On-site wastewater treatment and reuses in Japan

On-site wastewater treatment poses a challenging problem for engineers. It requires a balance of appropriate levels of technology and the operational complexity necessary to obtain high-quality effluent together with adequate reliability and simplicity to accommodate infrequent maintenance and monitoring. This review covers how these issues have been addressed in on-site wastewater treatment in Japan (termed johkasou). On-site systems in Japan range from outmoded designs that discharge grey water directly into the environment to advanced treatment units in high-density areas that produce reclaimed water on-site. Japan is a world leader in membrane technologies that have led to the development of on-site wastewater treatment units capable of water-reclamation quality effluent. Alternative ideas being pursued for on-site technologies also include separate waste stream collection, which would provide for more efficient treatment and reuse. Night soil treatment plants, where sludge from on-site systems is treated, are also distinctive to Japan, serving 37 million people. Japan has governmental regulations in place to ensure routine inspections of on-site units; furthermore, subsidies are available to reduce the cost of on-site systems for building owners. Lessons learned in on-site wastewater treatment in Japan have applications worldwide, from regions where water is scarce, to high-density areas in developing countries that currently lack sewer infrastructures.