Enhancing denitrification under external salt stress in constructed wetlands for coastal wastewater treatment☆

Water eutrophication, one of the primary environmental threats to global coastal wetlands, significantly disrupts salinity balance and ecological functions, leading to water quality deterioration, biodiversity loss, and a severe threat to the stability and sustainability of wetland ecosystems. Salt ion stimulation can impact nitrogen conversion processes and microbial community distribution in traditional biological treatments. Constructed wetlands (CWs) often face the challenge of insufficient carbon sources when treating coastal wastewater with a low C/N ratio. Meanwhile, algae in eutrophic waters of rivers and lakes can greatly enhance nutrient removal processes in CWs, not only through adsorption but also by serving as a source of algal carbon. This study aims to investigate the purification efficiency and decontamination mechanisms of CWs in treating low C/N ratio eutrophic water under salinity conditions. The study also examines the production and release of microcystins (MCs) under varying salt ion concentrations and C/N ratios. Biological analyses were conducted to illustrate the interaction between bacterial community diversity and nitrogen conversion genes. The results reveal that salinity stimulation influences nitrification and denitrification processes by altering microbial community distribution in CWs, while also inhibiting the production and release of MCs. Under 3 parts per thousand salinity stimulation, the nitrogen removal rate increases by 10 %, and the phosphorus removal rate reaches 80 % and mitigates the inhibitory effect of algae on the denitrification process. These findings suggest that CWs can effectively recycle algae under salt ion stress without causing excessive accumulation of MCs and offer a sustainable and environmentally friendly approach for coastal wetland restoration.