(a) Ventilation. A nitrifying trickling filter must include forced ventilation to distribute airflow throughout the underdrain area. The minimum design airflow rate must be the greater of:
  (1) 50 pounds of oxygen provided per pound of oxygen required at average organic loading, based on stoichiometry; or
  (2) 30 pounds of oxygen provided per pound of oxygen required at peak organic loading, based on stoichiometry.
(b) Temperature. The engineering report must justify the temperature used in the design equations. A design may include deep towers or other means to minimize recirculation while providing a design hydraulic dosing intensity that lessens the effects of temperature on removal efficiency.
(c) pH. The engineering report must document that the design recirculation rates are appropriate for dealing with the effects of pH.
(d) Predation. A nitrifying trickling filter must include a means for control of biomass predators, such as snails.
(e) Hydraulic Application Rates. A nitrifying trickling filter must operate at a design dosing intensity of at least 1.47 gallons per minute per square foot and provide operational control of dosing intensity.
(f) Media. Cross-flow synthetic media is required for a new tertiary nitrification filter or for the nitrifying section of a new combined nitrification filter.
(g) Tertiary Nitrification Filters. A trickling filter treating influent that has a five-day biochemical oxygen demand (BOD5 ) to total Kjeldahl nitrogen (TKN) ratio of equal to or greater than 1.0 and soluble BOD5 of less than or equal to 12 milligrams per liter (mg/l) is a tertiary nitrification filter.
  (1) Design Justification. The engineering report must include process design calculations and selection criteria of kinetic coefficients for a tertiary nitrification trickling filter. The design calculations and selection criteria must be justified by operating data from an existing trickling filter of similar construction and operation.
  (2) Media biotowers. A tertiary nitrification trickling filter design must minimize pH depression due to recirculation. A tertiary nitrification trickling filter must use either:
    (A) a single tower at least 20 feet tall; or
    (B) a series of towers less than 20 feet tall. The design must include provisions to readily switch the operating sequence of the trickling filters if multiple towers are operated in series.
(h) Combined BOD5 and Nitrification Filters. A trickling filter intended to perform nitrification and treating influent having a BOD5 to TKN ratio of less than 1.0 or soluble BOD5 of less than 12 mg/l is a combined BOD5 and nitrification filter.
  (1) Design Justification. The engineering report must justify the projected treatment efficiency and other design parameters of a combined BOD5 and nitrification trickling filter by including operating data from any existing trickling filter of similar construction and operation.
  (2) BOD5 Removal Requirements. A combined BOD5 and nitrification filter must achieve effluent total BOD5 of less than 15 mg/l.
  (3) Recirculation. A combined BOD5 and nitrification filter design must enable a high recirculation rate with turndown capability.

Source Note: The provisions of this §217.183 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254