1. (a) Inlets.

   1. (1) A primary clarifier inlet must provide uniform flow and stilling.
   2. (2) Vertical flow velocity through an inlet stilling well must not exceed 0.15 feet per second at peak flow.
   3. (3) An inlet distribution channel must not have a dead-end corner and must prevent the settling of solids in the channel.
   4. (4) An inlet structure must allow floating material to enter the clarifier.

2. (b) Scum removal.

   1. (1) A primary clarifier must have scum baffles and a means of collecting and disposing of scum.
   2. (2) A primary clarifier must discharge scum to a sludge digester or another method of disposal approved by the executive director.
   3. (3) The discharge of scum to any open drying area is prohibited.
   4. (4) A primary clarifier with a design flow equal to or greater than 25,000 gallons per day (gpd) must include a mechanical skimmer.
   5. (5) A primary clarifier with a design flow less than 25,000 gpd may use hydraulic differential skimming, only if the scum pickup is capable of removing scum from the entire operating surface of the clarifier.
   6. (6) A scum pump must be specifically designed for this purpose.

3. (c) Effluent weirs.

   1. (1) An effluent weir must prevent turbulence or localized high vertical flow velocity in the primary clarifier.
   2. (2) A weir must be located to prevent short circuiting flow through a primary clarifier.
   3. (3) A weir must be adjustable for leveling.
   4. (4) Weir loading, for a facility with a design flow of 1.0 million gallons per day (mgd) or less, must not exceed 20,000 gpd per linear foot of weir length.
   5. (5) Weir loading for a facility with a design flow in excess of 1.0 mgd must not exceed 30,000 gpd peak flow per linear foot of weir length.

4. (d) Basin sizing.

   1. (1) The surface area of a facility's clarifier(s) determines the proper overflow rates.
   2. (2) The actual clarifier size is based on the larger of the two surface area calculations: peak flow rate or design flow surface loading rate.

   3. (3) The design criteria for primary a clarifier in subparagraphs (A) and (B) of this paragraph are based upon a minimum side water depth of 10.0 feet.

      1. (A) Maximum surface loading:

         1. (i) must not exceed 1,800 gpd per square foot at peak flow;
         2. (ii) must not exceed 1,000 gpd per square foot at design flow; and
         3. (iii) does not include recirculation flow.

      2. (B) Detention Time.

         1. (i) Detention time at peak flow must be no less than 54 minutes (0.9 hour).
         2. (ii) Detention time at design flow must at least 108 minutes (1.8 hours).
         3. (iii) Overflow rate and side water depth (SWD) may be adjusted from a minimum of 10 feet a maximum of 18 feet, as long as the detention time remains unchanged.

         4. (iv) The detention time must be based on the effective volume and the overflow rate of a circular or rectangular clarifier.

            1. (I) The effective volume includes all liquid above the sludge blanket.
            2. (II) For a cone bottom tank, the top of the sludge blanket is considered to be at the top of the cone.
            3. (III) For a flat bottom tank, a sludge blanket of 3.0 feet must be allowed for development of maximum return sludge concentration.
5. (e) SWD. The minimum SWD for primary clarifiers is 10.0 feet.
6. (f) Freeboard. The walls of a primary clarifier must extend at least 6.0 inches above the surrounding ground surface and must provide a minimum freeboard of 12.0 inches at peak flow.

7. (g) Drains.

   1. (1) A primary clarifier must have the capability of draining completely to an appropriate point in the facility.
   2. (2) A portable dewatering pump is acceptable for complete dewatering.
8. (h) Accessibility. A primary clarifier must be accessible to facilitate routine operation and maintenance.
9. (i) Five-day biochemical oxygen demand (BOD₅) Removal. A design shall be based on no greater than 35% BOD₅ reduction, unless a higher efficiency is justified by a pilot study or data from a similar full-scale operation.

10. (j) Sludge Transfer.

    1. (1) A primary clarifier unit must include mechanical sludge collection equipment designed to rapidly remove sludge and transfer it for subsequent processing.
    2. (2) A gravity sludge transfer pipe must be at least 8.0 inches in diameter.

Source Note:The provisions of this §217.129 adopted to be effective August 28, 2008, 33 TexReg 6843.