Start-up and microbial mechanisms of low-voltage electrochemically integrated constructed wetlands: Effect of inoculated source

Abstract

Nitrogen (N) and phosphorus (P) species in wastewater treatment plants (WWTPs) effluents are important sources of nutrient input to water bodies, resulting in deteriorated eutrophication. The combination of electrochemical technology and constructed wetland provides emerging prospects for tertiary treatment of WWTPs effluents. In this work, three bench-scale electrochemically integrated vertical flow constructed wetlands (E-VFCWs) were established in parallel with different inoculated sources. Among them, the E-VFCW inoculated with anaerobic sludge (AN) of a swine wastewater treatment plant exhibited significantly shorter start-up period (9 d), followed by anoxic sludge (A) from a municipal WWTP (18 d), and no inoculum (NI) exhibited the longest start-up duration (30 d). In addition, AN group allowed the highest removal efficiencies (NO3--N (97.0 ± 1.6 %), TN (83.3 ± 4.6 %), and PO43--P (93.3 ± 3.3 %)) in tertiary wastewater treatment, and lower concentrations of SO42--S and total Fe in effluents. Results of microbial structure and Tax4Fun suggested that multi-path metabolisms including H2-, Fe(II)-, and FemSn-dependent denitrification might facilitate NO3--N reduction in E-VFCWs. The microbial mechanisms that AN exhibited the highest denitrification efficiency in the E-VFCWs may include the highest bacterial copies number and the more abundant denitrifying genes. Moreover, it should be noted that electron transfers mediated by sulfur (S) cycle might significantly enhance NO3--N reduction in E-VFCWs, especially in AN group. To conclude, the study offers new microbial insights into N conversion and S cycling patterns in electrochemically integrated systems in response to inoculated source.