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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.

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  (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).

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  (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 properly. For facilities with a design capacity greater than or equal to 0.5 mgd and in which recirculation is included in design computations for BOD5 removal, recirculation shall be provided by variable speed pumps and a method of conveniently measuring the recycle flow rate shall be provided.

    (F) Surface loading. The engineering report shall include calculations of the maximum, design, and minimum surface loadings on the filter(s) in terms of mgd/acre of filter area per day (for the initial year and design year). Hydraulic loadings of filters with crushed rock, slag, or similar media shall not exceed 40 mgd/acre based on design flow. The minimum surface loading shall not be less than 1.0 mgd/acre. Loadings on synthetic (manufactured or prefabricated) filter media shall be within the ranges specified by the manufacturer.

  (6) Underdrain system.

    (A) Underdrains. Underdrains with semicircular inverts or equivalent shall be provided and the underdrainage system shall cover the entire floor of the trickling filter. Inlet openings into the underdrains shall provide an unsubmerged gross combined area of at least 15% of the surface area of the filter.

    (B) Hydraulics. Underdrains and the filter effluent channel floor shall have a minimum slope of 1.0%. Effluent channels shall be designed to produce a minimum velocity of two feet per second at average daily flow rate of application to the trickling filter.

    (C) Drain tile. Underdrains for rock media trickling filters shall be either vitrified clay or precast reinforced concrete. The use of half tile for underdrain systems is unacceptable.

    (D) Corrosion. Underdrain systems for synthetic media trickling filters shall be resistant to corrosion.

    (E) Ventilation. The underdrain system, effluent channels, and effluent pipe shall be designed to permit free passage of air. Drains, channels, and effluent pipes shall have a cross-sectional area such that not more than 50% of the cross-sectional area will be submerged at peak flow plus recirculation. Provision shall be made in the design of the effluent channels to allow the possibility of increased hydraulic loading. The underdrain system shall provide at least one square foot of ventilating area (vent stacks, ventilating holes, ventilating ports) for every 250 square feet of rock media filter plan area. Ventilating area for synthetic media underdrains will be provided as recommended by the manufacturer, but shall be at least one square foot for every 175 square feet of synthetic media trickling filter plan area.

    (F) Maintenance. All flow distribution devices, underdrains, channels, and pipes shall be designed so they may be maintained, flushed, and properly drained. The units shall be designed to facilitate cleaning of the distributor arms. A gate shall be provided in the wall to facilitate rodding of the distributor arms.

    (G) Flooding. Provisions shall be made to enable flooding of the trickling filter for filter fly control; however, consideration will be given by the commission to alternate methods of filter fly control provided that the effectiveness of the alternate method is verified at a full scale installation. This information shall be submitted with the plans and specifications.

    (H) Flow measurements. Means shall be provided to measure flow to the filter and recirculation flows.

(f) Rotating biological contactors (RBC).

  (1) General.

    (A) RBC units shall be covered and ample ventilation provided. Working clearance of approximately 30 inches should be provided within the cover unless the covers are removable, utilizing equipment normally available on site. Enclosures shall be constructed of a suitable corrosion-resistant material.

    (B) The design of the RBC media shall provide for self-cleaning action due to the flow of water and air through the media. Careful selection of media that will not entrap solids should be made.

    (C) The RBC tank should be designed to minimize zones in which solids will settle out.

    (D) RBC media should be selected which is compatible with the wastewater. Selection of media can be critical where the wastewater has an industrial waste portion which either significantly increases the wastewater temperature or contains a chemical constituent which may decrease the life of the RBC media.

  (2) Design.

Cont'd...

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