(a) Inlets.
  (1) A primary clarifier inlet must provide uniform flow and stilling.
  (2) Vertical flow velocity through an inlet stilling well must not exceed 0.15 feet per second at peak flow.
  (3) An inlet distribution channel must not have a dead-end corner and must prevent the settling of solids in the channel.
  (4) An inlet structure must allow floating material to enter the clarifier.
(b) Scum removal.
  (1) A primary clarifier must have scum baffles and a means of collecting and disposing of scum.
  (2) A primary clarifier must discharge scum to a sludge digester, or must use an alternative method of disposal approved in writing by the executive director.
  (3) The discharge of scum to any open drying area is prohibited.
  (4) A primary clarifier with a design flow equal to or greater than 25,000 gallons per day must include a mechanical skimmer.
  (5) A primary clarifier with a design flow less than 25,000 gallons per day must use a mechanical skimmer or hydraulic differential skimming. If hydraulic differential skimming is used, it must be capable of removing scum from the entire operating surface of the clarifier.
  (6) A pump used for pumping scum must be specifically designed to pump scum.
(c) Effluent weirs.
  (1) An effluent weir must not cause turbulence or a localized vertical flow velocity that would pull solids over the effluent weir in the primary clarifier.
  (2) An effluent weir must be designed to prevent hydraulic short circuiting through a primary clarifier.
  (3) An effluent weir must be level, and must be designed to be adjustable for re-leveling.
  (4) Effluent weir loading must not exceed 20,000 gallons per day at peak flow per linear foot of weir length for a wastewater treatment facility with a design flow of 1.0 million gallons per day or less.
  (5) Weir loading must not exceed 30,000 gallons per day at peak flow per linear foot of weir length for a wastewater treatment facility with a design flow greater than 1.0 million gallons per day.
(d) Primary clarifier basin sizing.
  (1) The surface area of a primary clarifier must be used to determine the proper weir overflow rates.
  (2) The actual primary clarifier size must be based on the larger of two surface area calculations: peak flow rate and design flow surface loading rate.
  (3) Surface loading rates and detention times for a primary clarifier in subparagraphs (A) and (B) of this paragraph are based upon a minimum side water depth of 10.0 feet.
    (A) Surface loading rates, excluding recirculation flow, must not exceed:
      (i) 1,800 gallons per day per square foot at peak flow; and
      (ii) 1,000 gallons per day per square foot at design flow.
    (B) Detention Time.
      (i) Detention time at peak flow must be at least 54 minutes (0.9 hour).
      (ii) Detention time at design flow must be at least 108 minutes (1.8 hours).
      (iii) Side water depth may be adjusted from a minimum of 10 feet to a maximum of 18 feet, as long as the detention time remains unchanged.
      (iv) The detention time must be based on the effective volume and the weir overflow rate of a circular or rectangular clarifier.
        (I) The effective volume includes all liquid above the sludge blanket.
        (II) For a cone bottom tank, the top of the sludge blanket is considered to be at the top of the cone.
        (III) The design of a flat bottom tank must include additional depth for a sludge blanket with a thickness of 3.0 feet.
(e) Side Water Depth. The minimum side water depth for primary clarifiers is 10.0 feet. For a primary clarifier, the side water depth is defined as:
  (1) the water depth from the top of the cone in a cone bottom tank to the water surface; or
  (2) the water depth from 3.0 feet above the bottom of a flat bottom tank to the water surface.
(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.
(g) Drains.
  (1) A primary clarifier must have the capability of draining completely without bypassing secondary treatment.
  (2) A portable dewatering pump may be used for complete dewatering.
(h) Accessibility. A primary clarifier must be accessible for operation and maintenance.
(i) Five-day biochemical oxygen demand (BOD5 ) Removal. The design of a primary clarifier must 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.
(j) Sludge Transfer.
  (1) A primary clarifier unit must include mechanical sludge collection equipment designed to remove sludge in a manner that will minimize odor development and transfer the sludge for subsequent processing.
  (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; amended to be effective December 4, 2015, 40 TexReg 8254