The Mixture Rule: To (Hazardous) Waste or Not to Waste?
The Resource Conservation and Recovery Act, commonly known as RCRA, is one of the most complicated rules for environmental professionals to learn.
One of the more specialized and interesting aspects of RCRA is the contentious Mixture Rule. It was originally included in the regulation in 1980, but it was challenged in court and subsequently vacated in 1991. Then in March 1992, the Rule was reinstated on an interim basis and finalized as a rule in May 2001.
In simplistic terms, there are three elements of the Mixture Rule:
- If a characteristic hazardous waste is mixed with a solid waste, the resulting mixture is a hazardous waste, unless it does not exhibit one of the following characteristics: toxicity, reactivity, ignitability, or corrosivity.
- If a hazardous waste is listed because it exhibits a characteristic of ignitability, corrosivity, and/or reactivity (i.e., an ICR-listed waste) and is mixed with a solid waste, the resulting mixture is a hazardous waste, unless it does not exhibit a characteristic. There are 29 ICR-listed wastes, mostly consisting of P- and U-listed wastes. There is one F-listed waste, F003, which includes the nonhalogenated spent solvents, the most common ICR waste.
- If a listed waste (other than the 29 ICR-listed wastes) is mixed with a solid waste, the resulting mixture is a hazardous waste. This is true even if the resulting mixture contains only small amounts of listed waste and/or low concentrations of hazardous constituents.
Importantly, the Mixture Rule only applies to mixtures of hazardous and solid wastes. This rule does not apply to hazardous waste and products or to mixtures of hazardous waste and soil, groundwater, surface water, or other environmental media. The contained-in policy applies to these situations, which is the subject for another article. The Mixture Rule also does not apply when two or more hazardous wastes are mixed.
How the Mixture Rule Works
Below are some examples to illustrate the concepts above.
If an F003 spent solvent hazardous waste spills and is soaked up with an absorbent pad, what is the waste status of the contaminated absorbent pad, assuming the pad is not ignitable?
If the listed waste (F003) is mixed with “contaminated absorbents,” a solid waste, and no longer exhibits the characteristic (ignitability), the contaminated absorbent is not an F-listed waste via the Mixture Rule. Therefore, it would be discarded as contaminated debris under the contained-in policy.
If spent methanol solvent (F003) hazardous waste is discharged to a facility’s wastewater treatment system, is all the mixed wastewater hazardous?
Unless the mixed wastewater stream exhibits a characteristic, it would be considered non-hazardous.
The Special Case of Bevill Wastes
There are special mixture rules for Bevill wastes and oil and gas wastes. Bevill wastes include three classes that are excluded from the hazardous waste regulation, and they are named after the representative who introduced them into Congress. Bevill wastes include:
- Fly ash, bottom ash, slag, and flue gas emissions control waste from the combustion of fossil fuels
- Wastes from the extraction, beneficiation, and processing of ores and minerals
- Cement kiln dust
With Bevill wastes, the Mixture Rule works almost the same as it does if you mixed a solid waste with a hazardous waste.
- If a characteristic hazardous waste is mixed with a Bevill waste, the resulting mixture is hazardous only if it exhibits a hazardous waste characteristic not exhibited by the Bevill waste alone.
- If an ICR-only listed waste is mixed with a Bevill waste, the resulting mixture is hazardous only if it exhibits a hazardous waste characteristic not exhibited by the Bevill waste alone.
- If a listed hazardous waste (other than an ICR-listed waste) is mixed with a Bevill waste, the resulting mixture is a listed hazardous waste.
Oil and gas exploration and production wastes are also excluded from hazardous waste regulation and the Mixture Rule, and these wastes are essentially identical to the Mixture Rule for Bevill wastes. Below are some examples for how they should be treated:
An F003 spent solvent is mixed with a Bevill waste that exhibits the toxicity characteristic for cadmium. The resulting mixture is nonignitable but toxic for cadmium. Is it hazardous?
No, the only characteristic exhibited by the mixture is attributed to the Bevill waste alone.
A waste caustic solution, with the waste code D002 is mixed with an excluded oily waste in a pit. If the mixture has a pH greater than 12.5, is it a hazardous waste?
Yes, it is a D002 waste which is hazardous.
Boiling Down the Mixture Rule
When in doubt, the Mixture Rule boils down to a few cardinal rules:
- If a non-hazardous waste is mixed with a hazardous waste that is either characteristic or mixed with an ICR waste, and the resulting mixture exhibits a characteristic, then the mixture is a hazardous waste.
- If a non-hazardous waste is mixed with a listed waste, the mixture becomes a listed hazardous waste.
- Bevill wastes and oil and gas exploration and production wastes follow the above with some exceptions, as noted earlier.
If you are interested in learning more about the Mixture Rule and how it may apply to your facility or operations, please contact Industrial Hygiene Project Manager Laura Herron at Laura.Herron@APTIM.com.
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