1. (a) Mixing Requirements.

   1. (1) Chlorination Unit. A chlorination unit must be constructed so that the applied chlorine (Cl₂) is thoroughly mixed with the wastewater prior to entry into a Cl₂ contact chamber.
   2. (2) Mixing Zones. A mixing zone within a Cl₂ contact basin of an existing facility must not be considered as part of the volume needed for disinfection.
   3. (3) Cl₂ and Sodium Hypochlorite (NaClO). A disinfection system must apply the Cl₂ 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^-1.

   4. (4) Sulfur Dioxide (SO₂) and Sodium Bisulfite (NaHSO₃):

      1. (A) The mixing for an SO₂ and NaHSO₃ system must ensure compliance with all relevant permit requirements.
      2. (B) A disinfection system must provide a mean velocity gradient (G value) of at least 250 per second^-1.

2. (b) Disinfection Contact Basins.

   1. (1) A Cl₂ contact basin must provide a minimum Cl₂ contact time of 20 minutes at the peak flow.
   2. (2) The design of a Cl₂contact chamber must prevent short-circuiting and that the wastewater is retained in a contact basin for at least 20 minutes.
   3. (3) The report must include supporting data from a contact basin design model, performance data of a similar design, or a field tracer study.

3. (c) Dechlorination Contact Time.

   1. (1) A disinfection system must have sufficient mixing and contact time between the disinfected wastewater and a dechlorinating agent to ensure continuous compliance with the permitted Cl₂ limits.
   2. (2) A design 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.