1. (a) Application Rates. With one unit out of service, the peak application rate to any deep bed, intermittently backwashed, granular media filtration unit must not exceed twice the design application rate.

   1. (1) Single Media.

      1. (A) The design filtration rate for single media (sand) filters must not exceed three gallons per minute (gpm) per square foot (sf) of media surface.
      2. (B) The maximum filtration run time between backwash periods is 6.0 hours.
   2. (2) Dual Media. The design filtration rate for a dual media (anthracite and sand) filter must not exceed 4.0 gpm/sf of media surface.
   3. (3) Mixed Media. 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.

2. (b) Media Design.

   1. (1) A filter underdrain system must include a graded gravel layer with a minimum depth of 15 inches, or other filter media support material unless a filter media other than gravel is justified in the report.
   2. (2) Uniformity coefficient of media used in a filter must be 1.7 or less.
   3. (3) The particle size distribution for dual and mixed media filters must perform a hydraulic grading of material during backwash that will result in a filter bed with a pore space graded from progressively coarse to fine from the top of the media to the supporting layer.
   4. (4) Media depths for the various filter types must conform to the values in the following table, unless other media depths are justified in the report with an analysis of the backwash rates:
      Attached Graphic
      

3. (c) Backwash Systems.

   1. (1) Flowrate and Media Expansion.

      1. (A) A backwash system must allow a media expansion of at least 20%.
      2. (B) A single media filter must provide a minimum backwash flowrate of 6.0 gpm/sf of media area.
      3. (C) A dual and mixed media must provide a minimum backwash rate of 15 gpm/sf of media area.
      4. (D) Backwash times must be at least 10 minutes but not more than 15 minutes, unless the report justifies a different time.

   2. (2) Surge Control.

      1. (A) A wastewater treatment facility that does have flow equalization or other means of surge control must have a backwash tank.
      2. (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.
      3. (C) A design 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 facility headworks.
      4. (D) An enclosed backwash tank must be vented.

   3. (3) Pumps.

      1. (A) Pumps for backwashing filter units must deliver the required rate with the largest pump out of service.
      2. (B) A backup pump must be available on-site.
      3. (C) A valve arrangement for isolating a filter unit for backwashing must be accessible for maintenance.
      4. (D) A backwash system employing automatic controls must include a manual override system.

   4. (4) Supplemental Systems.

      1. (A) A single media filter system must include an air scour system or combination air and water scour system in addition to an up-flow backwash water system.
      2. (B) A dual or mixed media filter system must include either a surface air or water scour system.
      3. (C) Air scour system flowrates must be at least 3.0 standard cubic feet per minute per square foot of media surface area but not more than 5.0 scfm/sf of media surface area.
      4. (D) 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.
4. (d) Underdrain System. An underdrain system must provide a uniform distribution for filter backwash without plugging or exceeding the manufacturer's recommendation for maximum headloss.

5. (e) Tank Design.

   1. (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. (2) A wash water trough must have a minimum freeboard of 3.0 inches during the maximum backwash flowrate.

6. (f) Controls.

   1. (1) The filter operation controls may be manual or automatic.
   2. (2) Control indicators must be visible to a facility operator while adjusting the controls.
   3. (3) An automatically controlled system must include a manual override system.
   4. (4) Each 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.