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Mechanical Composting Indore Process |
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Mechanical Composting (Indore Process)
Another method of composting known as 'mechanical composting' is becoming popular. In this, compost is literally manufactured on a large scale by processing raw materials and turning out a finished product. The refuse is first cleared of salvable materials such as rags, bones, metal, glass and items which are likely to interfere with the grinding operation. It is then pulverised in a pulverising equipment in order to reduce the size of particles to less than 2 inches.
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The pulverised refuse is then mixed with sewage, sludge or nightsoil in a rotating machine and incubated. The factors which are controlled in the operation are a certain carbon-nitrogen ratio, temperature, moisture, pH and aeration. The entire process of composting is complete in 4-6 weeks. This method of composting is in vogue in some of the developed countries.
Composting is a microbial process that converts putrefiable organic waste materials into a stable, sanitary, humus-like product that is reduced in bulk and can be used for soil improvement. Composting is accomplished in static piles, aerated piles, or continuous feed reactors. The static pile process is simple but relatively slow, typically requiring many months for stabilisation. Odour and insect problems can be controlled by covering the piles with a layer of soil, finished compost, or wood chips. Unless turned several times, the finished compost is rather uneven in quality. Under favourable conditions, self-heating in static piles typically raises the temperature inside a compost pile to 55-60°C or 'above in 2-3 days. After a few days at peak temperature, there is a gradual temperature decline.
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Oxygen concentration in the compost is usually five times lower than in ambient air, even when the piles are mechanically turned. Some compost piles are often mechanically turned to maintain aerobic conditions. Turning of a compost pile may cause a secondary temperature rise brought about by the replenishment of the exhausted oxygen supply. Turning also helps to make the compost more uniform, because otherwise the thermophilic processes are restricted to the core of the compost pile.
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Following the thermophilic phase are several months of ‘curing' at mesophilic temperatures. During this period, the thermophilic populations decline and are replaced by mesophiles that survived the thermophilic period. Because of the slowness of the composting process, large amounts of land are required, a disadvantage in densely populated urban areas.
The aerated pile process achieves substantially faster composting rates through improved aeration. The aeration is maintained by suction of air through perforated pipes buried inside the compost pile. This design achieves at least partial oxygenation of the pile, but temperature control is inadequate. Inside the pile, temperatures rise to self-limiting levels of70-80°C. This can be improved by reversing the airflow from suction to injection. Thermostats placed inside the pile control blower operation, starting when the temperature exceeds 60°C.
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The injection of air not only oxygenates the pile but cools it sufficiently to avoid a self-limiting rise in the temperature. The heat generated by the biodegradation process is effectively used in evaporating water and results in a dryer and more stable compost. The aerated pile process goes to completion in about 3 weeks.
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Wood chips, if used as bulking agents, are removed from the final product by screening and are reused. The composting process could be hastened considerably by enriching the input airstream with pure oxygen. Although technically the concept of using pure oxygen seems attractive, it is highly doubtful that the returns would justify the sharply increased cost.
Composting can be accomplished more rapidly in a bioreactor. It requires about 20,000 cubic feet of air per ton or organic matter per day for efficient composting. This process forms a uniform and stable product, but it also requires a high initial investment. Composting in the ,reactor is accomplished in 2-4 days. A part or all of the reactor is maintained at thermophilic temperatures, using the heat produced in the composting process. After processing in the reactor, the product requires 'curing' for about a month prior to packaging and shipment.
Regardless of the process design, conducting the composting process in the thermophilic temperature range is desirable because it speeds the process and destroys pathogens that may be present in faecal matter and in sewage sludge. The aerobic oxidation reactions catalysed by microorganisms produce heat thus raising the temperature inside a compost pile to 76-78°C. Temperatures this high are actually inhibitory to biodegradation. Maximal thermophilic activity occurs between 52°C and 63°C. Aeration or turning may be adjusted to prevent excessive self-heating. Periodic water spraying can also reduce the temperature. Having sufficiently high temperatures is critical, however, for killing human pathogens because much of the material in compost piles, such as diapers, contains human faecal matter. Inadequate temperatures can lead to human health problems, especially if the piles are mechanically turned and particles become airborne.
The composting process is initiated by mesophilic heterotrophs. As the temperature rises, these are replaced by thermophilic forms. Thermophilic bacteria prominent in the composting process are Bacillus stearothermophilus, Thermomonospora, Thermoactinomyces and Clostridium thermocellum. Imporant fungi in the thermophilic composting process are Geotrichum candidum, Aspergillus fumigatus, Mucor pusillus, Chaetomium thermophile, Thermoascus auranticus and Torula thermophila.
Compost is a good soil conditioner and supplies some plant nutrients but cannot compete with synthetic fertilisers in agricultural production. If sewage sludge is a major component of the compost mixture, the finished compost may contain relatively high concentrations of potentially toxic heavy metals, such as cadmium and chromium. Compost finds unrestricted application in parks and gardens for ornamental plants and in land reclamation, such as for strip-mining reclamation and highway beautification projects.
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