Frequent Questions about the Waste Reduction Model (WARM)
This webpage provides a brief overview of responses to frequent questions about the Waste Reduction Model. Further information is available in the WARM documentation.
On this page:
- What information do I need to use WARM?
- The WARM results show negative values for greenhouse gas (GHG) emissions, energy or economic impacts. What does this mean?
- The WARM results show “INPUT ERROR: Make sure tons managed equals tons generated.” How do I fix this error?
- The WARM results show “INPUT ERROR: Please select dry digestion”. How do I fix this error?
- Why aren’t some materials management practices available for certain materials?
- What happens when I select the “National Average” landfilling scenario under question 5?
- Can WARM be used as a GHG inventory tool?
- Why are economic impacts from anaerobic digestion and composting sometimes lower than the economic impacts from landfilling and combustion?
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., the presence of a 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, and
- Waste transport distances.
The WARM results show negative values for GHG emissions, energy, or economic impacts. What does this mean?
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 and fuel consumption avoidance). When the results in WARM show a negative value for GHG emissions, energy or economic impacts, they indicate that managing a particular material using the selected management practice results in avoided GHG emissions, energy or economic impacts from a life-cycle perspective. A negative value for the total or incremental change in GHG emissions, energy or economic impacts indicates that the alternative scenario results in a net decrease in life-cycle GHG emissions, energy or economic impacts compared to the baseline scenario.
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 generated in the baseline scenario. In order to remove this error, the 20 tons of avoided waste generation in the alternative scenario must be included under the Source Reduction column as shown in the corrected alternative scenario below:
| Tons Generated | Tons Source Reduced | Tons Recycled | Tons Landfilled | Tons Combusted | Tons Composted | Tons Anaerobically Digested | |
|---|---|---|---|---|---|---|---|
| 100.0 | 20.0 | 80.0 | NA | NA |
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 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. 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, Energy and Economic Factors Used in WARM.
Why aren’t some materials management practices available for certain materials?
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 the management practice and material combination.
For example, recycling low-density polyethylene (LDPE) occurs in the United States, but EPA was unable to locate the 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, 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, Energy and Economic Factors Used in WARM.
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 you select either the “National Average” or the “No LFG Recovery” options, 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.
Can WARM be used as a GHG inventory tool?
No, while WARM emissions estimates are intended to provide estimates of GHG measurement, WARM is not intended to be used for inventory purposes. Its proper use entails comparing the current waste management practice with an alternative waste management practice, and obtaining the estimated potential impacts associated with changes in waste management practices. For more information, EPA 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.
Why are economic impacts from anaerobic digestion and composting sometimes lower than the economic impacts from landfilling and combustion?
EPA notes that one limitation of this version’s economic analysis is that composting and anaerobic digestion economic impacts are undervalued in WARM factors because they are not inclusive of life-cycle impacts. In WARM Version 15, the indirect activity only includes activities involved in the value chain of the direct processes, such as collection, sorting and transportation. WARM Version 15 does not include indirect impacts in the industries that supply or interact with the primary industries. WARM Version 15 also does not include induced activity such as the increased spending of workers who earn money due to the increased economic activity. This means that WARM economic factors do not account for the impact of reduced-input costs and energy usage that are otherwise captured as part of the GHG emissions and energy impacts that are estimated in WARM.
For additional information on direct and indirect economic impacts in WARM, please refer to the Economic Impacts chapter as part of the Documentation for Greenhouse Gas Emission, Energy and Economic Factors Used in WARM.