(a) This section applies to the traditional approach for sizing an aeration basin and clarifier, and is based on empirically-derived design values that have been used historically as standard engineering practice.     
      (b) Aeration Basin Sizing.      
        (1) An aeration system must be designed to maintain a minimum dissolved oxygen concentration of 2.0 milligrams per liter (mg/l) throughout the aeration basin at the maximum diurnal organic loading rate determined in §217.32(a)(3) and Figure: 30 TAC §217.32(a)(3) of this title (relating to Organic Loading and Flows for New Wastewater Treatment Facilities). The executive director may consider alternative dissolved oxygen specifications for designs that include biological nutrient removal or tapered aeration for energy conservation, which must be approved in writing.       
        (2) Based on the calculated organic load, the aeration basin volume must be designed to ensure that the organic loading on the aeration basin does not exceed the organic loading rates in the following table:        
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        (3) When identifying the aeration basin temperature for the process design in Table F.1. in Figure: 30 TAC §217.154(b)(2), the owner must use the average of the lowest consecutive seven-day mean aeration basin temperature from a wastewater treatment facility with similar characteristics. For purposes of this subsection, a similar wastewater treatment facility:                  (A) is located within 50 miles of the wastewater treatment facility where the planned aeration basin or system will be installed;                   (B) uses the same placement of the aeration basin (in-ground or above-ground); and                   (C) has any other characteristics required by the executive director in writing.         
      (c) Clarifier Sizing.      
        (1) The following table establishes the maximum surface loading rates and the minimum detention times that must be used to determine the size of a clarifier:        
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        (2) A clarifier must meet both the detention time and weir overflow rate criteria in Table F.2. in Figure: 30 TAC §217.154(c)(1).                  (A) When calculating weir overflow rates for a clarifier, return activated sludge flow must not be used in the calculation of the maximum weir overflow rate, in compliance with Table F.2. in Figure: 30 TAC §217.154(c)(1).                   (B) When calculating the weir overflow rate for a clarifier, the surface area of the stilling well may be included as part of the clarifier surface area.         
     Source Note:        The provisions of this §217.154 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254