Secondary containment is a basic engineering control to prevent a chemical or oil spill. There are some misconceptions, though, regarding secondary containment requirements. In terms of oil-based storage, these misconceptions can lead to inadequate containment capacity, significantly more containment capacity than needed, or simply not providing the right containment level when containers are grouped.
The Environmental Protection Agency's Spill Prevention, Control and Countermeasure (SPCC) dictation does not specifically quantify the requirements for secondary containment; it simply stipulates that a facility needs to "provide a secondary means of containment for the entire capacity of the largest single container and sufficient freeboard to contain precipitation." The EPA determined that, for freeboard, the more appropriate method of secondary containment is a matter of engineering practice and did not quantify a percentage or specific storm event for engineers to calculate freeboard when completing secondary containment designs. The containment capacities provided in the table below are based on industry guidance and best practices. I have visited dozens of facilities and seen several misconceptions during facility walk-throughs over the years. Here are some of the common mistakes people make when selecting containment systems, and how to correct them:
Overcoming These Five Common Secondary Containment Misconceptions
Any pre-fabricated containment pallet has sufficient secondary containment for a 55-gallon drum.
Often times, these pre-fabricated containment pallets have a spill volume of 20 to 30 gallons. Secondary containment needs to be at least the volume of the container and sufficient freeboard1 for precipitation. For indoor storage, the industry norm is 110 percent of the container's capacity. For a single drum, that would be 66 gallons of containment capacity.
Each 55-gallon drum stored indoors needs to have its own 66-gallon capacity secondary containment pallet.
While you can always go above and beyond what is required, you only need to size a secondary containment system to hold the single most comprehensive container plus freeboard for rain. Four 55-gallon drums stored together indoors on a single containment pallet would only need 66-gallons of containment volume.
An outdoor tank has sufficient secondary containment with a concrete containment system capable of containing 100 percent of the tank's capacity.
For outdoor containers, you have to account for precipitation in sizing your secondary containment setup. Unless a more rigorous local or state regulation presides, engineers typically use a rule of thumb for deciding appropriate outdoor containment capacity. The most common methods to provide freeboard for precipitation are either 125 percent of the container's volume OR the container's size plus volume that would be collected over the containment device's footprint during a 25-year, 24-hour storm event.
A facility can store multiple drums within a concrete containment structure where other equipment and containers are stored.
While this may be a satisfactory practice, storing additional items within the secondary containment structure reduces its containment space. If the structure was designed with a minimum 110 percent containment capacity, storing extra items within the containment construction could reduce the containment capacity below the needed amount.
A facility has sufficient containment if a berm is around the entire oil storage area on a sloped ground.
For a sloped floor, determining the actual containment capacity is crucial. If the height of the berm does not account for the slope, the secondary containment system's capacity may be much smaller than you think. A large portion of the containment volume can be wasted due to the slope on the floor. Also, maintaining the curb/berm elevation is critical to maximizing the containment system's capacity.