Wastes as Renewable Source of Energy
Waste is the spoilage, loss or destruction of either matter or energy, which is unusable 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
, 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. Some of the renewable sources of the organic materials are given in Table 19.5.
Composition of Wastes
Waste is a general term which embraces all types of wastes irrespective of constituents and phases. Therefore, composition of waste differs with differing nature, phases and sources. It may be inorganic, organic or mixed types. Organic wastes play a major role in being renewed and becoming a source of energy. Composition of organic materials is given under 'composition of biomass (see Composition of biomass
Table 19.5. Some renewable sources of biomaterials
Sources of Wastes
||Coir, shells and pith
||Coir board, coir fiber, xylose
||Single cell protein (SCP), Fuel
||Husk, bran, straw
||Bran oil, vitamin E, SCP,
||SCP, fuel/fire, alcohol
||SCP, Fuel (alcohol)
||Seeds and leaves
Industries generate various types of wastes/ by-products which contain sufficient amount of energy. Some of the industrial wastes are described below:
(I) Sugar cane (Saccharum officinarum) :
- Paper Mill : The wastes are bisulphite liquor and lignocellulosic pulp.
- Chemical Industries : The chemical wastes are maleic anhydride and phthalic anhydride.
- Oil Refineries : They produces wastes as gas, oil, paraffins (n-alkanes), olefins (n-alkenes) and other hydrocarbons.
- Cotton Mills : Cotton mills produce the cotton seeds and fibers as wastes.
- Food Industry : Waste materials of food industries are the collagen meat packaging waste and lactoserum (a by-product of cheese making food industry).
- Dairy : Dairy industry is one of the important industries which requires special attention, as far as treatment and disposal of wastes are concerned. Dairy wastes contain milk whey, butter milk, unused skim milk, plant washings and traces of detergents. The waste is a dilute solution or suspension containing lactose, protein, fat and minerals. Therefore, dairy wastes serve a food substrate for production of single cell protein, lactic acid, vitamins, ethyl alcohol and alcoholic beverages (Garg et al, 1985).
- Sugar Mills and Distilleries
Sugarcane belongs to family Gramineae. It is a tall, perennial grass, the stems of which are the source of cane syrup.
India is the largest producer of sugarcane in the world. Sugar cane is grown in many states of the country. There are about 400 sugar mills and 127 distilleries in working order; besides, about 88 new sugar mills are coming up soon. Enormous quantity of wastes in the form of bagasse, molasses, and press mud are produced by sugar mills.
Bagasse is the cellulosic material of sugar cane produced after extraction of sugary juice. It is used for the production of fuel, alcohols, single cell protein, and in minipaper mills also. Molasses is an important by-product of sugar mills which contains between 50-55 per cent fermentable sugars, mainly sucrose, glucose and fructose.
Fig. 19.5. Production of single cell protein, ethanol and biogas from the distillery wastes.
Every tonne of sugar produced gives about 190 liters of molasses, and about 280 liters of ethanol can be produced per tonne of molasses. Molasses is used for the production of animal feed, liquid fuel and alcoholic beverages.
About 732 million liters of alcohol per annum is produced from distilleries. For every liter of ethanol (rectified spirit) about 12-15 liters of effluent, reddish brown to dark in color, is generated which has a high biological oxygen demand (BOD) of about 40,000 mg/l. It is because of the presence of organic materials in high amount. Production of single cell protein, ethanol and biogas from the distillery effluent is shown in Fig. 19.5. For the utilization of distillery wastes works are being done at National Sugarcane Institute (Kanpur), National Environmental Engineering Research Institute (Nagpur), and Department of Microbiology, Hissar University (Hissar).
Importance of using bagasse as an alternative source of energy for sugar manufacturers has been realized in recent years. The Engineering Staff and College, Hyderabad organized a workshop in October, 1990 on "strategies for the development of co-generation of electric power in sugar factory". The Indian Renewable Energy Development Agency (IREDA) sanctioned around 97 projects in this field. Uttar Pradesh Government launched a programme for producing alternative energy from sugar cane waste in sugar mills. The first project of its kind in the country is being installed with the Sheh Road Sugar Mill in Bijnor. It will produce 6 MW electricity from the sugarcane waste. In this way fuel consumption would be reduced from 40 to 58 per cent.
(II) Sugar beet (Beta vulgaris).
Sugar beet is a member of Chenopodiaceae. It is a biannial herb with fleshy leaves and swollen roots. For sugar production good crops of sugar beet can be had in Rajasthan, Punjab, Haryana and Uttar Pradesh.
In India, a large area is occupied by usar
(sodic) land. It is a characteristic feature of sugar beet that it can grow in sodic soil. It is a short duration plant. The underground parts i.e.
tubers, contain high contents of sugar. Like sugarcane, it is also a good source of ethanol. Studies on alcohol production from sugar beet are in progress in the U.S.A., New Zealand, Australia, and India (at the NBRI and CDRI, Lucknow).
: In agriculture, a huge amount of residues/wastes are produced which, however, are thrown into field because of non-availability of technologies for utilization at village level. Some of the agricultural wastes produced from various plants are given in Table 19.3. However, residue/wastes of some crops are utilized in multifarious ways, and their future prospects are discussed below :
(I) Paddy (Oryza sativa): Paddy plant produces paddy and straw ; the products of paddy are rice, husk and bran. Husk is the outer most hard coat of paddy. Bran is the thin papery layer present between husk and rice.
Out of total (about 80 million tonnes) production of paddy, about 16-18 million tonnes of husk (Le. 20-25 per cent of paddy) are produced per annum. Due to lack of utilization technology bran is thrown as waste, and probably in some part it is used as cattle feed. Recently, bran has become a source of rice bran oil, whether edible or non-edible. Paddy husk is used as fuel in rice mills and villages as well.
Recent analysis of paddy husk of different species has shown the presence of high caloric value i.e. 3200-3500 KCal/Kg, which can replace about 10 million tonnes of coal per annum. It has been found that 1 tonne of husk can replace 450 liters of furnace oil (Gupta, 1984).
To meet the demand of energy and minimize the pressure on fossil fuel, many countries like Japan, China, Philippines, Indonesia, India have given special attention to develop technologies for utilization of paddy husk. Recently, the Central Fuel Research Institute (C.F.R.I.), Dhanbad has developed a process to produce oxalic acid from cellulosic matter and silica from mineral matter.
India has started a thermal power plant from paddy straw, which is first of its kind in the world which can generate 62 million unit of energy per annum. A rice straw-fired Thermal Power Plant was set up at a village, Jalkheri (Punjab) in March 11, 1989. It has been done as a joint venture of the Department of Non-conventional Energy Source (DNES), Punjab State Electricity Board and Bharat Heavy Electricity, Limited. Moreover, Punjab Agricultural University (Ludhiana) and IIT (Kanpur) have also developed a husk-fired plant. Similarly, Paddy Processing Research Centre (Tiruvarur) has developed methods for parboiling and milling operation in rice mill to extract about 92 per cent potential energy of husk.
(II) Tapioca (Manihot esculenta) : Tapioca, commonly known as cassava, is a member of Euphorbiaceae. It is a small shrub producing tubers inside the soil. Tubers are rich in starch. Therefore, this plant is exploited commercially for starch, sago and flour. It is an important source of alcohol also.
For these purposes, tapioca is cultivated in Kerala, Tamil Nadti, and Karnataka. Of the total production, Kerala alone contributes 80 per cent tapioca in the country. It grows in alkaline soil in marginal and infertile lands.
Tapioca can yield alcohol about 180 liters/tonne in contrast with sugarcane which produces about 70 liters/tonne. Central Tuber Crop Research Institute (CTCRI), Trivandrum has carried out work on alcohol production from tapioca. Starch is extracted from the tubers which is then converted by alpha amylase and gluco-amylase into sugar. Sugar is then fermented into alcohol.
: Forests contribute a considerable amount of biomass which could be variously used as a source of energy. Lignocellulosic contents also varies from 60-75 per cent of dry weight. Soft wood has higher lignin to cellulose ratio than hard wood. Residues/wastes generated from forestry are wood chips, saw dust, dried tree branches, tree twigs, tree-bark, leaf litter, etc.
(C) Municipal Sources
: High amount of municipal wastes are generated from cities, as a result of anthropogenic activity which are thrown near cities in open lands or in rivers. These wastes again become a serious hygenic problem as it contain high amount of organic matter and pathogenic microorganisms. Municipal and domestic wastes include sewage and sludge, garbage, horse dung, cattle dung and wastes from animal slaughter houses.
(a) Sewage and Sludge : Sewage is a product of water which are thrown away after its use. Treatment of sewage results in generation of another waste, which is called as sludge. Sludge is a solid matter in the settling tanks of sewers, and other treatment operations in a sewage treatment plant.
There are 142 class I cities in India which produce around 9,000 million liters of sewage per day. However, there is no sewage treatment facilities in about 70 class I cities such as Srinagar, Ranchi, Dhanbad, Bhopal, Jabalpur. Although methods for single cell protein production from sewage oxidation pond and irrigation of agriculture crops by treated water have been developed (see Fig. 18.1), yet this facility is available only at limited places. Moreover, if 900 million liters of sewage is converted into sullage gas per day from the major cities, 20 per cent of their energy demand could be met (see Biogas production from Municipal wastes) (Sharma, 1984).
(b) Urban (City) Garbage : Garbage potential of Indian cities i& quite high. It is estimated that production of city garbage in India is about 41,000 tonnes per day, the annual production is about 15 million tones. Nonetheless, city garbage produced in Bombay, Madras and Calcutta is comparable to that of developed countries. In India, garbage is thrown near the city. In some cities like Madras, Calcutta, Delhi, Baroda, Jodhpur, etc. Municipal solid waste composting plants are in operation (Shrama, 1984).
Moreover, the Central Mechanical Engineering and Research Institute, Durgapur has established a first pilot plant to produce electricity by using city garbage. The plant has a capacity to use about 500 kg garbage/ha, as a result of which about 5 Kw h electricity can be generated. Process of electricity production is the conversion of garbage, anaerobically, into biogas and in turn into electricity.