Lesson 11. Structural aspects of biogas plants

Biogas technology is an appropriate technique to convert non-conventional energy into conventional energy i.e. suitable organic matters in to end use forms of energy such as heat, mechanical and electrical energy.  Biogas is a neat, clean, combustible and pollution-free gaseous fuel.  The technology is simple and appropriate for both rural and urban areas as it converts the organic matter available both in rural and urban areas.  

Biogas plant has multiple applications for the benefit of individual households, community level and also industrial areas.  Some of the important applications of biogas plant are:

♣    Cooking fuel

♣    Fuel for lighting

♣    As engine fuel

♣    Enriched organic manure

♣    To trap insects at the farm

Types of Biogas Plants

There are many designs and models of biogas plants in operation with each one are having some special characteristics; however, each popular model has some basic components.  The basic difference in models are the storage structure such as floating drum and fixed drum biogas plants.  Other than these, based on the flow of influent slurry, the biogas plants may be classified as Janata model, horizontal plug-flow model, continuous vertical flow reactors etc. have some special components for proper functioning of these designs.

Basic components

The basic components of plants which are common to most of the models are:

  1. Digester or fermentation chamber

  2. Gas holder or gas storage chamber

  3. Inlet (pipe or tank)

  4. Outlet (pipe or tank)

  5. Mixing tank

  6. Gas outlet pipe

  7. Plant dome or roof structure

  8. Manhole or Digester cover

Types of Designs

a. Floating gasholder digester (KVIC)

This type was developed in India by Khadi and Village Industries Commission.  It runs on a semi-continuous mode and uses mainly cattle dung as input material. The gasholder is made of steel, although new materials such as fibre reinforced plastics (FRP) have already been introduced.  THis KVIC biogas plant digester was a vertical cylinder provided with partition wall except for the small sizes of capacity 2 and 3 m3 of gas per day.  The main characteristic of this type is the need for steel sheets and welding skill.


Fig.1. Floating drum biogas plant

b. Fixed dome digester

Fixed dome digesters run on a semi-continuous batch mode, which digest animal and human wastes as well as plant wastes. It is usually built below ground level, hence it is easier to insulate in a colder climate. The digester can be built using several materials such as bricks, cement mortar, concrete, lime concrete and lime clay.  This facilitates the introduction and use of local materials and manpower. 


Fig.2. Fixed dome (Deenbandhu) biogas plant

After construction

a.  General


Initial filling of the slurry should be done after 2 weeks of curing of the plant. Care should be taken to avoid the entry of sand and gravel with the slurry in to the plant.

Fill the plant with properly mixed dung slurry. (mix ratio, dung : water =1:I) Recommended quantity of slurry mixed (i,e. 25 kg. dung with 25 litres of water  per  cubic  meter  plant) should be added daily.

Do not allow scum to form in the digester, otherwise the production of gas will be reduced. Once the manure pit gets filled up with ejected slurry from biogas plant, it should be removed to the field or compost pit.

The initial gas obtained should be let out because it will contain more of Carbon dioxide.

i. KVIC (Floating drum) model biogas plant

When charging fresh slurry in the bigger sized floating drum  type plants,  make sure that  it  is  filled equally on both sides of the central partition wall. After the initial filling of the digester tank, drum moves upwards due to gas pressure. Regular loading of the plant should be commenced after this.  Rotate the gas holder once or twice every day in order to break the scum.

ii. Deenbandhu biogas plant

During the initial filling, fill the plant with slurry up to second step level of the outlet (bottom of the gas chamber). The regular loading of the plant should be commenced only after automatic ejection of the slurry through the outlet opening. Proper loading of the plant will avoid the scum formation because of the slurry movement. The entire biogas plant should be covered with soil to a minimum thickness of 15 cm.


From the gas outlet pipes, remove the condensed water at regular intervals.


The right type of burner designed for bio - gas (as per ISI specification) should be used for efficient utilization of gas.  Do not open the gas valve before lighting the gas appliances.  Burner holes and jets should be cleaned with kerosene and small iron wire to avoid blocking of gas. Wide bottom utensils should be preferably used so that loss in heat will be minimum. Pressure cooker should be used to reduce the gas  consumption.

Adjust the flame by turning the gas cock and air regulator till blue flame is obtained.  Do not test the biogas flammability at the top of the biogas plant.

(Source : TNAU-Bioenergy-BDTC manual)

Last modified: Wednesday, 9 April 2014, 5:35 AM