Johkasou is a unique, decentralized domestic wastewater treatment system that has been developed in Japan. It primarily relies on biological treatment, while also utilizing the metabolic processes of microorganisms to purify domestic wastewater generated from houses and other types of buildings. At the end of FY2023, the number of users had surpassed 11 million. It can thus be posited that, in addition to sewerage systems, Johkasou plays a vital role in Japan’s social infrastructure. This report reviews specific features of a decentralized wastewater treatment system such as inflow fluctuation, periodic maintenance, and desludging. It also offers insights into the technological advancement trends currently taking place, including compacting, advanced treatment, and energy conservation. The paper further explores possible directions for future advancements in the areas of disinfection, sludge treatment/disposal, and maintenance in the context of depopulation.