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Environmental and economic life cycle assessment of sewage sludge treatment processes Zhuang, Ziyi

Abstract

The management of sewage sludge is a major global issue due to the presence of contaminants in the sludge, such as heavy metals and polybrominated diphenyl ethers (PBDEs), which are harmful to human health and the environment. Due to these concerns, this study aims to create a decision-support tool for municipalities when evaluating alternative sludge treatment techniques. The environmental and economic impacts of four common treatment techniques (anaerobic digestion, incineration, composting and pyrolysis) and three end-of-life uses (landfill, agricultural application and energy recovery) are evaluated by the use of life cycle assessment (LCA) and life cycle costs analysis (LCCA). In order to deliver credible results, the uncertainties inherent in LCA and LCCA are assessed via probabilistic approaches. The global warming potential (GWP) for each scenario is studied by using the LCA method. The results demonstrate that pyrolysis has the lowest (deterministic) GWP after capturing environmental credits due to energy recovery and fertilizer substitution. Incineration is the worst option in terms of GWP, primarily due to the greenhouse gas (GHG) emissions from the process. The findings from the probabilistic analysis indicate that pyrolysis process and agricultural application of anaerobically digested sludge can achieve net negative GHG emissions under some circumstances. The economic assessment shows that composting has the lowest life cycle costs among these studied technologies due to its low capital investment costs. Incineration is the least preferred alternative due to its high waste management and transportation costs. The results also indicate that capital costs are the most dominant contributor to life cycle costs across all technologies. Pyrolysis process can generate more profits compared to the other alternatives given that valuable resources, such as energy, fertilizer and fuel, can be recovered from the process. Overall, by considering both environmental impacts and economic costs, this study suggests that pyrolysis is the most environmentally optimal and economically affordable sewage sludge treatment method due to its low life cycle costs and desirable performance in terms of GWP. The incineration process is the worst option since it is the most expensive option and has the highest GHG emissions among these considered treatment processes.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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