1. (a) Trickling filters are classified according to applied hydraulic loading, including recirculation, in million gallons per day (mgd) per acre of filter media surface area and influent organic loadings in pounds of five-day biochemical oxygen demand (BOD₅) per day per 1,000 cubic feet of filter media, The following factors must be the basis for the selection of the design hydraulic and organic loadings:

   1. (1) strength of the influent wastewater;
   2. (2) effectiveness of pretreatment;
   3. (3) type of filter media; and
   4. (4) treatment efficiency required.

2. (b) A trickling filter is classified as:

   1. (1) a roughing filter, which provides at least 50% but not more than 75% removal of soluble BOD₅;
   2. (2) a secondary treatment filter, which provides the required settled effluent BOD₅ and total suspended solids (TSS);
   3. (3) a combined BOD₅ and nitrifying filter, which provides the required settled effluent BOD₅, ammonia-nitrogen (NH₄-N), and TSS; or
   4. (4) a tertiary nitrifying filter, which provides the required settled effluent NH₄-N, if the influent to a trickling filter is a clarified secondary effluent.
3. (c) The following table lists the hydraulic and organic loadings for different classes of trickling filters:
   Attached Graphic
   

4. (d) Pretreatment.

   1. (1) A trickling filter must have upstream preliminary treatment units that:

      1. (A) remove grit, debris, suspended solids, oil, and grease;
      2. (B) have particles with a diameter greater than three millimeters; and
      3. (C) control the release of hydrogen sulfide.
   2. (2) A primary clarifier equipped with scum and grease removal devices must precede a rock media trickling filter.

5. (e) Rock Filter Media.

   1. (1) Materials.

      1. (A) Rock media using crushed rock, slag, or similar material containing more than 5% by weight of pieces with their longest dimension three times greater than the least dimension is prohibited.

      2. (B) Rock media must conform to the following size distribution and grading. Mechanical grading over a vibrating screen with square openings must meet the following:

         1. (i) passing 5.0 inch sieve - 100% by weight;
         2. (ii) retained on 3.0 inch sieve - 95-100% by weight;
         3. (iii) passing 2.0 inch sieve - 0.2% by weight;
         4. (iv) passing 1.0 inch sieve - 0.1% by weight; and
         5. (v) the loss of weight by a 20-cycle sodium test, as described in American Society of Civil Engineers' Manual of Engineering and Engineering Practice No. 13, must be less than 10%.

   2. (2) Placement.

      1. (A) Rock media must be at least 4.0 feet deep at the shallowest point.
      2. (B) Dumping rock media directly on a filter is prohibited. Rock media must be placed by hand to a depth of 12 inches above the underdrains. The remainder may be placed by belt conveyor or an equivalent mechanical method.
      3. (C) Crushed rock, slag, and other similar media must be washed and screened or forked to remove clay, organic material, and fines prior to placement.
      4. (D) The placement of any material must not damage the underdrains.
      5. (E) Vehicles and equipment are prohibited from driving over the filter media.

6. (f) Synthetic (Manufactured or Prefabricated) Media Materials.

   1. (1) Any synthetic media material must be used in accordance with all manufacturer's recommendations.

   2. (2) Synthetic media material may be considered innovative or nonconforming technology and may be subject to §217.7(b)(2) of this title (relating to Types of Plans and Specifications Approvals).

      1. (A) Suitability. The suitability of synthetic media material must be evaluated based on experience with an installation treating wastewater under similar hydraulic and organic loading conditions. The report must include a relevant case history involving the use of the synthetic media.
      2. (B) Durability. A synthetic media must be insoluble in wastewater and resistant to flaking, spalling, ultraviolet degradation, disintegration, erosion, aging, common acids and alkalis, organic compounds, and biological attack.

      3. (C) Structural Integrity.

         1. (i) A structural design must support the synthetic media, water flowing through or trapped in voids, and the maximum anticipated thickness of the wetted biofilm.
         2. (ii) The synthetic media must support the weight of a person, unless a separate provision is made for maintenance access to the entire top of the trickling filter media and to the distributor.
      4. (D) Placing of Synthetic Media. Modular synthetic media must be installed with the edges matched as nearly as possible to provide consistent hydraulic conditions within the trickling filter.

7. (g) Filter Dosing.

   1. (1) Suitable flow characteristics must be used for the application of wastewater to a filter by siphon, pump, or gravity discharge from preceding treatment unit.
   2. (2) A filter must be designed to control instantaneous dosing rates under both normal operating conditions and filter-flushing conditions.
   3. (3) The distributor speed and the recirculation rate must be adjusted for the dosing intensity as a compensatory measure under low-flow conditions. The following table provides design ranges of dosing intensity for normal usage periods and for flushing periods:
      Attached Graphic
      
   4. (4) A design may be based on instantaneous dosing intensity for rotary distributors using the equation in the following figure:
      Attached Graphic
      

8. (h) Distribution Equipment.

   1. (1) A design must include a rotary, horizontal, or traveling wastewater distribution system that distributes wastewater uniformly over the entire surface of a filter at the design and flushing dosing intensities.
   2. (2) A design must include filter distributors that operate properly at all anticipated flow rates.
   3. (3) A design must not deviate from the design dosing intensity by more than 10%.
   4. (4) A new trickling filter or upgrade of an existing trickling filter must include electrically driven, variable speed a filter distributor to allow operation at optimum dosing intensity independent of recirculation pumping.
   5. (5) If an existing rectangular trickling filter is retrofitted with rotary distributors, any media that will not be fully wetted must not be considered part of the required effective treatment area.
   6. (6) The center column of a rotary filter distributor must have adequately sized overflow ports to prevent water from reaching the bearings in the center column.
   7. (7) A filter distributor must include cleanout gates on the ends of the arms and an end spray nozzle to wet the edge of the media.
   8. (8) The filter walls must extend at least 12.0 inches above the top of the ends of the distributor arms.
   9. (9) The use of a mercury seal in a distributor of a trickling filter is prohibited in a new facility. If an existing treatment facility is materially altered, any mercury seal in a trickling filter must be replaced with an oil or mechanical seal.
   10. (10) The minimum clearance between the top of the filter media and the distributing nozzles is 6.0 inches.
   11. (11) Rotary distributors must capable of operating at speeds as low as one revolution per 30 minutes.
   12. (12) A trickling filter with a height or diameter that does not allow distributors to be removed and replaced by a crane must provide jacking columns and pads at the distributor column.

9. (i) Recirculation.

   1. (1) Low Flow Conditions.

      1. (A) A design must include minimum recirculation during periods of low flow in order to ensure that the biological growth on the filter media remains active at all times.
      2. (B) A design must include the minimum recirculation in the evaluation of the efficiency of a filter, if it is part of a proposed specified continuous recirculation rate.
      3. (C) Minimum flow to the filters must equal to or greater than 1.0 mgd per acre of filter aerial surface and must keep the distribution nozzles properly operating.
      4. (D) The minimum flow rate for a design using hydraulically driven distributors must keep rotary distributors turning at the minimum design rotational velocity.
      5. (E) For a facility designed with a capacity equal to or greater than 0.4 mgd and recirculation for BOD₅ removal, the recirculation system must include variable speed pumps and a method of conveniently measuring the recycle flow rate.

   2. (2) Compensatory Recirculation.

      1. (A) A design must provide recirculation to supplement influent flow if design and flushing dosing intensities are not achieved solely by the control of distributor operation.
      2. (B) Controls for the distributor speed and recycle pumping rate must provide optimum dosing intensity under all anticipated influent flow conditions.

   3. (3) Process Calculations. The report must:

      1. (A) describe a design that propose removal of the remaining organic matter by recirculation;
      2. (B) identify the effect of dilution of the influent on the rate of diffusion of dissolved organic substrates into the biofilm; and
      3. (C) identify the effect of reduced influent concentrations on reaction rates in each section of a filter having first order kinetics.
   4. (4) Maximum Recirculation Rate. A recirculation rate may exceed four times design flow if calculations to justify the higher rate are included in the report.

   5. (5) Configuration.

      1. (A) In a facility with influent that has constant organic loadings, a system must use direct recirculation of unsettled trickling filter effluent.
      2. (B) A design must ensure that the distributor nozzles can handle the recirculated sloughed biofilm.
      3. (C) In a facility with variable influent organic loadings, effluent must recirculate from a final clarifier to either a primary clarifier or a trickling filter to equalize organic loading.

10. (j) Average Hydraulic Surface Loading.

    1. (1) The report must include calculations of the maximum, design, and minimum area cross-section surface loadings on the filters in terms of million gallons per acre of filter area per day for the initial year and the design year.

    2. (2) The average hydraulic surface loadings of a filter with crushed rock, slag, or similar media must not:

       1. (A) exceed 40 mgd per acre based on design flow, except in roughing applications;
       2. (B) be less than 1.0 mgd per acre; and
       3. (C) be within the ranges specified by the manufacturer.

11. (k) Underdrain System Design.

    1. (1) A trickling filter must include an underdrain with semicircular inverts that cover the entire floor.
    2. (2) An underdrain must be vitrified clay or pre-cast reinforced concrete.
    3. (3) An underdrain constructed of half tile is prohibited.
    4. (4) Underdrain inlet openings must have a gross cross-sectional area greater than 15% of a filter's surface area.
    5. (5) A modular synthetic media design must be supported above a filter floor by beams and grating with support and clearances in accordance with the media manufacturer's recommendations.

12. (l) Underdrain Slopes.

    1. (1) An underdrain and filter effluent channel floor must have a minimum slope of 1%.
    2. (2) An effluent channel must produce a minimum velocity of 2.0 feet per second at design flow rate to a trickling filter.
    3. (3) The floor of a new trickling filter using stackable modular or synthetic media must slope toward a drainage channel on slope of at least 1% and not more than 5%, based on filter size and hydraulic loading.

13. (m) Passive Ventilation.

    1. (1) The effluent channel and effluent pipe of an underdrain system or a synthetic media support structure must permit free passage of air.
    2. (2) Any drain, channel, or effluent pipe must have a cross-sectional area with not more than 50% of the area submerged at peak flow plus recirculation.
    3. (3) The effluent channels must accommodate the specified flushing hydraulic dosing intensity and allow the possibility of increased hydraulic loading.
    4. (4) A ventilation system may include an extension of an underdrain through a filter sidewall, a ventilation opening through a sidewall, and an effluent discharge conduit designed as a partially full flow pipe or an open channel.
    5. (5) A vent opening through a trickling filter walls must include hydraulic closure to allow flooding of a filter for nuisance organism control.
    6. (6) A passive ventilation design must provide at least 2.5 square feet (sf) of ventilating area per 1,000 lbs of primary effluent BOD₅ per day.
    7. (7) An underdrain system for a rock media filter must provide at least 1.0 sf of ventilating area for every 250 sf of plan area.
    8. (8) The minimum required ventilating area for a synthetic media underdrain is the area recommended by the manufacturer.
    9. (9) The ventilating area must be the greater of 1.0 sf per 175 sf of synthetic media area or 2.6 sf per 1,000 cf of media volume.

14. (n) Forced Ventilation.

    1. (1) Forced ventilation is required for a trickling filter designed for nitrification, for a trickling filter design with a media depth in excess of 6.0 feet, or for any location where seasonal or diurnal temperatures do not provide sufficient difference between the ambient air and wastewater temperatures to sustain passive ventilation.
    2. (2) A design must specify the minimum airflow for forced ventilation and optimized process performance, and the report must include any calculation associated with this determination.

    3. (3) A down-flow forced ventilation system must include a provision for:

       1. (A) the removal of entrained droplets: or
       2. (B) the return of air containing entrained moisture to the top of a trickling filter; and
       3. (C) a reversible fan or other mechanism to reverse the airflow when a wide temperature difference between the ambient air and wastewater create strong updrafts.
    4. (4) A ventilation fan and the associated controls must withstand flooding of a filter without sustaining damage.
    5. (5) The following equation and the values in Table G.3 determine the minimum airflow rate:
       Attached Graphic
       

15. (o) Maintenance.

    1. (1) Cleaning and Sloughing.

       1. (A) A flow distribution device, an underdrain, a channel, and a pipe must allow maintenance, flushing, and drainage.
       2. (B) A trickling system must hydraulically accommodate the specified flushing hydraulic dosing intensity and must facilitate cleaning and rodding of the distributor arms.
       3. (C) A trickling filter system must prevent recirculation of sloughed biomass in pieces larger than the distributor nozzle opening or the filter media voids.

    2. (2) Nuisance Organism Control. A trickling filter system must control nuisance organisms by operation of trickling filters at proper design dosing intensities, with periodic flushing at higher dosing intensities.

       1. (A) Filter Flies.

          1. (i) The structural and hydraulic design of a new trickling filter must enable flooding of the trickling filter for fly control.
          2. (ii) The executive director may approve an alternate method of fly control for a filter that exceeds 6.0 feet in height if the effectiveness of the alternate method is verified at a full-scale installation and documented in the report.
       2. (B) Snails. A trickling filter system must minimize areas where sludge may accumulate. The system must include a low-velocity, open channel between a trickling filter and final clarifier for manual removal of snails.
    3. (3) Corrosion Protection. A design must minimize corrosion and use corrosion-resistant materials for all equipment and construction of a trickling filter, including ventilation equipment and covers.
16. (p) Flow Measurements. A trickling filter system must include a means to measure the flow to a filter and the recirculation flow.

17. (q) Odor Control. A trickling filter system must use ventilation with periodic flushing at a higher dosing intensity to minimize potential odor.

    1. (1) Covers.

       1. (A) The executive director may require an owner of a facility with a history of odor complaints to install a cover over a new, expanded or materially altered trickling filter.
       2. (B) A cover must allow access to the entire top of the filter media and to the distributor for maintenance and removal.
       3. (C) A covered trickling filter must have a forced ventilation system with a scrubber or an adsorption column for odor control.

    2. (2) Stripping. A trickling filter with high influent organic loading must have forced ventilation in a down-flow mode to minimize odor. Odorous off-gases may be:

       1. (A) recycled through a trickling filter;
       2. (B) used to ventilate a tertiary nitrifying trickling filter in an up-flow mode;
       3. (C) diffused into an aeration basin; or
       4. (D) treated separately for odor control using a scrubber or an adsorption column.
18. (r) Final Clarifiers. The size of the final clarifiers for a facility with a trickling filter must allow for the required effluent total suspended solids removal at the maximum influent flow and the maximum recirculation with all pumps in operation.

19. (s) Report Requirements.

    1. (1) The report must specify the filter efficiency formula used in the design calculations.
    2. (2) The report must include the operating data from any existing trickling filter of similar construction and operation at the facility to justify the projected treatment efficiency, kinetic coefficients, and other design parameters.
    3. (3) The report may include more than one set of applicable design equations to allow crosschecking of predicted treatment efficiency.

Source Note:The provisions of this §217.182 adopted to be effective August 28, 2008, 33 TexReg 6843.