Leachate can be any water that once it has drained through a medium takes up chemicals and solid materials during its passage. The term leachate is most often used in connection with landfills. Landfill leachate is contaminated 'dirty' water that is produced when rainwater comes into contact with waste materials on the area of the landfill. It contains a large number of different contaminants, probably the most significant of which is ammonia.
The second most common type of leachate encountered is the black odorous run-off from manure heaps and from some composting facilities.
If leachate is allowed to leak from a landfill it will usually cause pollution both locally around the waste, and it may form a plume of contamination within groundwaters it enters and a plume of groundwater pollution may move away from the landfill over time to contaminate wells and any drinking water taken from them.
Leachate forms from both the combination of liquids that are dumped in a tip or landfill, and liquids that form through decomposition of wastes, as precipitation filters through the wastes. It is a liquid which is mostly organically contaminated but which will also contain low levels of most of the liquids disposed of in the landfill from which it emanates.
Sometimes leachate can be produced by a landfill, which is sealed by a low permeability capping layer. That is normally the result of a rise in pressure on the landfill when additional loads are placed on the landfill forcing compression of the structure or the presence of excess water.
Leachate is produced by the percolation of precipitation through a landfill (from rainfall and snowmelt) once it penetrates the landfill's daily, intermediate, or final cover. However, the quantity that penetrates a well vegetated cover is lower than many expect, due to the evaporation from the surface, which will include the transpiration from the leaves of he foliage on he surface.
As the water passes vertically downward through the waste mass, it comes into contact with the waste, picking up chemical contaminants and biological impurities as it goes, and the deeper the waste he stronger it gets. It also gets stronger if it stands from a long while in the waste which is not highly surprising.
There are two main types of leachate produced in landfills which contain biological municipal solid waste (MSW). These are known as acetogenic leachate and methanogenic leachate. The methanogenic type is often black in color always smelly and may smell of bad eggs. Methanogenic only has only a slight smell and is brown or golden colored.
Acetogenic leachate is the young leachate which is produced in a landfill first. It has a very high Chemical Oxygen Demand (COD) which can be as high as hundreds of thousands of milligrams per litre for short periods, soon after the cells of he organic waste break open or "lyse" and the complex compounds which make up live cell tissue drain out of the cells.
The demand for oxygen in a modern quite rapidly filled landfill, is so intense that within a few months of deposition a new cell of waste will lack oxygen within the airspaces. Oxygen will be present in the waste which is then said to be in an anoxic condition.
Over time the original oxygen in the waste and in the leachate becomes depleted as biological fermentation proceeds, and at some point ancient bacteria which have always been present in airless bogs and swamps and lie dormant in our environment multiply and take over the reaction within he waste.
These are known as methanogenic bacteria. Why are they called that? Well, it is simple really! They produce the gas known as methane!
All that brings me around to the point where I can now define methanogenic leachate. Yes. You have guessed it. Methanogenic leachate is the leachate that is produce by a methane producing anaerobic landfill. By the time it has become methanogenic however, the process of decomposition by fermentation has reduced the COD to quite possibly 1/100 th of its maximum value, or even 1/1000 th.
However, the leachate is hardly any less toxic to aquatic life, because the ammonia present in dissolved and gaseous forms remains high, and thus as we stated earlier is one of the most important contaminants in leachate.
That is the story of leachate from young (acetogenic) to old (methanogenic).
Visit Steve Last's web site for Part 2 of this article at The Leachate Web Site. he is an expert on leachate treatment and has been designing and building leachate treatment plants for more than 20 years.
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