Closed biological treatment process
Closed biological treatment process means a tank or surface impoundment where biological treatment occurs and air emissions from the treatment process are routed to either a control device by means of a closed vent system or to a fuel gas system by means of hard-piping. The tank or surface impoundment has a fixed roof, as defined in 63.111 of this subpart, or a floating flexible membrane cover that meets the requirements specified in 63.134 of this subpart.
40 CFR § 63.111
All terms used in this subpart shall have the meaning given them in the Act, in subpart F of this part, and in this section, as follows.