Solid waste in sanitary landfill is undergone by many chemical, biological and physical changes. The most important changes are as follows:
- Biological decay of organic material either aerobically or anaerobically with the emission of gases and liquids
- Chemical oxidation of materials
- Release of gases from the landfill and lateral diffusion of gases
- Movement of liquids by differential heads
- Leaching and dissolving of organic and inorganic materials by water and lechate movement through the fill
- Movement of dissolved material by concentration gradient
- Irregular settlement of materials by merging of materials in spaces
1) DECOMPOSITION IN LANDFILL
The waste that is placed in the landfill is undergone by bacterial decomposition. At first the decomposition occurs in aerobic condition because air is soon lost. Long term decomposition occurs in anaerobic conditions. The source of aerobic and anaerobic organisms that deals with decomposition is soil that is used as final cover material daily.
The rate of decomposition of organic material depends upon their properties and to much extent on the moisture conditions. The organic material in solid waste is classified into 3 categories:
- those containing cellulose and derivatives of cellulose
- those not containing cellulose and their derivatives
- plastics, rubber and leather
Cellulose is major part of organic waste e.g. paper, straw, string and plant tissues. Non cellulose organics are proteins, carbohydrates and fats. Mineral salts in less amount and moisture are attached with these materials.
The end result products from anaerobic decomposition are intermediate volatile organic acids, partially stable organic materials and many gases.
2) GASES IN LANDFILL
Gases in landfill consist of air, ammonia, carbon dioxide, carbon monoxide, hydrogen, hydrogen sulphide, nitrogen, methane and oxygen. The main gases evolved from anaerobic decomposition of organic material are carbon dioxide and methane. The high carbon dioxide concentration is due to aerobic decomposition. It occurs until the oxygen in air is depleted. After this anaerobic decomposition occurs. After 18 months the composition of gas varies. If the landfill is not vented the percentage of methane increases.
The total volume of gas released during anaerobic decomposition is estimated by various methods. One is for example if all the organic materials have the formula CaHbOcNd then total volume could be estimated with the assumption of its complete conversion into carbon dioxide and methane.
3) LECHATE IN LANDFILLS
Lechate is defined as liquid that percolates through solid waste and has extracted dissolved or suspended materials from it. In landfills the liquid is produced from decomposition of waste and from external sources such as groundwater, rainfall, surface drainage and water from underground springs.
When lechate percolates through solid waste that undergoes decomposition, both biological and chemical materials are picked up.
Quantity of lechate is direct function of the amount external water entering the landfill. If landfill is constructed properly then production of lechate could be eliminated.
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| COMPLETE LANDFILL |
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