In the Sustainable Development Goals, environmental sanitation has become a global issue, greatly expanding upon a more narrowly defined sense of sanitation framing it as merely sanitation facilities. Under the Citywide Inclusive Sanitation （CWIS） initiative, the importance of a decentralized sanitation infrastructure is not only being recognized at the rural level, but is also gaining mainstream recognition in urban areas where non-sewered sanitation systems are entering the conversation. However, septic tanks are now in question due to the fact they are a major source of greenhouse gas emissions in urban sanitation services. Furthermore, climate resilience in the water, sanitation, and hygiene （WASH） sector has also recently become increasingly important. In response to these issues, Japan’s experience with night-soil collection and treatment provides a social model for achieving improved environmental sanitation over a short period of time. Moreover, a unique decentralized sanitation ecosystem has been established by organizing various hardware and software technologies/systems, enabling proper operation and maintenance of sophisticated Johkasou technologies. The experiences of creating such an ecosystem have great lessons to offer low- and middle-income countries. Furthermore, as an advanced disaster response country, Japan’s wisdom on disaster resilience could also be applied to enhance the climate resilience of WASH.

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.

Japanese Johkasou systems have been recognized worldwide for their excellence as decentralized wastewater treatment facilities. As their widespread use increases not only in developing countries but also in developed countries, the number of Japanese Johkasou installed overseas has seen rapid expansion over the past decade. Although urbanization and population growth spurred by economic development in developing countries is currently changing the landscape briskly, large-scale sewage facilities have not seen much progress, resulting in the increased demand for decentralized wastewater treatment facilities in these regions. Another reason for such rapid growth lies in the fact that the Japanese government, Johkasou manufacturers, and related companies have been actively cooperating to expand the Johkasou systems overseas. Along with this overseas expansion, international standardization of decentralized wastewater management is becoming one of the challenges. In light of this, efforts are also being made to reflect the technology and know-how of the Japanese Johkasou systems within the international standards framework.

The 2024 Noto Peninsula Earthquake occurred on January 1, 2024. The earthquake recorded a seismic intensity of 5+ in various areas, with Shika Town and Wajima City recording a seismic intensity of 7, and the northern part of the Noto Peninsula recording a seismic intensity of 6-. This earthquake caused great damage to houses and lifelines, and there are still areas where the response to collapsed houses and the restoration of lifelines have not progressed sufficiently. In the cities and towns of the Noto Peninsula, where damage to houses and other structures was great, there are many Johkasou installed by the cities and towns, but the damage to Johkasou caused by this earthquake is said to be the highest among all earthquake disasters to date. Some of the damaged Johkasou could not be used due to leaks, etc., and victims are unable to use flush toilets, so the restoration of Johkasou is urgently needed. In addition, disaster response and recovery require a large number of human resources, but there is still a shortage of human resources among Johkasou-related personnel in Ishikawa Prefecture alone. This paper provides an overview of the state of Johkasou damage and the public and private sector activities toward recovery based on the information collected so far, and also introduces examples of these activities.

The management of water and sanitation infrastructure under Japan’s current phase of depopulation has already become an urgent issue for the country. This paper focuses on water and sanitation infrastructure in small municipalities and rural areas where population decline is being felt significantly. It discusses possibilities for and issues surrounding more decentralized and community-managed systems, mainly referring to examples of small-scale water supply systems that have been established in Hokkaido. In addition, the paper considers resource recycling models in small-scale sanitation infrastructure being implemented in Japan amid the issue of declining population. Site-specific considerations become very important when considering the future of sanitation infrastructure in low-population density areas. It is, therefore, important to have a wide range of flexible measures available and to be prepared with a methodology for comparing these various options.

Toilet with attached treatment unit is attracting attention as an environmentally friendly, resource-recycling solution that can be used in natural areas where there is no place to safely discharge of waste. Such toilets have already been put into successful practical use in mountain areas, riverbeds, event venues, construction sites, and other places where the existing infrastructure cannot be relied upon. Despite the positive points, there have been several cases where the toilets have malfunctioned due to conditions of excessive usage or insufficient maintenance. This paper summarizes the disposal technology and other features of the toilet with attached treatment unit, introduces installation cases that have till now been undertaken, and outlines evaluations and issues that have arisen during the process of publicly demonstrating this technology. One further point to note is that this technology is attracting attention as an effective means for the alleviation of poor toilet conditions at evacuation shelters in disaster-stricken areas and natural disaster sites. The paper also outlines matters necessary for best utilizing these technologies in various places along with the challenges for establishing them as a new means of social infrastructure with a future outlook.