List of Biogas Plants Developed in India

A number of plant designs have been developed by different organisations and individuals in India which are either modifications of movable drum or fixed type or entirely different new designs.

Some broad details and distinctive feature of these plant design are given below:

1. Deenbandhu Biogas Plant:

Action for Food Production (AFPRO), an NGO, in 1984 developed a low cost fixed dome plant called ‘Deenbandhu Model’ meaning ‘friend of the poor.’ The history of this model dates back to 1981 when the AFPRO organised a training programme aided by F&AO/UNDP at Varanasi where AFPRO presented 8 alternative designs of 2 m³ capacity plants which following some modifications in 1984 were later standardised and promoted as Deenbandhu model.

It is appropriate for using all types of wastes and minimises biogas losses from inlet chamber and ensures maximum utilisation of digester volume thereby making the plant operate at designed HRT. This model is 30 per cent cheaper than Janata model and 45 per cent less than the KVIC plants of comparable capacities. It is 15-30 percent cheaper than the Pragati Model developed by BORDA and UNDARP (United Socio- Economic Development of Research Programme).

Storage capacity is 33 per cent of daily gas production. It has curved bottom and a hemispherical top which are joined at their bases with no cylindrical portion in between. Displaced slurry following fermentation moves to the outlet displacement chamber as there is no displacement space on the inlet side. An inlet pipe connects mixing tank with the digester. The plant design is based on gas yield of 0.04 m³ per kg of fresh dung, and HRT of 40 days in plains and 55 days in hilly regions.

Cattle dung slurry prepared in 1: 1 ratio with water is fed upto the level of second step in the outlet tank which is also the base of the outlet displacement chamber. As the gas generates and accumulates in the empty portion of the plant, it presses the slurry of the digester and displaces it into the outlet displacement chamber.

The slurry level in the digester falls whereas in the outlet chamber it starts rising. This fall and rise continues till the level in the digester reaches the upper end of the outlet opening, and at this stage the slurry level in the outlet tank reaches the height of discharge opening. Any unused gas beyond this stage escapes through the outlet tank.

The diameter of the base is almost the same as the diameter of JBPs of identical capacities. For example, in a 2 m³ plant the bottom with a large curvature has a base diameter of 2.55 m³ which is the same as in a JBP. Effluent discharge opening in the outlet displacement chamber is at least 150 mm (6 inch) lower than the bottom of the gas outlet which is flushed with ceiling.

It helps to prevent entry of gas or its foam into the gas pipeline taken as a trade-off between low discharge opening and high dome. For plant bottom, base diameter to rise of arch ratio is kept as 5: 1. Design of Deenbandhu model (1, 2, 3, 4 and 6 m³ capacities) was approved by the DNES in 1986 for countrywide popularisation under the NPBD. Based on AFPRO study whereas gas yield per unit weight of feed is same for both Deenbandhu and JBP the productivity per unit volume of digester is higher by 20 to 40 per cent in Deenbandhu plant for all months.

2. Gayatri Model:

Gayatri model was developed by the Government Imple­ments Factory, Bhubaneswar. It eliminates the use of brick dome and mini­mises the use of cement. In place of brick dome, a pre-fabricated fibre-glass reinforced plastic (FRP) dome of the same dimension is used for gas collection. A pilot-scale Gayatri model plant of 2 m³ capacity as shown in Fig. 4.11 has been functioning satisfactorily at the Government Implements Factory, Bhubaneswar.

3. TNAU Model:

Tamil Nadu Agricultural University at Coimbatore evolved four designs of biogas plants with a view to achieve among others reduction in retention period. One of its model is similar to the fixed dome plant of the Janata type with a flat bottom whereas the other model resembles the Nepalese version of the Chinese model with curved digester bottom and Ferro-cement dome.

The others two designs are mobile plants while one being cubical the other cylindrical in shape. Special feature of all these plants is low retention period of 15 days which is achieved by periodic addition of micro-organisms which speed up the digestion process.

4. Krishna Biogas Plant:

Shivsaaan Renewable Research Institute has developed an alter­native fixed dome type plant made of prefabricated concrete which it named as Krishna biogas plant (Fig. 4.11a). The plant consists of a digester with its dome and the lower cylindrical portion cast monolithically in reinforced cement concrete (RCC).

The inner surface of the structure that comes in contact with biogas under varying pressures is coated with an epoxy paint to prevent possible gas leakage. In addition, a single compensatory slurry displacement tank, also cast in RCC is provided over the dome to account for increase in gas pressure. Besides reducing overall costs, Krishna model overcomes several limitations commonly associated with conventional fixed dome plants such as frequent gas leakage.

Use of pre-fabricated concrete structure achieves faster construction and closer adherence to time-bound programme of constructing a specified number of plants through more realistic scheduling of production, transporta­tion and erection of plant sequences. By February, 1991, the Shivasadan Renewable Research Institute had set up 725 Krishna biogas plants. The cost of the Krishna model is claimed to be 15 to 20 per cent lower than the corresponding KVIC model.

The Krishna plant was originally designed for 30 days HRT as per MNES specifications for the Maharashtra region. It is possible to connect a latrine unit to the biogas plant so that night-soil could be digested along with the cattle dung. Details of digester volume and gas holding volume for the plants of 2, 3, 4 and 6 m³ gas production capacities are given in Table 4.15. It may be noted while the digesters volume is 1.5 times of this nominal size, the gas holding volume is half the nominal size.

The gas balancing tank is automatically filled up by the digested substrate twice a day coinciding with the two meals times following increase in gas pressure inside the digester. As the gas is consumed for cooking or heating, the digested substrate returns back to the digester through automatic balancing. Digested slurry is directly fed to the compost pit through spent slurry drain pipe.

5. Manipal Model:

This model has features of both Indian and Chinese models. It has a rectangular masonry digester with an inclined RCC roof. There is no separate gasholder unlike in earlier Indian designs and larger inclined space above the slurry level provides storage area for resulting biogas to accumulate.

Digester can be built, partly inside or fully above ground depend­ing upon ground conditions. Variation in pressure is achieved by a shallow ferro-cement cover dipped in water seal on sloping roof slabs. Inlet and outlet pipes are selected according to feed material availability and desired mode of plant operation (Fig. 4.12).

Resulting biogas is taken out with the help of a pipe. This model is more economical vis-a-vis., KVIC design economical as it replaces mild steel gasholder of the KVIC design with a small ferro-cement gasholder. Furthermore, ferro-cement gasholder can be of uniform size regardless of plant capacity which makes it amenable for standardisation and pre-fabrication. In addition sloping RCC roof and rectangular masonry digester achieve sufficient ease in plant construction and maintenance.

6. Spherical Biogas Plant:

A fixed-dome spherical low-cost plant design was developed by the Tata Energy Research Institute (TERI), New Delhi. Besides being a low-cost model it is also durable on account of its shape which can withstand gas pressure (Fig. 4.12a). One of the additional features of this plant is that it uses a nylon net for automatic breaking of the scum formation over the slurry surface within the digester.

This plant is designed for a retention period of 50 days with a maximum pressure of 2 ft. 6 inches (75 cm). Digester and gasholder together form a complete sphere in which the top portion serves as the gasholder. Mixing tafik and outlet tank are on either side of the digester. The gasholder is protected by an additional layer of tiles which are laid over the bricks. A GI-pipe is provided on top of the dome and a valve is fitted to it to regulate gas flow.

A manhole of 2 ft x 2 ft is provided at digester top which facilitates entry and also serves as opening to the outlet tank Organic wastes are mixed with water for preparing slurry in the mixing tank. Once uniformly mixed slurry is prepared, lid is opened and the slurry flows into the spherical digester through the inlet pipe. The slurry inside the digester may contain lighter particles like grass and other waste materials.

These particles float on top surface of the slurry and form a hard layer which obstructs gas generation. For preventing formation of this hard scum layer, a nylon net is spread inside the digester just above the manhole opening.

During digestion process, biogas is produced and accumulated in the gasholder. The slurry level within the digester goes down due to pressure of the accumulated gas and rises up with its consumption. This movement causes slurry to pass through the nylon net which breaks up the scum.

7. Astra Model:

Centre of Applications of the Science and Technology in Rural Areas (ASTRA), a unit of the Indian Institute of Science, Bangalore, developed a plant model which has come to be known as the ASTRA model. It is a floating drum type biogas plant which utilises a solar water heater and a solar still for obtaining higher gas yield. A schematic view of the ASTRA model is shown in Fig. 4.13.

The ASTRA model needs reduced retention period, incurs low cost and causes higher gas yield due to the facility for maintaining higher slurry temperature particularly during winter. Digester and gasholder dimen­sions can be optimally selected that correspond to the minimum plant cost. Mild steel sides of the gasholder are extended by 0.3 m³ above the black painted roof to form a 0.3 m³ deep water pond on gasholder top.

The pond is covered with a polythene sheet. By making the transparent cover assume a tent like shape, the roof-top solar heater serves as solar still to yield distilled water. Two experimental ASTRA plants of 2 and 6 m³ capacity with their performance analysed over a period of one year were marginally superior to the matching KVIC design with their costs less by 40 per cent.

8. Belur Math Model:

This model was developed by Swami Vimuktananda at Ramakrishna Mission at Belur Math (West Bengal). Its distinctive feature lies in having a separate gasholder independent of digester.

A schematic view of this design is shown in Fig. 4.14. Gasholder consists of two cylindrical tanks placed opposite to one another with lower one filled with water. Slurry is normally fed to the digester every day. Resulting biogas following anaerobic fermentation is passed through a pipeline to the gasholder and from there through another pipeline to the place of use.

9. Neeri Model:

This model evolved by the National Environmental Engineer­ing Research Institute (NEERI) at Nagpur is a modified version of the movable drum type biogas plant. Its digester made of brickwork partly lies below ground and partly above. It is specially designed to run on night-soil.

There is a facility for periodic sludge removal into sludge drying bed where it is dewatered and dried. A schematic view of the plant is shown in Fig. 4.15. Supernatant of the digester and underflow of drying beds are further treated in a stabilisation pond.

10. Bajwa-KVIC Model:

The Bajwa-KVIC model also called the Ganesh model is an improved version of the KVIC model. Whereas the KVIC model uses the mild steel and cement for its construction, the modified Bajwa-KVIC model uses only a few mild steel round bars and no cement. Its digester is made of bamboo strips, mild steel bars- and low density polyethylene sheets.

Gasholder is made of high density polyethylene sheets. Gasholder is a floating drum guided by a central guide-pipe as in the KVIC model. Gasholder is very light and does not require periodical maintenance. It is possible to carry out repair locally with the help of kerosene-fired torch.

Plant construction is easier since locally available materials like bamboo-split can be used and costly items like cement and mild steel are used bare minimum. LDPE and HDPE sheets are found very effective in checking water seepage than cement or brick apart from ease to carry.

Life of the plant however following exposure of LDPE gasholder to ultraviolet rays is comparatively less. A 2-m³ capacity plant has been installed at the Government Implement Factory, Bhubaneswar and is reportedly working satisfactorily.

11. Jyoti Top Loaded Digester:

Design of this model (Fig. 4.17) was developed by the Jyoti Solar Energy Institute at Vallabh Vidyanagar (Gujarat) which is particularly appropriate for utilising agricultural waste materials and crop residues. In this model which is a modified version of the KVIC model, wastes are admitted from the gasholder top leading to the slurry surface with the help of a plunger arrangement.

Care is taken to make inlet pipe sufficiently long so that it always touches the slurry surface for preventing gas leakage through inlet. For achieving adequate dispersion of wastes as well as avoiding rotation of wastes as a whole, stirrer with paddles near the surface and passing through central guide pipes are provided.

Gasholder can move up and down inside the peripheral channel close to upper part of the digester depending upon the gas pressure. Jyoti model is helpful in alleviating the problem of scum formation which is quite severe in digestion of forest and crop residues. The plant design obviates the need for fine shredding and drying of the residues before feeding them.

12. Jwala Biogas Plant:

This design was developed by the AMM Murugappa Chettiar Research Centre, Madras. It has cylindrical digester which is largely kept below ground level. Gasholder is made up of low density polyethylene (LPDE) sheet which is fixed on a geodesic balloon.

Composite dome is held inside a water seal to avoid gas leakage. Scum-breaking is achieved with the help of a stirrer which passes through a central guide-tube. It has a partition wall which is tangentially touched by the inlet pipe. This model is relatively inexpensive as no costly materials are involved in its construction.

13. Mud Jar Biogas Plant:

Centre of Science for Villages, Wardha, deve­loped a low cost biogas plant which resembles a ‘Surahi’ local name for a mud pot which can be built by a village potter and hardly needs any maintenance. Ten mud jars of 300 litre capacity each connected in series were used as digesters and resulting gas was sufficient for meeting cooking needs of a family of five for four hours use at a stretch.

Based on 1982 price level, the plant cost was Rs. 650 which is less than one-fourth of the traditional masonry built biogas plant of the Indian design. Mud jars were connected with three inch thick clay pipes. The first mud jar was attached to a small pot to serve as inlet for feed. Similarly the last jar was connected to a pot to act as the plant outlet.

Whereas inlet and outlet pots were placed above the ground, three-fourth portion of the intermediate mud jars (digesters) were embedded within the ground (Fig. 4.19). Partial underground placement of digesters minimises the problems of leaks and plant breakage.

Upper one-fourth portion of the jars act as gasholder where gas is stored. For preventing gas seepage through the porous walls of mud jars, their upper exposed portion can be painted with coal tar or locally developed paint made out of sap of trees and lime. Generated gas ca« either be taken out through flexible tubes or collected in leak-proof balloons. Mud jar plants can prove valuable in areas that possess good soil properties.

14. ‘Kachra’ Biogas Plant:

Dr. K.P. Goswami, an eminent agricultural scien­tist, developed a plant what has come to be known as ‘Kachra’ biogas plant arising out of years of work at the Punjab Agricultural University, Ludhiana and later Gujarat Agricultural University at Anand. The need to develop a special plant arose following difficulty in fermenting straw like oversized agricultural wastes which float and cause clogging in the digester.

The ‘Kachra’ biogas plant can work on any plant material or animal wastes in any proportion but it works more efficiently with leafy materials than woody materials. Initially a bench scale plant (2-3 m³ biogas per day capacity) was designed and developed which was later on modified.

The earlier design was made of iron sheets and included three adjacent rectangular masonry chambers, 213 cm deep and 183 cm wide with a portion 150 cm deep kept underground. It had two 243 cm long digester tanks separated by a partition wall and having a 40-cm wide gate opening.

The two tanks were provided with separate stirrers for vertical stirring of digester slurry. The installation cost of the original double chambered ‘Kachra’ biogas plant was one and half times more than the conventional plant of the KVIC design of identical capacity. The economics of plant operation was, however, matching and comparable with that of a conventional plant.

Development activities were continued as a result of which double chamber in earlier design was replaced by a single chamber. The modified design consists of a 2.5 x 2 x 1.65 m³ rectangular brick masonry digester covered with a rectangular gasholder (Fig. 4.20). The outlet is built contiguous to the digester which is connected through two windows of size 45 x 45 cm² near the digester bottom.

Slurry level in the digester is maintained by overflow channel in the exit chamber. Gas is collected in free-floating type rectangular gasholder. The plant is preferred to be operated on a single type of feedstock, be it, paddy straw, wheat straw or water-hyacinth alone with stirring done horizontally.

15. Flexible Biogas Plant (FLXI):

Conventional underground digesters are not appropriate for thousands of miles along the coastlines in India, Pakistan and Bangladesh and several other countries with vast coastlines. Recurrent floods in the vicinity of Ganges, Indus and Brahmaputra river banks adversely affect the success of biogas programme in these densely populated regions. In these areas balloon type flexible biogas plants made of neoprene-rubber material could provide an effective solution (Fig. 4.24).

Flexi plants can be easily installed above ground, conveniently transported to isolated areas, quickly installed at any time and can be built up at low cost. In India the Swastik Company, Pune, supplies flexible Biogas plants briefly called FLXI. Gas production per filling quantity is comparable to the similar range from conven­tional biogas plants. Playing children or roaming buffaloes may cause leakages but can be easily repaired.

Growing or splitting families may carry or replace their balloon plant whereas with conventional plants it becomes a lost invest­ment. Economics of the plant may improve if more plants are made by the Swastik Company. A minimum of five operation years is needed to amortize the investment.

However as the average life span of the Swastik plant is barely eight to ten years, long term return on investment for conventional underground digesters is accordingly higher. Hence the Swastik plant is not an alternative to conventional plant where these are found appropriate but it should merely be viewed as a substitute option where conventional plants cannot be built under­ground.

Ramakrishna Mission Technical Institute, Mylapore (Madras) has recently designed and constructed a small portable biogas plant. It consists of a flexible bag made of a layer of PVC black sheet on inside and a layer of woven material often used in a fertiliser bag outside. The volumetric capacity of the bag is 2.2 cubic metres. Input slurry consisting of equal parts by volume of cow-dung and water is poured into the bag through an opening.

Inlet pipe dips sufficiently into the slurry to prevent gas escape through it. Digested slurry is discharged through bent pipe whose bends help to maintain the desired slurry level. When fresh slurry is poured in, an equal amount of digested slurry is displaced through the discharge or outlet tube.

Biogas is collected in the bag which gradually expands it. Gas needs to be continuously utilised to prevent building up of excessive gas pressure as otherwise it may cause gas escape through the inlet or outlet tubes.

Plants Approved by the MNES under NPBD:

Under the NPBD a total six types of biogas plants have been approved by the MNES. This includes KVIC movable drum type, Janata fixed dome type, and Deenbandhu fixed dome type, Pragati movable drum type, ferrocement and Swastik biogas plants. Initially only two models, namely, KVIC movable drum type and Janata fixed type were approved for promotion.

As the programme made headway, four more models were added out of which Deenbandhu, a low cost fixed dome type has surpassed all others. Cross-sectional view of Pragati Model is shown in Fig. 4.20b.

Material requirement for constructing different capacity Pragati model is given in Table 4.16. The Pragati model though it is of movable drum type but it has a spherical digester with a conical bottom.