Landfill Methane - A Conflicted Concept
Contents |
Introduction
Managing existing organic waste in landfills, while also addressing the disposal of new organic waste, is a challenge. While organics already in landfills need to be managed to mitigate threats to groundwater and to minimize methane leakage, the government has indicated that the best long-term solution is organic waste diversion. The ECO supports this philosophy.
The “divert organics” philosophy, however, is being undermined by policies and regulations currently in place. In our 2008/2009 Annual Report, we expressed concerns about the conflicting messages that these policies may be sending to municipalities with regard to organic waste. These conflicting messages are a function of questionable modeling assumptions that underpin the design of landfill gas collection systems and the inherent incompatibility of landfill management priorities relating to energy production, groundwater protection and GHG emissions control.
5.2 Questionable Design/Modeling Assumptions
The difficulty in obtaining reliable field measurements of uncontrolled methane releases (referred to as ‘fugitives’) makes an accurate inventory of GHG emissions from landfills difficult to achieve and has led to a proliferation of models to estimate GHG emissions.
Current Emissions Models Are Inaccurate
Landfills (Figure 1) can extend over tens of hectares and, with their base extending approximately 20 metres into the ground, may be many tens of metres high.
Given their size, it is difficult to measure the fugitive emissions escaping from landfills. They leak into the atmosphere through cracks, tears and broken seams along the sides and top of the structure, and can escape through leachate collection trenches and piping from the bottom of the facility. Without actual fugitive emissions data, modeling is needed to predict fugitive releases from the facility. The variables in such a model must include a number of factor inputs, such as the quantities (and types) of waste-in-place, operating parameters, moisture conditions, and related environmental conditions.
Like balancing a chequebook, a “mass balance analysis” attempts to estimate fugitive releases by accounting for all the material entering and leaving the landfill system. In addition to estimating the total gas potential in a landfill over its biologically-active lifespan, this requires knowledge of:
- The annual gas generated based on decomposition rates;
- Of that generated, the amount of landfill gas captured (via the gas collection system), sequestered (in the waste mass), and oxidized (in the overlying earthen cover) which then needs to be subtracted from the estimate of gas generated to determine net (fugitive) releases; and,
- The gas capture rate.
In essence, the challenge is to solve for the following equation:
Total Annual Gas Generated = Gas Captured/Assumed Capture Rate
Of the three parameters noted above, it is usual to have solid data only on the amount of gas captured. There are often reasonable estimates available for the total gas generated and decomposition rates, but only theoretical calculations of sequestration, oxidation and collection efficiency (the gas capture rate), and no reliable information at all on fugitive emissions. As such, the value of these models in determining the landfill sector’s contribution to Ontario’s GHG inventory is questionable.
Collection Efficiencies Much Lower Than Assumed
The ECO noted in our 2008/2009 Annual Report that the efficiency of a landfill gas control system depends on many factors, including the placement of the collection pipes and the permeability of the containment materials around the landfill. There is conflicting opinion in the literature regarding gas capture rates. The U.S. Environmental Protection Agency (U.S. EPA) has assumed that landfill gas collection efficiency is 75 per cent. This unconfirmed assumption has been incorporated directly into Ontario’s GHG inventories.
The initial basis for the U.S. EPA’s 75% efficiency estimate is based on what the EPA assumed are the best – not the average – gas collection efficiencies. Some landfills perform optimally, while others may have less efficient or incomplete gas control systems. Technical reports from independent sources indicate that instantaneous gas collection efficiencies range between 34 and 50 per cent, averaging at approximately 40 per cent.
Additionally, the 75 per cent figure is based on what optimal systems can achieve when the efficacy of a gas control system is at its highest. This is a period after the final cover is installed and continues while the cap is maintained. However, gas starts to be generated between five and twenty days after organic waste is buried. In landfills, food scraps tend to decompose first, followed by paper products and textiles, creating gas and leachate. Further, in Ontario, the collection systems may not be installed or become operational for several years. The IPCC has determined that the best collection systems operated at the optimum times (when the landfill is sealed) may achieve efficiencies greater than 90 per cent. However, the IPCC also noted that not all landfills perform optimally and that “there are fugitive emissions from landfilled waste prior to and after the implementation of active gas extraction” such that “estimates of ‘lifetime’ recovery efficiencies may be a low as 20%”.
Problems Estimating Total Gas Potential
The lifetime gas generation potential (LGGP) of a landfill is calculated by measuring the organic fraction of the municipal wastes contained therein. While the standard assumption has been that the LGGP of organic waste is 100 cubic metres per tonne (m3/t), there is considerable variation in that estimate. By isolating the degradable organic fraction for further analysis, researchers postulate that LGGP can vary by a factor of 300 per cent, ranging between 100 and 310 m3 of total gas/t of waste.
The issue is complicated further when trying to estimate the fraction that methane comprises of the total gas potential per tonne of waste. It has been noted that “[t]here is no method for determining methane potential that is without fault.” Ontario assumes that 50 per cent of total landfill gas is methane; however, observed methane ratios in landfill gas are reported to range from 35 to 60 per cent.
Calculation of an accurate methane gas potential figure requires reliable waste composition and sequestration data, both of which are often lacking. The IPCC’s 1996 Guidelines state that, “[t]he degradable organic content (DOC) of the waste has a large impact on the potential methane generation value. Small variations in the DOC inputs can result in large variations in the overall methane estimates.” The implication here is that variations in degradable organic content will generate large errors in the estimates of uncontrolled (fugitive) releases of methane from landfills – a situation exacerbated further by uncertainties regarding moisture levels and distribution in landfills.
Uncertainties Regarding Moisture Levels and Distribution
Liquids are not evenly distributed in landfills. Municipal solid waste is highly heterogeneous, heavily compacted, interspersed with daily cover, and often confined in plastic bags, all of which create preferred paths for water flow. Estimates are that liquids only reach 23 to 34 per cent of the waste mass. This means that there is inadequate moisture for complete decomposition. In-coming wastes usually contain not much more than 20 per cent moisture. However, complete biological conversion requires 60 to 80 per cent moisture. This level of moisture, essential for bacteria growth, metabolism, and nutrient transport, is necessary to optimize the generation of methane.
Ordinarily, landfills might achieve average moisture levels of, perhaps, 35 per cent (none of which would be evenly distributed). In Ontario, landfills that employ leachate recirculation to protect groundwater, as well as bioreactor landfills can significantly increase moisture levels. More moisture translates into greater amounts of methane gas generated. If, however, collection efficiencies are as low as 40 per cent as suggested above, then a much larger volume of methane gas must be leaking from landfills as fugitive emissions. The ECO has cautioned that these uncontrolled releases of methane and other GHGs “could reduce, offset or even exceed the potential environmental gains from landfill gas capture and power generation.”
Table 6 compares the fugitive methane releases from a hypothetical landfill with a metered annual capture of 10,000 m3 of methane. A capture rate of 75 per cent, as assumed by MOE, yields an annual uncontrolled methane leakage rate of 3,333 m3. However, if the assumed collection efficiency drops to 40 per cent, then the fugitive methane leakage rate increases by 4.5 times to 15,000 m3, all other factors being equal. If the lifetime collection efficiency is as low as the IPCC has suggested – 20 per cent – then the fugitive release rate increases by a factor of 12. While the ECO recognizes that these are estimates, they illustrate the uncertainty regarding the true impacts of landfilling organic wastes.
| Factors | Scenarios | |||||
|---|---|---|---|---|---|---|
| A | B | C | Units | |||
| Methane Captured | 10,000 | 10,000 | 10,000 | m3/yr | ||
| Methane Concentration Ratio | 50% | 50% | 50% | |||
| Capture Rate | 75% | 40% | 20% | |||
| Oxidation Rate | 10% | 10% | 10% | |||
| Sequestration Rate | 0% | 0% | 0% | |||
| NMOC* | 0.5% | 0.5% | 0.5% | |||
| Outputs | ||||||
| Fugitive Methane | 3.333 | 15,000 | 40,000 | m3/yr | ||
| Fugitive NMOCs | 33.3 | 150.0 | 400.0 | m3/yr | ||
| * Non-methane organic compound Source: Center for a Competitive Waste Industry, 2011. |
So, we are left with what amounts to a landfill operational/design conundrum:
- High moisture levels, only present some of the time in landfills, are a prerequisite for gas to be generated
- An impermeable cover or cap is necessary to create the vacuum pressures needed for gas collection to work properly
- The cap prevents the entry of precipitation, reducing moisture levels
- When moisture levels drop, gas generation tapers off, leaving an undetermined but likely significant fraction of organic waste susceptible to future decomposition
- Post-closure, when the cap is no longer being actively maintained, it will eventually degrade and crack
- Cap failure allows moisture to re-enter the site, re-activating biological activity in the remaining organic waste and the generation of methane gas
- This ‘new’ methane will appear as fugitive releases into the atmosphere.
Other Concerns
There is considerable disagreement regarding the extent to which methane generated in landfills is destroyed through oxidation in the overlying soil layer. The U.S. EPA assumes that 10 per cent of the methane generated in a landfill is oxidized in the soil layer that tops a closed cell. However, oxidation rates drop if a composite cap has been installed under the soil blanket. In that case, landfill gases concentrate along cracks and tears that can appear in the plastic sheeting. Such high flux emissions quickly overwhelm the capacity of the topsoil to oxidize the escaping methane. Further, in Ontario, a correction for colder winter temperatures would likely need to be applied. However, the ECO is unaware of any field studies of oxidation in colder temperatures that have been cited in Canada’s or Ontario’s GHG inventory reports.
There is also debate about the role of carbon sequestration in the remaining lignin not decomposed in landfill organic matter. While the U.S. EPA has suggested a 10 per cent sequestration rate based on one laboratory test, more recent research contradicts this by showing that the actual sequestration rate ranges between 0.8 and 9.4 per cent. As such, the uncertainty surrounding the roles of oxidation and sequestration of methane in landfills further obscures the actual fugitive methane releases from landfills. If we assume the lower oxidation rates noted above, then this means that even higher fugitive methane releases from landfills could be occurring than depicted in Table 6.
To summarize, many of the key technical assumptions that underpin landfill gas control practices in Ontario have never been properly tested or verified in the field. This calls into question the methodologies and assumptions determining the waste sector’s contribution to provincial GHG emissions. For example, if collection efficiencies are 40 per cent on average instead of 75 per cent, as noted above, then the province is significantly underestimating fugitive releases from landfills. It also calls into question the rationale for landfill energy production as an appropriate component of a climate change mitigation strategy.
5.3 Conflicting Regulatory Requirements
Landfill Gas Collection and Control Regulation (Ontario Regulation 232/98 – Landfilling Sites, made under the Environmental Protection Act)
Amendments to Ontario’s landfill regulations, promulgated in 2008, require landfill facilities above a prescribed capacity (1.5 million m3) to install gas collection systems. This affects 32 major public and private landfills in Ontario representing just over 300 million m3 of permitted capacity. The methane gas collected may be flared (burned) or extracted for energy production.
Beginning June 1, 2010, eligible landfills were required to submit an annual written report with respect to the previous year’s operation of the “landfill gas collection, venting or use facilities” that includes the following information:
- the total landfill gas volume collected at the site during the year;
- the percentage of the volume that was methane gas;
- the reduction in methane emissions from the landfill site associated with the burning or use
of landfill gas during the year (expressed in tonnes of CO2e and based on a GWP of 21 for methane gas);
- a description of how sound scientific or engineering principles have been used to support
these statements; and,
- all calculations and information that support the statements.
It should be noted that, with the exception of “total landfill gas volume collected by the facilities at the site during the year”, the other required information can only be estimated based on the same scientific or engineering principles that the ECO has called into question above.
Diversion Efforts - Waste Diversion Act, 2002 Review
In 2009, MOE placed a policy proposal on the Environmental Registry (#010-8164) and the links to a minister’s report entitled “From Waste to Worth: The Role of Waste Diversion in the Green Economy – Minister’s Report on the Waste Diversion Act, 2002 Review”. The intent was to propose policy changes to Ontario’s waste management framework that increase waste diversion while delivering “environmental and economic outcomes.” The minister’s report set the context for Ontario’s current diversion approach as follows: “The WDA promotes waste reduction, reuse and recycling, and prohibits programs from promoting the burning, landfilling, or land application of designated material.” (emphasis added) “Branded organics”, although not defined, are proposed in the report as a designated material that should be considered for inclusion in a long-term (five-year) schedule for diversion. The report concluded by recognizing the challenges in “moving existing programs to the new framework” and encouraged interested parties to provide feedback. A decision notice with regard to a review of the Waste Diversion Act, 2002 had yet to be posted as of April 2011.
Climate Change Action Plan Targets
Given that the waste sector historically contributes between 3 to 4 per cent of Ontario’s GHG emissions, in 2009 the government announced it would introduce a regulation to phase in new requirements for methane capture and energy production in landfills.
According to the government’s CCAP Annual Report 2009–2010, landfill methane gas collection for new, expanding or operating landfills is anticipated to achieve GHG reductions of 1.7 Mt CO2e by 2014, and 2.1 Mt CO2e by 2020. These projected reductions, however, may be more than offset by unintended fugitive releases as discussed above.
Conflicting Priorities
These apparently divergent landfill policy directions beg the question: What is the government trying to accomplish? Is it control of GHGs? Is it energy production? Is it the stabilization of landfills to limit their contaminating lifespans? Or, is it the diversion of organics away from landfills altogether? Are these goals and objectives compatible? To the extent that they require substantially different landfill design parameters and operating requirements, the ECO believes that they are not compatible.
As described above, the models relied upon to measure fugitive methane releases do not accurately represent what is happening in landfills. Without detailed waste inventories, it is impossible to determine the total methane potential of landfilled organics. The end result is conflicting compliance issues. For example, the requirement for infiltration rates of greater or equal to 150 millimetres of water per year in O. Reg. 232/98 may conflict with the control of methane because it produces greater volumes of methane, more of which may be escaping as fugitives. These high rates of permeability, along with the negative pressures generated by gas control systems, will work against each other. They offer additional pathways for fugitive methane leaks while also risking the draw down of air into the landfill that may either dry out the cells, kill the anaerobic bacteria that generate the methane and/or mix with the methane to create an explosive combination.
The ECO concluded in our 2008/2009 Annual Report that the best way to deal with GHGs from landfills is to reduce or ideally eliminate, on a go-forward basis, the amount of organic matter that ends up in them. The ECO also noted that the only way to reconcile this policy objective with efforts devoted to the generation of landfill energy projects should be “within the context of an overall solid waste management strategy.” The strategy, yet to be developed, will need to balance the equally important goals of controlling groundwater contamination from leachate, controlling the release of methane into the atmosphere, and determining the most environmentally appropriate method to destroy the methane captured from existing wastes-in-place.
The ECO is also concerned about the mixed signals being sent to Ontario municipalities. The Ontario Power Authority’s inclusion of landfill gas among the renewable energy sources eligible for 20-year guaranteed Feed-in Tariff contracts is a case in point. On the one hand, Ontario’s municipalities are responding to the dual concerns about landfill GHGs and threats to groundwater from leachate contamination by accelerating organics diversion efforts. But, the requirement to install gas capture systems in smaller capacity landfill sites, at considerable capital outlay, may prompt operators to seek an increased stream of organics to feed their gas collection systems to generate electricity and revenues to recoup these costs.
Despite the government’s best intentions to mitigate the impacts of methane emissions from landfills, a renewed emphasis on landfill methane for energy production may make matters worse by increasing fugitive releases – with the unintended consequence of erecting marketplace barriers to landfill alternatives such as diversion.
What Needs to Happen
Management options for existing wastes-in-place are urgently required. While there are well-established alternatives to landfilling for new discards (including composting, anaerobic digestion, and thermal conversion technologies such as pyrolysis) that do not create uncontrolled methane releases, there is no real alternative to existing wastes-in-place which must be managed to mitigate environmental impacts.
Diverting New Discards
Every municipality in Ontario successfully separates about one-third its residents’ bottles, cans and newspapers for recycling. Implementing green bin programs for food scraps, pet wastes and soiled paper is the next logical step. Experience demonstrates that even higher levels of organics diversion are feasible. Regardless of the efficiency of a gas control system, organics diversion is always more effective in preventing the release of methane. Diversion will always produce greater GHG reduction benefits than flaring or energy production at landfills, no matter what assumptions are used. The challenge of existing wastes-in-place, however, still remains.
Managing Existing Wastes-in-Place
Ontario’s Climate Change Action Plan assumes uncritically that recovering the energy value in landfill gas is inherently preferable to flaring it, especially if the energy produced displaces electricity generated through the burning of fossil fuels. However, this encourages landfills to amend operating practices to increase the generation of methane to fuel their energy production facilities. This harkens back to the landfill gas design conundrum described earlier. The paradox is:
- the proportion of methane in landfill gas generated at landfill sites kept dry would be too
low to economically operate the reciprocating engines that typically generate electricity (not enough methane); and,
- the operational changes needed to increase gas generation and methane concentration
also serve to degrade gas collection efficiency while increasing fugitive emissions over both the short and long term.
With methane’s high GWP, particularly over the short term, a small increase in fugitive emissions could overwhelm the benefits from lower CO2e emissions associated with the displacement of electricity generated by fossil fuels. Further, once the province phases out the use of coal in 2014, the contributions of electric power from landfill energy to the grid could be displacing other, cleaner sources of power.
Landfills that are properly operated should strive to minimize infiltration of liquids and maximize gas capture for flaring only. This will ensure that the wastes remain as close to biologically inactive as possible and prevent hazardous compounds from being released, thereby posing less of a threat to the environment.
The Next Steps
There are serious deficiencies in the mathematical models used by the government to calculate the generation of landfill gas over the course of a facility’s biologically active lifetime. The ECO believes that the promotion of landfill energy options over organic waste diversion compromises the achievement of CCAP GHG reduction targets. This is particularly true in the near term when the methane gas generated by organic wastes in landfills brings the planet closer to a dangerous “tipping point” (see Appendix 4). Thus, MOE must move quickly to develop a solid waste management strategy that clarifies how existing wastes-in-place will be treated while, on a go-forward basis, articulates the timing and commitments to divert all future organics from landfills. This strategy should be informed by an immediate revisiting of modeling assumptions behind projected GHG reductions from landfills facilitated through Ontario-specific field studies.
The ECO believes that the projected cumulative GHG reductions of 2.1 Mt at 2020 from landfill methane gas collection are, at best, optimistic and, at worst, may be completely negated due to an increase in fugitive methane releases. We have shown that gas collection systems for energy production require major modifications to how a landfill is managed to ensure a continuous supply of methane; modifications that increase the volume of methane that may escape as fugitive emissions.
With regard to future organics, diversion will always produce greater GHG reduction benefits. Existing wastes-in-place, on the other hand, must be managed to ensure that the wastes remain as biologically inactive as possible, with currently installed gas collection systems flaring the methane captured.
