(a) Application Rates. With one unit out of service, the peak application rate to any deep bed, intermittently backwashed, granular media (deep bed) filtration unit must not exceed twice the design application rate.      
        (1) Single Media (Sand).                  (A) The design filtration rate for single media (sand) filters must not exceed three gallons per minute per square foot (gpm/sf) of media surface.                   (B) The maximum filtration run time between backwash periods is 6.0 hours.         
        (2) Dual Media (Stratified Anthracite and Sand). The design filtration rate for a dual media (anthracite and sand) filter must not exceed 4.0 gpm/sf of media surface.       
        (3) Mixed Media (Non-stratified Anthracite, Sand, Garnet, or Other Materials). The design filtration rate for mixed media (non-stratified anthracite, sand, garnet, or other materials) must not exceed 5.0 gpm/sf of media surface.       
      (b) Media Design.      
        (1) A deep bed filter underdrain system must include a graded gravel layer with a minimum depth of 15 inches. Filter support media other than gravel may be used if justified in the engineering report.       
        (2) The uniformity coefficient of media used in a deep bed filter must be 1.7 or less.       
        (3) A dual or mixed deep bed filter must be designed to maintain stratification of media during backwash, based on the particle size distribution.       
        (4) Media material depths for the various deep bed filter types must conform to the values in the following table, unless other media depths are justified in the engineering report with an analysis of the backwash rates:        
          Attached Graphic         
      (c) Backwash Systems.      
        (1) Flowrate and Media Expansion.                  (A) A backwash system must allow a fluidized media expansion of at least 20% by volume.                   (B) A single media deep bed filter must provide a minimum backwash flowrate of 6.0 gpm/sf of media area.                   (C) Dual and mixed media deep bed filters must provide a minimum backwash rate of 15 gpm/sf of media area.                   (D) Backwash times must be at least 10 minutes but not more than 15 minutes, unless the engineering report justifies a different time.         
        (2) Surge Control.                  (A) A wastewater treatment facility that does not have flow equalization or other means of surge control must have a backwash tank.                   (B) A surge control device must prevent increases in flow greater than 15% of the design flow of the upstream treatment units if backwash is taken directly to the headworks of the wastewater treatment facility or the influent lift station.                   (C) The design of a surge control device must be based on calculations that demonstrate the slug effects of backwash water and that demonstrate treatment capabilities are not diminished with the return of backwash water to the headworks of the wastewater treatment facility or the influent lift station.                   (D) An enclosed backwash tank must be vented.         
        (3) Pumps.                  (A) Pumps for backwashing deep bed filters units must deliver the required flow rate for backwash operations according to paragraph (1) of this subsection with the largest pump out of service.                   (B) A backup pump must be available on-site.                   (C) A valve arrangement for isolating a deep bed filter unit for backwashing must be accessible for maintenance.                   (D) A backwash system employing automatic controls must include a manual override system.         
        (4) Supplemental Systems.                  (A) A single deep bed media filter system must include either an air scour system or a combination air and water scour system in addition to an up-flow backwash water system.                   (B) A dual or mixed deep bed media filter system must include either a surface air or water scour system that meets one of the following requirements for air or water scouring.                      (i) Air scour system flowrates must be at least 3.0 standard cubic feet per minute per square foot (scfm/sf) of media surface area but not more than 5.0 scfm/sf of media surface area.                       (ii) Water scour system flowrates must be at least 0.5 gpm/sf of media area but not more than 2.0 gpm/sf of media area.           
      (d) Underdrain System. An underdrain system must provide a uniform distribution for filter backwash without plugging or exceeding the manufacturer's recommendation for maximum head loss. The manufacturer's recommendation for maximum head loss must be included in the engineering report.     
      (e) Tank Design.      
        (1) The bottom of a wash water collection trough must be a minimum of 6.0 inches above the maximum elevation of the expanded media during backwash.       
        (2) A wash water collection trough must have a minimum freeboard of 3.0 inches during the maximum backwash flowrate.       
      (f) Controls.      
        (1) The deep bed filter operation controls may be manual or automatic.       
        (2) Control indicators must be visible to a wastewater treatment facility operator while adjusting the controls.       
        (3) An automatically controlled system must include a manual override system.       
        (4) Each deep bed filter unit must have a head loss indicator.       
     Source Note:        The provisions of this §217.191 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254