the potential for aeration of msw landfills to accelerate completion

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Waste Management 28 (2008) 1039–1048

The potential for aeration of MSW landfills to accelerate completion

Charlotte Rich, Jan Gronow, Nikolaos Voulvoulis *

Centre for Environmental Policy, Imperial College London, London SW7 2BP, UK

Accepted 5 March 2007Available online 24 May 2007

Abstract

Landfilling is a popular waste disposal method, but, as it is practised currently, it is fundamentally unsustainable. The low short-termfinancial costs belie the potential long-term environmental costs, and traditional landfill sites require long-term management in order tomitigate any possible environmental damage. Old landfill sites might require aftercare for decades or even centuries, and in some casesremediation may be necessary. Biological stabilisation of a landfill is the key issue; completion criteria provide a yardstick by which thesuccess of any new technology may be measured. In order for a site to achieve completion it must pose no risk to human health or theenvironment, meaning that attenuation of any emissions from the site must occur within the local environment without causing harm.Remediation of old landfill sites by aerating the waste has been undertaken in Germany, the United States, Italy and The Netherlands,with considerable success. At a pilot scale, aeration has also been used in newly emplaced waste to accelerate stabilisation. This paperreviews the use of aerobic landfill worldwide, and assesses the ways in which the use of aerobic landfill techniques can decrease the risksassociated with current landfill practices, making landfill a more sustainable waste disposal option. It focuses on assessing ways to utiliseaeration to enhance stabilisation. The results demonstrated that aeration of old landfill sites may be an efficient and cost-effective methodof remediation and allow the date of completion to be brought forward by decades. Similarly, aeration of newly emplaced waste can beeffective in enhancing degradation, assisting with completion and reducing environmental risks. However, further research is required toestablish what procedure for adding air to a landfill would be most suitable for the UK and to investigate new risks that may arise, suchas the possible emission of non-methane organic compounds.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

In the UK the primary disposal method for commercial,industrial and municipal solid waste is landfilling, withapproximately 100 million tonnes of solid waste sent tolandfills each year in England and Wales: this represents23% of the total waste in England and Wales, and 60%of municipal solid waste (MSW) (Environment Agency,2005).

The overriding aim of landfilling is to dispose of wastewithout causing harm; however, a secondary aim is toachieve a state where management and monitoring are nolonger necessary. This removes any environmental con-cerns, eliminates operating costs and can allow the landto be reused, this last being an important consideration

0956-053X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2007.03.022

* Corresponding author. Tel.: +44 20 7594 7459; fax: +44 20 7581 0245.E-mail address: [email protected] (N. Voulvoulis).

in a densely populated country such as the UK. A site isdetermined complete when its undisturbed state poses norisk to either human health or the environment, althoughunder this definition a complete site is not necessarily anuncontaminated one; pollutants remain in the waste andmay be released if the site is disturbed.

In the USA a landfill is considered biologically stabilisedwhen it is producing negligible gas, any leachate producedis not considered to be a pollution hazard, and maximumsettlement has occurred; this tends to correspond to thecompletion of anaerobic microbial activity (Anex, 1996).In England and Wales the Environment Agency uses a riskassessment approach. The criteria for completion are notabsolute, as two landfills might for example produce simi-lar leachate but be in areas of differing sensitivity, forexample by having different hydrogeological characteristicsand hence different pathways for pollutants to follow, or byhaving differently sensitive receptors of those pollutants.

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1040 C. Rich et al. / Waste Management 28 (2008) 1039–1048

What is an acceptable level of risk needs to be decided inadvance; in England and Wales certificates of completionfor sites other than inert sites are few and far between.

Factors assessed in the risk assessment for completioninclude: quantity, quality and generation rate of leachate;generation, flow and concentration of gas; trace composi-tion of the gas; potential for leachate or gas to be generatedin future; physical stability of the waste and associatedstructures; and presence of particular problem wasteswhich could present a risk in the future (EnvironmentAgency, 2003). Any new landfill technique that could bringabout improvements in any of these measures would serveto bring forward stabilisation, leading to a more sustain-able landfill practice.

One technique that can help to achieve this is aeration,either of new waste immediately following emplacementor of older sites as a remediation measure. A variety of pro-jects worldwide have tested the possibilities of landfill aer-ation in achieving different objectives (for examplereduction of emission potential with a view to reducingthe aftercare period (Heyer et al., 2005), remediation ofold abandoned sites (Cossu et al., 2003), and odour reduc-tion (Jacobs et al., 2003)), and have found the results to besuccessful. Aeration reduces the long-term emission poten-tial at an early stage, which is a more sustainable optionthan the current practice of reliance on the longevity ofhighly engineered liners to contain waste with a high pollu-tion potential.

In order to assess the benefits of any new landfill tech-nique, there needs to be a meaningful framework thatcan be applied equally to a traditional landfill and newtechniques; the concept of completion provides a yardstickby which to judge performance. Measuring the success ofaerobic landfill techniques in terms of the requirementsfor completion allows the performance of aerobic tech-niques to be assessed relative to both current and potentialfuture anaerobic landfill practices.

Until now, aerobic landfill techniques have not beenused in the UK. However, the increasing problem of tradi-tional landfills drives a need to find new waste solutions forthe country, and the success of projects elsewhere leads tothe conclusion that may be aeration could be applied in theUK. This paper reviews such elements of landfill aerationworldwide as are pertinent to UK application, and assessesthe ways in which the use of aerobic landfill techniques candecrease the risks associated with current landfill practices,making landfilling a more sustainable waste disposaloption. It focuses on assessing ways to utilise aeration toenhance stabilisation in UK landfills.

1.1. Aerobic landfill: processes and products

A traditional anaerobic landfill has five stages (Fig. 1);the process starts with an aerobic stage, moves throughthree anaerobic stages and finishes with the reintroductionof aerobic conditions. These stages are not consecutive,instead occurring to a large extent simultaneously, with dif-

ferent stages being more dominant in different parts of thelandfill at different times, requiring transport processes.The conditions in each stage are generated by the domi-nance of different consortia of bacteria producing a rangeof different products, some of which are utilised at subse-quent stages in the process. At the beginning of a landfill’slife, the substrate is freely available and hydrolysis productspredominate. At the end, the reactions will be limited bysubstrate availability and this substrate will be the difficultto degrade material. Although traditional landfill involvesprocessing significant amounts of biodegradable waste,the products of the different stages are not present in abun-dance, as they are almost immediately consumed by themethanogenesis. In those cases the production rates are usu-ally slower than the speed of the transport processes.

When used as a remediation technique, the aerobic land-fill technique is an interruption of the five-stage process; themicrobial activities that run to completion in an anaerobiclandfill are interrupted when aerobic conditions are intro-duced artificially. Hence there may be a higher concentra-tion of material present that would normally be removedbefore the final aerobic stage commenced.

Under aerobic conditions the positive redox potentialaffects both metal mobility and the mobility of organiccompounds. Other reactions are also affected, for examplefermentation reactions in an anaerobic landfill producelarge amounts of acids, thus reducing pH; this in turnaffects solubility and sorption properties of metals andorganic contaminants. Omitting this form of degradationmay significantly alter the characteristics of the productsformed, and hence the possible emissions.

Aerobic landfills lead relatively rapidly to a state wherethe only organic compounds are hardly- or non-degradablecompounds, with very low residual gas potential (Heyeret al., 2003).

Landfill management options are not split simply intoeither aerobic or anaerobic techniques. There is a contin-uum of techniques ranging from a traditional anaerobiclandfill to a fully aerobic landfill (Table 1). Key differencesbetween the many options can be summarised by compar-ing anaerobic, semi-aerobic and aerobic techniques (Table2).

1.2. The potential for aerobic landfill to assist with

stabilisation

In order for a site to achieve completion, it must pose norisk to human health or the environment; this is quantifiedby assessing such factors as: quantity, quality and genera-tion rate of leachate; generation, flow and concentrationof gas; trace composition of the gas; potential for leachateor gas to be generated in future; physical stability of thewaste and associated structures; and presence of particularproblem wastes which could present a risk in the future(Environment Agency, 2003).

Aerobic landfill techniques can bring about improve-ments in most of these criteria. In old abandoned landfill

Table 1Different types of landfill in terms of the extent of aerobic activity

Anaerobic Traditional anaerobic landfill: with or without LFG or leachate collection.Leachate recirculation: used in traditional anaerobic sites to improve leachate quality, reduce leachate quantity and increase LFGproduction by circulating nutrients and bacteria through the waste mass.Bioreactors: designed quickly to transform and degrade organic waste, by adding liquid (and sometimes air) (US Environmental ProtectionAgency, 2004). Has the effect of maximising LFG production.Semi-aerobic landfill, or the Fukuoka method: air is drawn into the waste mass through the leachate collection system due to thetemperature differential between the inside of the waste mass and the ambient air. The air moving into the waste mass creates aerobicconditions, thus further elevating the temperature. The passive nature of the process is beneficial in terms of energy costs.Overdrawing: similar to semi-aerobic landfill; used to remediate old landfill sites. LFG is pumped out until ambient air is drawn in througha passive aeration well and/or via the surface of the waste. While the Fukuoka method is used from emplacement, overdrawing is appliedto existing sites. Not suitable for sites with waste depth >10 m.Hybrid landfill: takes advantage of aerobic techniques to increase the degradation rate, while retaining the benefits of LFG production. Airis artificially pumped into the waste, prolonging the initial aerobic phase and allowing aerobic decomposition. Some hybrid landfills useleachate recirculation or other methods of moisture addition. Gas is collected from the lower levels while the upper levels degrade at ahigher rate, and methanogenesis starts earlier. Energy recovery from LFG can start sooner than in traditional landfill.Aerobic landfill: the landfill body is completely aerobic throughout the life of the site: aeration continues from the emplacement of thewaste onwards, until the landfill body is completely stabilised.

Process Products

Waste organic fraction

Aer

obic

Stage I Hydrolysis/Aerobic

degradation CO2, H2O Aer

obic

Stage II Hydrolysis and Fermentation Organic acids H2, CO2, H2O

Ammoniacal nitrogen

Stage III Acetogenesis Acetic acid H2, CO2

Ana

erob

ic

Stage IV Methanogenesis CH4, CO2

Ana

erob

ic

Aer

obic

Stage V OxidationCO2, H2O

Aer

obic

Gases Leachate

Fig. 1. The five stages of waste degradation in anaerobic landfill (Williams, 1998).

C. Rich et al. / Waste Management 28 (2008) 1039–1048 1041

sites where aerobic techniques are to be used as a remedia-tion measure, the quantities of both gas and leachate arealready reduced relative to the early stages of landfill, sincethe majority of the degradation has already taken place,but both still need collecting and treating for years, if notdecades. This long tailing-off period has been shown tobe significantly reduced using aerobic techniques (Heyer

et al., 2005) although there are certain pollutants thatmay remain problematic, for example ammonia–nitrogen(Berge et al., 2005).

Aeration achieves improvement in several measures ofleachate quality, including BOD5, COD, concentrationsof volatile organic compounds (VOCs) and other contam-inants (Read et al., 2001c; Ritzkowski and Stegmann,

Table 2Traditional anaerobic landfill versus aerobic techniques (all data taken from existing landfills; none available for aeration of fresh waste)

Traditional anaerobic Fukuoka method Aerobic

Process Five stages: aerobic, fermentation,acetogenesis, methanogenesis,oxidation

Passive drawing of air into wastemass due to temperature gradient

Aerobic conditions achieved by forcing air into waste mass

Temperaturerange

30–65 �C (optimum range for LFGgeneration 30–45 �C)

40–50 �C (Yoshida, 2005) 40–70 �C (Ritzkowski et al., 2003) ideally 54–66 �C (Readet al., 2001a)a

Most UK landfill sites operate at30–35 �C

pH range 5–9 (7–8 during methanogenesisstage; ideally 6.8–7.5)

Ideally above �8 (Lee et al.,1994)

7.5–8.5 (Stessel and Murphy, 1992): fewer acids areproduced than in anaerobic landfill, as fermentationreactions are limited

Timescale Estimates vary from decades tomillennia

30 years (Chong et al., 2005) 2–3 years (Cossu, 2005) 4–5 years (Prantl et al., 2005); otherattempts at estimates less successful

Emissions CO2, CH4, H2O, trace pollutants CO2, H2O, trace pollutants CO2, H2O, trace pollutants

a These two readings were obtained from sites with quite different operating characteristics.


2005b). Leachate is one of the main pathways for contam-ination to be passed from the landfill to the environmentand, via groundwater, to humans, so it has a key role inthe risk assessment that is performed when the operatorseeks completion.

The volume of leachate requiring treatment has beenshown to be reduced in aerobic landfill due to evaporation,which is attributed to the elevated temperature of the wastemass (Read et al., 2001c). In some cases the leachate isrecycled as a means of maintaining the correct balance ofmoisture in the waste mass (Read et al., 2001b). The timefor which leachate requiring treatment is produced isreduced, with this aspect of aftercare being required forseveral decades less than under anaerobic conditions (Ritz-kowski and Stegmann, 2005b).

The emission potential of landfill gas is a key issue whenapplying for a certificate of completion. Gas monitoring ofaerated landfills shows that methane production is reduced.Usually the aim of using aerobic techniques is to introduceaerobic conditions throughout the waste mass in order topromote accelerated stabilisation, in which case methanelevels must be reduced rapidly and kept at a minimum;anaerobic conditions must not be allowed to be re-estab-lished as the methane produced could then form an explo-sive mix with the oxygen. Data from the Modena landfill,however, show that even a month after an intended breakin aeration, methane, although it was being generated, wasnot at levels as high as before aeration had been initiated(35% before start, 15% 1 mo after aeration ceased) (Cossuet al., 2003), indicating that this might not be a problem. Itimplies that anaerobic conditions were partly resumed, butwere not present throughout the waste mass; further confir-mation of the exact implications is required.

Under aerobic conditions the carbon that would, underanaerobic conditions, have been emitted as methane isinstead emitted as carbon dioxide. Gas data shows thatwhenever methane levels drop, carbon dioxide levels rise,and vice versa (Read et al., 2001b; Ritzkowski et al.,

2003). The intended break in aeration at Modena con-firmed this relationship: as the methane concentration rosefollowing the break the carbon dioxide levels fell (Cossuet al., 2003).

Other gaseous emissions from a traditional landfillinclude hydrogen sulphide and VOCs (Hamoda, 2006).While aeration may achieve reductions in these emissions,there are others that may increase as a result of the aerobicprocess itself. Carbon monoxide concentrations in wastehave been shown to increase as the waste becomes aerobic(Powell et al., 2006); CO can have human health implica-tions and has a role in the tropospheric formation of ozone.Other non-methane organic compounds (NMOCs) includehazardous air pollutants (HAPs) and other greenhousegases, and it is unclear whether their emission increasesor decreases following aeration (Powell et al., 2006).Ammonia–nitrogen, which is toxic to aquatic species,may be a significant pollutant in leachate from bioreactorlandfills (Warith and Takata, 2004), and this problem isexacerbated by the lack of a clear understanding of the fateof nitrogen in bioreactors (Berge et al., 2005). Ammonia–nitrogen is currently removed in leachate treatment by airstripping, among other techniques (Kargi and Pamukoglu,2004; Stephenson et al., 2004; Uygur and Kargi, 2004), andwhile aeration itself encourages nitrification, it in fact doesnot solve the ammonia–nitrogen problem as the resultantincreased waste temperatures may inhibit nitrification,although to what degree is unclear (Berge et al., 2005).However, lysimeter studies have not found this to be aproblem, observing instead that ammonia–nitrogen canbe removed under aeration (Prantl et al., 2006); a pilot-scale landfill has shown the same (Bilgili et al., 2006).Clearly further research is required to establish how theseresults might be translated to a full-scale application.

The rate of settlement of the waste is greater due to theincrease in degradation under aerobic conditions (Ritzkow-ski et al., 2003). A large part of this height reduction is dueto subsidence following consolidation rather than an even

Table 3Investment and operating costs for three German aerobic landfill sites(Heyer et al., 2003)

Kuhstedt Neumuhle Milmersdorf

Investment and operating costs(€)a

650,000 650,000 640,000

Total landfill volume (m3) 220,000 420,000 580,000Specific costs (€/m3) 3.0 1.5 1.1

a These costs include infrastructure, construction site equipment, gaswells, ductworks, gas distributing station, compressing station, waste airtreatment, operating costs for 2–3 years. They do not include monitoring,planning, approval certificates, reporting or documentation costs.


settlement across the whole waste mass, while many of thehigher settlement measurements originate from areas wherethe aeration equipment is installed (Ritzkowski et al.,2003).

Odour problems are not explicitly listed by the Environ-ment Agency as one of the considerations for completion,but the list is not exhaustive. In fact, odour complaintsfrom local residents are a common problem for landfillsites, and their elimination would be a clear benefit foroperators. Some projects have explicitly aimed for odourreduction (Jacobs et al., 2003). Odours at aerobic sites havebeen reported to be less pungent, and of an organic nature,as opposed to the hydrogen sulphide and ammonia that aretypical of anaerobic sites (Read et al., 2001b).

Aerobic landfill techniques cannot affect the presence ofproblem wastes whose degradation is not improved underaerobic conditions. Nonetheless, aeration of a landfillshould lead to improvement in most of the factors underconsideration in landfill completion applications.

Aerobic techniques have been used successfully in manycountries worldwide, with a range of aims. In the UnitedStates aerobic landfill techniques can be used to save alarge void space for ongoing use as landfill. It has beenproposed that aeration be combined with landfill mining,which would lead to landfills with longer lifespans; manyresearchers regard this as a type of sustainable landfill(Read et al., 2001a). In Germany aerobic landfilling isused to bring forward stabilisation (Stegmann et al.,2003). Accelerated stabilisation can reduce leachate andgas emissions to below the levels required for completion.One Italian project used aerobic techniques to stabiliselandfills prior to excavation, enabling the construction ofa railway line through a landfill site (Cossu et al., 2003);another combined pre-treatment, aeration and flushingto optimise the advantages of each of the three technolo-gies (Cossu, 2003). In The Netherlands aeration has beenused to achieve odour reduction prior to re-opening anold landfill (Jacobs et al., 2003). While some countries(e.g., the US (Read et al., 2001c)) add moisture as wellas air, usually in the form of recycled leachate, othersdo not (for example Germany (Ritzkowski et al., 2003)).There are, naturally, operational differences between sitesas well as between countries, and while these lead to diffi-culties when comparing data, they will also act as guideswhen deciding what procedure should be followed in theUK.

1.3. The potential for use of aerobic landfill in the UK:

drivers, economic considerations and potential obstacles

The opportunities for aerobic landfill techniques to beadopted as a waste management solution in the UK willbe dictated by factors such as the legal framework andthe costs of aeration. Should aeration be used in the UKthere would be further impacts on waste management,and these should be considered concurrently with thebenefits.

Aerobic landfill techniques have been used in several EUmember states with considerable success. The LandfillDirective aims to reduce the amount of biodegradablewaste in MSW going to landfill. However, much biode-gradable municipal waste (BMW) has already been depos-ited in landfills and the Landfill Directive is aimed at MSWonly, so the amount of biodegradable waste in landfillsremains significant.

In fact, legislation in the EU paves the way for greaterseparation of waste, which could lead to more efficientuse of aerated sites. The Landfill Directive anticipates thepossibility of separating biodegradable and non-biodegrad-able waste; it currently demands the separation of wasteinto inert, non-hazardous and hazardous classes, butallows for further subdivision within each category. It givesexamples of criteria that can be used, and one of these is‘‘requirements or limitations on the biodegradability ofthe organic waste components’’ (European Council,1999), which is tailor-made for diverting inappropriatewaste away from sites intended for aerobic treatment.Hence the existing legislative framework is open to use insupport of aeration techniques.

The costs of aeration of an existing landfill site havebeen estimated at 1–3 €/m3 (Heyer et al., 2003); Table 3shows the costs for three German sites. The higher costsof 2–3 €/m3 are expected only in cases where conditionsare unfavourable in some way, for example small sites orthose where a suitable infrastructure does not exist. Whenoperation is optimised, costs are expected to be approxi-mately 0.5–1 €/m3 (Heyer et al., 2003). US estimates ofcosts also show the potential for considerable savings,mainly based on the extra availability of space allowingsites to remain open for longer (Read et al., 2001b). Inaddition, prolonging the life of a landfill site delays theneed to look for a new site, another expensive area of land-fill operation. Some data have been projected for a newsemi-aerobic landfill in Malaysia, giving costs of US$8.89per tonne of waste, compared to US$7.89 per tonne atanaerobic Malaysian sites (Chong et al., 2005). This esti-mate includes land costs, which obviously will varydepending on the country where the landfill is built. More-over, due to the differences between this process and theaeration techniques used in Europe and the US so far, thesedata should be treated with caution.


The cost-effectiveness of aeration will vary depending onthe method of aeration chosen and the requirements ofeach individual site. Moreover, most estimates of the costsof aerobic landfilling have, so far, been either hypotheticalor based on the characteristics of particular site. Generalis-ing from these data is inadvisable, and further long-termresearch is required to clarify exactly what the costs andsavings of aeration will be.

The costs of aeration are offset by several savings,including: reduced requirements for a surface cap (insteada lower-cost cap can be installed, suitable for the antici-pated lower emissions); lower gas and leachate treatmentcosts; reduction of aftercare period by at least several dec-ades; and earlier recultivation and after-use.

Frequently one of the funding problems facing opera-tors of old closed landfills is that during the operation ofthe site no funds were allocated for its aftercare (Stegmannet al., 2003). Funds for this can come from two mainsources of income, both of which dry up long before thesite achieves completion: gate charges and the sale ofenergy derived from LFG.

Aeration can allow both of these sources of revenue tobe preserved. The rapid stabilisation of waste due to aera-tion may create more space in the site, allowing it to con-tinue taking waste for longer than would be possible ifanaerobic conditions were in place, hence increasing reve-nue from gate prices. The use of methane as a source ofenergy has been referred to as a ‘‘least-cost method of mit-igating existing environmental problems’’ (Stessel andMurphy, 1992). In fact it mitigates those problems onlyfor as long as it remains profitable.

LFG production inevitably slows until its use as asource of energy is no longer economically viable, but afterthis point it is still produced in large enough quantities toneed treating before it can be emitted to the atmosphere.Estimates of how long before obtaining energy fromLFG ceases to be profitable vary considerably, rangingfrom 10 to 15 years (Ritzkowski et al., 2003) upwards,depending on the size and other characteristics of the site.By shortening the ensuing unprofitable period, aeration ofa landfill can save on gas treatment costs. To retain theincome, aerobic techniques could be used in a way not dis-similar to those used in remediation. The period of time forwhich the anaerobic landfill is sufficiently productive ofmethane can be used to bring money in, and aerobic tech-niques are then introduced, in order to stabilise the wastewithin a short period of time.

It is clear that there is potential for significant financialbenefits with the use of aerobic techniques, since the costsof long-term monitoring and aftercare outweigh the costsof aerobic treatment. There are some problems with theaeration of waste, however, although these can beovercome.

Heavy metals are not immobilised in an aerobic landfillas they are in an anaerobic landfill, which could lead toproblems with the leaching of these toxic elements. Themobility of heavy metals in waste is high in the initial aer-

obic phase of a traditional landfill, but decreases as theoxygen levels decline, and under the reducing conditionsof anaerobic stages heavy metals tend to be retained withinthe waste. In the final aerobic phase the mobility does notincrease as the metals tend to have formed hydroxides andsulphides in the anaerobic phase; the majority of these arenot readily soluble and so are not released once aerobicconditions are re-established (Binder and Bramryd, 2001;Revans et al., 1999). Some metals are partly mobilisedwhen the waste is aerated, but appear to be partly adsorbedby the residual wastes (Ritzkowski and Stegmann, 2003).

In a landfill that is aerated throughout its life, metalmobility would not decline in the same way as in the earlystages of an anaerobic landfill, and thus there would be arisk of the release of heavy metals. Similarly, there is evi-dence from the field of wastewater treatment that some pes-ticides are better degraded anaerobically than aerobically;the biodegradation of organochlorines, for example, relieson an initial anaerobic stage to allow reductive dechlorina-tion (Lester and Birkett, 1999). This would not be a prob-lem where aerobic conditions are artificially reintroducedlater in the life of a landfill, provided the anaerobic condi-tions that prevail in the earlier stages have done so for longenough to permit degradation of pesticides. However, in asite aerated from the start it is likely that some pesticideswithin the waste might not be adequately degraded andmight be retained in the landfill body for decades. This issimilar to the problem experienced with the anaerobic land-fill, of the pollution potential remaining high even after asignificant period of time, if only for this one group ofpollutants.

Those pesticides which require anaerobic conditions areunlikely to be degraded efficiently in conventional leachatetreatment, as this is aerobic. In a traditional landfill thisdoes not pose a problem as the anaerobic conditions inthe waste have allowed these pesticides to biodegrade.However, a site that is aerobic throughout will not providethis opportunity. Similarly, if not immobilised under anaer-obic conditions the heavy metals within the waste would bemore prone to leaching out, further increasing the pollu-tant load on a leachate treatment system. Hence leachatetreatment systems as currently used at anaerobic landfillsites are not suitable for application to aerobic landfillsas they stand. Revision of leachate treatment systems mightsolve the problems introduced by the aeration of waste,while late aeration of waste would bring the benefits of aer-obic techniques but without creating new problems inleachate treatment.

It is clear that an aerobic landfill can decompose readilydegradable wastes within a short timescale; this raises thequestion of what happens to the less readily degradablewaste. It has been recommended that ‘‘inert and recalci-trant’’ waste be diverted away from landfills intended foraerobic treatment, in order to permit the effects of acceler-ated stabilisation and increased settlement to be used totheir greatest advantage (Read et al., 2001b). This wouldallow more accurate prediction of the behaviour of the


landfill, since all the waste within it would behave inapproximately the same way. One issue that remains iswhether, with less non-degradable waste present, the land-fill would settle further, and how easy it would be to con-trol this process. The recalcitrant waste also providesstructure and pore volume to allow distribution of oxygenthroughout the waste mass, and its removal could causeproblems in the same way as it does in biowastecomposting.

2. Discussion

It is apparent from the research conducted to date thataeration of waste causes it to stabilise more quickly thanif left untreated. The key question here is whether thismight be incorporated into waste management in the Uni-ted Kingdom, with the overall aim of achieving completionearlier and thus producing more sustainable landfillpossibilities.

If a landfill is considered to pose ‘‘an unacceptable riskto the environment over the very long-term and cannotrealistically be expected to ever reach completion, thenthe permit should not be issued’’ (Environment Agency,2003). What this implies is that all landfills that are permit-ted can be expected to achieve completion at some point.The problem is when this might happen; landfills may takewell over a century to achieve biological stabilisation.

A parallel problem is that many of the engineering com-ponents of a landfill will deteriorate long before the full dis-charge of the contaminant load. Stabilisation has beendescribed as finding a balance between the residual con-taminant load and the attenuation ability of the environ-ment (Hall et al., 2004). Assessments of the length oftime required to reach this balance for some contaminantsestimate in millennia rather than decades or centuries.

Undoubtedly the need for liners will not be eliminated,but there is the possibility that they could be less rigorouslyconstructed if it could be proven that the waste would befully degraded within a given period of time. Liners wouldneed to be sufficiently impermeable in the short-term tocope with the emission potential of the waste in the earlystages of an aerobic landfill, since it remains high in theshort term. However, since the waste could be stabilisedwithin a much shorter time scale, it is conceivable thatthe longevity of a liner could be deliberately reduced, off-setting some of the expense of implementing acceleratedstabilisation techniques.

However, there is little, if any, consensus on when linerfailure might occur; a figure of 250–300 years has beenreported, but without a solid scientific justification (Halland Drury, 2002). Other estimates range from 20 to 2000years. In addition, the idea of a liner failing instantaneouslyat some point is almost certainly unrealistic; Hall andDrury suggest the use of a half-life. The modelling of land-fill behaviour has to incorporate liner failure, but with suchvariation in the perspectives on liner failure the debate onhow to model it will continue for some time yet. An accel-

erated stabilisation technique that brought the completiondate of a landfill to under a century would make a largepart of this debate academic.

There could be many advantages to introducing aerobictechniques to the UK; the question remains of how bestaeration might be applied to achieve these benefits. Thebest option appears to be to use aerobic techniques toaccelerate the stabilisation of a site after it has been anaer-obic for some 20 or so years. This would combine the tra-ditional anaerobic approach, allowing energy generationfrom LFG (representing a considerable income stream)with the rapid stabilisation of waste and reduction in emis-sion potential given by aerobic treatment. This approachavoids some of the problems associated with aerating alandfill throughout its life. Neither the lack of degradationof certain pesticides, nor the increased heavy metal mobil-ity under aerobic conditions, would be an issue with lateaeration. The late aeration could also provide opportuni-ties for more comprehensive ammonia–nitrogen removalthan that predicted when waste is aerated throughout itslife (Berge et al., 2005).

It is beyond the scope of this paper to discuss in detailwhat would be the best operational procedure for air addi-tion in the UK, for example whether to add moisture, as inthe US, or not, as in Germany. There is a clear need forUK-focused research to investigate the operational param-eters within which a UK aerobic site might operate, andhence what procedures would be most effective withinthose parameters.

As the Landfill Tax rises, landfill gate prices willincrease, and landfill operators will be forced into greatercompetition with one another; the economics of runninga landfill site in the UK are only going to get moredemanding. Aftercare will remain high on the list of prior-ities, at least from an environmental perspective, but it isnot environmental benefits that motivate operators toact. For aeration to be undertaken willingly by operators,it needs to be economically attractive, and the savings inaftercare costs can contribute to this attractiveness. Fur-ther research is required on all costs and savings associatedwith aeration, however, as so much of the data accumu-lated so far is projected rather than measured.

The different emphases of the various research projectsand trial cells create a confusing picture, where some resultsremain unconfirmed. Considerable further research isrequired before a full picture can be obtained of the futureof aerobic landfill techniques. So far, all field research hastaken place using existing landfill sites, so that many of thesites that are run as full-scale aerobic landfills are usingwaste that has been anaerobic for some time. There are,therefore, questions yet to be answered about the behav-iour of waste that has never been anaerobic. Lysimeterstudies tend to include this possibility, but the conclusionsdrawn from full-scale studies cannot be absolute until full-scale studies are undertaken using new waste.

One of the more problematic issues is the fate of heavymetals in aerobic landfill. There is clear evidence that heavy


metals are immobilised under anaerobic conditions, and,crucially, that they remain immobilised once aerobic condi-tions return (Revans et al., 1999). However, under condi-tions that have been aerobic from the start, thisimmobilisation does not occur. To what extent this couldbe a risk in the operation of aerobic landfills remains tobe seen. Also requiring considerable investigation is theissue of non-methane organic compounds emitted to theatmosphere.

Some projects report incomplete aeration of the landfillbody (Read et al., 2001a), while others guarantee aerobicconditions throughout (Ritzkowski et al., 2003). Thereare no details to support either of these claims, but it isin fact one of the key issues when operating an aerobiclandfill. It may be that a certain proportion of the wastemass will remain under-aerated in all cases, and the invest-ment necessary to resolve this would be too great for toolittle gain. This is a question that will be answered as aera-tion is applied to more large-scale sites. The different mech-anisms used to aerate the waste may be responsible for thedifferences in their performance, and hence may be animportant factor when deciding what method would bemost appropriate for application in the UK.

Carbon conversion as a means of measuring the successof the aeration of the waste mass has been investigated(Ritzkowski et al., 2003, 2006; Ritzkowski and Stegmann,2005a). The theory behind this method is sound but ithas yet to be used extensively at other sites. Furtherresearch is required on rates of carbon conversion beforethis method can be used in full-scale projects as the primarymeans of measuring success.

Applied to new landfill sites, aerobic landfill techniquescan bring considerable benefits, and, when used as a reme-diation measure on contaminated land or in conjunctionwith anaerobic landfill (as a hybrid), the adverse environ-mental effects of landfills can be mitigated. In addition,the long and unprofitable period of time at the end of alandfill’s life can be reduced significantly. However, furtherresearch on the implementation of aerobic techniques inthe UK is needed before the full implications of its use willbe certain.

3. Conclusions and recommendations

Over 20 years ago Japanese research advocated the ben-efits of semi-aerobic landfill techniques, quoting advanta-ges such as improved leachate quality, a reduction inhazardous gases, faster stabilisation of solid waste thanunder anaerobic conditions and a low overall cost of oper-ations. Recently operators in several countries have exper-imented with the use of aeration, and many have come tosimilar conclusions. Improvements in leachate and gasquality, and faster stabilisation, have been demonstratedin projects that have had a variety of aims and emphases,and which have been cost-effective.

Cost is one of the difficulties with any change in landfillstrategy: the costs experienced now do not correspond to

those in the future. The low monetary cost of traditionalanaerobic landfill does not reflect its true cost, while otherwaste disposal options are often sidelined in favour of land-fill on account of their expense despite their greater sustain-ability. The financial expense of aeration can be set againstboth the current and future costs of anaerobic landfill, andit is examination of the latter that shows the benefits of aer-ation. The improvements to leachate quality, reductions ingas production and accelerated stabilisation of waste saveon aftercare and remediation.

In addition aerobic techniques have been successfullyused in a range of remediation projects. These projectshave demonstrated that, given the right conditions, aera-tion can provide an economic and comprehensive solutionfor sites in the UK that need remediation; the success ofthis remediation approach also demonstrates the viabilityof using planned aeration later in the life of a landfill asa hybrid option.

Late aeration would allow the anaerobic conditions inearly stages to fulfil various important functions: immobil-isation of heavy metals within the waste mass; the initialdegradation of certain problematic pesticides; and produc-tion of large quantities of methane, allowing energy recov-ery and its associated income. Hence some of the benefitsof the anaerobic phase of landfill are retained, but the longaftercare period is reduced and the advantages of late aer-ation, such as ammonia–nitrogen removal, remain. Sinceenergy recovery will be possible at many landfill sites forsome years yet, this means that current use of aerobic tech-niques in the UK would be restricted to remediation of oldabandoned sites and the shortening of the aftercare periodat sites that are no longer suitable for energy recovery.

As they are used now, hybrid landfills cut down theunproductive time at the beginning of a landfill’s life,bringing forward the onset of methanogenesis with theaim of using LFG for energy as soon a possible. Thisapproach does nothing to change what happens at theother end of the life of the landfill, the long period whereenergy recovery is unprofitable, but gas and leachate treat-ment is necessary. To invert the procedure, using aerobictechniques at the end of the life of the site, would cut awaynot a couple of unproductive weeks at the beginning of thelandfill’s life, but a couple of unproductive and potentiallycontaminating decades, if not centuries, at the end.

It is therefore recommended that this approach is inves-tigated further: implementing aerobic landfill techniques asremediation in the short term, but not introducing aerationto newer sites until later in their lives. Further research isrequired in a range of areas linked to the aerobic landfill,but the literature currently available suggests that theremay be benefits to be had in both the short- and long-term,making more detailed investigation and large-scale testingworthwhile.

The use of aerobic landfill will have implications forwaste management strategies. For example, waste separa-tion might be necessary in order to improve the conditionsfor aerobic techniques to take their effect, or it might be


that the standards for liner construction could be relaxed interms of the required longevity. Again, further research isneeded to explore these implications before aeration canbe recommended.

As has been shown in the projects undertaken in othercountries, aerobic techniques have been used successfullywith a variety of aims. There are sites in the UK that needremediation, and while aeration will not suit all situations,it has been proven to be appropriate for a range of remedi-ation needs. Aerating old sites presents problems, such ashow to inject the air, or to collect leachate in those siteswithout leachate collection systems already in place. How-ever, aeration projects undertaken so far have had the sameobstacles, which have proved to be surmountable.

Predictably in such a relatively newly popular field thereare a number of areas that remain under-researched; in factalmost all areas require consolidation, but some merit par-ticular focus. The behaviour of different contaminantsunder both aerobic and anaerobic conditions needs clarifi-cation: in particular heavy metals and pesticides, as well asemissions of substances that are not routinely monitored intraditional landfill, such as gaseous emissions of non-meth-ane organic compounds. How to measure the effectivenessof the aeration will also have to be confirmed in order forsites to be run as efficiently as possible, and operationaldetails, such as projected timescales and mechanisms forair addition, need to be clarified. The financial costs of aer-ation are, as yet, mostly discussed hypothetically, and oneparticularly important research need is the clarification ofexactly what these costs will be.

Despite the remaining uncertainty surrounding both thetheory and the practice of aerobic landfill, it is clearly a toolthat has the potential to be used to improve the sustainabil-ity of landfill in the UK.

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