Copyright Bureau of Sewerage Tokyo Metropolitan Government
The history of sewerage systems in Japan goes back to at least the late seventh century. At the archaeological sites of two ancient capitals, Fujiwara-kyo (established in 694 A.D.) and Heijo-kyo (established in 710 A.D.), ditches were found along both sides of the roads, and are believed to have been used as part of sewerage systems. In the Edo Period (1603-1867), open sewer ditches were laid throughout the city of Edo to collect wastewater from households and rainwater for release into rivers and trenches. Later, in the 1950s and 1960s, when Japan was experiencing rapid economic growth, water pollution in public waters became a serious problem, raising demands for the development of proper sewerage systems.
Currently, 1,496 out of 1,822 municipalities in Japan conduct their own sewage treatment, and the sewage systems serve 69.3 percent of the total Japanese population. The energy consumed by sewerage systems, however, has increased in proportion to the spread of these systems. Their total annual electricity consumption in fiscal 2004 was about seven billion kilowatt-hours (kWh), approximately 0.7 percent of total national consumption.
As stated in "Sewerage Vision 2100," a policy report drawn up by the Ministry of Land, Infrastructure, Transport and Tourism in September 2005, the government shifted its sewerage policy from a focus on expanding coverage to one of wastewater control for the 21st century, which involves the creation of networks for a sound water cycle and resource recycling system. One of the basic policies included in the report was the creation of "passage of resources" to utilize sewerage's functions to recover and supply resources in order to make sewage treatment plants energy self-sufficient and to address global warming.
Committee Created to Promote Use of Sewage Sludge
As sewerage systems have become more widely introduced, the volume of sewage sludge has also continued to increase. In fiscal 2004, approximately 75 million tons of sewage sludge were generated (pre-treatment amount, 97 percent water content). Although only about 1 percent of total sewage sludge volume will eventually be buried in landfill sites after additional treatment, no further increases in the amount of sludge for burial can be tolerated, since landfill site space in Japan is limited. On the other hand, sewage sludge can, fortunately, be transformed into carbon neutral biomass fuel. Given that sewage sludge is stable in both quantity and quality and can be recovered solely at sewage treatment plants, it is an ideal material to utilize as a resource for biomass fuel. The sewage sludge recycling rate -- which indicates the percentage of total sewage sludge volume effectively recycled as biomass fuel -- increased to 70 percent in fiscal 2005 from 38 percent in fiscal 1996.
Below are examples of pioneering efforts being conducted by sewage treatment plants in Japan, drawing information from previous JFS articles and a report titled "Toward the Creation of Resource Paths," which was released by the ministry in March 2007.
Transforming Sewage Sludge into Fuel to Generate Power
The Sunamachi Water Reclamation Center is one of the 13 sewage treatment centers in the 23 wards of metropolitan Tokyo. Located in Koto ward, the center started operation of a sludge carbonization facility at its Tobu Sludge Plant in November 2007. Sewage sludge, which used to be incinerated and buried in landfill sites, is now carbonized and gasified into biomass fuel in this facility.
The plant is capable of processing 300 tons per day of dehydrated sludge generated by wastewater treatment. Annually, 99,000 tons of dehydrated sludge are dried in a dryer and then carbonized in a carbonization furnace, producing 8,700 tons of solid biomass fuel. The fuel is then transported to the Nakoso Power Plant in the Joban Joint Power Co. (in Iwaki, Fukushima Prefecture), where the fuel is mixed with coal at the rate of about 1 percent of the coal, and the resulting mixture is burned in the No. 7 generator unit (output of 250,000 kW).
As a result, about 9 percent of the sludge generated in Tokyo annually can be used as an energy source. This sludge treatment method makes it possible to have a reduction of about 37,000 tons of carbon dioxide (CO2) emissions a year, compared to conventional sludge combustion methods. Moreover, by reducing the use of coal in the power plant, 9,200 tons of CO2 emissions can also be avoided every year. Thus, the total reduction of CO2 emissions is estimated at about 46,200 tons a year.
At the Mikasagawa Sewage Purification Center, located in Hakata, Fukuoka Prefecture (on the southern island of Kyushu), biosolids-derived fuel (BDF) has been produced since the end of fiscal 2000, with the aim of reducing the amount of sewage sludge. BDF is a coal-like solid biomass fuel produced by mixing sewage sludge with discarded edible oil under reduced pressure and then heating (at about 100 degrees Celsius) and drying the resulting material. About 2,300 tons of BDF were produced in fiscal 2007, which is effectively used not only as fuel but also as raw material for fertilizer.
Biomass Co-combustion with BDF Cuts CO2 Emissions from Coal Thermal Plant
The Electric Power Development Co., known as J-Power, has conducted combustion tests on coal mixed with BDF, using the No. 1 generator unit (output of one million kW) at its Matsuura Thermal Plant in Nagasaki Prefecture, Kyushu, and has confirmed that the maximum mixture rate of BDF should be 1 percent (about 90 tons per day). The tests have also shown that burning 1,200 tons of BDF can reduce the use of 1,100 tons of coal, which will result in a reduction of 2,600 tons of CO2 emissions. J-Power has been burning the mixed fuel since fiscal 2006, in which it used about 1,800 tons of BDF.
Making Effective Use of Sludge Digestion Gas
At another sewage treatment center in Tokyo, the Morigasaki Water Reclamation Center, sewage sludge is condensed in a thickener tank and thermophilically digested (at about 50 degrees Celsius) in a chamber, so that the volume of the sludge is reduced while organic matter is gasified into methane. The center has been using it as fuel to generate electricity since April 2004.
The power generation capacity fueled by the methane is 3,200 kW, covering about half the total electric power used in the center. This results in cutting 600 million yen (U.S.$5.7 million) of electricity costs a year. As generated by biomass, this electricity is regarded as green power which has an added value of environmental friendliness -- that is, it helps reduce about 4,800 tons of CO2 emissions a year. The Tokyo Metropolitan Government became the first Japanese municipality to participate in the Green Power Certification System, in which the environmental added value is sold in the form of Green Power Certificates, and persons who purchase the certificates are considered to be buying electricity generated by a natural energy source. In this way, the metropolitan government is contributing to the expansion of the green power market.
Sewage gas generated from the sludge treatment facility in the Higashinada Sewage Treatment Plant (Higashinada ward, Kobe, Hyogo Prefecture) was mainly used as a boiler fuel, but 30 percent of the gas was burnt off as waste. The city of Kobe has succeeded in purifying this waste sewage gas for use as a fuel for natural gas vehicles. Sewage gas consists of methane (about 60 percent), CO2 (about 40 percent), and a small amount of impurities such as siloxane and hydrogen sulfide. The city started a verification test on removing CO2 and impurities from the gas, together with Kobelco Eco-Solutions Co., in November 2004 at the sewage gas purification plant built in the Higashinada Sewage Treatment Plant. The removal method is to apply high pressure to the gas to dissolve CO2 and impurities in the water. The test was successful, producing a biogas with 98 percent methane content. This natural gas became known as Kobe Biogas, a name selected in a public competition.
Road tested in 2005, a city bus has been running on Kobe Biogas in the Higashinada ward of the city since October 2006. Japan's first biogas facility that includes a purification plant, gas tanks, and a gas station was built into the Higashinada Treatment Plant, and it began full-scale operation in April 2008. Its output is 2,000 cubic meters per day, sufficient to fuel 40 city buses running 50 kilometers per day. The use of Kobe Biogas reduces about 1,200 tons of CO2 emissions per year. The main vehicles that use it are city buses, garbage trucks, road-maintenance vehicles, trucks to transport de-watered sewage sludge cakes, sewer pipe-flushing vehicles, and company delivery trucks.
Bio-Methane from Sewage Sludge to Be Used as Bus Fuel
Small Hydropower Generation
The micro-hydro unit at the Kasai Treatment Center in Edogawa ward, Tokyo, has been in operation since the autumn of 2004, using the discharged water from the plant's outlet (average discharge: 0.67 cubic meters per second; effective head: 5.05 meters) at the rated output of 24 kW, producing 140,000 kWh per year, or 0.2 percent of the power consumed by the plant. The Morigasaki Treatment Center mentioned above has two micro-hydro units with a rated output of 95 kW and one with a rated output of 4 kW, which have been in operation since June 2005, producing about 800,000 kWh per year, or 0.7 percent of the electricity consumed by the plant, and contributing to a CO2 reduction of 300 tons per year.
Meanwhile, the outlet of the city of Kobe's Suzurandai Sewage Treatment Plant discharges only 0.185 cubic meters of water per second, but its effective head is 65 meters. Making use of this drop, it has been generating 490,000 kWh of electricity per year, or 13.6 percent of the power it consumes, since April 2002. More recently, the micro-hydro unit at the Ishida Sewage Treatment Plant in the city of Kyoto has been in operation at a rated output of 9.4 kW since June 2007, producing an estimated 80,000 kWh of electricity per year.
(Written by Kiyoshi Koshiba)