Composting PDF

EnvironmentallAspects of Microbiology 213 Chapter28 COMPOSTING Composting is a thermophilic, aerobic decomposition process,in which solid substrates like municipal waste,...

EnvironmentallAspects of Microbiology 213 Chapter28 COMPOSTING Composting is a thermophilic, aerobic decomposition process,in which solid substrates like municipal waste, raw sewage, sludge can be degraded over a period of weeks by a succession of microbial population to yield a stable form of organic matter. After such a biodegradation, a dark brown, granular, humus like end producttis formed, which is called as compost. The primary objective of composting is to convert an unstable, offensive material into a stable end product. Thus it is an effective method of domestic refuse disposal. When compost is used as a soil conditioner it also provides a means of recycling solid wastes. The difference between anaerobic digestion of sewage sludge and composting is that the latter is an aerobic process. SUBSTRATES Substrates for composting include a range of materials from municipal refuse to sewage sludge. Paper, garbage, agricultural wastes like leaves and even human faecal material in some cases can be composted. For producing a stable compost the organic substrate must cOntain adequate nitrogen. A reasonable carbon : nitrogen ratio is about 50. Rubbish, the faecal material, has a very high content of arbonaceous materials and has too high C:N ratio. To make CXCelent compost the C:N ratio of rubbish can be lowered by mixing With garbage, sewage sludge, cow manure or wastes from fish farms of canners. 214 Environmental Aspects of Microbiolog THE PROCESS The composting process involves five major stages. They are : salvage, grinding and homogenization, degradation, drying and curine andfinishing. The entire process may take one to three months to produce a good compost. (a) Salvage Domestic as wellas municipal refuse contain non biodegradable materials like glass, ceramic,plastic and metals in addition to other biodegradable organic materials. So they have to be removed. Ferrous materials can be renmoved magnetically. Non compliance materials like plastics are removed by hand - sorting (manual picking). Other very large objects are separated by mechanical screening. (b) Grinding, pulping and homogenization For effective composting the substrates should be a finely divided granular material, which permits aeration, mixing and invasion by the degradative microorganisms. Particles of size about 4+ 7em will be suitable and this can be achieved by grinding of shredding. Alternatively, wet pulping method can also be used which produces a slurry containing about 5% solids. But this requires drying to about 50% solids to be suitable for composting. Sometimes agitation in the initial stages of composting cause the break down of fibres in the material to be composted. This reduce the pretreatment required. A moisture level of about 40 to 50% is ideal for goo composting. Low moisture content may inhibit microbial action. So,  Enyironmental Aspects of Microbiolopy 215 this stage moisture content has to be adjusted. The required ontent can be attained by adding water. Sewage sludge can moisture also be nd to attain required moisture and in this case the final product will be of a higher grade. The sludge may be raw, combination of these types and the nature of dewatered, digested or a resulting compost will depend on the sludge used.Generally raw primary or cludge will be preferred to digested sludge because they cansecondary be easily dewatered and has a higher nutrient content. When municipal refuse and sewage sludge are combined for composting, the ratio of refuse to sludge should be approximately 2 : 1 by weight. (c) Biological degradation There are different methods of composting the commonly used local method involves piling up of the substrates into large heap. Bulking agents like large wood chips or barks can be mixed with the substrates to facilitate aeration. After the required period of composting, about 21 days, bulking agents are separated from the compost and the compost is ready for use. The most primitive and simplest method is to pile the material into windrows, which are heaps resembling pyramids. Windrows are of about 3cm high with a base of 9m width. The mixture is turned regularly with mechanical shovels to promote aeration. After four to SIX Weeks, depending on moisture and climate, a stable compost is formed. Windrow composting has anumber of disadvantages. Since it is done in the open air i is significantly affected by the local climate. Often the compost generates odors which may be difficult to  Environmental. Aspects of 216 (M icrobiol control and this may rise complaints from local people. Windro oE) composting requires large land area and therefore unsuitable for urban. An alternative method of composting involves mechanical mixing and aeration inside closed digesters. A number of processet are used in mechanical composting and all of them have equipments to control temperature, 'pH andC:N ratio. The most well known process is the Dano process. This employs drums which are about 30m long and 5 m in diameter. The drums rotate slowly to facilitate mixing and aeration. The advantages of these mechanical composting are (1) they do not suffer the problems of windrow composting (2) with better control over mixing, aeration and temperature the composting process can be completed in a shorter period than that required for windrow composting. (d) Curing and drying The compost must be stabilized completely before it is added to the soil. Otherwise, it will remove the nitrogen, which is available to the plant, from the soil in the final stages of its degradation. In case of windrow composting, to ensure complele stabilization the compost is cured for a period of two weeks in shallow piles. In mechanical composting, curing periods can range upto three weeksor may be absent. (e) Finishing The type of finishing required is largely dependent on the method of marketing or disposal of the compost. Often, the composs  Environmental. Aspects ofI fMicrobiology 217 will be mechanically dried to a moisture content of less than 30% for agricultural or horticultural use. The consistency of the compost is this also important. For this reason, regrinding and screening of the compost may be done to remove large pieces of debris. When sold in emallquantities, the compost may be pelletized before bagging. Sometimes the compost is pasteurized to kill pathogens before sale to ehe farmer. In contrast, if the compost is to be disposed off by landfilling no finishing will be required at all. MICROBIOLOGYOF COMPOSTING Composting involves different groups of microorganisms which act on the substrates in succession Although the decomposition is mainly carried out by the thermophilic microorganisms, the initial process is caried out by mesophilic microorganisms. In the initial stages of composting the heap of materials to be composted will be at ambient temperature and the pH will be more or less neutral. At first the biodegradable materials, such as proteins and carbohydrates, are decomposed naturally by the mresophilic microorganisms. The degradation products are usually simple organic acids which causes a drop in pH. As the initial reactions proceed heat is produced which nses the temperature within the heap. The insulating effect of heap further causes an increase in temperature. However, the maximum lemperature achieved and the time taken to achieve it depend on various factors like composition of organic wastes, moisture content, Size of heap and degree of aeration and agitation. As the temperature Increases, the activity of mesophiles are gradually reduced and finally about 60°Cthe mesophiles are kiled by the high temperature. They ay become the source of nutrients for higher forms like protozoa, 218 Environmental Aspects of rotifers and nematodes. They may even add to the total Microbiology organic content. At this temperature, thermophlic microorganisms take the degradation process. Hemicellulose, cellulose and lignin a degraded rather slowly. Thermophilic bacteria like and Thermoactinomyces and and thermophilic fungi such a Thermonospora Thermoascus carry out these reactions. Once the readily degradable materials are degraded, the reaction rate slows down. Eventually the temperature decreases once again to the mesophilic range through heat loss from the surface of the heap. Once again the mesophiles, either through re-invasion from outside or through germination of heat resistant spores, dominate. The thermophilicprocess has certain advantages as follows : - acceleration of the process - killing of pathogenic microorganisms -destruction of ungerminated seeds of weeds. Although the pathogenic microorganisms are destroyed,| sometimes spores of the fungi Aspergillus may be found in the vicinity of composting areas. However, any adverse effects caused by these microorganisms are highly localized and hence there will not be any threat to residential areas. In certain cases, starter culture from old compost fermentations may be used as an inoculum for new ones. PROBLEMS OF COMPOSTING AND THEIR REMEDIES Composting procedure may sometimes be associated wit certain problems which make the process unsatisfactory.  fMicrobiology Environmental,Aspects of! 219 Glass, metal, polythene bags and other non-biodegradable matter |. have to be separated from the refuse. This may be done mechanically or by hand befor before and after composting. 2. Farmers may refuse to use compost if it includes night soil. There needs a market and low price for the compost. 3 a Organic refuse and night soil may contain fly eggs and larvae before they come to the process. To prevent them from hatching the compost has to be turned often so that the surface materials are brought to the hot centre. More over the process should be carried out on a hard surface to prevent the larvae from burrowing into the soil. ADVANTAGES In spite of the above said problems composting has its own advantages. -Composting can be carried out with simple equipment and very little |training. Wellsuited to many tropical environments It is particularly appropriate for small cities and towns near to agricultural areas.