Anaerobic digestion is decomposition and decay process by which organic
matter is broken down to its simpler chemicals components under anaerobic
conditions. Anaerobic microorganisms digest the organic matter in digestion
tank in WTP under anaerobic conditions, and produce methane (60%), carbon
dioxide (35%), and small amount of hydrogen, nitrogen, and hydrogen sulphide,
as well as trace amounts of other gases.

Sludge Digester

Generally, digester gas is utilized as a fuel for boilers to heat the digester tank.
Excess gas burns with burning equipment and exhaust gas is released to the
atmosphere.  Under the condition that lots of excess gas is obtained in large
WTP or by using high efficiency digester tank, energy saving in total system
can be expected by utilizing the gas energy.

Boiler for Steam Injection into Digester for Warming

As for the utilization of digester gas, direct utilization such as a fuel for sludge
incinerator, and boilers for hot water supply or air conditioning is desirable
because of its high efficiency.

Recently, power generation systems using digester gas has been widely adopted.
In WTP in Yokohama city, most of digester gas is used as a fuel for gas engine
to produce electric power, and the generation capacity in Hokubu WTP is
about 5,000kW.  Moreover, heat recovery system from exhaust gas or cooling
water are installed in the generation systems, and recovered heat is effectively
used in WTP. In this system, electricity is generated by heat engine and at
the same time byproduct heat is effectively recovered.This system is called
cogeneration system, and its energy efficiency is high.

Gas Engine Fuel Cells

The methane contained in digester gas can also be used as a fuel for Fuel cells.
Fuel cells are power generating systems that produce DC electricity by combining
hydrogen and oxygen (from the air) in an electrochemical reaction. usually, pure
hydrogen is expensive and hydrogen is generated from digester gas or natural gas.
The principle of fuel cell is the reverse reaction of electrolysis of water, an
electrolytic process which decomposes water into oxygen and hydrogen gas
with the aid of an electric current. Byproduct of fuel cells is only the water and
this system is good for environment.

Sludge Digester in Yokohama City

In Kobe City, Biogas of which methane concentration is 98% has been successfully
produced from digester gas (methane concentration: 60%) after eliminating the
impure substances. The quality of Biogas is almost same as city gas, and the
biogas is used as a fuel forvehicles.Generally, Biogas needs to be upgraded in
order to obtain;
A higher caloric value
No enhancement of corrosion due to high level of hydrogen sulphide.
A gas without any mechanically damaging particles.
Since 2006, municipal bus using Biogas as a fuel, has been operated, and
it is scheduled to increase the Biogas production in WTP.

Operating municipal bus using Biogas as a fuel in Kobe


Future full scale plan for producing and supplying Biogas in WTP in Kobe

Digester gas, one of the useful biomass produced by anaerobic digestion in
Wastewater Treatment Plant can contribute to the prevention of global
warming and sustainable development, and several effective measures for
gas utilization have been developed in Japan.


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