Sulfur cycle contributes to stable autotrophic denitrification and lower N2O accumulation in electrochemically integrated constructed wetlands: Electron transfers patterns and metagenome insights

Excessive discharge of nutrients from wastewater treatment plants (WWTPs) is an important pollutant source of eutrophic water bodies. In this work, three electrochemically integrated horizontal flow constructed wetlands (E-HFCWs) were developed for advanced nutrients removal from WWTPs effluent with different S/N ratios. In E-HFCWs, PO43-, NO3--N and TN removal percentages at current of 0.2 A and hydraulic retention time (HRT) of 24 h did not differ significantly as S/N ratios altered. When fed with low, middle and high concentrations of SO42--S wastewater during this period, PO43--P removal percentages respectively reached 99.3 %+/- 0.9 %, 99.2 %+/- 1.1 % and 99.0 %+/- 1.4 %, NO3--N removal percentages respectively reached 99.5 %+/- 0.5 %, 99.6 %+/- 0.4 % and 99.4 %+/- 0.8 %, and TN removal percentages respectively reached 92.0 %+/- 2.5 %, 90.8 %+/- 3.4 % and 91.2 %+/- 2.7 %. This work highlighted that sulfur cycle played crucial roles in improving nitrogen removal stability as current or HRT decreased in higher S/N ratio groups. The formed sulfur ferrites under higher current or HRT condition served as electron reservoir, and would resupply electron for denitrification when electron supplied by electrolysis was deficient. In addition, the higher S/N ratio groups allowed significantly lower N2O accumulation, which was accordance with the concept of carbon neutral. Based on metagenome results, the occurrence of more abundant sulfur-oxidizing denitrifying genes and bacteria (e.g., Thiobacillus) in higher S/N ratio groups under lower current or HRT further demonstrated the significant roles of sulfur cycle in stable autotrophic denitrification performance in E-HFCWs. Overall, this work provides perspective on the future practical application for the regulation of nitrogen removal stability enhancement and N2O emission reduction in electrochemically integrated bioreactors.

Automated summary

This study developed three electrochemically integrated horizontal flow constructed wetlands (E-HFCWs) for nutrients removal from wastewater treatment plants (WWTPs) effluent with different S/N ratios. The results showed that the removal percentages of PO43-, NO3--N and TN did not differ significantly as S/N ratios altered. The sulfur cycle played a crucial role in improving nitrogen removal stability in E-HFCWs. Furthermore, the higher S/N ratio groups allowed significantly lower N2O accumulation, which was in line with the concept of carbon neutral. Metagenome analysis demonstrated the significant role of sulfur cycle in stable autotrophic denitrification performance in E-HFCWs.