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Chlorination & measurement

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Chlorination & measurement

  1. 1. CHLORINE MEASUREMENT Praful Hanmante
  2. 2.  Most commonly used for disinfection of water. CHLORINE AS DISINFECT
  3. 3. Cl2 when applied to water  forms hypochlorous acid and hydrochloric acid Cl2 + H2O ↔ HOCl + H+ +Cl- [pH dependent reaction] Ca(OCl)2 + 2H2O ↔ 2HOCl + Ca(OH)2 NaOCl + H2O ↔ HOCl+NaOH HOCl = Hypochlorous Acid Ca(OCl)2 = Calcium hypochlorite Ca(OH)2 = Calcium Hydroxide NaOCl = Sodium hypochlorite
  4. 4. HOCl ↔ H+ + OCl- [pH dependent reaction]  Killing effiency of HOCl is 40 -80 times larger than OCl - . Lower pH favors HOCl.  HOCl = Hypochlorous acid  OCl - = Hypochloride ion  Free chlorine = HOCl + OCl- FREE CHLORINE
  5. 5.  HOCl + NH3 ↔ NH2Cl (monochloramine) + H2O  HOCl + NH2Cl ↔ NHCl2 (dichloramine) + H2O  HOCl + NHCl2 ↔ NCl3 (trichloramine) + H2O  Combined chlorine=monochloramine+dichloramine+ trichloramine  Total chlorine = combined chlorine + free chlorine COMBINED CHLORINE
  7. 7. MEASUREMENT TECHNIQUES  DPD Colorimetric Method Amperometric Method
  8. 8. Add buffer and indicator solution to a known volume of water. After mixing, measure wavelength of light transmitted through sample to determine colour change and thus, amount of free residual chlorine in the sample. Advantages Price. High pH is not a limiting factor. EPA approved DPD measurement. Disadvantages Uses reagents.. High maintenance. No pH or temperature output. COLORIMETRIC METHOD
  10. 10.  Free Residual Chlorine— (DPD), The reaction takes place at a buffered pH of 6.3 to 6.6.  Total Residual Chlorine— By adding potassium iodide to the sample, chloramines in the sample oxidize iodide to iodine, which then oxidizes the DPD indicator to the magenta color at a buffered pH of 5.1.
  11. 11. A sensor consisting of a membrane (which allows HOCL to migrate through it), two dissimilar metal electrodes, and an electrolyte are submersed into the water sample. A free residual chlorine reading is derived based on mV changes experienced by the sensor. Advantages Ease of use No reagents or buffers added Disadvantages pH dependence AMPEROMETRIC METHOD
  12. 12. How does the chlorine sensor work  Current is proportional to diffusion rate, which is proportional to concentration cathode membrane anode (Ag/AgCl) electrolyte (KCl) electrons HOCL HOCL + H+ + 2e- Cl- + H2O 2Ag + 2Cl - AgCl + 2e- Amperometric sensor measures only HOCL
  14. 14. FREE CHLORINE AND THE PH PROBLEM  Three ways to solve the problem...  Restrict applications to only those having constant pH  Use acid to adjust the pH of the sample  Measure pH continuously and perform automatic correction 350 400 450 500 550 600 650 7.0 7.5 8.0 8.5 9.0 9.5 sensitivity,nA/ppm pH at 25 C
  15. 15. 15 The reagent solution...  Add an acid to the sample to lower pH to about 6.  Acid converts OCl to HOCl, which the sensor measures.  Disadvantages – requires sample conditioning system – requires reagents acid reagent bottle reagent pump sensor HOCl + OCl HOCl
  17. 17. The Rosemount Analytical approach Use continuous pH correction Good between pH 6.0 and 9.5. – Below pH 6.0, don’t need correction. – Above pH 9.5, usually there is too little HOCl available to measure. If pH varies less than 0.2 peak-to-peak, don’t need continuous pH correction. If pH varies more than 0.2 peak-to-peak, use continuous pH correction. free chlorine sensor pH sensor
  18. 18. Free chlorine sensor - 499ACL-01  Easy to replace membrane - no tools required  Range: 0 - 20 ppm  Linear range: 0 - 6 ppm  Linearity: 2% (typical). Following calibration in the linear range, the measured concentration of any point in the linear range is within about ±0.1 ppm of the expected value.  Electrolyte life: about three months  Variopol quick disconnect option makes replacing the sensor easy. cable cable grip machined plug, receives cable grip O-rings MNPT threads, pipe tape not shown
  19. 19. 19 Determination of total chlorine  No single sensor can be used to determine all the different forms of total chlorine.  The only way to measure total chlorine is to convert it into a form the sensor can measure.  Requires a sample conditioning system. Most manufacturers use an off-line system. acetic acid buffer with potassium iodide total chlorine iodine iodine sensor to waste The concentration of iodine produced by the reaction between total chlorine and potassium iodide is directly proportional to the concentra-tion of total chlorine in the sample.
  20. 20.  Can be used as disinfectant  Chloramines: effective vs. bacteria but NOT viruses.  Chloramines not powerful enough to form THMs.  Last a lot longer in the mains than free chlorine, 20 CHLORAMINES
  21. 21. Activated carbon  C + 2Cl2 + 2H2O → 4HCl + CO2  When dissolved in water, sodium bisulfite (SBS) is formed from SMBS (Sodium metabisulfite) : Na2S2O5 + H2O → 2 NaHSO3 2NaHSO3 + 2HOCl → H2SO4 + 2HCl + Na2SO4 DECHLORINATION
  22. 22. 22 Dechlorination Remember: You’re measuring the absence of chlorine, not chlorine. chlorine sensor dechlorination chemicals chlorinated water chlorinated water sample (for calibrating and checking sensor response) dechlorinated water sample
  23. 23. Installation Linear velocity in basin is typically not high enough for a good measurement. Basins and tanks: Rarely is there enough flow through a tank to provide adequate stirring. better sample point Best practice is to use an extracted sample and the FCL arrangement for flow control
  24. 24.  FLOW  Loss of flow  Air bubbles  Membrane fouling FACTOR AFFECTING
  25. 25. Maintenance Current depends on diffusion rate through the membrane. chlorine membrane cathode clean surface fouled surface A dirty or fouled membrane blocks the passage of chlorine. The result is low readings and sluggish response to concentration changes.
  26. 26. 26 Sample flow  Diffusion rate (ie sensor current) depends on the concentration of chlorine at the surface of the membrane.  As the sensor operates, it depletes the chlorine at the membrane surface.  Diffusion from bulk liquid is not adequate to replace loss, so current drops. cathode membrane sample cathode membrane sample cathode membrane sample NO FLOW
  27. 27. 27 Sample flow (cont’d)…  Flowing sample keeps the concentration of chlorine at the surface equal to the bulk concentration.  Sensor current is proportional to the concentration of chlorine in the bulk solution. cathode membrane sample FLOW cathode membrane sample
  28. 28. 28 Sample flow is important  Normally, sensor current is a non-linear function of flow.  There is a minimum flow above which the sensor current is more or less independent of flow.  Each type of sensor has an optimum flow. sensor current flow minimum flow rate
  29. 29. 29 Automatic cleaning systems wastewater aeration basin oxygen sensor sensor sensor cable spray nozzle membrane water or air line normal operation spray cleaner activated recovery period normal operation Commonly used with oxygen sensors.
  30. 30. 30 3 ELECTRODE
  32. 32. 33 Drinking water filter plant T OZ ozone generator screens raw water flocculant ozone destruct T ozone contact chamber settling basin backwash pump to screens backwash reclaim pond NaOH NaOCl pHCl T PC mixer contact tank T
  33. 33. 34 Distribution system filter plant NaOCl Cl Cl
  34. 34. 35 Groundwater source NaOCl Cl Cl well
  35. 35. 36 Desalination/wastewater reclamation plant NaOCl media filter T C C C Cl RO modules calcite filter dCl NaHSO3 membrane filter pH Cl T reject *SCS921A **FCL * ** PC
  36. 36. 37 Waste treatment plant raw waste grit chamber primary sedimentation tank screens to sludge system to sludge system aeration basin secondary sedimentation tank dechlorination tank blower NaOCl activated sludge to sludge system to aeration basin chlorine contact chamber DODO CldCl T Cl typically total chlorine