LANDFILL GAS MANAGEMENT

Ray LombardLombard & Associates

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INTRODUCTION

Modern sanitary landfill sites can be managed to operate like large anaerobic digestors and thequality of the leachate that such sites produce improves in a surprisingly short time. In thecoastal areas of KwaZulu-Natal, where the rainfall is relatively high, sanitary landfill sitesclosely mimic the flushing bioreactor models that have been researched by Cossu, Robinson,Knox and others in Europe1.

However, one of the serious by-products of this approach is the generation of substantialvolumes of landfill gas as a result of biodegradation of putrescible wastes. If landfill gas is notmanaged correctly it impacts negatively on the receiving environment.

The technology of landfill gas recovery has advanced considerably in the last decade. Robustand adaptable systems have been developed to cope with the variable supply and quality ofbiogas from landfill sites. This technology provides for the safe destruction of landfill gas bymeeting strict environmental emission standards. Further, various options exist for the use oflandfill gas:-

• as thermal energy for processes in industry, whether in heating water or raising steam;• as thermal energy for domestic heating in climates that are colder than our own;• as a supplementary fuel for incineration plant;• for the generation of electricity generation and in one case, • in South Africa, as a raw material chemical feed stock for the commercial production of

cyanide.

In South Africa, the relatively low cost of energy is a serious constraint on the viability of mostof the above uses and the primary economic benefit of landfill gas extraction, whether by flaringthe gas or recovering it for use, lies in extending the life of landfill sites. This occurs whenairspace is conserved due to accelerated settlement of the landfill as a result of optimising thecatabolism of the biodegradable fraction of the waste.

The Bisasar Road Class GLB+ Landfill is managed by Durban Solid Waste. A detailed landfillgas recovery investigation was carried out in order to determine the potential gas yield from thissite. Landfill gas problems were identified and the process of managing their impacts has begun.

During the study period it was shown that the Bisasar Road Landfill site life, which was aplanned 37 years given the applicable deposition rates, would be extended by some 7 yearswhich would result in a saving to the Municipal Operating Budget of almost R 60 million.

Initially, a landfill gas curtain system, consisting of six (6) landfill gas wells and a modularHofstetter EGH-01 pump and flare station, was installed to protect the weigh-bridge and the siteoffices from landfill gas migration. Later, a more extensive degassing plant, consisting oftwenty-four (24) landfill gas wells, a gas pump and flare station (including two Hofstetter flares -one 500 Nm3h-1 pilot unit and the other a 2 000 Nm3h-1 slave unit), was installed to cover onethird of the landfill surface area. The operation of these two systems has been extensivelymonitored over a period of two years in the case of the gas curtain and one year in the case of thepurpose-built plant. In addition to monitoring gas quality and flow rates, monthly surveyreadings have been taken to monitor the settlement that has taken place on the site over the same

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period.

WHAT IS LANDFILL GAS

Modern sanitary landfill operations, by definition, involve the controlled spreading, compactionand covering of refuse which rapidly establishes anaerobic conditions. The occurrence oflandfill gas is well known and has been extensively researched and documented world wide.Putrescible wastes contain readily biodegradable carbon compounds and include the categoriesof waste commonly referred to as “Domestic”, “Industrial & Commercial” and “Civic Amenity”.Inert or builders' wastes will also produce gas if they contain paper (e.g. old cement bags), card-board, sawdust, wood and vegetation2. At least one so-called “inert waste” site has already beenshown to be generating landfill gas where a CH4 concentration of 14% by volume has beenrecorded in a monitoring borehole located within that site2.

1.0 Landfill Gas Production

Anaerobic breakdown of organic material results in the production of methane (CH4),carbon dioxide (CO2) and other volatile organic compounds. The resulting gas mixture issaturated with moisture and is referred to as landfill gas. The following composition istypical of landfill gas:-

Component % by Volume

CH4 64CO 2 34N2 2

At 25°C landfill gas typically contains 1.8% by mass of water (H2O) and has a density of1.295 kg m-3. The gas contains trace amounts of volatile fatty acids (VFA) that areresponsible for the typical sour odour associated with landfill gas. Landfill gas from anactive extraction scheme usually has a CH4 content of 40 - 50%. The authors have foundthat in the water surplus areas of KwaZulu-Natal, landfill sites that are anaerobic, with adepth of refuse greater than 2 m and sufficiently moist, will begin to generate significantvolumes of landfill gas within a period after 6 - 9 months after deposition whichcompares with the situation reported by Falzon in Australia3.

2.0 Landfill Gas Properties

Methane (CH4) is a colourless, odourless, asphyxiant, flammable, non-toxic gas that islighter than air with a vapour density of 0.6. CH4 is explosive between the concentrationsof 5% - 15% by volume in air. This concentration range is referred to as the explosiverange with the two extremes being referred to as the lower and upper (UEL) explosivelimits respectively2.

Carbon dioxide (CO2) is a colourless, odourless, non-flammable, toxic gas that is heavierthan air with a vapour density of 1.53. At a level of 3% by volume in air breathingbecomes laboured with resultant headaches. Generally, if the patient can be removedfrom the exposure recovery will usually be rapid. At a level of 5 - 6% by volume in air

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these symptoms become severe and at 10% by volume in air visual disturbances, tremorsand loss of consciousness can occur. The accepted safety limit for CO2 is 1.5% byvolume in air and concentrations above this limit are regarded as hazardous.Hydrogen sulphide (H2S) is a highly toxic flammable gas with a characteristic offensiverotten egg odour. At a level of 50 vpm in air H2S dulls the olfactory system and the gasis no longer detectable making it even more dangerous. H2S is almost as toxic ashydrogen cyanide (HCN). Concentrations from 20 - 150 vpm cause eye irritation and onprolonged exposure pulmonary oedema. Higher levels of around 500 vpm causeheadache, dizziness, excitement, staggering gait, diarrhoea, dysuria and may result inbronchitis and bronchopneumonia. Concentrations in the range of 800 - 1 000 vpm arefatal within a period of 30 minutes3.

The CO2 and VFA components of landfill gas are aggressive to concrete, brick mortarand mild steel. These materials must therefore be protected when used in a situationwhere landfill gas can be expected. Typical protection measures for masonry and mildsteel include epoxy coatings and high density polyethylene lining materials.

Landfill gas will displace oxygen from enclosed spaces making entry to them extremelyhazardous. It is often not clearly understood by landfill operators that their activities arecontrolled by the Occupational Health and Safety Act and that they may be prosecutedfor failing to observe elementary safety precautions. Landfill gas can severely damageplant growth in migrations pathways due to a lack of O2 in the root zone. This occursboth by physical displacement and microbiological use of O2 in the conversion of CH4 toCO2. Entry to enclosed spaces should be prohibited until the atmosphere has been testedfor oxygen (O2) content and the presence of flammable gas. If landfill gas is permitted toaccumulate in low lying or enclosed spaces it will produce an atmosphere that is bothexplosive and hazardous to life. As a guideline, entry should not be made if the O2

content is outside the range 20 - 21% by volume or the flammable gas exceeds 20% ofthe lower explosive limit (LEL)2. Adherence to these limits will ensure that exposure tothe maximum concentration limit for CO2 is not breached.

3.0 Moisture

The moisture content of the waste placed in a landfill is an important factor that affectslandfill gas production. The moisture content of waste in a landfill may change over timedepending on the rainfall, the control of run-off and the management of the site. Ingeneral the higher the moisture content the greater the gas production. In a relativelysmall landfill containing five hundred thousand tons, the production of landfill gas mayreach up to 600 N m3 h-1. During the acidophilic or facultative anaerobic phase ofbiodegradation in landfill the catabolism will produce excess water, some of which willlater be re-absorbed during the methanogenesis or obligate anaerobic phase.

4.0 Variability of Landfill Gas Emissions

The following conditions will cause variation in landfill gas emission and composition5:-• rising atmospheric pressure• falling atmospheric pressure• rate of change of atmospheric pressure• barometric pressure history prior to start of pressure change

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• rising aquifer levels• falling aquifer levels• migration through biologically or chemically active media.Rapidly falling atmospheric pressure results in increased gas flows with increased CH4

content. The more stable the pressure prior to a fall the greater the increase in gasmigration because gas accumulates during periods of stable atmospheric pressure. Theauthors have noted that the zone of greatest variability in landfill gas composition is thetop 2 to 3 m of the landfill. A variation in landfill gas migration related to rainfall hasbeen has also been noted, where gas migration decreases during the dry season in asummer rainfall area. It is felt that the converse may also apply in a winter rainfall area,such as the Western Cape, when wet conditions will promote gas migration.

The CH4 component of landfill gas will biologically oxidise to CO2 in the upper layers ofthe soil cover profile. Consequently rising levels of CO2 may give an early warning ofCH4 migration. Normal soil also contains CO2 from microbial metabolism. Thus, thepresence of CO2 is not always an indicator of landfill gas and must be viewed in the lightof similar soil circumstances unaffected by landfill.

5.0 Odour

Landfill gas contains over a hundred trace compounds that can be malodorous andpersistent in that they tend to become absorbed onto textiles such as clothes, curtains etc.These odours are strongest where decomposing waste is exposed to the atmosphere.Consequently, exposure of previously landfilled waste must be prevented where possible.Typical malodorous compounds are hydrogen sulphide (H2S), esters, terpenes,mercaptans and volatile fatty acids (VFA). These trace compounds are described asvolatile organic compounds (VOC). The normal landfill gas odour is that of the VFAcomponent but the reduced sulphur containing compounds such as H2S and mercaptanscan also influence the odour of the gas depending on the types of waste that have beenlandfilled. Typical odour threshold levels of VOC in mg m-3 are shown below:-

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Typical odour threshold levels of VOC in mg m-3

VOC OdourThreshold

BenzeneDecanesdichlorodifluoromethaneHeptanesLimonenesMethylene ChlorideNonanesOctanesPropylbenzenesTerpenesTolueneTrichloroethylene1.1.1TrichloroethaneUndecaneXyleneEthylbenzene1.2 DichloroethaneButylbenzenes

9 0001 000-100 000577502 0001 000-1 000700115 000800 000800400200250 000-

Walk over levels of CH4 found on typical landfills in Hong Kong are shown below6:-

Normal Occasional Maximum0 - 100 vpm(0 - 69.1 mg/m3)

100 - 500 vpm(69.1 - 345.5 mg/m3)

10 000 vpm(6 910 mg/m3)

For a typical landfill gas CH4 content of 50% by volume these walk over levels indicatedilutions of landfill gas at the landfill surface of 1 000 - 10 000 times. Studies in the UKof three domestic landfill sites have indicated that a dilution in the order of 435 timeswould be sufficient to reduce the concentration of toxic landfill gas components belowtheir relevant occupational exposure limits. Studies in the USA have reached a similarconclusion.

MONITORING LANDFILL GAS

1.0 Landfill Gas Detection

Detection equipment used is either portable hand held for field work or wall mounted andfixed for continuous monitoring in structures or enclosed spaces. Portable equipment forCH4 analysis comprises three types of detectors.

The first type of detector for low level work in the presence of oxygen, operates on theprinciple of catalytic combustion on a sensor called a pellistor. This detector is normallycalibrated to read within the range of 0 - 100% of the LEL, i.e. 0 - 5% CH4. The tworanges are not completely interchangeable as the LEL will depend on the oxygen content

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of the atmosphere being tested. It must be stressed that this type of equipment will givea low or possibly zero result for CH4 or other combustible gas should the O2 level bebelow about 15%.

The second type of detector is based on the differing thermal conductivity of gases, and iscalibrated for a particular gas, e.g. CH4, and does not give a differing response based onthe O2 content of the atmosphere being tested.

The third type of detector is based on infra red absorbance of CH4 and CO2, this type isindependent of oxygen concentration. The most modern instruments also contain anelectrochemical cell for detection of O2.

Equipment selected for use on landfill sites must be able to measure CH4, CO2, O2 andatmospheric pressure.

2.0 Permit Requirements

A typical permit specification by the Department of Water Affairs & Forestry for landfillgas states:-“The Permit Holder shall implement adequate measures to the satisfaction ofthe Regional Director, to ventilate or to prevent lateral migration of CH4 gas generatedin the site so that build up of dangerous concentrations is prevented. The concentrationof flammable gas outside the waste disposal area and inside the Site shall not exceed 1%by volume in air and the concentration of CO2 should not exceed 0.5% by volume in air,amended for Standard Temperature and Pressure.”

Structures exist around most landfill sites which provide enclosed spaces in which toanalyse for the presence of landfill gas. The presence of landfill gas in these structureswill indicate the need for a formalised monitoring programme.

3.0 Installation of Gas Monitoring Probes

Shallow monitoring probes may be installed around the perimeter of the site. Otherpoints such as the leachate sumps, sewer manholes, ground water monitoring boreholesand drains should be included in the monitoring programme. The selection of sites forprobes must be based on potential risk to structures, people and preferential gas migrationpathways.

Deeper and permanent boreholes are advocated by the DWA&F which also specifieslevels in the 'Permit to Operate' of 1.0% CH4 and 0.5% CO2 measured at boreholes withinthe site boundary but outside the waste fill. Such boreholes must be at least 1 metredeeper than the deepest waste. Further, the DWA&F require that monitoring of LFGemission from a closed landfill take place until the levels shown above are met within thewaste body itself over a two year period. This may take anything from 30 - 100 years ifLFG is not actively extracted.

4.0 Ongoing Monitoring

The selected sites should be monitored for landfill gas on a regular basis, e.g. 3 monthly.Regular monitoring will provide information for the development of landfill gasmigration trends. The most frequent trends observed include:-

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• CH4 and CO2 concentrations exhibit a classic inverse relationship with change inpressure.

• Consistent evidence of landfill gas migration towards a particular boundary of the site.• Consistent levels of CH4 within the LEL may be observed in certain probes.

If consistent evidence of landfill gas migration to a boundary is observed, then it is oftengood practice to install further probes along this boundary to determine the area overwhich the migration is occurring.

WHY EXTRACT LANDFILL GAS?

The result of the continuous generation of biogas is a build-up of landfill gas pressure in the sitewhich drives the migration of the gas to the surface of the site, the interface with the soil andstrata on which the site is located and thus the landfill impacts negatively on its receivingenvironment.

Variations in atmospheric pressure due to large weather systems, e.g. cold fronts, cyclones andinter tropical convergence zone formations exacerbate landfill gas migration. The escaping gasimpairs the working of the landfill, pollutes the receiving environment with offending andunpleasant odours, causes plant die-back which impairs site rehabilitation work, increases thefire hazard on the landfill and creates explosive mixtures with air in confined spaces where gasmay accumulate, e.g. basements of buildings and services such as storm water drainage, sewers,telephone cables. Many cases of explosions caused by landfill gas have been recorded but notalways publicised.

The awareness of landfill gas problems in South Africa is increasing and has caused theDepartment of Water Affairs & Forestry (DWAF) to issue permits to operate landfill sites, interms of the Environment Conservation Act (Act 73 of 1989), which contain clauses that enforcethe monitoring and control of landfill gas.

Guidance on how to achieve monitoring and control can be found in the United Kingdom’sDepartment of the Environment’s Waste Management Paper No.27 (WMP 27). A first steptowards control is the installation of monitoring probes and boreholes in land outside the landfillsite to detect the migration of landfill gas. WMP 27 indicates that the levels of landfill gas levelsoutside the site should be less than 1% CH4 and 1.5% CO2 by volume in air. If these limits areexceeded then measures to reduce them must be implemented by the site operator.

DETERMINING GAS YIELDS

The information in Appendix I illustrates the expected landfill gas yield per tonne of waste in alandfill containing putrescible waste. The appended graphs, labelled Bisasar Road Specific GasProduction and Gas Quantity, illustrate the expected landfill gas yield per tonne of waste in theBisasar Road GLB+ landfill. Based on European experience gas production rises to a peakwithin about three years of waste deposition and then declines for a period of up to fifty years.At the peak there may be over 30 m3 of gas being produced in a year by a single tonne of waste.

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1.0 Methods

The yield of landfill gas from a site may be estimated from two different sources.:-• Theoretical yield from accurate refuse acceptance data regarding the mass and types of

waste landfilled.• Pumping trials using normal portable extraction equipment.

Yields should be estimated using both methods where possible.

1.1 Theoretical Yield

The data available regarding waste acceptance in most cases is limited as most landfillsin South Africa do not operate using weigh-bridges. Where weigh-bridge data isunavailable, information based on estimated original ground levels and current landformmay give an idea of in-place volume. This waste volume together with anticipatedmetabolisable carbon content can be used to predict a yield. Events such as fires withinthe refuse will render the yield calculations inaccurate.

The following typical parameters are used as part of the input to the yield model:-• Annual mass of waste landfilled• Estimated metabolisable carbon• Proportions of metabolisable carbon available in the short, medium and long term• Decay half-life of the short, medium and long term components• Average landfill temperature• CH4 content of residual gas

Theoretical calculations normally require validation through a pumping trial to ensurethat yields are not over or under estimated.

1.2 Gas Pumping Trial

A pumping trial will involve the installation of 4 - 8 normal extraction wells into selectedareas of the landfill, connecting these wells to a 250 Nm3 h-1 portable pump and flare unitmodified for monitoring purposes. The unit is a normal one in all other aspects ofconstruction, operation and safety.

1.2.1 Wells - Wells and well-heads are installed as normal for a gas extraction scheme. Wellsare sunk to the base of the landfill using either impact piling or rotary coring techniques.Installation of the test wells provides extremely useful information on potential problemslikely to be encountered in full scale extraction schemes.

Well-heads are also of the normal type with facilities for measuring the followingparameters:-• Gas pressure• Gas temperature• Gas velocity

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• Gas composition

1.2.2 Leachate - In common with the experience of other vendors of landfill gas plant inEurope, the authors have found that the accumulation of leachate within gas wells canseverely restrict collection efficiencies. Thus the gas extraction well-heads installed mustbe capable of having leachate pumping systems fitted. The pumping systems must becapable of operating under explosive, hot, water saturated atmospheres and at low or noflow rates. A leachate pumping system was included as part of the original trial carriedout on the Bisasar Road landfill and provided valuable information on flows, whichdemonstrated the need for a leachate pumping system on bother the gas curtain and theextensive extraction system.

1.2.3 Trial monitoring - The Bisasar Road trial was monitored on a regular basis for gas flowsand quality and leachate flow. The trial concentrated on exhausting the "residual" gas andestablishing the equilibrium gas flow for the site. This exercise took eight (8) weeks.Thereafter, the trial continued for a further twelve (12) weeks to monitor equilibrium gasyield and quality. The measured flows and qualities were then integrated with the theoreticalmodel and the necessary changes made to the input parameters to match actual and predictedyields. The confirmed yield of gas and leachate was used to produce a concept design forongoing management use. Part of the concept design and report covered the potentialeconomic use of the anticipated gas yield.

The results of the pumping trial were used to:-• Design an active gas extraction scheme.• Design gas cut off curtain schemes.• Estimate leachate generation.• Establish if gas yield may be viable as an alternative energy source

GAS YIELD POTENTIAL AND MANAGEMENT STRATEGY

Many factors affect gas production.

One group of factors is derived from the type and quantity of waste landfilled:-• Percentage of organic carbon in the waste• The ease of decomposition of the waste• Moisture content of the waste.

Another group of factors is related to the daily management of those waste inputs:-• Daily cover and type• Degree of compaction• Infiltration of precipitation• Fires• Capping programme• Waste temperature

The final group of factors are related to manipulation of the refuse landfilled in order to

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maximise gas generation:- • Optimisation of waste moisture content• Addition of high moisture content materials such as sewage activated sludge waste• Addition of wastes known to enhance gas generation such as phenolic sludges• Sub-cap leachate recycling with or with out pre-treatment.

In the event that active gas extraction is installed landfill management must ensure themaximisation of gas yields by incorporating the strategies listed above. Production gas wellsare installed at inter-well distances or centres that allow for optimum gas quality. Gas isextracted from these wells at rates that also optimise quality. It is essential that air ingress isminimised to achieve optimum gas quality.

Consequently 100% of the gas generated may not be extracted. Gas collected from individualwells must be actively managed to achieve acceptable gas quality as too high a rate of extractionwill introduce oxygen back into the landfill and inhibit landfill gas production. Various studieshave however shown that a small amount of oxygen entering the landfill may be beneficial to gasproduction2.

1.0 Benefits of Active Gas Extraction

1.1 Migration Control

Active gas extraction is the only effective method of controlling migration from a landfillsite. This control is effected by use of a specifically designed gas well curtain in order toprevent migration to specific high risk areas. Such areas will generally be those withpreferential migration pathways.

The gas wells for environmental control involve the creation of a gas control curtainrequiring the location of gas wells at smaller centres than for gas production wells.Curtain wells often produce gas with lower CH4 and higher O2 concentrations in order toprevent gas escaping. Such gas must usually be flared separately in order to prevent gasquality problems for downstream users. In some circumstances special "pump anddisperse" systems may be required where gas is of too low a quality to be flared safely.These systems have additional safety measures installed to allow them to pumppotentially explosive mixtures.

1.2 Odour Control

Odours are always a problem for landfill operators. The normal odour associated withfresh waste can only be controlled by rapid compaction and cover. The typical odourassociated with landfill gas is not the result of its major components CH4 and CO2 buttrace volatile reduced sulphur, volatile fatty acids and volatile amines. Since landfill gasis the carrier for these compounds and the mechanism of their dispersal the only effectivemanagement is active extraction and flaring or use of the gas as a thermal energy source.

1.3 Settlement

Landfill geotechnical studies have lately become an important area of research in landfillengineering. These have shown that landfill settlement has two major components:-

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• Immediate or physical settlement due to the loading of each new layer of waste placedon top of the older layers. This is most evident as each new lift is placed.

• Long term or creep settlement which is associated strongly with biodegradationassociated mass transfer in landfill.

The removal of landfill gas has resulted in settlements of 2 m or more in a 30 m deeplandfill being recorded. This settlement arises as a result of the gas, leachate andcondensate removal which allows elements in the waste body to collapse. Settlementwill not be uniform because the waste is a not a homogeneous mixture. Settlement ratesin areas subjected to active extraction in the Bisasar Road landfill vary from 15 - 30 mmper metre of landfilled waste depth over a period of some 400 days. A settlement curve for waste of a similar composition (52% builders rubble and soil)deposited on a nearby site (Springfield Park) demonstrated a settlement in the range of 5 -9 mm per m refuse depth landfilled over a period of 700 days7. This reflects long termsettlement rather than physical settlement. An extraction scheme running at 1 000 Nm3 h-

1 for 365 days will remove around 11 000 tons of gas. If gas extraction is undertakenduring the life of the site then this settlement will extend the site life by 10% or moreresulting in a considerable cost saving.

1.4 Biological Stability

The amount of gas generated by any particular site is finite. This finite volume can beallowed to disperse naturally over a long period of time, namely 30 - 50 years, or can beactively extracted over 10 - 15 years. In both instances the waste mass can beconsidered stable once the finite gas volume has been generated. At this stage thelandfill no longer requires monitoring in terms of the Permit to Operate.

1.5 Leachate Extraction

Waste in a landfill may be described as being composed of a series of porous lens shapedelements. These lenses are separated by a network of structural voids or macropores.Liquid flow takes place within the interior of the lenses and the structural voids. Wastecompaction increases as additional layers of refuse are built-up and microbiologicaldegradation takes place. Compaction increases with depth while permeability decreases.The concept of a continuous leachate piezometric level in landfilled waste is not valid.Pumping tests have shown highly variable responses to draw down in adjacent wellswithin 10 - 15 m of pump test wells. Water level response varied from zero draw downto a draw down of 6 m below the water level in the pump test well.

A landfill may be described as being a complex mix of flow regimes in multiple layerscomprising leaky, confined, and unconfined aquifers interconnected with aquitards andaquicludes8. It is normally necessary to remove leachate from gas wells in order toensure efficient gas removal. Due to the extreme variation in leachate levels, wellsnormally fill from the many perched leachate horizons within the waste mass. The liquidlevel within a well frequently reaches levels of 1 m or less from the landfill surface.Under these circumstances active gas extraction becomes almost impossible unlessleachate is pumped from the wells. The authors have found that leachate level control isbest performed on combined gas and leachate wells rather than in separate ones due tothe complex flow regimes.

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Pumping leachate from a landfill reduces the potential for leachate to reach theenvironment and removes waste breakdown products further hastening waste massstabilisation. The pumped leachate may require pre-treatment prior to disposal to anormal waste water treatment works.

1.6 Other Environmental Benefits

Both CH4 and CO2 contribute to the "Greenhouse" effect9. Green house gases compare interms of their radiative effect with that for CO2 as follows:-

Component Relative Rating

CO2 1CH4 20 - 30 CFCs 10 000

CH4 is 20 - 30 times more efficient in radiating energy back to earth than CO2. There isnormally a trace component of CFCs in landfill gas derived from the disposal of aerosolcans, CFC blown styrene foams, refrigerators and air conditioner leaks. Collection andcombustion of landfill gas converts the CH4 to CO2 and water. The contribution oflandfill gas to the green house effect is thereby reduced. CFCs are destroyed in flareswith the destruction efficiency depending on flare temperature and design.

Municipal solid waste contains almost 30% biodegradable carbon of which two thirdsmay be converted to landfill gas. If the gas is not collected and flared there is a verysubstantial contribution to the greenhouse effect from the CH4/CFC component. There isa further reduction in greenhouse gas generation if the energy value of CH4 replacesfossil fuel use.

Global estimates for landfill CH4 production are around 40 x 106 tons in 1995. Theoverall global production figure is estimated at 375 x 106 tons. High efficiency gascollection and energy recovery schemes are essential in reducing CH4 emissions.

1.7 Energy Replacement

Landfill gas typically contains up to 50% CH4 and has calorific value of 16 - 18 MJ Nm-3.The conversion of thermal energy to electricity via a reciprocating spark ignition enginehas an efficiency of 33 - 38% depending degree of use of exhaust heat. The rates varydepending on the consumer and consumption. Thus, each case has to be treated on itsown merits. For example:-

In the case of Pietermaritzburg Msunduzi TLC, generation of 1 MW of power requiressome 700 Nm3 h-1 of landfill gas at 50% CH4. Installation of a 900 kW capacity sparkignition engine, generator and switch gear is estimated to cost R 3 000 000. Amortisingthis cost over 5 years at 19% interest plus maintenance gives an annual running cost of R1 231 000. Using the PMLC C1 rate of R0.106 per kWh and monthly MD charge of R42.12 per kVA on 90% availability gives a potential income of R 1 232 000.

In the case of the Durban Metropolitan Area, the situation is complicated by the cost

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recovery strategies employed by the Metro Electricity utility. The appended graphs,labelled BISASAR ROAD Energy potential and Collectable gas yield, illustrate theenergy value of the landfill gas that may be recovered from that landfill. As has beennoted in developed countries in Europe, where energy costs are, in general higher, than inSouth Africa, electricity generation is not economic unless special tariff rates are paid forlandfill gas based power. By comparison the United Kingdom Non-Fossil Fuel Optionattracts a preferential tariff of R 0.23 - 0.29 kWh depending on whether generation is onpeak or off peak. UK generation costs are the equivalent of around 8 - 10 c/kWh-1. Thislower cost is largely related to the much lower interest rate prevailing in that country.

Direct thermal use of the gas requires 3% - 4% additional energy to compensate for theenergy lost in heating the non-combustible CO2 component. The direct use of gas inapplications such as cement kilns, asphalt hot mix plants, brick kilns, glass furnaces,incinerators or steam raising is the most economic within a South African context.

Our current electrical energy price does not cover the cost of generating electrical powerexcept possibly as standby to replace diesel use or maximum demand lopping.

Emissions from landfill gas combustion can be made to comply with the most stringentEuropean Standards providing correct flare technology or ignition control in sparkignition engines is selected. Meeting these strict criteria is more costly.

THE BISASAR ROAD PROJECT

1.0 Investigation

1.1 Landfill DTM

Historical survey data obtained from the City of Durban’s Survey Department wascaptured by manual input. This information was used to produce a digital terrain model(DTM) of the landfill site reflecting the original topography of the site , the current landform and the final geometric design of the site. The DTM was used to position theexploratory wells and, later, the gas curtain extraction and production wells for thelandfill gas extraction scheme.

1.2 Pumping Trial

Demonstration pilot wells were initially established during 1991 to evaluate the presenceand concentrations of biogas in the landfill. These wells were sited at the lower end ofthe landfill immediately above and adjacent to the main site stability berm. Due to thepresence of a vast perched water body within the landfilled waste extreme difficulty wasexperienced using a large machine auger.

Obstacles within the waste such as concrete and tyres frequently resulted in refusal atrelatively shallow depth. Thus, only two wells were established during the demonstrationperiod. During the demonstration period landfill gas was also monitored in theweighbridge offices and other facilities adjacent to the landfill. The landfill gas yieldedby the two wells, together with the results of the gas survey, was sufficient to convince

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Durban Solid Waste to carry out a full gas extraction pumping trial.

In addition to the pilot wells installed during the demonstration project another eightwells were established, of which only two proved useful, using a FrankiPile Type PilingRig. This method was not subject to the refusals experienced with the augur method.These wells were sited using the DTM described above in order to maximise the wastefill depth in order to ensure anaerobic conditions and landfill gas production. AHofstetter EGH-03 M mobile test pump and flare station was installed and connected tothe gas collection system connected to the extraction wells. However, upon commissioning the respective gas wells, it soon became evident that theproblem of perched water bodies within the landfill was widespread on this site. Severaldifferent de-watering systems were tested with mixed but inconclusive results. Theextraction wells were re-located until relatively “dry” positions were found where thewells did not become flooded with perched leachate and the landfill gas pumping trial re-commissioned.

The pumping trial was carried out over a period of six months. It took 6 weeks for thegas flow rates and the quality of the landfill gas to attain a steady state. Initially, residualbiogas that had built up within the landfill had to be extracted and microbial catabolismhad to be stimulated to produce the sustainable gas yield that the gas pumping trial wasdesigned to measure. Having stabilised all the gas wells at a flow rate the followingmeasurements were carried out at regular intervals:-

• %CH4

• Gas velocity• Volumetric flow rate• External temperature• Barometric pressure

Flow rates at both the well heads and the pump and flare station were varied in order todetermine the sustainable gas yield . These results formed the input which was used tocalculate the specific gas yield for the landfill site as reported in Appendix II.

2.0 Landfill Gas Curtain Scheme

2.1 Objectives

2.1.1 The primary objectives of the landfill gas management system are:-• to achieve the minimum practical risk of explosion, fire, asphyxiation, odour or

detrimental effect on the health of people, animals or plants due to the presence ofLFG on or through migration off the portion of the site adjacent to theoffices/weighbridge accommodation.

• To exclude as far as is practicable, all landfill gas from buildings, services, ducts andenclosed spaces on and off that portion of the site adjacent to the offices/weighbridgeaccommodation.

• Where gas cannot be fully excluded, to maintain an adequate monitoring regimewithin the affected area to ensure safety at all times and to enable remedial measuresto be taken.

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2.1.2 The secondary objectives are:-• To prevent unnecessary ingress of air into the site so as to minimise the risk of

underground combustion and to optimise the generation of methane.• To utilise the LFG as an energy source.

It must be emphasised that the primary function of the LFG management system is toensure safety. Safety must not be compromised in the interests of thermal energy usage.

2.2 System Construction

The system consists of six (6) gas wells at 36 m centres some 23 m from the crest of theretaining berm on the eastern side of the site adjacent to the site office and weighbridgeaccommodation. The wells comprise a 160 mm HDPE slotted 10% open area gasrecovery pipe surrounded by graded 19 mm stone. The recovery pipe enters an FRST300 well head which is connected to 110 mm HDPE gas collection piping. Gas flowfrom wells may be individually controlled by the Hofstetter MRV 100 regulator.Sampling points for determination of gas velocity, temperature and pressure are part ofthe MRV regulator.

The wells are connected to a Hofstetter EGH 01A/N pump and flare station capable ofextracting and flaring 250 Nm3 h-1 landfill gas containing from 25 - 50% CH4. The flareis adjustable for optimum flow rate and flame temperature stoichiometry.

Each gas well contains a leachate removal system. This system consists of a leachatecirculating pump and supply/return piping to an eductor in each well. The circulatingpump is fed from a header tank. The pump is protected from running dry by a low levelcut out. This cut out only operates in the automatic mode and there is no manualoverride. Make up to the header tank is from fresh water via a float valve. Excessleachate overflows to sewer.

Condensate generated in the knock-out system at the pump and flare station is alsodischarged to sewer. The knock-out system contains a water seal to prevent air ingress..

2.3 Monitoring and Recording Procedures

A comprehensive programme of monitoring is an essential element in ensuring that theuncontrolled migration of LFG out of that portion of the site adjacent to the weigh-bridge/office accommodation does not pose a threat to safety of the environment.

DSW ensure that all instruments required for carrying out the necessary monitoring areavailable. The type of instrumentation available must be suitable and approved for theintended use. Instruments and equipment must only be used in accordance with themanufacturers guidelines and must be maintained and calibrated in accordance with hisspecifications.

The following item of equipment is required for monitoring the scheduled parameters:-

(i) A portable combined CH4, CO2 and O2 detector capable of measuring thesegases in landfill gas mixtures on both the LEL and percent volume scales.

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(ii) A digital manometer for measuring pressure differentials to within anaccuracy of 1 mbar.

(iii) A vane anemometer for measuring gas velocities in collection piping. Thisunit should also be capable of measuring temperature and of providing timeaveraged results as well as instantaneous readings.

All these instruments must be intrinsically safe.

2.4 Normal Operating Procedures

2.4.1 Gas wells - The gas production wells are sunk into the landfill with a 400 mmFrankiPile piling rig. A 160 mm NB, Class 6, HDPE, 10% open area slotted gasrecovery pipe is inserted and the annulus filled with 19mm, no fines, non acid reactive,stone. A FRST-300 well head fitted with an MRV-100 gas regulator is place over therecovery pipe. The well is sealed with a 2.0mm HDPE skirt and protected with 1250 mmdiameter concrete manhole rings.

The six gas wells may be "tuned" for gas quality to provide the required flow to preventmigration of gas from their area of influence. The flow regulation is performed with theMRV-100 regulator. The methane level in gas drawn from a well must be measuredwithin 24 - 48 hours after any flow adjustment. Normally CH4 level should not beallowed to go below 25% v/v. The MRV-100 regulating unit is fitted with a 1" BSP ballvalve/sampling adapter for measuring gas velocity and methane content. In addition a 0 -50 oC bimetal strip temperature indicator has been inserted in the 2" BSP fitting tomeasure gas temperatures. Wells producing large quantities of gas will show gastemperatures above 40oC, whilst those producing lesser quantities of LFG or with ingressof air will show temperatures below 30oC.

2.4.2 Leachate extraction - The leachate eductor system receives supply and return waterthrough the well cap. This water can be heard when the eductor is operating correctly.Blockages of the eductor nozzles are relatively common due to ingress of plastic, sandgrit etc. A blockage may be detected by the following symptoms:-• no discernible water noise when circulating pump is running; • higher than usual pump discharge pressure;• low gas flows from that well as a result of leachate build up;• higher than usual methane levels in adjacent monitoring wells as a result of low LFG

flow from the flooded well.

The leachate eductor system must be operated as an integral part of the gas wellmanagement. Each eductor has a leachate pumping capacity of 1000 l h-1. This leachatewill overflow from the tank to sewer during normal operation. Should the level ofleachate in the wells be below the eductor then there will be gas ingress to the system andgas release in the header tank.

The header tank must be regarded as potentially explosive and oxygen deficient at alltimes. The pump timer must be adjusted to accommodate leachate ingress to the wells asconditions change. The circulating water temperature should not rise above 45 oC.

2.4.3 Pump and flare station - Before starting the flare after any maintenance the blower system

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must be purged of air. The unit starts after a self check cycle. The gas valve opens and theignition transformer is energised. If flame presence is established by the UV monitor thennormal operation is established. If the flame is not detected the unit shuts down. The unitwill re-start a further 4 times. Thereafter the plant will shut down completely. There are 4fault lights to indicate the reason for a shut down.

These are:-

• High condensate level• High blower temperature >80oC• High blower current• Flame out

Once the fault has been rectified the re-set button is pressed and the unit may be re-started. The combustion conditions may also be adjusted to attain minimum carbonmonoxide formation. The flame must be set to run with excess air. Common flameconditions are identified as follows:-

Poor combustion - Flame noise hardly discernible, low flame temperature, CO formationand odour formation.

Ideal combustion - Flame noise distinctly discernible, steady flame with temperature>800oC, no CO formation and no odour formation.

Excess air - Flame unsteady and fluttering, flame goes out, flame temperature too high>900oC and very noisy.

The flame will require adjustment on each occasion that well flow rates are altered inorder to properly combust the changed quality and quantity of gas being extracted.

3.0 Landfill Gas Extraction Scheme

3.1 System Construction

3.1.1 Gas extraction - The system comprises 24 gas wells in two parts. The production wellsection has 20 wells at 50 m centres and the gas curtain section has 4 wells at 36 m centresin an arc on the western side of the landfill adjacent to the informal settlement area.

Each well consists of a 160 mm HDPE slotted 10% open area gas recovery pipe in a 410mm OD percussion bored well. The recovery pipe enters an FRST 300 well head which isconnected to 180 mm HDPE gas collection piping. Gas flow from wells may beindividually controlled by the Hofstetter MRV 100 regulator. Sampling points fordetermination of gas velocity, temperature and pressure are part of the MRV regulator. Thewell heads are fitted with a bi-metallic strip temperature gauge monitoring gas temperature.The wells are connected to a Hofstetter 2 500 Nm3 h-1 pump and flare station. This station isenclosed in a container which houses a Roots type positive displacement blower and 2 flaresof 500 and 2 000 Nm3 h-1 capacity. The station contains continuous gas analysis equipmentand the unit is controlled using a Programmed Logic Controller (PLC).

3.1.2 Leachate extraction - Each gas well also contains a leachate removal system. This system

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consists of a leachate circulating pump and supply/return piping to an educator in each well. Thecirculating pump is fed from a header tank. The pump is protected from running dry by a lowlevel cut out and its suction is consequently always flooded. This cut out only operates in theautomatic mode and not on hand. Make up to the header tank is from fresh water via a floatvalve. Excess leachate overflows to the sewer.

The system is PLC controlled to extract leachate from 5 different leachate circuits.

3.1.3 Condensate removal - Condensate generated in the knockout system at the pump and flarestation is discharged to sewer. The knock out system contains a seal to prevent ingress. Thewater seal must be full at all times.

3.2 Normal Operating Procedures

3.2.1 Gas wells - Each gas production well was sunk into the landfill with a 410 mm FrankiPiletype piling rig. A 160 mm NB, Class 6, HDPE, 10% open area slotted gas recovery pipe isinserted into each pile driven hole and an FRST-300 well head fitted with an MRV-100 gasregulator is placed over the recovery pipe. The well is sealed with a flexible seal using abentonite plug and the well head assembly is protected with 1 250 mm diameter concretemanhole rings and a manhole lid.

The gas wells may be "tuned" for gas quality to provide the required flow to preventmigration of gas from their area of influence. The flow regulation is performed with theMRV 100 regulator. This item is a rotary piston valve adjusted by screwing the plug in orout of the valve body. The rotary piston is exposed by unscrewing the cap. Flow changesare made by adjusting piston depth with the special spanner provided and stored in the flarestation control box.

The methane level in gas drawn from a well must be measured within 24 - 48 hours afterany flow adjustment. Normally CH4 levels should not be allowed to go below 25% v/v. theASOM analyser unit in the flare station is set to provide and alarm at 30% CH4 and will tripthe plant at 25% CH4.

The MRV-100 regulating unit is fitted with a 1" BSP ball valve/sampling adapter formeasuring gas velocity and methane content. In addition a 0 - 50 oC bi-metal striptemperature indicator has been inserted in the 2" BSP fitting to measure gas temperatures.

Wells producing large quantities of gas will show gas temperatures above 40 oC, whilstthose producing lesser quantities of LFG or with ingress of air will show lower temperaturesand may be below 30 oC.

3.2.2 Leachate extraction - The leachate eductor system receives supply and return water throughthe well cap. This water can be heard when the eductor is operating correctly.

Blockages of the eductor nozzles are relatively common due to ingress of plastic, sand gritetc. and may be detected by the following symptoms:-

• no discernible water noise when circulating pump is running• higher than usual pump discharge pressure• low leachate discharge to sewer• low gas flows from that well as a result of leachate build-up

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• higher than usual methane levels in adjacent monitoring wells as a result of low LFGflow from the flooded well.

The eductor system must be operated as an integral part of the gas well management. Theeductor system is powered by two pumps feeding 5 lines off a common manifold. Themanifold is split with a cross connection valve to allow either pump to run all five lines.

The take-off from the manifold to feed lines is controlled with electrically driven actuatedball valves operated via signals from a PLC. The PLC controls the duration, sequence andfrequency of operation of the ball valves. The pumps are protected from running dry byusing cut outs on low tank level and low flow. The return flow from the eductors enters theholding tank for re-circulation. Excess leachate drains to sewer through a turbine watermeter for flow recording.

Each pump has its own switch gear which can select "Auto" or "Manual" operation and eachvalve can be activated manually to open or close. Whilst any pump or valve is beingoperated manually none of the safety systems operate. On no account must any pump beoperated manually unless there is an adequately experienced person present at all times.Each eductor has a leachate pumping capacity of 1 000 l h-1. This leachate will overflowfrom the tank to sewer during normal operation. Should the level of leachate in the wells bebelow the eductor then there will be gas ingress to the system and gas release in the headertank. The header tank must be regarded as potentially explosive and oxygen deficient at alltimes.

The pump timer must be adjusted to accommodate leachate ingress to the wells asconditions change. Run the pump for several shorter periods to keep leachate levels low andto prevent the circulating water temperature from rising as a result of heat exchange with thehot LFG. The circulating water temperature should not rise above 45 oC. Records ofleachate flow and pump pressure must be kept on a regular basis.

3.2.3 Pump and flare station - The manufacturer’s operations and maintenance manual must beadhered to at all times. This manual is provided in 3 files.

File 1/3 Process StationFile 2/3 Electrical ControlFile 3/3 Appendices

These files provide full details of the manufacturers specifications, operating procedures andmaintenance requirements for the plant.

3.3 Routine Maintenance Procedures

3.3.1 Gas wells and leachate extraction - It is recommended that maintenance be performed aspart of a maintenance contract offered by Envirogas Management Systems (Pty) Ltd.

3.3.2Pump and Flare Station - It is recommended that this be performed as part of amaintenance contract offered by Envirogas Management Systems (Pty) Ltd. The normalmaintenance interval is every 2000 hours. Blower lubricating oil is however onlychanged every 4000 hours or every second service. The typical service will covercleaning and/or changing filters, changing oil, sensor and analyser calibration, ignition

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system performance testing, assessment of the mechanical and electrical condition of allcomponents.

CONCLUSIONS

Landfill gas causes several problems, including odour propagation, vegetation die-back,explosive conditions in enclosed spaces and asphyxiating conditions in enclosed spaces. Landfillgas migration is strongly influenced by falls in atmospheric pressure. The best method ofcontrolling gas migration is active gas extraction with a pump and flaring of the extracted gas.This is carried out in two forms:-

Firstly, the installation of gas barrier curtains where there is a known migration pathway or riskto property and life,

Secondly, through the installation of production wells where the majority of the gas can becollected of a quality which enables it to be used as an economic source of energy. It may beused directly as thermal energy or converted into electricity.

Active gas extraction is only possible when the gas wells do not contain leachate. Shouldleachate be present it must be extracted from the landfill.

The only way to establish whether leachate pumping is required is by undertaking a pumpingtrial which assesses both leachate and gas production potential in the landfill site. Active landfillgas extraction/leachate pumping and flaring or utilisation from a landfill provides the followingbenefits:-

• Landfill gas migration control• Odour control• Increased rate of settlement resulting in additional air space for landfilling• Increased rate of waste stabilisation• Reduction in leachate migration• Reduction in Greenhouse gas emission• Fossil Fuel Energy Replacement

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BIBLIOGRAPHY

1. Robinson, H., Sustainable Waste Management : is there a future for landfills? Proc. ofWasteCon’96, Institute of Waste Management, Durban 17 - 21 September 1996.

2. Waste Management Paper No. 27 "Landfill gas" Second Impression 1992, UnitedKingdom Department of the Environment.

3. Falzon, J., Landfill gas: An Australian Perspective, Proceedings SARDINIA 97, Vol. II,Page 487-496.

4. "Dangerous Properties of Industrial Materials", Ed N Irving Sax, 6th Edition, 1984.

5. BKS Report, Monitoring and projection of settlement on the Springfield ParkDevelopment - 1986.

6. Scott Wilson Kirkpatrick and Partners, January 1992, Tseung Kwan O and SENTLandfill, Landfills Investigation, Final Report, Environment Protection Department HongKong.

8. Bendz, D.P., et al, "Flow Regime in Landfills - Implications for Modelling", ProceedingsSardinia '97, Vol. II, pp 97 - 108.

9. Meadows, M. et al, "Global Methane Emissions from Solid Waste Disposal Sites", Proc.Sardinia 97, Vol. IV, pp 3 - 10.

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