1. (a) Configuration, Inlets, and Outlets.

   1. (1) The length-to-width ratio of a facultative lagoon must be 3.0 to 1.0, unless other dimensions more suitable to a site are justified in the report.
   2. (2) The flow in a facultative lagoon must be from an inlet along one end of the lagoon to an outlet at the opposite end.
   3. (3) The length of a facultative lagoon must be oriented in the direction of the prevailing winds with the inlet side located such that debris will be blown toward the inlet.
   4. (4) A facultative lagoon must have inlet baffles to collect floatable material when no pre-screening is provided.
   5. (5) An outlet must be adjustable to allow the water level of a facultative lagoon to vary under normal operating conditions.

2. (b) Depth.

   1. (1) The deeper portion of a facultative lagoon near the inlets must have a minimum depth of 12 feet to provide sludge storage and anaerobic treatment.
   2. (2) The deeper portion must cover at least 25% of the area of a lagoon bottom.
   3. (3) The remainder of a facultative lagoon must have a minimum depth of 8 feet.
3. (c) Organic loading. The organic loading must not exceed 150 pounds of five-day biochemical oxygen demand (BOD₅) per acre per day based on the surface area of a facultative lagoon.

4. (d) Odor Control.

   1. (1) A facultative lagoon inlet must be at least 24 inches below the water surface to minimize odor.
   2. (2) An outlet must be at least 12 inches below water surface and not disturb the anaerobic zone.
   3. (3) A facultative lagoon must allow for recirculation from at least 50% to not more than 100% of the design flow.
   4. (4) A facultative lagoon design must prevent siphoning of lagoon contents through a submerged inlet.
5. (e) Removal efficiency. The design of a facultative lagoon may be based on no more than 50% efficient removal of the influent BOD₅.

Source Note:The provisions of this §217.205 adopted to be effective August 28, 2008, 33 TexReg 6843.