Advancing in grey water footprint evaluation of mixed multiple outlets: The efficacy of a weighted approach vs. conventional method

The stress exerted on aquatic environments by wastewater discharge outlets, including those from industrial parks, can be quantified through a refined assessment of the grey water footprint (GWF). The conventional total GWF (TGWF), widely used in practice, sums the GWFs of mixed outlets without identifying a critical pollutant (CP), which is essential for effective pollution control and management strategies. A novel methodology was developed to calculate the weighted total grey water footprint (W-TGWF), integrating all discharge outlets by considering dilution effects and identifying the final CP. This study, conducted over four months from October 2022 to January 2023, evaluated TGWF and W-TGWF simultaneously for mixed outlets of treatment processes, Activated Sludge (AS) and Membrane Bioreactor (MBR), operated in parallel to treat wastewater from an industrial park in Isfahan province, central Iran. The mixed influent wastewater at the equalization tank, representing the total inlet GWF, ranged from 1.55 to 2.06 million cubic meters per month (MCM/month). For mixed outlets, W-TGWF consistently yielded lower values than TGWF, showing reductions of 22%, 13%, 26%, and 5% from October to January. The periodic average W-TGWF was 15,304 m 3 /month based on NH 4 as the CP. In contrast, TGWF reached 18,909 m 3 /month, 24% higher, due to the simultaneous influence of NH 4 and F parameters without definitive CP identification. These findings show that TGWF tends to overestimate environmental stress and obscure pollutant prioritization. The weighted approach offers a more accurate and ecologically meaningful framework for assessing mixed GWFs, with direct implications for water management and regulation.