Q. Does a hazardous waste storage tank need to be emptied every 90 days (or 180 days), or can a ”mass balance approach” be taken?
A. EPA stated in the January 11, 1982 issue of the Federal Register (47 FR 1250) that generators must remove all wastes from the tank within 90 days from the time he first places waste in the “empty” tank. The discussion goes on to say that the tank is considered empty “when its contents have been drained to the fullest extent possible…but it is not expected that 100% of the wastes will always be removed.”
The EPA recognized that tank systems do not always allow for complete drainage due to flanges, screens, or siphons. Therefore, as general guidance, the EPA stated that a tank is considered empty when “the generator has left the tank’s drainage system open until a steady, continuous flow has ceased.”
In October 12, 2006, the EPA was asked whether a “mass balance approach” was acceptable in proving that no waste was accumulated in a storage tank for more than the applicable time limit. In their answer, the EPA stated that their interpretation of 40 CFR 262.34(a)(1)(ii) does allow for this mass balance approach.
The conclusion is that tanks can be operated in one of two ways: in a “batch process or in a continuous flow process.”
The batch process refers to when a tank would receive a “batch” of hazardous waste, or subsequent batches, and the tank is fully emptied within 90 days from the date the first drop of hazardous waste was placed in the tank.
The continuous flow process is when a tank receives hazardous waste on an ongoing basis. In this scenario, the EPA discusses a “mass balance approach.” The “key parameters” for this approach take into account things such as: the daily throughput of hazardous waste, the time period the waste resides, and the volume of the tank.
The EPA considered that under a continuous flow process where periodically emptying the tank is impractical, a generator could comply with the 90-day accumulation limit by measuring the volume of hazardous waste that flows into and out of the tank and showing that an amount of hazardous waste at least equal to the volume of the tank flows through during each 90-day period. This is called a“mass balance approach.”
Another important thing to note is that it is the generator’s responsibility to prove the continuous flow process does not allow waste to accumulate more than 90- (or 180-) days and meets all the requirements applicable to the tank. The EPA discusses examples of ways to prove this, such using equipment to monitor material entering and leaving the tank and recording the results.