<<Prev Rule

Texas Administrative Code

Next Rule>>
TITLE 30ENVIRONMENTAL QUALITY
PART 1TEXAS COMMISSION ON ENVIRONMENTAL QUALITY
CHAPTER 317DESIGN CRITERIA PRIOR TO 2008
RULE §317.4Wastewater Treatment Facilities

are based upon a side water depth of ten feet and shall be considered acceptable.

Attached Graphic

  (6) Sidewater depth (SWD). The minimum SWD for conventional primary and intermediate clarifiers is seven feet. All final clarifiers shall have a minimum SWD of eight feet. Final clarifiers having a surface area equal to or greater than 1,250 square feet (diameter equal to or greater than 40 feet) must be provided with a minimum SWD of 10 feet.

  (7) Hopper bottom clarifiers. Hopper bottom clarifiers without mechanical sludge collecting equipment will only be approved for those facilities with a permitted design flow of less than 25,000 gallons per day. The required SWD for hopper bottom clarifiers may be computed using the following equation: SWD = 160 QD + 4, where SWD equals required SWD in feet and QD equals design flow in million gallons per day. Furthermore, SWD as computed previously for any flow may be reduced by crediting the upper one-third of the hopper as effective SWD if the following conditions are met:

    (A) clarifier surface loading rate is reduced by at least 15% from maximum loading rate as per paragraph (5) of this subsection;

    (B) influent stilling baffle and effluent weir are designed to prevent short circuiting;

    (C) detention time at peak flow is at least 1.8 hours for secondary treatment and 2.4 hours for advanced treatment; and

    (D) an appropriate form of flow equalization is used.

  (8) Sludge collection equipment. All conventional clarifier units that treat flow from a treatment plant facility with a design flow of 25,000 gallons per day or greater shall be provided with mechanical sludge collecting equipment. Hopper bottom clarifiers must have a smooth wall finish and a hopper slope of not less than 60 degrees.

  (9) BOD5 removal. It shall be assumed that the BOD5 removal in a primary clarifier is 35%, unless satisfactory evidence is presented to indicate that the efficiency will be otherwise. In plant efficiency calculations, it shall be assumed that the BOD5 removal in intermediate and final clarifiers is included in the calculation for the efficiency of the treatment unit preceding the intermediate or final clarifier.

(e) Trickling filters.

  (1) General. Trickling filters are secondary aerobic biological processes which are used for treatment of sewage.

  (2) Basic design parameters. Trickling filters are classified according to applied hydraulic loading in million gallons per day per acre (mgd/acre) of filter media surface area, and organic loadings in pounds of biochemical oxygen demand (BOD) per day per 1,000 cubic feet of filter media (lb BOD/day-1,000 cu ft). The following factors should be considered in the selection of the design hydraulic and organic loadings: strength of the influent sewage, effectiveness of pretreatment, type of filter media, and treatment efficiency required. Typical ranges of applied hydraulic and organic loadings for the different classes of trickling filters are presented in the following table for illustrative purposes. The design engineer shall submit sufficient operating data from existing trickling filters of similar construction and operation to justify his efficiency calculations for the filters, and a filter efficiency formula from a reliable source acceptable to the commission. The formula of the National Research Council may be used when rock media is used in the trickling filter(s).

Attached Graphic

  (3) Pretreatment. The trickling filter treatment facility shall be preceded by primary clarifiers equipped with scum and grease removal devices. Design engineers may submit operating data as justification of other alternative pretreatment devices which provide for effective removal of grit, debris, suspended solids, and excess oil and grease. Preaeration shall be provided where influent wastewater contains harmful levels of hydrogen sulfide concentrations.

  (4) Filter media.

    (A) Material specifications for rock media. The following are minimum requirements.

      (i) Crushed rock, slag, or similar media should not contain more than 5.0% by weight of pieces whose longest dimension is greater than three times its least dimension. The rock media should be free from thin, elongated, and flat pieces and should be free from dust, clay, sand, or fine material. Rock media should conform to the following size distribution and grading when mechanically graded over a vibrating screen with square openings:

        (I) passing five-inch sieve--100% by weight;

        (II) retained on three-inch sieve--95% to 100% by weight;

        (III) passing two-inch sieve--0.2% by weight;

        (IV) passing one-inch sieve--0.1% by weight;

        (V) the loss of weight by a 20-cycle sodium sulphate test, as described in the American Society of Civil Engineers Manual of Engineering and Engineering Practice Number 13, shall be less than 10%.

      (ii) Rock media shall not be less than four feet in depth (at the shallowest point) nor deeper than eight feet (at the deepest point of the filter).

    (B) Synthetic (manufactured or prefabricated) media.

      (i) Application of synthetic media shall be evaluated on a case-by-case basis. Suitability should be evaluated on the basis of experience with installations treating similar strength wastewater under similar hydraulic and organic loading conditions. The manufacturer's recommendations shall be included, as well as case histories involving the use of the media.

      (ii) Media shall be relatively insoluble in sewage and resistant to flaking or spalling, ultraviolet degradation, disintegration, erosion, aging, all common acids and alkalies, organic compounds, biological attack, and shall support the weight of a person when the media is in operation.

      (iii) Media depths should be consistent with the recommendations of the manufacturer.

    (C) Placing of media.

      (i) The dumping of media directly on the filter is unacceptable. Instructions for placing media shall be included in the specifications.

      (ii) Crushed rock, slag, and similar media shall be washed and screened or forked to remove clays, organic material, and fines.

      (iii) Such materials should be placed by hand to a depth of 12 inches above the underdrains and all material should be carefully placed in a manner which will not damage the underdrains. The remainder of the material may be placed by means of belt conveyors or equally effective methods approved by the engineers. Trucks, tractors, or other heavy equipment should not be driven over the filter media during or after construction.

      (iv) Prefabricated filter media shall be placed in accordance with recommendations provided by the manufacturer.

  (5) Filter hydraulics.

    (A) Dosing. Wastewater may be applied to the filters by siphons, pumps, or by gravity discharge from preceding treatment units when suitable flow characteristics have been developed.

    (B) Distribution equipment. Settled wastewater may be distributed over the filter media by rotary, horizontal, or travelling distributors, provided the equipment proposed is capable of producing the required continuity and uniformity of distribution over the entire surface of the filter. Deviation from a calculated uniformly distributed volume per unit surface area shall not exceed 10% at any portion of the filter. Filter distributors shall be designed to operate properly at all flow rates. Excessive head in the center column of rotary distributors shall be avoided, and all center columns shall have adequately sized overflow ports to prevent the head from building up sufficiently for the water to reach the bearings in the center column. Distributors shall include cleanout gates on the ends of the arms and shall also include an end nozzle to spray water on the wall of the filter to keep the edge of the media continuously wet. The filter walls shall extend at least 12 inches above the top of the ends of the distributor arms.

    (C) Seals. The use of mercury seals is prohibited in the distributors of newly constructed trickling filters. If an existing treatment facility is to be modified, any mercury seals in the trickling filters shall be replaced with oil or mechanical seals.

    (D) Distributor clearance. A minimum clearance of six inches shall be provided between the top of the filter media and the distributing nozzles.

    (E) Recirculation. In order to insure that the biological growth on the filter media remains active at all times, provisions shall be included in all designs for minimum recirculation during periods of low flow. This minimum recirculation shall not be considered in the evaluation of the efficiency of the filter unless it is part of the proposed specified continuous recirculation rate. Minimum flow to the filters shall not be less than 1.0 mgd/acre of filter surface. In addition, the minimum flow rate must be great enough to keep rotary distributors turning and the distribution nozzles operating Cont'd...

Next Page Previous Page

Link to Texas Secretary of State Home Page | link to Texas Register home page | link to Texas Administrative Code home page | link to Open Meetings home page