Can saturation levels’ optimization abate the pressure of C/N ratio variation and low temperature on vertical flow constructed wetland for nitrogen removal?

Currently, it remains unclear whether optimizing saturation levels can mitigate the adverse effects of carbon-to-nitrogen (C/N) ratio fluctuations and low temperatures on nitrogen removal in vertical flow constructed wetlands (VFCWs). To answer this, eight VFCWs with different saturation levels were operated under different influent C/N ratios in different seasons. The impacts of saturation levels on water purification performance and its underlying mechanisms were then investigated. The results showed that optimizing saturation levels significantly enhanced nitrogen removal efficiency ( P < 0.05). At low (C/N = 3.3) and middle (C/N = 6.3) ratios, a 77.7 % saturation level achieved optimal TN removal efficiencies of 53.0 % and 81.5 %, respectively. However, at high C/N ratios (10.9), saturation levels ≤ 55.5 % proved more effective, exhibiting TN removal rates ≥ 79.2 %. When subjected to low temperatures (< 15 °C), the 77.7 % saturation level maintained the highest TN removal efficiency (40.0 %). Notably, TN removal efficiency showed significant quadratic relationships with saturation levels at low and middle C/N ratios ( P < 0.05), while showing significant negative linear correlations at high C/N ratio ( P < 0.05). Partial saturation operation created optimal redox conditions for functional microbial communities in VFCWs. Specifically, ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were enriched in the unsaturated zone, while denitrifying bacteria thrived in the saturated zone . This spatial stratification of microbial populations effectively explains the observed pollutant removal patterns. Furthermore, redox potential and temperature emerged as the primary environmental factors governing microbial composition, population, networks, and nitrogen-related functional bacteria. These findings provided new insights into promoting nitrogen removal efficiency of VFCWs by saturation levels’ optimization. • At low and middle C/N ratios, high saturation level was better for TN removal. • At high C/N ratio, TN removal was optimal at half saturation levels or lower. • Under low temperatures, TN removal was promoted at high saturation level. • Quantitative correlation between saturation levels and removal efficiency was established. • Optimal saturation levels enhanced microbial distribution, composition, and network.