Environmental impact of integrating decentralized urine treatment in the urban wastewater management system: A comparative life cycle assessment

As municipal wastewater treatment regulations become more stringent, integrating source-separated urine treatment into centralized urban wastewater management offers a ′hybrid′ solution. However, it is not clear how the environmental impacts of such hybrid systems compare to highly efficient centralized wastewater treatment plants (WWTPs) with low N2O emissions and electricity use. In this study, a consequential life cycle assessment was used to compare the environmental impact of three urine hybrid wastewater treatment systems which combine decentralized urine treatment with a highly efficient central WWTP — to a centralized WWTP treating mixed wastewater (baseline). The studied urine treatment systems include partial nitrification & distillation, struvite precipitation & stripping/scrubbing, and partial nitritation/anammox. Additionally, the contribution of urine alkalinization to the overall impact was quantified. The results show that at least one hybrid scenario showed a lower environmental impact in 8 out of the 10 assessed impact categories. Global warming potential and marine eutrophication were found to be higher than the baseline. Additionally, it was identified that urine alkalinization increased the environmental impact of the treatment system in 7 out of the 10 impact categories. A Pareto frontier analysis was developed to guide decision makers on where hybrid solutions could be used as a strategy to reduce global warming impacts of conventional WWTPs. It was realized that using N₂O emission factors of 75 WWTPs, 87% of centralized WWTPs had lower CO₂ emissions compared to partial nitrification & distillation, and 91% compared to partial nitritation/anammox hybrid solutions. However, at energy demands of 1 kWh/PE and 2 kWh/PE, both hybrid solutions showed lower emissions than all the studied WWTPs. The study highlights the potential of hybrid wastewater treatment solutions to address specific environmental challenges in wastewater management and a strategy to reduce global warming impacts in WWTPs with high N₂O emissions and electricity use.