1. (a) If a sludge thickener(s) is used, following criteria are required:

   1. (1) Capacity. The maximum monthly sludge production rate must be used as the basis for sludge thickening system sizing and design.

   2. (2) Flexibility.

      1. (A) A sludge thickening system must have a bypass.

      2. (B) A facility with a design flow greater than 1.0 million gallons per day (mgd) must have:

         1. (i) at least dual sludge thickening units;
         2. (ii) an alternate means of thickening; or
         3. (iii) an alternate disposal method.

2. (b) Specific Requirements for a Mechanical Gravity Thickener.

   1. (1) Equipment Features.

      1. (A) A mechanical gravity thickener must have:

         1. (i) a low-speed stirring mechanism for continuous mixing and flocculation within the zone of sludge concentration;
         2. (ii) sludge storage, if sufficient storage is unavailable in other external tankage; and
         3. (iii) a means of controlling the rate of sludge withdrawal.
      2. (B) A mechanical thickener may use a chemical addition or dilution water feed system.
      3. (C) A scraper mechanical train must be capable of withstanding any expected torque load. The normal working torque load must not exceed 10% of the manufacturer's recommended torque load.

   2. (2) Design Basis.

      1. (A) A mechanical thickener design must be justified in the report.
      2. (B) The executive director may require data from a pilot study or similar sludge thickening unit operating under similar conditions.
      3. (C) The thickener overflow rate must be at least 400 gallons per day per square foot (gpd/sf) but no more than 800 gpd/sf.
      4. (D) The minimum side water depth for a mechanical thickener is 10 feet.
      5. (E) A circular thickener must have a minimum bottom slope of 1.5 inches per foot.
      6. (F) The peripheral velocity of a scraper must be at least 15 feet per minute but no more than 20 feet per minute.
      7. (G) A mechanical thickener design must minimize the potential for short-circuiting.

3. (c) Dissolved Air Flotation (DAF) Thickener.

   1. (1) Equipment Features.

      1. (A) A DAF basin must have a bottom scraper that function independently of the surface skimmer.
      2. (B) A recycle pressurization system for a DAF basin must use effluent or secondary effluent instead of potable water.
      3. (C) A DAF basin must have a polymer feed system. A feed system must meet the requirements of §217.247(r) of this title (relating to Chemical Pretreatment of Sludge).
      4. (D) A DAF basin must be located in a covered building with positive air ventilation.

   2. (2) Design Basis.

      1. (A) A DAF basin design must be justified in the report.
      2. (B) The executive director may require data from a pilot study or similar DAF operating under similar conditions.
      3. (C) The hydraulic loading rate must not exceed 2.0 gallons per minute (gpm) per square foot (sf).
      4. (D) The solids loading rate must be at least 1.0 pound but not more than 4.0 pounds per hour per sf.
      5. (E) The air to solids weight ratio must be at least 0.02 but not more than 0.04.
      6. (F) A retention tank system must have a minimum pressure of 40 pounds per square inch gauge.
      7. (G) A skimmer must have multiple or variable speeds that allow an operational range of at least 1.0 foot per minute (fpm) but not more than 25.0 fpm.

4. (d) Centrifugal Thickener.

   1. (1) A centrifugal thickener design must be justified in the report.
   2. (2) The executive director may require data from a pilot study or similar centrifugal thickener operating under similar conditions.
   3. (3) A centrifugal thickener must be preceded by pretreatment to prevent plugging of a nozzle or excessive wear in the bowl.
   4. (4) The centrate is subject to §217.242 of this title (relating to Control of Sludge and Supernatant Volumes).

5. (e) Specific Requirements for Gravity Belt Thickeners.

   1. (1) Equipment Features.

      1. (A) Gravity belt thickeners must include a wash water system (60 pounds per square inch minimum) capable of providing 60 gpm per meter of belt width belt. Booster pumps may be employed to achieve design conditions.
      2. (B) Gravity belt thickeners must include a polymer feed system that meets the requirements of §217.247 of this title.
      3. (C) A filtrate drainage system must be sized to remove the full hydraulic capacity of a gravity belt thickener without accumulation or ponding.

   2. (2) Design Basis. Gravity belt thickener sizing must be based upon the following criteria, unless otherwise justified in the report:

      1. (A) maximum solids loading of 1,250 pounds per meter of belt width; or
      2. (B) maximum hydraulic loading 250 gpm per meter of belt width.
   3. (3) Gravity belt thickener filtrate is subject to the requirement in §217.242 of this title.

Source Note:The provisions of this §217.248 adopted to be effective August 28, 2008, 33 TexReg 6843.