Impact of Wastewater Treatment Plant Discharge on Water Quality of a Heavily Urbanized River in Milan Metropolitan Area: Traditional and Emerging Contaminant Analysis

Wastewater treatment plants (WWTPs) can still be considered point sources of contamination into receiving aquatic ecosystems, especially for many emerging contaminants, which require additional treatments for their removal. In this study, the impact of a WWTP on the water quality of a river located in the metropolitan area of Milan, Northern Italy, was investigated. A wide range of emerging contaminants (i.e., perfluorinated compounds, pharmaceuticals, and synthetic fragrances) and traditional contaminants (i.e., heavy metals, nutrients, and microbiological parameters) were analyzed, both in the river water and in the wastewater at the inlet and outlet of the WWTP, with the aim of evaluating removal efficiency and the risk for the riverine ecosystem. The results showed that wastewater treatment acts differently on the analyzed compounds, effectively removing nutrients, bacteria, few pharmaceuticals, and most heavy metals, but leaving others unchanged such as perfluorinated compounds and synthetic fragrances, that are thus discharged into the receiving river, especially during rain events due to the activation of sewer overflows. The calculation of the Risk Quotient for organic compounds confirmed the negative impact of the WWTP effluent on the chemical quality of the river water, with a consequent potential ecological risk for riverine biota. This study also verified that certain traditional contaminants (i.e., total nitrogen (TN), total phosphorous (TP), thermotolerant coliforms, Escherichia coli), and contamination tracer (i.e., chloride (Cl), boron (B), and MBAS (Methylene Blue Active Substances) could be effectively measured in real time rather than through classical laboratory analysis and could support timely risk assessment.