Bio-gas.

Bio Gas (Methane gas):

In 1776, for the first time, Italian physicist, Volta demonstrated methane in Marsh gas; generated from organic matter in the bottom sediments of ponds and streams. In Nepal first biogas plant was installed byFather B.R.Sauboll in 1955 in the Godavari with the development of micro model digester made of oil drums. However, a serious approach to install biogas plant has started only after 1975 and by the end of 2001, 70, 270 biogas plant has been installed in remote hills in Nepal.

Under anaerobic condition, the organic materials are converted through microbiological reactions into gases and sludge(fertilizer). The mixture of different gases produced by microorganisms under anaerobic digestion of organic compounds is known as Biogas. Biogas is the mixture of gases composed of Methane (50-80%), CO₂ (15-40%),Nitrogen (4%), H₂S(1%) and traces of H₂, O₂, and CO, produced by anaerobic microbial digestion of organic matter. Anaerobic digestion occurs in nature in the sediments of lakes and ponds and in the rumens of cattle. The caloric value of biogas is 5871 Kcal/m*3 with 63% methane and is around 8600 Kcal/m*3 with 80% methane. The ignition temperature of biogas is 180*C. Biogas burns with clear blue flame and can be used to generate electricity. It can be purified to yield a good grade methane with efficiently operated digester and the substrate, 3m*3 of methane can be produced from 10kg biomass.

Fig: Biogas digester

#Substrate for Biogas:

The usual substrates for biogas production are:

1. Waste products of animal husbandry (animal excreta) and agricultural wastes: These are solid wastes rich in cellulose and lignocellulose. In Nepal and other many developing countries cattle, dung (gobar) is used to produce biogas, locally called gobar gas. In Chin, pig-dung is used as the major substrate.
2. Industrial and Food processing waste : These arise from sugar, potato, vegetable and fruit processing, brewery and distillery waste and as whey from cheese production. They may be nonparticulate to high strength wastes.
3. Domestic and Municipal wastes: These are in the form of garbage and sewage than 40% and pH of around 7. If needed, N, P and trace elements should be added. Water hyacinth (Eichornia crasipies) , an aquatic weed with huge biomass is a good substrate for methane gas production. With high C:N ratio and low lignin content the yield of methane is high. Another aquatic weed Azolla can also be used.

Gobar gas plant is based on excreta of cattle and farm animals which contain 20% inorganic solids. The inorganic materials are reduced to 10% by mixing dung with water in the ratio of 1:1. The feeding rate of typical dung based biogas plant is up to 3500 kg/day. The spent slurry at about 2% V/V of the fresh slurry is added back to maintain the microbial population. Human excreta and kitchen wastes can also be used.

Temperature : 35-38*C

pH : 7

Time consumed :2-4 weeks when cow dung is used.

Microbial production of Methane/ Biogas.

Methane is the most abundant constituent of biogas. The microbial generation of methane , appropriately referred to as 'methanogenesis' from biomass occurs in following phases:

1. Hydrolysis or solubilisation
2. Acidogenesis and Acetogenesis

Hydrolysis or solubilisation:

It is the initial stage in which the feedstock or substrate is solubilized by water and microbial enzymes. The organic materials to be decomposed (cattle dung or other organic materials) are mixed with water to make the slurry. The complex polymers in the feedstock are hydrolyzed into the simpler compounds (low molecular weight soluble products). The hydrolysis of complex organic compounds is done by hydrolytic fermentative bacteria which include both obligate and facultative anaerobic bacteria. These microorganisms hydrolyze and ferment the organic materials (Eg. cellulose, starch and other polysaccharides, proteins, lipids, etc. The enzyme amylase and glucosidase hydrolyse the glycosidic bond in starch and glycogen forming the monosaccharides. The chitin and cellulose are respectively, into their monomeric forms. The lipases and esterases hydrolyze fats and lipids. The fats, oils, and other lipids are degraded to yield fatty acids and glycerol. The proteolytic enzymes called proteases catalyse the hydrolysis of proteins or polypeptides. Many proteases specifically cleave the bond between specific amino acids. They hydrolyse the polypeptides into peptides and then peptides to amino acids. Thus in the process of solubilization, polysaccharides are hydrolyzed into monosaccharides, proteins and polypeptides into amino acids, fats/ lipids into glycerols and fatty acids i.e the polymers into individual monomeric units.

Cellulose ⇒ Cellobiose ⇒ Glucose (Endo & Exoglucanase) (Glycosidase)

Fats/ Lipids ⇒ Glycerol + Fatty acids (Lipases & Ester oils)

Proteins ⇒ Peptides ⇒ Aminoacids (Proteinase) (Peptidase)

Microorganisms involved:

Clostridium acetobutylicum , Clostridium propionicum , Eubacterium limosum, Bacteriods ,etc.

The proteolyticClostridium spare considered responsible for the anaerobic decomposition of aminoacids.

Acidogenesis and Acetogenesis:

This phase is characterised by the production of an array of organic acids like formic acids, acetic acid, propionic acids, butyric acid, long-chain fatty acids, alcohols, CO2 and H2 through different fermentative pathways. The simple monomeric molecules formed during the hydrolytic phase are utilised by the fermentative bacteria to produce these compounds.

1. The anaerobic degradation of proteins leads to the formation of many amino acids. The anaerobic degradation or fermentation of amino acids by anaerobic bacteria like Clostridium spp also lead to the formation of organic acids.

Examples:

1. Glutamic acid → Methylaspartic acid → Citramalic acid → Pyruvic acid + Acetic acid(Weng & Jeris)
2. Alanine + Glycine → Pyruvic acid → Acetyl coA → Acetyl phosphate → Acetic acid (Stickland reaction)

The obligate H2 producing bacteria (Syntrophic H2 producing bacteria ) breakdown the fatty acid oxidisers includeSyntrophomonas & Syntrophobacter. The other group of bacteria oxidises H2 by reducing CO2 to acetic acid. Some of the acetogenic bacteria also convert alcohol into acetic acid.

Organisms :Selenomonas, Succinomonas, Succinovibrio, Bacteroides, Desulfovobrio, Butyrivibrio, Lacnosporaetc.

#Factors affecting Methane gas production :

1. Temperature: The methane gas production can occur at thermophilic and mesophilic temperature range.
2. pH : For the production of sufficient amount of methane, optimum pH of the digester is should be maintained between 6-8 as the acidic medium lowers methane formation.
3. Carbon:Nitrogen ratio: Improper C:N ratio is 30% . Amendment of N₂ or C substances should be done exogenously, according to chemical nature of substances use in fermentation.
4. Creation of anaerobic condition: It is obvious that methane production takes place in strictly anaerobic condition, therefore the digesters should be totally airtight.
5. Slurry : Proper solubilization of organic substrates i.e. the ratio between solid and water should be 1:1 when the substrate is household things like dung.
6. Sedding / Enrichment with starter bacteria: In the beginning, seeding of slurry with the small amount of sludge of another digester activates methane evolution of sludge of another digester activates methane evolution. Sludge contains acetogenic and methanogenic bacteria.

# Applications / Uses of Biogas:

The uses of Biogas with its applications are described below :

1. Direct combustion (Eg : For heating and cooking)
2. Generating electricity : Electricity may be produced by modified internal combustion engines or gas turbines.
3. Purification and conversion into compressed gas or liquid form : The gas may be purified (pure CH4) and can be solid as compressed gas or liquid. However, the purification process is complex and expensive.

#Advantages of Biogas:

1. The technology is much cheaper and simpler than those for other biofuels and it is ideal for small scale application.
2. Waste materials can be used as the substrate, thus there will be a reduction in pollution and pollutants.
3. Recovery of product (i.e methane) is spontaneous and it is separated automatically from the substrate.
4. Any biodegradable substrate can be used without aseptic condition for biogas production.
5. Anaerobic digestion inactivates pathogens and reduce the incidence of water-borne diseases.

#Disadvantages of Biogas :

1. The product value is rather low, making it unattractive for commercial production.
2. It has the low yield.
3. The biogas contains some gases as impurities which are corrosive to combustion engines.

# Reference:

1. Alexander, M. Introduction to Soil Microbiology. Academic Press, 1961.
2. Rangaswami, G and Bagyaraja PT. Agricultural Microbiology. 2nd. Prentice Hall of India, 1993.