Contributing to CO2 Reductions by Turning Sewage Sludge Into Green Energy

Biomass energy can make effective use of waste materials that would normally be thrown away while greatly contributing to CO2 reductions. In this article, we focus on a wastewater treatment system that recycles resources being promoted by the Tokyo Metropolitan Government's Bureau of Sewerage as one measure against global warming. Suwanmake

Biomass Power Generation Using Sludge

Wastewater treatment plants contribute to a sustainable water cycle by treating wastewater discharged from homes and factories and returning the cleaned water to rivers and oceans. While a component of public infrastructure that is indispensable to our daily lives, maintaining and promoting sewerage projects requires a large amount of energy and emits large volumes of greenhouse gases.

The greatest impact of all comes from the greenhouse gases emitted during sludge incineration. Sludge refers to the muddy waste formed when matter suspended in wastewater settles or rises to the surface, which is often disposed of by incineration or at landfills. Sewerage providers have been asked to reduce the amount of carbon dioxide (CO2) and nitrous oxide (N2O) emitted during sludge incineration as well as the amount of incinerated ash disposed of at landfills for the sake of the environment.

Under such circumstances, the method of reusing sludge as a resource by the Tokyo Metropolitan Government's Bureau of Sewerage has been attracting attention. The Morigasaki Water Reclamation Center, located in Ota City, Tokyo, is the largest wastewater treatment plant in Japan, consisting of two facilities in the east and the west. Here, sludge generated in the wastewater treatment process is heated in digester tanks where its organic matter is converted into methane gas, a process which has successfully reduced the amount of sludge needing to be incinerated.

This methane gas is further used as a source of fuel for the center's power generation facilities. It is used to generate about 20 million kWh of electricity a year, equivalent to 20% of the electricity used at the Morigasaki Water Reclamation Center. This use of methane gas is said to have the effect of reducing greenhouse gas emissions by about 10,000 tons a year.

Sludge digester tanks are kept at about 50°C, fermenting sludge for about 20 days to generate methane gas.

The methane gas generated is stored in tanks. It is also used as fuel for humidifying the sludge digester tanks.

28% Reduction in Greenhouse Gases

The Tokyo Metropolitan Government's Bureau of Sewerage formulated "Earth Plans" in 2004, 2010, and 2017 as a measure for preventing global warming through which it systematically promotes countermeasures for energy. "In order to achieve our goals, we've been proactively developing technologies and taking other measures to drastically reduce our energy consumption," says Muneyoshi Osamu, Director for Energy and GHG Measures Promotion at the Bureau's Planning Section, Planning and Coordination Division.

Earth Plan 2017 set a goal of reducing greenhouse gas emissions by 25% from FY2000 levels by FY2020 and by 30% or more by FY2030. They achieved a 28% reduction in FY2020, exceeding their target by 3 percentage-point.

At the Morigasaki Water Reclamation Center, they are also striving to produce renewable energy, such as through digestion gas power generation, micro hydraulic power generation, and solar photoelectric generation. Muneyoshi says that they plan to accelerate their efforts with alternative energy, which include expanding their use of these sources.

"The Morigasaki Water Reclamation Center and the Nanbu Sludge Plant, a nearby sludge incineration facility, will cooperate to continue steadily reducing greenhouse gas emissions moving forward."

Muneyoshi Osamu, Bureau of Sewerage, Tokyo Metropolitan Government.

One of the reasons why biomass energy has not become widespread is that it is costly to transport and collect. However, the advantage of this system is that sewage sludge is collected right at the treatment plant, so there is no need to build new collection and transport systems.

Moreover, the Morigasaki Water Reclamation Center's methane gas power generation project was the first sewerage project in Japan implemented under a Private Finance Initiative (PFI). By utilizing private sector funds and technical capabilities, they have been able to reduce costs and provide high-quality public services.

Expanding the use of biomass energy will lead to reduced industrial waste and CO2 emissions and also further help nurture Japan's future industries. It will be interesting to see how the government and private sector will come together to promote projects moving forward.

Photos by Matsuda Maki / Translation by Amitt