(a) A volume-flux design must size an aeration basin and clarifier on the relationship between the volume-flux of solids in the secondary clarifier, the sludge volume index (SVI), and the sludge blanket depth. The following design approach may be used as an alternative to the traditional design approach. If the volume-flux design approach is used, it must be used consistently throughout the design. No other design method may be used in combination with the volume-flux design method.      
        (1) A design may base the aeration tank volume and the clarifier volume on a mixed liquor suspended solids (MLSS) and floc volume (at SVI of 100) for the required minimum solids retention time.       
        (2) Larger values of MLSS require less aeration tank volume and greater clarifier volume.       
        (3) By examining a range of values of the MLSS and the floc volume, the most favorable arrangement for a wastewater treatment facility may be selected.       
        (4) When using the volume-flux design method, the size of an aeration basin and a clarifier must be in accordance with the requirements of this section.       
      (b) Design approach.      
        (1) Determine the solids retention time (SRT) needed to meet the permit requirement for five-day biochemical oxygen demand (BOD                 5                ) and ammonia-nitrogen (NH                 3                -N) effluent limitations.       
        (2) Select a trial value mixed liquor floc volume (for example, MLSS at an SVI of 100).       
        (3) Using the design organic loading rate, the required SRT and yield, and the trial MLSS, determine the aeration tank volume.       
        (4) Using the trial value of mixed liquor flow volume, determine the clarifier area.       
        (5) For clarifiers overloaded in thickening at the peak flow, determine the final MLSS during storm flow and the resulting sludge blanket depth.       
        (6) Observing effluent limitations, determine the side water depth (SWD) and volume of the clarifier.       
        (7) Repeat the steps in paragraphs (2) - (6) of this subsection at different mixed liquor floc volumes and select the most favorable conditions for the wastewater treatment facility design.       
      (c) Aeration Basin Sizing.      
        (1) For a wastewater treatment facility that does not require nitrification, the minimum SRT is as follows:                  (A) for a wastewater treatment facility with an effluent BOD                     5                    monthly average limitation of 20 milligrams per liter (mg/l), the minimum SRT is three days;                   (B) for an extended aeration wastewater treatment facility with an effluent BOD                     5                    monthly average limitation of 20 mg/l, the minimum SRT is 22 days;                   (C) for a wastewater treatment facility with an effluent BOD                     5                    monthly average limitation less than 20 mg/l, the minimum SRT is 4.5 days; and                   (D) for an extended aeration wastewater treatment facility with an effluent BOD                     5                    monthly average limitation of less than 20 mg/l, the minimum SRT is 25 days.         
        (2) For a wastewater treatment facility that requires nitrification, the minimum SRT is based on the winter reactor temperature as set forth in §217.154(b) of this title (relating to Aeration Basin and Clarifier Sizing--Traditional Design) and the values of SRT and net solids production (Y), as listed in Table F.8. in Figure: 30 TAC §217.164(c)(3). The maximum BOD                 5                loading limitation for a single-step aeration process is 50 pounds (lb) BOD                 5                per 1,000 cubic feet (cf) and for the first step of multi-step aeration process facilities is 100 lb BOD                 5                /1,000 cf.       
        (3) An above-ground steel or fiberglass tank requires 2 degrees Celsius lower minimum operating temperature than a wastewater treatment facility utilizing a reinforced concrete tank. A wastewater treatment facility must be designed for an MLSS concentration of at least 2,000 mg/l but no more than 5,000 mg/l. The net solids production, (Y), in the following table includes both coefficients for yield and endogenous respiration:        
          Attached Graphic         
        (4) To calculate the SRT, divide the safety factor by the maximum growth rate as shown in the following equation. The safety factor includes the design factor for the ratio of average to maximum diurnal ammonia loading. A value of 3.0, as recommended in the United States Environmental Protection Agency manual                 Nitrogen Control,                is used in calculating the values in Table F.8. in Figure: 30 TAC §217.164(c)(3).        
          Attached Graphic         
        (5) To determine the aeration basin volume, select a trial value of MLSS. The aeration basin volume is calculated as the maximum value from the following equations:        
          Attached Graphic         
      (d) Clarifier Sizing.      
        (1) A clarifier basin size is based on volume-flux from the floc volume of solids entering the clarifier.       
        (2) Biological solids may occupy different volumes for the same mass of solids as indicated by the SVI.       
        (3) For purposes of determining weir overflow rates for clarifier sizing, the design flow and the peak flow must include any return flows from units downstream of the clarifier, including flow from skimmers, thickeners, and filter backwash.       
        (4) A clarifier must be sized to prevent overloading under any design condition.       
        (5) The settling velocity of the mixed liquor solids must equal or exceed the two-hour peak weir overflow rate.       
        (6) A clarifier must be sized to prevent overloading in the thickening process at the design flow.       
        (7) The wastewater treatment facility's operation and maintenance manual must state the design maximum mixed liquor floc volume.       
      (e) Determine Weir Overflow Rate and Area. The values in Table F.9 in Figure 1: 30 TAC §217.164(e)(2)(I) determine the maximum surface loading rates. The MLSS concentration must include the same concentration used for sizing the aeration basin. The design must be based on the underflow rate. The design must include calculations for maximum weir overflow rate for the clarifier at the peak flow (Table F.9. in Figure 1: 30 TAC §217.164(e)(2)(I)), the aeration basin MLSS concentration, and a selected underflow rate. The area of the clarifier is determined by the following equation:      
        Attached Graphic       
        (1) Determine Volume of a Clarifier. The volume of a clarifier must exceed the values determined from the minimum side wall depth (SWD) in Equation F.9. in Figure: 30 TAC §217.164(e)(1) or the minimum detention time in Equation F.10. in Figure: 30 TAC §217.164(e)(1):        
          Attached Graphic         
        (2) Dimensions for Clarifiers Designed for Solids Storage Capabilities. The design of a clarifier that may be overloaded in thickening at the design flow must include the ability to store solids during peak flow events. The design must be based on the values in Table F.9. in Figure 1: 30 TAC §217.164(e)(2)(I), Table F.10. in Figure 2: 30 TAC §217.164(e)(2)(I), and Table F.11. in Figure 3: 30 TAC §217.164(e)(2)(I). The process for designing a clarifier based on this concept must be completed as follows:                  (A) Determine the area of a clarifier. The area calculations must be based on the trial MLSS value selected for the sizing of the aeration basin in paragraph (1) of this subsection. The area of a clarifier must exceed the greater of the areas determined by Equation F.11. or Equation F.12. in Figure: 30 TAC §217.164(e)(2)(A):          
            Attached Graphic           
          (B) The final MLSS value must be the result of the transfer of solids from an aeration tank to a clarifier at the peak flow. A clarifier design must allow for rates of flow that will transfer solids from an aeration tank to a clarifier if the clarifier becomes overloaded in thickening until the mixed liquor solids are reduced to the concentration that no longer causes the overload.                   (C) Using Table F.11. in Figure 3: 30 TAC §217.164(e)(2)(I) and the selected underflow rate, the MLSS concentration at peak flow is determined using the following equation:          
            Attached Graphic                     (D) Determine depth of sludge blanket at peak flow. The depth of a sludge blanket is determined by the aeration basin volume, the change in MLSS, the area of the clarifier, and the concentration of the blanket solids at the selected underflow rate as shown in the following equation:          
            Attached Graphic                     (E) Determine the SWD. The SWD of a clarifier is the maximum value resulting from the following conditions:                      (i) 10 ft, unless a lower depth is allowed by §217.152(g) of this title (relating to Requirements for Clarifiers);                       (ii) 3.0 times the sludge blanket depth; and                       (iii) minimum detention time per the following equation:            
              Attached Graphic                       (F) Determine clarifier volume. The volume of a clarifier is the surface area multiplied by the SWD determined in subparagraph (E) of this paragraph.          
            Attached Graphic                     (G) The formulas for Equation F.17. in Figure: 30 TAC §217.164(e)(2)(G)(i); Equation F.18. in Figure: 30 TAC §217.164(e)(2)(G)(ii); and Table F.10. in Figure 2: 30 TAC §217.164(e)(2)(I); calculate the rates that are equal to the settling velocity of activated sludge at various floc volume concentrations. For values less than 30%, the floc volume is the 30-minute settled volume in an unstirred one-liter graduated cylinder. For values greater than 30%, the sample is diluted so that the settled volume is at least 15% but not more than 30%, and the result multiplied by the dilution factor.                      (i) For floc volume less than 40%, use the following equation; or            
              Attached Graphic                         (ii) For floc volume greater than 40%, use the following equation:            
              Attached Graphic                       (H) Table F.9. in Figure 1: 30 TAC §217.164(e)(2)(I) and Table F.11. in Figure 3: 30 TAC §217.164(e)(2)(I) are based on an analysis of the floc volume-flux, i.e. floc volume times settling velocity, calculated from Equation F.17. in Figure: 30 TAC §217.164(e)(2)(G)(i) and Equation F.18. in Figure: 30 TAC §217.164(e)(2)(G)(ii). Table F.11. in Figure 3: 30 TAC §217.164(e)(2)(I) is a tabulation of the maximum concentration of the underflow at different underflow rates. Equation F.19. is for Table F.11. in Figure 3: 30 TAC §217.164(e)(2)(I).          
            Attached Graphic                     (I) Table F.9. in Figure 1: 30 TAC §217.164(e)(2)(I) determines the weir overflow rate that, along with the underflow rate and MLSS, determines the same floc volume-flux as shown in Table F.11. in Figure 3: 30 TAC §217.164(e)(2)(I).          
            Attached Graphic           
            Attached Graphic           
            Attached Graphic           
     Source Note:        The provisions of this §217.164 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254