BIOGAS<br /><ul><li>Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen.
Biogas originates from biogenic material and is a type of biofuel.
One type of biogas is produced by anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste and energy crops.
This type of biogas comprises primarily methane and carbon dioxide.
The other principal type of biogas is wood gas which is created by gasification of wood or other biomass.
This type of biogas is comprised primarily of nitrogen, hydrogen, and carbon monoxide, with trace amounts of methane.</li></ul>Biogas can be used as a low-cost fuel in any country for any heating purpose, such as cooking. <br />It can also be used in modern waste management facilities where it can be used to run any type of heat engine, to generate either mechanical or electrical power. <br />Biogas is a renewable fuel, so it qualifies for renewable energy subsidies in some parts of the world.<br />Production:<br />Biogas is practically produced as landfill gas (LFG) or digester gas.<br />A biogas plant is the name often given to an anaerobic digester that treats farm wastes or energy crops.<br />Biogas can be produced utilizing anaerobic digesters. <br />These plants can be fed with energy crops such as maize silage or biodegradable wastes including sewage sludge and food waste. <br />The process of biogas production is explained using ‘gobar gas’ as an example.<br />Gobar gas plants are based on excreta of cattle and other farm animals, which contains about 20% inorganic particles.<br />The level of dost particles is reduced to about 10% by mixing the dung with water in 1:1 ratio.<br />The feeding rate of a typical dung based biogas plant is at the rate of 3500kg dung per day.<br />Generally spent slurry at about 2%(v/v) of the fresh dung slurry is added back to maintain the microbial population.<br />Calcium ammonium nitrate at the rate of 1% (w/w) of the dung is added to the slurry.<br />In addition to cow dung, human excreta and kitchen waste can also be used.<br />Addition of human excreta markedly increases biogas output, perhaps due to its higher nitrogen content, which supports microbial growth.<br />The optimal temperatures for biogas production are between 35-38°C.<br />Lower temperatures lead to lower gas yields, and at 15°C biogas production may come to a halt.<br />Therefore, biogas production during winters and in colder regions requires thermal insulation or heating of the digesters.<br />The pH of slurry should be around 7.<br />Under favourable conditions, the biogas yield may be up to 60 liters/kg of dung.<br />Biogas technology raw materials:<br /><ul><li>Raw materials may be obtained from a variety of sources - livestock and poultry wastes, night soil, crop residues, food-processing and paper wastes, and materials such as aquatic weeds, water hyacinth, filamentous algae, and seaweed.
Different problems are encountered with each of these wastes with regard to collection, transportation, processing, storage, residue utilization, and ultimate use.
Industrial and food processing waste:these arise from sugar, potato, vegetable and fruit processing, brewery and distillery wastes, and whey from cheese production.
Animal excreta and agricultural wastes:these are solid wastes rich on cellulose and lignocelluloses.
Agricultural biomass like straw, bagasse, etc. show poor digestibility and often high C : N ratio.
Domestic and municipal wastes:these are in the form of solid wastes and sewage respectively.</li></ul>Biochemistry: <br />Digestion:<br />Digestion refers to various reactions and interactions that take place among the methanogens, non-methanogens and substrates fed into the digester as inputs. <br />The breaking down of inputs that are complex organic materials is achieved through three stages as described below: <br />Stage 1: Hydrolysis: The waste materials of plant and animal origins consist mainly of carbohydrates, lipids, proteins and inorganic materials. <br />Large molecular complex substances are solubilized into simpler ones with the help of extracellular enzyme released by the bacteria. <br />This stage is also known as polymer breakdown stage. <br />For example, the cellulose consisting of polymerized glucose is broken down to dimeric, and then to monomeric sugar molecules (glucose) by cellulolytic bacteria. <br />Stage 2: Acidification: The monomer such as glucose which is produced in Stage 1 is fermented under anaerobic condition into various acids with the help of enzymes produced by the acid forming bacteria. <br />The principal acids produced in this process are acetic acid, propionic acid, butyric acid and ethanol. <br />Stage 3: Methanization: The principle acids produced in Stage 2 are processed by methanogenic bacteria to produce methane. <br />The reactions that takes place in the process of methane production is called Methanization and is expressed by the following equations. <br />CH3COOHAcetic acid --> CH4Methane + CO2Carbon dioxide 2CH3CH2OHEthanol + CO2Carbon dioxide --> CH4Methane + 2CH3COOHAcetic acid CO2Carbon dioxide + 4H2Hydrogen --> CH4Methane + 2H2OWater <br />Microbiology:<br />Several hundred species of microorganisms are involved in the anaerobic digestion and biogas production.<br /><ul><li>Hydrolytic and fermentative bacteria:this group includes both obligate and facultative anaerobes, and may occur up to 108-109 cells/ml of sewage sludge digesters.
They remove the small amounts of O2 present and create anaerobic conditions.
These bacteria hydrolyze and ferment the organic materials, e.g., cellulose, starch, proteins, sugars, lipids etc., and produce organic acids, CO2 and H2.
Syntrophic H2 producing bacteria:these bacteria break down organic acids having greater than 2 carbon atoms in their chain to produce acetate, CO2 and H2.
Methanogenic bacteria:this group of bacteria converts acetate, and CO2 + H2 into methane.
Thus methanogens remove the H2 produced by obligate H2 producing bacteria, thereby lowering the H2 partial pressure and enabling the latter to continue producing H2.
Methanogenic bacteria are the strictest possible anaerobes known.
They may occur up to 106-108 cells/ml of the slurry in digesters.
Acetogenic bacteria:these bacteria oxidize H2 by reducing CO2 to acetic acid, which is then used up by methanogens to generate methane, CO2 and H2.
The micro-organisms community depends on a lot of different factors – like pH in the media, temperature, and feeding material.But a few things they have in common:</li></ul>they are strictly anaerobic (would be inhibited or even killed by oxygen) <br />they have their optimum proliferation rate near pH 7 <br />reducing agent must be present <br />temperature should be at least 30°C <br />degradation of organic compound takes place in four successive steps (hydrolysis, acidogenesis, acetogenesis, methanogenesis)<br />Composition of biogas:<br />CompoundChem %MethaneCH450-75Carbon dioxideCO225-50NitrogenN20-10HydrogenH20-1Hydrogen sulfideH2S0-3OxygenO20-2<br />Bio Gas Plant would serve many purposes such as:<br />1. Environment friendly disposal of waste, which is need of hour considering mass pollution everywhere. <br />2. Generation of fairly good amount of fuel gas, which will definitely support the diminishing energy resources. <br />3. Generation of high quality manure, which would be weed less and an excellent soil conditioner. This is very important for replenishing fast decreasing resources of productive soils. It must be noted that need for replenishing the soil with high quality organic manure has been identified in plan documents.<br />4. Biogas is a colourless, odourless and inflammable gas. The gas generated in this plant can also be used as a source of natural gas.<br /> 5.During the digestion process, the waste is kept without oxygen for a period of 15-50 days at a temperature of 25-35°C. These conditions are sufficient to inactivate some of the pathogenic bacteria, viruses, protozoa etc.<br />Factors affecting biogas:<br />pH value:<br />The optimum biogas production is achieved when the pH value of input mixture in the digester is between 6 and 7. <br />In the initial period of fermentation, as large amounts of organic acids are produced by acid forming bacteria, the pH inside the digester can decrease to below 5. <br />This inhibits or even stops the digestion or fermentation process. <br />Methanogenic bacteria are very sensitive to pH and do not thrive below a value of 6.5. <br />Temperature. <br />The methanogens are inactive in extreme high and low temperatures. <br />The optimum temperature is 35 degrees C. <br />When the ambient temperature goes down to 10 degrees C, gas production virtually stops. <br />Satisfactory gas production takes place in the mesophilic range, between 25 degrees to 30 degrees C. <br />Proper insulation of digester helps to increase gas production in the cold season. <br />Loading rate. <br />Loading rate is the amount of raw materials fed per unit volume of digester capacity per day. <br />If the plant is overfed, acids will accumulate and methane production will be inhibited. <br />Similarly, if the plant is underfed, the gas production will also be low. <br />Retention time. <br />Retention time (also known as detention time) is the average period that a given quantity of input remains in the digester to be acted upon by the methanogens. <br />A digester should have a volume of 50 to 60 times the slurry added daily. <br />The retention time is also dependent on the temperature and upto 35 degrees C, the higher the temperature, the lower the retention time. <br />Toxicity. <br />Mineral ions, heavy metals and the detergents are some of the toxic materials that inhibit the normal growth of pathogens in the digester. <br />Small quantity of mineral ions (e.g. sodium, potassium, calcium, magnesium, ammonium and sulphur) also stimulates the growth of bacteria, while very heavy concentration of these ions will have toxic effect. <br />Similarly, heavy metals such as copper, nickel, chromium, zinc, lead, etc. in small quantities are essential for the growth of bacteria but their higher concentration has toxic effects. <br />Likewise, detergents including soap, antibiotics, organic solvents, etc. inhibit the activities of methane producing bacteria and addition of these substances in the digester should be avoided.<br />Carbon-nitrigen (C:N) ratio:improper C:N ratio lowers methane production. Maximum digestion occurs when C:N ratio is 30:1.<br />Creation of anaerobic conditions:it is obvious that methane production takes place in strictly anaerobic condition, therefore, the digesters should be totally airtight.<br /> <br />Advantages:<br /><ul><li>Cheaper and simpler technology than other biofuels.
Aseptic conditions are not needed for operation.
Any biodegradable matter can be used as substrate.
Anaerobic digestion inactivates pathogens and parasites.</li></ul>Disadvantages:<br /><ul><li>The product value is rather low.
The process is not very attractive economically on large industrial scale.
The biogas yields are lower due to the dilute nature of substrates used.
Bioas contains some gases as impurities, which are corrosive to the metal ptsof nternal combustion engine.</li></ul>Status of biogas production in India: <br /><ul><li>Many developing countries are encouraging for installation of biogas plants to meet the demand of fuel.
India is one of the pioneer countries in biogas technology where biogas research and plant construction has been carried out over the past 30 years.
In 1951, for the first time, biogas plants were constructed with the target of 8,000 units before 1973.
Government of India launched an all India coordinated project with the target of 1,00,00 units by 1978, but the number could reach to 50,000 units only.
Up to 1993, non-conventional energy development agency (NEDA) of Uttar Pradesh has installed about 100 night soil based biogas plant throughout the state.
The sewage plant at Okhla has 15 digesters of 5665 m3 capacity each and produces 17,000m3 gas per day.
The gas generated is equivalent to about 10,000 liters of kerosene per day, where 1 m3 biogas has energy potential equal to about 2/3 liters of kerosene.
The Dadar sewage treatment plant produces about 2800 m2 biogas per day from sewage.
In Sonepat district of Haryana, over 300 biogas units of 2 to 10 m3 capacity have been set up for cooking and lighting purposes.
The Haryana government provided 25% subsidy per plant for setting up the units.
Himachal Pradesh government set up the National Project on biogas, under which about 12, 292 units in 1987-89 and 3,505 units in 1989-90 were setup.
National sugar institute (Kanpur) has developed methods for production of biogas from bagasse and other agricultural residues.
In this plant, 12 steel digesters, each having capacity of 50 m3, are set up to which about 14 tones of mixture of agricultural wastes and cattle manure and 28 tones of water are fed to begin the biogas production.