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big is not beautifuel.

To become eligible to receive the investment subsidy provided by His Majesty’s Government of Nepal under the Biogas Support Programme (BSP), only the 2047 design GGC model plants of 4, 6, 8, 10, 15 and 20 cubic meters capacity should be constructed. If the design and size of the plant other than mentioned above is chosen, the subsidy is not allowed. The table below gives some relevant data about these six different sizes of biogas plants.


Table 1
Size of Plant
Daily Fresh Dung (Kg)
Daily Water Liters
Approx. No. Cattle Required
2 – 3
3 – 4
4 – 6
6 – 9
9 – 14
14 and more
* Plant size is the sum of digester volume and gas storage
** Based on a hydraulic retention time of 70 days

A biogas plant consists of five main structure or components. The required quantity of dung and water is mixed in the inlet tank and this mix in the form of slurry is allowed to be digested inside the digester. The gas produced in the digester is collected in the dome, called as the gasholder. The digested slurry flows to the outlet tank from the dig through the manhole. The slurry then flows through overflow opening to the compost pit where it is collected and composted. The gas is supplied to the point of application through the pipeline.

Before deciding the size of plant, it is necessary to collect dung for several days to determine what is the average daily dung production. The amount of dung daily available helps in determining the capacity of the plant. For example, if 55 kg of dung is collected daily, a 8 m3 plant has to be selected. It should be kept in mind that at least six kg dung is required for 1 m3 of plant capacity. The important point to be considered is that the size of plant has to be selected on basis of available dung not on the family size.

If a plant is underfed, the gas production will be low; in this case, the pressure of the gas might not be sufficient to displace the slurry in the outlet chamber. This means that amount of slurry fed into the digester is more than the amount of slurry thrown out from the outlet. This will cause the slurry level to rise in the digester; gasholder and it may eventually enter to the gas pipe and sometimes, to the gas stove and lamp while opening the main valve. Therefore, the slurry should always be fed according to the prescribed amount as indicated, in the above table.

2. Construction Materials

If the construction materials to be used in the plant construction such as cement, sand, aggregate etc. are not of good quality, the quality of plant will be poor even if design and workmanship involved are excellent. In order to select these materials of best quality, their brief description regarding the specifications has been given hereunder. (The list construction materials are given in annex 1).

a) Cement

The cement to be used in the plant construction has to be of high quality portland cement from a brand with a good reputation. It must be fresh, without lumps and stored in a dry place. Bags of cement should never be stacked directly on the floor or against the walls but wooden planks should be placed on the floor to protect cement from dampness.

b) Sand

Sand for construction purpose must be clean. Dirty sand has a very negative effect on the strength of the structure. If the sand contains 3% or more impurities, it must be washed. The quantity of impurities especially the mud in the sand can be determined by a simple test using a bottle. This is called the ‘bottle test’. For this test, small quantity of sand is put in the bottle. After this, water is poured in and the bottle is stirred vigorously. The bottle is than left stationary to allow the sand to settle down. The particles of sand are heavier than that of mud so it settles down quickly. After 20-25 minutes, the layer of mud verses sand inside the bottle are measured. Course and granular sand can be used for concreting work but fine sand will be better for plastering work.

c) Gravel

Gravel should not be too big or very small. It should not be bigger than 25% of the thickness of concrete product where it is used in. As the slabs and the top of the dome are not more than 3″ thick, gravel should not be larger than 0.75″ (2 cm) in size. Furthermore, the gravel must be clean. If it is dirty, it should be washed with clean water.

d) Water

Water is mainly used for preparing the mortar for masonry work, concreting work and plastering. It is also used to soak bricks/stones before using them. Besides these, water is also used for washing sand and aggregates. It is advised not to use water from ponds and irrigation canals for these purposes, as it is usually too dirty. Dirty water has an adverse effect the strength of the structure; hence water to be used must clean.

e) Bricks

Bricks must be of the best quality locally available. When hitting two bricks, the sound must be clear. They must be well baked and regular in shape. Before use, bricks must be soaked for few minutes in clean water. Such bricks will not soak moisture) from the mortar afterwards.

f) Stones

If stones are to be used for masonry work, they have to be clean, strong and of good quality. Stones should be washed if they are dirty.


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