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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.

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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
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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
chlorine injection and mixing
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
chlorine injection and mixing1
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
proper contact basin dimensions
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
baffling
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
modeling tracer testing
Modeling & Tracer Testing

= no chlorine

= complete mix

= no chlorine

= complete mix

inlets and aeration
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
sludge control
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
scum removal
Scum Removal
  • Scum and grease have high chlorine demand
  • Create potential for escaping contact basin and affecting sample results
analyzers and alarms
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
outfall design
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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Questions

?

Tim Brodeur, PE

Tim.brodeur@aecom.com