Leachate

Leachate is the liquid that drains or 'leaches' from a landfill; it varies widely in composition regarding the age of the landfill and the type of waste that it contains.^[1] It can usually contain both dissolved and suspended material.

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Generation of Leachate

The generation of leachate is caused principally by precipitation percolating through waste deposited in a landfill. Once in contact with decomposing solid waste, the percolating water becomes contaminated and if it then flows out of the waste material it is termed leachate.^[2] Additional leachate volume is produced during this decomposition of carbonaceous material producing a wide range of other materials including methane, carbon dioxide and a complex mixture of organic acids, aldehydes, alcohols and simple sugars.

These risks of leachate generation can be mitigated by properly designed and engineered landfill sites, such as sites that are constructed on geologically impermeable materials or sites that use impermeable liners made of geotextiles or engineered clay. The use of linings is now mandatory within both the United States and the European Union except where the waste is genuinely inert. In addition, most toxic and difficult materials are now specifically excluded from land-fill. However despite much stricter statutory controls leachates from modern sites are found to contain a range of contaminants that may either be associated with some level of illegal activity or may reflect the ubiquitous use of a range of difficult materials in household and domestic products which enter the waste stream legally.

Composition of Leachate

When water percolates through the waste, it promotes and assists process of decomposition by bacteria and fungi. These processes in turn release by-products of decomposition and rapidly use up any available oxygen creating an anoxic environment. In actively decomposing waste the temperature rises and the pH falls rapidly and many metals ions which are relatively insoluble at neutral pH can be become dissolved in the developing leachate. The decomposition processes themselves release further water which adds to the volume of leachate. Leachate also reacts with materials that are not themselves prone to decomposition such as fire ash and cement based building materials changing the chemical composition. In sites with large volumes of building waste, especially those containing gypsum plaster, the reaction of leachate with the gypsum can generate large volumes of hydrogen sulphide which may be released dissolved in the leachate and may also form a large component of the landfill gas.

In landfill that receives a mixture of municipal, commercial, and mixed industrial waste, but excludes significant amounts of concentrated specific chemical waste, landfill leachate may be characterized as a water-based solution of four groups of contaminants ; dissolved organic matter (alcohols, acids, aldehydes, short chain sugars etc.), inorganic macro components (common cations and anions including sulfate, chloride, Iron, aluminium, zinc and ammonia), heavy metals (Pb, Ni, Cu, Hg, , and xenobiotic organic compounds such as halogenated organics, (PCBs, dioxins etc.). ^[3].

The physical appearance of leachate when it emerges from a typical landfill site is strongly odours yellow or orange coloured cloudy liquid. The smell is acidic and offensive and may be very pervasive because of hydrogen, nitrogen and sulfur rich organic species such as mercaptans

Leachate management

In older landfills and those with no membrane between the waste and the underlying geology, leachate is free to egress the waste directly into the groundwater. In such cases high concentrations of leachate are often found in nearby springs and flushes.

More modern landfills in the developed world have some form of membrane separating the waste from the surrounding ground and in such sites there is often a leachate collection series of pipes laid on the membrane to convey the leachate to a collection or treament location. However all membranes are porous to some limited extent so that over time low volumes of leachate will cross the membrane. The design of land-fill membranes is such that such low volumes should never have a measurable adverse impact on the quality of the receiving groundwater. A more significant risk may be the failure or abandonment of the leachate collection system. Such systems are prone to internal failure as landfills suffer large internal movements as waste decomposes unevenly and thus buckles and distorts pipes. If a leachate collection system fails, leachate levels will slowly build in a site and may even over-top the containing membrane and flow out into the environment. Rising leachate levels can also wet waste masses that have previously been dry triggering off a new way of active decomposition and leachate generation. Thus what appears to to be a stabilised and inactive site can become re-activated and restart significant gas production and exhibit significant changes in finished ground levels.

One method of leachate management that was more common in uncontained sites was leachate re-circulation in which leachate was collected and re-injected into the waste mass. This process greatly accelerated decomposition and therefore gas production and had the impact opf converting some leachate volume into landfill gas and reducing the overall volume of leachate for disposal. However it also tended to increase substantially the concentrations of conservative materials making it a more difficult waste to treat.

In some older landfills,leachate was directed to the sewers, but this can cause a number of problems. Toxic metals from leachate passing through the sewage treatment plant concentrate in the sewage sludge making it difficult or dangerous to dispose of to land without incurring a risk to the environment. In Europe regulations and control have improved in recent decades and toxic wastes are now no longer permitted to be disposed of to the Municipal Solid Waste landfills, and in most developed countries the metals problem has diminished. Paradoxically, however, as sewage treatment works discharges are being improved throughout Europe and many other countries, the sewage treatment works operators are finding that leachates are difficult waste streams to treat because they contain very high ammoniacal nitrogen concentrations, they are usually very acidic , they are often anoxic and, if received in large volumes relative to the incoming sewage flow, the lack of Phosphorous in particular can result in nutrient starvation for the biological communities that perform the sewage treatment processes making leachate a difficult to treat waste stream. However, within ageing municipal solid waste landfills this may not be a problem due asthe pH returns close to neutral after the initial stage of acidogenic leachate decomposition. Many sewer undertakers limit maximum ammonical nitrogen^[4] concentration in their sewers to 250 mg/l to protect sewer maintenance workers, as the WHO's maximum occupational safety limit would be exceeded at above pH 9 to 10, which is often the highest permitted alkalinity of permitted sewer discharges.

Many older leachate streams also contained a variety of synthetic organic species and their decomposition products, some of which had the potential to be acutely damaging to the environment.

Environmental impact

The risks from waste leachate are due to its high organic contaminant concentrations and high ammoniacal nitrogen. Pathogenic microorganisms and toxic substances that might be present in it are often cited as the most important, but pathogenic organism counts reduce rapidly with time in the landfill, so this only applies to the most fresh leachate.

Most landfills containing organic material will produce methane, some of which dissolves in the leachate. This could in theory be released in weakly ventilated areas in the treatment plant. All plants in Europe must now be assessed under the EU ATEX Directive and zoned where explosion risks are identified to prevent future accidents. The most important requirement is the prevention of discharge of dissolved methane from untreated leachate when it is discharged into public sewers, and most sewage treatment authorities limit the permissible discharge concentration of dissolved methane to 0.14 mg/l, or 1/10th of the lower explosive limit. This entails methane stripping from the leachate.

The greatest environmental risks occur in the discharges from older sites constructed before modern engineering standards became mandatory and also from sites in the developing world where modern standards have not been applied. There are also substantial risks from illegal sites and ad-hoc sites used by criminal gangs to dispose of waste materials. Leachate streams running directly into the aquatic environment have both an acute and chronic impact on the environment which may be very severe and can severely diminish bio-diversity and greatly reduce populations of sensitive species. Where toxic metals and organics are present this can lead to chronic toxin accumulation in both local and far distant populations. Rivers impacted by leachate are often yellow in appearance and often support severe overgrowths of sewage fungus

Other Types of Leachate

Leachate can also be produced from land that was contaminated by chemicals or toxic materials used in industrial activities such as factories, mines or storage sites. Composting sites in high rainfall also produce leachate.

See also

• Eluvium
• Illuvium
• Landfill mining

References

1. ^ Henry, J.; Heinke, G. (1996) Environmental Science and Engineering, Prentice Hall, ISBN 0-13-120650-8
2. ^ Washington State Departmentof Ecology, Solid Waste Landfill Design Manual
3. ^ Present and Long-Term Composition of MSW Landfill Leachate: A Review "Peter Kjeldsen, Morton A. Barlaz, Alix P. Rooker, Anders Baun, Anna Ledin, and Thomas H. Christensen"
4. ^ Dictionary of Environmental Science and Technology - Third Edition - Andrew Porteous - ISBN 0-471-63470-0, pg 25.