Requirements for Miscellaneous Hazardous Waste Units

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What is Subpart X?

Subpart X refers to miscellaneous units, which are a unique category of hazardous waste management units, regulated under the Resource Conservation and Recovery Act (RCRA) and title 40 Code of Federal Regulations (CFR) part 264 subpart X. These units do not fit within the definition of the more typical waste management units described in part 264 (containers, tanks, incinerators, etc.). In order to be permitted, subpart X units must meet environmental performance standards, while other part 264 units must meet specific technology standards.

In developing the subpart X regulations, the Agency wanted to promulgate a new set of general standards that would cover the diverse technologies and units not yet covered in part 264. To accomplish this goal, the subpart X regulations are general, not technology-specific. Since miscellaneous units are subject to site-specific design and operating requirements, RCRA requires that owners and operators applying for a permit provide the implementing agency with detailed information on unit design and potential environmental impacts.

In sum, miscellaneous units are required to be located, designed, constructed, operated, maintained, and closed in a manner that will prevent any unsafe releases into the:

  • groundwater
  • subsurface environment
  • surface water
  • wetlands
  • soil surface
  • air

This media-based or pathway-based approach ensures that any potential problems arising from these units are addressed. 

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What Types of Units are Subject to Subpart X?

Units subject to subpart X include:

  • Geologic repositories other than injection wells
  • Deactivated missile silos other than injection wells or tanks
  • Thermal treatment units other than incinerators, boilers, or industrial furnaces
  • Open burning and open detonation (OB/OD) of waste explosives
  • Chemical, physical, biological treatment units

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What does Subpart X not Regulate?

Units not subject to subpart X include:

  • Units regulated under other parts of 264 or 265 (e.g., containers, tanks, landfills, surface impoundments)
  • Units excluded from permitting under parts 264 and 270
  • Underground injection wells regulated by 40 CFR part 146
  • Research, development and demonstration units covered under part 270​

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Resources and Policy Letters for Subpart X

The following resources provide more information about subpart X:

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