| Microbial electrochemical systems(MESs)can convert the chemical energy in the organic pollutants into electricity by utilizing exoelectrogens as biocatalyst,and simultaneously recover energy in the form of electricity.As a promising technology for wastewater treatment and energy recovery,MESs have intensively been researched in the field of electrode material modification,reactor configuration and extracellular electron transfer mechanism.However,most MESs exhibit decline in electricity generation,high in energy consumption and poor performance in simultaneous carbon and nitrogen removal when treating organic wastewater containing complicated substrates,which hinder their practical application on wider scope.Therefore,improving the degradation efficiency of complicated organic matters and reducing the energy input for nitrogen removal are key factors in optimizing MESs configurations.Based on these criterions,microbial electrochemical systems being able to accomplish carbon and nitrogen removal and simultaneous net energy output were developed in this study.Syntrophic interactions between microbial communities and their influence on organic degradation process,as well as mechanism for nitrogen removal were fully discussed,which were essential for establishing enhanced wastewater treatment technology based on MESs.A continuous stirred microbial electrochemical system(CSMES)was developed to enhance complicated substrates removal and electricity generation,which was designed by integrating continuous stirred tank reactor(CSTR)and MES.Maximum power densities of 583 ± 9,562 ± 7,533 ± 10 and 572 ± 6 m W/m2 were obtained by each cell of the system when it was operated in a four-independent circuit mode at an OLR of 12 kg COD/m~3/d.COD removal,methane production rate and energy recovery efficiency were 87.1 ± 1.1%,1.48 ± 0.15 L/L/d and 32.1% of the CSMES,which were superior to the parallelized operating CSTR.Pyrosequencing analysis demonstrated that current generation led to acclimation of Geobacter(14.5%)on CSMESAnode,which was the most predominant known exoelectrogen.Due to the relatively complicated bacterial communities,the CSMES possessed high relative abundance both in acetoclastic methanogens(52.1%)and hydrogenotrophic methanogens(47.0%),while the CSTR was only dominated by acetoclastic methanogens(79.1%).Syntrophic process that occurred between anaerobic digestion in the complete mixing zone(CMZ)and current generation in the microbial electrochemical zone(MEZ)enhanced the removal of volatile fatty acids(VFAs)and released their inhibition on methanogens,which consequently increased the diversity and activity of the methanogens,enhanced organic removal of CSMES and increased its energy recovery efficiency.Brewery wastewater was used as feed to investigate the degradation mechanism of complicated organic compounds in CSMES.The CMZ was used for hydrolysis and acidification,where 79.1 ± 5.6% of soluble proteins and 86.6 ± 2.2% of soluble carbohydrates in brewery wastewater were first degraded by fermentative bacteria(Clostridium and Bacteroides,19.7% and 5.0%),with short-chain VFAs as the main products for methanogens(Methanosaeta and Methanobacterium,40.3% and 38.4%).A portion of VFAs as well as the undegraded soluble proteins and carbohydrates were transferred to the MEZ along the hydraulic path,where they were further utilized by acetogenic bacteria(Syntrophobacter,20.8%)and exoelectrogens(Geobacter,12.4%)for electricity generation,which polished the effluent quality and recovered electric energy from the brewery wastewater.The cascade degradation process was essential for CSMES to maintain its electricity production constant when dealing with complicated organic compounds,while a clear spatial distribution of the four functional microbial groups and syntrophic interaction among them was the biological foundation of this cascade degradation process.An air-enriched biocathode microbial electrochemical system(ABMES)was developed based on siphon principle for effluent drainage,which was used to reduce the energy input of MESs for nitrogen removal.In order to enhance the oxygen adsorption ability during the intermittent air enriched process,columnar activated carbon(CAC)with half of its surface coated by polytetrafluoroethylene(PTFE-coated CAC)was fabricated as biocathode material.The ABMES with PTFE-coated CAC biocathode achieved a maximum power density of 8.2 ± 0.8 W/m~3,which was 39% higher than that of the untreated CAC biocathode.The higher accessibility of air due to the hydrophobic surface enhanced oxygen transfer to the cathodic biofilm and consequently increased its air-capture rate(29.7 ± 0.6 L/m~3)by 54 ± 3.8%,which was the primary cause for the better performance of the PTFE-coated biocathode.Internal resistance analysis under both reoxygenation and oxygen sufficient conditions suggested the higher biocatalytic activity of the cathodic biofilm was a minor factor for the better performance of the PTFE-coated CAC biocathode.Nitrogen removal was achieved in the cathode chamber of ABMES through simultaneous nitrification and denitrification(SND).The cathodic oxygen reduction reaction,nitrification and denitrification process were influenced by the external resistance,COD/N ratio and frequency of catholyte feeding-draining process.At external resistance of 5 ? and frequency of 8 cph,the ABMES achieved a maximum power density of 8.9 ± 0.2 W/m~3 and removed 53.2 ± 3.8% of the total nitrogen(TN)when treating reject water with a relatively low COD/N ratio of 2.5.The intermittent air enriched method consumed 14.3% of the total energy produced,which obviated energy-intensive aeration process for nitrogen removal.Electron balance analysis showed that the combination effects between heterotrophic den itrification and bioelectrochemical denitrification contributed to the higher TN removal of ABMES under low COD/N ratio condition.The CSMES-ABMES combination system was used to treat swine wastewater to investigate its performance in terms of carbon and nitrogen removal.The COD and SS concentration in swine wastewater decreased to 6745 ± 522 mg/L and 2441 ± 185 mg/L after coagulation-flocculation pretreatment with polyaluminium chloride.The CSMES was responsible for COD and SS removal while the ammonia ni trogen and TN were removed by the ABMES.The CSMES-ABMES combination system removed 97.7 ± 4.5% of the COD,94.4 ± 5.8% of the SS,85.1 ± 3.3% of the ammonia nitrogen and 43.8 ± 2.3% of the TN,and simultaneously achieve d a net energy production of 1.298 k Wh/m~3,demonstrating the combination system could accomplish simultaneous carbon and nitrogen removal and be operated in an energy self-sufficient manner. |
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