Landfill leachate is generated as a result of precipitation, surface run-off, and infiltration of groundwater percolating through a landfill. The discharge of landfill leachate can lead to serious environmental problems, since it contains a large amount of organic matter(both biodegradable and non-biodegradable carbon), ammonia-nitrogen, heavy metals, chlorinated organic and inorganic salts in landfill leachate composition. Although some of these pollutants can be degraded by microorganisms, the limitation of common biological processes has made it difficult to meet the correlative discharge standard.
Landfill leachate varies widely in composition, it usually contains both dissolved and suspended material. The generation of leachate is caused principally by precipitation percolating through waste deposited in a landfill. As leachate first emerges, it can be black in colour, anoxic, and possibly effervescent, with dissolved and entrained gases.
The leachate should be treated before discharged due to the risk from its high organic contaminant concentrations and high concentration of ammonia. (4 reasons why landfill leachate is difficult to treat?) Pathogenic microorganisms that might be present in it. In the US and the European Union, it is mandatory to use impermeable liners except where the waste is deemed insert.