Every one of us is familiar with the sight of biogas plants or sludge digesters near farmlands, sewage treatment plants and occasionally housing complexes. It is also notable that they are constantly seeded with farm wastes/sludge slurry to generate gaseous mixture. It principally contains gases: methane, carbon dioxide, nitrogen, hydrogen, hydrogen sulfide, water vapor but lacking oxygen. These gaseous byproducts are collectively referred as 'Biogas'. Methane constitutes maximum percent proportion as compared to other gaseous ingredients of biogas composition. Biogas has been in use by mankind as a fuel (for domestic and industrial processes) since long time. It is actually the cheapest form of renewable energy source available today for both developed and developing countries in the world. For very peculiar reasons, it is also one of the efficient and ecofriendly strategies for biological waste treatment, disposal and its management. The production of biogas is a typical natural process of anaerobic breakdown of organic matter. Since it is anaerobic (oxygen absent), it is also termed as fermentation or anaerobic digestion.
Substrates for biogas production: Biogas generation requires substrates, which are organic and biodegradable. Organic substrates are actually waste generated from various domestic and industrial sources. Domestic wastes from kitchen like leftover food, rotten vegetables/fruits, crop silage, plant residues, cattle dung, green manure, litter and other agricultural wastes; wastes from meat processing and dairy industry; municipal garbage including sewage and night soil also contribute as major organic substrates for biogas production.
Biological agents involved: The word "Bio" in the 'Biogas' itself indicates that it is produced by enzyme catalyzed chemical reactions of biological agents which are none other than microorganisms; especially anaerobic bacteria. Both facultative and strictly anaerobic eubacteria as well as archaebacteria are involved in biogas formation. Depending upon their functional catabolic activities during biogas formation, anaerobic bacteria are roughly grouped as hydrolytic, acidogenic, acetogenic and methanogenic bacteria. Methanogenic bacteria represent principal archaebacterial genera such as Methanobacterium, Methanococcus, Methanogenium, Methanosarcina, Methanospirillum and Methanothrix, all are strictly anaerobic. Eubacterial genera, which are hydrolytic, acidogenic and acetogenic, include Acetivibrio, Clostridium, Cellulomonas, Bacillus, Bacteroides, Butyrivibrio, Ruminococcus and Eubacterium spp.
Biochemical mechanism of production: Biogas is generated via four different and stepwise biochemical reactions catalyzed by enzymes of anaerobic bacteria (biological agents) described in previous paragraph. These bioagents feed upon the feedstock gradually processing it to intermediate molecules including sugars, hydrogen, and acetic acid, before finally being converted to biogas. Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to be fully effective. It is therefore common practice to introduce anaerobic microorganisms from materials with existing populations, a process known as "seeding" the digesters. Anaerobic bacteria containing natural materials include sewage sludge or cattle slurry which are usually utilized for 'seeding' purpose. Following are the stepwise biocatalytic reactions of biogas production:
1. Hydrolysis: Initial feedstock is made up of energetic but high molecular weight polymeric constituents such as carbohydrates (cellulose, starch), proteins (nitrogenous compounds) and lipids (oils, fats). In order to access energy from these polymers, feeding bacteria enzymatically breaks down them to simpler and accessible form as simple sugars, amino acids and fatty acids respectively. This breakdown is known as hydrolysis and catalyzed by enzymes of feeding bacteria. Enzymes like cellulase, amylase, protease and lipase are some of the enzymes which catalyze the hydrolysis. Examples of hydrolytic bacteria are Bacillus, Cellulomonas and Eubacterium etc.
2. Acidogenesis: Bacteria from the genera Propionibacterium, Butyrivibrio, Acetivibrio are acidogenic and facultatively anaerobic. They convert the products of hydrolysis (sugar and amino acids) into carbon dioxide, hydrogen, ammonia, hydrogen sulfide, volatile fatty acids and organic acids like propionic, butyric acids and acid alcohols.
3. Acetogenesis: Acetogenic bacteria such as Clostridia and Acetivibrio produce acetic acid from organic acids generated in previous step along with ammonia, hydrogen, carbon dioxide and occasionally long chain volatile fatty acids (butyrate, propionate). During acetogenesis, oxygen dissolved in slurry is consumed by bacteria creating anaerobic environment favorable for growth and action of obligate anaerobic methanogens or methanogenic bacteria.
4. Methanogenesis: Generation of methane in gaseous form is the final and important step of biogas formation. Volatile fatty acids produced in previous step are first converted to hydrogen, carbon dioxide and acetate which are thereafter used by methanogens. Methanogens utilize acetate and hydrogen from acetogenesis as carbon and energy source. Methanogens thus eventually convert products of acetogenesis to methane, carbon dioxide and water.
Ubiquity of biogas generation on the earth:
Biogas generation has been detected as one of the integral processes associated with methanogenesis that occurs in anaerobic natural habitats on the Earth. Some of them include cattle rumen, flooded paddy fields, swamps, wetlands, marshlands, mud ponds, marine sediments, stagnant waters and lakes with high degree of eutrophication and earth's crust. Methanogenic archaebacteria principally inhabit these natural sites and significantly contribute to biogas formation.
The need to increase utilization of biogas:
Biogas is practically in use for various domestic and industrial applications. Its application as a fuel is prominent in cooking appliances, refrigeration and can be a suitable substitute for all fossil fuels. Biogas energy can be used as backup for solar cells. Waste material which is fed to generate biogas is also efficiently converted to high quality organic byproduct. This product is actually a potential fertilizer and can be developed for crop improvement and other agricultural purposes. Biogas generation is also one of the natural and effective means of biodegradation of waste materials which are usually dumped in the ground untreated. So it would also help to reduce ground water, soil and air pollution and a great solution for rural and urban waste management. The most important benefit that mankind would receive from increased utilization of biogas is: we can trap methane gas for above useful applications which is otherwise wasted as harmful greenhouse gas.
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