Waste is the spoilage, loss or destruction of either matter or energy, which is unsuable to man. Gradually increasing civilization through industrialization and urbanization, has led to increase in generation of wastes into environment from various sources. Waste generation is, therefore, a necessary outcome of consumption, and also because of insufficient process, general ignorance, wasteful habits and social attitudes (Ray, 1979).
Ray (1979) classified the wastes into energy wastes and material wastes. The main source of energy in the developed and developing countries is petroleum oil, followed by coal. In India, about 50 per cent oil is imported each year. Coal mines are concentrated only in a few regions. Coal is used in generation of electricity, steam engines and fire. Most potential energy of coal is wasted during electric generation in thermal power plants. Thermal loss in India is about 20-30 per cent because of lack of suitable technologies. Based on the chemical nature, material wastes are of various types;
(i) Inorganic wastes (those generated by metallurgical and chemical industries, coal mines, etc.),
(ii) Organic wastes (agricultural products, dairy and milk products, slaughter houses, sewage, forestry, etc.), and
(iii) Mixed wastes (those discharged from industries dealing with textiles, dyes, cake and gas, plastic, wool, leather, petroleum, etc.). The inorganic wastes may be recovered by chemical/ mechanical treatment, whereas organic and mixed wastes require biological as well chemical treatments.
Moreover, the wastes occur in three states, the solid, liquid and gaseous ones. The solid wastes can be burnt, thermally decomposed, anaerobically digested to get methane and other combustible gases or biologically converted to a variety of products. Liquid wastes are most troublesome, because of the presence of non-retractable chemicals, and their further return to environment through surface waters. Gaseous wastes include the toxic gases such as NO2, NO2, NH3, CO2, CO2, SO2, etc. When concentration of these gases increases in the atmosphere they cause gaseous pollution, which has its bad impact on plant and animal lives. The organic wastes and residues become a source of renewable energy in multifarious ways.