Empirical study on determinants of sewerage and decentralized treatment coverage in Japan: urban infrastructure implications

Wastewater treatment service levels vary across Japan despite near-universal potable water supply. This study examines whether population density and fiscal capacity are associated with prefectural variation in (i) the population served by sewerage and (ii) the total population served by sewage treatment, including decentralized systems such as Johkasou (on-site treatment tanks). Using data for 47 prefectures in 2002, 2007, 2012, 2017, and 2022, we estimate separate cross-sectional regressions for each year. Population density is more strongly associated with sewerage coverage, whereas fiscal capacity is more strongly associated with total sewage treatment coverage. The findings support a core–periphery interpretation: sewerage is relatively advantageous in dense areas, while decentralized systems remain important in low-density settings. We discuss implications for differentiated planning of sewer renewal, combined sewer overflow (CSO) mitigation, and decentralized treatment under population decline, interpreting the results as observational associations, not causal effects.