Nutrients Mass Balance in an Integrated Constructed Wetland–Microbial Fuel Cell System Using Activated Carbon

Abstract

This study investigated the nutrient mass balance in an integrated constructed wetland–microbial fuel cell (CW–MFC) system, using activated carbon as both CW substrates and MFC anodes. Results indicated that in the closed-circuit CW–MFC system, nitrogen (N) primarily accumulated in substrates, accounting for 47.16%, and was ultimately removed via microbial denitrification, accounting for 48.37%, while plant uptake contributed to only 2.28% removal of N. Phosphate (P) was predominantly removed by substrates accumulation, amounting to 96.81%, and <2% of P was removed via plant uptake and microbial assimilation. MFCs facilitated a 5.3% enhancement in TN removal within CWs by bolstering the microbial denitrification at cathodes and microbial assimilation of NH₃-N at anodes. The conventional microbial denitrification and MFCs contributed to 43.10% and 5.27% removal of N, respectively. Plants facilitated the removal of NH₃-N by 5.5% and TN by 3.4% in CW–MFC systems, primarily through indirect mechanisms. Neither MFCs nor plants significantly influenced the removal of organic N and orthophosphate ( ${\text{PO}}_4^{3-}$ -P) in CW–MFC systems. Activated carbon significantly contributed to the high removal of P in CW–MFC systems due to its exceptional adsorption capacity and to N removal through the synergistic effects of its adsorption capacity, electrochemical properties, and biofilm-mediated degradation. Three electroactive bacteria genera, Citrobacter, Geobacter, and Pseudomonas constituted 15.0–16.6% of the microbial community on AC anodes in closed-circuit CW–MFC. Nine nitrifying genera, Nitrospiraceae, Nitrosomonas, Candidatus Nitrososphaera, Candidatus Nitrosotenuis, Nitrolancea, Candidatus Nitrosotalea, Nitrosospira, Candidatus Nitrosoarchaeum, and Nitrosomonadaceae constituted 2.5–8.5% of the microbial community across different components of CW–MFCs, and only Denitrobacterium (0.6–2.0%) was detected as denitrifying bacteria.