Research On The Performance And Operation Optimization Of Constructed Wetland-Microbial Fuel Cell Systems

The constructed wetland-microbial fuel cell（CW-MFC）systems have emerged in recent years with the aim of treating wastewater while simultaneously producing electricity.The current research on CW-MFCs mainly refers to the effects of one or two factors on the electricity output and organics removal of CW-MFCs.In this study,lab-scale bio-cathode single-chamber upflow vertical subsurface flow CW-MFC systems were built,aiming to conduct in-depth research on the characteristics and operation optimization of CW-MFCs for treating domestic wastewater.It is expected that the results from this study will provide reliable theoretical analysis and experimental data support for CW-MFCs treating domestic wastewater.The main conclusions were as follows.Hydraulic retention time was the most influential factor for the removal of organics,nitrogen and phosphorus in CW-MFCs;effluent reflux ratio and DO concentration in the cathode zone affected organics and nitrogen removal by affecting the internal DO distribution in CW-MFCs;the external resistance had an extremely significant and decisive influence on the electricity output of CW-MFCs,and significantly affected organics and nitrogen removal,and greenhouse gases（CO₂,CH₄ and N₂O）emission in CW-MFCs by affecting the output current;the main mechanism of electricity production in CW-MFCs was by direct transfer of electrons to the anode by electrogens growing on the electrode,and it was mainly by influencing the anoxic/anaerobic micro-environment in the aerobic cathode area that the volume ratio of granular graphite in the substrates affected TN removal in CW-MFCs;the ambient temperature significantly affected the removal of organics and nitrogen,electricity generation,and greenhouse gases emission in CW-MFCs.Incorporating the MFC significantly improved CODcr removal efficiency（by7.5%–15.0%）in CW-MFCs through enhancing the degradation of organics by electrogens on anodes,in the meantime,the electrogens on anodes competed with methanogens for organics and thus decreased CH₄ emission from CW-MFCs by22%–37%;the bacteria on MFC cathodes can use the produced electrons to reduce nitrite and nitrate,which significantly improved TN removal efficiency（by9.6%–17.8%）in CW-MFCs as well as decreased N₂O emission by 18%–39%;the MFC could reduce the dependence of denitrification on organic carbon sources in CWs,influent C/N ratio=3 could result in a TN removal of>90%and the lowest emission of CO₂ equivalents（4.17 g/m²·d）in CW-MFCs;the physio-chemical processes of substrates were the primary way of phosphorus removal in CW-MFCs.The comprehensive optimal operating conditions in favor of wastewater treatment of CW-MFCs,which were obtained by the orthogonal experiment and mechanism analysis,were:volume ratio of granular graphite（or other adsorption materials with rich porous structure）in the substrates’surface to 5cm（the cathode zone）≥10%,DO concentration in the cathode zone=1.5 mg/L,hydraulic retention time=1.5 d,effluent reflux ratio=50%,and external resistance≤250Ω;under the comprehensive optimal operating conditions the average removal rates of COD_Cr,NH₃-N,TN,PO₄^3--P and TP in CW-MFCs were 88.23%,88.33%,90.91%,88.53%and 87.04%,respectively;based on the results of orthogonal experiment,the pollutants degradation regression model obtained by stepwise regression showed relatively accurate prediction results about the average pollutants removal rates in CW-MFCs under different operating conditions.