12 Methods and data sources
13 Solid waste
13.1 Methods
Solid waste emissions for Minnesota and Wisconsin are calculated using two different methods due to a difference in data availability.
13.1.1 Minnesota
The previous iteration of this inventory calculated Minnesota’s solid waste emissions by multiplying activity totals by emissions factors from the EPA’s Emissions Factor Hub. This update instead uses methodologies recommended by the Intergovernmental Panel on Climate Change (IPCC) (“2006 IPCC Guidelines for National Greenhouse Gas Inventories” 2006).The methodologies were selected to align with best practices for community-wide inventories using the IPCC recommendations and the guidance of the Global Protocol for Community-Scale Greenhouse Gas Inventories (Fong et al. 2021).
Landfill
The IPCC suggests two alternatives for calculating landfill emissions, a first order decay model and a methane commitment model. The first order decay model is often used for larger-scale inventories, such as the US Federal Inventory, and requires waste data going back to 1950. Given the data available and the scope of this inventory, we chose to instead use the simpler methane commitment model to calculate county-level emissions for Minnesota.
The methane commitment model calculates methane emissions from landfills for a given year by multiplying municipal solid waste totals by a methane generation potential and adjusting for oxidation and methane flaring,
\[Emissions_{CH_4} = [(MSW \times L_0 )-rec] \times (1 – ox) \]
\(MSW\), or the amount of municipal solid waste processed in landfills, is reported on a county level by MPCA’s SCORE report (?sec-mpca-score).
It is then multiplied by a methane generation potential \(L_0\). In some processes, the amount of methane recovered from landfills, either through methane flaring or landfill gas to energy programs, is subtracted here. Due to data concerns and best practices recommendations, we have chosen not to include methane recovery in our Minnesota emissions calculations. Learn more in Section 13.2.5.
After subtracting methane recovered, emissions are multiplied by \(1-ox\) to account for oxidation in the landfill. Our oxidation value is assigned the IPCC default of 0.1.
\(L_0\), the methane generation potential, is calculated as follows:
\[L_0 = MCF × DOC_f × F × 16/12 \times DOC\] where
- \(MCF\) = methane commitment factor. Assigned IPCC default of 0.5 for managed, semi-aerobic landfills.
- \(DOC_f\) = fraction of degradable organic carbon degraded. Assigned IPCC default of 0.6.
- \(F\) = fraction of methane in landfill gas. Assigned IPCC default of 0.5.
- \(16/12\) = Methane (CH4) to carbon (C) ratio (atomic weight)
- \(DOC\) = degradable organic carbon. Calculated based on local waste makeup data from MPCA’s 2013 Statewide Waste Characterization study, using the equation \(DOC = ( 0.4 \times \text{paper/textiles}) + ( 0.17 \times \text{non-food organics}) + ( 0.15 \times \text{food waste}) + ( 0.3 \times \text{wood/straw})\).
Compost
Compost produces both methane and nitrous oxide. Emissions are calculated by multiplying waste activity totals by emissions factors divided between aerobic and anaerobic digesters. Since Minnesota only has one anaerobic digester that is outside the inventory area, we assumed 0% anaerobic digestion within the inventory area (source).
\[Emissions_{CH_4} = MSW_{compost} \times 10 \times 10^{-3}\]
\[Emissions_{N_2O} = MSW_{compost} \times 0.6 \times 10^{-3}\]
As in other sections, MSW activity data comes from MPCA’s SCORE report. The emissions factors of 10 and 0.6 come from IPCC default values.
Incineration
Since incineration data is reported to SCORE as Waste to Energy, it is assumed that all incineration in the MSA is considered Waste to Energy.
Incineration of waste produces CH4, CO2, and N2O emissions. However, the Global Protocol for Community-Scale Greenhouse Gas Emission Inventories reports negligible CH4 emissions for continuous incineration facilities.
\[Emissions_{CO_2} = (MSW_{incinerated} \times E_i + MSW_{onsite} \times E_o) \times FCC \times FFC \times 44/12\]
\[Emissions_{N_2O} = MSW_{incinerated} \times EF_{N_2O} \times 10^{-6}\]
where:
\(MSW_{incinerated}\) = municipal solid waste incinerated, as reported by SCORE.
\(E_i\) = efficiency of combustion for incineration. Assigned IPCC default of 95%.
\(MSW_{onsite}\) = municipal solid waste burned onsite, as reported by SCORE.
\(E_o\) = efficiency of combustion for onsite burning. Assigned Greenhouse Gas Protocol default of 71%.
\(FCC\) = fraction of carbon content in MSW. Assigned IPCC default of 40%.
\(FFC\) = fraction of fossil carbon in MSW. Assigned IPCC default of 40%.
\(44/12\) = Ratio of carbon dioxide (CO2) to carbon (C) by atomic weight
\(EF_{N_2O}\) = aggregate N2O emission factor for MSW. Assigned GHG Protocol default of 50 g N2O/ metric tons waste for continuous and semi-continuous incinerators.
City-level estimates
Given that the most granular solid waste data available is at the county level, waste emissions estimates are allocated to cities by population. County emissions are multiplied by the city or municipality’s share of county population.
This means that city-level estimates do not account for differences in waste collection programs between cities. The data may, for example, allocate organics emissions to cities that do not have organics processing programs. For this reason, we do not provide a breakdown of city-level emissions by source, and instead use only the total solid waste emissions. This should provide a relatively accurate estimate of total solid waste emissions for your community.
Limitations
Because the methane commitment method for landfill emissions calculates emissions slightly differently than the IPCC-encouraged First Order Decay model, landfill results may differ slightly from sources that use First Order Decay, such as the EPA’s National Inventory and its State Inventory Tools. Both methods are accepted as valid ways to estimate solid waste emissions.
MPCA SCORE does not report activity data for waste generated and processed by industry.
Emissions are not calculated for waste that is recycled, as any emissions generated in the recycling process come from the energy use of the facilities or transportation and are accounted for in other sectors of this inventory.
13.1.2 Wisconsin
Wisconsin emissions are calculated by interpolating and scaling down state-level data from the Wisconsin DNR (Wisconsin DNR 2021).
This 2021 inventory estimates landfill and waste-to-energy emissions for the years 2005 and 2018, including methane recovery offsets. In order to fill in missing years, emissions between 2005 and 2018 were linearly interpolated. Due to the small amount of change in emissions, it was assumed that emissions from 2018 to 2021 were constant. These emissions were then allocated to counties based on population.
13.2 Data sources
13.2.1 MPCA Waste Characterization Study
In 2013, the MPCA contracted with Burns & McDonnell Engineering Company, Inc. to conduct a waste characterization study of landfill waste, an update to an earlier study in the year 2000. For simplicity, we have chosen to set waste proportions equal to those 2013 values for all years in this inventory.
The study sampled waste makeup from six waste disposal facilities across the state and used the results to model statewide totals. More details can be found in study documentation.
This data is a high-quality state-level dataset.
For consistency, the values in this study have been compared to the IPCC default waste breakdown for North America. Relevant values were found to be consistent within 90% confidence intervals (as reported in the MPCA study), with the exception of food waste. IPCC values attribute 33.9% of landfill waste to food waste, while the 2013 report only finds that 17.8% of landfill waste is food waste. There are many possible explanations for this discrepancy, including differences in waste breakdown across the North American region and the possibility that the IPCC’s numbers reflect additional food waste processed in organics facilities, which would not be included in the MPCA study.
Due to the fact that it is more recent and more specific to the Minnesota region, we have chosen the MPCA study as the source of truth in this case.
13.2.2 Wisconsin Greenhouse Gas Emissions Inventory
Waste emissions for Wisconsin counties were unavailable in the same detail as the MPCA data. Thus, we estimated total waste emissions based on statewide emissions estimates, allocated by population.
The most recent Wisconsin Greenhouse Gas Emissions Inventory was done in 2021 by the Wisconsin Department of Natural Resources. Included in the solid waste data for this source are emissions from landfills and waste combustion, taking into account the emissions reduced by landfill gas collection for gas-to-energy use or flaring. This inventory does not, however, include emissions generated from composting or recycling.
The emissions for solid waste in this report were calculated using the EPA’s State Inventory and Projection Tool, a tool designed to help states calculate greenhouse gas emissions (USEPA 2024). Default values provided by the tool were used except in the case of default Mixed Solid Waste population tonnage values, which were replaced by data from the Wisconsin state DNR Annual Waste Tonnage Report (Wisconsin DNR 2021).
For 2018, the Wisconsin DNR reported 2.2 million metric tons carbon dioxide equivalent (MMTCO2e) generated through landfilling and solid waste management.
In the process of analysis, this statewide estimate was apportioned to the county level based on county population data, as detailed in Chapter 12.
13.2.3 Limitations
- Since data reported directly from the counties was unavailable for Wisconsin, the solid waste data used here reflects a disaggregation of state-level data and may not be reflective of the specific mix of waste generated by Pierce and St. Croix counties.
- Data collected in Wisconsin’s emissions inventory only represents waste disposed of in landfills or waste combustion facilities, and does not include organics. Composting data is unavailable for Wisconsin counties.
13.2.4 Comparison to similar datasets
The US EPA completes yearly state-level estimates of emissions for each state, which combined sum to the totals reported in the US Greenhouse Gas Emissions Inventory. The data for these estimates and the US inventory can be explored at the GHG Inventory Data Explorer. The EPA’s total of landfill emissions for Wisconsin for 2018 was 2.450 MMTCO2e, not far off from the Wisconsin DNR’s 2.2 MMTCO2e. The EPA’s estimate for 2021 was 2.422 MMTCO2e. More details can be found in Section 15.2.
Since the EPA completes an inventory for the entire US and its methods may not reflect the specific nuances of emissions in each state, we elected to use the data from the Wisconsin DNR for this inventory.
13.2.5 EPA Methane Recovery Data
The EPA generates methane flaring and landfill gas to energy data for each state as part of its State Inventory Tool for solid waste. This data is collected as part of the Landfill Methane Outreach Program.
However, since this data is collected on a national level, there are potential discrepancies with state-level and especially regional breakdowns of methane recovery. Due to these concerns, we have chosen to exclude this data source from our inventory and instead use the IPCC default of 0 methane recovery. This means that our inventory may overestimate emissions from solid waste landfills.
14 Wastewater
14.1 Methods
14.2 Data sources
15 Data Validation
15.2 Comparison with other inventories
15.2.1 US Greenhouse Gas Emissions Inventory
The United States EPA conducts a comprehensive yearly estimate of greenhouse gas emissions from multiple sectors and gases. It also publishes statewide totals consistent with the national inventory. These emissions totals are consistent with international standards for greenhouse gas accounting, although they may differ from inventories completed at the state level for various reasons.
US Inventory data for the waste sector in both Minnesota and Wisconsin was downloaded from the Greenhouse Gas Inventory Data Explorer and processed in R script: epa_inventory_data.R, where it was apportioned from state to county level by population.
Here, we compare federal inventory data with our current data as well as a previous version of the Met Council inventory, which instead of using IPCC protocols multiplied SCORE data by the EPA’s emission factors.