When is a UST considered "empty"?

A UST is considered "empty" when all materials have been removed such that any remaining residue does not exceed 2.5 cm. This definition is significant because it establishes a clear standard for what it means to empty a tank, ensuring that residual materials do not pose a risk to the environment or create hazards during further assessments, inspections, or potential decommissioning.

The threshold of 2.5 cm is important as it recognizes that completely removing every micro-particle of liquid may not be feasible. Therefore, the focus is set on minimizing the risk by ensuring that any remaining materials are below a specified level, thereby aligning with safety and regulatory protocols regarding underground storage tanks.

In contrast, defining a tank as "empty" merely based on the absence of liquid present, without considering residue, would not adequately address potential environmental concerns associated with residual contamination. Similarly, disconnecting a tank or merely cleaning and inspecting it does not meet the definition of "empty" without addressing the actual contents left inside the tank.