Section 49 CFR 173.29 contains the PHMSA regulations for the transportation of empty packagings that last contained hazardous materials.  As a shipper of hazardous materials you must be aware of 173.29(a) states that an empty packaging with the residue of a hazardous material will be regulated the same as a full container. In order for your package to be considered truly empty and not regulated, it cannot contain any hazmat residue.

This how-to guide describes the steps involved to make sure your hazmat package is completely empty with no hazmat residue and therefore not regulated as hazmat.

1. Sufficiently clean out any hazmat residue or vapors from the package.
2. If the package cannot be sufficiently cleaned, refill the package with a material which is not hazardous, to such an extent that any residue remaining in the package no longer poses a hazard.
3. Remove, obliterate, or securely cover any hazmat marking, label, or placard from the package.