How, When, and Why to Use the Hazardous Waste Mixture Rule

Unless specifically excluded, there are three ways a solid waste becomes a hazardous waste:

• When the waste first meets an F, K, P, or U listing description in 40 CFR 261, Subpart D;
• When the waste exhibits one or more of the four characteristics of hazardous waste – ignitability, corrosivity, reactivity, and toxicity as identified in 40 CFR 261, Subpart C; OR
• Through the “mixture rule.”

What Is the RCRA Hazardous Waste Mixture Rule?

Found at 40 CFR 261.3(a)(2)(iv), the hazardous waste “mixture rule” states that if you mix a listed hazardous waste (one with F, K, P, or U codes) with any other solid waste, the entire mixture is now a listed hazardous waste.

When Does Mixing Wastes Make Sense?

When would a hazardous waste generator want to mix wastes? One of the most common scenarios is a consolidation drum in a central accumulation or satellite area, where a generator might mix compatible wastes in the same container. For example, if a generator mixed an F005 spent toluene with unused benzene (U019), the waste in that drum would now carry both listed waste codes in addition to the D001 for ignitability and D018 for the toxicity characteristic of benzene.

Scientists working in laboratories might also mix discarded, left-over chemicals that are compatible with one another.

Since it is typically more expensive to treat acute hazardous wastes (P-listed wastes and several F-listed wastes with “H” indicated as the hazard code), it is advisable not to mix these types of wastes with solid waste or other non-acute hazardous wastes.

What Are the Benefits of Mixing Wastes?

When are wastes excluded as hazardous wastes if they are mixed? The regulations at 40 CFR 261.3(d) state that a solid waste that exhibits a characteristic of hazardous waste (i.e., I, C, R, or E) stops being a hazardous waste if it does not exhibit any characteristic.

Mixing Characteristic Hazardous Waste

How might this occur? A generator might use the exclusion at 40 CFR 270.1(c)(2) for elementary neutralization to neutralize a corrosivity-only waste on site. This treatment requires mixing the waste with a neutralizing agent. Once the waste is not corrosive, it is no longer a hazardous waste.

It is also allowable to add waste to absorbent, or absorbent to waste, when it initially accumulates without a RCRA permit. So, if you added unused alcohols (D001 for ignitability) to absorbent material in a container, and it is no longer a liquid with a flash point of <140 degrees F, then it is no longer a hazardous waste.

Mixing Listed Hazardous Waste

When does a listed hazardous waste stop being a hazardous waste? If the waste is listed for “T” (toxic) or “H” (acutely hazardous), or if it exhibits the toxicity characteristic at 40 CFR 261.24, there’s no way out. If, however, the waste is listed only for ignitability (I), corrosivity (C), or reactivity (R), and it no longer exhibits those characteristics, then it is no longer a hazardous waste.

For example, if spent acetone—an F003 waste, listed only for ignitability—spills on the floor and you use absorbent pads to clean it up so there is no free liquid remaining, the absorbent pads would not be a hazardous waste.

The Big Picture

If a solid waste is no longer a hazardous waste, it is not subject to container management rules, accumulation time limits, and manifesting. However, waste excluded in this manner is still subject to land disposal restriction treatment and recordkeeping requirements of 40 CFR 268. [See 40 CFR 261.3(d)(1) and (g)(3)]

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Tags: hazardous, mixture rule, RCRA, RCRA Training, waste, waste minimization