From organic load to nitrate legacy: grey water footprint as a long-term benchmark for treated sugar industry wastewater

Sugar production generates high-strength wastewater that poses a persistent threat to aquatic ecosystems, particularly in water-scarce regions. While wastewater treatment plants (WWTPs) are deployed to mitigate this impact, their performance is traditionally evaluated through pollutant removal efficiency, a metric that can mask the significant residual environmental pressure of the final effluent. This study employs the grey water footprint (GWF) to conduct a holistic, five-year (2021-2025) assessment of a sugar production facility in Isfahan, Iran, quantifying the volume of freshwater required to assimilate the pollutant load from both raw and treated wastewater. The analysis of raw wastewater revealed an exceptionally high pollution load, culminating in a Nitrate-dominated GWF of 167.3 m³ per ton of sugar produced. Following treatment via an upflow anaerobic sludge blanket (UASB) system, the effluent was monitored for key pollutants (BOD, COD, Nitrate, Phosphate). Diagnostic ratio, Principal Component Analysis (PCA), and a 95% confidence interval uncertainty analysis collectively and unequivocally identified Nitrate as the most unstable and critical parameter, consistently dictating the annual GWF. The treatment process achieved a substantial reduction, lowering the average GWF to 1.4 m³/ton, a decrease of over 99%. It was demonstrated that the GWF provides a more accurate and environmentally relevant benchmark than removal percentages alone, transforming the perception of treatment efficacy. The findings underscore the imperative to optimize treatment trains for robust Nitrate removal and to integrate GWF metrics into regulatory frameworks for sustainable water resource management in the sugar industry and analogous sectors.