Chlorine Contact Chambers; Chlorine Mixing and Contact Time for Ct

1. 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

2. 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

3. Disinfection Performance Comparison: Free Chlorine vs. Chloramines

4. 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

5. 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

6. Injection and Mixing

7. 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

8. 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

9. Modeling & Tracer Testing = no chlorine = complete mix = no chlorine = complete mix

10. 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

11. 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

12. Scum Removal • Scum and grease have high chlorine demand • Create potential for escaping contact basin and affecting sample results

13. 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

14. 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.

15. Questions ? Tim Brodeur, PE Tim.brodeur@aecom.com