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How to Determine Chlorination Contact Time

How to Determine Chlorination Contact Time. Training for Engineers Prepared by R. M. McKeown. Wangdue Phodrang Pilot Programme 18 th April, 2014. Contents. What is chlorine contact time? Step 1. Determine theoretical contact time - Use standard tables

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How to Determine Chlorination Contact Time

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  1. How to Determine Chlorination Contact Time Training for Engineers Prepared by R. M. McKeown Wangdue Phodrang Pilot Programme 18th April, 2014.

  2. Contents • What is chlorine contact time? • Step 1. Determine theoretical contact time • - Use standard tables • Step 2. Determine actual contact time • - Determine detention time • - Determine pH, temperature and chlorine concentration of water • Step 3. Compare theoretical and actual contact time • -If needed, implement operational changes to ensure contact time is achieved 2 10/11/2014

  3. Contents What is chlorine contact time? 3 10/11/2014

  4. CONTACT TIME Chlorine needs time to kill microorganisms CONTACT TIME 4 10/11/2014

  5. CONTACT TIME Contact time (CT) = Residual chlorine (mg/L) x detention time (minutes) e.g. 0.5 mg/L x 30 minutes CT = 15 mg/L.min 5 10/11/2014

  6. Contents • What is chlorine contact time? • Step 1. Determine theoretical contact time • Use standard tables 6 10/11/2014

  7. Step 1: Determine theoretical contact time • Determine theoretical contact time needed (AWWA, 1991): • Example, • - Temperature 15oC • - pH 7.5 • - Chlorine 0.8 mg/L CT 88 mg/L.min required 7 10/11/2014

  8. Step 1: Determine theoretical contact time • If you do not use the tables: • For water at 250C and where pH <8.0 • - Must ensure a minimum contact time of 15 mg/L.min is achieved. • e.g. 30 minutes detention x 0.5 mg/L chlorine residual • For water at 150C and where pH <8.0 • - Must ensure a minimum contact time of 30 mg/L.min is achieved. • e.g. 60 minutes detention x 0.5 mg/L chlorine residual • => This will kill majority of harmful microorganisms 8 10/11/2014

  9. Contents • What is chlorine contact time? • Step 1. Determine theoretical contact time • - Use standard tables • Step 2. Determine actual contact time • - Determine detention time in clear water basin • - Determine residual chlorine concentration of water 9 10/11/2014

  10. Step 2: Determine actual contact time • Determine actual detention time: • Detention time = Volume tanks/Plant flow • 1,500,000 L/ 1680 L/min • 892 minutes • Chlorine residual is 0.8 mg/L: • Important to measure at end of process!!! • Determine actual contact time: • Contact time = Chlorine (mg/L) x detention time • 0.8 mg/L x 892 min • 713 mg/L.min • =>Must assess short-circuiting potential • =>Must assess lowest operating level of basin CHLORINE DOSE 1.5 mg/L CHLORINE 0.8 mg/L 10 10/11/2014

  11. Contents • What is chlorine contact time? • Step 1. Determine theoretical contact time • - Use standard tables • Step 2. Determine actual contact time • - Determine detention time in clear water basin • - Determine pH, temperature and chlorine concentration of water • Step 3. Compare theoretical and actual contact time • -If needed, implement operational changes to ensure contact time is achieved 11 10/11/2014

  12. Step 3. Compare Theoretical and Actual Contact Times • Actual > Theoretical • 713 mg/L.min 88 mg/L.min (ideal) • Appropriate contact time for good disinfection • If actual is < theoretical => operational changes • Increase chlorine dose • Reduce plant flow rate (increase detention time) 12 10/11/2014

  13. Note: Short Circuiting • Ideal mixing: • Uses full volume of tanks = maximum detention time (maximum contact time!) • Detention time = Volume tanks/Plant flow • 1,500,000 L/ 1680 L/min • 892 minutes CHLORINE 13 10/11/2014

  14. Note: Short Circuiting • Short-circuiting: • Uses less volume of tanks = less detention time (less contact time!) • Detention time = Volume tanks/Plant flow • e.g. 750,000 L/ 1680 L/min • 446 minutes CHLORINE 14 10/11/2014

  15. Baffling factor • Standard factor that considers the short circuiting potential of your tank: 15 10/11/2014

  16. Baffling factor • So if you calculate your contact time to be e.g. 88 mg/L.min • And you have an ‘Poor’ baffling factor • Contact time is actual 88 x 0.3 • 26 mg/L.min 16 10/11/2014

  17. Note: Basin operating level • If basin only 50% full, then detention time is also reduced by 50%: • = Less detention time (less contact time!) Basin 50% Full (750 m3) Basin 100% Full (1500 m3) • Detention time = Volume tanks/Plant flow • 750,000 L/ 1680 L/min • 446 minutes • Detention time = Volume tanks/Plant flow • 1,500,000 L/ 1680 L/min • 892 minutes 17 10/11/2014

  18. Calculating contact time • Must always use minimum operating level of your basin • Must always use lowest chlorine residual you have recorded • Must use a baffling factor if you do not have ideal mixing conditions • => Builds in a ‘worst case scenario’ margin of safety so you do not over estimate your contact time 18 10/11/2014

  19. Where to dose chlorine for maximum contact time? • => Dose as early in the system as possible • => Must always dose when the plant is running • => Ensure all tank inlet pipes receive dosed water • =>Ensure you maximise detention time CHLORINE 19 10/11/2014

  20. Note: Short Circuiting CHLORINE 20 10/11/2014

  21. Ideal chlorine dose and contact time … • Will be different for every site • Ensure a minimum of 15 to 30 min.mg/L where pH is < 8.0 (i.e. 30 to 60 minutes and 0.5 mg/L chlorine residual) • Ensure chlorine is dosed early and maximum detention time is achieved • Avoid short-circuiting 21 10/11/2014 www.egwater.vic.gov.au

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