Effects of flow mode on the performance and microbial community in tidal flow constructed wetland treating mariculture wastewater

This study aimed to investigate the influence of flow mode (down-flow and up-flow) on the removal of organic matter and nitrogen, and their transformation processes in two tidal flow constructed wetlands (TFCWs) fed with simulated mariculture wastewater with 2.4% salinity and wastewater without adding salt (0.0% salinity). Downflow mode was beneficial for COD removal (98.28-99.62%), while up-flow mode improved the denitrification capacity and benefited the removal of total inorganic nitrogen (TIN) (81.46-88.26%). High salinity decreased the NH4+-N removal (70.42-73.05%) compared with 0.0% salinity (80.57-86.04%), and lead to higher NO2xe213; -N accumulation. High-thought sequencing indicated flow mode, salinity and depth along the TFCWs together alter the composition and abundance of functional bacteria in the wetlands. Higher abundance of ammonia oxidizing bacteria (AOB) was observed under down-flow mode (0.3-3.7%) compared with up-flow mode (0.2-0.9%) in two TFCWs. Denitrifying bacteria (DNB) enriched in 38-43 cm depth layer in saline system and 0-5 cm depth layer in freshwater system, with the relative abundance of 18.3-63.8%. High salinity inhibited the growth of AOB and nitrite oxidizing bacteria (NOB), while DNB could adapt to salinity stress and maintained high abundance in saline system. The enrichment of halotolerant/halophilic bacteria and diverse nitrogen removal pathways provided a solid foundation for the efficient nitrogen removal in TFCW treating mariculture wastewater.