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Frequent Questions about the Waste Reduction Model (WARM)

This page provides a brief overview of responses to frequent questions about the Waste Reduction Model (WARM). Further information is available in the WARM documentation.

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What Information Do I Need to Use WARM?

The most basic information needed to run WARM includes data on the amount of waste handled by material type (e.g., aluminum cans, corrugated containers), the associated waste management practice currently used (recycling, composting, combustion, anaerobic digestion, or landfilling) and the alternative practice (which could include source reduction).

The tool provides default data on the “national average” materials management practices, such as the prevalence of landfill gas recovery, average landfill gas collection efficiency, and transportation distances. However, the results in WARM will be more accurate if users enter site-specific information, to the extent that it is readily available. Additional user inputs include:

  • Landfill characteristics (e.g., presence of landfill gas collection system)
  • Anaerobic digestion characteristics (e.g., wet or dry digester)
  • Material characteristics (e.g., produced from virgin materials or with recycled inputs)
  • Location for regional electricity grid
  • Waste transport distances

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The WARM Results Show Negative Values for GHG Emissions or Energy. What Does This Mean?

For each materials management practice in WARM accounts for sources of emissions (e.g., combustion of fossil fuels) and emissions sinks (e.g., forest carbon storage, avoid fuel consumption). When the results in WARM show a negative value for GHG emissions or energy, this indicates that managing that material using the selected management practice results in avoided GHG emissions or energy from a life-cycle perspective. A negative value for the total or incremental change in GHG emissions or energy indicates that the alternative scenario results in a net decrease in life-cycle GHG emissions or energy compared to the baseline scenario.

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The WARM Results Show “INPUT ERROR: Make Sure Tons Managed Equals Tons Generated”. How Do I Fix This Error?

In WARM, the same tonnage of materials must be analyzed across both the baseline and alternative scenarios. This error message occurs when the tons of materials entered on the “Analysis Inputs” sheet are not equal for both the baseline and alternative scenarios. To avoid this error, ensure that, for each material, the total tons assigned to different management practices adds up to the value in the “Tons Generated” column. Any decrease in generation should be entered in the Source Reduction column as a positive value and any increase in generation should be entered in the Source Reduction column as a negative value.

For example, assume a situation in which 100 tons of aluminum cans are landfilled in the baseline scenario:

Material  Tons Recycled  Tons Landfilled  Tons Combusted  Tons Composted  Tons Anaerobically Digested   Tons Generated
Aluminum Cans              100.0    NA  NA   100.0

Next, in the alternative scenario, only 80 tons of aluminum cans are generated, with all of those cans recycled:

Tons Generated     Tons Source Reduced   Tons Recycled   Tons Landfilled   Tons Combusted   Tons Composted   Tons Anaerobically Digested  
100.0                    80.0       NA   NA Tons managed does not equal baseline generation.

WARM cannot calculate results for the alternative scenario above because the total tons managed does not match the tons managed in the baseline scenario. In order to remove this error, the 20 tons of avoided waste generation in alternative scenario must be included under the Source Reduction column as shown in the corrected alternative scenario:

Tons Generated    Tons Source Reduced  Tons Recycled  Tons Landfilled  Tons Combusted  Tons Composted  Tons Anaerobically Digested
100.0   20.0                 80.0      NA  NA

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The WARM Results Show “INPUT ERROR: Please Select Dry Digestion”. How Do I Fix This Error?

In WARM, two types of anaerobic digestion are modeled: wet digestion and dry digestion. Dry digestion applies for all organic materials included under the anaerobic digestion management practice (i.e., food waste, yard trimmings, and mixed organics). However, wet digestion can only be applied for food waste materials and cannot be applied for yard trimmings materials and mixed organics. This error occurs when tons are entered for yard trimmings or mixed organics while “Wet Digestion” is selected under question 8a on the “Analysis Inputs” sheet. In order to remove this error and model digestion of yard trimmings, select “Dry Digestion” under question 8a. For additional information on anaerobic digestion in WARM, please refer to the Anaerobic Digestion chapter as part of the Documentation for Greenhouse Gas Emission and Energy Factors Used in WARM.

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Why Aren’t Some Materials Management Practices Available for Certain Materials?

For some combinations of materials and management practices in WARM are not available due to the lack of available data. To model the management of materials in WARM, EPA relies on recent, comprehensive estimates of the life-cycle impacts of material production and end-of-life management. When sufficient data are not available, WARM cannot include that management practice and material combination. For example, recycling low-density polyethylene (LDPE) occurs within the United States, but EPA was unable to locate complete data necessary for modeling LDPE recycling in WARM. For additional guidance on options for addressing these gaps, please refer to the guidance document on “Using WARM Emission Factors for Materials and Pathways Not in WARM”, available from the Additional User Guidance documents.

For other materials, management using certain practices is not currently technologically possible. For example, non-organic materials like metals cannot be composted or anaerobically digested. Similarly, the mixed MSW and recyclables are household materials that are typically discarded or recycled, respectively. Due to their diverse component mixture, these material types cannot be "source reduced" in WARM. Mixed organics includes yard trimmings, which are not considered to be manufactured and therefore cannot be source reduced.

For additional information on the management pathways available for each material, please refer to the Documentation for Greenhouse Gas Emission and Energy Factors Used in WARM.

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What Happens When I Select the “National Average” Landfilling Scenario Under Question Five?

The emissions from landfilling depend on whether the landfill where your waste is disposed has a landfill gas (LFG) control system.  If you do not know whether your landfill has LFG control, select the "National Average" option under question 5 to calculate emissions based on the most recent estimates for the proportion of landfills with LFG control in the United States. If either the “National Average” or “No LFG Recovery” options are selected, questions 6a and 6b do not apply and you should proceed to question 7. However, if you select “LFG Recovery”, you can further refine how WARM models landfilling by responding to questions 6a and 6b.

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Can WARM Be Used as a GHG Accounting or Inventory Tool?

Although emissions estimates provided by WARM are intended to support voluntary GHG measurement and reporting initiatives, WARM is intended as planning tool and not an accounting tool. Its proper use entails comparing the current waste management practice with an alternative waste management practice and obtaining the impacts associated with changes in waste management practices. For further information, EPA has prepared a short summary on the differences between life-cycle analysis (such as WARM uses) and GHG inventories, available from the Additional User Guidance documents.

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