With the possibility of serious health problems and disastrous environmental consequences, the need to understand the often mysterious nature of landfill fires is greater than ever. A serious landfill fire results in the downgrading of a ‘controlled’ landfill to ‘uncontrolled’ status, or in practical terms the waste mass becomes inadvertently reconnected to the environment. All the costs and effort of engineering a perfect containment system are wasted if fugitive emissions, often including dioxin and untreated leachate, are released through a perforated cap or liner.
Landfill fires vary in scale from minor outbreaks on the surface, to massive tyre conflagrations with the potential to cause environmental incidents. In human terms, the uncontrolled atmospheric emissions arising from these fires, which often continue for years, are potentially lethal with well-proven acute and chronic health impacts. Recent landfills are very largely comprised of combustibles such as plastic and textiles, which maintain their fuel value into virtual perpetuity.
How do landfill fires start?
For surface fires the reasons are obvious – a heat source of some type has contacted the surface, for instance deposits of hot wastes, lightning, or arson. For deep-seated fires (below 4.5 metres) the initiation mechanisms are quite different. Accidentally initiated fires normally start for one of three reasons:
- Spontaneous Combustion: where a buried heat source, resulting from biological decomposition or chemical oxidation, produces a rise in temperature if the waste mass cannot dissipate the heat faster than it is being produced – a process known as ‘thermal runaway’. The life cycle of a landfill includes two periods of significant temperature rise which unfortunately coincide with elevated oxygen levels and, during the first period, maximum settlement when the landfill mass is prone to collapse and further ingress of oxygen.
- Spontaneous landfill combustion has been traced to a batch of mercury cell batteries which short-circuited during the final settlement of a landfill, and to cotton rags soaked in aluminum paint. Combustion accelerants can also help to make the party go with a zing, the dregs of distilled alcoholic drinks bottles are an example. Spontaneous combustion however, might not be the most common initiator of landfill fires. A well respected landfill fire expert in British Columbia believes that less than 5 % of fires are initiated by spontaneous combustion.
- Piloted Ignition: from a point heat source, happens when ignited waste is buried in the landfill.
Deliberately initiated fires are still common in the Third World as a means of preserving and recovering airspace. The practice is risky as sites are often located within walking distance of populated residential areas susceptible to atmospheric emissions. The most worrying form of deliberately initiated fires is arson – most tyre fires are caused by out-of-hours acts of arson. Landfill arson is sometimes easy to detect but extremely difficult to successfully prosecute.
Landfill operators therefore need to be vigilant for hot deposits during the working life of landfills. Knowing their clients and their arisings, enforcing good working practices, maintaining adequate out-of-hours security and being especially vigilant during the ‘hot’ aerobic phases of the landfill lifecycle will all help to reduce the likelihood of a fire.
Deep-seated landfill fires do not ‘burn’ in the accepted sense. These fires are a form of combustion, known as pyrolysis, where the thermal reaction takes place in an oxygen-starved environment. The combusting material is consumed very slowly and at low temperature. As the waste is heated it begins to devolatalize (you might visualize volatiles as the vapor given off by a candle at the instant it is blown out). The volatiles are either incompletely combusted into other species for example carbon monoxide, dioxin from PVC, hydrogen sulfide from gypsum drywall board, or re-deposited on the surface of cold wastes lying in front of the advancing temperature front. Once devolatalization is complete the remaining fuel, in the form of fixed carbon (visualize this as the charred wick on a candle after it is blown out) can remain hot, under starved oxygen conditions, for years. Mattresses and children’s toys are designed to smolder in a fire and for this reason some operators now refuse to accept them.
Deep-seated landfill fires can expand in two different ways known as ‘confined’ and ‘unconfined’ progression:
- Confined landfills: are formed from multiple layers of waste deposited in thin strata which are compacted by a landfill compactor fitted with sheep’s foot rollers. The rollers tend to re-align the waste into a form which is more permeable horizontally than vertically. These layers are sandwiched between layers of daily cover. In this case, a fire will tend to expand horizontally rather than vertically. A confined fire might be indicated by a shallow collapse, surrounded by tension cracks, at the surface.
- Unconfined landfill: In this case there are no horizontal constraints and a fire will progress vertically upwards producing a dangerous sinkhole at the surface.
Atmospheric emissions from landfill fires are often dismissed as a nuisance. The following are two examples of just how serious the ‘nuisance’ is:
- Dioxin emissions: A program held in the UN, considers that for the foreseeable future, non-industrial, uncontrolled combustion, mainly comprised of landfill fires and illegal barrel burning, will remain the most significant source of persistant organic pollutants (POPs) in the form of polychlorinated dibenzo-p-dioxins (PCCDs) and polychlorinated dibenzofurans (PCCFs)
- Gases and Vapors: landfill fires emit a toxic cocktail of ‘Most Wanted’ fugitive gases including formaldehyde, hydrogen cyanide, hydrogen sulfide, nitrogen oxides and many others . Visible smoke might not be visible since compacted waste acts as a good particulate filter, but fugitive gases are able to percolate towards the surface. A particular problem with smoke, which is largely unburned carbon, is particles that have become activated, in the form of an adsorbent, with a huge appetite for mopping-up ‘most wanted’ contaminants. Very small particles, known as Sub PM2.5s (smaller than 2.5 millionths of a meter in diameter) are capable of remaining airborne for days, and together with adsorbed contaminants will pass directly into the bloodstream once inhaled.
- Groundwater emissions: This is an interesting issue. An uncontrolled release of leachate can occur, even to an otherwise dry site, if groundwater is admitted through a perforated basal liner. Manufacturers of HDPE / LLDPE liners recommend an upper temperature limit of between 60°C and 71°C. Exceeding these temperatures, for even a short time, causes depletion in the membrane antioxidants and a spectacular loss of service life. At 10°C the service life of a liner might be 375 years; at 60°C the service life will have decreased to around 20 years.
Landfill fire detection ( Are our Landfills equipped with security services? )
Surface fires are easy to detect. Deep seated fires can sometimes be predicted and can often be detected at a surprisingly early stage. The scope of this article doesn’t allow a full discussion so we’ll stick to the basics.
We know that, in a landfill, increased heat heralds the onset of the aerobic phases of biodegradation and with it, the presence of oxygen. An alert operator will be aware of heightened risks during these phases. Elevated oxygen levels occur for lots of different reasons including over-abstraction of landfill gas, cap perforation by burrowing animals or poor drilling techniques. A natural phenomenon occurs during a reversal in atmospheric pressure following a period of low pressure. Landfills behave like giant lungs, inhaling and exhaling gas under the right conditions.
The most cost effective method of detecting a fire is through intelligent observation by site staff – an odor similar to a barbecue, gas wells which seem to have fallen out of vertical alignment, breaks or heat softening of landfill gas pipelines, or an unexplained need to change the lubricating oil in landfill gas engines. In addition to this, the operator should institute benchmarking and continuous reviews of key site variables to identify unexpected changes:
- unexpected changes in the topographic shape of the landfill obtained by using ‘cut and fill’ volumetric comparisons from sequential topographic surveys
- changes in vertical temperature profiles obtained from thermocouple arrays. Operators should become nervous of a fast rise (ie ΔT) of greater than 3°C or when the temperature reaches 60°C. At 75°C the site is almost certainly on fire
- any unexpected changes in landfill gas analysis; both in concentration and relative proportions;
1. changes in methane and carbon dioxide might indicate an imminent transition between the aerobic and anaerobic phases
2. changes in nitrogen possibly indicating that the oxygen in the air originally contained within the waste mass has been consumed leaving the original nitrogen content ‘stranded’ in the landfill
3. changes in hydrogen content might indicate a water-shift reaction where carbon monoxide and water react to form carbon dioxide and hydrogen (this occurs naturally in bog fires).
4. changes in carbon monoxide concentration might evidence incomplete combustion.
- changes in leachate analysis – slight increases in the level of nitrogen, pH, conductivity, heavy metals and Chemical Oxygen Demand (COD) together with increases of Total Dissolved Solids (TDS) and ash are all indicators that something is wrong.
Once a fire is suspected, confirmation can be obtained by performing a thermal scan of the site. Thermal scans might help determine the location and extent of a fire but, as we have discussed earlier, the heat from a confined fire might not appear over the epicenter of the fire. Thermal imaging will only reveal heat emanating from the surface. This effect can be used to good effect by using a scan to look at the surface of landfill gas pipes (LFG) laid on the ground. LFG pipes containing warm gas show up clearly. Since one knows the direction of flow of the gas in the system, a quick review of the pipe network might show the approximate location of a fire; even if the landfill surface appears to be at ambient temperature.
Landfill fire treatment
No one can tell you how to put out a landfill fire – there isn’t a single solution for all. It’s an interesting observation that the method which finally extinguishes the fire is not usually the first method tried. Landfill fires often defy common sense and this is complicated by the fact that the fire crew may never have been on a landfill and site operators may never have put out a fire.
Landfill fires incorporating a high proportion of tyres can become uncontrollable in a few minutes and are very difficult to extinguish – a burning truck tyre can be completely immersed in water for 15 seconds and still reignite, when exposed to air, due to the shape of the tyre. Fire extinguishing tactics are usually based around the following options:
- Excavate and douse
- inert gas injection, using nitrogen or carbon dioxide
- Cryogenic (ground freezing)
- Water-based techniques including water injection
- Foam-based techniques.
These methods require a very high level of skill and courage by the plant operators and the continuous input of professional support workers including fire fighters, health and safety specialists, geotechnical engineers and the like. The dangers of plant falling into the excavation cannot be overstressed – the combination of increased pore pressure from fire fighting waters, overloading of the excavation rim and unloading of the toe of the excavation will inevitably conspire to form dangerously unstable geotechnical conditions.
The Mauritians Authorities revealed a worrying lack of understanding by licensing authorities, of their role in fire risk reduction together with a lack of landfill fire competency by site operators and the emergency services.
The means to reduce the environmental and financial impacts of a fire, must be incorporated within the site plans to include both the technical design and in the site operating procedures. The tangible and intangible costs of a fire can be significantly reduced by forward planning:
- Reasonable site licence conditions incorporating fire-avoidance and fire-detection clauses
- Site engineered with fire suppression features (stored water, granular minerals stockpile, etc.)
- Operating regimes designed to identify sources, benchmark site variables to identify changes in – topography, temperature, gas, etc and high levels of site observation, security, etc
- Incident command and control plan, occasional site visits, location of suitable hire plant – extended boom excavators, pumps, foam availability, breathing apparatus, oil booms, lighting towers, etc
- On site resources – hand tools, rakes, shovels, rescue equipment, and camera to gather perishable evidence.
The early diagnosis and treatment of a landfill fire is essential if the cost is to be minimized. The means of continuous monitoring and availability of treatment resources, which should both be mandatorily required and continuously audited by the licensing authority, must be borne by both the operators and the government.
Therefore, a need has been identified for proactive joined-up thinking in landfill fire reduction strategies by encouraging a higher degree of professional competency and leadership amongst licensing authorities and increased training-based competency of site operators, and emergency services.
Video by Jameel Peerally