Regulation Of Metabolities In Mixed Culture Fermentation

Most of the organic matter will be converted to methane in mixed culture fermentation(MCF)and the rest could produce biomass and recalcitrant compounds.Clearly,some other valuable products(alcohols,volatile fatty acids,etc.)will be produced via MCF if the methanogenic process is inhibited.However,these biofuels are required further separation and purification to be used for downstream processes.Therefore,a series of parameters have been controlled to regulate of fermentation products distribution in this study,such as temperature,pH and ammonium concentration.The results provide a theoretical reference for the production of fatty acids and methane via MCF.The main contents and results include:1.This study aimed to develop an in-situ sludge pretreatment method by increasing the temperature from thennophilic to extreme thermophilic condition in a single-stage anaerobic digester.The results revealed that a stable performance was obtained within the temperature range of 55?-65?,and the maximum methane yield of 208.51±13.66 mL/g VS was obtained at 650C.Moreover,the maximum extent of hydrolysis(33%)and acidification(27.1%)was also observed at 65?.However,further increase of temperature to 70? did not improve the organic conversion efficiency.Microbial community analysis revealed that Coprothermobacter,highly related to acetate oxidisers,appeared to be the abundant bacterial group at higher temperature.A progressive shift in methanogenic members from Methanosarcina to Methanothermobacter was observed upon increasing the temperature.This work demonstrated single-stage sludge digestion system can be successfully established at high temperature(65?)with stable performance,which can eliminate the need of conventional thermophilic pretreatment step.2.Temperature is an important factor affecting sludge digestion performance.In this study,the effects of solid retention time(SRT)on a thennophilic(65?)single-stage wasted activated sludge(WAS)digester were investigated.The result showed that the optimum SRT was 6 days with methane yield of 186.16 mL/g VS.It was found that SRT had little effect on the hydrolysis and volatile solids(VS)destruction.Longer SRT promoted more recalcitrant compounds release and impaired the acidification,leading to the low methane yield.The microbial community analysis revealed that the dominant pathway for methane production at 65? with different SRTs was syntrophic acetate oxidizing bacteria-hydrogenotrophic methanogens(SAOB-HM)and acetoclastic methanogens were absent in the system.Form economic point of view,the net energy can be positive when the total solids(TS)concentration of the feed sludge is above 23 g/L.3.In this study,a mixture of primary and wasted activated sludge was fermented in a semi-continuous reactor aiming for enhanced volatile fatty acids(VFAs)production.The reactor was subjected to a stepwise pH increase from 7 to 10 during approximately 130 days of operation.The result revealed that the maximum acidification was obtained at pH 8.9(21%)resulting in the maximum production of VFAs(423.22±25.49 mg COD/g VSS),while the maximum hydrolysis efficiency was observed at pH 9.9(42%).The high pH was effective in releasing dissolved organic matter(DOM)including protein,carbohydrate,building blocks and low molecular weight(LMW)neutrals.More LMW DOMs were released than high molecular weight(HMW)DOMs fractions at higher pH.pH 9.9 favored hydrolysis of HMW DOMs while it did not enhance the acidogenesis of LMW DOMs.The microbial community analysis showed that the relative abundance of phyla Actinobacteria and Proteobacteria increased with the increased pH,which may lead to the maximum hydrolysis at pH 9.9.At pH 8.9,class Clostridia(59.16%)was the most dominant population where the maximum acidification(21%)was obtained.This suggested that the dominance of Clostridia was highly related to acidification extent.The relative abundance of Euryarchaeota decreased significantly from 58%to 2%with increased pH.4.Acetate is an important industrial chemical and its production from wastes via mixed culture fermentation(MCF)is economic.In this work,the effect of hydraulic retention time(HRT)on acetate production from tofu processing wastewater(TPW)in extreme-thermophilic(70?)MCF was first investigated.It was found that long HRT(>3 days)could lead to less acetate production while stable acetate production was achieved at short HRT(3 days)with the yield of 0.57 g-COD/g-CODTPW.The microbial community analysis showed that hydrogenotrophic methanogens(mainly Methanothermobacter)occupied up to 90%of archaea at both HRTs of 3 and 5 days.However,Coprothermobacter,the main acetate-degraders,decreased from 35.74%to 10.58%of bacteria when HRT decreased from 5 to 3 days,supporting the aggravation of syntrophic acetate oxidation in long HRT.This work demonstrated that HRT was a crucial factor to maintain stable acetate production from TPW in extreme-thermophilic MCF.5.High-purity propionate production from glycerol in mixed culture fermentation(MCF)induced by high ammonium concentration was investigated.Fed-batch experiments revealed that higher ammonium concentration(>2.9 g/L)had simultaneous negative effects on acetate and propionate degradation.Propionate production and yield was up to 22.6 g/L and 0.45 g COD/g COD glycerol,respectively,with a purity of 96%.Sequential batch experiments demonstrated that the yields of propionate were 0.3±0.05,0.32±0.01,and 0.34±0.03 g COD/g COD at a glycerol concentration of 2.78,4.38,and 5.56 g/L,respectively,and the purity of propionate was 91%-100%.Microbial community analysis showed that the phylum Firmicutes dominated the bacterial community at different glycerol concentrations.However,the Methanosaeta population decreased from 46%to 6%when glycerol concentration increased from 2.78 to 5.56 g/L,resulting in lower acetate degradation rate.