The role of grey water footprint in urban wastewater management: from pollution to reuse.

Abstract

The grey water footprint (WF_grey), defined as the volume of freshwater required to dilute pollutants below ambient water quality standards, has gained increasing relevance as a qualitative indicator of wastewater impact on receiving water bodies (RWB). This study evaluates the WF_grey using real data from a municipal wastewater treatment plant (WWTP) and its associated RWB over an eight-year period (2016-2023). A wide range of pollutants were considered, including organic compounds, solids, nutrients, and heavy metals. The analysis considered three scenarios: (a) untreated wastewater discharge, (b) treated effluent discharge, and (c) treated wastewater reuse for agricultural purposes. Results indicate that the WWTP significantly reduces pollutant loads, with scenario (b) showing an average 8-fold decrease in WF_grey compared to scenario (a). Ammonium (in scenario (a)) and phosphorus (in scenario (b)) emerged as the primary contributors to WF_grey, highlighting the limited phosphorus removal due to the absence of dedicated chemical or biological treatments. Lead was found to be the most impactful heavy metal in several instances, underlining the importance of monitoring low-concentrated contaminants. Ammonium and total nitrogen showed the highest removal efficiency, whereas phosphorus removal was relatively inefficient, reinforcing the need for future plant upgrades. Furthermore, the reuse scenario (c) demonstrated the potential of nutrient-rich treated effluent for irrigation, particularly during dry summer months, offering dual benefits of pollution reduction and resource recovery. Overall, WF_grey proved to be a robust and accessible indicator for evaluating WWTP performance, identifying critical pollutants, guiding plant optimization, and supporting sustainable discharge and reuse strategies. Future developments should expand monitoring to emerging contaminants such as pharmaceuticals, pesticides, microplastics, and additional heavy metals to ensure comprehensive environmental protection.