The Mixture Rule: To (Hazardous) Waste or Not to Waste?

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:

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:

With Bevill wastes, the Mixture Rule works almost the same as it does if you mixed a solid waste with a 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 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.