(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 (BOD 5 ) 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) BOD 5 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 BOD 5 ;
(2) a secondary treatment filter, which provides the removal of pollutants required to meet the effluent limits for BOD 5 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 BOD 5 and nitrifying filter, which provides the removal of pollutants required to meet the effluent limits for BOD 5 , ammonia-nitrogen (NH 3 -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 NH 3 -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.
(d) Pretreatment.
(1) A trickling filter must have upstream preliminary treatment units that:
(2) A primary clarifier equipped with scum and grease removal devices must precede a rock media trickling filter.
(e) Rock Filter Media.
(1) Materials.
(2) Placement.
(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.
(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:
(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).
(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.
(2) Compensatory Recirculation.
(3) Process Calculations. The engineering report must:
(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.
(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:
(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.
Cont'd...