Twin Cities MSA Greenhouse Gas Inventory

9.1 Introduction

In 2021, waste activities over the entire United States generated emissions of 169.2 MMTCO₂e, or 2.7 percent of total U.S. greenhouse gas emissions (USEPA 2023). Solid waste emissions primarily consist of CH₄, emitted in large part by landfills and organics composting, but also include CO₂ and N₂O generation. In the metropolitan area, waste incineration accounts for a significant portion of solid waste management and therefore emissions. This inventory also includes emissions generated in the management of recyclables.

Wastewater emissions account for direct CH₄ and N₂O emissions from the treatment of municipal wastewater, and not additional indirect anthropogenic emissions such as the electricity needed to operate processing plants.

9.2 Methods

9.2.1 Solid Waste

The process for calculating solid waste emissions slightly differs between Minnesota and Wisconsin counties, due to a difference in data availability. Both state’s use state or federal data sources, the highest quality data rank (Table B.2).

For Minnesota counties, waste generation totals are allocated by sector from Minnesota Pollution Control Agency (MPCA) SCORE data (MPCA 2022). Totals for each sector are multiplied by the appropriate EPA-provided emissions factor to provide an estimate of emissions from that source and county. Learn more about MPCA SCORE in Section 10.1.

For Wisconsin counties, state-level emissions data as calculated by the Wisconsin DNR is allocated to the relevant counties by population (Wisconsin DNR 2021).

The emissions for each county \(c\) are proportional to its fractional share of Wisconsin’s population.

\[Emissions_c = Emissions_{total} \times \frac{Pop_c}{Pop_{total}}\]

9.2.2 Wastewater

We used state level estimates of municipal wastewater emissions (metric tons CO₂e) from the EPA state inventory and projection tool (USEPA 2024). Due to incomplete data, industrial wastewater emissions were not estimated. Emissions were apportioned to each county, \(c\), based on its fractional share of its state’s population.

\[Emissions_c = Emissions_{state} \times \frac{Pop_c}{Pop_{state}}\]

9.3 Results

Emissions from waste is the smallest of the three sectors in the Twin Cities MSA. Waste generated 1.49 MMtCO₂e of emissions in the Twin Cities MSA in 2021. Solid waste, including landfills, recycling, and organics, generates the largest share of emissions in the waste sector, with municipal wastewater treatment comprising a smaller share of waste emissions.

Waste emissions in Minnesota have declined nearly 40% since 2005, due to a variety of factors including gas capture technologies and aging waste in open landfills (MPCA 2023). Waste emissions in Wisconsin show no significant change from 2005 to 2018, possibly due to increased waste generation offsetting reductions from gas capture (Wisconsin DNR 2021). The Metropolitan Council, which oversees wastewater services for the majority of the urbanized extent of the metropolitan region, continues to work to reduce wastewater emissions across its 9 wastewater treatment plants.

Figure 9.1: Solid waste and wastewater county emissions

Figure 9.2: Solid waste and wastewater county emissions by category

9.3.1 Wastewater

Wastewater generated 0.33 MMtCO₂e of emissions in the Twin Cities MSA in 2021.

Figure 9.3: 2021 wastewater emissions

9.3.2 Solid waste

Solid waste generated 1.15 MMtCO₂e of emissions in the Twin Cities MSA in 2021. Of that total, 57.5% of emissions came from landfill, 26.5% from waste to energy facilities, and the remaining 16.0% from organics and recycling.

Figure 9.4: 2021 solid waste emissions

Greenhouse gas emissions from solid waste are dominated by the landfill sector. In Hennepin, Ramsey, and Washington counties, municipal centers where a significant portion of waste is incinerated, waste-to-energy or incineration makes up a large fraction of emissions as well.

Figure 9.5: 2021 solid waste emissions by category

9.4 Correlation with related data

We would expect counties with a higher population to have higher solid waste emissions.

Figure 9.6: County population and solid waste emissions

9.5 Comparison with other inventories

9.5.0.1 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.

Figure 9.7: Solid waste emissions comparison: Met Council and US GHG Inventory

Wastewater comparisons are very consistent, with the US GHG Inventory consistently estimating more emissions than the EPA derived Met Council estimate.

Figure 9.8: Wastewater emissions comparison by county, EPA-derived Met Council and US GHG Inventory. US inventory consistently higher than Council.

Solid waste emissions differences vary by county; while Ramsey, Hennepin and Washington county estimates from the US GHG Inventory are higher than the Council estimates, the opposite is true for Dakota and Anoka counties. This is likely related to the fact that Ramsey, Hennepin and Washington counties process a significant portion of their waste in waste-to-energy facilities, reducing the amount of emissions accounted for under the landfill category.

Figure 9.9: Landfill emissions comparison by county, Met Council and US GHG Inventory. Results vary by county.