Recent challenges in Japan’s sewerage sector, such as population decline, reduced revenues, staff shortages, and aging facilities, have raised concerns over the long-term sustainability of wastewater treatment services. In this context, Public–Private Partnerships（ PPP） have been strongly promoted as a potential solution, aiming to improve efficiency by leveraging private sector expertise and flexibility. However, empirical evidence on whether PPP actually enhances operational efficiency remains limited. This study applies the Malmquist Index, a methodology widely used in management science, to analyze productivity changes in 285 wastewater treatment plants across Japan from fiscal year 2011 to 2021. Using cost, greenhouse gas emissions, and treatment performance as indicators, the results show that PPP introduction does not automatically improve efficiency across all facilities. Nonetheless, significant efficiency improvements were observed in large-scale and aging plants where PPP was newly introduced. These findings suggest that the key significance of PPP lies not only in cost or environmental optimization but in sustaining wastewater services under increasing social and demographic pressures.

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