Life cycle assessment indicators of urban wastewater and sewage sludge treatment

The world is facing a water quality crisis resulting from continuous population growth, urbanization, land use change, industrialization, unsustainable water use practices and wastewater management strategies, among others. In this context, wastewater treatment (WWT) facilities are of vital significance for urban systems. Wastewater management clearly plays a central role in achieving future water security in a world where water stress is expected to increase. Life cycle assessment (LCA) can be used as a tool to evaluate the environmental impacts associated to WWTPs and improvement options. In this study, LCA is applied to compare the environmental performance of different scenarios for wastewater and sludge disposal in a WWT plant located in Southern Italy. The first scenario (BAU, Business As Usual) is based on the present sludge management performed within and outside the case-study plant: after mechanical treatment, dewatered sludge is transported by truck to a landfill for final disposal, while treated water is released to a river. The second scenario (B) assumes a partially circular pattern, with anaerobic fermentation of sludge to biogas, biogas use for electricity and heat cogeneration, integrated by additional thermal energy from previously recovered waste cooking oil (WCO), electricity and heat feedback to upstream WWT steps (including sludge drying), and final disposal of dried sludge to landfill and water to river. The third scenario (C) suggests an improved circular pattern with gasification of the dried sludge to further support heat and electricity production (with very small delivery of residues to landfill). The fourth scenario (D) builds on the third scenario in that the volume of treated wastewater is not discharged into local rivers but is partially used for fertirrigation of Salix Alba fields, whose biomass is further used for electricity generation. In doing so, the water P and N content decreases and so does the water eutrophication potential. Finally, a renewable scenario (E) is built assuming that the electricity demand of the WWT plant is met by a green electricity mix, for comparison with previous options. The most impacted categories in all scenarios result to be Freshwater Eutrophication Potential (FEP) and Human Toxicity Potential (HTP). Increased circularity through recycling in scenarios B and C reduces the process contribution to some environmental impact categories such as Global Warming Potential (GWP) and Fossil Depletion Potential (FDP), but does not provide significant improvement to FEP. Fertirrigation in scenario D lowers FEP by about 60% compared to the BAU scenario. Furthermore, HTP is lowered by almost 53%. Finally, other options are discussed that could be also explored in future studies to evaluate if and to what extent they could further improve the overall performance of the WWT plant.