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Wastewater Treatment Plants & Bacteria: Strategies for Compliance
Chlorine Contact Chambers; Chlorine Mixing and Contact Time for CtHarris County Wastewater SymposiumWastewater Treatment Plants & Bacteria: Strategies for ComplianceTim Brodeur, PEApril 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
Proper Contact Basin Dimensions– Ideal Reactor: Plug-
flow• 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 short-circuiting
– Smooth corner fillets• Promotes plug flow,
reduces areas for regrowth
Wastewater Treatment Plants & Bacteria: Strategies for Compliance
Modeling & Tracer Testing
= complete mix = no chlorine= complete mix = no chlorine
Wastewater Treatment Plants & Bacteria: Strategies for Compliance
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.
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