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 DOI: 10.1177/0734242X9701500502

1997 15: 453Waste Manag ResK. Westlake

Possibility or Pipe-Dream?−−Sustainable Landfill  

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, 453-

SUSTAINABLE LANDFILL—POSSIBILITY OR PIPE-DREAM?

K. Westlake

Centre for Hazard and Risk Management, Loughborough University, Loughborough,Leicestershire LE11 3TU, U.K.

(Received 18 December 1995, accepted in revised form 18 July 1996)

Too often a sustainable landfill is described in terms of operational technique (e.g.bioreactor landfill or repository for pretreated waste) rather than the more appropriategoal of managing a landfill such that the environmental risk is acceptable. Thetechnique that achieves the lowest risk landfill will vary according to a number offactors including the waste composition, climate and local geology/hydrogeology,and will vary from country to country, region to region, and site to site. A trulysustainable landfill is one in which the waste materials are safely assimilated intothe surrounding environment, whether or not they have been treated by biological,thermal or other processes, and which manages gas-related problems so as to

minimize the environmental impact. This is more likely to be achieved in containmentlandfills, but recognizing that liner failure will occur ultimately, and that in the longterm, the escape of waste materials and their products of degradation is inevitable.Appropriate site selection, design and management is crucial to the attainment ofmore sustainable waste management. @ 1997 ISWA

Key Words—Sustainable development, sustainable landfill, risk assessment, bio-reactor, pretreatment.

1. Introduction

&dquo;Sustainable development&dquo; was defined in 1987, in the report of the World Commissionon Environment and Development (The Brundtland Report), as &dquo;development whichmeets the needs of the present without compromising the ability of future generationsto meet their own needs&dquo;.

This report identified the characteristics of sustainable development as:

(1) the maintenance of the overall quality of life;(2) the maintenance of continuing access to natural resources; and(3) the avoidance of lasting environmental damage.

Landfill has been defined (ISWA 1992) as &dquo;the engineered deposit of waste onto andinto land in such a way that pollution or harm to the environment is prevented and,through restoration, land provided which may be used for another purpose&dquo;. But whatis a sustainable landfill?

In the above definition, there is a requirement that &dquo;pollution or harm to theenvironment is prevented&dquo;, yet in the landfill environment prevention can never beguaranteed. In this context, pollution and contamination must be differentiated. Forthe purpose of this discussion, &dquo;contamination&dquo; represents the introduction by maninto the environment of potentially dangerous substances which do not necessarily

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constitute an environmental risk. Where a risk is present the term &dquo;pollution&dquo; is used.This differentiation was recognized by Westlake (1995) when describing a sustainablelandfill in general terms as &dquo;a landfill designed and operated in such a way thatminimises both short-term and long-term environmental risks to an acceptable level&dquo;.This definition recognizes that while environmental pollution is not acceptable, theprevention of environmental contamination is not possible, nor is it necessarily desirable.This risk-based definition recognizes that because national and regional hydrogeology,topography, weather, flora, fauna and a range of other factors can vary, appropriatetechnology and techniques for one region may not be appropriate for another. The keyelement of a sustainable landfill is that differences in the above, and in the specificnature of landfill hazards, are recognized within an environmental risk assessmentprocess, so that site-specific risks can be measured and used to support appropriatesite selection, design, management, and control.

It seems that too often nowadays landfill policy and principles are politicallymotivated, or are determined along the lines of national preference: in spite of thefacts! Thus, preferred waste management methods are split along national lines, andsustainable landfill is defined in terms of the preferred landfill method (e.g. bioreactoror pretreatment) rather than as a principle to be achieved by the most appropriatemeans for that region or country. However, Waste Management Paper 26A whichprovides guidance on landfill design, construction and operational practice in the U.K.(DoE, 1995) has now replaced more prescriptive guidance with a risk-based approachto the subject, and is one example of the increasing awareness and importance of riskmanagement within the waste industry. The most appropriate technique may varyconsiderably even within developed countries, and will be significantly different indeveloping countries where national culture and infrastructure, climate and the natureof waste arisings (and a plethora of other factors) will be important determinants ofthe means by which sustainable landfill development is achieved. The key factor shouldbe that whatever the means, the end result should be risk reduction to an acceptable level.This will necessarily account for economic factors and would not impose inappropriate,expensive, high-technology solutions on poor countries who could use money moreeffectively elsewhere, including pollution prevention and control in areas of greaterrisk.For a sustainable landfill, risk reduction can be achieved in a number of different

ways, but all of which require effective control of waste degradation processes andappropriate landfill siting, design, engineering, and management. In the developedworld the principles of landfill practice have changed considerably since the 1970s andthree major principles of landfill design and management have been recognized; theseare &dquo;dilute and attenuate&dquo;, otherwise known as &dquo;dilute and disperse&dquo;, &dquo;containment&dquo;,and &dquo;entombment&dquo; (or &dquo;dry-tomb&dquo;).

2. Principles of landfill

2.1 Dilute and attenuate

Dilute and attenuate is the principle of landfill disposal for unconfined sites, with littleor no engineering of the site boundary, in which leachate formed within the waste isallowed to migrate into the surrounding environment. This principle relies uponattenuation of the leachate both within the waste and in the surrounding geology, bybiological and physico-chemical processes. Dilution within groundwater further reduces

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the risk posed by the migrating leachate, but by definition, necessarily contaminatesthat groundwater. For the dilute and attenuate principle to be effective, the associatedrisk should be deemed to be acceptable. Such conditions may occur, for example, whenthe groundwater into which the leachate is discharging cannot be used for public supply,and where the risk to other sensitive receptors is also acceptable, or where there areno other sensitive receptors. However, the fact that public water supply has beencontaminated by landfill leachate (e.g. Anon 1994) is physical evidence of generalknowledge that site-specific risk assessments have not been conducted in the past.The advantages of the dilute and attenuate landfill are that there is no requirement

for expensive landfill lining/engineering, and as liquids formed within the site migratefrom the base, there is no requirement for leachate collection and treatment facilities.These &dquo;advantages&dquo; necessarily have contamination implications.

2.2 Containment landfill

Containment landfill requires a much greater degree of site design, engineering andmanagement, and exercises a greater degree of control over the hazards associated withthe disposal of waste to landfill. In the developed world, containment landfill is nowthe accepted means of disposal to land, although the degree of engineering to achievecontainment, and the management of water and other parameters varies considerably.To some, the use of an engineered clay liner represents containment landfill, yet theclay is specified in terms of its permeability and thus, by definition, is not a true

containment system, but rather relies on controlled leakage. In many circumstances,the degree of containment provided by a system such as this, coupled with the low rateof release, may control the environmental risks to an acceptable level and no furthercontainment measures may be necessary. In other circumstances, more effective con-tainment may be necessary, in which composite liner systems and multiple liner systems,which utilize geomembranes in conjunction with natural materials, may be required.Such combination liners may be required for example, for use with particularlyhazardous or refractile wastes where degradation/stabilization rates within the site arerelatively slow, or in circumstances where sensitive receptors are particularly vulnerable.The uses, advantages and disadvantages of different liner systems have been discussedby Westlake (1995).The underlying principle of containment landfill is that liquids (leachate) generated

within the waste should not be allowed to migrate beyond the site boundary. Thecontainment of leachate implies, in most cases, that leachate will collect and will requiretreatment. This has placed new requirements upon the effective management of landfills.A containment site has been defined (ISWA 1992) as a &dquo;landfill site where the rate ofrelease of leachate into the environment is extremely low. Polluting components inwaste are retained within such landfills for sufficient time to allow biodegradation andattenuating processes to occur, thus preventing the escape of polluting species at anunacceptable concentration&dquo;. Although the containment principle refers to liquidmanagement, a truly sustainable landfill will also require that gas emissions are managedin such a way as to minimize the environmental impact.

2.3 Entombment

Entombment or the dry-tomb approach to landfill, in which moisture is, as far as

possible, excluded from the waste, is implicit, in the U.S.A., under RCRA sub-title

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&dquo;D&dquo; requirements. The principle of the &dquo;dry-tomb&dquo; approach is that through preventionof liquid infiltration, the waste will remain dry, will not decompose, and will notproduce a polluting leachate or gas. In so doing, this approach effectively stores wastein perpetuity (or for as long as the containment system remains intact), in a relativelydry form. Waste storage in this way accepts that no attenuation of waste will occurand argues that either: I

(1) storage creates the opportunity for development of new technologies to deal withthe stored waste in a more appropriate way at some point in time in the future; or

(2) that the engineered containment will remain to always ensure that no infiltrationof liquids will occur, and thus potentially polluting leachate will not be produced.

2.4 Principle of choice

When considering sustainable landfill, it is important to consider time scales beyondthose of a single generation, and the prevention of pollution in 30 years time is of littlevalue if the pollution incident is simply delayed to some time beyond. Thus, the dry-tomb landfill cannot be considered as sustainable, as upon engineering failure, theproduction of landfill gas and leachate will ultimately occur at some point in the future,whereupon pollution may occur.

Landfills operated on a dilute and attenuate principle may represent a sustainableoption, if a site-specific risk assessment has shown the risks to be acceptable. However,there are likely to be few circumstances where the uncontrolled release of landfill gasand leachate to the surrounding environment does not pose some risk of environmentalpollution. Where some degree of control can be exercised, e.g. through the use ofengineered natural materials as landfill liners, the dilute and attenuate practice may bejustified.

However, the containment principle would appear to offer the most sustainableoption for landfill development in most circumstances, but in supporting the containmentprinciple, it must be recognized that the containment barrier will ultimately fail,whereupon landfill gas and leachate may be released to the environment. This representsa hazardous event. In general terms, a hazard is any event which has the potential tobe harmful. When a pathway is present by which a sensitive receptor (target) can beharmed by that event, then there will be an associated risk. Thus, when designing forcontainment barrier failure, the environmental risk may be reduced by reducing thehazard, by removing a sensitive receptor, or by controlling the hazard migrationpathway such that the receptor is not affected. For existing landfills, the latter twooptions will be very difficult, if at all possible, or may be very expensive. Thus, hazardreduction appears to be the most easily achievable means of ensuring sustainable landfilldevelopment. When siting and designing new landfills, control of the pathway elementis feasible and may be achieved through the use of appropriate site location and bythe use of appropriately designed liners.As hazard reduction outside the landfill will be achieved by biological and physico-

chemical processes in the surrounding strata which cannot be controlled, hazardreduction should under most circumstances be achieved before the containment barrierfails. According to local conditions and how &dquo;containment&dquo; is defined, a natural claybarrier may provide sufficient containment to satisfy the requirements of sustainablelandfill.

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The hazard reduction within the landfill may be achieved either by degrading(stabilizing) the wastes within the site, or by pretreating the wastes prior to landfill toreduce the hazard before emplacement. Whichever method is employed, it must be

recognized that the containment principle for sustainable landfill development is onebased on short-term containment which ultimately relies on the dilute and attenuateprinciple for long-term wastes management, and that the prime objective of a trulysustainable landfill is the reassimilation of the waste into the surrounding environment.This concept has been discussed previously by Campbell (1992). This concept is alsoimplicit in the definition of the containment landfill described above where perhaps thekey phrases &dquo;rate of release ... is extremely low&dquo; and &dquo;at an unacceptable concentration&dquo;,for this implies that the containment barrier is not entire throughout the life of the siteand that migration of leachate may occur, but that the associated risk is acceptable.

3. Bioreactor or pre-treatment

For effective waste management, the choice between within-site waste stabilization and

pretreatment should be made in the light of local conditions, waste composition, volumeof waste arisings and other relevant factors.

3.1 Bioreactor

The underlying principle of the bioreactor landfill is that by optimizing operationalcontrol and environmental conditions within the waste (especially moisture content),more rapid and complete degradation of waste may be achieved. The general objectiveis to produce a &dquo;stable waste&dquo; within a reasonable time scale, often quoted as within30 years of emplacement, and thus ensure that the risk to the environment will be atan acceptable level when liner failure occurs. However, the figure of 30 years has beenderived variously in different countries, and there is no single good reason why thisfigure should be the accepted goal. The only sensible requirement is that short-termrisks should be controlled and that when the containment system breaks down, theassociated risk to the environment should be acceptable. Thus, the durability of theliner system will be a key factor. In all circumstances, the period for waste stabilizationshould, where possible, be reduced to the shortest possible time scale in order tominimize environmental risks and reduce to a minimum the influence of factors suchas loss of institutional control. In this case, economic factors as well as technology andmanagement factors will influence control over this process.Although a completely stabilized waste may not necessarily be required for the

attainment of acceptable environmental risk, the stabilization (degradation) processeswill produce landfill gas which will require effective management, and the more stablethe final waste then the more gas that will be produced. Landfill gas may present arisk by virtue of its toxic, corrosive, asphyxiating and explosive/flammable properties.Estimates by the Intergovernmental Panel on Climate Change (IPCC 1992) also suggestthat landfill contributes 8-20% of total anthropogenic methane emissions and thuscontributes significantly to global warming. However, the impact on global warmingmay be reduced through the use of landfill gas as a fuel for heat and energy productionin which the methane is burned producing the relatively less damaging carbon dioxide.Recognizing that landfill gas collection is a relatively inefficient process, there will besome escape to atmosphere of methane from landfills. In choosing a sustainable optionfor landfill management, the effects of this process, other emissions and disamenity

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effects (and other negative impacts), as well as any net benefits, would have to becompared with alternatives such as incineration. Ensuring too that account is taken ofthe subsequent affect on landfill leachate-related risks. In this regard, the concept ofBest Practicable Environmental Option (BPEO) is important.BPEO was defined by the Royal Commission on Environmental Pollution (1988) in

Britain as &dquo;the outcome of a systematic consultative and decision-making procedurewhich emphasizes the protection of the environment across land, air and water. TheBPEO procedure establishes, for a given set of objectives, the option that provides themost benefit or least damage to the environment as a whole, at acceptable cost, in thelong term as well as in the short term&dquo;. This approach accounts for emissions to allmedia and will help in the determination of the option that presents the lowest overallenvironmental risk. It can have an important role in the determination of the mostsustainable waste management strategies.Comparison of these factors will not be undertaken easily, but will be necessary if

the full environmental impact of landfills is to be assessed. Knowledge of landfillprocesses suggest that an increase in gas production will lead to a reduction in thepollution potential of leachate, through the removal of leachate biochemical oxygendemand (BOD) and thus a balance must be struck between the relative impact of each.A comprehensive review of landfill gas has been undertaken by Gendebien et al. (1992).

For the bioreactor landfill, a number of key factors will be critical to its success.Walker (1993) has discussed problems associated with the removal of ammonia fromleachate and the long time required for ammonia levels to reach acceptable con-centrations (approximately 550 years). Such difficulties have led to the introduction ofthe concept of the flushing bioreactor (Harris et al. 1994) in which liquid flushing isused to remove degradation products and other compounds, including inorganics fromthe waste mass. Knox (1990b) showed that for a given set of conditions, between 5 and7 bed-volumes of water will have to pass through most landfills receiving degradablewaste in order to reduce ammoniacal nitrogen levels to dischargeable concentrations.According to Harris et al. (1994), if this is to be achieved within 30 years then &dquo;themean hydraulic retention time must be less than 5 years, and for it to be effective, thepassage of water must be reasonably uniform, reaching the whole of the waste masseliminating zones of dead volume&dquo;. This will require irrigation rates equivalent to2000 mm per annum or more, which is much greater than the effective rainfall in manyareas where landfills are found. However, if leachate treatment and recirculation areemployed then the equivalent of the required bed-volume changes can be achievedwithout the need for further water addition. The application of leachate recirculationto sustainable development has been discussed in more detail by Knox (1996).An alternative bioreactor concept-the fermentation/leaching wet cell (F/L wet-

cell)-has been developed by Lee and Jones-Lee (1993). This concept also aims tostabilize effectively the waste and leach the soluble potentially polluting components.The design requires that the waste is shredded prior to emplacement to try to ensurecontact of the liquid with all waste components. According to Lee and Jones-Lee(1993), who cite Ham (1975), this will also eliminate the need for daily cover, and hasthe potential to increase the capacity of landfill by about 20%. The other key featureof F/L wet-cell is the use of a clean water system beneath the clay of the compositeliner to maintain movement of water up through the clay, thus preventing any leachateleakages to escape from the containment system.One of the major problems with this and other concepts of using the landfill as a

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treatment process is that while the theory is apparently sound, and may, in some caseshave been demonstrated at laboratory scale, it has not been proven in practice. All theevidence that is currently available suggests that to achieve a stable, relatively non-polluting waste in an acceptable period of time would be extremely difficult. Even instudies at the large Brogborough (U.K.) test cells (ETSU 1993), where some successfulmanipulation of waste reactions has been achieved, it may be many years before astable waste is produced, if indeed this ever occurs. The influence of factors such aswaste shredding, pre-composting and waste compaction have been discussed by Westlake(1995), while the influence of hydraulic characteristics, leachate recirculation and relatedfactors have been discussed by Knox (1996). While there are many uncertainties, thereare sufficient encouraging results to warrant further investigation, if not at this stage,whole-hearted support of the bioreactor concept.

3.2 Pretreatment

In most circumstances, incineration is likely to be the favoured pretreatment option,although composting and anaerobic digestion may also be used in some circumstances.There are good arguments in favour of the incineration of combustible waste as avolume reduction process, especially when coupled to combined heat and power systemsand effective control of waste incineration gases. However, if incineration is chosen asa pretreatment option, then a further question arises; namely, should the ash be disposedto monofill, or should it be co-disposed with household waste? The answer to thisquestion may be important in determining whether the bioreactor or pre-treatmentscenarios predominate. It is important to recognize that the answers are, to an extent,mutually exclusive as the incineration of MSW removes the &dquo;medium&dquo; for effective co-disposal.While the incineration of MSW will remove most (but not all) of the organic material,

there are still likely to be problems associated with high levels of chloride, metals,sulphate and pH (according to the type of gas clean-up process) that would makesustainable disposal difficult to achieve. The ash from incineration (including the flyash) could be disposed to a containment landfill that was dedicated to ash disposal,and in which liquid infiltration was prevented. As discussed above, such containmentsystems must be expected to fail ultimately; in the absence of any attenuating mechanismsduring storage, environmental pollution will result. However, where a site-specific riskassessment has been undertaken, safe assimilation into the environment may be possible.As an alternative, the ash could be stabilized through processes such as solidificationprior to landfill. Yet even after solidification, there is a risk that some toxic species willremain mobile, although the risk will be considerably less than monodisposal ofuntreated ash. Further treatment processes such as solidification would also significantlyincrease the cost of disposal. An alternative which does not appear to have beenconsidered so far is monodisposal in a containment cell with high-rate liquid addition,collection, treatment (including pH modification to enhance leaching) and recirculation,such that upon liner failure, toxic elements of concern have been reduced to acceptablelevels.Whether pretreatment is effected by incineration, biological processing (such as

composting), or by other means, the subsequent disposal to landfill will require carefulmanagement and control such that the risks to the environment are acceptable. Theeffort required for effective control will depend upon the waste stream and the

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pretreatment process, and will be a significant factor (but one of many factors) indetermining the most appropriate waste management option. However, concerns oververy high chloride levels in incinerator ash have already led some landfill operators tosuggest that they will not be prepared to accept incinerator ash for monodisposal inlandfill.

4. Conclusion

There is no such thing as a &dquo;no risk&dquo; waste disposal operation, and a sustainablelandfill cannot be defined in terms of its design and operation. There is no simpleanswer to whether pretreatment or bioreactor approaches to landfill are the mostsustainable. The important concept is that a sustainable landfill should be sited, designedand operated in a way that is appropriate to the local conditions and which reducesthe associated risks to an acceptable level. In this context, the setting of politicallydetermined, prescriptive requirements for landfill design and operation are inappropriateat best and may be detrimental to the objectives of sustainable landfill development.Similarly, strict adherence to a waste management hierarchy, regardless of, for example,economic markets (e.g. for recycled goods) and of regional- or waste-specific variations,may not represent the most effective and lowest risk option for waste disposal.At the present time we do not have the knowledge and data to support choices

between bioreactor landfill and pretreatment. We should be working towards gainingthe knowledge and understanding that, at the planning stage, will allow informed

decisions to be made on the least risk, most sustainable option for a particular location.The concept that containment of any description will ultimately fail is important tothe successful development of a sustainable landfill, in which eventual reassimilationof the wastes into the surrounding environment is the ultimate goal.Through research and development, our understanding of the processes involved in

the production, properties, migration, attenuation and control of landfill gas andleachate have increased considerably in the past decade, such that the risk of pollutionfrom a modern landfill has significantly decreased, provided that landfill activities areconducted in a well-planned, operated and controlled manner. However, there is a needfor yet more research, and there is an especial need to examine the optimization oflandfill reactions for enhanced waste stabilization on a large-scale, and to examine safeand effective ways of disposing incinerator ash that may exhibit very high levels ofchloride, sulphate and a range of metal ions. The concept of the landfill as a treatmentprocess (whether for treated or untreated waste), which will allow eventual re-as-similation into the surrounding environment, seems to offer the best chance of trulysustainable waste management, for it does not rely on the long-term integrity ofcontainment systems. However, the data that will be required to assess effectively thisconcept will take many years to accumulate. If alternative disposal methods are adoptedin the meantime, it may be very difficult to change at some time in the future.The development of a truly sustainable landfill will be important to the safe and

effective management and control of waste in the future. Landfill is an essential partof an integrated waste management strategy, without which effective waste managementwill not be possible. Whether or not we are prepared to pay in the short term the pricefor truly sustainable landfill development remains to be seen. The long-term benefitsare unquestioned.

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Department of the Environment (1995) Waste Management paper 26B: Landfill design, constructionand operational practice. London, UK: HMSO.

ETSU (1993) Landfill gas enhancement studies: the Brogborough test cells. ETSU report B/B5/00080/rep, Harwell Laboratory, Oxfordshire, U.K.

Gendebien, A., Pauwels, M., Constant, M., Ledrut-Damanet, M.-J., Nyns, E.-J., Willumsen, H.-C., Butson, J., Fabry, R. & Ferrero, G.-L. (1992) Landfill Gas from Environment to Energy.Directorate General XVII, CEC, Luxembourg. ISBN 92-826-3672-0.

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Knox, K. (1996) Leachate recirculation and its role in sustainable development. IWM Proceedings[U.K], March, pp. 10-15.

Lee, G. F. & Jones-Lee, A. (1993) Landfill and groundwater pollution issues: "dry tomb" vs.wet-cell landfills. In Proceedings Sardinia ’93, Fourth International Landfill Symposium, S.Margherita di Pula, Cagliari, Italy, 11-15 October.

RCEP (Royal Commission on Environmental Pollution) (1988) Best Practicable EnvironmentalOption. 12th report, Cm 310. London, U.K.: HMSO.

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