The sewage system in the city of Yokohama faces several sustainability challenges in terms of its service. Firstly, aging sewage pipes and infrastructure not only directly affect the quality of sewage service, but they also expose daily life and socio-economic activities in the city to danger, such as sink holes in roads due to damage to pipes, that need to be prevented.

Secondly, the City pursuits environmentally sustainable sewage system. For instance, despite of the system thoroughly covering the city area, eutrophication in enclosed water bodies still remained. Moreover, the large amount of sludge generated everyday in the treatment plants reaches about 20 percent of total industrial waste.

To cope with the aging sewage systems, a renewal plan has systematically been developed. A target was set to reduce the dependency rate on corporate bond to improve the financial basis for the sewage services. Private finance initiative (PFI) was also introduced to reduce the cost of recycling sludge. Daily management of systems has been strengthened to prolong the life of facilities and pipes, and a database of data and information on the sewage systems is being developed.

The renewal plan also includes the introduction of advanced treatment technology which enables for treated wastewater to be utilized for various purposes in the city. For example, ozonized water is utilized for toilets, small artificial streams, and air conditioning systems (AC), while sand filtered water is reclaimed for cleaning the wastewater treatment plants and toilets.

Moreover, sludge generated from the wastewater treatment process can also be recycled. After collection, they are transported to two sludge treatment plants, and then subject to thickening, digestion, and dehydration processes. Gas generated during the digestion process is recovered as a source of energy, and utilized for electricity generation and fuel for incineration in the treatment plants. The ash after incineration has also been used as raw material for improved soil and cement. Local governments and sewage management bodies may decrease the significant volume of final disposal by cautiously processing the sludge and recycling it to fertilizer for farm land, construction materials, and a source of energy.

The sewage coverage rate increased from 85 percent in 1989 to 99.8 percent in 2013. No untreated wastewater flows into rivers, and river water quality has significantly improved. BOD values in major rivers in the city decreased 40 – 87 percent in 2013 from those in 1984. The number of treatment plants applies advanced treatment process increased from two in 1996 to eight in 2013. As the result, the removal rate of nitrogen and phosphorus increased.

In addition, small streams where ozonized reclaimed water is used provide excellent waterfront and leisure space for citizens. The efforts to pursue sustainability for sewage systems also contributes to serve as inundation mitigation during floods.

Water quality in rivers, streams and coastal waters in Yokohama has improved due to its sewage system. Improved water quality also resulted in the increase of diversity of fish in the main rivers. Studies showed that fishes such as killifish, salmon and freshwater minnows returned.

Photo by Amritanshu Sikdar on Unsplash