The substrate configuration influences pollutant removal in constructed wetlands: from the aspects of submerged status of substrate and carbon-felt distribution

Abstract

Redox regulation dominates the pollutant removal in constructed wetlands (CWs). To enhance efficient and cost-effective nitrogen removal, this study intended to build an unsaturated zone and add carbon-felt material for electron donor/acceptor adjustment. The unsaturated zone heights (0, 10, 20 cm) and carbon-felt distribution patterns (evenly scattered (CW_SE), continuously linked (CW_L), and head-tail linked like microbial fuel cells (CW_MFC)) were simultaneously adjusted. Moreover, their effects and underlying microbial mechanisms on water purification were investigated. Results indicated that CWs with a 20 cm unsaturated zone achieved over 99% ammonia nitrogen removal. CW_SE facilitated optimal pollutant-microbe contact, enabling efficient in-situ electron utilization for 64.27% total nitrogen removal through simultaneous nitrification-denitrification and anammox. In CW_L, continuous carbon-felt distribution allowed efficient electron transport at a relatively macro-area and enhanced electron consumption by oxygen at the surface, leading to superior ammonia oxidation (82.97%) in the middle area of CW_L. Conversely, CW_MFC facilitated direct electron transfer through the whole CW, enriched Geobacter at the top and Vibrio at the bottom, achieving 84.23% total nitrogen removal through nitrification-denitrification under high oxygenation. This study elucidated microbial community niche differentiation in CWs mediated by carbon-felt electron transport and proposed optimal application scenarios for different carbon-felt configurations.