The Mixture Rule

The RCRA "mixture rule" provides guidelines for determining when mixtures of different kinds of waste must be regulated as hazardous waste. To make this determination, you need to consider certain criteria, as follows:

• If a characteristic hazardous waste is mixed with a solid waste, the resulting mixture is only considered a hazardous waste if that characteristic is still present in the mixture. If the mixture no longer exhibits the characteristic, it is not considered a hazardous waste.
• If a listed hazardous waste is mixed with a solid waste, the resulting mixture is considered a hazardous waste.
• If a listed or characteristic waste is mixed with something other than a solid waste, that mixture is NOT considered a hazardous waste.

Often, treatment (such as incineration) of a hazardous waste produces a residue or sludge that is different from the original waste, and may or may not be considered as hazardous waste:

• If the residue or sludge is derived from a characteristic waste, the residue is only considered hazardous if it still exhibits one of the four characteristics (even if it is now a different characteristic).
• If the residue or sludge is derived from a listed waste, the residue is still considered hazardous, even if it is only listed for a single characteristic and the residue no longer exhibits that characteristic.

It is important to distinguish between mixtures of wastes as they relate to the mixture rule and the RCRA definition of "mixed waste" that involves radioactive materials, which we discussed earlier.

To learn more about RCRA safety standards visit our RCRA Refresher Online Training web page.