hydrogen is produced mainly from fossil fuels,biomass and water using chemical or bio-logical processes.One such biological process is anaerobic hydro-gen fermentation which can utilize highly concentrated organic wastewater and biomass,such as municipal solid wastes and sew-age sludge as raw material (Das and Veziroglu,2001).Press mud is another potential waste source for hydrogen production.About 4%of crushed sugar cane is converted to press mud,and annually about 4.2million tons of press mud is available from the sugar industry in India (Saravanane and Radjaram,1998).It is an acidic,thus mildly corrosive,compressed fibrous waste that contains 5–15%sugar (Partha and Sivasubramanium,2006)and 84%is bio-degradable.Earlier studies on press mud examined the feasibility of methane production by methanogenesis at pH of 8(Saravanane and Radjaram,1998).In contrast,hydrogen production is favoredat an acid pH (G´omez et al.,2006;Mohanakrishna et al.,2010).As a semi-solid material,press mud cannot be fermented unless water or sewage is added.Fermentation can be carried out in anupflow anaerobic sludge blanket (UASB)process since this anaero-bic treatment system has a high treatment efficiency and a short hydraulic retention time (HRT)(Chang and Lin,2004).Fermenta-tion in a continuous stirred tank reactor does not seem practical since this type of reactor was unable to maintain high level of fer-mentative biomass for hydrogen production and its specific hydro-gen production rate was low (Yu et al.,2002).Dilution is a key parameter in anaerobic fermentation as low dilution leads to prod-uct inhibition and high dilution leads to wash out of biomass in reactor (Han and Shin,2004).In the present study water and sew-age were used to dilute press mud to allow efficient fermentation.2.Methods 2.1.SeedSeed sludge was collected from a municipal wastewater treatment plant maintained by the Public Works Department,Pondicherry,India.It was sieved through a wire mesh of Ø0.5mm to remove solid materials that may block the flow in the pump.Cow dung was mixed with water at a 1:2ratio and digested under anaerobic conditions for 30days at a pH between 5and 6by adding HCl/NaOH as needed.The digest was filtered through wire mesh (Ø0.5mm)to remove fibrous materials,mixed with seed sludge at 4:1ratio,and heated to 70°C for 1h to inhibit the methanogens,and used as seed.The characteristics of the seed (fermented cow dung diluted with sludge)are given in Table 1.0960-8524/$-see front matter Ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.biortech.2010.11.075⇑Corresponding author.Tel.:+914132643007,mobile:+919443748471;fax:+914132643008.E-mail addresses:radjaram@ (B.Radjaram),saravananae@ (R.Saravanane).1Tel.:+914132655281x210,mobile:+919345156037;fax:+914132655101.2.2.Feed substrateThe characteristics of press mud and sewage are given in Table2. In preliminary experiments it was determined that a minimum dilution of1:7.5of press mud and water was necessary to obtain a hydrogen fermentation substrate that can befiltered and ulti-mately used in a continuousflow system.Therefore,feeds for reac-tor R1were prepared for three series of experiments by diluting 4kg press mud with30,40and50L of water to obtain dilution ratios of1:7.5,1:10,and1:12.5,respectively.Feeds for reactor R2 were prepared at the same dilution ratios by adding sewage instead of water.The press mud/sewage and press mud/water mixtures were allowed to soak for4h,filtered through wire mesh (Ø0.5mm)to removefibers and thefiltrate was fed into the reac-tors.The characteristics of the feed substrate are given in Table1.2.3.Bioreactor systemTwo UASB reactors of volume20L each were used in this study. The cylindrical plexiglas reactors had an internal diameter10cm and a height of190cm(H/D>10)with a gas liquid separator at the top(diameter,20cm;height,20cm)with an effluent port at 15cm from the bottom of the gas liquid separator.Seven sampling ports were evenlyfixed over the entire height of the column at 25-cm interval.The gas liquid separator housed an inverted funnel connected to a wet gas meter through aflexible tube for measuring the gasflow rate.The reactor had provisions for feeding from the bottom and removal of effluent from the top.The reactor received feed from a feed tank through a peristaltic pump(Ravel RH P100L) as shown in the schematic diagram(Fig.1).The feed substrate in the feed tank was stirred at every20min for3min with a mechan-ical stirrerfitted in the tank to prevent settlement of solids.2.4.OperationReactors R1and R2were operated with press mud diluted with water and sewage,respectively.HRT wasfixed at30h since a high-er HRT does not lift the particulate substrate in the reactor column. The experiments were started in R1and R2with a1:7.5dilution and performance was observed till steady state was reached in both reactors.The operation was continued at steady state for30 additional days.Then,the dilution was changed to1:10and the performance was monitored for30days.Finally the dilution was changed to1:12.5for another30–34days.The pH of the reactors was between5and6,by adding HCl/NaOH as needed with the feed and the reactors were operated at ambient temperatures (30–38°C)without heating or cooling.2.5.Analytical methodsTemperature,pH and biohydrogen production were measured daily with a thermometer,pH probe and wet gas meter,while the gas composition was analyzed using a gas chromatograph (Nucon GC).The total volatile solids(VS),chemical oxygen demand (COD),volatile fatty acid(VFA),alkalinity,etc.were estimated oncea week by standard methods(APHA,1995).3.Results and discussion3.1.Start-up of UASB reactorDuring the start-up period,the UASB system was fed continu-ously with diluted substrate at a concentration starting from 2500to10,000mg COD/L to reach an organic loading rate(OLR) of8–20g/L/day.The pH in the reactor decreased from6.5to5.5 during the start-up period,while hydrogen production(Fig.2b) and COD removal efficiency(Fig.2c)gradually increased.Biohy-drogen production started on day29in R1,with an uneven output for76days.After76days,both biohydrogen production and COD removal efficiency stabilized in the system,reaching approxi-mately1600mL/day and70%,respectively(Fig.2b and c).Hydro-gen production started on day34in R2with an unsteady output up to day60.After60days both hydrogen yield and COD removal efficiency stabilized.These results revealed that anaerobic acti-vated sludge of reactors R2and R1possessed good acid-tolerance and stable hydrogen production ability,which indicated good acclimatization.3.2.Continuous hydrogen productionR1reached steady-state conditions,defined as consistent hydrogen production with a variation of less than10%,after 90days(Fig.2b).The hydrogen production was1900mL/day with a1:7.5diluted feed.When,the dilution was changed to1:10onTable1Characteristics of seed and feeds of R1and R2for dilutions of1:7.5,1:10and1:12.5.Substrate pH TS TDS TSS TVS TVDS TVSS Total COD Alkalinity VFASeed 6.7610.9 2.058.85 6.81 5.810.4019.03 1.82 Reactor R11:7.5 6.467.7 2.25 5.45 4.651 3.6512.4812.68 1.50 1:10 5.017.7 2.45 5.25 5.5 2.4 3.117.8815.85 5.22 1:12.5 5.6515.4 2.213.2 4.20.8 3.429.95 6.34 1.68Reactor R21:7.5 5.29.97.6 2.3 4.25 2.27 1.9810.5627.36 2.04 1:10 5.187.2 1.5 5.7 3.90.5 3.432.0347.550.34 1:12.5 6.12 6.6 5.620.98 3.2 2.480.728.3219.03 5.52All units are in g/L except pH.Table2Characteristics of press mud and sewage.Parameters a Press mud SewagepH 4.5–57.2COD(%)117.613.6C/N ratio24.0417.5Total solids(%)297.4Moisture content(%)71–Total volatile solid(%)84 5.8Organic carbon(%)48.80 6.1Nitrogen(%) 2.05 1.96Phosphorous(%)0.65–Potassium(%)0.28–Sodium(%)0.18–Calcium(%) 2.7–Sulphate(%) 1.07–Sugar(%)3–Wax(%)1–a Average values.2774 B.Radjaram,R.Saravanane/Bioresource Technology102(2011)2773–2780day120,production gradually reached5000–6000mL/day by day 136.After144days it reached7210mL/day andfinally attained a maximum of7820mL/day during days145–150(Fig.2b).When the dilution was changed to1:12.5on day150,gas production de-clined and did not stabilize(Fig.2b).Maximum hydrogen produc-tion was4.887L/kg press mud at1:10dilution in R1.Reactor R2 took60days to reach steady-state conditions.At a1:7.5press mud:sewage ratio,maximum hydrogen production was1990mL/ day on day89.When the dilution was changed to1:10on day 90,the gas yield dropped below1000mL/day for1week time, but increased thereafter(Fig.4b).The highest hydrogen production of7960mL/day occurred on day115.At a dilution of1:12.5,max-imum hydrogen production was3880mL/day(Fig.4b).Maximum hydrogen of4.990L/kg press mud added was evaluated at a1:10 dilution in R2.These yields are lower than those obtained with wheat feed and sweet sorghum fermentations which were56L/ kg feed at HRT of15h(Hawkes et al.,2008)and10.41L/kg feed (Georgia et al.,2008),respectively;however,the use of such sub-strates for hydrogen production is questionable since they are also used as animal feed.The yields were higher than those achieved with Rhodobacter sphaeroides RV which reached only1.4–1.6L/L reactor volume/day(Fascetti and Todini,1995)and1.3mL/mL por-ous glass media/h(Tsygankov et al.,1994)and required input of light.Earlier studies had demonstrated that sewage sludge diges-tion can produce hydrogen at the rate of3.75mL/min(Nicolau et al.,2008)and that acclimatized anaerobic activated sludge had a hydrogen producing ability as high as10.4m3H2/m3reactor/ day in a continuous reactor with an available volume of9.6L (Ren et al.,1995.The co-digestion experiments with press mud and sewage demonstrated that sewage was able to support hydrogen production since it yields4.99L/kg press mud feed,i.e.0.5m3/m3reactor volume/day.H2%gradually increased with experiment days and reached a maximum of50–55%during consistency periods of study in reac-tors1and2(Figs.2and4b).3.3.Assessment of COD loading rate,%reduction and specific hydrogen production rateCOD removalfluctuated in the initial start-up stage and later stabilized after80days of operation in R1(Fig.2c).COD reduc-tion was60–77%,70%and86%at1:7.5,1:10,and1:12.5dilu-tions,respectively(Fig.2c).The high COD reduction was possible due to the large column height(H/D>10)which pro-vided a long mixing path during substrateflow from inlet to outlet.Filling the column to about one third its heights with biomass allowed maintenance of adequate numbers of hydro-gen-producing bacteria in the reactor.Specific biohydrogen pro-duction rates(SBPR)were10.98,40.6mL,and 2.58mL/g COD reduced/day at maximum hydrogen production at1:7.5,1:10, and1:12.5dilution,respectively(Fig.3a).In R2,COD loading rates varied between6and25g/L/day(Fig.4c)and COD reduc-tions were36–41%,55–61%,and40–41%at the1:7.5,1:10,and 1:12.5dilutions,respectively(Fig.4c).Anaerobic fermentation of food waste in leaching bed reactor gave biohydrogen yield of 21.2–41.5mL/g COD at an HRT of25h(Kim et al.,2004).Biohy-drogen yield was26.13mol/kg COD reduced in molasses fer-mentation(Ren et al.,2006)and 1.8–2.3mM/g COD fed in cheese processing waste water(Yang et al.,2007)whereas in our study41mL/g COD reduced was achieved from a wasteB.Radjaram,R.Saravanane/Bioresource Technology102(2011)2773–27802775substrate by co-digestion.SBPR in R2was20–37,35–41,and 39–51mL/g COD reduced/day at1:7.5,1:10,and1:12.5dilu-tions,respectively(Fig.5a).With a decrease in the dilution ra-tio,substrate becomes dense and product inhibition occurs due to excess VFA accumulation in the reactor,but higher dilu-tion led to wash out of bacteria and decreased biohydrogen pro-duction.Han and Shin(2004)observed a similar outcome in theirstudy.of experiments,(b)biohydrogen yield and%H2,(c)COD loading rate and%reduction,(d)VS loading rate and%reduction of reactor3.4.Appraisal of Volatile solid loading rate,%reduction and SBPRVolatile solid(VS)loading rates reached a maximum of13g/L/ day in R1(Fig.2d).VS reduction was55%,90%,and69%at1:7.5,1:10,and1:12.5dilutions,respectively(Fig.2d).Hydrogen yield gradually increased after75days and was consistently around 1600±150mL/day at a1:7.5dilution with a SBPR of12.77mL/g VS reduced/day on peak output.Though the SPBR was as highas32.81mL/g VS reduced/day,it was not constant.SBPR was62.66 and17.92mL/g VS reduced/day at a1:10and1:12.5dilution, respectively(Fig.3a).The substrate in the feed tank underwent slow hydrolysis which resulted in VS accumulation during the initial stages up to30days(Fig.2d).In R2,VS loading rate varied between3and16g/L/day(Fig.4d).VS reductionfluctuated in the start-up stage and stabilized under steady state condition.VS reduction was between53–63%,57–59%,and40–57%at the 1:7.5,1:10,and1:12.5dilutions,respectively(Fig.4d).SBPR was highest at the1:10dilution as187mL/g VS were reduced per day (Fig.5a).Co-digestion of municipal solid waste and slaughter house waste gave H2yield of52.5–71.3mL/g VS reduced(G´omezet al.,2006)and9.33L H2/g VSS/day was observed in rice winery waste water(Yu et al.,2002),whereas in our co-digestion H2yield was187mL/g/VS reduced.As the reactor had a height/diame-ter>10,1.9times more than the normal UASB reactor(Castello et al.,2009)the biomass concentration increased from the initial level of150to a maximum of1200mm.The digested substrate of low densityfloats and reaches the upper portion of reactor as evident by separation and collapsing of biomass layer in reactor which will be removed at regular period.Haandel van and Lettinga (1994)noted similar observation in theirstudy.3.5.Observation of VFA,alkalinity and pHThe VFA concentration in the effluent of R1was high during the start-up stage and declined gradually(Fig.3c)indicating acid to hydrogen conversion after30days.The later increase was likely due to the accumulation of sludge in the reactor by the digested substrate mass.The sludge was removed daily at a rate of2L at the sampling port and this process reduced VFA accumulation to below15g/L.Before the production increased above1000mL/ day,a separation or parting of biomass layer in the reactor oc-curred at300mm from the inlet port.Subsequently the biomass level was separated by150mm gap of liquid.This separation oc-curred once in every3days showing an effective anaerobic reac-tion by upward movement of digested substrate.The separation layer lasted for4–12h,as the gas rose to the surface and particles settled.The VFA level was constant at a1:10dilution with 9516mg/L(Fig.3c).Sporadically discharging sludge is a feasible means of dealing with sludge production from a biological system. In order to appraise the performance of a UASB reactor,it is imper-ative to know whether the reactor is operated at its utmost sludge holdup.Hence,the reactor could be operated by periodically dis-charging excess sludge,before sludge attains a maximum level (Haandel van and Lettinga,1994).VFA production followed an un-steady profile during start-up in R2,but reached consistency dur-ing steady state gas yield(Fig.5c).The VFA of the effluent was 19,202,180,and240mg/L at1:7.5,1:10and1:12.5dilutions, respectively(Fig.5c).Alkalinity was lower at the1:10dilution in R1(9516mg/L)than at the other dilutions and gave maximum biohydrogen yield (Fig.3d).In R2,alkalinity levels of the feedfluctuated between 60and41g/L during the start-up period,rose to57g/L at the 1:10dilution and declined to19g/L at the1:12.5dilutions (Fig.5d).The hydrogen yield was maximum at elevated alkalinity of feed(57g/L)and effluent(38g/L)while VFA levels were low.Feed pH was maintained between5and6by acidic press mud, an added advantage for fermentation.Studies on municipal solid waste and slaughter house waste fermentation found5–6as opti-mum pH(G´omez et al.,2006)and food waste with sewage sludge co-digestion was at a pH of6(Kim et al.,2004)indicating particu-late substrates requires a pH range of5–6.Due to anaerobic fer-mentation,the pH of the effluent was always near6.3.6.Assessment of VSS/TSS and VFA/alkalinity ratiosThe volatile suspended solids to total suspended solids ratio (VSS/TSS)in the UASB reactor denotes the biomass components of the sludge and variation in VSS/TSS indicates change in biomass components.A VSS/TSS ratio of0.6–0.8has been reported in UASB reactor treating sewage(Haandel van and Lettinga,1994)and0.76 for a HRT of24h in sucrose treated in a UASB reactor was reported by Chang and Lin(2004).At a1:10dilution,a VSS/TSS ratio of1.1 (Fig.3b)and1.45–1.5(Fig.5b)gave maximum biohydrogen yield for R1and R2,respectively.At very low VSS/TSS ratios,the hydro-gen yield decreased due to high sludge productivity which affected the fermentation process.A low VSS/TSS ratio indicates the organic fraction degradation.VFA/alkalinity ratios of0.01for R2and of0.03 for R1in1:10diluted press mud(Figs.3and5b)provided for max-imum hydrogen yields.The concentration of VFA in the feed and effluent showed a considerable increase with increasing organic loading rates(Figs.3and5c).4.ConclusionsDigestion of press mud and sewage at a1:10dilution and at a 30h HRT resulted in the production of approximately2.75L H2/kg press mud.As press mud is of no use other than as soil fertilizer,it could be fully exploited to produce hydrogen.From4.2million tons of press mud available annually in India,it is estimated that 11.55million m3hydrogen could be produced.Only limited acid/ base additions were necessary to maintain a pH of5–6,resulting in cost savings.ReferencesAPHA,1995.Standard Methods for the Examination of Water and Wastewater.American Public Health Association,New York,USA.Castello,E.,Garcı´a y Santos,C.,Iglesias,T.,Paolino,G.,Wenzel,J.,Borzacconi,L., Etchebehere,C.,2009.Feasibility of biohydrogen production from cheese whey using a UASB reactor:links between microbial community and reactor performance.Int.J.Hydrogen Energy34,5674–5682.Chang,F.Y.,Lin,Chiu-Yue,2004.Biohydrogen production using an upflow anaerobic sludge blanket reactor.Int.J.Hydrogen Energy29,33–39.Das, D.,Veziroglu,T.N.,2001.Hydrogen production by biological processes:a survey of literature.Int.J.Hydrogen Energy26,13–28.Fascetti,E.,Todini,O.,1995.Rhodobacter sphaeroides RV cultivation and hydrogen production in a one-and two-stage chemostat.Appl.Micobiol.Biotechnol.44, 300–305.G´omez,X.,Mor´an,A.,Cuetos,M.J.,S´anchez,M.E.,2006.The production of hydrogen by dark fermentation of municipal solid wastes and slaughterhouse waste:a two-phase process.J.Power Sources157(2),727–732.Georgia,A.,Gavala,Hariklia N.,Skiadas,Ioannis V.,Angelopoulos,K.,Lyberatos, Gerasimos,2008.Biofuels generation from sweet sorghum:fermentative hydrogen production and anaerobic digestion of the remaining biomass.Bioresour.Technol.99,110–119.Haandel van,A.C.,Lettinga,G.,1994.Anaerobic Sewage Treatment.Wiley,Inc., London.Han,S.K.,Shin,H.S.,2004.Biohydrogen production by anaerobic fermentation of food waste.Int.J.Hydrogen Energy34,5674–5682.Hawkes,F.R.,Forseya,Helen,Premiera,Giuliano C.,Dinsdalea,Richard M.,Hawkesa, Dennis L.,Guwya,Alan J.,Maddya,Jon,Cherrymana,Samantha,Shineb,James, Autyc,David,2008.Fermentative production of hydrogen from a wheatflour industry co-product.Bioresour.Technol.99,5020–5029.Kim,S.H.,Han,Sun-Kee,Shin,Hang-Sik,2004.Feasibility of biohydrogen production by anaerobic co-digestion of food waste and sewage sludge.Int.J.Hydrogen Energy29,1607–1616.Mohanakrishna,G.,Kannaiah Goud,R.,Venkata Mohan,S.,Sarma,P.N.,2010.Enhancing biohydrogen production through sewage supplementation of composite vegetable based market waste.Int.J.Hydrogen Energy35,533–541. 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