Evaluation of environmental and biogas production performance for sewage sludge and food waste treatment by life cycle assessment with sensitivity and uncertainty analysis

Abstract

Sustainable treatment of sewage sludge and food waste is absolutely necessary to avoid negative environmental impacts and to recover valuable resources. Anaerobic digestion (AD) is a commonly used technology for sewage sludge and food waste waste treatment, while anaerobic co-digestion (coAD) has received more attention in recent years due to its improved biogas production performance for energy generation compared to AD treatment. In order to evaluate and determine a sustainable treatment method for sewage sludge and food waste by considering AD and coAD as treatment methods, life cycle assessment (LCA) can be applied for the evaluation. However, the LCA results are often subject to uncertainty which may lead to wrong conclusions for decision makers. Sensitivity and uncertainty analysis can be undertaken to tackle the uncertainty and hence improve the reliability of the results. This study aims to evaluate the use of AD and coAD for sewage sludge and food waste treatment based on their environmental and biogas production performance using LCA. In order to tackle the uncertainty involved, sensitivity and uncertainty analysis are further conducted to identify the key process parameters and propagate the uncertainty involved in the study. Macau is selected as one of the reference cities in the study in order to evaluate the environmental and biogas production performance of AD and coAD for sewage sludge and food waste treatment. In the study, the proposed AD and coAD scenarios are compared to the Macau’s current incineration scenario. Since incineration scenario is one of the treatment scenarios studied, energy production performance is used as one of the indicators for comparison instead of biogas production performance. A sustainable waste treatment strategy is then identified through a combination of waste treatment methods identified for Macau. Based on the data collected, the assumptions made, and the system boundary defined, the coAD scenario and the AD scenario (i.e. for the remaining food waste) outweigh other scenarios, producing the lowest environmental impacts and the highest energy production through biogas production. With the combination of these two selected scenarios, the proposed waste treatment scenario improves the performance in human health, ecosystems, and energy production by 36%, 13%, and 61%, respectively compared with the existing incineration scenario in Macau. Moreover, the proposed scenario also has better performance even if the generation of sewage sludge increases, with almost 2 times better than the existing incineration scenario in human health categories. Further, Hong Kong is chosen as the other reference city for studying the uncertainty involved in the treatment of sewage sludge and food waste. This study identifies the key process parameters in various waste treatment systems using a proposed selection approach with sensitivity analysis and further propagates the parameter uncertainty by Monte Carlo simulation. The result shows that coAD treatment contributes the highest environmental benefits to Hong Kong. With the use of the proposed selection approach, it is found that the electricity generation efficiencies in different waste treatment facilities, such as the incineration plant and the anaerobic digestion plant, have the greatest sensitivity to the result. Uncertainty analysis is further conducted by propagating the uncertainty of the key process parameters identified. With 10,000 iterations by Monte Carlo simulation for uncertainty propagation, the coAD scenario with a combined cycle gas turbine (CCGT) system for biogas utilization has a 95% probability of achieving at least -5.32 × 10⁴ kg avoided CO₂e emissions, while the probability of the second best scenario, which applies a combined heat and power (CHP) system for biogas utilization, achieving the same avoid emissions is below 5%. It is concluded that the use of CCGT is more advantageous than CHP for biogas utilization in Hong Kong. The findings of this research can provide an additional approach to the decision makers in developing the policy framework for sustainable sewage sludge and food waste treatment in Macau and Hong Kong. In summary, the methodologies and results of this study can provide comprehensive material for other areas planning sustainable sewage sludge and food waste treatment.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology