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

   1. (1) The length-to-width ratio of a facultative lagoon must be 3.0 to 1.0.
   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 of the lagoon.
   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 of a facultative lagoon must cover at least 25% of the area of the lagoon bottom.
   3. (3) The remainder of a facultative lagoon must have a minimum depth of 8.0 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 the 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 the water surface, but not further below the water surface than a distance equal to one-half the depth of the lagoon at the outlet.
   3. (3) The design of a facultative lagoon must allow for recirculation of at least 50%, but not more than 100% of the design flow.
   4. (4) The design of a facultative lagoon must prevent siphoning of lagoon contents through a submerged inlet.
5. (e) Removal efficiency. The design of a facultative lagoon must not be based on more than 50% removal of the influent BOD₅.

Source Note:The provisions of this §217.205 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254.