Measures for Improvement of Combined Sewer Systems in Yokohama

In Japan, combined sewer systems have been installed in about 200 cities across the country, mainly in the big cities which began constructing sewerage at an early date. Such systems enabled a prompt diffusion of sewerage and therefore helped to raise the quality of water in public water areas in urbanized districts. Along with the rise in water quality in such areas in recent years, however, the problem of water pollution due to overflow from combined sewer systems in wet weather has come to the fore. In the process, the preparation of measures to improve combined sewer systems has become an agenda item nationwide.In Yokohama, combined sewers serve about one-fourth of the city area. The city has been installing stormwater tanks for temporary storage of the highly polluted first-flush wastewater in order to improve water quality in public water areas. For further improvement, it initiated an ad-hoc project for combined sewer systems in fiscal 2003. In addition, as measures for emergency improvement that can be promptly taken and have immediate effect, it began constructing disinfection facilities and drying grit chambers and other facilities in fiscal 2002.A simulation of the effects of the plan for improvement with stormwater tanks using the total yearly runoff pollution load of separate sewers as the target found that the rate of attainment would be about 93% in the event of completion of the entire plan and 72% in that of completion of the targeted five-year plan. Water quality sampling data from the Kanazawa Pumping Station, where the aforementioned emergency measures have already been taken, indicate that the fecal coliform group concentration in the public water area 24 hours after the end of direct discharge was below the requisite standard (1,000/100 ml).