(a) Design.
  (1) If evaporative lagoons are used, the wastewater treatment facility must have at least two evaporative lagoons.
  (2) The primary evaporative lagoon must provide at least 60% of the total surface area of the total evaporative lagoon system.
  (3) The number and size of evaporative lagoons must provide adequate evaporation for the design flow during periods of low evaporation. For the purposes of this chapter, the low evaporation rate used in design must be the lowest annual evaporation rate in the past 25 years from the Texas Water Development Board's precipitation and lake evaporation dataset for the quadrangle where the evaporation lagoon is located.
(b) Odor Management. An evaporative lagoon must be located so that odors from the evaporative lagoon will not cause a nuisance.
(c) Configuration, Depth, and Loading.
  (1) An evaporative lagoon may be constructed in a round, square, or rectangular shape. The corners of a square or rectangular shaped evaporative lagoon must be rounded in order to minimize accumulation of floating materials.
  (2) The depth of an evaporative lagoon is dependent on its location within the lagoon system, as set forth in subparagraphs (A) and (B) of this paragraph.
    (A) The maximum operating depth for a primary evaporative lagoon is 5.0 feet, but the area around an inlet must be designed with additional depth for solids deposition according to the criteria in §217.205 of this title (relating to Facultative Lagoons).
    (B) The maximum operating depth for a secondary evaporative lagoon is 8.0 feet.
  (3) Evaporation and Organic Loading.
    (A) The size of an evaporative lagoon system must be based on the evaporation rate for the site and a maximum allowable organic loading rate.
    (B) The evaporation loss must be calculated by using the Penman-Monteith method or a comparable, established method.
    (C) An evaporative lagoon system must be sized to account for the influent flows and precipitation from a 25-year frequency, one-year rainfall event in accordance with §309.20(b)(3)(B) of this title (relating to Land Disposal of Sewage Effluent), unless the engineering report includes an alternate method of disposing of the wastewater, along with supporting documentation.
    (D) The five-day biochemical oxygen demand (BOD5 ) loading on a primary evaporative lagoon must not exceed 150 pounds of BOD5 per acre of surface area per day.
(d) Embankment. The embankments for an evaporative lagoon must be constructed in accordance with §217.203(f) of this title (relating to Design Criteria for Natural Treatment Facilities).
(e) Inlet and Outlet Structures.
  (1) An influent line for an evaporative lagoon must terminate into a manhole located along the embankment edge.
  (2) An inlet manhole invert must be a minimum of 6.0 inches above the maximum high water level of a primary evaporative lagoon or provide other means to prevent backflow into an upstream treatment unit.
  (3) A submerged discharge pipe must extend from a manhole along and anchored to the bottom of an evaporative lagoon.
  (4) An inlet discharge pipe must discharge onto a concrete apron in a depression near the center of the primary evaporative lagoon to prevent scour. A concrete apron must be at least 2.0 square feet in surface area, be at least 8.0 inches thick, and be resistant to the corrosive effects of a wastewater environment.
  (5) Inlet and outlet structures for an evaporative lagoon must be designed and constructed in a manner that allows the water surface elevation to be varied during normal operating conditions.

Source Note: The provisions of this §217.208 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254