It requires the existence of a reliable market for the recovered materials. Although the recovery of certain materials such as aluminium cans large plastic bottles can be profitable. Development of market depends upon quality of the materials, overall industry capacity and the cost of competing raw materials.
Some important solid wastes that have been successfully reclaimed are:
1. Construction materials from wastes
Construction materials can be produced from wastes such as silt from water works, red mud from aluminium industry, agricultural wastes, fly ash from thermal power plants, etc.
2. Utilizing agricultural wastes
Agricultural wastes can be used for the manufacture of paper and card-boards. Sugarcane bagasse an agricultural waste is a chief source of cellulose; but it is burnt away as a cheap fuel. For a better economy, it is an essential step to get some by products like protein from such wastes. This will not only save the protein deficiency, but also reduce the wastes.
3. Medicines from agricultural wastes
Furan compounds occur widely in nature and are cheap raw materials. Filial is readily available from agricultural wastes such as corn com and oak hulls. Furfural (which is produced commercially by the reaction of corn cobs with sulphuric acid) is the basic material used for the synthesis of nitrofurans. These are important germicides used for treating cattle diseases.
4. Utilizing slaughter- houses wastes
Waste products of slaughter houses can also be utilized. Blood can be used in pharmaceutical industry; and hides and skins can be used be used for leather production.
5. Recovery of heavy metal ions
Presence of toxic heavy metal ions in industrial wastes is major concern. These metals can be recovered by chemical treatment or bio-extractive technology (i.e., bioleaching).
6. Liquid fuels form agricultural wastes
The rising cost of petroleum has renewed the need for the production of ethanol by fermentation of agricultural wastes for use as a liquid fuel.
7. Energy from industrial and urban wastes
Huge quantities of urban, municipal and industrial wastes produced are released into the environment with little or no treatment in developing nation, resulting in environmental pollution. These waste can be utilizes as an enormous source of potential energy and thus help in reducing emissions.
8. Conversion of agricultural wastes into cheap and efficient fuel
Indian scientists, in collaboration with Dutch researchers, have developed a technology of converting agricultural wastes like rice husk and ground nut shells into briquettes to be used as an efficient, economical and non-polluting fuel. The technology (known as screw press technology) for briquetting biomass compacts low density agricultural residues, leaving a hole in the centre. The hole acts in enhancing the combustion characteristics of the biomass in the new technology, dried biomass is crushed and pre heated to about 100-2000C, preheating softens the biomass and also reduces the load on the screw. The material is than fed into a screw extruder press where a revolving screw compacts the material which is further hardened into briquettes. These briquettes are virtually smoke free and have ash content of 2. 10%.
Hence agricultural wastes can produce huge quantities of briquettes which are potential environmental friendly fuel resources.
9. Rubber from old tyres
American chemists have identified some strains of sulphur mounting bacteria "seuephotobees" from the hot springs of yellow stone national pack. These sulphur loving bacteria consume the sulphate from type wastes and leave the polymer backbone of carbons intact during recycling.
10. Proteins from cellulose waste
Experiments carried out at the Louisiana state university (LSU), USA, have shown the possibility of commercializing a process for marking proteins from waste cellulose.
The LSU process has two main stages. First, the cellulose materials such as crop wastes, urban wastes are treated with strong alkali using cobalt chloride catalyst and heated from 25 to 1000C. by doing so the lignin over cellulose fibres is destroyed. The cellulose matter is then neutralized with an acid and led to fermentation chamber to be attacked by cellulomonas bacteria. The bacterial enzymes disintegrate the cellulosic chains to from disaccharides and render them digestible by micro-organisms.
11. Utilizing aquatic weeds
Aquatic weeds such as water hyacinth (eichhornia) can be utilized by converting it into fertilizers, biogas, animal feed, paper, etc.
12. Paper:
Generally wastes paper contains grit, sand, ink, tar, and rubber bands etc. which have to be separated before further treatment of paper. This recycled paper can be used for printing only a few times because with each recycle the fibres become somewhat shorted and more rayed. The manufacture of paper board is the main use of waste paper.
13. Aluminium:
Due to high cost of imported aluminium ore (bauxite) and the development of an extensive collection and processing infra-structure is the most successful recycled product.
14. Class:
The use of increased cullet in glass production results in energy saving for the manufacture. (Each 10% increased in cullet results into 2% reduction in energy consumption).
Wastes glass can be used as an aggregate in asphalt instead of crushed stone. The finished product exposes no jagged edges and is skid resistant.
15. Plastics:
Plastics are not bio-degradable. Hence they remain intact in disposal operations such as sanitary landfills.
One possible method of recycling plastic is to use it with the raw plastic during the production of new packs. But mixed plastic cannot be recycled in this manner so their recycling may be accomplished by reusing the plastic in another form. For example the union carbide company in the USA has pyrolysed plastics to produce hard and soft waxes, greases, adhesives and tars.
When plastics are exposed to ultra-violet light, certain chemical groups absorb the light and release energy. This energy breaks the polymer chain so that the plastic loses its strength, becomes brittle and is broken up by the wind and rain. Hence it can be decomposed by normal soil bacteria into basic constituents.
16. Waste oil utilization and recovery:
Waste oil generated from lubricants and hydraulic fluids is one of the more commonly recycled materials. Annual production of waste oil in the US is of the order of 4 billion litres. Around half of this amount is burned as fuel and lesser quantities are recycled or disposed as waste. The collection, recycling, treatment and disposal of solid waste oil are all complicated by the fact that it comes from diverse, widely dispersed sources and contains several classes of potentially hazardous contaminants.
17. Recycling wastes oil:
The processes used to convert wastes oil to a feedstock hydrocarbon liquid for lubricant formulation. The first of these used distillation to remove water and light ends that have come from condensation and contaminant fuel. The second, or processing step, may be a vacuum distillation in which the three products are oil for further processing, a fuel oil cut, and a heavy residue. The processing step may also employ treatment with a mixture of solvents including isopropyl and butyl alcohols and methylethyl ketone to dissolve the oil and color. The third step employs vacuum distillation to separate lubricating oil stocks from a fuel fraction and heavy residue. This phase of treatment may also involve hydro finishing, treatment with clay and filtration.