Mechanisms and Pilot-Scale Validation of Iron-Loaded Biochar-Based Tidal Flow Constructed Wetlands for Enhanced Deep Nitrogen Removal from Wastewater Treatment Plant Effluent
To address low nitrogen removal efficiency in wastewater treatment plant (WWTP) effluent due to insufficient carbon sources and weak reoxygenation in conventional constructed wetlands (CWs), a tidal flow-subsurface flow integrated CW using iron-loaded biochar (BC-TF) as substrate was developed, with zeolite-based CW as control. Simulated wastewater experiments, water quality monitoring, nitrification/denitrification intensity assays, and high-throughput sequencing were employed. Results showed that tidal flow operation significantly enhanced removal of total nitrogen (TN) and ammonia nitrogen (NH4+-N), and increased nitrification intensity. Addition of iron-loaded biochar significantly improved TN and nitrate nitrogen (NO3−-N) removal, with BC-TF achieving an average TN removal of 86.03%, significantly higher than other groups (P<0.001). Microbial analysis revealed Proteobacteria, Actinobacteria, and Bacteroidetes as key phyla; iron-loaded biochar increased microbial abundance and diversity in tidal flow wetlands, while tidal flow alone reduced bacterial diversity. Pilot-scale experiments confirmed that tidal flow increased dissolved oxygen and nitrogen removal. This study is the first to combine iron-loaded biochar with tidal flow-subsurface flow CWs, systematically revealing the synergistic nitrogen removal mechanism of 'iron-loaded biochar-tidal flow-microorganisms', clarifying the role of iron-nitrogen coupling, and validating engineering applicability via pilot tests. The combination enhances reoxygenation, supplements carbon sources, and optimizes microbial community structure, effectively improving deep nitrogen purification of WWTP effluent, providing technical reference for tailwater treatment.