Technologies For Methane Production Enhancement And The Influencing Mechanism On The Anaerobic Microbial Community

Considering the demand of water-treatment industry in energy saving and carbon reduction,it is urgent to develop a series of new low-energy-consumption wastewater treatment systems.In these systems,anerobic methane production is an important technology to recover methane from raw wastewater or by-products(sludge).By further coverting methane to heat or electricity and supplying it to other processes,it is possible to achieve the reduction of whole system's energy consumption.However,the perfermances of existing anaerobic biotechnologies in producing methane from domestic wastewater are not satisfying.For example,anaerobic processes are generally inefficient when treating dilute wastewater or under psychrophilic condition,and are sensitive to inhibitory substances in wastewater,and generally require a long period to startup.Behind all these technical bottlenecks,the core problem is that methane-producing flora is highly sensitive to environmental factors and is slow to grow and metabolize under adverse environmental conditions(eg.low strength,low temperature,inhibitory factors).“Introducing bioelectrolysis system” and “introducing conductive materials” into anaerobic digestion system,are two promising strengthening methods for affecting anaerobic microbial community and simulating methanogens.Therefore,this study investigated the effects of these two methods and applied them in anaerobic processes to enhance methane recovery from wastewater.Firstly,two groups of membrane-less microbial electrolysis cells(MEC)were started up under closed-circuit(RCC)and open-circuit(ROC)condition,respectively,to simulate the anerobic systems with(without)bioelectrochemistry.It was found that the COD removal and the methane production rate of RCC were increased by 38% and 1.38 times,respectively,compared with ROC.The enrichment process of functional microbial populations in two systems were tricked by dynamic simulation.It was found that hydrogenotrophic methanogens and electrochemical-active bacteria were enriched,while the enrichment of acid-producing bacteria was limited in RCC.The results of electron balance analysis of RCC and ROC after startup,showed that fermentation type altered in RCC,the proportion of electrons transferred into acetate and hydrogen increased from 54% to 72% and electrons remained in propionate and butyrate decreased from 36% to 18%.In addition,about 16% transferred from acetate to hydrogen due to the bioelectrolysis process,which resulted in a significant improvement in methane production efficiency.Secondly,the influences of two conditions – applied voltage and azo dye acid orange 7(AO7),on methane production process in anaerobic biofilm systems with conductive carriers,were investigated.It was found that applying conductive granular graphite as fillers can induce the formation of direct interspecies electron transfer(DIET)between methanogens and exoelectrogenic bacteria,and thus promote the methane prduction efficiency.Compared with the system with nonconductive carriers,the methane production activity and methane production rate increased 8.3-and 3.8-times,respectively,by applying conductive fillers.In systems with conductive fillers,applying external voltage significantly influenced archaeal community structure,while adding AO7 in influent caused bacterial community structure shift and enriched considerable exoelectrogenic bacteria,so each condition slightly enhanced methane recovery efficiency.Applying two conditions simultaneously,however,generated a synergistic effect,specifically enriched genera which were related with DIET methane-producing process,such as Desulfobulbus,Enterococcus,Methanosarcina and Desulfuromonas,thereby significantly promoted methane production performance,whose methane production activity and methane production rate increased 1.5-and 1.4-times,respectively,compared with control group without voltage or AO7.Finally,A UASB-MEC coupling reactor was established by combining a up-flow anaerobic sludge bed(UASB)reactor and a MEC.Key parameters of MEC unit and electrodes position were optimized.It was found that increasing the dimensional ratio between cathode and anode,applying a 0.9 V external voltage and using nickel mesh as cathode significantly improved methane-producing and electrochemical performance of MEC unit,and placing electrodes above the sludge bed(in aqueous phase)is more advantageous than placing them in sludge bed.Sludge fermentation liquid was treated in optimized UASB-MEC reactor,whose methane production rate and methane yield increased 53% and 86% compared with normal UASB,and start-up period shortened from 45 days to 22 days.A modified anaerobic filter(MAF)was established by applying conductive granular graphite as fillers,and applying voltage and adding AO7 during the start-up stage.MAF achieved a methane production rate of 0.2 m3/(m3·d)and a methane yield of 0.13 m3/kg COD under HRT of 4 h.Compared with conventional anaerobic biofilter,the methane production rate and methane yield of MAF increased 42-and 13-times domestic wastewater.Experimental results proved the feasibility of strengthening methane production efficiency by introducing bioelectrochemcial systems or introducing conductive materials into anaerobic systems,which provided new technical ideas for recovering energy from domestic wastewater.