Wastewater Treatment Plants & Bacteria: Strategies for Compliance 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 (Cl 2, 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: 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 % of particles with embedded coliforms varies by plant & treatment process WERF Study (96-CTS-3) TSS only gives part of the picture; consider PSD analysis for plants that need HLD

Wastewater Treatment Plants & Bacteria: Strategies for Compliance Effluent TSS characteristics vary by plant Plant TypeOxidation Ditch Pure Oxygen Plant MCRT40 days2 days # particles > 10 μm 10 4 /100 ml10 6 /100 ml oxidation ditch pure oxygen Particle size distribution (PSD) analysis WERF Study (04-HHE-5)

Wastewater Treatment Plants & Bacteria: Strategies for Compliance Data collection recommendations for high level disinfection projects ParameterRaw Influent Secondary Clarifier Effluent Filtered Effluent Disinfected Effluent Fecal coliforms Particle size distribution UV transmittance (UVT) Build data base over at least a year (seasonal effects) Allows for more informed decision-making during HLD design

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 ? Tim Brodeur, PE