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Wastewater treatment plant (WWTP) effluent is discharged into rivers as supplemental water, which may result in ecological risk. This study compares the element composition and microbial community of WWTP effluent and natural surface water (NSW) and reveals the potential ecological risk of WWTP effluent discharge. Twenty recently upgraded WWTPs and three relatively large reservoirs in Zhengzhou city, China, were selected. The contents of N, P, S, K, Ca, Mg, B, Si, Na, Fe, Mn, Co, Ni and Sn were significantly higher in WWTP effluent than in NSW, while those of Mo, V, Pb and Cd were significantly lower. There was no significant difference between WWTP effluent and NSW in terms of the element imbalance index (IMI) (representing the extent of imbalance of element proportions) relative to the class IV surface water quality standard (the control standard for most Chinese rivers). The macronutrient IMI relative to the Hoagland formula was significantly lower in WWTP effluent than in NSW, and WWTP effluent discharge could significantly lower this index in NSW; this may be an important cause of primary productivity explosion. The microbial diversity was significantly higher in WWTP effluent than in NSW. The predicted relative abundances of mobile genetic elements and oxidative-stress-tolerant phenotypes were significantly higher in WWTP effluent than in NSW, whereas the abundance of gram-negative phenotypes was significantly lower, and that of potential pathogenic phenotypes was slightly lower. The effluent from upgraded WWTPs exhibited a low risk of pathogen diffusion but a high risk of antibiotic resistance gene diffusion. The element composition and microbial community should be considered when evaluating the ecological risk of WWTP effluent discharge. [Display omitted] • WWTP effluent discharge can alleviate the macronutrient imbalance in the receiving water. • Alleviation of elemental imbalance can lead to problems related to eutrophication. • Upgraded WWTP effluent discharge has a low risk of pathogen diffusion. • Upgraded WWTP effluent discharge has a high risk of antibiotic resistance gene diffusion. [ABSTRACT FROM AUTHOR]

Numerous studies have been conducted on the advanced treatment of wastewater treatment plant (WWTP) effluent by constructed wetlands (CWs); however, no studies have focused on the effect of the naturalization function of CWs on WWTP tailwater, i.e., reducing imbalances in the elemental composition and microbial community, compared with natural surface water (NSW). Therefore, the possible naturalization functions of CWs were evaluated by comparing the elemental composition, microbial structure and function in NSW with the influent and effluent of CW. Surveys were conducted in seventeen full-scale wetlands constructed to treat WWTP tailwater in eastern China, and the largest lake in the region, Taihu Lake, was selected as the reference for NSW. The imbalance index (IMI) was proposed to characterize the difference in the elemental proportions between tailwater and the references. The results showed that the IMI values for the WWTP effluent ranged from 0.51 to 0.10 and from 0.95 to 0.37 for macroelements and microelements, with average values of 0.30 and 0.62, respectively. When macronutrient IMI>0.27 or micronutrient IMI>0.43, the CWs alleviated the element imbalance; otherwise, the CWs increased the IMI and aggravated the imbalance. With the Hoagland formula as the reference, a similar result was obtained. CW treatment reduced the difference in bacterial composition between tailwater and NSW. According to FAPROTAX and BugBase function prediction, CW treatment reduced the difference in the relative abundance of microbial functions related to C, N, and S cycles, pathogenicity and antibiotic resistance genes between tailwater and NSW. The CW area and hydraulic retention time were significantly positively correlated with the naturalization index. Therefore, the CW system can naturalize tailwater in terms of both elemental composition and microbial composition and function, indicating the necessity of re-evaluating CW functions in treating WWTP tailwater. [Display omitted] • Tailwater naturalization is making its biochemical composition closer to groundwater. • The naturalization function of CWs for tailwater was evaluated. • CW treatment generally alleviated the imbalance of elemental composition in tailwater. • CW treatment naturalized the bacterial community characteristics of tailwater. • CW area and hydraulic retention time influence CW naturalization effectiveness. [ABSTRACT FROM AUTHOR]