(a) Mixing Requirements.      
        (1) Mixing Zones. A mixing zone within a chlorine contact basin must not be considered as part of the volume needed for disinfection. A mixing zone must be designed to ensure that the chlorine is thoroughly mixed with the wastewater before entering a chlorine contact chamber, as described in paragraph (2) of this subsection.       
        (2) Chlorine and Sodium Hypochlorite Application. A disinfection system must apply the chlorine gas or solution in a highly turbulent flow regime created by in-line diffusers, mechanical mixers, or jet mixers. Effective initial mixing for the mean velocity gradient (G value) in the area of turbulent flow must exceed 500 per second. A serpentine disinfection channel may be used in place of turbulent initial mixing if the length-to-width ratio is at least 40-to-1 and complete mixing is demonstrated by a dye test.       
        (3) Sulfur Dioxide and Sodium Bisulfite Application.                  (A) The mixing for a sulfur dioxide or sodium bisulfite system must ensure compliance with the effluent limits in the wastewater permit.                   (B) A disinfection system must provide a mean velocity gradient (G value) of at least 250 per second.         
      (b) Chlorine Contact Basins.      
        (1) A chlorine contact basin must provide a minimum chlorine contact time of 20 minutes at the peak flow.       
        (2) A chlorine contact basin must prevent short-circuiting to ensure that the wastewater is retained in a chlorine contact basin for at least 20 minutes at peak flow.       
        (3) A rectangular chlorine contact basin must have rounded corners.       
        (4) If a wastewater treatment facility is designed with more than one chlorine contact basin:                  (A) the design must provide a means of verifying the chlorine contact time and residual chlorine in each basin; and                   (B) separate sampling points must be provided after each chlorine contact basin, unless the effluent from the basins is commingled at a single sampling point.         
        (5) The design of an aerated chlorine contact basin must include an analysis of the chlorine feed rate required to offset chlorine volatilization.       
        (6) The engineering report must include supporting data from a chlorine contact basin design model, performance data of a similar design, or a field tracer study.       
        (7) A chlorine contact basin must include a drain to facilitate removal of accumulated settled solids.       
      (c) Dechlorination Contact Time.      
        (1) A dechlorination system must have sufficient mixing and contact time between the disinfected wastewater and a dechlorinating agent to ensure continuous compliance with the chlorine limits in the wastewater permit.       
        (2) A dechlorination system must prevent short-circuiting and provide a minimum contact time of 20 seconds at the peak flow.       
     Source Note:        The provisions of this §217.281 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254