|(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.
(1) A trickling filter must have upstream preliminary
treatment units that:
(A) remove grit, debris, suspended solids, oil, and
(B) remove particles with a diameter greater than three
(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.
(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
(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
(E) Vehicles and equipment are prohibited from driving
over the rock filter media.
(f) Synthetic (Manufactured or Prefabricated) Media
(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
(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
(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
(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.
(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
(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
(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.
(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
(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
(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.