Performance And Mechanism Of Nitrogen Removel And Electricity Generation In Anammox-MFC

Serious enrichment of reactive nitrogen and lack of organic carbon sources are major obstacles to wastewater nitrogen removal im China.Anaerobic ammonium oxidation（Anammox）shrinking the requirement for aeration and external carbon sources by converting ammonium（NH4+）and nitrite（NO2-）directly to nitrogen gas（N2）,has become a burgeoning nitrogen removal technology in recent decade.However,nitrate accumulation in anammox reactors has always been a limiting factor of its total nitrogen removal efficiency,which makes advanced nitrogen removal difficult under low-strength wastewater.Microbial fuel cells（MFC）utilize exoelectrogens as anodic biocatalysts to oxidize pollutants in wastewater and synchronously generate electricity.The development of anaerobic ammonia oxidation microbial fuel cell（ANAMMOX-MFC）is conducive to advanced nitrogen removal and energy recovery,which meets the development direction of nitrogen removal technologies.In this study,the ANAMMOX-MFC was constructed,their performances and working mechanisms were investigated,and a novel method optimizing nitrogen removal and power generation by adding subsequent carbon source was demonstrated.The main results are as follows:（1）The ANAMMOX-MFC was constructed and ran in sequencing batch（HRT 24h）.The nitrogen removal rate fluctuated between 67%and 97%in the start-up phase,and the voltage dropped from 392 mV to 57 mV.After 48 days of operation,the nitrogen removal rate stabilized above 95%and the voltage fluctuated between 21 mV and 62 mV.After 65 days of operation,the phenomenon of voltage rose in the second half of one cycle was observed.Long-term discharge experiments showed that the voltage continued to rise and gradually stabilized.Gas chromatography（GC）test showed that the anode solution contained volatile fatty acids（VFA）,which confirmed that the VFA production by hydrolysis of the anode biofilms led to the voltage swell.（2）Influent COD concentration less than 100 mg·L^-1 can promote denitrification and enhance nitrogen removal effect,more than 100 mg·L^-1 can inhibit anammox process,and the nitrogen removal effect became worse.Increasing influent COD concentration(100～150 mg·L^-1)had not observed the rise of output voltage,indicated that it had little effect on electricity generation.（3）The addition of subsequent carbon source can promote the performance of ANAMMOX-MFC in nitrogen removal and power generation.Before the addition of subsequent carbon source,the concentration of total nitrogen（TN）in the effluent of the anode chamber（COD/N-0.65）was less than 5.6 mg·L^-1,the removal rate of TN was above 95%,and the maximum power density was 0.01 Wm^-3,.After 100 mg COD/L sodium acetate was added as the subsequent carbon source when the device was operated 12 h,the concentration of effluent TN of the anode chamber（COD/N～1.5）was less than 1.0 mg～L^-1,the removal rate of TN was above 99%,and the maximum power density was 1.41 Wm^-3.Subsequent carbon source concentration has an effect on ANAMMOX-MFC power generation.Subsequent COD concentration less than 20 mg·L^-1 had no effect on power generation,more than 40 mg·L^-1 can promote power generation,and more than 80 mg·L^-1 can further enhance power generation.（4）Batch tests showed that the anammox and denitrification occurred synchronously in the first half of one operation cycle with the addition of subsequent carbon source,that is,the subsequent carbon source did not directly participate in the nitrogen removal,but only promoted the power generation.No nitrate accumulated at the end of the cycle,which solved the nitrate accumulation in anammox system.The subsequent carbon source as the substrate of heterotrophic bacteria,induced the activity of heterotrophic exoelectrogens and improved the power output of the system.was innovatively used as a "control switch" of high power generation for ANAMMOX-MFC.（5）High-throughput 16S rRNA genes sequencing on anode biofilms showed that Pseudomonas in replace of Comamonas became the main exoelectrogens after adding subsequent carbon source,and it was responsible to be a boost in power generation.In addition,the abundance of Candidatus Brocadia decreased.This study represented a proof of concept for the feasibility of the anammox synchronous denitrification in an integrative anode chamber under low C/N ratios,and provided an optimization strategy of electricity generation.These studies will broaden the current understanding of the interaction between anammox bacteria and heterotrophic bacteria and lay the foundation for the ANAMMOX-MFC wastewater treatment process scientific design and operation.