Comparison of black water source-separation and conventional sanitation systems using life cycle assessment

Abstract

Abstract The municipal sanitation system based on black water source-separation (BWS) is often advanced as a viable environmental alternative to conventional systems (CONV). However, current studies have not led to any formal conclusions on the environmental advantage of such a system. The objective of this study is to compare the environmental performances of BWS and CONV in terms of environmental damage using the life cycle assessment method. The functional unit is to ensure wastewater and kitchen refuse collection and treatment and by-product (digestate/sludge and biogas) recycling for one inhabitant for one year. The results show that BWS generates higher impact scores than CONV: 100%, 89%, 24% and 25% more for the human health, ecosystem quality, climate change and resources indicators, respectively, when metal emission impacts from fertilizers are excluded. If metal emission impacts were accounted for the conclusions are reversed for human health and ecosystem quality. The exclusion of metal emission impacts from fertilizers for the interpretation of the results is based on acknowledged overestimation and high degree of uncertainty of (eco)toxicity impacts by existing assessment methods. However, even with such exclusion, the impact scores of both systems for the ecosystem quality indicator are still not significantly different because of the large contribution of metal emissions from the background data, which still remained in the inventory. Depending on the grid mix and organic fertilizer transport distance assumptions, the study conclusions may be inverted for the climate change and resources indicators, since BWS may obtain lower impact scores than CONV. The main contributors to BWS impact scores are ammonia emissions from applied digestate and digestate management (transport, storage and spreading). Suggestions for significant enhancements are required for BWS to attain better environmental performances than CONV.