Integrated life-cycle data envelopment analysis for techno-environmental performance evaluation on sludge-to-energy systems

Sludge-to-energy systems have been widely recognized as a favourable approach to treat sewage sludge and recover energy from the energy-rich biomass waste simultaneously. An inclusive efficiency benchmarking tool for sludge-to-energy systems is lacking. This study developed a life-cycle data envelopment analysis framework, by integrating life-cycle assessment and data envelopment analysis, for evaluating sludge-to-energy systems. The life-cycle data envelopment analysis approach highlighted the strong linkage between sludge treatment and energy systems and included all the essential performance metrics in benchmarking the efficiency of the sludgeto-energy systems. Using life-cycle data envelopment analysis, this study compared the relative operation efficiency of nine Hong Kong sludge-to-energy systems and seven systems in other urban cities based on the volatile solids reduction, energy recovery, energy use, chemical consumption, sludge residues generation and direct environmental emissions. The results showed that 44% and 69% of the sixteen sludge-to-energy systems were efficient in terms of overall and pure technical efficiency, respectively. The life-cycle data envelopment analysis results 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 life-cycle data envelopment analysis framework is widely applicable for guiding decision-making on enhancing sludge-to-energy systems worldwide.