Composting is a natural process of biodegradation and nutrient recycling. It is slow but gradual fermentation process in which biological components like microorganisms, nematodes or worms, protozoa and variety of insects participate along with abiotic factors like temperature, pH, humidity, salinity and nutrient concentration. Composting reactions are induced when right amount of humidity, temperature and nutrition are available. Compost making organisms are indigenous and derive their nutrition from the organic/inorganic waste. Waste material consists of complex carbohydrates from vegetables, wood chips, bark, straw, leaves and animal dung; protein in the form of egg shells, hair, skin, meat/fish/poultry waste, urine, solid sewage matter or plant debris. Waste frying oils from kitchen, dairy waste or soaps and detergents from laundry may be present as fatty waste material. Nematodes and protozoa help to cut or chop waste material into smaller pieces. This mechanical helping occurs before the start of composting process and is very mandatory to create suitable 'feed' for further microbial action. Microbes involved in composting reaction do not have similar nutrient/ oxygen/ temperature or pH requirements. They are very diverse in their growth requirements but still work in coordination and balance to initiate and further carry out complete composting process. Naturally, composting is a continuous, synergistic biological reaction which is halted only when nutrients are not adequate. However, microbial flora of the compost adapts easily to this low or scanty nutrient status and remains dormant till a new batch of feed in the form of organic waste is added to restart the process. In other words, composting is biological breakdown in succession and utilization of 'waste' material by microorganisms and protozoans. Commercially, composting is carried out in a composter bin or on open ground like agricultural plot having waste piled or dumped in pits. Bacteria are integral biological constituents of composting process from its start to finish; so it would be interesting to know about different types of bacteria involved and their peculiar role in compost formation.
Bacterial groups functional during composting process: Composting process is initiated and carried out by functionally active microorganisms of which about 80% constitute bacteria. Three physiological groups of bacteria which work within definite temperature range are: aerobic (10-400C), mesophilic (45-550C) and anaerobic/thermophilic bacteria (upto 800C). At the start of process, aerobic bacteria consume oxygen present in air filled waste pile for their growth. Cellulose, chitin, lignin, hemicellulose and starch degrading bacteria including strains of Actinomyces predominate in this phase. Growing bacteria simultaneously breaks down complex carbohydrates present in the organic waste with the help of enzymes such as cellulase, chitinase, diastase or pectinase. High respiratory activity followed by oxygen consumption depletes oxygen concentration resulting in the formation of slightly anaerobic or oxygen lacking conditions. Mesophilic microbes such as Cellulomonas and Bacillus grow under such microaerophilic conditions utilizing the carbohydrates broken down by aerobic activity. Mesophiles break down protein containing wastes by enzymes proteinase and protease. Microaerophilic conditions are vanished gradually with mesophilic activity and the waste environment becomes anaerobic and here anaerobic microbes that need no oxygen grow predominantly. Anaerobic phase is also indicated by rise in temperature from 600C to as high as 800C, species of thermophilic actinomycetes like Thermoactinomyces, Streptomyces; extremely thermally resistant species of Thermus and Deinococcus; methane generating or methanogenic bacteria, Methanobacterium spp. which are also thermophilic, grow at such high temperatures. Aerobic decomposition of organic waste matter ends up in the formation of simple sugars like glucose, ethanol and amino acids. Increase in initial temperature upto 70-800C is a characteristic feature of aerobic composting process. On the contrary, sometimes anaerobic bacteria predominate in waste to be composted and lead to anaerobic decomposition of organic matter. Waste in such case is degraded into organic acids without rise in temperature beyond 450C. Such acidic compost is detrimental for crop growth and hence not suitable for crop application. This compost may contain harmful pathogenic bacteria, fungi either in vegetative or sporulated state as low temperature of the compost favors their growth; in aerobic composting process, however rise in temperature kills pathogenic strains and thus is safe for crop application. In nature, composting process is completed or cured by 1month to 2 years approximately. During this stage of curing, compost is cooled and stabilized again by microbial activity and here also bacteria have a major role to play. Sporulated bacilli and actinomycetes are germinated to active vegetative cells to break down remaining waste that has been more resistant to decompose into final humus.
Bacterial groups present in mature compost: Black color, smooth texture, earthy odor and right amount of moisture makes the compost an ideal growth medium especially for bacteria. It is one of the natural sources of nutrients especially stable humic substances to be utilized by different types of bacteria. It is decomposed state of complex inorganic and organic compounds and hence rich source of C, N and energy needed for bacterial growth. Mature compost may contain organic material which can continue to degrade at very slow rate by mesophilic fauna at ambient temperatures. Porous nature of compost facilitates sufficient aeration and supports multiplication of fermentative aerobic bacteria. Thus mature or cured compost consist of mesophilic, aerobic, occasionally metal tolerant and thermophilic bacteria. Pathogenic coliform bacteria like E.coli and Salmonella typhi are never detected in cured good quality compost. Both thermotolerent and mesophilic actinomycetes like Nocardia, Streptomyces, Pseudocardia, Thermomonospora, Thermoactinomyces and Streptomyces; mesophilic heterotrophs such as Bacillus, Alcaligenes, Pseudomonas, Serratia and Proteus including diazotrophs, phosphate solubilizers, iron chelating bacteria and anaerobic nonpathogenic species of Clostridium have been detected in the compost. Their numbers in particular population have been found in the range of 1 thousand to 10 billion cfu/gm of compost but their populations are always dynamic. Such compost can be used as inoculum to culture new batch of compost. This bioculture can be added directly to composter fed with organic waste, can be used as soil amendment, fertilizer for organic farming or one can also maintain it to be used in number of future compost inoculations. This is very economical as there is no need to buy costly culture from market to inoculate new batch of waste to be composted. Forming a compost of good quality can be a good business for agriculture entrepreneurs.
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