(a) Inlets.      
        (1) A clarifier must have an inlet valve or gate.       
        (2) A clarifier inlet must provide uniform flow and stilling.       
        (3) A transfer pipe must not trap or entrain air.       
        (4) Vertical flow velocity through an inlet stilling well must not exceed 0.15 feet per second at peak flow.       
        (5) An inlet distribution channel must be designed to prevent the settling of solids in the channel.       
      (b) Pumped Inflow.      
        (1) For a wastewater treatment facility with pumped inflow, a clarifier must be able to accommodate all flow without overloading or overtopping the clarifier effluent weir.       
        (2) A clarifier must hydraulically accommodate peak flows without adversely affecting treatment in the clarifier or in subsequent treatment units.       
      (c) Scum removal.      
        (1) A clarifier must include scum baffles and a means for scum collection and disposal.                  (A) A clarifier at a wastewater treatment facility with a design flow equal to or greater than 10,000 gallons per day must use a mechanical skimmer.                   (B) A clarifier at a wastewater treatment facility with a design flow less than 10,000 gallons per day may use either mechanical skimming or hydraulic differential skimming. Hydraulic differential skimming may only be used if the scum pickup is capable of removing scum from the entire operating surface of the clarifier.         
        (2) Scum collected from a clarifier in a wastewater treatment facility using an activated sludge process or an aerated lagoon must be:                  (A) discharged to an aeration basin or digester; or                   (B) disposed of using any method that complies with Chapter 312 of this title (relating to Sludge Use, Disposal, and Transportation).         
        (3) If scum contains debris or foreign materials that were not removed by preliminary treatment, debris and foreign materials must be removed from the scum before the scum is discharged to a digester or applied to the land. Otherwise, scum containing debris or other foreign materials must be sent to a landfill or a sludge-only monofill.       
        (4) Scum collected from a clarifier in a wastewater treatment facility not using an activated sludge process and not using an aerated lagoon must be discharged to a sludge digester or must be disposed of using any disposal method that complies with Chapter 312 of this title.       
        (5) Discharge of scum to a drying area that is open to the atmosphere is prohibited.       
        (6) Storage or holding of scum in a containment vessel that is open to the atmosphere is prohibited. Scum must be contained in a vessel that is not open to the atmosphere or must be routed to a treatment unit in the wastewater treatment facility for further processing.       
        (7) A pump used for pumping scum must be specifically designed to pump scum.       
      (d) Clarifier Effluent Weirs.      
        (1) A clarifier effluent weir must prevent turbulence or localized high vertical flow velocity in a clarifier.       
        (2) A clarifier effluent weir must be located a minimum of 6.0 inches from an outer wall or baffle, except for a clarifier effluent weir and launder that is attached to a wall. A clarifier effluent weir must prevent the short-circuiting of flow throughout the clarifier.       
        (3) A clarifier effluent weir must be level, and must be adjustable to allow re-leveling of the weir and to provide for minor changes to the water surface elevation in the clarifier.       
        (4) For a wastewater treatment facility with a design flow of less than 1.0 million gallons per day, the clarifier effluent weir loading must not exceed 20,000 gallons per day at the peak flow per linear foot of weir length.       
        (5) For a wastewater treatment facility with a design flow equal to or greater than 1.0 million gallons per day, the clarifier effluent weir loading must not exceed 30,000 gallons per day at the peak flow per linear foot of weir length.       
        (6) A center-feed circular clarifier must have effluent weirs around the entire perimeter of the clarifier.       
      (e) Sludge Pipes.      
        (1) The transfer of sludge from a clarifier to a treatment or processing unit must not negatively affect treatment efficiency of the unit that receives the sludge.       
        (2) A sludge pipe must be a minimum of 4.0 inches in diameter.       
        (3) The flow velocity in a sludge pipe must be greater than 2.0 feet per second for a wastewater treatment facility with a design flow greater than 150,000 gallons per day. For a wastewater treatment facility with a design flow of 150,000 gallons per day or less, the flow velocity in the sludge pipe must be greater than 0.5 feet per second. The executive director may consider approving lower velocities in writing for a wastewater treatment facility that uses a biological nutrient removal process, on a case-by-case basis.       
        (4) A sludge pipe must be accessible for cleaning.       
        (5) A means to remove a blockage from all sludge pipes must be provided at the wastewater treatment facility.       
      (f) Sludge Collection Equipment.      
        (1) A clarifier that is part of a wastewater treatment facility with a design flow of 10,000 gallons per day or greater must include mechanical sludge collecting equipment.       
        (2) A clarifier that is part of a wastewater treatment facility with a design flow of less than 10,000 gallons per day must include manual equipment designed to move settled sludge to the sludge collection pipe, unless mechanical sludge collecting equipment is provided.       
      (g) Side Water Depth.      
        (1) For a secondary clarifier, the side water depth is defined as:                  (A) the water depth from the top of the cone in a cone bottom tank to the water surface; or                   (B) the water depth from 2.0 feet above the bottom of a flat bottom tank to the water surface.         
        (2) The minimum side water depth for a clarifier with a mechanical sludge collector is:                  (A) 10.0 feet if the surface area is equal to or greater than 300 square feet; and                   (B) 8.0 feet if the surface area is less than 300 square feet.         
        (3) A clarifier with a hopper bottom must have a minimum side water depth of 8.0 feet, not including the hopper and sump. The corresponding surface area and surface loading relationships in Figure: 30 TAC §217.152(g)(3) must be used.        
          Attached Graphic         
        (4) The hopper portion of a hopper bottom clarifier must have a vertical depth of at least 4.0 feet.       
      (h) Restrictions on Hopper Bottom Clarifiers.      
        (1) A hopper bottom clarifier is prohibited for use in a wastewater treatment facility with a design flow equal to or greater than 10,000 gallons per day.       
        (2) Each hopper cell of a hopper bottom clarifier must have individually controlled sludge removal equipment.       
        (3) A hopper bottom clarifier must have a smooth wall finish.       
        (4) A hopper bottom clarifier must have an upper hopper slope of not less than 60 degrees from horizontal.       
      (i) Restrictions on Short Circuiting. The influent stilling baffle and effluent clarifier weir must prevent hydraulic short circuiting.     
      (j) Return Sludge Pumping Capacity.      
        (1) The capacity of a return sludge pumping system must be calculated based on the area of the activated sludge clarifier or clarifiers, including the stilling well area.       
        (2) The return sludge pumping capacity must be equal to or greater than the clarifier underflow rate in gallons per day per square foot (gpd/sf) with the largest pump out of service.       
        (3) A return sludge pumping system must be capable of pumping at least 200 gpd/sf but not more than 400 gpd/sf.       
        (4) The return sludge pumping capacity must be controlled via throttling, variable speed drives, or multiple pump operation.       
     Source Note:        The provisions of this §217.152 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254