Integrated constructed wetland-river (ICWR) system for efficient nitrogen removal from secondary effluent:...

To address the challenge of balancing high efficiency and cost-effectiveness in nitrogen removal from secondary effluent, a novel integrated constructed wetland-river (ICWR) system was developed in this study. This system comprised four sequential units: a first-stage constructed wetland (CW1), a first-stage river reach (R1), a second-stage constructed wetland (CW2), and a second-stage river reach (R2), arranged as CW1-R1-CW2-R2. The performance of ICWR system in purifying real secondary effluent from a wastewater treatment plant (WWTP) as well as its cost-effectiveness and carbon footprint were evaluated. The ICWR system achieved an annual average TN removal rate of 65.0 % for secondary effluent with a low C/N ratio (2.06) and TN concentrations of 7.74-16.90 mg/L. Compared to conventional tertiary treatment processes like denitrification biofilters (DNBFs) and standalone constructed wetlands (CWs) under similar conditions, the ICWR system demonstrated 26.62 % and 28.05 % higher TN removal rates, respectively. It also maintained strong performance during cold season, with a TN removal rate of 54.6 %, about 40.0 % higher than standalone CWs. Seasonal synergy was evident, with CW1 contributing most in warm periods and R2 dominating in cold seasons. Moreover, the ICWR system reduced treatment costs by 78.6 % and 64.0 % compared to DNBFs and CWs, and lowered carbon emissions by 55.4 % on average relative to DNBFs. This study presents an innovative and practical solution for efficient, economical, and eco-friendly advanced nitrogen removal, particularly suitable for cold climates and economically disadvantaged regions. It serves as an alternative to traditional grey tertiary treatment processes within WWTPs, offering a nature-based engineering approach that is implemented outside the WWTP.