There are no Federal requirements with time constraints which would force generators to begin closure activities after wastes have been removed from the accumulation unit. You should contact your state implementing agency for a determination on this issue.
It is important to note that there is no clarification on the management of residues in empty tanks, only empty containers (40 CFR §261.7 ). The clarification that is available on the determination of "empty" in tanks for purposes of accumulation is specific to accumulation only and does not apply to the management of residues.
A tank will be considered "empty" when its contents have been drained to the fullest extent possible. Since many tank designs do not allow for complete drainage due to flanges, screens, or syphons, it not expected that 100% of the wastes will always be removed. There may be cases where a tank is never "completely empty" (47 FR 1250; January 11, 1982).
Eventually, the generator must comply with the closure performance standards of Section 265.11 and the standards for disposal and decontamination of equipment, structures and soils as per Section 265.114.
In addition, the following guidance memorandum offers clarification on the issue of empty accumulation tanks:
Memo, Skinner to Devine; August 31, 1982 (RCRA Online #12062)
This memorandum can be found at the RCRA Online Database at the following URL: www.epa.gov/rcraonline.