| Treating low-organic strength wastewater at low-temperatures(below 20 ?)is recognized as one of the main challenges for the development and application of anaerobic digestion(AD).Under such conditions,significant inhibition on anaerobic digester sludge could result in decreases in both the substrates utilization efficiency and the maxium growth rate of microorganisms.Therefore,the key solutions to address the issue are promoting the electron transfer efficiency,enhancing the microbial metabolic activity,and accelerating the pollutants degradation in AD system.However,studies focusing on enhancing low-temperature AD in the aspect of improving the microbial metabolic activity were few.Herein,bio-electrochemical AD systems were developed to achieve high performance of low-temperature AD of low-organic strength wastewater,by enhancing the electron transfer process,and,by promoting the metabolic activity and evolution of microbial communities.Main findings are summarized as follows:This study provided a new approach by using bio-electrochemical systems(BES)to improve the performance of AD for treating low-organic strength wastewater and recovering energy in the form of CH4.The BES had a relatively higher durability against temperature shock.At an optimal applied voltage of 0.4 V,the BES significantly enhanced both the COD removal and CH4 yield efficiency compared with those of the control at operational temperatures of 20,12,and 8 ?.Energy balance evaluation indicated that the net energy obtained from the BES was higher than the energy recovered from the control,implying that operation of BES was economically available.Analysis of microbial physiological characteristics elucidated that electrochemical effects stimulated the production of EPS,suggesting a faster metabolism activity of biomass in the BES.16S rRNA sequencing showed that genera of H2-utilizing methanogens,Methanobacterium,Methanoregula,and Methanospirillum,dominated the consortia at low temperatures.On the surfaces of cathodes,considerable enrichments of H2-producing bacteria,Acetobacteroides and Anaerolinea were observed,proposing the formation of H2-mediated syntrophic communities in the BES.The effects of graphene(Gr)on methanogenesis from AD was investigated.Short-term results showed that graphene(25 and 100 mg/g VSS)had significantly positive effects on methane production rate by 17.0%and 51.4%,respectively.Further investigation clarified that acetate-consuming methanogenesis was enhanced.After 55 day's operation at room temperature(from 20 to 10 ?),the methane production rate with 25 mg/g VSS of Gr was 14.3%higher than that of the control,while the addition of 100 mg/g VSS of Gr showed a slight inhibition on methane yield.Illumina sequencing data showed that the archaeal community structure remained fairly constant as the incubated sludge with graphene at low temperature,in which Methanoregula,Methanoseata and Methanospirillum were the dominant species.Besides,the enrichment in the genus of Geobacter was observed with graphene,suggesting that the direct interspecies electron transfer between Geobacter and Methanoseata might be promoted.The methanogenesis performance,nanoparticles(NPs)transformation and methanogenic community development in anaerobic digester sludge under MnO2 NP supplementation were demonstrated in this study.MnO2 NPs(400 mg/g VSS)stimulated the CH4 yield by 42%with a final CH4 proportion of 81.8%of the total gas production.Meanwhile,the coenzyme F420 and ETS activities showed positive correlation with MnO2 NPs concentration.Microbial Mn reduction and oxidation occurred in conjunction with methanogenesis,resulting in transformation of the shape of the MnO2 NPs from wire-like to globular particles.Microbial community analysis indicated that the relative abundances of genera Methanobacterium,Methanosaeta,and Methanosarcina were higher in the presence of MnO2 NPs.Moreover,a new and different crucial synergy within the methanogenic community was formed with low-abundance consortia driving Mn respiration coupled to methanogenesis in AD.Finally,this study prepared modified electrodes with Gr/PPy and MnO2 NPs/PPy.BES with modified electrodes was developed to investigate their effects on the performance of methanogenesis from AD.Results showed that the electro-conductibility of modified electrodes increased about 50%than that of the blank carbon felt.At an operational temperature of 20 ?,the COD removal efficiencies and CH4 yield rates of the BES with Gr/PPy and MnO2 NPs/PPy modified electordes were 12.7%,25.6%,and 43.9%,66.3%higher than that of the control.Although the performance of all reactors decreased as the temperature dropping to 12 ?,the CH4 yield rates of Gr/PPy and MnO2 NPs/PPy were still 22.8%and 39.0%higher than that of the control.Further EPS and ETS activity analysis indicated that the modified electrodes could stimulate the metabolic activity of the activated sludge.SEM observation showed that the modified electrodes beared higher specific surface area,benefiting the attachment and formation of dense biofilms on the surface of electrodes.16S rRNA sequnceing results demonstrated that H2-utilizing methanogens dominated in the BES,and the influence of Gr/PPy and MnO2 NPs/PPy differed on the microbial structure of biofilms. |
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