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RULE §217.182Trickling Filters--General Requirements

(a) Trickling filters are classified according to applied hydraulic loading, including recirculation, in million gallons per day 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 used as the basis for the selection of the design hydraulic and influent organic loadings:

  (1) BOD5 concentration of the influent wastewater;

  (2) effectiveness of pretreatment;

  (3) type of filter media; and

  (4) treatment efficiency required.

(b) A trickling filter may be classified as:

  (1) a roughing filter, which provides at least 50%, but not more than 75% removal of soluble BOD5 ;

  (2) a secondary treatment filter, which provides the removal of pollutants required to meet the effluent limits for BOD5 and total suspended solids (TSS) of effluent set 1 or 2 in §309.4 of this title (relating to Table 1, Effluent Limitations for Domestic Wastewater Plants);

  (3) a combined BOD5 and nitrifying filter, which provides the removal of pollutants required to meet the effluent limits for BOD5 , ammonia-nitrogen (NH3 -N), and TSS of effluent set 2N or 2N1 in §309.4 of this title; or

  (4) a tertiary nitrifying filter, which provides removal of NH3 -N, if the influent to the trickling filter is a clarified secondary effluent.

(c) The following table lists the hydraulic and organic loadings for different classes of trickling filters.

Attached Graphic

(d) Pretreatment.

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

    (A) remove grit, debris, suspended solids, oil, and grease;

    (B) remove particles with a diameter greater than three millimeters; and

    (C) control the release of hydrogen sulfide.

  (2) A primary clarifier equipped with scum and grease removal devices must precede a rock media trickling filter.

(e) Rock Filter Media.

  (1) Materials.

    (A) Rock filter media composed of crushed rock, slag, or similar material is prohibited if more than 5% of the media, by weight, consists of pieces with their longest dimension measuring more than three times greater than their shortest dimension.

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

      (i) passing 5.0 inch sieve - 100% by weight;

      (ii) retained on 3.0 inch sieve - 95 to 100% by weight;

      (iii) passing 2.0 inch sieve - 0.2% by weight;

      (iv) passing 1.0 inch sieve - 0.1% by weight; and

      (v) the loss of weight by the 20-cycle test, as described in American Society of Civil Engineers' Manual of Engineering and Engineering Practice No. 13, must be less than 10%.

  (2) Placement.

    (A) Rock filter media must be at least 4.0 feet deep at the shallowest point.

    (B) Dumping rock filter media directly on a trickling filter is prohibited. Rock media must be placed by hand to a depth of 12 inches above the underdrains. The remainder of the rock filter media may be placed by belt conveyor or an equivalent mechanical method.

    (C) Crushed rock, slag, and other similar media must be washed and screened or forked to remove clay, organic material, and other fine particles prior to placement.

    (D) The placement of any material must not damage the underdrains.

    (E) Vehicles and equipment are prohibited from driving over the rock filter media.

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

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

  (2) Synthetic media material may be considered innovative or non-conforming technology and may be subject to §217.7(b)(2) of this title (relating to Types of Plans and Specifications Approvals), and requires executive director approval in writing.

    (A) Suitability. The suitability of synthetic media material must be evaluated based on performance data from a wastewater treatment facility with similar media operating under similar hydraulic and organic loading conditions. The engineering report must include a relevant case history involving the use of the synthetic media.

    (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.

    (C) Structural Integrity.

      (i) The structural design of synthetic filter media must support the synthetic media, water flowing through or trapped in voids, and the maximum anticipated thickness of the wetted biofilm.

      (ii) The synthetic filter media must support the weight of an individual, unless a separate provision is made for maintenance access to the entire top of the trickling filter media and to the distributor.

    (D) Placing of Synthetic Filter Media. Modular synthetic filter media must be installed with the edges of the modules matched as closely as possible to provide consistent hydraulic conditions within the trickling filter.

(g) Trickling Filter Dosing.

  (1) Dosing rates to a trickling filter must be within the design dosing rate range, even if the trickling filter receives flow from a siphon, pump, or gravity discharge from a preceding treatment unit.

  (2) A trickling filter must be designed to control instantaneous dosing rates under both normal operating conditions and filter-flushing conditions.

  (3) The distributor speed and the recirculation rate of a trickling filter 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 both normal usage periods and for flushing periods:

Attached Graphic

  (4) A design may be based on instantaneous dosing intensity for rotary distributors using Equation G.1. in Figure: 30 TAC §217.182(g)(4).

Attached Graphic

(h) Distribution Equipment.

  (1) The design of a trickling filter 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) A design must include filter distributors that operate properly at all anticipated flow rates.

  (3) A design must not deviate from the design dosing intensity by more than 10%.

  (4) A new trickling filter or an upgrade of an existing trickling filter must include an electrically driven, variable speed filter distributor to allow operation at optimum dosing intensity independent of recirculation pumping.

  (5) If an existing rectangular trickling filter is retrofitted with rotary distributors, any filter media that will not be fully wetted must not be considered part of the required effective treatment area.

  (6) The center column of a rotary trickling filter distributor must have emergency overflow ports that are sized to prevent water from reaching the bearings in the center column.

  (7) A filter distributor must include cleanout gates on the ends of the distributor arms and an end spray nozzle to wet the edges of the trickling filter media.

  (8) The trickling filter walls must extend at least 12.0 inches above the top of the ends of the distributor arms.

  (9) The use of a mercury seal in a distributor of a trickling filter is prohibited in a new wastewater treatment facility. If an existing wastewater treatment facility is materially altered, any mercury seal in a trickling filter must be replaced with an oil or mechanical seal.

  (10) The minimum clearance between the top of the trickling filter media and the distributing nozzles is 6.0 inches.

  (11) Rotary distributors must be capable of operating at speeds as low as one revolution per 30 minutes.

  (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.

(i) Recirculation.

  (1) Low Flow Conditions.

    (A) The design of a trickling filter must include a mechanism to maintain minimum recirculation during periods of low flow to ensure that the biological growth on the filter media remains active at all times.

    (B) For all trickling filters with continuous recirculation, the design must include the minimum recirculation rate in the evaluation of the efficiency of the filter.

    (C) Minimum flow to the filters must be equal to or greater than 1.0 million gallons per day per acre of filter aerial surface and must ensure the proper operation of the distribution nozzles.

    (D) The minimum flow rate for a trickling filter design using hydraulically driven distributors must keep rotary distributors turning at the minimum design rotational velocity.

    (E) For a wastewater treatment facility designed with a design flow equal to or greater than 0.4 million gallons per day and recirculation for BOD5 removal, the recirculation system must include variable speed pumps and a method of conveniently measuring the recycle flow rate.

  (2) Compensatory Recirculation.

    (A) The design of a trickling filter must provide compensatory recirculation to supplement influent flow if design and flushing dosing intensities are not achieved solely by the control of distributor operation.

    (B) Controls for the distributor speed and recycle pumping rate must provide optimum dosing intensity under all anticipated influent flow conditions.

  (3) Process Calculations. The engineering report must:

    (A) provide design details about removal of the remaining organic matter by recirculation;

    (B) identify the effect of dilution of the influent on the rate of diffusion of dissolved organic substrates into the biofilm; and

    (C) identify the effect of reduced influent concentrations on reaction rates in each section of a filter having first order kinetics.

  (4) Recirculation Rate. A recirculation rate may exceed four times the design flow if calculations to justify the higher rate are included in the engineering report.

  (5) Configuration.

    (A) In a wastewater treatment facility with influent that has constant organic loadings, direct recirculation of unsettled trickling filter effluent must be used.

    (B) A design must ensure that the distributor nozzles can handle the recirculated sloughed biofilm.

    (C) In a wastewater treatment 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.

(j) Average Hydraulic Surface Loading.

  (1) The engineering report must include calculations of the maximum, design, and minimum surface loadings on the trickling filters in terms of million gallons per acre of filter area per day for the flow expected in the initial year and at full capacity.

  (2) The average hydraulic surface loadings of a trickling filter with crushed rock, slag, or similar media:

    (A) must not exceed 40 million gallons per day per acre based on design flow, except in roughing applications;

    (B) must not be less than 1.0 million gallons per day per acre; and

    (C) must be within the ranges specified by the manufacturer.

(k) Underdrain System Design.

  (1) A trickling filter must include an underdrain with semicircular inverts that cover the entire floor of the trickling filter.

  (2) An underdrain must be constructed of vitrified clay or pre-cast reinforced concrete.

  (3) An underdrain constructed of half tile is prohibited.

  (4) Underdrain inlet openings must have a gross cross-sectional area greater than 15% of a trickling filter's surface area.

  (5) A modular synthetic media design must be supported above a trickling filter floor by beams and grating with support and clearances in accordance with the trickling filter media manufacturer's recommendations. The manufacturer's recommendations must be included in the engineering report.

(l) Underdrain Slopes.

  (1) An underdrain and trickling filter effluent channel floor must have a minimum slope of 1%.

  (2) An effluent channel must produce a minimum velocity of 2.0 feet per second at the design flow rate to a trickling filter.


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