Landfill emissions

This presents information on how a bottom-up estimate of methane from NZ landfills was generated. Original code is available on Github at https://github.com/manaakiwhenua/stocking-rates

MfE has documentation on NZ-specific emissions factors here: https://environment.govt.nz/assets/publications/climate-change/New-Zealands-Greenhouse-Gas-Inventory-1990-2021-Chapters-1-15.pdf (Chapter 7: Waste, p.304; particularly section 7.2 (p.309) Solid Waste disposal (5.A)).

The method for determining bottom-up landfill emissions is based on that information, in conjunction with the equations published by the IPCC: https://www.ipcc-nggip.iges.or.jp/public/2019rf/pdf/5_Volume5/19R_V5_3_Ch03_SWDS.pdf (Helpful MS Excel spreadsheets are also available: https://www.ipcc-nggip.iges.or.jp/public/2006gl/vol5.html)

Available information

• Landfill site inventory
  • MfE dashboard: https://environment.govt.nz/facts-and-science/waste/waste-facilities-and-disposal/ providing name, location and type of landfills
  • Data was obtained by personal request to Logan Ashmore (logan.ashmore@mfe.govt.nz) as the dashboard does not have a download functionality (2024-03-18)
  • Ivan Chirino-Valle (ivan.chirino-valle@mfe.govt.nz) was also a helpful contact
  • There are some errors in this dataset, such as a small number of mislocated sites and one instance of a mistaken type (a cleanfill labelled as a municipal landfill). Known errors have been corrected.
  • All non-class 1 landfills are disregarded.
• Waste activity data
  • Given as KG per person per month
  • 2009-07 until 2023-12
  • Extrapolated (backwards thorugh time) until 1990-01
• Population data
  • 2022 estimated resident population (https://datafinder.stats.govt.nz/layer/115047-new-zealand-2022-estimated-resident-population-grid-250-metre-2023/)
• Estimated composition of waste to municipal landfills
  • Table 7.2.5 of the Inventory, p. 318
  • Given as percentage of waste composition for each of 8 waste categories with particular emissions factors
  • 1990-2022, noting that values have not changed since 2018.
• Sites with gas recovery
  • Compiled from https://environment.govt.nz/assets/Publications/Files/Measuring-Emissions-Detailed-Guide-2020.pdf
  • Supplemented by manual compilation for more recent sites, and large sites have been verified (some had installed LFG since that was published)
• Climate information
  • Whether sites are "wet" or dry" is determined by reference to national-scale (rather than site-specific) climate data, as recorded in the NZ Environmental Data Stack.
  • Wet sites are assumed to be those with a ratio of "Annual Precipitation" to "Potential Evapotranspiration" that is greater than 1.
  • Potential evapotranspiration (PET) is taken as the FAO Penman-Monteith equation.

The best possible information, a temporal record of site-specific actiity data (i.e. the waste actually processed by each landfill, over time, and its composition) is unfortunately not available. The method therefore is largely an attempt to estimate this activity information, before applying the stated emissions equations.

Model summary

1. Landfill sites are geolocated.
   1. Landfill sites are attributed a "wet" or "dry" site characteristic.
   2. Landfill gas recovery system information is attributed to landfills.
2. Each population grid (250m² grid cells) is assigned to its nearest landfill (Euclidean allocation). This is done on a monthly basis over the time series to account for landfills opening and closing (but the population distribution itself is static).
3. Equations for estimating methane emissions from landfills are applied on a monthly basis, but ouputs are the emissions generated for the 12 months prior to that point (rather than monthly emissions). These equations take the form of a first order decay (FOD) model.
4. This monthly point data (i.e. landfill sites, with methane emissions over time) is then converted to raster form, to match a grid used in inverse modelling. (The same grid used in https://datastore.landcareresearch.co.nz/dataset/nz-monthly-enteric-methane-emissions-2017-2022) Output units are Gg CH₄.

The 2006 IPCC Guidelines present the basic concept of First Order Decay (FOD) as "...The basis for the calculation is the amount of Decomposable Degradable Organic Carbon (DDOCm) as defined in Equation 3.2. DDOCm is the part of the organic carbon that will degrade under the anaerobic conditions in SWDS. It is used in the equations and spreadsheet models as DDOCm. The index m is used for mass. DDOCm equals the product of the waste amount (W), the fraction of degradable organic carbon (DOC) in the waste, the fraction of the degradable organic carbon that decomposes (DOCf ), and the part of the waste that will decompose under aerobic conditions (prior to the conditions becoming anaerobic) in the SWDS, which is interpreted with the methane correction factor (MCF)...". The parameter that is related to aerobic condition is expressed in terms of MCF. The guidance on the use of MCF in different management conditions of SWDS is updated in Table 3.1 (Updated). Currently some countries use active aeration or aerobic stabilization of managed landfills at large scale as an abatement measure (e.g., Germany and the United States). Decomposition rate of the organic matter under aerobic condition is about 3-4 times higher than that under anaerobic condition (Ishigaki et al. 2003; Ritzkowski & Stegmann 2012). Rapid aerobic decomposition reduces DOC available for anaerobic decomposition.

Output sense checking

In 2021, the Solid waste disposal source category contributed 2,578.2 kt CO2-e (80.2 per cent) of total emissions from the Waste sector. Emissions from Solid waste disposal in 2021 were 740.0 kt CO2-e (22.3 per cent) below the 1990 level of 3,318.2 kt CO2-e. While there is year- to-year variation, this net decrease is the result of two contrary trends. First, population and economic growth have driven ongoing increases in the total and per capita amount of municipal waste generated but, secondly, changing composition over time and improved landfill management practices, particularly LFG recovery, have offset this increase, resulting in emissions peaking in 2002. — Ministry for the Environment. 2023. New Zealand’s Greenhouse Gas Inventory 1990–2021. Wellington: Ministry for the Environment. (p311)

For the 12 months to Jan 2022, our method estimates emissions of 83.5 kT CH₄, which is approximately equivalent to 83.5 * 25 = 2087.5 kT CO₂-e, which is 80.1% of the stated emissions. Accounting for the ±40% uncertainty in managed landfill activity data, and the compounding ±40% uncertaintly for emissions factors (Table 7.2.9, that suggests a plausible range of 751.5–4091.5 kT CO₂-e, indicating that the values are in the correct order of magnitude but nevertheless still subject to significant sources of systematic error.

An under-estimation is expected with this methodology and available data because:

• We assume that landfills currently with LFG recovery have always had LFG recovery for their lifetime (due to lack of available information about when this capacity is installed).
• It is likely that some landfills are missing from the record; particularly small, closed municipal waste facilities (which may still emit LFG). There has been a trend of consolidation to larger waste facilities, and the smaller ones are unlikely to have LFG recovery. Although this does not change the volume of waste, it may still bias emissions downwards due to possible under-allocation to small sites without LFG recovery.

Some assumptions and limitations have uncertain direction with respect to emissions estimates:

• The model assigns population to landfills based on Euclidean allocation. There is no available information about the actual allocation of population centres to landfills, nor the actual capacity of landfills. As such, there is significant uncertaintly in the apportionment of waste to particular landfills, and therefore both the location of emissions, and the total volume of emissions (e.g. if too much waste is allocated to a wet site without LFG recovery, or vice versa).
• The population distribution is assumed static. (However closed landfills are not considered when allocating population to the nearest landfill.)
• Classification of landfill sites into "wet" and "dry" sites is not based on site-level information, such as rain gauges. It is also based on long-term climate information.

Please acknowledge the funding from the NZ Space Agency through the MethaneSAT Project (C01X2023) which ultimately made this possible.