Treatment of low C/N wastewater containing phthalates in algal pond-constructed wetlands: Roles of extracellular polymeric substances and microbiome

Abstract

Carbon source deficiency is the crucial factor constraining conventional biological nitrogen removal in low C/N wastewater. Therefore, this study investigated nutrients and dimethyl phthalate (DMP) removal and mechanisms in an algal pond-constructed wetland system treating wastewater at different low C/N ratio. The findings indicated that the system exhibited optimal comprehensive treatment performance at a C/N ratio of 5. Under this condition, the removal efficiencies of ammonia nitrogen, total nitrogen and DMP reached 98.5 %, 56.8 % and 78.0 %, respectively, while nitrous oxide emissions were significantly reduced. Scenedesmus obliquus exhibited good growth. Meanwhile, extracellular polymeric substances (EPS) secreted by microorganisms were mainly soluble EPS, which served as a slow-release carbon source to supplement the electron donors required for denitrification and enhanced DMP biodegradation. Although an elevated C/N ratio reduced microbial community biodiversity and abundance, it significantly enhanced the relative abundance of functional microbes related to nitrogen transformation. Notably, the levels of denitrification functional genes (norB, nosZ, nirK, nirS) all improved, indicating enhanced genetic and metabolic potential for denitrification. These findings indicate moderately optimizing the influent C/N ratio can effectively and synergistically remove multiple pollutants and reduce greenhouse gas emissions in the integrated system, offering a theoretical foundation for the co-treatment and ecological remediation of multiple pollutants in low C/N wastewater.