wastewater treatment plants & bacteria: strategies for compliance chlorine contact chambers;...

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


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


Disinfection Performance Comparison:

Free Chlorine vs. Chloramines


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 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


Injection and Mixing


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– 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

= complete mix = no chlorine= complete mix = 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


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 and grease have high chlorine demand– Create potential for escaping contact basin and

affecting sample results


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– 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.


Questions

?Tim Brodeur, PE

[email protected]

wastewater treatment plants & bacteria: strategies for compliance chlorine contact chambers;...

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