Sustainability evaluation framework for effective decision-making in urban wastewater and sludge treatment systems

Abstract

Sustainable water and wastewater systems are of increasing importance globally when facing the challenges of rapid urbanization, population growth and economic development. In the mission of achieving more sustainable urban water and wastewater management, application of water systems with the best-available environmental, economic and social performance is necessary. An evaluation framework that considers the characteristics of urban cities, designs of water systems and paradigm shift in wastewater treatment is demanded for. This research study developed a sustainability evaluation framework using life-cycle assessment (LCA) based techniques to assist effective decision-making in urban wastewater and sludge treatment systems. An eco-efficiency analysis (EEA) framework was developed through integrating LCA and life-cycle costing (LCC) techniques to evaluate sewage sludge treatment options in urban cities. The framework was demonstrated in a case study of six sewage sludge management scenarios in Hong Kong. Consideration of land resource, which could be trivial in rural areas, was revealed to be crucial in urban cities. Furthermore, detailed assessment based on actual data of transportation distances was significant to avoid up to 187,000 tonnes inaccuracies in estimated GHG emissions. Sludge treatment scenario adopting anaerobic digestion (AD), dewatering, incineration and reuse in cement production was the most favorable option in the case study. By the inclusive evaluation of sludge treatment scenarios instead of individual treatment technologies, the EEA provides comprehensive and informative results and is widely applicable for sustainable urban sludge management. An innovative EEA framework was developed for evaluating non-potable water supply systems. Four scenarios including freshwater flushing, seawater flushing, greywater recycling using aerobic membrane bioreactor (MBR) and anaerobic fluidized-bed MBR (AFMBR), were analyzed in a case study in Hong Kong. The EEA framework included detailed engineering designs of the systems for building comprehensive and reliable inventories. Results revealed the AFMBR greywater reuse scenario to be the most eco-efficient option as the system is capable of energy recovery, recycling of water resource and reduction of sewage treatment loadings. This study demonstrated the EEA framework as an effective tool to guide water management towards sustainability and provided a basis for further research on the application of greywater recycling systems on a larger scale. A life-cycle data envelopment analysis (LC-DEA) framework was developed for evaluating sludge-to-energy (STE) systems. The framework highlighted the strong linkage between sludge treatment and energy systems and included all essential performance metrics, namely volatile solids reduction, energy recovery, energy use, chemical consumption, sludge residues generation and direct environmental emissions, in benchmarking the efficiency of STE systems. Results showed that 44% and 69% of the sixteen STE systems were efficient in terms of overall and pure technical efficiency, respectively. The LC-DEA also informed the appropriate strategies for improving efficiency, such as increasing energy recovery, reducing energy use and scaling up/down the systems, for the less efficient systems. The framework is widely applicable for guiding decision-making on enhancing STE systems worldwide. In summary, this research study contributed to the development of LCA-based sustainability evaluation framework that informs decision-making in sewage sludge treatment, non-potable water supply systems and STE systems in urban cities.