Super Team

Bio Gas

What should be kept in mind while constructing a bio gas tank properly ?


The precise ratio of Cement, Sand and ¾ inch aggregate material in concrete mixture used in construction of any type of biogas digester should be 1:3:6. Water should be added at the rate of 8 gallons per 50 Kg. bag of cement.

In the case of Chinese digester the concreting should be done to the bottom of the digester, upper level of the digester wall, bottom of the outlet pit, bottom of the mixing pit and the lid of the digester.
In the case of Lankan Batch type digester the concreting should be done to the bottom of the digester, upper level of the digester wall, bottom of the gas holder and lid of the digester.

The thickness of the walls should be 6inches and make it 9inches if the dampness prevails in the area.
Construction of main components of the digester Mixing pit is either circular or rectangular in shape and into which dairy or piggery discharge get collected before entering it into the digester. The bottom of the mixing pit is sloping outward so that it acts as a sand trap which prevents sand and stones entering the digester.

Main openings of the digester- In Lankan type of digesters the only opening available is on the top of the dome and through which raw materials can be fed into the digester. The diameter of the opening is 32-36 inches and its height is about 12inches.

In Chinese type of digester the main opening, which is about 23 inches wide is located on the dome.
Outlet pit is located about 2 inches below the lower level of the dome. The outward flow of slurry into the outlet pit takes place through the opening located on the wall of the digester. When the gas pressure inside the digester increases the materials inside the digester get discharged into the outlet pit through this opening. This mechanism helps to regulate the pressure inside the system.

Dome The upper part of both Chinese and Lankan type of digesters is dome shape. The opening which facilitates the removal of gas is located on the top of the dome. Therefore, the digester dome is an important component and needs special attention during the construction.

Height of the dome- Multiply the diameter of the digester in inches by 0.22 to determine the height of the dome of any type of digester.

Materials used in construction of dome

Use a mixture of lime, sand and cement for brick work. Prepare a mixture of lime and sand at the rate of 1:4 and leave it for a day for conditioning in wet form to use in plastering. The plastering has to be done to prevent any leakage of gas in the digester. Therefore it is recommended to use 5 layers of plastering as follows:

First layer        1:4 above mixture & Cement @ 4:1
Second layer   1:4 above mixture & Cement @ 4:2
Third layer      1:4 above mixture & Cement @ 4:3
Forth layer      Cement and Sand @1;1
Fifth layer       Cement and Padlow 2%

Plastering of items such as inside wall and bottom of the Lankan Batch type digester and upper part of the wall and the dome of Chinese type digester should be continued until full completion.

Construction of lid of the main opening and sealing the unit

Use 1:3:6 concrete mixes to a thickness of about 21/2 inches with a wire mesh reinforcement for the construction of the main lid of the digester. Before fixing the lid to main opening, the unit should be allowed 15 days for curing. In order to prevent any gas leakage seal the unit using a wet mixture of Fine soil (10parts) Fine sand (1part) and Lime (2parts).

Application of raw materials

Except for Lankan type digester, the raw material is fed into the digester via inlet pipe from mixing pit. In the case of Sri Lankan Batch type digester the raw material is fed into the digester in bulk form before sealing the unit.


Contact Us

Janathakshan Project

Practical Action Regional Office (Sri Lanka, India, Pakistan),

No 05, Lionel Edirisinghe Mawatha,

Colombo-5, Sri Lanka.


Tel : 94-11-2829412

Fax : 94-11-2856188

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