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Photovoltaic electrolysis improves nitrogen and phosphorus removals of biochar-amended constructed wetlands

Gao, Yan, Yan, Cheng, Wei, Ruping, Zhang, Wen, Shen, Jianing, Wang, Mingxuan, Gao, Bin, Yang, Yicheng, Yang, Liuyan
Ecological engineering 2019 v.138 pp. 71-78
adsorption, ammonium nitrogen, anodes, biochar, cathodes, chemical precipitation, constructed wetlands, electrochemistry, electrolysis, energy flow, flocculation, hydrogen-oxidizing bacteria, ions, iron, nitrate nitrogen, nitrogen, phosphates, solar energy, wastewater treatment
To deeply remove nitrogen (N) and phosphorus (P) from constructed wetlands (CWs), the authors found that CWs can be combined with the use of photovoltaic energy for best results. Thus, electrolysis-integrated, biochar-amended, horizontal (subsurface)-flow-constructed wetlands (E-BHFCWs) were combined with free-water, surface-constructed wetlands (FWSCWs) to remove N and P from secondary effluent water of a wastewater treatment plant (WWTP). Photovoltaic energy was used for electrolysis to intensify the removal of nitrate nitrogen (NO3−-N), ammonium nitrogen (NH3-N), and phosphate phosphorus (PO43−-P) in E-BHFCWs. In addition, FWSCWs were constructed to further remove NO3−-N, NH3-N, PO43−-P and iron ions in effluent of E-BHFCWs and to enhance DO concentration. For the E-BHFCWs combined with FWSCWs, the removal rates of NO3−-N, NH3-N, and PO43−-P were 73.28%, 53.11% and 67.58% respectively. The removal amounts of NO3−-N, NH3-N, and PO43−-P were about 211.08 kg N/a, 64.81 kg N/a and 10.46 kg P/a by E-BHFCWs. Photovoltaic electrolysis in E-BHFCWs was able to enhance NO3−-N removal through electrochemical reduction by iron cathodes, enriched hydrogenotrophic bacteria in the substrate, and by modified biochar substrate in situ with iron ions. Meanwhile, NH3-N was mainly removed by modified biochar adsorption, nitrifying microbes, and plant uptake. The removal rate of PO43−-P was also enhanced due to the formation of ferric ions by the anodization of sacrificial iron anodes, causing chemical precipitation, physical adsorption, and flocculation of PO43−-P. Thus, E-BHFCWs combined with FWSCWs are capable of effectively promoting removals of NO3−-N, NH3-N, and PO43−–P and provide a new insight into a new energy flow that can enhance N and P removals of CWs.