Developing a life cycle composite footprint index for sustainability prioritization of sludge-to-energy alternatives

Sludge-to-energy technologies can achieve sewage sludge treatment and energy recovery simultaneously. Having a comprehensive assessment for the related technologies can contribute to the decision-making process and sustainable development of sludge management industry. In this paper, a life cycle composite footprint index was proposed, including energy recovery, carbon emissions, water consumptions, nitrogen and sulfur flows. Related methodology framework was constructed to evaluate the sustainability performance of sludge-to-energy alternatives on the composite footprint index. Fuzzy Best-Worst Method (BWM) and fuzzy AHP method were applied to obtain the weights and the overall scores. A case study was carried out applying the established framework to assess six sludge-to-energy scenarios, covering dewatering, composting, drying, incineration, incinerated ash melting, and dewatered sludge melting by the life cycle composite energy-carbon-water index. Results showed that dewatered sludge melting was the most preferred option owing to the considerable quantity of energy production, while drying process was the undesired one because of the unsatisfactory performance on energy recovery and carbon emissions. Sensitivity analysis and uncertainty analysis were carried out to study the impacts of changing weights on different aspects and the influence of changing energy recovery amount from anaerobic digestion, lower heating value and carbon content in sewage sludge toward the sustainability assessment of the sludge-to-energy technologies.