1. (a) Oxygen Requirements (O₂R) of wastewater.

   1. (1) An aeration system must be designed to provide a minimum dissolved oxygen concentration in the aeration basin of 2.0 milligrams per liter (mg/l).
   2. (2) Mechanical and diffused aeration systems must supply the O₂R calculated by Equation F.2 located in paragraph (3) of this subsection or use the recommended values presented in Table F.3 in paragraph (3) of this subsection.
   3. (3) The O₂R values in Table F.3 in the following figure use concentrations of 200 mg/l five-day biochemical oxygen demand (BOD₅) and 45 mg/l ammonia-nitrogen (NH₃-N) in Equation F.2 in the following figure:
      Attached Graphic
      

2. (b) Diffused Aeration System. An airflow design must be based either paragraph (1) or (2) of this subsection.

   1. (1) Design Airflow Requirements - Default Values. A diffused air system may use the following table to determine the airflow for sizing:
      Attached Graphic
      

   2. (2) Design Airflow Requirements - Equipment and Site Specific Values. A diffused air system may base calculations of the airflow requirements for the diffused air equipment in accordance with subparagraphs (A) - (D) of this paragraph.

      1. (A) Determine Clean Water Oxygen Transfer Efficiency.

         1. (i) A diffused air system may have a clean water oxygen transfer efficiency greater than 4% only if the full scale diffuser performance data from a certified testing laboratory or sealed by an independent licensed professional engineer demonstrates the diffuser's transfer efficiency.
         2. (ii) A testing laboratory or licensed engineer shall use the oxygen transfer testing methodology described in the most current version of the American Society of Civil Engineers (ASCE) publication, A Standard for the Measurement of Oxygen Transfer in Clean Water.
         3. (iii) A diffused air system with a clean water transfer efficiency greater than 18% for a coarse bubble system and greater than 26% for a fine bubble system is considered an innovative technology and is subject to §217.7(b)(2) of this title (relating to Types of Plans and Specifications Approvals).
         4. (iv) A design for clean water transfer efficiencies obtained at temperatures other than 20 degrees Celsius must be adjusted for a diffused air system to reflect the approximate transfer efficiencies and air requirements under field conditions by using the following equation:
            Attached Graphic
            

      2. (B) Determining Wastewater Oxygen Transfer Efficiency (WOTE).

         1. (i) The WOTE must be determined from clean water test data by multiplying the clean water transfer efficiency by 0.65 for a coarse bubble diffuser and by multiplying the clean water transfer efficiency by 0.45 for a fine bubble diffuser.
         2. (ii) The executive director may require additional testing and data to justify actual WOTE for a facility treating wastewater containing greater than 10% industrial wastes.
      3. (C) Determining Required Airflow (RAF). The RAF must be calculated using the following equation to determine the size needed for a diffuser submergence of 12.0 feet. If the diffuser submergence is other than 12.0 feet, a diffused air system must correct the RAF detailed in subparagraph (D) of this paragraph.
         Attached Graphic
         
      4. (D) Corrections to RAF based on varying diffuser submergence depths. If the diffuser submergence is not 12.0 feet, the design must specify the adjustment of the minimum airflow rate as calculated in subparagraph (C) of this paragraph by multiplying the calculated values by the factors in the following table:
         Attached Graphic
         

   3. (3) Mixing Requirements for Diffused Air. The air requirements for mixing must be calculated using:

      1. (A) Design of Municipal Wastewater Treatment Plants, Chapter 11, a joint publication of the ASCE and the Water Environment Federation, for mixing requirements; or
      2. (B) provide mixing air at a rate greater than or equal to 20 scfm/1000 cf for a coarse bubble diffuser and greater than or equal to 0.12 scfm /square foot (sf) for a fine bubble diffuser.

   4. (4) Blowers and Air Compressors.

      1. (A) A blower and a compressor must have sufficient capacity to provide the required aeration rate for biological treatment and the air requirements of any supplemental unit.
      2. (B) The report must include blower or compressor calculations that show the actual air requirements for the expected temperature range, including both summer and winter conditions, and the impact of the actual site elevation on the air supply.
      3. (C) A diffused air system must have multiple compressors arranged to provide an adjustable air supply to meet the variable organic load on the facility.
      4. (D) The compressors must be capable of handling the maximum design air requirements with the largest single unit out of service.
      5. (E) A blower unit and a compressor unit must restart automatically after a power outage, or a telemetry system or an auto-dialer with battery backup must notify an operator of any outage.
      6. (F) A design must specify blowers or air compressors with sufficient capacity to handle air intake temperatures that may exceed 100 degrees Fahrenheit (38 degrees Celsius), and pressures that may be less than standard (14.7 pounds per square inch absolute).
      7. (G) A design must specify the capacity of a motor drive necessary to handle air intake temperatures that may be 20 degrees Fahrenheit (-7 degrees Celsius) or less in a location that experiences temperatures below 20 degrees Fahrenheit (-7 degrees Celsius).

   5. (5) Diffuser Systems - Additional Requirements.

      1. (A) Diffuser Submergence.

         1. (i) A submergence depth for any diffuser must meet the minimum depths in the following table, for a new facility:
            Attached Graphic
            
         2. (ii) A diffuser submergence depth for any material alteration or expansion of an existing facility may vary from the values in Table F.6 in clause (i) of this subparagraph to match existing air pressure, delivery rate, and hydraulic requirements.
         3. (iii) A submerged depth for a diffuser of less than 7.0 feet is prohibited.
      2. (B) Grit Removal. A facility that uses diffusers and has wastewater with high concentrations of grit must include a grit removal unit upstream of an aeration process or must include multiple trains that may be taken out of service to allow for grit removal.

      3. (C) Aeration System Pipes.

         1. (i) Each diffuser header must include an open/close or throttling type control valve that can withstand the heat of compressed air.
         2. (ii) An air header must be able to withstand temperatures up to 250 degrees F.
         3. (iii) The capacity of an air diffuser system, including pipes and diffusers, must equal 150% of design air requirements.
         4. (iv) The design of an aeration system must minimize head loss. The report must include a hydraulic analysis of the entire air pipe system that quantifies head loss through the pipe system and details the distribution of air from the blowers to the diffusers.
         5. (v) An aeration system may use non-metallic pipes only in the aeration basin, but the pipes must be a minimum of 4.0 feet below the average water surface elevation in the aeration basin.

3. (c) Mechanical Aeration Systems.

   1. (1) Required Airflow - Equipment and Site Specific Values. The airflow requirements for a mechanical aeration system must be calculated in accordance with subparagraphs (A) and (B) of this paragraph.

      1. (A) Determine Clean Water Oxygen Transfer Efficiency.

         1. (i) The report must include the oxygen transfer efficiency rate for the mechanical equipment.
         2. (ii) Clean water oxygen transfer rate must not exceed 2.0 pounds of oxygen per horsepower-hour, unless justified by full scale performance data conducted by a certified testing laboratory or sealed by an independent, licensed professional engineer using the oxygen transfer testing methodology described in the most current version of the ASCE publication, A Standard for the Measurement of Oxygen Transfer in Clean Water.
         3. (iii) A proposed clean water transfer efficiency in excess of 2.0 pounds of oxygen per horsepower-hour is innovative technology and subject to the requirements of §217.7(b)(2) of this title (relating to Types of Plans and Specifications Approvals).

      2. (B) Determine Wastewater Oxygen Transfer Efficiency.

         1. (i) The report must include data to justify actual wastewater transfer efficiency.
         2. (ii) A design must include an estimate of the wastewater transfer efficiency from the clean water transfer efficiency by multiplying the clean water transfer efficiency by 0.65 for all mechanical aeration equipment for a facility treating greater than 10% industrial wastes.

   2. (2) Mixing Requirements.

      1. (A) A mechanical aeration device must provide sufficient mixing to prevent deposition of mixed liquor suspended solids (MLSS) under any flow condition.
      2. (B) A mechanical aeration device must be capable of re-suspending the MLSS after a shutdown period.
      3. (C) Mechanical aeration devices with channel or basin layout must have a minimum of 100 horsepower per million gallons of aeration basin volume or 0.75 horsepower per thousand cubic feet of aeration basin volume.

   3. (3) Mechanical Components.

      1. (A) Process reliability.

         1. (i) Each basin must include a minimum of two mechanical aeration devices.
         2. (ii) A mechanical aeration device must meet the maximum design requirements for oxygen transfer with the largest single unit out of service.
         3. (iii) A mechanical aeration device must automatically restart after a power outage, or a telemetry system with battery backup or an auto-dialer with battery backup must notify a facility operator or owner.

      2. (B) Operation and maintenance.

         1. (i) A mechanical aeration device must have two speed or variable speed drive units, unless another means of varying the output is provided.
         2. (ii) A mechanical aeration device may use single-speed drive units with timer-controlled operation, if the device also includes an independent means of mixing.
         3. (iii) A facility operator must be able to perform routine maintenance on the aeration equipment without the potential of coming into contact with raw or partially treated wastewater.
         4. (iv) Any bearing, drive motor, or gear reducer must be accessible and be equipped with a splash prevention device.
         5. (v) Any gear reducer must have a drainage system to prevent operator contact with mixed liquor.

Source Note:The provisions of this §217.155 adopted to be effective August 28, 2008, 33 TexReg 6843.