What is Leachate? Landfill Leachate Explained

A classic image of leachate draining out of the landfilled
waste in a landfill

Leachate is formed when water passes through the waste in the landfill cell. The precipitation can be from rain, melted snow or the waste itself. As the liquid moves through the landfill many organic and inorganic compounds, like heavy metals, are transported in the leachate. This moves to the base of the landfill cell and collects.

To minimise the amount of leachate generated, modern landfills are built in sections, with a low permeability cover placed over the waste as soon as possible to limit the infiltration of rainwater. Present-day landfills also have low permeability liner systems and collection pipes to remove the leachate that forms so that it can be taken to a waste water treatment plant. A modern landfill that is properly sited with respect to the local geology and that has a properly designed and constructed liner, leachate collection system, and low permeability cover has limited potential to contaminate groundwater.

The amount of leachate produced is directly linked to the amount of precipitation around the landfill.2 The amount of liquid waste in the landfill also affects the quantity of leachate produced. A large landfill site will produce greater amount of leachate than a smaller site.

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 number of different contaminant, probably the most significant of which is ammonia. Ammonia is toxic to fish and other aquatic life and therefore it must therefore be strictly controlled.

Water that infiltrates and percolates through landfills produces leachate, which may contain undesirable and toxic chemicals. The character of the leachate depends to a large degree on the type of waste stored and the hydrological, and chemical conditions of the landfill. Construction practices for modern landfills has significantly reduced the threat of leachate contaminating water resources. v

Although the number of landfills in the United States has decreased steadily from 8,000 in 1988 to 1,767 in 2002, landfills are much larger and total landfill capacity has remained relatively constant. Furthermore, many historic landfills exist and these can still pose a risk of environmental contamination. A subsurface leachate plume can form in locations where the landfill site produces leachate which leaks into an alluvial aquifer. On many occasions the leachate will become filtered and be self-treated in the rocks of the aquifer, but some toxic contaminants are much more persistent than other and many only need to be present in very low concentrations before they can have damaging effects on health - especially the health of children.

Some landfills have accepted municipal wastes for more than 60 years until being closed. Now that is a lot of potential contamination if it gets into the ground and enters boreholes and contaminates drinkign water, as has often happened. Ground water at the site is contaminated by dozens of organic compounds, many of which are toxins and carcinogens. Investigations at sites like these are always necessary to focus on the interactions between the contaminant plume, the aquifer, and any rivers to which the aquifer discharges, and whether the plume may one day reach wells and boreholes. The are numerous municipal landfills where in the past this has been a problem and new problems, while not so common nowadays due to modern landfill lining systems, still do occur.

So, it is clear that the careful management of leachate, the liquid byproduct of landfills, is an important component of landfill operations. Unlike domestic and other industrial wastewaters, the quantity and quality of leachate varies widely. The reason for the wide range is the result of both environmental (local rainfall and evaporation conditions) and operational factors (how much surface area is open to the rain etc).

Leachate, that's water containing dissolved chemicals; is the term applied particularly to fluids escaping from waste-disposal sites. The waste-disposal sites are by definition designed to minimise adverse effects of waste disposal, minimising by lining and collecting the leachate produced. However, many are poorly designed and are leaking tehse liquids, which are contaminating groundwater. Actually, landfill leachate is the liquid that is the product of the liquid content of the waste, infiltrating precipitation, and groundwater if the waste is below the groundwater table or they leak out of the bottom of the landfilled waste and drop into the water tble.

Leachate is a complex mixture of dissolved and colloidal organic matter and inorganic compounds and ions. Materials placed in landfills include such things as municipal garbage (trash), demolition debris, sludge from wastewater treatment plants (but not in the EU any longer), incinerator ash (in hazardous waste landfills), foundry sand and other foundry waste, and toxic and hazardous materials.

Treatment of leachate derived from landfill sites can be problematic due to factors such as toxicity, and changes in the influent composition over quite short periods due to weather conditions. As a landfill ages, the biodegradable organic proportion of the wastewater tends to decrease whilst the ammonia concentration increases. This can often lead to a treatment plant, designed for one type of leachate, receiving a very different wastewater to that which it was designed to treat.

According to some recently held studies, a 10 acre landfill will have average leakage rate around 0.2 to 10 gallons per day. Thta is simply due to the fact that for engineers to prevent any leaks over very large areas is no readily achievable when a small hole a few millimetres across can leak enough to cause real problems. Therefore, for any effective landfill, to limit how much leachate will leak out to have the leachate collection system, which whisks the leachate away quickly before it can seep out of an hole, is vital.

When water dribbles down through the landfill. It slowly travels towards the bottom of the landfill. Once there it can be pumped out with the help of a series of pumps and pipes. These pipes are usually vertical but can be laid along the bottom of the landfill, from which the leachate extraction system collects the leachate. In some cases leachate collection pipes have become crushed by the huge weight of the enormous amount of garbage in most modern landfills, plus the heat in the landfill. Unless the right matrials are used corrosion will further damage the extraction pipework, weaken it, and allow blockages to occur. If this happens and contaminated water or waste remains in the landfill, it will result into the build up of fluids just like the bathtub at bathtime.

To control the eleacahte and keep the amount produced low, cover or a low-permeability (clay or equivalent material) cap is placed over modern landfills with the added advantage that landfill gas can then be captured and utilised. It is like an umbrella over the landfill so the water can be kept away. Generally it is designed in a way that it has several sloped layers to protect landfill from rain. These layers can be of clay or a membrane liner. For more protection, the low-permeability cover is coated with a thin layer of gravelly or sandy soil. In order to protect the low-permeability cover, this layer of soil is further overlaid by topsoil in which vegetation can grow roots. Absence of cap or cover can allow the rain to enter in the landfill and hence leachate could build-up to the level where bathtub is overwhelmed.

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