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Report
Chlorine Contact Chambers;
Chlorine Mixing and Contact Time for Ct
Harris County Wastewater Symposium
Wastewater Treatment Plants & Bacteria: Strategies for Compliance
Tim Brodeur, PE
April 26, 2011
Wastewater
Treatment Plants & Bacteria:
Strategies for Compliance
Chlorine Contact Chambers
Identify operational and design improvements associated
with the disinfection process to help struggling WWTPs
meet the new Coliform requirements
• Goals of this Section:
• Identify issues that promote ineffective bacteria
reduction
• Identify operational solutions that can be implemented
tomorrow
• Identify design solutions and retrofits for existing
WWTPs
• Identify design solutions to be implemented on your
next “Greenfield” design
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Disinfection
Performance
Comparison:
Free Chlorine
vs. Chloramines
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Chlorine Injection and Mixing
– Design considerations
• Proper Mixing
– Reduces off-gassing (Cl2, if gas is used)
– Reduces formation of di- and tri-chloramines
– Increases formation of hypochlorous acid (stronger disinfectant); less
hypochlorite ion
• Recommended level of mixing:
– G-value: 500 s-1: Required by Texas CEQ
– In-pipe injection ahead of a hydraulic jump w/ 2 ft. head loss
• Which is a better source of water for creating your chlorine
solution: pre-injection point water or post-injection point water?
– Post Injection
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Chlorine Injection
and Mixing
– Operation considerations
• Relocation of chlorine
injection point further
up-stream
– Results in longer contact
time
• Achieve break-point
chlorination
– Results in a stronger
disinfection agent
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Injection
and Mixing
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Proper Contact Basin Dimensions
– Ideal Reactor: Plugflow
• 2-log removal greater
than improper
reactors
– Minimum length to
width ratio: 40:1
• 72:1 (L:W) provided
95% plug flow*
*Marsile and Boyle (1973)
– Depth to width ratio:
1:1
– Consider wind effects
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Baffling
– Operation considerations
• Addition of retro-fit
baffles
– Design considerations
– Longitudinal in lieu of
horizontal baffling
• Horizontal baffles result
in greater back mixing
and potential for
producing areas of
stagnation and shortcircuiting
– Smooth corner fillets
• Promotes plug flow,
reduces areas for
regrowth
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Modeling & Tracer Testing
= complete mix
= complete mix
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
= no chlorine
= no chlorine
Inlets and Aeration
• Influent Pipe Design
– Increase influent pipe diameter up-stream of interface with
contact basin
– High velocities entering the basin can cause short circuiting
• Proper Post-Aeration Procedures
– Operation considerations
• Aeration strips chlorine residual
• Vigorous aeration causes short circuiting
• Consider only utilizing aeration at end of contact basin
– Design considerations
• Operators still like flexibility; and still want aeration throughout
basin for flexibility
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Sludge Control
– Operation considerations
• Keep settled sludge levels under control
• Sludge :
– Increases chlorine demand,
– Shields bacteria
– Promotes regrowth
– Design considerations
• Addition of multiple drains
• Sloped floors to drains
• In certain cases: Don’t extend baffling to floor
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Scum Removal
– Scum and grease have high chlorine demand
– Create potential for escaping contact basin and
affecting sample results
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Analyzers and Alarms
– Operator
considerations
• Ammonia analysis
pre-chlorine injection
• Free chlorine analysis
at contact basin weir
• Just use Free and Total
chlorine analysis with
Grab Ammonia
samples
– Design considerations
• On-line chlorine
analyzer with alarm
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Outfall Design
– Regrowth of bacteria can
be significant between
the outfall weir and end
of outfall pipe.
– Regrowth is promoted in
the outfall pipe by an
abundant food source
and lack of predatory
protozoa.
– Outfall lengths should be
limited when possible.
– Residence times should
be reduced by properly
sized outfall pipes.
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Questions
?
Wastewater Treatment Plants & Bacteria:
Strategies for Compliance
Tim Brodeur, PE
[email protected]

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