Nature has a provision for destroying and disposing off wastes and dead plants and animals. This decay or decomposition is carried out by tiny micro-organisms called bacteria. Making of farm-yard manure (FYM) and compost is also through decomposition of organic matter (OM). When a heap of vegetable or animal waste and weeds etc., die or decompose at the bottom of backwater or shallow lagoons, bubbles can be noticed rising to the surface of water. Sometimes these bubbles burn with dancing flame at dusk. This phenomenon has puzzled man for a long time. It was only during the past hundred years that Scientists unlocked this secret as the decomposition process. The gas thus produced was and is still called "Marsh Gas." The technology of harnessing this gas under artificially created conditions is known as Biogas Technology.
 

Biogas Technology has a very significant role to play in integrated agricultural operations, rural sanitation, large-scale dairy farms & sewage disposal etc. It is estimated that cattle dung, when passed through a Biogas unit, yields 30-40% more net energy and about 35-45% more Nitrogen in manure as compared with that obtained by burning dung cakes and ordinarily prepared compost, respectively. Besides, from a biogas plant both the products are obtained. There are about 250 million bovine (cattle and buffalo) population in India and one biogas unit for small family requires about 3-5 cattle heads, thus about 10 million family size plants fed on cattle and buffalo dung can be installed. Overall as per this estimate of NCAER, total energy produced by livestock excreta amounts to about 80% of the rural fuel requirement.

BIOGAS PLANT

Biogas is a mixture of a few gases, such as Methane, Carbon dioxide, Hydrogen etc., formed as a result of anaerobic digestion of organic wastes. A biogas plant is commonly described as underground masonry, well shaped fermentation tank, connected with inlet and outlet tanks and covered by an inverted floating or fixed gas storage tank.
 

Biogas generation is a process widely occurring in nature and can be described as a biological process in which biomass or organic matter, in the absence of Oxygen, is converted into Methane and Carbon dioxide. It is characterized by low nutrient requirement, and high degree of waste stabilization process where biogas is one of the two useful products; the other being enriched organic manure in the form of digested slurry. It is essentially a three stage process involving following reactions:

1) Hydrolysis 2) Acid formation 3) Methane generation.

For all practical purposes the first two steps are often defined as a single stage, i.e. hydrolysis and acid formation stages are grouped as acid formation stage. Micro-organisms taking part in this phase are termed as acid formers. As a group, these organisms are rapidly growing and are not much dependent upon surroundings. Products of first two stages serve as the raw material for the third stage where organic acids are utilized as carbon source by Methane forming micro-organisms, which are also known as Methanogens. These Methanogens are more susceptible to their surroundings. The tolerated pH range is 6.8 to 7.5 with optimum 7.0. Any departure from this range is inhibitory. Atmospheric Oxygen is extremely toxic for methanogens, as they are strict anaerobes.
 

There are several parameters which affect the anaerobic digestion / gas yields and they can be divided into two parts:
 

1. Environmental factors :


There are a few environmental factors which limit the reactions if they differ significantly from their optimum levels. Factors of most interest are s (a) temperature, (b) pH and (c) nutrient contents of the raw materials,
 

1. Temperature :


It is a factor which affects most small & medium size biogas installations in developing countries. There are three zones of temperature in which biogas is produced by anaerobic fermentation of organic matter, viz.: 1) Mesophillic, 2) Thermophillic and 3) Psycrophillic zones. The optimum temperature of digester slurry in Mesophillic zone is 35^oC, 55^oC in Thermophillic zone and 10^oC in Psycrophillic zone. In different temperature zones different sets of microbes, (bacteria) especially the methanogens remain active; whereas the other two groups of microbes either remain dormant and thus more or less inactive as far as the anaerobic digestion is concerned or get killed. However, the rate of fermentation is much faster at high temperature. Most rural household biogas plants (digesters) in developing countries operate at ambient temperatures, thus digester slurry temperature is susceptible to seasonal variation but is more dependent on the ground temperature than the atmospheric temperature. As a result, gas output in winter falls by up to 50 %. Below a slurry temperature of 10^oC all the reactions cease to take place but revive gradually with the rise in temperature.
 

2. pH


The pH range suitable for gas production is rather narrow i.e. 6.6 to 7.5. Below 6.2 it becomes toxic. pH is also controlled by natural buffering effect of NH₄⁺ and HCO₃^-ions. pH falls with the production of volatile fatty acids (VFAs) but attains a more or less constant level once the reaction progress.
 

3. Nutrient Concentration


Biogas producing raw materials can be divided into two parts i.e. 1) Nitrogen rich and 2) Nitrogen poor. Nitrogen concentration is considered with respect to carbon contents of the raw materials and it is often depicted in terms of C to N ratio. Optimum C/N ratio is in the range of 25 to 30:1. In the case of cattle dung the problem of nutrient concentration does not exist as C/N ratio is usually around 25:1.
 

2. 0perational Factors


Operational factors contributing to the gas production process are: (a) retention time (RT) - also referred as detention or residence time, (b) slurry concentration and (c) mixing.
 

1. Retention Time (RT)


It is the period during which any organic matter is subjected to the anaerobic environment and reaction in a biogas digester. When the organic matter is fed in the form of slurry the term used is Hydraulic Retention Time (HRT); whereas if it is fed in the solid form (usually 20-30% TS), the term used is Solid Retention Time (SRT). Retention time is proportional to the temperature of the process. At 15-30^oC retention period is 40-55 days, at 35-37^oC, 20 days and at 55^oC 6-10 days. Digester as it is equal to retention time multiplied by the volume of daily feed.
 

2. Slurry concentration


This is denoted by dry matter concentration of the inputs. The optimum level for cattle dung slurry in the range of 8 to 10% and any variations from this result in lower gas output. Mixing four parts of dung with five parts of water forms a slurry with dry matter concentration of about 9%, whereas 1 part of dung to 1 part to water would give a slurry concentration of 10%. This also affects the loading rate etc.
 

3. Mixing & Stirring

1. Batch digester


In this process the whole digester is filled with raw materials for gas production along with some starting (seed) material. This is allowed to ferment and produce gas over a certain length of time and when gas yields become very low the digester is emptied of all the sludge which can be supplied as manure. In this system gas production begins at a low level and goes on increasing only to drop down again after reaching the peak. Because of variable gas production level, high cost and periodic emptying and filling of digesters, this process has not become popular. Examples of these digesters are small size garbage plant and crop-residues plant.
 

2. Semi-continuous digester


The rural household digesters are fed once a day and the fresh input displaces the same volume of spent materials from the digester. Every day a certain quantity of fresh inputs is fed into the digesters which is expected to remain in the digester for a prescribed retention time and produces gas over this length of time before being discharged.
 

3. Semi-batch digester

It was in or around the year 1945 that Scientists at Indian Agricultural Research Institute (IARI), New Delhi worked on semi-continuous flow digesters and in the year 1961 Khedi and Village Industries Commission (KVIC) patented a design which is being popularized by various agencies in many countries. This design consists of a deep well shaped underground digester connected with inlet and outlet pipes at its bottom, which are separated by a partition wall dividing the 3/4th of the total height into two parts. A mild steel gas storage drum is inverted over the slurry which goes up and down around a guide pipe with the accumulation and withdrawal of gas. Now FRP and ferro-cement gas holders are also being used in the KVIC plant.
 

The Janata model is a fixed roof biogas plant which was developed by PRAD in 1978. This is also a semi-continuous flow plant. The main feature of the Janata design is that the digester and gas holder are part of a composite unit made of bricks and cement masonry. It has a cylindrical digester with dome shaped roof and large inlet and outlet tanks on two sides. It requires shuttering for making the dome shaped roof and a skilled & trained master mason is a must for the construction of a Janata Biogas plant. This plant costs about 20-30% less than the KVIC floating drum type plant.
 

1. Laying the gas distribution pipeline

1. Pipe and fittings to be used for laying gas distribution system must be of best quality. It is important from safety point of view and needs to be paid more attention for in-the-house connections. Extra emphasis must be given to the selection of valves to be employed.
2. All underground pipes should be coated with protective paints to avoid corrosion. Underground pipes should be about 1 foot below the ground level.
3. As far as possible only gradual bends (not sharp elbows) should be used for 90^o turns in the pipeline, This reduces pressure drop.
4. Only gate valves, plug valves and ball valves should be used for gas pipeline to minimize pressure loss during flow of gas through the valves.
5. For connecting the burners with gas pipeline use of transparent polyethylene tubes should be avoided and only neoprene rubber tube should be used.
6. Biogas is saturated with water vapour and slight fall in temperature causes its condensation in the pipeline. Therefore, adequate arrangements to remove the condensate must be made at the time of pipe laying. All the pipes must have some gradient and at all the low points water removers should be installed. Water accumulation in pipe results in drop in pressure which causes reduction in flow rate. The water remover can be of two basic types :

1. manually operated water remover


A schematic diagram of this type of water remover is depicted in Fig. l. It is a 'T' joint at the lowest point of a certain section of gas pipeline. The vertical branch of the 'T' is kept in a perpendicular downward direction and it is connected to one foot long piece of pipe. The other end of this pipe is either plugged or fitted with a valve. The condensate in the pipeline will flow into this pipe and will be drained off manually at an interval of a week or ten days or as guided by experience.



Fig. 1 : Schematic diagram of water remover
 

2. Automatic water removal siphon

1. The whole gas distribution system should be divided in a few sections so that anyone of them can be isolated from rest of the pipeline if it were to be repaired. This can be done by providing UNIONS at points where bends have been employed.
2. Above ground pipe should be only along the walls and not hanging free. It should be hooked all along the walls especially on both sides of valves with the help of clamps at every two meters or so and no pipe should sag at any point. There should be a continuous slope in the directions of water remover.
3. Gas cock in the houses should be out of the reach of children.
4. At the time of installation whole pipeline should be tested for any leakage at a pressure of 1 kg/cm², if possible.
5. Burners should be connected in such a way that gas taps are in the front so that to operate the burner the user does not have to take her/his hand over the burner.
6. Sketch of sample layout for pipeline from biogas plant to house is shown in Fig. 3. Normally, at least one water remover for 100 m pipe length should be installed. Details of in-the-house connections are not shown in the figure as it will vary from house to house. However, all the points mentioned above must be kept in mind while laying pipeline in the house.

1. Biogas requirement for cooking is 8 to 10 cft (0,25 to 0.3 m³) gas per head per day. Standard biogas stoves consume 16 cft (0.425 m³) gas per hour.
2. Biogas lamp consumes 4-5 cft (0.15 m³) gas per hour.
3. Dual fuel (diesel & Biogas) engines consume 15-16 cft (0.425 m³) gas per hour per hp.