Digestion gas system
(sewage sludge digestion gas power generation system for Hitachi-Takahagi Wide Area Sewage Water Works Association (Ibaraki Prefecture))

Greenhouse effect due to methane gas emissions from sewage treatment plants are reduced by a digestion gas power generation plant that has been widely used.

Sewage sludge digestion gas power generation systems, which use methane gas as their fuel which is produced by loading thickened sewage sludge into the digestion tank, are in widespread use in cities of equivalent size to Japanese prefectural capitals.
As methane gas is said to have 21 times as high a global warming effect as CO₂. There are two effects due to the recovery and use of methane gas for power generation viz. prevention of global warming and energy saving*. Its advantage of economic efficiency has also been reevaluated in light of the recent steep increase in crude oil prices.

One approach to the effective use of methane gas is to check whether it is possible to increase its volume and thereby enhance its economy.
The Ishi Purification Center of the Hitachi-Takahagi Wide Area Sewage Works Association, located along the north coast of Ibaraki Prefecture, has recovered 20% of the produced digestion gas as heat energy and used it for the humidification of the digestion tank, and incinerated the rest of the digestion gas.

The center considered introducing a cogeneration system capable of recovering all digestion gas as heat and electric energy. The primary purpose of introducing the system was the reduction of greenhouse effect due to this gas and at the same time, maintaining profitability was also a greater concern. JFE Engineering has the largest share in the field of digestion gas power generation, having constructed many digestion gas power generation systems in Tokyo and Yokohama, and has also accumulated technology for peripheral systems, such as desulfurization systems and systems for removing siloxane contained in shampoos (which turns into silica when burned and causes damage to engines and other units), in addition to engines and generators.

Based on this experience, JFE Engineering studied the reduction of capital costs and operating costs from a variety of aspects, and proposed a total operation system with a digestion gas engine power generation system as its core. Among other factors, the stable operation and the approximately 20% reduction in maintenance costs, thanks to the siloxane removing system which was highly appreciated. Construction work began in August 2004. The operation system was completed and began operation at the end of March 2005, and has been operating in good condition since then. According to our plan, the new technology will achieve a reduction of about 50% of electricity charges and about 40% of the volume of greenhouse effect due to gas emissions.
Digestion gas power generation is adopted widely at sewage treatment plants in larger cities, but JFE Engineering would also like sewage treatment plants in local areas to consider introducing digestion gas power generation which helps to contribute towards conservation of the global environment and also adds economic value. JFE Engineering is sure to propose a solution backed by its broad experience and reliable technology.

Until March 2007, JFE Engineering also continued research on increasing the volume of produced gas by mixing sewage sludge with other waste in collaboration with the Japan Institute of Wastewater Engineering Technology. This research was aimed to ferment biomass with high moisture content, such as raw garbage from households and the food service industries, with sewage sludge and recover methane gas. The new technology is expected to be put to practical use soon.
This technology faces some issues that need to be resolved, such as establishing a system to sort and collect raw garbage from households as flammable garbage, and formulating a legal system to treat general garbage and industrial waste together. However, JFE Engineering would like to participate in the creation of a system suitable for each community with its unique technology as the core.

• ※ International agreements state that biogas, such as digestion gas, and the CO₂ emissions produced by burning it are not included in the subject substances of greenhouse effect gas.

Digestion gas power generation PFI project [bay eggs Corporation]

The North Sludge Recycling Center uses digestion gas produced by sludge digestion as fuel and a gas engine to generate electricity.

Cost reduction

Thanks to the project’s adoption of the PFI system, a VFM (Value for Money) of 8.5% was achieved.

Effective use of energy

A total efficiency of 70.6% was achieved, including digestion gas power generation and the use of produced heat energy.

Reduced environmental burdens

CO₂ emission was reduced by 588 tons a year (25.2%) as a result of the increase of electricity generated.

PFI project

What is PFI (Private Finance Initiative)?

PFI is a new project system that provides public services under the initiative of private enterprises.Efficient and effective public services can be provided by designing, constructing, maintaining, and managing public facilities with the funds, technology, and administrative know-how of private enterprises.

The city of Yokohama and bay eggs Corporation entered into a specific project contract on the Digestion Gas Power Generation Construction Project.
Bay Eggs Corporation will undertake the construction of the facility as well as 20 years of operation and maintenance (until March 31, 2030).

Bay Eggs Corporation is a special purpose company established for the purpose of the project concerned, with investments by JFE Engineering Corporation (70%) and Toshiba Corporation (30%).

Electric power and hot water are produced by a digestion gas engine using digestion gas generated from sewage sludge.