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Simultaneous to the publication of the final National Emission Standards for Hazardous Air Pollutants for Boilers (the Boiler NESHAP) and the Commercial and Industrial Solid Waste Incinerator (CISWI) Rules, EPA  issued a new regulation that for the first time defines what materials are a non-hazardous solid waste when combusted.  This rule, Solid Wastes Used as Fuels or Ingredients in Combustion Units (40 CFR  Part 241), was developed in response to a court decision requiring EPA to clearly define which combusted materials meet the definition of solid waste.  For a facility to determine if a boiler is subject to the Boiler NESHAP or the CISWI Rules, it is necessary to determine whether the material combusted in the unit meets the definition of a solid waste.    

If a material meets the definition of traditional fuel, the analysis is simple:  the combusted material has never been discarded and cannot be a solid waste.  However, the definition of traditional fuel is narrow, and with the exception of on-spec used oil, requires the combusted material come from virgin materials.  Thus, coke oven gas generated during the production of coke from coal is a traditional fuel, as it comes from a virgin material, but landfill gas is not a traditional fuel, as it comes from non-virgin material.

For those combusted materials not meeting the definition of traditional fuel, Subpart B of this Rule identifies procedures a facility must follow to determine if a Non-Hazardous Secondary Material (NHSM) that is combusted is, or is not, a solid waste.

Solid Waste and NHSM

The NHSM rule references the Resource Conservation and Recovery Act (RCRA) definition of solid waste, as codified at 40 CFR 258.2.  Per the RCRA definition, a solid waste is Solid Waste_Tiresdefined as “any garbage, or refuse, sludge from a wastewater treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, including solid, liquid, semi-solid, or contained gaseous material resulting from industrial, commercial, mining, and agricultural operations, and from community activities… .”  If the material has not been discarded, then the combustion unit is not burning a solid waste.  The NHSM rule provides a procedure to determine whether or not a NHSM has been discarded.  In the preamble to the NHSM Rule, EPA generally contends (via the legitimacy criteria) that if a NHSM has a higher level of contaminants than a “traditional fuel,” the NHSM is a solid waste.  

A secondary material is defined as any material that is not the primary product of a manufacturing or commercial process, and can include post-consumer material, off-specification commercial chemical products or manufacturing chemical intermediates, post-industrial material, and scrap.  A NHSM is defined as a secondary material that, when discarded, would not be identified as a hazardous waste under 40 CFR Part 261. Examples of secondary materials that are not hazardous waste could include on-site industrial generated trash, old corrugated cardboard (OCC) rejects, wastewater treatment plant sludge, and contained gases that are routed to a combustion device.   

In general, the final rule identifies NHSM burned in combustion units as solid wastes unless the material falls into one of the following categories and meets a specific set of legitimacy criteria.

  • Material used as a fuel that remains within the control of the generator
  • Scrap tires that have not been discarded and are managed under the oversight of an established tire collection program  
  • Resinated wood that has not been discarded
  • Material used as an ingredient (defined as a NHSM that is a component in a compound, process, or product)
  • Material produced from the processing of NHSM.  Processing is defined as any operations that transform discarded NHSM into a non-waste fuel or non-waste ingredient product.  Processing includes operations that remove or destroy contaminants, significantly improve the fuel characteristics of the material, chemically improve the as-fired energy content, or improve the ingredient characteristics. Shredding alone does not constitute processing.

EPA may grant a non-waste determination, on a case-by-case basis, for a NHSM used as a fuel, not managed within the control of the generator, not discarded, and thus not a solid waste when combusted. 

Legitimacy Criteria

Any NHSM that is not a traditional fuel must meet the following legitimacy criteria to avoid classification as a solid waste.  The legitimacy criteria differs slightly upon whether the material is used as a fuel or as an ingredient.

  • Legitimacy Criteria for Materials Used as a Fuel:
    • Managed as a valuable commodity such that the storage of the NHSM does not exceed reasonable time frames and releases to the environment are prevented
    • Meaningful heating value (met presumptively if 5,000 Btu/lb or greater) and used as a fuel in a combustion unit that recovers energy
    • Contains contaminants at levels comparable in concentration to or lower than those in traditional fuels which the combustion unit is designed to burn
  • Legitimacy Criteria for Materials Used as an Ingredient:
    • Managed as a valuable commodity such that the storage of the NHSM does not exceed reasonable time frames and releases to the environment are prevented
    • Provides a useful contribution to the production or manufacturing process
    • Used to produce a valuable product or intermediate
    • Results in products that contain contaminants at levels comparable in concentration to or lower than those found in traditional products manufactured without the NHSM 

Even if the NHSM can meet the legitimacy criteria described above, the moment the NHSM is discarded, it meets the definition of a solid waste.

For both types of NHSM, the most challenging aspect is the last criteria, which requires comparable contaminant concentrations.  For ingredients, this legitimacy criterion is likely fairly straightforward to meet, as products made with NHSM ingredients are unlikely to differ materially in contaminant levels from those made with virgin ingredients.  However for fuels, the comparable contaminants criterion appears much more complex and challenging.  EPA discusses this topic in the preamble, but offers minimal guidance on material conclusions.

Understanding the Term “Contained Gaseous Materials”

The RCRA definition of solid waste includes “ … other discarded material, including … contained gaseous material.”   EPA has not included a definition of contained gaseous materials in the RCRA regulations.  Historically, EPA has treated “contained gaseous material” as gases that are stored in containers such as a cylinder of nitrogen.  EPA has not treated gaseous materials in pipelines as contained gaseous materials.2  In the response to comments for the proposed NHSM Rule, EPA indicated that “we are unable to find any Agency reasoning supporting previous EPA interpretations that only gases in containers may be considered “contained.” … EPA cannot see how gaseous secondary material that is generated in any particular system and is somehow sent to a gas-fired boiler, even through a pipeline, can be considered an uncontained gas.”3  The implication of EPA’s opinion on what is considered a contained gaseous material is significant for many industries such as pulp and paper mills and landfill gas-to-energy operations.  For example, each facility that routs a vent stream from a process unit to a combustion device may have to determine whether the vented gas meets the legitimacy criteria described above, unless the vented gas qualifies as a traditional fuel.  It will likely be very difficult, if not impossible, for certain industries to demonstrate that the vented gas contains contaminants at levels comparable or less in concentration to that of the traditional fuel the combustion unit is designed to burn.

To further illustrate this point, commenters requested clarification on how EPA would treat biogas, landfill gas, and sewage gas under the NHSM Rule.  EPA responded that “the combustor using the material is expected to use the framework described in … 40 CFR 241.3.”4  The only logical conclusion that can be made from EPA’s response is EPA expects that any facility that combusts a vent stream should follow the procedure stipulated in the NHSM rule.

As an example, it is common for a pulp and paper mill to combust non-condensable gases (NCG) in a boiler, as is required under NESHAP Subpart S.  It is highly unlikely that a pulp and paper mill will be able to demonstrate that the hydrogen sulfide content in NCG is less than that of the traditional fuel fired in the combustion unit.  The main reason their pulp and paper mill route NCG to the boilers is to reduce the amount of pollution that would be vented to the atmosphere by using the combustion device for control.  However, under the recent guidance on what is considered a contained gaseous material, burning NCG in a combustion device may result in a determination that the emission unit is subject to the CISWI rules instead of the Boiler MACT.

It is important to note that the conclusions reached for contained gaseous material are based on recent EPA guidance as a definition is not included in any of the RCRA regulations for contained gaseous materials.


EPA has prepared a number of Material Characterization Papers in order to assist industry with determining if a fuel stream meets the definition of a solid waste.5   The Material Characterization Papers include a definition of the respective secondary material, the annual quantities generated and used, the management and combustion processes, and the anticipated secondary material composition and impacts.  

The NHSM rule introduces a level of complexity for facilities that had previously used materials generated on site as fuel in boilers and process heaters.  Examples of materials where rule applicability depends on the NHSM rule include:

  • Contained gases (e.g., landfill gas and process vent gas routed to combustion units)
  • Wastewater treatment residuals
  • Container/packaging rejects

These materials (and many others) must now be evaluated against the legitimacy criteria to determine whether or not they are solid waste and whether the combustion devices that use the materials as fuel are subject to NESHAP or CISWI rules.  Every facility should take the following steps for non-traditional fuels combusted:

  1. Determine which materials remain within the control of the generator, or are ingredients. 
  2. For those non-ingredient materials not within control of the generator, if they are not (a) scrap tires or resinated wood, or (b) produced from re-processing, the facility must petition EPA to find that the material has not been discarded.
    •  If EPA finds the material has not been discarded, then consider the legitimacy criteria
    • If EPA finds otherwise, the material is solid waste 
  3. Evaluate the legitimacy criteria for each NHSM from Step 1 and Step 2 

Each facility should review all of its streams that are routed to a combustion device and confirm if the streams are a traditional fuel or if the legitimacy criteria must be reviewed for any secondary materials.

1  Per Section 129(h) of the Clean Air Act, if an emission unit becomes subject to a Section 129 Regulation, such as the CISWI Rules, the emission unit will not be subject to the Section 112 Regulations (NESHAP Subparts).  As an example, if a cement kiln becomes subject to the CISWI Rules, the cement kiln will no longer be subject to the Portland Cement NESHAP.
2  Per 40 CFR 60.2265, “contained gaseous material” has been defined as “gases that are in a container when the container is combusted.”  However, EPA has removed this term and definition in the 2011 CISWI Rules.
3  “Responses to Comments: EPA-HQ-RCRA-2008-0329.
4  “Responses to Comments: EPA-HQ-RCRA-2008-0329.