| (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.
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