Electron flow assisted COD removal in wastewater under continuous flow conditions using microbial electrochemical system

Highlights: • The MES with continuous flow and raw sewage show enhance COD removal by electron flow. • The initial raise of current density contributed to most of the COD removal enhance. • The anode function restriction level was 200 mg L⁻¹ for COD and 120 mg L⁻¹ for SCOD. • Only 7% of energy in reduced COD was converted to sludge resulting a low yield rate. • Sludge yield rate of 28 mg g⁻¹-COD was obtained with 30 days sludge retention time. Microbial electrochemical system (MES) was an emerging technology for synchronous power generation and pollution removal. During current generation, the COD removal performance of air-cathode MES fed with domestic wastewater was greatly enhanced, compared with the condition absent of current. Based on the simulation of first-order kinetics, the COD removal rate was investigated with the increasing of current densities from 0 to 1.61 ± 0.13 A m⁻² by external resistances regulation. In this process, the overall COD removal kinetics constants were increased by 110% from 0.075 ± 0.005 to 0.157 ± 0.010 h⁻¹. The background COD reduction unrelated to current generation was greatly stimulated by the initial rising of current density (from 0 to 0.273 ± 0.003 A m⁻²) and then limited by the substrate competition with exoelectrogens. The effluent with low COD concentration was found to restrict the power output of downstream anodes with a COD limit level of 200 mg L⁻¹ and SCOD of 120 mg L⁻¹. In MES, only a small fraction (−1-COD with a sludge retention time of 30 days. Therefore, the low sludge yield and electron-flow-assisted COD removal should be recognized as the two main advantages of MES rather than power generation in wastewater treatment.