Biogas Plant

 

A biogas plant consists of a well shaped, underground tank T called digester, which is made of bricks, and has a dome- shaped roof D, also made of cement and bricks. The disgester is a kind of sealed tank in which there is no air (or oxygen). 

The dome of the digester tank acts as a gas- holder or storage tank for the biogas. There is a gas outlet S at the top of the dome having a valve V. On the left of the disgester tank is a sloping inlet chamber I and the right side is a rectangular outlet chamber O, both made of bricks.

The inlet chamber is for introducing fresh dung slurry into the main digester tank whereas the outlet chamber is for taking out the spent dung slurry after the extraction of biogas. The Intel chamber is connected to a mixting tank M while the outlet chamber is connected to the overflow tank F.

We will now describe the working of a biogas plant. Cow-dung and water are mixed in equal proportions in the mixing tank M to prepare the slurry. This slurry of dung and water is fed  into the digester tank T through the inlet chamber I. The digester tank is filled with dung slurry upto the cylindrical level , the dome being left free for the collection of biogas. It takes about 50 to 60 days for the new gas- plant to become operative (start functioning). 

Durning this period, the cow- dung undergoes degradation by anaerobic bacteria in the presence of water (but absence of oxygen) with the gradual evolution of biogas. This biogas starts collecting in the dome. As more and more biogas collects in the dome, it exerts pressure on the slurry in the digester tank, and forces the spent slurry to go into overflow tank F, through the outlet chamber O. Form the overflow tank, the spent slurry is removed gradually.  The spent dung-slurry, left after the extraction of biogas, is rich in nitrogen and phosphorus compounds hence forms a good manure.

One the gas- plant becomes operative, more fresh dung slurry is added to the digester tank regularly and this leads to the continuous biogas production. The biogas which has collected in the dome of the disgester tank is taken out through the outlet S and supplied to village homes through a network of pipes to be used as a cooking gas.

Please note that through mostly we use cow-dung in biogas plants, but human excreta ( human faeces) can also added alongwith cow- dung in biogas plants. Agricultural wastes, vegetable wastes, poultry droppings, and paper scrap can also be used for producing biogas (Sewage us the dirty drain water containing human excreta). Biogas can also be obtained by the action of anaerobic bacteria 🦠 on domestic sewage in the absence of air. The biogas obtained from the degradation of domestic sewage is also called sewage gas. So, at many places in our country, domestic sewage is disgested in large scale utilisation of biowastes and sewage materials for producing biogas can provide us a safe and efficient method of waste disposal besides supplying energy and manure.

The important uses of biogas are give below:

1. Biogas is used as a fuel for cooking food. When biogas is burned in a gas stove, it produces a lot of heat. 

This heat is used for cooking food and for other domestic heating purposes. Biogas is good domestic fuel because of the following reasons:

1. Biogas burns without smoke and hence does not cause air pollution.
2. Biogas has a high calorific value. That is, biogas produces a large amount of heat per unit mass.
3. Biogas burns completely without leaving behind any residue (unlike wood, charcoal or coal). So, it is a clean fuel.
4. There is no storage problem for biogas as it is supplied by pipes directly from the gas plant.
5. Biogas is cheaper than most common fuels.

2. Biogas is also used for lighting.

3. Biogas is used as a fuel to run engines. At many places, the engines of water pumping sets used for irrigation are run on biogas, instead of diesel.

4. Biogas is used for generating electricity.

Biogas is a renewable source of energy. Since India has a large population of cattle and other farm animals (called livestock) to provide animal dung, therefore, biogas can became a steady source of energy in our rural areas. Before we end this discussion on biomass as a source of energy, we would like to compare the biomass and Hydroelectricity as sources of energy.

• Biomass is renewable source of and Hydroelectricity is also a renewable source of energy.
• The use of biomass by burning causes air pollution but the use of Hydroelectricity does not cause any pollution.
• Biomass energy can be obtained without using any special but Hydroelectricity can be produced only by establishing hydro- power plants.