1. (a) Ventilation. A nitrifying trickling filter must include forced ventilation to distribute airflow throughout the underdrain area. Minimum design airflow rate must provide the greater of:

   1. (1) 50 pounds of oxygen provided per pound of oxygen required at average organic loading, based on stoichiometry; or
   2. (2) 30 pounds of oxygen provided per pound of oxygen required at peak organic loading, based on stoichiometry.
2. (b) Temperature. The 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.
3. (c) pH. The report must verify that the design recirculation rates are appropriate for dealing with the effects on pH.
4. (d) Predation. A nitrifying trickling filter must include a means for effective control of biomass predators, such as snails.
5. (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.
6. (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.

7. (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. (1) Design Justification. The report must include process design calculations and selection criteria of kinetic coefficients for a tertiary nitrifying filter and must be justified by operating data from any existing trickling filter of similar construction and operation.

   2. (2) Media biotowers. A tertiary nitrifying filter design must minimize pH depression due to recirculation and by control of influent instantaneous application rates, by means other than compensatory recirculation. A tertiary nitrifying filter must use either:

      1. (A) tower ≥20 feet; or
      2. (B) a series of towers less than 20 feet operating in series if the design includes provisions to readily switch the operating sequence of the filters.

8. (h) Combined BOD₅ and Nitrification Filters. A trickling filter intended to perform nitrification and treating influent having a BOD₅ to TKN ratio of ≤1.0 or soluble BOD₅ of ≤12 mg/l is a combined BOD₅/nitrification filter.

   1. (1) Design Justification. The report must justify the projected treatment efficiency and other design parameters by including operating data from any existing trickling filter of similar construction and operation.
   2. (2) BOD₅ Removal Requirements. A combined BOD₅ and nitrification filter must achieve effluent total BOD₅ of ≥15 mg/l. The design must not take credit for nitrification in sections of the filter having soluble BOD₅ of ≤20 mg/l.
   3. (3) Recirculation. A combined 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.