Question

Draw and explain fixed dome type biogas plant.

Answer 1

Fixed Dome Type Biogas Plant, also commonly known as the Janta model (in India) or the Chinese model.

Diagram of a Fixed Dome Biogas Plant

This diagram shows a cross-section of the plant, which is typically built underground.

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Key Components and Their Functions

1. Inlet Tank (or Mixing Pit): This is where the raw organic material (like cow dung, kitchen waste) is mixed with water to create a uniform mixture called "slurry."

2. Inlet Pipe: This pipe connects the Inlet Tank to the Digester. The prepared slurry is fed into the main chamber through this pipe.

3. Digester Tank: This is the main body of the plant, a large, underground, cylindrical chamber. It is here that the anaerobic digestion (decomposition of organic matter by bacteria in the absence of oxygen) takes place.

4. Dome (Gas Holder): This is the fixed, dome-shaped roof of the digester, made of brick and cement or concrete. It is an immovable part of the structure. The biogas produced during digestion, being lighter than the slurry, rises and gets trapped in this dome. The dome acts as the gas storage area.

5. Gas Outlet Pipe & Valve: A pipe is fitted at the top of the dome with a valve to control the flow of biogas. This pipe carries the gas to the point of use, such as a kitchen stove or a gas lamp.

6. Outlet Tank (or Displacement Pit): As fresh slurry is added through the inlet, and as gas accumulates in the dome, the digested slurry is displaced and pushed out into this chamber. The level of slurry in the outlet tank indicates how much gas is stored in the dome.

7. Slurry: The mixture of organic waste and water. The undigested mixture is called "influent," and the digested, nutrient-rich liquid that comes out is called "effluent" or "bio-slurry."

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How it Works (Working Principle)

The operation of a fixed dome biogas plant is a continuous process:

1. Preparation and Feeding: Cow dung (or other organic waste) and water are mixed in a 1:1 ratio in the Inlet Tank to form a slurry.
2. Digestion: The slurry flows into the Digester Tank. Inside the sealed, oxygen-free digester, anaerobic bacteria start to break down the complex organic matter. This process takes a few weeks to start and produces biogas.
3. Gas Collection: The biogas produced is primarily a mixture of methane (CH₄, 55-70%) and carbon dioxide (CO₂, 30-45%). Being lighter than the slurry, the gas rises and collects in the space under the dome.
4. Pressure Buildup: As more gas collects, it exerts pressure on the surface of the slurry inside the digester.
5. Displacement: This gas pressure forces the digested slurry to move out of the digester and into the Outlet Tank. Therefore, a high slurry level in the outlet tank means the dome is full of gas and the pressure is high.
6. Gas Usage: When the gas valve is opened, the pressure in the dome pushes the biogas through the pipes to be used for cooking or lighting. As gas is used, the pressure drops, and the slurry level in the outlet tank goes down.
7. Fertilizer Production: The digested slurry (effluent) that overflows into the outlet tank is a high-quality, nutrient-rich organic fertilizer that can be used for crops.

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Advantages and Disadvantages

| Advantages | Disadvantages |
| ------------------------------------------------ | ---------------------------------------------------------- |
| No Moving Parts: Low wear and tear. | Variable Gas Pressure: Pressure depends on the volume of gas stored. |
| Low Maintenance: Simple design requires little upkeep. | High Initial Cost: Construction requires skilled masonry work. |
| Long Lifespan: Can last for 20+ years if built properly. | Difficult Repairs: Cracks in the underground dome are hard to detect and repair. |
| Saves Space: Being underground, it doesn't occupy surface land. | Construction Challenges: Requires specific soil conditions and skilled labor to prevent gas leaks. |
| Good Thermal Insulation: The ground protects the digester from daily temperature fluctuations. | Gas Storage is Limited: The volume of the dome limits how much gas can be stored. |

Answer 2

Diagram of fixed dome type biogas plant:

Explanation of layout of biogas plant by the method of fermentation conversion
1. Foundation :
Biomass plant consists of pit excavated to desire size & depth, The foundation is
nothing but the base of digester. It is made with the help of cement, concrete.
2. Digester:
 It is container made up of bricks, sand & cement. Digestor tank is undergrounded to
increase the efficiency.
 In the digester, decomposition of biomass takes place due to anaerobic bacteria to
produce biogas.
 Quantity of gas produced is depend open type of waste & temperature.
3. Dome (Balancing Tank) :
It is the roof of digestor in which biogas is collected.
4. Mixing Tank:
It is the tank placed on the top of inlet chamber in which animal, sanitary waste & water are mixed properly to make slurry.
5. Inlet Chamber:
It is to admit slurry into digesteor chamber through pipe due to gravity.
6. Outlet Chambers:
When generated biogas is high then it increases pressure downwards to slurry.
Due to pressure of gas, slurry comes upward automatically through pipe which is collected in outlet chamber.
The residue (slum) left is used as fertilizer (valuable)
7. Gas Outlet pipe:
It is an outlet pipe fitted at the top of the dome of the digestor to take the biogas for utilization.
The valve is provided to control the flow of biogas.
8. Mixing or Stirring:
The decomposition process can be speed up by stirring the slurry from the top
of the dome with the help of stirrer which is at digester chamber.