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AbstractThe aim of the paper is to presentsome fundamental conditions for thelandfill leachate generation, anddiscuss possible variations due to thecomposition of the deposited refuse.Consideration is given to changes inthe refuse, and its (future) influenceon the leachate composition withrespect to impurities. The majorfactors defining the composition arediscussed as well as the change intime of the leachate. However, thispaper is limited to what is defined asanaerobic phases inside the landfill. Inan extended perspective a “second”long lasting aerobic phase is likely tooccur. Some comments are given onthe leachate composition as found at anumber of Swedish landfills, typicallycontradicting a number of widespread“convictions”.

SammanfattningDenna uppsats diskuterar någragrundläggande villkor för lakvatten-bildning och dess sammansättning.

Speciellt belyses hur en deponis inne-håll av typiska föroreningar förändrasmed tiden. Särskild uppmärksamhetägnas förhållandena i deponins inre,och att deponin kan och bör ses somen anaerob reaktor, med påverkan pålakvattnets sammansättning. Särskiltbetonas, att metallerna fastläggs somsulfider under anaeroba förhållanden,liksom en långtgående hydrolyseringav kvävet äger rum. Slutligen visasfrån en anläggning, som följts nogaunder ett antal år, att det biologiskaslammet från reningsanläggningen harlåga eller mycket låga halter av de”vanligen” diskuterade riskablaföroreningsvariablerna.

IntroductionThe problem of waste handling is asold as mankind. The first writtendirective on waste handling isprobably the statement found in OldTestimony (Deut. 23: 12 – 13), whereinstructions are given on how to dealwith faeces. Another concern aboutsolid waste and refuse was expressed

Landfill leachate, generation,composition, and some findingsfrom leachate treatment atSwedish plants

By Stig Morling

Stig Morling is MSc employed at SWECO VIAK ABP.O. Box 34044 SE 100 26 STOCKHOLM, SWEDEN

E-mail address: stig.morling@sweco.se

by the north African philosopher IbnKhaldoun in the 14-th century, statingabout his fellow Arabs: “It is thedesert that follows the Arab, not theArab following the desert” (Freequotation) However, the waste pro-blem resembles most of the otherurban environmental problems bybasic conditions such as potentialhealth threats, odour problems(provided that organic matters weredisposed), the issues of collection andtransportation, and so forth.

The by far most common method tohandle solid waste has been – and isstill – seen in a global perspective – tocollect and deposit it in various typesof landfill. One inevitable problemcreated by the depositing the solidwaste is the formation of (landfill)leachate leaving the deposit andcausing potential water pollution. Theproblem has been identified in allindustrialised countries. The problemwas clearly identified during the1970-ies in Sweden.

In the following the discussion willbe limited to the landfill leachateproblem linked to landfills containingorganic wastes. This may be seen asobsolete from a bureaucratic point ofview, as the EU has implemented aprohibition to deposit organic matter.However, as will be described in thefollowing, a sanitary landfill willproduce leachate long time after itsclosure, and the leachate will containconsiderable concentrations of pollut-ing agents. So, even if a landfill isabandoned the responsibility tohandle the leachate will remain for avery long time.

Another aspect – easily forgotten –

is that the landfill technology will bedominant for many countries aroundthe world, independent of any EUdirectives! Thus the followingconsiderations will have relevance ina wide perspective.

Objectives of the paperThe objective of this paper is toprovide a short outline of leachategeneration and its composition, withspecial relevance for outlining ofrelevant treatment technologies. Thepaper focuses on the conditions in theSwedish theatre, as the work in the USand Europe on leachate treatment isoften governed by far more stringenteffluent standards than found inSweden. This fact has also resulted ina focus on very disparate treatmentmethods, as presented below. Anotherclear consequence may be that theapproach to leachate treatment notalways has been supported by aprocess engineering viewpoint.

Leachate generationLeachates from landfill are generatedby a number of factors, such as:

• Infiltration of ground water;• Infiltration of leachate into the

ground (a potential pollution ofthe ground water may occur);

• Rainfall (precipitation);• Water from the deposited waste,

mainly due to the static pressure;• Evaporation from the site.

Older landfills often were operated ina rather unsophisticated way; themanagement and operation seldomincluded adequate protection devices

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and with large open deposit areaswhere the waste was disposed. This inturn means that many “old” landfillsare exposed to comparatively largeamounts of water, emanating from thedifferent sources as defined above.

Some basic points that define theinfluence of rainwater are – apartfrom the magnitude and frequency ofthe precipitation – are the landfill area

directly exposed to receive rainwaterand allow it to percolate into thelandfill and the shape of the landfillallowing rainwater to “run off” fromthe landfill area as surface water. InFigure 1 is presented a schematicpicture of the water balance in alandfill. The figure is taken from aDoctoral Thesis presented by SamiSerti (2000).

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Figure 1. Shematic scheme of how leachate is genarete

Even if the water balance over adefined period of time – normally ayear – shows that the evaporationfrom the area is bigger than theprecipitation it may be essential tofocus on a shorter time when studyingthe water balance. As an example: Innormally dry areas very heavy rain-falls with short duration may cause alarge amount of leachate from thelandfill. This “run off” must beaddressed in a proper technical way –

either by storage lagoons or a “simpletechnique” for treating the leachate.The latter alternative may be interest-ing if there is a potential to treat theleachate is such a way that it may beused for irrigation. A crude leachate ishighly susceptible to be unfit forirrigation purposes.

By and by it has become moreapparent that landfill leachatemanagement called for a deeperunderstanding of the processes within

the landfills. A good understanding ofthe “inner” environmental processesin a landfill would facilitate theplanning of the landfill leachatemanagement. It would also provideneeded knowledge of the short termand long term composition of theleachate composition. And finally thiswould provide “input” data forleachate treatment design.

Processes defining theleachate composition´A sanitary landfill passes through fourstages with respect to the internal bio-logical process performance. The firstthree phases may be defined andcharacterised as follows; see Table 1.

The fourth phase that is labelled the“humic phase”. The knowledge of thisphase is limited as very few observedlandfills have entered this phase, seeSerti (2000), as it is expected to occurmore than 100 years – perhaps manycenturies after the closure of a sani-tary landfill. Thus as Serti hasdescribed in (3) most of the outlined(future) changes of the leachatecomposition is based not on obser-vations but on analogies and rationalhypotheses based on chemistry. In thefollowing the discussion will berefined to the three first phases in aland fill with special attention to theconditions during the second and thirdphases.

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First phase: Aerobic phase Duration some weeksCharacterisation of landfill leachate pH ~ 8

High levels of heavy metals

Second phase: Acidic (anaerobic) phaseDuration some yearsCharacterisation of landfill leachate pH ~ 5

High concentration of VFAHigh levels of BODRatio COD/BOD is low: 1.3:1 – 2.0:1High levels of NH4-N, organic N and PO4-P,High levels of heavy metals

Third phase: Methane phase (anaerobic)Duration > 100 yearsCharacterisation of landfill leachate pH ~ 7

Low concentration of VFALow levels of BODRatio COD/BOD is high 20:1 – 10:1High levels of NH4-N; Moderate to low levels of organic NVery low levels of PO4-PLow to very low levels of heavy metals, apart from Fe and Mn

Table 1. Simplified characterisation of the biological performance in a landfill related todisposal time, after Dr Sami Serti (2000)

It would be kept in mind that this“phasing” of the sequential processesin landfill is related to a number ofconditions, such as:

• The solid waste composition –especially if the solid wastecontains large or small amountsof organic matters, more or lesseasily degradable would influ-ence the velocity in the aerobic/anaerobic reactions;

• The formation of leachate, andits ability to transport matterswithin the landfill;

• The ambient temperature –climatic conditions. As anexample may be mentioned anewly opened landfill site inOujjda, Maroc, where –according to observations bySWECO engineers - the Methanephase seems to have startedwithin half a year from theopening of the deposit.

• The arrangement of the landfill –if the landfill is arranged withrather small deposit cells that areclosed and sealed after only oneor two years the anaerobic condi-tions would most likely beaccelerated. This in turn would“convert” the landfill cell into ananaerobic reactor.

As found in the table a landfillleachate treatment management mustconsider the two last phases, asmodern landfills are operated with anumber of cells, thus producing alandfill leachate of varying age - fromless than one year to several decades.

Another way to illustrate thecomplex reactions in a landfill isfound in Sami Serti (2000). TheFigure 2 present an illustration of alandfill with macro and micro condi-tions in a landfill. The figure presentsboth the short term and long terminfluences on the solid waste, and thusthe conditions for the leachate crea-tion and composition.

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Figure 2. Processes in landfills

A typical modern landfill designscheme is presented in Figure 3,showing a cross section of a landfillcell. It would be kept in mind that the

cell will be covered after completion,and anaerobic processes will beenhanced.

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Figure 3. Typical modern landfill design scheme

Some outlines oftreatment technologiesThree major factors emerging in themid 1980-ies and early 1990-iescontributed to the development oflandfill leachate treatment methods:

• A growing concern regarding thelandfill leachate composition,inter alia heavy metals contentand complex organic compounds,such as dioxins. To what extentsuch a concern was well-founded may be disputed;

• The insight of the environmentalimpact from non-oxidised nitro-gen (especially ammonia nitro-gen) became apparent;

The development of landfill leachatetreatment technologies in Swedenmay, somewhat simplified, be definedby five different main tendencies:

• A co-treatment with municipalwastewater in a “classic” treat-ment facility;

• Different treatment optionsbased on “simple” methods, suchas recycling the landfill leachateto the landfill, irrigation of“energy forest” areas, usingconstructed or natural wetlandsor infiltration;

• Adopted and modified classicbiologic treatment methods, toobtain efficient landfill leachatetreatment;

• Chemical physical treatmentmethods; such as ammoniastripping, chemical precipitationand activated carbon filtration.

• Use of “advanced” treatmentmethods, such as reversedosmosis and/or “hyper filtration”.

All these methods are currently in usearound Sweden. The methods will notbe discussed in detail in this paper;only one aspect with respect to treat-ment technologies will be discussed.Some of the very profound conside-

rations with respect to leachatecomposition are discussed andquestioned.

Landfill leachatecompositionThe following Table 2 illustratestypical composition of landfillleachate from Swedish plants. TheTable includes both a large landfill inthe western part of Sweden (calledTrestad, operated by TRAAB) andnew and old landfills.

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Large landfill (old) New landfill Old landfill

Number of observationsp 7.2 5 - 6 8 - 9Conductivity, mS/m 543 50 – 1,400 50 – 1,400Alkalinity, mekv/l 543Cl-, mg/l 920 (5) – 1,300 1,000 – 6,000BOD7, mg/l 27 1,000 -2,000 10 - 800COD, mg/l 480 1,000 -30,000 500 – 4,000Total P, mg/l 1.1 < 24 0.1 – 4.0NH4-N, mg/l 240 150 - 560 80 -370Total N, mg/l 330 800 100 - 400Suspended Solids, mg/l 5 n.a. n.a.

Table 2. Typical composition of leachates, from Swedish landfills

The overall picture of the landfillleachate composition is confirmed byan investigation made by Glixelli(2003). The report presents literaturedocumentations, covering reportsfrom Germany, Great Britain, Polandand Turkey on leachate composition.These reports show a vide range inconcentrations of the pollutants. Insome cases the referred figures aredivided into the disposal times(phases) as described above.

Organic contentAs discussed above is the organiccontent in the leachate “timedependant”. The most strikingdifference between the leachatecomposition from a “new” and “old”landfill is the ratio COD/BOD, andalso the content change of BOD. Thisis related to the anaerobic decompo-sition.

As pointed out this stage willnormally change into the methane

phase after a rather limited time, whenmost of the degradable organics aredecomposed of organics into methanegas and carbon dioxide. The ratioCOD/BOD increases and ends upbeing very high – often in the vicinityof 20/1. These circumstances will inturn influence the selection ofadequate treatment methods for theleachate. The anaerobic conditionsalso support the creation of metalsulphides, being one important reasonfor the long term leachate compo-sition with respect to heavy metalcontent.

Nutrient content in leachateMost leachates are rich in nitrogenand also normally contain low to verylow concentrations of phosphorous.The nitrogen content may, as shownabove be in the range 100 – 800 mgtotal N/l– typical levels found inSwedish leachate investigations.Spinoza and others report substanti-ally higher levels for leachates,according to Magnus Montelius(1996) is the range 50 – 50 000 mg/lof total N. The nitrogen ammoniumpart of the total nitrogen increases bytime, mainly due to anaerobichydrolysis of organic nitrogen intoammonia. Already during the acidicphase in the landfill the ammoniacontent represents the major part ofthe total nitrogen. For an old landfilloperated at methane phase theammonia nitrogen represents 85 to 95 %of the total nitrogen content in theleachate.

The phosphorous content on theother hand is found to be low to verylow in most leachates; see Table 2.

The phosphorous is to a large extenthydrolysed, and found as phosphates.The content is normally not sufficientto support an aerobic biological treat-ment; when such a treatment ispreferred an addition of phosphoricacid is arranged.

Chloride and other salt componentsLeachates from most landfills have arather high salt content, especiallywhen compared with municipalwastewater in Europe. For arid areas,such as Northern Africa the matter iseven more relevant. The salinity incrude municipal wastewater in thecity of Sfax, Tunisia is about 5 000 to7 000 mg/l.

The high salinity in leachates repre-sents problems in at least two ways:

• If the receiving stream is verypoor (small water flow) and avery limited dilution is expectedespecially the chloride content inthe leachate may constituent adischarge problem;

• High salinity and especially highchloride content create a verycorrosive environment, andwhen a treatment plant is built itbecomes essential to chose noncorrosive materials for theprocess equipment and adequateprotection for the concrete,provided that the plant is builtwith concrete reactors.

• The high chloride content willalso affect the COD analysis.This has been handled by addingmercury to the sample whenanalysing the COD. Never the

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less at very high chloride contentthe COD analysis will beseverely affected and the resultmay be dubious.

Other salinity components, such asSO4

- will add to concrete corrosion.

Heavy metal contentIn Figure 4 is illustrated in a graphicalway how some of the constituents inthe leachate change by time, especi-ally worth is looking at the heavymetal content in the leachate. An often

not well founded statement regardingthe leachate composition is that theheavy metal content is high. Asillustrated above this statement is nottrue when it comes to leachateemanating from a landfill in the“methane stage”. The figure illustratesthese conditions further. A number ofobservations at leachate treatmentplants in Sweden also support thestatement that heavy metals are notfound at high concentrations in theleachates.

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Figure 4. Development of leachate quality, pH and redox potential

As mentioned above the heavy metalcontent in landfill leachate has been aconcern. Thus the content has beenanalysed at a number of timesthroughout the operation time. Theresults of 11 different analyses showthe following: Only at very fewoccasions are heavy metal concen-trations been found that exceeds the

level for potable water in Sweden.Noticeable exceptions are Fe and Mnwith concentrations exceeding theconsent value for drinking water.Apart from this observation only fewanalysis are found with valuesexceeding the potable water qualityconsent value. This statement may beillustrated for Cd; see Figure 5.

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Figure 5. Cd content in treated landfill leachate at Köping SBR plant shown in increasingconcentration, not by time

This statement is further illustrated bya summary of analysis at the Isätra

landfill, town of Sala, some 120 kmnorth west of Stockholm; see Table 4.

The sludge in the biological facility,based on SBR (Sequencing BatchReactor technology) was accordinglyinvestigated with respect to the heavymetal content. Also in this case wasfound low to very low concentrationsof the “most susceptible” metals. In

Table 5 the measured concentrationsare compared with the Swedishguidelines for sludge quality related toagricultural use. The results are foundat the Köping leachate treatmentfacility, some 180 km west ofStockholm.

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Heavy metal Maximum 75 %:s Median- 25 %:s Minimum Limit value forvalue percentil value percentil value potable water,

Swedish standards

mg/l mg/l mg/l mg/l mg/l mg/lIron, Fe 13.0 8.5 6.5 5.1 3.5 1.0Manganese, Mn 1.9 1.4 1.3 1.2 0.5 0.3Zink, Zn 0.44 0.21 0.16 0.12 0.08 0.3Cobalt, Co 0.008 0.006 0.006 0.005 0.004Chromium, Cr 0.03 0.02 0.02 0.02 0.01 0.05Cadmium, Cd 0.00041 0.00028 0.00021 0.00012 0.00005 0.001Copper, Cu 0.027 0.014 0.012 0.009 0.007 0.20Nickel, Ni 0.03 0.02 0.02 0.02 0.02 0.05Led, Pb 0.019 0.008 0.004 0.003 0.002 0.010Arsenic, As 0.380 0.006 0.004 0.003 0.001 0.01Mercury, Hg 0.0002 0.0002 0.0002 0.0001 0.0001 0.001

Table 4. Heavy metal concentrations in leachate at Isätra landfill, Sweden. Water intoleachate treatment facility, observation period October 2001 through September 2002 (10observations).Values exceeding the limit values for potable water are presented in Italianbold.

Sludge from landfill Swedish EPAleachate treatment guidelikes

Lead 5.1 < 100Cadmium 1.0 < 2Copper 99 < 600Chromium, tot 7.7 < 100Mercury 0.06 < 2,5Nickel 7.7 < 100Zink 71 < 800

Table 5. Sludge content of heavy metals at a small Swedish leachate treatment facility, seeMorling (2006) compared with reuse requirements for agricultural use (mg/kg TS)

Other pollution parametersLeachate is normally regarded as apotentially toxic matter. This state-ment may be confirmed or rejected for

each leachate by toxicity tests. For thecase in Köping, Sweden and theleachate planning toxicity tests wereconducted. It was found that the

untreated leachate was toxic. Refe-rence: Laboratory tests conducted at amunicipal water laboratory, see Dahl(1998). After biological treatment bynitrification it was found that thetoxicity was substantially lower, oreven not easily detectable (communi-cation from Anita Höglund-Eriksson,at VAFAB, the owner and operator ofthe plant). The toxicity has often beenrelated to the presence of complexorganic matters, such as chlorinatedorganics (PCB dioxins). A problemconnected with this issue is that theconcentration of these compounds isoften found to be lower than theaccuracy of the analysis method. Thisin turn does not imply that theleachate is not toxic, only that theanalysed compound can not bedetected with accuracy. A normallymet concern is that the sludge wouldbe contaminated by these complexcompounds. At the Köping plant thebiological sludge has been analysedwith respect to some of these com-pounds. The outcome from three testsshows the following:

“Seven different PCB-compoundsregarded as potentially hazardous –have been analysed at threeoccasions. The concentrations onthese PCB-compounds were foundlow to very low. The analyses showedthat the sum of these seven com-pounds were < ∑ 0.02 mg/kg TS at allthree occasions. The Swedish EPAguidelines for agricultural usestipulates ∑ PCB < 0.4 mg/kg TS.

The nonylphenol concentration hasbeen measured in the sludge at threeoccasions. The results found were the

following: 12 mg/kg TS (2000-08-16);3.6 mg/kg TS, (2001-05-04) and 3.1mg/kg TS, (2002-04-19). Again theselevels would be regarded as low, oreven very low in comparison with theSwedish EPA criteria fornonylphenol; < 50 mg/kg TS.”, seeMorling (2006).

Discussion andconclusionsA good understanding of the condi-tions for landfill leachate treatmentstarts in an understanding of theleachate generation and the processesinside the landfill. The identifiedprocess phases inside the landfill–aerobic, acidic (anaerobic), methane(anaerobic) and humic (aerobic) willall determine different compositionsof the leachate. By these four phasesthe two intermediate are of centralimportance for the decisions onleachate management. Some centralpoints with respect to leachate gene-ration and its polluting potentials aresummarized as follows:

• It is essential to establish a modelfor water balance for a land filland from this model try toestimate the short term and longterm leachate generation. Amongthe most important factors arethe annual and peak precipitationfigures, the evaporation. Whenplanning a new landfill it wouldbe indispensable to prevent thetransportation of ground waterinto the landfill and the perco-lation of leachates into the ground.

• The understanding of themethane phase is crucial both

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.

.

with respect to the potential ofenergy content in the generatedgas and how to address leachatemanagement;

• The crucial question regardingleachate handling with respect toenvironmental protection is howto quantify potentially hazardouscompounds. The complex or-ganic matters, such as dioxinsare often found at concentrationsbelow the accuracy level of theanalysis method.

• The often advocated standpointthat the heavy metal content inleachates is “high” is often founddisputable, apart from Fe andMn. These metals are found inconcentrations considerablyhigher than the permit levels forpotable water (in Sweden).

• In the Swedish theatre there hasbeen a lack of understanding fora process oriented perspective onleachate treatment. This may bereflected in the two examplesgiven in this paper, whereinsufficient knowledge in pro-cess engineering points out dubiouspathways for process design.

• An important part in leachatetreatment planning would be toperform treatability tests.

• The needs for realistic criteria oneffluent standards may includetoxicity tests on treated water.

• The sludge quality with respectto polluting agents, both heavymetals and complex organiccompounds would be investi-gated from leachate treatmentfacilities, in order to quantify thecontent.

AcknowledgementsDr Sami Serti at SWECO VIAK hasgiven the outlines for the charac-terisation of the biological perfor-mance in a landfill and also valuablecontributions on the outlines of thispaper.

References1. Sami Bozkurt (Serti) “Assessment

of the Long-Term Transport Pro-cesses and Chemical Evaluation inWaste Deposits”, Doctoral Thesis,Royal Institute of Technology,Stockholm, 2000

2. William Hogland and others “Landfilling, First Edition” Departmentof Water Resources Engineering,Lund Institute of Technology/LundUniversity, 1996

3. Glixelli, Thomas M. (2003)“Treatment of ammonium-richwaste streams with deammonifi-cation process”, Master of ScienceThesis, Cracow University ofTechnology, Cracow and RoyalInstitute of Technology, Stockholm

4. Stig Morling (2006) “Swedishexperiences of landfill leachatetreatment using Sequencing BachReactors for nitrogen removal”lecture given at IPSI conference inBled, Slovenia, December 2006(under publication)

5. Lennart Dahl and others: “Norsaavfallsanläggning, Reningsförsökangående lakvatten, Nyköping1998-05-14) performed at theMunicipal laboratory at NyköpingWWTP

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