Study On Medium Chain Fatty Acids Biosynthesis Base On Anaerobic Fermentation Products Of Waste Activated Sludge

Anaerobic fermentation is an excellent strategy for energy and organic matter recycling from sludge.At present,most studies focus on the conversion of sludge into low-value-added products such as methane,hydrogen,and short chain fatty acids(SCFAs)during anaerobic fermentation,while research on high-value-added products such as medium chain fatty acids(MCFAs)is currently rare.Therefore,this study explores the feasibility of producing MCFAs by using the waste activated sludge(WAS)as a substrate during anaerobic fermentation.The main conclusions are as follows:The effect of different pretreatment on SCFAs production during anaerobic fermentation was investigated.Four commonly used pretreatment methods,namely thermal-alkaline(THALK),ozonation(OZN),electrolysis(EC)and Na Cl O-promoted electrolysis(EC-AOP),could facilitate WAS solubilization and SCFAs production.Among these,THALK group had the highest dissolved organic carbon(DOC)and SCFAs production.EEMFPARAFAC and FT-ICR-MS were employed to characterize the transformation of dissolved organic matter(DOM)in WAS fermentation liquors at molecular level.Proteins and lipids were the dominant DOM in the pretreated WAS.Overall,these results demonstrated that THALK effectively enhanced release of SCFAs and DOM,which is the best pretreatment in this study.In the pure culture,the ethanol/carboxylate ratio and substrate concentration had significant effects on the yield of MCFAs.The highest yield of MCFAs was found when ethanol/carboxylate ratio was 2:1 and total carbon(TC)concentration was 600 m M.In this condition,the shortest lag phase(4 d),the highest caproate concentration(8.9 g/L)and the best carbon efficiency(89.3%)were achieved.Then,fermentation liquor of THALK pretreated sludge was used as substrate to produce MCFAs.The yield of caproate was 6.2 g/L,which was 70%lower than that based on pure substances,due to the effect of DOM in fermentation liquor,like humic acids.However,caproate production in this study was higher than that of other biological wastes,such as vegetable and fruit wastes,which indicated that it is feasible to produce MCFAs with WAS fermentation liquor as substrate in pure culture.In open culture,the MCFAs yield and microbial community diversity were significantly affected by the HRT and ethanol/acetate ratio in continuous reactors.When the HRT was 5 d and ethanol/acetate ratio was3:1,the yield of caproate was highest(1400 mg/(L·d)).After 120 d of domestication,the abundance of Clostridium sensu stricto was as high as65.2%,which positively related to the production of MCFAs,indicating that the medium chain fatty acid-producing bacteria became dominant and stable in the system.When fermentation liquor of THALK pretreated sludge was used as fermentation substrate,the yield of MCFAs was maintained at a stable level,which was 1.9 g/(L·d).The mechanism analysis revealed that the fermentation liquor used as substrate could enhance the fatty acid biosynthesis pathway(FAB)and reverse β-oxidation pathway(RBO)during the chain elongation(CE)process,improving the production rate of MCFAs.These results indicated that it is feasible to produce MCFAs with WAS fermentation liquor as substrate in open culture.