Global Environment Centre Foundation

Search by Keywords
Home > NETT21 > Technologies for Resource/Energy Recovery from Sewage Sludge

Data Entry: June 2011
Name of Technology Methane Fermentation System of Sewage Sludge and Raw Garbage, and Carbonization-activation for Utilization
Technology Owner Kawasaki Heavy Industries, Ltd.
General Description This technology aims to achieve two goals:
- To make beneficial use of methane gas recovered via methane fermentation from a mixture of sewage sludge and organic waste, such as kitchen refuse, received from local communities, viewing a wastewater treatment plant as a facility for using local biomass; and
- To promote carbonization-activation and recycling of fermentation residue, or dewatered sludge.
This technology system integrates two processes, which are methane fermentation of sewage sludge-biomass mixture followed by power generation using the resultant biogas and carbonization-activation of fermentation residue.
The use of biomass is under assessment in Japan, but progress is slow regarding the use of wet biomass due to its high water content which accounts for a high weight percentage. Methane fermentation is attracting great interest as a low-cost means of recovering energy from wet biomass. However, one drawback of the process is the treatment cost for filtrate from the dewatering machine after fermentation. In this regard, a wastewater treatment plant has large wastewater treatment equipment and consumes electricity generated within the plant, and so is expected to be a useful facility for making use of wet biomass. Moreover, methane gas can be used as supplementary fuel for carbonization or carbonization-activation of residual dewatered sludge to promote recycling without consuming fossil fuel. Furthermore, in the case of receiving kitchen refuse included in general domestic waste, the refuse incineration plant can be downsized when rehabilitated.
Components of Technology The four major components of this technology are: (1) Pretreatment equipment, (2) Methane fermentation tank, (3) Biogas power generating equipment and (4) Carbonization-activation equipment.

These components have the following functions.
(1) Pretreatment equipment
Receives kitchen refuse and other biomass, removes foreign matter and solubilizes the biomass mixed with sewage sludge.
(2) Methane fermentation tank
Performs methane fermentation of the solubilized liquid and another portion of sewage sludge to produce biogas.
(3) Biogas power generator
Refines biogas, generates power by gas engine and supplies electricity to plant facilities.
(4) Carbonization-activation equipment
Dries dewatered digested sludge (dewatered sludge) and produces activated carbon.

Actual conditions may vary depending on the local situation. For example, the type of biomass that the plant can receive may vary or the plant may receive dewatered sludge from other plants for treatment. This technology allows for optimization, such as considering whether or not to incorporate every component or changing the capacity to reduce the final cost


Scopes of both technologies

The difference between Case 1 and Case 2 is whether the wastewater treatment plant has a digestion tank or not. In Case 1, assuming that the plant has no existing digestion tank, the project builds a compact horizontal methane fermentation tank, which is already used in some locations for treating livestock manure. Case 2 assumes that the wastewater treatment plant already has a digestion tank.

Features of Technology

This technology is designed to receive wet biomass including kitchen refuse, recover a larger quantity of methane gas and earn revenue from treatment. By producing valuable activated carbon from dewatered sludge without disposing of it as industrial waste and earning revenue from its sales, this technology substantially improves the economy of the overall system.
(1)Wastewater treatment plant receives kitchen refuse and other wet biomass.
→ Earns waste disposal fees.
(2)Performs methane fermentation of a mixture of wet biomass and sewage sludge, and uses the resultant biogas for high-efficiency power generation.
→ Increases the biogas recovery rate and reduces the amount of electric power purchased.
(3)Produces activated carbonized products from dewatered sludge.
→ Reduces disposal costs for dewatered sludge, and earns revenue from selling valuable products.
⇒ Substantially improves the economy of the system.


The features of this technology are detailed below.
(1) Pretreatment equipment
The pretreatment equipment is comprised of a crusher-separator and solubilization tank. The pretreatment equipment crushes kitchen refuse and separates and removes foreign matter. The kitchen refuse is then mixed with some sewage sludge and made soluble by the activity of microorganisms within a retention period of approximately 24 hours at 40 to 50°C. The viscosity of the mixture decreases through this process, meeting the required fluidity in the methane fermentation tank. In this process, part of the organic matter becomes organic acids via acid fermentation. Moreover, high-melting-point animal fats are also solubilized without becoming lumpy. A dryer, if installed in the plant, eliminates the need for a solubilization tank heater since dryer exhaust can warm the sewage sludge to be fed into the solubilization tank.

(2) Horizontal methane fermentation tank (Case 1)
The horizontal methane fermentation tank is a plug flow system and is free from short-circuiting unlike the complete mixing system. The plug flow system is suitable for methane fermentation reaction, since it is a consecutive reaction. Furthermore, synergistic effects can be attained by raising the operating temperature, thus allowing a compact fermentation tank to be used with a volume for a retention period of approximately seven days. Compactness also reduces the amount of heat radiation and hence heating energy in comparison with the conventional complete-mixing ones used for high-temperature fermentation. At the section where the raw material is fed in, some 40% of sludge is returned to prevent rancidification and maintain a high microorganism concentration. The retention periods required for different raw materials are easy to forecast by laboratory batch testing.

(3) Sludge heating tower for digestion tank
The scrubber-equipped gas-liquid contact tower allows dryer exhaust to come in direct contact with sewage sludge to condense the steam from the exhaust and also heat the sewage sludge. This system has been installed and is in operation at seven locations in Japan. Usually, 30 to 50% of the produced digestion gas is fired in a dedicated boiler to produce hot water or steam for heating. This technology system eliminates this need, achieving substantial digestion gas savings. Sludge in the digestion tank can be recycled to the gas-liquid contact tower depending on the tank temperature.

(4) Carbonization-activation equipment
Comprises a dryer and activated carbonization furnace. The dryer is a flash dryer using steam as medium. The activated carbonization furnace is an externally heated screw furnace. The dryer obtains heat from the exhaust of the activated carbonization furnace via a heat exchanger. The steam-rich exhaust is dehumidified in the sludge heating tower, leaving an extremely small quantity of odorous gas. The odorous gas is used for combustion in the activated carbonization furnace, eliminating the need for a hot air generating furnace and deodorization furnace. In the activated carbonization furnace, dried sludge and steam contained therein are sent to the lowest part of the screw and a high operating temperature is used to facilitate the activation reaction to produce adsorptive activated carbonized products. Activated carbonized products can be sold at high prices as a dioxin adsorbent. Moreover, carbonized products produced at a low operating temperature are used as soil improvers or fuel.

Schematic diagram of the technology

Evaluation of Technology

This technology was developed in the LOTUS Project (Lead to Outstanding technology for Utilization of Sludge Project) and evaluated and reviewed by the Sewerage Technology Development Project Committee under the Ministry of Land, Infrastructure and Transport in Japan in 2008.

[Link to the technology evaluation report]

Note: This technology was developed by Kawasaki Plant Systems, Ltd. and evaluated in 2008. Kawasaki Plant Systems, Ltd. has been consolidated into Kawasaki Heavy Industries Ltd. since 2010.

Contact Information Industrial Plant Sales Department (Chemical Plant), Plant & Infrastructure Company, Kawasaki Heavy Industries Ltd.
URL: http://www.khi.co.jp/english/kplant/business/environment/water/dirt.html
For inquiry: http://www.khi.co.jp/cgi-bin/other-q1_e.cgi?form-type=ind-plant&ca_no=30

Maintained by Global Environment Centre Foundation (GEC)