Impact of Subjective Choices on Life Cycle Assessment of Wastewater Sludge Treatment Processes

Life cycle assessment (LCA) has been used to evaluate wastewater treatment technologies, processes, and scenarios. The outcomes of LCA are affected by various uncertainties, including those from input data and those from the LCA model set-up (i.e., structural uncertainty). The impacts of structural uncertainty have not received adequate attention in research. The objective of this study was to quantify how the subjective choices embedded in the LCA model set-up affect the LCA outcomes of three wastewater sludge treatment processes. The subjective choices were based on LCA’s cultural theory that considers the three different human perspectives: egalitarian, hierarchist, and individualist. The three processes are multiple hearth incineration (MHI), fluid bed incineration (FBI), and anaerobic digestion (AD) for wastewater sludge treatment. This study revealed that for MHI, lower impacts are in the categories of human toxicity and marine ecotoxicity under the individualist perspective when compared to the other two perspectives, but higher impacts are in the categories of terrestrial acidification, terrestrial ecotoxicity, fresh water ecotoxicity, and marine ecotoxicity under the egalitarian perspective. Among the three subjective perspectives and the three studied wastewater sludge treatment processes, AD has the lowest and MHI has the highest environmental impacts. The results from this study revealed that the differences from subjective choices created large differences in LCA outcomes in climate change, human toxicity, ionizing radiation, terrestrial acidification, terrestrial ecotoxicity, and marine ecotoxicity. Findings from this study can benefit stake holders to understand the impacts of subjective choices and the limitation of LCA outcomes for making informed and sound decisions.