Closing the loop in wastewater treatment: Evaluating environmental and energy gains from sludge-to-resource technologies

The untreated sewage not only degrades water quality but also contributes to greenhouse gas emissions and harms human health and ecosystems. In India, 37% of sewage is treated daily in conventional sewage treatment plants (STPs), and this is expected to grow in the future. These STPs help treat sewage but produce sludge. The sludge is typically disposed of in landfills, which adds to the environmental and health risks. The present study employs life cycle assessment (LCA) to evaluate the impacts of STPs incorporating various sludge management techniques. Unlike conventional LCAs that rely on default emission values, this study integrates wastewater modelling using GPS-X TM with LCA using India-specific operational data to evaluate emissions, resource recovery, and energy generation. The LCA compares 14 sludge-to-resource pathways that incorporate 6 techniques: gasification, anaerobic digestion, pyrolysis, hydrothermal liquefaction, incineration, and disposal. The study shows that an STP can reduce emissions by 99.51%–99.75% compared to untreated wastewater. The study also shows that sludge treatment units (SPUs), anaerobic digestion, and gasification reduce the overall impact by 0.76 to 3.21 times that of disposal, while generating energy. While waste-to-energy technologies have significant trade-offs between environmental impact and energy recovery, the study finds that technologies such as anaerobic digestion, gasification, and sludge-based fertiliser production demonstrate the lowest environmental impacts. Considering other products, such as biochar and digested sludge, which can be utilised in agriculture, and sludge can be converted into an alternative fuel. The study concludes that there is a need for integrated evaluations of STP and SPU to address sustainability and infrastructure gaps in sewage treatment. • SBR produces a direct emission of 0.17 kg CO2 eq /m 3 . • SBR-based STP contributes to N 2 O emissions. • GHG emissions reduced by up to 23.55% across sludge utilisation pathways. • HTL is not environmentally sustainable for sludge treatment.