Biogas Production System
The biogas (mainly mixture of methane and carbon dioxide) is produced/generated under both, natural and artificial conditions. However for techno-economically-viable production of biogas for wider application the artificial system is the best and most convenient method. The production of biogas is a biological process which takes place in the absence of air (oxygen), through which the organic material is converted in to, essentially Methane (CH4) and Carbon dioxide (CO2) and in the process gives excellent organic fertilizer and humus as the second by-product. The one essential requirement in producing biogas is an airtight (air leak-proof) container. Biogas is generated only when the decomposition of biomass takes place under the anaerobic conditions, as the anaerobic bacteria (microbes) that live without oxygen are responsible for the production of this gas through the destruction of organic matter. The airtight container used for the biogas production under artificial condition is known as digester or reactor.

Composition of Biogas
Biogas is a colourless, odourless, inflammable gas, produced by organic waste and biomass decomposition (fermentation). Biogas can be produced from animal, human and plant (crop) wastes, weeds, grasses, vines, leaves, aquatic plants and crop residues etc. The composition of different gases in biogas :
Methane (CH4) : 55-70%
Carbon Dioxide (CO2) : 30-45%
Hydrogen Sulphide (H2S) : 1-2%
Nitrogen (N2) : 0-1%
Hydrogen (H2) : 0-1%
Carbon Mono Oxide (CO) : Traces
Oxygen (O2) : Traces

Property of Biogas
Biogas burns with a blue flame. It has a heat value of 500-700 BTU/Ft3 (4,500-5,000 Kcal/m3) when its methane content is in the range of 60-70%. The value is directly proportional to the amount of methane contains and this depends upon the nature of raw materials used in the digestion. Since the composition of this gas is different, the burners designed for coal gas, butane or LPG when used, as ‘biogas burner’ will give much lower efficiency. Therefore specially designed biogas burners are used which give a thermal efficiency of 55-65%.

Biogas is a very stable gas, which is a non-toxic, colourless, tasteless and odourless gas. However, as biogas has a small percentage of Hydrogen Sulphide, the mixture may very slightly smell of rotten egg, which is not often noticeable especially when being burned. When the mixture of methane and air (oxygen) burn a blue flame is emitted, producing large amount of heat energy. Because of the mixture of Carbon Dioxide in large quantity the biogas becomes a safe fuel in rural homes as will prevent explosion.

A 1 m3 biogas will generate 4,500-5,500 Kcal/m2 of heat energy, and when burned in specifically designed burners having 60% efficiency, will give out effective heat of 2,700-3,200 Kcal/m2. 1 Kcal is defined as the heat required to raise the temperature of 1 kg (litre) of water by 1 degrees Celsius. Therefore this effective heat (say 3,000 Kcal/m2 is on an average), is sufficient to bring approx. 100 kg (litre) of water from 20 degrees Celsius to a boil, or light a lamp with a brightness equivalent to 60-100 Watts for 4-5 hours.

Biogas Plant
Biogas Plant (BGP) is an airtight container that facilitates fermentation of material under anaerobic condition. The other names given to this device are ‘Biogas Digester’, ‘Biogas Reactor’, ‘Methane Generator’ and ‘Methane Reactor’. Technology not only provides biogas as a clean and convenient fuel but also an excellent and enriched bio-manure. Thus the BGP also acts as a miniature Bio-fertilizer Factory hence some people prefer to refer it as ‘Biogas Fertilizer Plant’ or ‘Bio-manure Plant’. The fresh organic material (generally in a homogenous slurry form) is fed into the digester of the plant from one end, known as Inlet Pipe or Inlet Tank. The decomposition (fermentation) takes place inside the digester due to bacterial (microbial) action, which produces biogas and organic fertilizer (manure) rich in humus & other nutrients. There is a provision for storing biogas on the upper portion of the BGP. There are some BGP designs that have Floating Gasholder and others have Fixed Gas Storage Chamber. On the other end of the digester Outlet Pipe or Outlet Tank is provided for the automatic discharge of the liquid digested manure.

DIGESTER
It is either an under ground Cylindrical-shaped or Ellipsoidal-shaped structure where the digestion (fermentation) of substrate takes place. The digester is also known as ‘Fermentation Tank or Chamber’. In a simple Rural Household BGP working under ambient temperature, the digester is designed to hold slurry equivalent to of 55, 40 or 30 days of daily feeding. This is known as Hydraulic Retention Time (HRT) of BGP. The designed HRT of 55, 40 and 30 days is determined by the different temperature zones in the country- the states & regions falling under the different temperature zones are already defined for India. The digester can be constructed of brick masonry, cement concrete (CC) or reinforced cement concrete (RCC) or stone masonry or pre-fabricated cement concrete blocks (PFCCB) or Ferro-cement (ferroconcrete) or steel or rubber or bamboo reinforced cement mortar (BRCM). In the case of smaller capacity floating gasholder plants of 2 & 3 m3 no partition wall is provided inside the digester, whereas the BGPs of 4 m3 capacity and above have been provided partition wall in the middle. This is provided for preventing short-circuiting of slurry and promoting better efficiency. This means the partition wall also divides the entire volume of the digester (fermentation chamber) into two halves. As against this no partition wall is provided inside the digester of a fixed dome design. The reason for this is that the diameter of the digesters in all the fixed dome models are comparatively much bigger than the floating drum BGPs, which takes care of the short-circuiting problems to a satisfactory level, without adding to additional cost of providing a partition wall.

GAS HOLDER OR GAS STORAGE CHAMBER
In the case of floating gas holder BGPs, the Gas holder is a drum like structure, fabricated either of mild steel sheets or ferro-cement (ferroconcrete) or high density plastic (HDP) or fibre glass reinforced plastic (FRP). It fits like a cap on the mouth of digester where it is submerged in the slurry and rests on the ledge, constructed inside the digester for this purpose. The drum collects gas, which is produced from the slurry inside the digester as it gets decomposed, and rises upwards, being lighter than air. To ensure that there is enough pressure on the stored gas so that it flows on its own to the point of utilisation through pipeline when the gate valve is open, the gas is stored inside the gas holder at a constant pressure of 8-10 cm of water column. This pressure is achieved by making the weight of biogas holder as 80-100 kg/cm2. In its up and down movement the drum is guided by a central guide pipe. The gas formed is otherwise sealed from all sides except at the bottom. The scum of the semidried mat formed on the surface of the slurry is broken (disturbed) by rotating the biogas holder, which has scum-breaking arrangement inside it. The gas storage capacity of a family size floating biogas holder BGP is kept as 50% of the rate capacity (daily gas production in 24 hours). This storage capacity comes to approximately 12 hours of biogas produced every day.

In the case of fixed dome designs the biogas holder is commonly known as gas storage chamber (GSC). The GSC is the integral and fixed part of the Main Unit of the Plant (MUP) in the case of fixed dome BGPs. Therefore the GSC of the fixed dome BGP is made of the same building material as that of the MUP. The gas storage capacity of a family size fixed dome BGP is kept as 33% of the rate capacity (daily gas production in 24 hours). This storage capacity comes to approximately 8 hours of biogas produced during the night when it is not in use.

INLET
In the case of floating biogas holder pipe the Inlet is made of cement concrete (CC) pipe. The Inlet Pipe reaches the bottom of the digester well on one side of the partition wall. The top end of this pipe is connected to the Mixing Tank.
In the case of the first approved fixed dome models (Janata Model) the inlet is like a chamber or tank-it is a bell mouth shaped brick masonry construction and its outer wall is sloppy. The top end of the outer wall of the inlet chamber has an opening connecting the mixing tank, whereas the bottom portion joins the inlet gate. The top (mouth) of the inlet chamber is kept covered with heavy slab. The Inlet of the other fixed dome models (Deenbandhu and Shramik Bandhu) has Asbestos Cement Concrete (ACC) pipes of appropriate diameters.

OUTLET
In the case of floating gas holder pipe the Outlet is made of cement concrete pipe standing at an angle, which reaches the bottom of the digester on the opposite side of the partition wall. In smaller plants (2 & 3 m3 capacity BGPs) which has no partition walls, the outlet is made of small (approx. 2 ft. length) cement concrete pipe inserted on top most portion of the digester, submerged in the slurry.
In the two fixed dome (Janata & Deenbandhu models) plants, the Outlet is made in the form of rectangular tank. However, in the case of Shramik Bandhu model the upper portion of the Outlet (known as Outlet Displacement Chamber) is made hemi-spherical in shape, designed to save in the material and labour cost. In all the three-fixed dome models (Janata, Deenbandhu & Shramik Bandhu models), the bottom end of the outlet tank is connected to the outlet gate. There is a small opening provided on the outer wall of the outlet chamber for the automatic discharge of the digested slurry outside the BGP, equal to approximately 80-90% of the daily feed. The top mouth of the outlet chamber is kept covered with heavy slab.

MIXING TANK
This is a cylindrical tank used for making homogenous slurry by mixing the manure from domestic farm animals with appropriate quantity of water. Thoroughly mixing of slurry before releasing it inside the digester, through the inlet, helps in increasing the efficiency of digestion. Normally a feeder fan is fixed inside the mixing tank for facilitating easy and faster mixing of manure with water for making homogenous slurry.

GAS OUTLET PIPE
The Gas Outlet Pipe is made of pipe and fixed on top of the drum at the centre in case of floating biogas holder BGP and on the crown of the fixed dome BGP. From this pipe the connection to gas pipeline is made for conveying the gas to the point of utilisation. A gate valve is fixed on the gas outlet pipe to close and check the flow of biogas from plant to the pipeline.

Functioning of a Simple India Rural Household Biogas Plants (BGPs)
The fresh organic material (generally in a homogenous slurry form) is fed into the digester of the plant from one end, known as Inlet. Fixed quantity of fresh material fed each day (normally in one lot at a predetermine time) goes down at the bottom of the digester and forms the ‘bottom-most active layer’, being heavier then the previous day and older material. The decomposition (fermentation) takes place inside the digester due to bacterial (microbial) action, which produces biogas and digested or semi-digested organic material.

As the organic material ferments, biogas is formed which rises to the top and gets accumulated (collected) in the Gas Holder (in case of floating gas holder BGPs) or Gas Storage Chamber (in case of fixed dome BGPs). A Gas Outlet Pipe is provided on the top most portion of the Gas Holder (Gas Storage Chamber) of the BGP. Alternatively, the biogas produced can be taken to another place through pipe connected on top of the Gas Outlet Pipe and stored separately. The Slurry (semi-digested and digested) occupies the major portion of the digester and the Sludge (almost fully digested) occupies the bottom most portion of the digester. The digested slurry (also known as effluent) is automatically discharged from the other opening, known as Outlet, is an excellent bio-fertilizer, rich in humus. The anaerobic fermentation increases the ammonia content by 120% and quick acting phosphorous by 150%. Similarly the percentage of potash and several micro-nutrients useful to the healthy growth of the crops also increase. The nitrogen is transformed into Ammonia that is easier for plant to absorb. This digested slurry can either be taken directly to the farmer’s field along with irrigation water or stored in a Slurry Pits (attached to the BGP) for drying or directed to the Compost Pit for making compost along with other waste biomass. The slurry and also the sludge contain a higher percentage of nitrogen and phosphorous than the same quantity of raw organic material fed inside the digester of the BGP.

Stratification (Layering) of Digester due to Anaerobic Fermentation
In the process of digestion of feedstock in a BGP many by-products are formed. They are biogas, scum, supernatant, digested slurry, digested sludge and inorganic solids. If the content of Biogas Digester is not stirred or disturbed for a few hours then these by-products get formed in to different layers inside the digester. The heaviest by-product, which is Inorganic Solids will be at the bottom most portion, followed by Digested Sludge, and so on and so forth as shown in the three diagrams for three different types of digester.

SCUM
Mixture of coarse fibrous and lighter material that separates from the manure slurry and floats on the top most layer of the slurry is called Scum. The accumulation and removal of scum is sometimes a serious problem. In moderate amount scum can’t do any harm and can be easily broken by gentle stirring, but in large quantity can lead to slowing down biogas production and even shutting down the BGPs.

SUPERNATANT
The spent liquid of the slurry (mixture of manure and water) layering just above the sludge, in case of Batch-fed and Semi Batch-fed Digester, is known as Supernatant. Since supernatant has dissolved solids, the fertiliser value of this liquid (supernatant) is as great as that of effluent (digested slurry). Supernatant is a biologically active by-product; therefore must be sun dried before using it in agricultural fields.

DIGESTED SLURRY (EFFLUENT)
The effluent of the digested slurry is in liquid form and has its solid content reduced to approximately 10-20% by volume of the original (Influent) manure (fresh) slurry, after going through the anaerobic digestion cycle. Out of the three types of digestion processes mentioned above, the digested slurry in effluent-form comes out only in semi-continuous BGP. The digested slurry effluent, either in liquid-form or after sun drying in Slurry Pits makes excellent bio-fertilizer for agricultural and horticultural crops or aquaculture.

SLUDGE
In the batch-fed or semi batch-fed digester where the plant wastes and other solid organic materials are added, the digested material contains less of effluent and more of sludge. The sludge precipitates at the bottom of the digester and is formed mostly of the solids substances of plant wastes. The sludge is usually composted with chemical fertilizers as it may contain higher percentage of parasites and pathogens and hookworm eggs of etc., especially if the semi-batch digesters are either connected to the pigsty or latrines. Depending upon the raw materials used and the conditions of the digestion, the sludge contains many elements essential to the plant life e.g. Nitrogen, Phosphorous, Potassium plus a small quantity of Salts (trace elements), indispensable to the plant growth- the trace elements such as boron, calcium, copper, iron, magnesium, sulphur and zinc etc. The fresh digested sludge, especially if the night soil is used, has high ammonia content and in this state may act like a chemical fertiliser by forcing a large dose of nitrogen than required by the plant and thus increasing the accumulation of toxic nitrogen compounds. For this reason, it is probably best to let the sludge age for about two weeks in open place. The fresher the sludge the more it needs to be diluted with water before application to the crops, otherwise very high concentration of nitrogen my kill the plants.

INORGANIC SOLIDS
In village situation the floor of the animals shelters are full of dirt, which gets mixed with the manure. Added to this the collected manure is kept on the unlined surface which has plenty of mud and dirt. Due to all this the feed stock for the BGP always has some inorganic solids, which goes inside the digester along with the organic materials. The bacteria can not digest the inorganic solids, and therefore settles down as a part of the bottom most layer inside the digester. The Inorganic Solids contains mud, ash, sand, gravel and other inorganic materials. The presence of too much inorganic solids in the digester can adversely affect the efficiency of the BGP. Therefore to improve the efficiency and enhance the life of a semi-continuous BGP it is advisable to empty even it in a period of 5-10 years for thoroughly cleaning and washing it from inside and then reloading it with fresh slurry.


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