Leachate is the liquid produced when water percolates through any permeable material. Silage leachate is a form of leachate which is a serious environmental problem, however, when people refer to leachate they are usually talking about the contaminated water that is produced by water percolating through waste.
Leachate is produced when water filters downward through a landfill, and as it does so it picks up dissolved materials from the decomposing wastes. Depending on characteristics of the landfill and the wastes it contains, the leachate may vary from being relatively harmless or extremely toxic.
Landfills with low permeability daily cover, in wet and high rainfall climates with high slopes (less than 20 to 1) are particularly susceptible. Leachate seeps also are possible if operators use recirculation to return concentrates from, for example, RO systems and we have known seeps to be particularly persistent where systems are inadvertently placed over well-compacted, former haul roads.
Landfill leachate can also be defined as liquid that leaks from a landfill and enters the environment. This liquid may either exist in the landfill as deposited, or it may be created after rainwater mixes with the chemical waste in in a landfill.
Designed to hold our garbage and to prevent it from contaminating our soil and our drinking water, the modern landfill has become a highly technical and complex structure. Contrary to what many might think, a landfill is not just a pile of garbage. Because these landfills are designed not to let the leachate escape and pollute the surrounding ground the landfills hold the leachate for a long while before it can descend through the waste and be removed from the bottom. This means that the modern landfill produces some of the strongest and most contaminated leachate waters ever produced anywhere.
Treatment of this polluted water is a complex task due to its nature. A typical leachate is highly contaminated with ammonia, organic contaminants, halogenated hydrocarbons and some heavy metals (although this can be overstated in leachate from modern well controlled landfills). Also, leachates commonly hold high concentrations of inorganic salts.
Treatment lagoons and leachate ponds are a method of leachate management, but they are usually ineffective fall all but the freshest and weakest landfill leachates.
In dry climates and arid areas, removal of the organic content from a highly polluted drainage (such as landfill leachate) is not sufficient. This is because of introducing the salts and other inorganic pollutants from the leachate into the natural water cycle, even at minimal concentrations, can lead to bio-accumulation (a growing potential of environmental pollution) in the resultant ecosystems which receive the discharge of treated leachate.
Additionally, although the biodegradability of leachate organic compounds declines with time, complex organic compounds, such as humic substances and manufactured chemical compounds, remain in solution.
Biological treatment is the most favourable procedure that should be used for leachate treatment, and biological treatment should also be used in those cases when chemical/physical treatment is also required. Biological leachate treatment is a relatively low cost process in which organics are degraded mainly to carbon dioxide, water, and biomass.
All those substances that are been eliminated using a pre-treatment of biological degradation no longer have to be treated by means of the much higher cost chemical/physical procedures available.
As the landfill age increases the treatment of the leachate it produces will mainly focus on the nitrification of ammonia. Biological denitrification can then be achieved subsequently when an external organic substrate is added to the leachate.
It is important to stress that the design criteria for sewage treatment plants cannot be used for this polluted water. For the design and operation of biological leachate treatment plants specific points have to be respected and a leachate treatment process expert is essential. For many adopting the advice of sewage treatment experts for leachate-treatment has resulted in inappropriate process designs, and wasted money, time and effort.