A landfill is a special facility designed for solid waste disposal into the ground. A well-engineered landfill is designed to ensure safety, avoid adverse effects on public health, and prevent leachate leakage into the soil or groundwater. Landfills may be categorized according to their design: open dumps, basic landfills, sanitary landfills, and bioreactor landfills. In addition, they can be classified based on the type of waste they accommodate, such as municipal, industrial, and hazardous wastes.
Open dumps are the simplest form of landfilling and the oldest technique of solid waste disposal. In these sites, waste is not classified before disposal nor covered after it. In addition, no specific criteria are followed for site selection, and no measures are taken to ensure their environmental safety.
Basic landfills are the most common type of landfills in developing countries. In these landfills, the waste is compacted and covered. Sometimes, these facilities feature a collection or flaring system to remove the gas produced by waste decomposition.
Sanitary landfills are engineered to provide more control over the various aspects of the landfilling process. In landfill operations a truck drives to, or up, on the existing waste, and deposits the waste. Other vehicles such as tractors spread the waste out. At the end of the operating day, cover material is placed on the new waste.
"Daily cover" comes from the soil dug at or near the landfill site. Sometimes, depending on availability and local costs, woodchips or compost material may be added to the soil and vendors sell special mixes designed to improve cover characteristics.
Typical dimensions are:
The ratio of waste to cover can be 4:1, so the cover soil ends up being a good portion of the material in the landfill. The overall height is typically 10 to 15 ft before that area of the landfill is "complete", and new waste is placed elsewhere.
In addition, they are designed with leachate control and gas collection systems (with 85% collection efficiency). There are three types of sanitary landfills: manual, semi-mechanized, and mechanized. Manual sanitary landfills are used in small towns with waste production of less than 15 tons per day, where it is not feasible to employ heavy equipment in the waste disposal processes. In this type, the waste compacting and disposal processes are manually executed by human operators. On the other hand, semi-mechanized sanitary landfills are used in medium cities, where the waste production is 16-40 tons per day. A mixture of human labor and heavy equipment, such as bulldozers is employed in the compaction and filling processes. Finally, the mechanized sanitary landfills have a more sophisticated engineering design, which is convenient for large cities with waste production of more than 40 tons per day. In this type, heavy machinery designed for solid waste disposal and treatment, such as solid waste compactors, loaders, and dump trucks are used.
The fourth type is the bioreactor landfill. Similar to sanitary landfills, bioreactor landfills are carefully designed. However, they are intended to improve the decomposition of the biodegradable fraction of the solid waste. Bioreactor landfills require additional controls on many parameters, such as temperature and pH, because biodegradation is a very sensitive reaction. There are two types of bioreactor landfills: aerobic and anaerobic. In both types leachate is collected and reinjected into the system along with other liquids to maintain the moisture content. In aerobic bioreactors, air is injected into the waste and the decomposition reaction is carried out by aerobic bacteria, while in the anaerobic type, the reaction is carried out in total absence of air by anaerobic bacteria, and the landfill gas, which contains high percentage of methane, is produced.
Sanitary landfills are more common than bioreactor landfills. Their essential components comprise: 1- a liner system, which prevents the leachate leakage to the soil and ground water; 2- a leachate collection and control system, which collects leachate and send it to the treatment facility; 3- a gas collection system, which collects the landfill gas and send it to storage or flares; 4- a cover, which is situated at the top of the landfill to prevent water penetration into the system; 5- a surface water drainage system to collect and drain all surface runoff from the area; and 6- a monitoring system to observe the quality of the ground water, air, soil, and landfill gas. In addition, any landfill site should have a closure plan, in which a detailed procedure for terminating the operations in the facility at the end of its lifetime should be listed.
Landfill location is an important design parameter. Site selection is a procedure that ensures choosing the most appropriate site for a landfill facility. It comprises choosing the appropriate criteria for the location, listing potential sites, collecting data about each site, screening the available sites to determine the best 2-3 choices, conducting environmental impact assessment for the selected sites, and final selection of the most appropriate site.
A landfill site should be located within appropriate distance from the nearest water source, e.g., rivers and lakes; food plain; habitation; highway; public area, e.g., parks; area with ground water tables > 2 m below ground level; or any area that could be harmed by the presence of a landfill.
Landfill capacity is calculated based on the current waste production in the area and that predicted for the planned lifetime of the landfill, and it should also consider the different facilities in the system, i.e., the calculated area should be increased by about 15%. Landfills can be classified according to their size as follows: small, medium, and large size landfills with areas of <5 ha, 5-20 ha, and > 20 ha, respectively.