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The purpose of this section of our web site is to inform newcomers to the subject about the most common liquid referred to as leachate - landfill leachate.
In fact the term "leachate" is so often applied to landfill leachate, both within the waste management industry and outside, that it is easy to forget that leachate is the term used for the liquid produced by the action of “leaching” when water percolates through any permeable material other than through landfilled wastes.
No matter how hard we try to avoid generating waste, by waste reduction, re-use, recycling, composting and many other methods of waste pre-treatment prior to landfilling, landfilling will continue and leachate generation is a problem which is here to stay.
Even if all the landfills could be closed, and the generation of new leachate could be stopped today, we would still have to manage the leachate from many thousands of existing operational and closed landfill sites for generations.
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Definition
The Chambers Dictionary contains the following entry:
leach'ate (noun) a liquid that has percolated through or out of some substance; a liquid that has been polluted or made toxic by percolating through rubbish; a solution obtained by leaching.
The United States EPA provides a more specific definition:
"Leachate is any liquid including any suspended components in the liquid, that has percolated through or drained from hazardous waste."
In this context hazardous waste can be very broadly defined and could for example include a salt store, or contaminated soils.
In this discussion we will concentrate on leachate derived from rubbish, and in general from municipal (domestic and commercial) solid wastes (also often called sanitary wastes).
The History of Leachate
Leachate has been generated ever since man first congregated to form settlements, and dug "middens" or pits, or created dung heaps. Of course the size of these was small and in all but clay soils, the leachate formed would have largely soaked away unheeded, in small volumes which would have created no significant impact other than potentially the positive enhancement to nearby plant growth by locally elevating nutrient levels.
The hazards posed by the lack of good drainage and treatment o f societies liquid wastes (sewage) remained by far the most acute, in all civilisations until the latter quarter of the 20th century, largely due to the vastly greater capability of sewage to provide a direct vector for human disease. When considering landfill leachate hazards, this distinction should always be made. Enteric organisms (of the stomach) and pathogenic organisms (capable of causing disease) are simply not present in leachate to anything like the same degree as in sewage.
This remained the case until the late 1950s in the US, and the mid-seventies in the more economically advanced nations within Europe and Asia. Until this time every home possessed one or more open fire, and many possessed a garden or yard, in which green wastes would either be burnt or routinely be composted in order to provide a free source of compost for the vegetable garden. Indeed, the perceived need for good compost or manure was such that, even during my post war (1950s) youth in South London, when deliveries made by house drawn vehicle resulted in the presence of "droppings" in the street, these would be assiduously collected by the residents for composting.
As a result pre-and post Second World war wastes comprised largely ashes, and inert materials, there were of course progressively more metals present from tin cans, but the degree to which putrescible kitchen wastes were present was low, and with plenty of alkaline material present to provide a buffering capacity the pH would seldom drop to bring the hazardous heavy metals into solution. In most towns in any event the sewer systems were combined foul and stormwater systems, so if local urban tips needed draining they would simply discharge to sewer. It was not a problem, as the sites were small and the contaminant strengths low, or comparable with domestic foul sewage.
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Click on this image to find out more about leachate history
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In addition, with road transport links as yet still poorly developed, landfill sites (tips or dumps) would be small, and with settlement rates low, these would often be redeveloped. In fact in historical times middens would be built upon as a matter of course and the town ground level would, in reality, gradually rise! Without this general build-up of material one wonders how much of current archaeological evidence would have survived!
The history of leachate as currently found throughout the world, really starts when the combination of greater affluence, central heating in cold climes to remove the previous high ash content, the arrival of plastics, and the recognition of the need for landfill site control and regulation, caused landfills to become much larger, and although more easily regulated, potentially much more environmentally damaging.
In the UK it was the implementation of the 1974 Local Government Act which finally, and indirectly, boosted the problem of landfill leachate into the consciousness of the ordinary man. Although under this act the local government departments responsible for waste disposal were also responsible for environmental standards enforcement (both poacher and gamekeeper), standards of environmental monitoring rose rapidly. For the first time there came the realisation that waste disposal to landfill in its modern guise could be much more damaging in some places than others, due to local geology and the proximity of water usage for public supply, and at some locations the impact on water resources, rivers and streams could be disastrous in terms of possible loss of resources and ecological damage to rivers and streams.
In the UK by the late 1970s research was underway, on a newly coined concept, which many said was no more than an ill-founded attempt to justify the unjustifiable. Landfill sites were being chosen with permeable underlying geological strata to avoid the build-up of leachate and these needed to be justified scientifically. It was the theory of "dilute, attenuate and disperse", or the use of the natural pollutant attenuation capacity of many soils to arrest the outward migration of contaminants for long enough to provide biological filtration and cation exchange, which would, it was thought, limit the impact of the landfill to a very restricted “unsaturated”  zone around the landfill. As we will see in later sections of this discussion this theory would depend on the capability of the unsaturated zone to remove ammoniacal nitrogen, and to do this either physical treatment (eg cation exchange) would need to be present, or the preservation of an aerobic (natural filter) zone under the landfill prior to discharge to groundwater would be essential.
The technical and economic debate continued until the 1970s in the UK, about the acceptability of "dilute, attenuate and disperse", while many European countries had already decided only to select sites in groundwater free clay geological conditions, or to seal each site with an engineered lining. The issue of whether to line landfills or not, was finally decided by the EU Landfill Directive, and now all the current member states must comply with the Directives policy which require all except truly inert landfills, to be lined with an impermeable membrane, to prevent the escape of leachate.
In the US the accent had also developed quite rapidly from lining in principle, into the use of multiple lining layers, most other nations have also since followed the same path, and landfill sites all around the world began to be built to comply, and began also to fill with the water percolating down through them from rainfall - and leachate as we know it today began to be formed.
Legislation and Leachate in Europe
In Europe the following legislation, as enacted into the laws of each member state, is of relevance when considering leachate discharges and the permitting of leachate treatment plants:-
- The EU Landfill Directive;
- The EU Groundwater Directive;
- The EU Urban Wastewater Directive - where discharges are pre-treated before discharge to public foul sewer;
- The EU Dangerous Substances Directive;
- The EU Water Framework Directive.
Potential Hazards when Leachate is Discharged to Sewers
The discharge of leachate to public foul sewers is common. Leachate is often too strong for acceptance at public Wastewater Treatment Works (WwTWs), but where these do accept leachate untreated, hazards exist from the possible presence of certain volatile materials.
These hazards usually arise as follows:-
- Leachate from methanogenic landfills will contain dissolved methane when it emerges, and this dissolved methane, if present in a poorly ventilated sewer, can in theory cause explosions;
- High concentrations of ammoniacal nitrogen often found in leachate may also (especially at high pH-values), cause health problems to sewer maintenance staff.
The process adopted for the removal of methane is called "methane stripping". Visit our Methane Stripping web site for further information, or here to see a case study of one of these plants, at Red Moss (Closed) Landfill, Horwich, Bolton.
Ammoniacal nitrogen, if present at a worst case pH of approximately 10 in a foul sewer would, at a concentration of 250 mg/l attain the permitted long term exposure limit, at 15 degrees Celsius. Many water companies therefore set a maximum permitted discharge concentration at this level. (The WRC Publication, TR142, The Determination of Safe Limits for the Discharge of Volatile Materials to Sewers, June 1980, Toogood & Hobson provides the derivation of the above.)
Back to top. Next page. See also Leachate Treatment - an Overview of Processes.
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