Impact of operating mode variability on pollutant removal and microbial dynamics in a stacked hybrid constructed wetland: Implications for performance optimization

This study innovatively developed a stacked hybrid constructed wetland, integrating the advantages of both free-water surface and subsurface flow constructed wetlands for enhanced treatment of sewage plant effluent. The effects of three different operation modes-Anoxic subsurface flow, Oxic subsurface flow, and Oxic subsurface flow with step-feeding-on sewage plant effluent treatment were thoroughly examined. Results indicated that all three modes exhibited excellent pollutant removal capabilities. Notably, the average removal rates of NH₄⁺-N, TP, and COD approached 100% under the Oxic subsurface flow with step-feeding mode, while the average removal rate of TN reached 48.18 ± 1.86%. Additionally, the findings show that different operating modes significantly influenced the primary functional microorganisms in the wetland system. Paenarthrobacter, Nitrospira, and Dechloromonas emerged as dominant bacteria in both the Oxic subsurface flow mode and the Oxic subsurface flow with step-feeding mode, contributing to a diversified nitrogen metabolism pathway. The stacked hybrid constructed wetland demonstrated robust pollutant removal efficiency and rich microbial diversity, making it suitable for the advanced treatment of sewage treatment plant effluents.