The sewage system has been contributing to the improvement of public water quality. The sewage coverage rate increased from 85% in 1989 to 99.8% 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% in 2013 from those in 1984. On the other hand, eutrophication in enclosed water bodies such as lakes and coastal waters still remains. To cope with this issue, Yokohama City introduced advanced treatment technology to remove nitrate and phosphorus contributing to eutrophication. 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.
Wastewater treatment plants do not eliminate harmful substances entirely from wastewater. Remnants of nitrate and phosphorus in treated water contribute to eutrophication especially when they are discharged into closed water bodies. To preserve rich water environment, all the wastewater needs to be treated by collective or individual means before being discharged to water bodies. Moreover, application of effective technologies and continuous monitoring are essential to enhance water quality.