Define leachate | Dictionary and Thesaurus

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« DefineleachableDefineleaches »Go to User ContributedDictionaryGo to ExtensiveDefinitionUser ContributedDictionaryEnglishEtymologyThe liquid produced when water percolates through anypermeablematerial.ReferencesExtensive DefinitionLeachate is the liquid that drains or 'leaches'from a landfill; itvaries widely in composition regarding the age of the landfill andthe type of waste that itcontains. It can usually contain both dissolved and suspendedmaterial.Generation of LeachateThe generation of leachate is causedprincipally by precipitationpercolating through waste deposited in a landfill. Once in contactwith decomposing solid waste, the percolating water becomescontaminated and if it then flows out of the waste material it istermed leachate. Additional leachate volume is produced during thisdecomposition of carbonaceous material producing a wide range ofother materials including methane, carbon dioxide and a complexmixture of organic acids, aldehydes, alcohols and simplesugars.These risks of leachate generation can bemitigated by properly designed and engineered landfill sites, suchas sites that are constructed on geologically impermeable materialsor sites that use impermeable liners made of geotextiles or engineeredclay. The use of linings isnow mandatory within both the UnitedStates and the EuropeanUnion except where the waste is genuinely inert. In addition,most toxic and difficult materials are now specifically excludedfrom land-fill. However despite much stricter statutory controlsleachates from modern sites are found to contain a range ofcontaminants that may either be associated with some level ofillegal activity or may reflect the ubiquitous use of a range ofdifficult materials in household and domestic products which enterthe waste stream legally.Composition of LeachateWhen water percolates through thewaste, it promotes and assists process of decomposition by bacteria and fungi. These processes in turnrelease by-products of decomposition and rapidly use up anyavailable oxygen creating an anoxic environment. In activelydecomposing waste the temperature rises and the pH falls rapidly andmany metals ions which are relatively insoluble at neutral pH can be becomedissolved in the developing leachate. The decomposition processesthemselves release further water which adds to the volume ofleachate. Leachate also reacts with materials that are notthemselves prone to decomposition such as fire ash and cement based building materialschanging the chemical composition. In sites with large volumes ofbuilding waste, especially those containing gypsum plaster, the reaction ofleachate with the gypsum can generate large volumes of hydrogensulfide which may be released in the leachate and may also forma large component of the landfill gas.In landfill that receives a mixture of municipal,commercial, and mixed industrial waste, but excludes significantamounts of concentrated specific chemical waste, landfill leachatemay be characterized as a water-based solution of four groups ofcontaminants ; 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, , andxenobiotic organiccompounds such as halogenated organics, (PCBs, dioxins etc.). .The physical appearance of leachate when itemerges from a typical landfill site is strongly-odoured yellow- ororange-coloured cloudy liquid. The smell is acidic and offensiveand may be very pervasive because of hydrogen, nitrogen and sulfurrich organic species such as mercaptans.Leachate managementIn older landfills and those with nomembrane between the waste and the underlying geology, leachate isfree to egress the waste directly into the groundwater. In suchcases high concentrations of leachate are often found in nearbysprings and flushes. As leachate first emerges it can be black incolour, anoxic and may be effervescent with dissolved and entrainedgases. As it becomes oxygenated it tends to turn brown or yellowbecause of the presence of Iron salts in solution and insuspension. It also quickly develops a bacterial flora oftencomprising substantial growths of Sphaerotilus.Membrane and collection for treatmentMore modern landfillsin the developed world have some form of membrane separating thewaste from the surrounding ground and in such sites there is oftena leachate collection series of pipes laid on the membrane toconvey the leachate to a collection or treatment location. For anexample of a treatment system with only minor membrane use, seeNantmelLandfill Site.All membranes are porous to some limited extentso that over time low volumes of leachate will cross the membrane.The design of landfill membranes is such that such low volumesshould never have a measurable adverse impact on the quality of thereceiving groundwater. A more significant risk may be the failureor abandonment of the leachate collection system. Such systems areprone to internal failure as landfills suffer large internalmovements as waste decomposes unevenly and thus buckles anddistorts pipes. If a leachate collection system fails, leachatelevels will slowly build in a site and may even over-top thecontaining membrane and flow out into the environment. Risingleachate levels can also wet waste masses that have previously beendry triggering off a new way of active decomposition and leachategeneration. Thus what appears to to be a stabilised and inactivesite can become re-activated and restart significant gas productionand exhibit significant changes in finished ground levels.Re-injection into landfillOne method of leachatemanagement that was more common in uncontained sites was leachatere-circulation in which leachate was collected and re-injected intothe waste mass. This process greatly accelerated decomposition andtherefore gas production and had the impact of converting someleachate volume into landfill gas and reducing the overall volumeof leachate for disposal. However it also tended to increasesubstantially the concentrations of contaminant materials making ita more difficult waste to treat.Removal to sewer systemIn some older landfills, leachatewas directed to the sewers, but this can cause anumber of problems. Toxic metals from leachate passing through thesewagetreatment plant concentrate in the sewage sludge making itdifficult or dangerous to dispose of to land without incurring arisk to the environment. In Europe regulationsand control have improved in recent decades and toxic wastes arenow no longer permitted to be disposed of to the Municipal SolidWaste landfills, and in most developed countries the metals problemhas diminished. Paradoxically, however, as sewage treatment worksdischarges are being improved throughout Europe and many othercountries, the sewage treatment works operators are finding thatleachates are difficult waste streams to treat because they containvery high ammoniacal nitrogen concentrations, they are usually veryacidic , they are often anoxic and, if received in large volumesrelative to the incoming sewage flow, the lack of Phosphorus inparticular can result in nutrient starvation for the biologicalcommunities that perform the sewage treatment processes makingleachate a difficult to treat waste stream. However, within agingmunicipal solid waste landfills this may not be a problem as the pHreturns close to neutral after the initial stage of acidogenicleachate decomposition. Many sewer undertakers limit maximumammonicalnitrogen concentration in their sewers to 250 mg/l to protectsewer maintenance workers, as the WHO'smaximum occupational safety limit would be exceeded at abovepH 9 to 10,which is often the highest permitted pH of permitted sewerdischarges.Many older leachate streams also contained avariety of synthetic organic species and their decompositionproducts, some of which had the potential to be acutely damaging tothe environment.Environmental impactThe risks from waste leachate are dueto its high organic contaminant concentrations and high ammoniacal nitrogen. Pathogenicmicroorganisms andtoxic substances thatmight be present in it are often cited as the most important, butpathogenic organism counts reduce rapidly with time in thelandfill, so this only applies to the most fresh leachate. Mostlandfills containing organic material will produce methane, some of which dissolvesin the leachate. This could in theory be released in weaklyventilated areas in the treatment plant. All plants in Europe mustnow be assessed under the EU ATEX Directive and zoned where explosion risks areidentified to prevent future accidents. The most importantrequirement is the prevention of discharge of dissolved methanefrom untreated leachate when it is discharged into public sewers,and most sewage treatment authorities limit the permissibledischarge concentration of dissolved methane to 0.14 mg/l, or1/10th of the lower explosive limit. This entails methane strippingfrom the leachate.The greatest environmental risks occur in thedischarges from older sites constructed before modern engineeringstandards became mandatory and also from sites in the developingworld where modern standards have not been applied. There are alsosubstantial risks from illegal sites and ad-hoc sites used bycriminal gangs to dispose of waste materials. Leachate streamsrunning directly into the aquatic environment have both an acuteand chronic impact on the environment which may be very severe andcan severely diminish bio-diversity and greatly reduce populationsof sensitive species. Where toxic metals and organics are presentthis can lead to chronic toxin accumulation in both local and fardistant populations. Rivers impacted by leachate are often yellowin appearance and often support severe overgrowths of sewagefungusOther Types of LeachateLeachate can also be produced fromland that was contaminated by chemicals or toxic materials used inindustrial activities such as factories, mines or storage sites. Composting sitesin high rainfall also produce leachate.ReferencesExternal linksAnintroduction to leachateLandfill LeachateFreeaccess leachate aeration system drafting software for diffusersActivated Carbon Landfill Leachate TreatmentSolidwaste. Washington Department of Ecologyleachate in Catalan: Lixiviatleachate in Spanish: Lixiviadoleachate in Persian: شیرابهleachate in French: Lixiviatleachate in Italian: Percolatoleachate in Portuguese:Chorume

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