What is Biodigester?

There are different types of biodigester in use. The widely used model is the fixed dome digester buried under the ground. The fixed dome biodigester consists of digester with a fixed nonmoveable gasholder,which sits on top of the digester. In this biogas process,a source of renewable energy is generated such as methane(CH4),bio-slurry,gas for cooking,lightening and much more.

The estimated costs of biodigeter depend on different sizes:
Biodigester size (m3) Total cost (US$) Subsidy from NBP (US$) Farmer  cost (US$) Use of biogas stove duration (Hour) Use of biogas lamp duration (Hour)
2 304 150 154 1 - 2 1 - 2
3 381 150 231 1 - 2 1 - 2
4 550 150 400 2 - 4 8 - 16
6 620 150 470 4 - 6 16 - 24
8 720 150 570 6 - 8 24 - 32
10 800 150 650 8 - 10 32 - 40
15 1100 150 950 10 - 15 40 - 60

Biodigester Model

There are different types of household biodigesters introduced by NBP. The widely used model is the fixed dome digester buried under the ground. The fixed dome biodigester consists of a digester with a fixed non-movable gasholder, which sits on top of the digester. When more gas is produced than consumed, slurry from the digester is displaced into the outlet chamber by virtue gas pressure. The gas pressure increases with the volume of gas stored and the rising height of the slurry level in the outlet chamber. Various models of fixed dome biodigesters are in use as shown in the next page. However, the basic principle of operation of these biodigesters is the same for all types. Currently NBP rolled out two types of fixed dome digester as below:

  • 1. Farmer’s Friend Model This biodigester’s size is having from 4m3 to 15m3 with biogas production 800 to 4,000 litres which need to fill the dung from 20kg-100kg per day for the household with 5-15 people.
  • 2. NBP S1 Model- Pro-Poor biodigesters This biodigester’s size is having from 2m3 to 3m3 with biogas production 600 to 800 litres which need to fill the dung from 15kg-20kg per day for the household with 3-5 people household.

Lagoon and floating drum models- Medium Scale Biodigester for R&D: Appropriate medium scale Biodigester plants with 2 different technologies piloted with animal farms which raising pigs 500-2,500 heads to produce electricity for daily consumption. Main activities include review and develop feasible models and piloting with proper monitoring and evaluation.

How does biogas work?

Biogas systems make use of a relatively simple, well-known, and mature technology. A biogas system comprises a large tank, or digester. Inside this tank, bacteria work symbiotically to convert organic waste into methane gas through the process of anaerobic digestion. Each day, the operator of a biogas system nourishes the bacteria inside the digester with household by-products such as market waste, kitchen waste, and manure from livestock. The methane gas produced by the bacteria inside biogas system may be used for cooking, lighting, and other energy needs. Waste that has been fully digested exits the biogas system in the form of organic fertiliser. Biogas has several external benefits: biogas replaces the unsustainable use of biomass, slows deforestation and reduces greenhouse gas emissions; emissions of methane (a strong greenhouse gas) associated with the decomposition of livestock manure are mitigated; the import of both kerosene and foodstuffs are reduced, thereby improving the trade balance; and healthcare costs are decreased through improved indoor air conditions and better environmental sanitation.