Rapid Composting of Municipal Solid Wastes

Composting is the best eco-compatible disposal option for municipal solid wastes. Composting helps in managing large quantities of organic wastes and provides nutrient rich compost, a well-known soil conditioner. It also creates employment opportunities both in rural and urban centers. MSW-derived compost is likely to improve soil properties because of its low bulk density, high water-holding capacity, slightly alkaline pH and high organic matter content. Although composting is an efficient means of organic waste management, requirements such as time, space and labour make this technology unattractive among the public and entrepreneurs. Shortening of composting period (rapid composting) while maintaining the quality of finished compost is therefore, suggested for the management and disposal of municipal organic wastes.

Composting Process

Composting is a biological degradation process during which conversion of organic matter into simpler units of carbon and nitrogen takes place. The decomposition of organic materials is carried out primarily by bacteria, yeasts and fungi. Other microorganisms such as ants, nematodes and oligochaete worms are also involved in the process of degradation. In general, the rate of organic matter degradation depends mostly on degrading microbes and their activities. Microorganims through different kinds of substrate specific hydrolytic enzymes promote the degradation of organic matter followed by the solubilisation of these degraded materials into simple water-soluble compounds.

Although composting is a biological process, it is controlled by many physico-chemical parameters. Some of the most influential parameters on composting are: 1) Temperature, pH, aeration, moisture, size, nature and volume of substrate, C/N ratio, C/P ratio etc. Composting is carried out naturally in pits or windrows. In-vessel and mechanical composting is also possible under controlled conditions. By and large, traditional methods adopt aerobic composting of MSW through measures like infrequent turnings or static aeration. These processes are laborious and time consuming as well. Rapid composting on the other hand takes only two to three weeks for the preparation of quality compost.

Rapid composting of municipal solid wastes

Rapid composting aims at enhancing the rate of organic matter degradation while keeping the nutrient content of the compost high.The approach of rapid composting is to keep all the vital parameters in optimum levels to speed up the process of composting.

Rapid composting of municipal solid wastes

Rapid composting aims at enhancing the rate of organic matter degradation while keeping the nutrient content of the compost high.The approach of rapid composting is to keep all the vital parameters in optimum levels to speed up the process of composting.

Berkley rapid composting (BRC) Process

BRC advocates shredding and frequent turnings of composting material. This method suggests that any organic material can be composted best if it is between ½ to 1 ½ inches in size. A size reduction of the particles and the resulting enlargement of the available specific surface can support the biological process. While chopping without much machinery support may be possible at smaller scales, mechanization may be necessary at large scale applications. Besides chopping of raw materials, Berkley method also uses mineral compounds like ammonium sulphate, chicken manure, urine and turning of the material on daily basis. The method takes two to three weeks.

Rapid composting through Effective Microorganisms (EM)

Indigenous microorganisms naturally undertake break down of organic matter in composting. Nevertheless, the inoculation of substrates with effective lignolytic and cellulolytic microorganisms may speed up the degradation of recalcitrant lignocelluloses leading to the rapid composting of MSW. White rot-fungi are known as the most efficient ligninolytic microorganisms. Phanerochaete chrysosporium is probably the best studied microorganism for lignino-celluloltic activity and is often used as a reference. Other well-known white-rot fungi, Coriolus versicolor show even higher efficiency and a wider range of lignino-cellulolytic activity.

The use of EM as activator brings down the composting duration from twelve weeks to four weeks. Another method of EM based rapid composting process is the “IBS rapid composting” for windrow composting.

An indigenous technology for composting the unseggregated municipal waste biomass directly on dumping sites involving reuse of the same dumping ground repeatedly was developed in Mumbai in 1990’s. The technology was developed by Ms.Excel Industries, Mumbai. The microbial slurry culture known as Celrich substrate DF BC.01 is added to the windrows to initiate rapid aerobic decomposition of waste biomass. The entire process of aerobic decomposition of the garbage is completed within four to six weeks and the process recovers over 90% of the organic manure as compost.

Rapid composting by forced aeration

Gas exchange is crucial during compositing because oxygen based microbial metabolism is more efficient than fermentation. Lack of oxygen is a common reason for composting failures. Oxygen concentrations are determined by rates of diffusion and microbial uptake. Passive diffusion within the interstitial atmosphere is too slow to supply a large composting mass with sufficient oxygen; therefore, active ventilation is required.

Mechanical forced aeration based methods like “ Aerated static pile” reduce the composting time period further, allow for higher, broader piles and have lower land requirements as well. The technology is commonly used for treatment of municipal sewage sludges.

Rapid composting of MSW by chemical addition

Recently, it has been reported that the addition of chemical agents such as phosphogypsum, bauxite residues etc. could enhance biodegradation of Municipal solid waste in the landfill. Since phosphogypsum is enriched with sulfate, it is reasonable to assume that a sulfate using bacterial colony present in landfills will use phosphogypsum as an energy source after oxygen is depleted. The use of phosphogypsum as landfill cover can enhance biological decomposition of MSW. Similarly, bauxite residue having good amount of trace elements can supply micronutrients to the growing microbes thereby increasing the rate of microbial activity. It is also found that bauxite addition increases pH, temperature and the rate of aeration of the composting mixture. All these changes are expected to facilitate the rate of degradation of organic waste. Addition of glucose as instant carbon source into the composting mixture was also found to be useful in accelerating the rate of decomposition.

The rate and quantum of garbage generation and problems associated with it warrants advanced management practices. Composting, undoubtfully, is an efficient way of managing municipal solid wastes; however, constraints such as time, space and man-power should be addressed scientifically for the wide spread adaptation of this technology.

Dr SPM Prince William, Scientist