Cold temperature mediated nitrate removal pathways in electrolysis-assisted constructed wetland systems under different influent C/N ratios and anode materials

Electrolysis had proven to be useful for the enhanced performance in constructed wetlands (CWs). While at cold temperature, the nitrate removal pathways, plant physiological characteristics and microbial community structure in electrolysis-assisted CWs were unclear. Therefore, the purification performance of three electrolysis-assisted horizontal subsurface-flow constructed wetlands (E-HSCWs) with different anodes and a control system in cold seasons were evaluated in this study. E-HSCWs showed a 2.02-83.21% increase of total nitrogen (TN) removal when compared to control, and the gaps were enlarged with increasing C/N (chemical oxygen demand/total nitrogen, COD/TN) ratios. Nitrite accumulation in E-HSCWs presented a first increase then went down trend with increasing C/N ratios, compared to a steady increase in control system. The optimum C/N ratio was 8 in E-HSCWs for both TN and COD removal. Moreover, Ti vertical bar IrO2-Ta2O5 (Ti) anode showed the highest potential for TN and COD removal. Less root weight, shorter root length and reduced TN and total phosphorus (TP) contents in roots were observed in wetland plants (Iris sibirica) of E-HSCWs. In E-HSCWs with Fe and C anodes, the nitrate removal was mainly accomplished by autotrophic denitrifier Hydrogenophaga. While in E-HSCWs with Ti anode, the synergistic effect of autotrophic denitrifier Hydrogenophaga and heterotrophic denitrifiers Acidovorax, Sim-plicispira, Zoogloea accounted for the nitrate removal. These results showed that E-HSCWs at proper C/N ratio of 8 would be promising for nitrate removal at cold temperature.

Automated summary

This study evaluated the purification performance of different anodes in electrolysis-assisted horizontal subsurface-flow constructed wetlands (E-HSCWs) during cold seasons. The results showed that E-HSCWs at a proper COD/TN ratio of 8 have the potential for efficient nitrate removal.