Leachate
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Leachate is the liquid produced when water percolates through any permeable material. It can contain either dissolved or suspended material, or usually both. This liquid is most commonly found in association with landfills where result of rain percolating through the waste and reacting with the products of decomposition, chemicals and other materials in the waste to produce the leachate. If the landfill has no leachate collection system, the leachate can enter groundwater, and this can pose environmental or health problems as a result. Typically, landfill leachate is anoxic, acidic, rich in organic acid groups, sulfate ions and with high concentrations of common metal ions especially iron. Leachate has a very distinctive smell which is not easily forgotten.
The risks from waste leachate are due to its high organic contaminant concentrations and high ammoniacal nitrogen. Pathogenic micro-organisms and toxic substances that might be present in it are often cited as the most important, but pathogenic organism counts reduce rapidly with time in the landfill, so this only applies to the most fresh leachate. These risks are greatly mitigated by properly designed and engineered landfill sites. For example, sites that are constructed on geologically impermeable materials or sites that use impermeable liners made of geotextiles or clay. The use of linings is now mandatory within both the United States and the European Union except where the waste is genuinely impermeable. In addition, toxic materials such as cadmium and toluene cannot be disposed of in landfills.
In older landfills, the leachate was directed to the sewers, but this caused problems. Originally one of these was the contamination by toxic metals that passed through the sewage treatment plant and eventually entered the environment. However, with improved regulation and control whereby toxic wastes are now no longer permitted to be disposed to Municipal Solid Waste landfills in Europe, and in most developed countries the metals problem has largely been solved. Paradoxically, however, as sewage treatment works discharges are being improved throughout Europe and many other countries, the sewage treatment works operators are finding that the very high ammoniacal nitrogen concentrations in leachate difficult to treat.
Another problem was that if the landfill contained large amounts of organic material then methane was produced, some of which dissolved in the leachate. This could in theory be released in weakly ventilated areas in the treatment plant and all plants in Europe must now be assessed under the EU ATEX Directive, and zoned where explosion risks are identified to prevent future accidents. The most important requirement is the prevention of discharge of dissolved methane from untreated leachate when it is discharged into public sewers, and most sewerage undertakers limit the permissible discharge concentration of dissolved methane to 0.14 mg/l, or 1/10th of the lower explosive limit. This entails methane purging or stripping.
Finally, leachate can contain high concentrations of ammonia which in theory can pose a health hazard to treatment plant workers, particularly in acidic leachate. However, within municipal solid waste landfills this is not a problem due to the pH remaining close to neutral after the initial stage of (acidogenic) leachate decomposition. Many sewer undertakers limit maximum ammonical nitrogen [1] concentration in their sewers to 250 mg/l to protect sewer maintenance workers, as the WHO's maximum occupational safety limit would be exceeded at above pH 9 to 10, which is often the highest permitted alkalinity of permitted sewer discharges.
Leachate can also be produced from land that was contaminated by chemicals or toxic materials used in industrial activities such as factories, mines or storage sites. Composting sites in high rainfall also produce leachate.
[edit] See also
[edit] External links
- An introduction to leachate
- Landfill Leachate
- Free access leachate aeration system drafting software for diffusers
- Activated Carbon Landfill Leachate Treatment
[1] Dictionary of Environmental Science and Technology - Third Edition - Andrew Porteous - ISBN 0-471-63470-0, pg 25.
