Performance of a Trickling-Bed Biocathode Microbial Electrochemical System Treating Domestic Wastewater and Functional Microbial Community Characteristics  
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School of Municipal and Environmental Engineering, Shenyang Jianzhu University, No.25 Hunnan Middle Road, Hunnan New District, Shenyang 110168, China; [EMAIL_ADDRESS] (H.W. ); [EMAIL_ADDRESS] (Z.M. ); [EMAIL_ADDRESS] (Y.L.) *Correspondence: [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (G.W. ); Tel. : +86-24-246-907-09 (L.C. ); +86-24-246-907-08 (G.W); +86-24-246-907-08 (G.W); +86  
useful products and remove unwanted compounds, such as nitrogen compounds, which are common pollutants in agricultural, industrial, and domestic wastewater [10; 11; 12; 13]. Since the anaerobic condition in the anode of an MES does not effectively facilitate total nitrogen removal, incorporating an aerobic process into an MES for nitrification becomes a key step for efficient nitrogen removal through bioachode MESs. The first demonstration of complete nitrogen removal in an MES was accomplished with the aid of an external biofilm-based aerobic reactor for nitrification [14]. In this system, the anodic effluent containing ammonia was pumped into an external aerated vessel, where ammonia was biologically oxidized to nitrate, and the nitrification liquid was subsequently flowed into the cathodic chamber for denitrification. Even though this \"loop-configuration\" successfully coupled nitrogen removal with energy recovery from acetate, it was unable to achieve low nitrogen levels in the effluent, largely due to the ammonia diffusion through the cation exchange membrane. To resolve this shortcoming, the subsequent design integrated the nitrification stage into the cathodic chamber, thus promoting simultaneous nitrification and denitrification (SND) in the same chamber [15]. Though a high nitrogen removal efficiency of 94.1 \(\pm\) 0.9% was obtained in the MES, extra energy should be provided for aeration of the catholyte. It was estimated that the energy consumed by an activated sludge-based aerobic process (0.3 kWh m\({}^{-3}\)) was about one order of magnitude higher than the average energy produced by an MES treating domestic wastewater [16]. To reduce energy consumption associated with aeration, some rotating biocathodes were developed, which were able to achieve an oxygen gradient through agitation of the electrolyte and, consequently, produced aerobic and anoxic zones for nitrification and denitrification [17; 18; 19]. Though the membrane-less design of these systems could reduce the construction cost, it would facilitate the contact between organic matter and nitrate, thereby leading to significant heterotrophic denitrification and inhibiting bioelectrochemical denitrification (carried out by autotrophic denitifying bacteria that are capable of accepting electrons from cathode). Consequently, the benefit of using MESs' electricity-generating feature for nitrogen removal would be weakened [12]. Further improvement of energy reduction was accomplished in a tide-type bioachode MES [20]. Using the siphon principle for periodical water drainage, the biocathode was able to achieve intermittent aerobic/anoxic conditions during a continuous feeding and periodical draining process. Though energy-intensive aeration was obviated, and net energy output was achieved in this tide-type bioachode MES, the biocathode material preparation procedure should be further simplified for practical application [21]. Biotrickling filters (BFs), a common biological treatment technology for gaseous volatile organic compound, have been coupled with MES for treating ethyl acetate in waste gas [22; 23; 24]. These previous results demonstrated the feasibility of BF-MES integrated systems for coupling the biodegradation of gaseous contaminants with the generation of electricity.