Quick safe and economical disposal of resistant wastes has become a necessity today. The enormous volume of stinking, ugly looking, perpetually expanding garbage dumps occupy large area of otherwise useful land for a considerable period of time. Even compost pits dug with costly labour fail to accommodate them. The only solution to the problem is to reduce the volume of the garbage to manageable amount. This can be done by:
1. Incineration of Wastes:
Incineration of solid wastes involves the combustion of garbage at a temperature usually above 200°C. It reduces the bulk of solids to ashes while the heat produced may also be utilized in some useful way. Burning of solid wastes, however, produces smoke and harmful vapours like hydrogen peroxide, carbon dioxide and monoxide, sulphur dioxide, various hydrocarbons, and a number of other toxic fumes which further pollute the atmosphere.
Moreover, wastes to be incinerated should contain enough material which burns. The solid wastes discarded in most of Indian cities have a very low calorific value as most of the combustible trash like paper, plastics, rubber; polythene etc. is picked by rag-pickers which throng the streets. The solid wastes which accumulate in Delhi have a calorific value as low as 600-800 per kg while many of the solid waste incineration plants in India are designed for burning trash with a calorific value higher than 1500 per kg.
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2. Pyrolysis and Vitrification of Solid Wastes :
Pyrolysis involves heating of solid wastes at high temperature (600°-800°C) in oxygen deficient conditions so that the trash does not burn but only smoulders. There is little change in the bulk of solid wastes and the product of pyrolysis is almost like charcoal which can be further used as fuel, if burning it does not produce any poisonous waste gases. As we have to provide heat for pyrolysis,the process is expensive. Vitrification often done in situ is a process which may be used for a mixtureof recalcitrant and refractory organic wastes which are unaffected by any other type of treatment.
Two electrodes are inserted in the heap of solid wastes and a very powerful electric current is passed through them. The heat generated melts mud, glass, plastics and other materials to produce a glass like fusion product which can be dumped anywhere since it produces little leachates and creates no problems. All these processes require large investments and high grade of technical expertise so thai the entire trash is dealt with effectively without creation of any further problems.
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Probably the best and the most economical approach to the problem of-solid waste disposal involve microbial degradation of solid wastes. Bio- degradable wastes can be conveniently handled in this way. Recalcitrant and refractory solids like plastics, polymers and many plasticizers create problems. These materials are synthetic substances and as such are often too new to the biological system. The microbial system simply lacks the enzymatic machinary needed for their decomposition.
Recent strides taken in the field of biotechnology has come to be of great help in this direction. Genetically engineered microbes have been produced which can decompose a number of organic compounds which were considered to be nondegradable earlier. It was only an Indian born American Scientist Chakrabarty A.N., who has patented for the first time a genetically engineered strain of bacteria, Pseudomonas, which decomposes a number of complex and toxic hydrocarbons. Efforts are also being made to produce bio-degradable plastic and polymers through genetically engineered microbes. Biologically produced plastic and polymers shall create much less problem as microbial system shall decompose them quickly and effectively into simpler constituents.
4. Sorting and Recycling of Solid Wastes Resistant to Degradation:
As much of the solid wastes generated these days consists of materials resistant to degradation, recycling and re-use appears to be the best solution to the problem. Recycling of paper, plastics and materials resistant to degradation and non-degradable wastes has been in practice since long. Sorting out of the re-usable stuff from heaps of waste material constitutes the first step in the process which may often involve much manual labour.
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This often renders the cost of the operation exorbitant and makes the final product if not costlier at least as much expensive as the fresh one. In a country like India poor raddi-wallahs or the garbage collectors make a living from the discarded solids and in the process do a commendable job by removing much of recalcitrant solid trash from the garbage dumps. Pieces of metal, glass, rubber, plastics of various types, etc. are thus removed to be recycled again as finished product.
However, the product derived from recycling operations is often not of the same quality as original ones. Paper, for example, derived from recycled stuff is of a rough quality and has to be used as packing material, in cartons, in corrugated boards etc. With little improvement in the processing and manufacturing technology a better quality paper can be made from the recycled stuff as has been done in Japan which recycles as much as 40% of its unwanted paper into new high quality material.
Similarly recycling of trash consisting of polyvinyl chloride (PVC) after retrieval from waste dumps does not yield good quality products. The thin polythene bags or the material of which polythene buckets and containers are made of can be re-used but the result is hard brittle material which has to be discarded soon.
The problem is, therefore, not solved but only delayed. A solution to this problem has been found in compatibilizer molecules which stick on these different types of plastic molecules making the recycled material sturdier, long lasting and having as much gloss and finish as the original one.
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The material of the transparent plastic bottles is, however, different from those of thin polythene bags. It consists of Polyethyline tetraphthalate (PET) which is capable of picking up poisonous substances present in waste dumps. Recycling wastes consisting of polyethyline tetraphthalate, though easier, is not without any risk. The poisonous material absorbed in waste dumps may persist in the final product. It may be released slowly in the material stored in the containers made of recycles material.