Construction of sewerage systems in the city of Yokohama has proceeded at a rapid pace ever since the first wastewater treatment plant went into operation in 1962. By 1984, all eleven of the planned wastewater treatment plants were in operation. By March 1989, the sewerage service rate had reached 80 percent, representing over 2.56 million of the city's residents.
Naturally, the expansion of the sewerage system was reflected in a commensurate increase in the quantity of sludge for treatment and disposal. Recognizing the critical importance of keeping abreast of this increase, the city has promoted the development of technology for more efficient treatment and disposal of sludge throughout the intervening years.
In recent years, the city has also seen extensive urbanization and mounting concern among its citizenry for conserving the environment. In response, it was decided to shift from the former decentralized method of sludge treatment (i.e., treatment of sludge at the plant generating it) to a method of centralized treatment at two sludge treatment centers, each built within a different wastewater treatment plant located in the waterfront area. Yokohama was the first city in Japan to adopt this form of centralized sludge treatment.
The construction of the centralized sludge treatment facilities made extensive use of results from a program of technological development. The centers feature the first egg-shaped digestion tanks, a high-concentration mode of digestion made possible by the use of centrifugal thickeners, power generator fueled with digestion gas, effective use of the waste heat from these generators, and the saving of fuel by the use of incinerators equipped with dryers.
The record for the first year of full-scale operation clearly demonstrated the merits of centralized treatment. The quantity of digestion gas generated at the center was double that of the quantity generated under the former method for an equivalent quantity of sludge. Power generation fueled by digestion gas supplied 64 percent of the center's own power needs. While operating at only one-fifth of the planned sludge capacity, the center boasted a savings of about 200 million yen for the year, and the treatment costs were less than half of the treatment cost of the same quantity utilizing the former method.
This report presents an account of the operational record for the first year of this center (the Hokubu Sludge Treatment Center) and the system of effective energy use that took place in it.