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ENVE 201 Environmental Engineering Chemistry 1

ENVE 201 Environmental Engineering Chemistry 1. CHLORINATION Dr. Aslıhan Kerç. Chlorination. Disinfection of public water supplies and wastewater effluents . To prevent spread of water borne diseases (?) Cholara, typhoid  by contamination of drinking water with wastewater

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ENVE 201 Environmental Engineering Chemistry 1

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  1. ENVE 201EnvironmentalEngineeringChemistry 1 CHLORINATION Dr. Aslıhan Kerç

  2. Chlorination • Disinfection of publicwatersuppliesandwastewatereffluents. • To prevent spread of water borne diseases (?) Cholara, typhoid  by contamination of drinking water with wastewater ChlorinationformsTHMs

  3. Alternative disinfectants : Chlorine dioxide Ozone Emergency chlorination w/hypochlorites (1850) Continuous chlorination of public water supplies  1904 ( Calcium Hypochloride)

  4. Calcium Hypochloride instable during storage limited usage Development of gaseous chlorine feeding facilities increased use Continual decline of waterborne disease

  5. Current increase in waterborne diseases: • Giardiasis • Cryptosporidium • Infectious Hepatisis ( viral infection ) Protozoa

  6. Chlorine Chemistry Chlorine compound used in disinfection • Chlorine gas Cl2 • Calcium Hypochlorite Ca(OCl)2 • Sodium hypochlorite NaOCl • Chlorine dioxide ClO2 (Cl- is not a disinfectant) For small applications

  7. Cl2 when applied to water  forms hypochlorous acid and hydrochloric acid Cl2 + H2O ↔ HOCl + H+ +Cl- (1) Stability constant for this rxn K = [HOCl][H+] [Cl-] / [Cl2] = 4.5*10 -4 @25 ° C

  8. Ionization : HOCl ↔ H+ + OCl- (2) K = [H+] [OCl-] / [HOCl] = 2.9* 10-8 @ 25 ° C Free available chlorine = [HOCl] + [OCl-] Variable w / temperature

  9. Distribution between these species is important • Killing effiency of HOCl is 40 -80 times larger than OCl - . Lower pH favors HOCl. • HOCl = Hypochlorous acid • OCl - = Hypochloride ion

  10. Percentage distribution of HOCl and OCl - : [HOCl]/ ([HOCl] + [OCl -] = 1 / ( 1+ ([OCl -] / [HOCl] )) = 1 / (1 + (Ki/ [H+] Hypochlorite salts : Ca(OCl)2 + 2H2O ↔ 2HOCl + Ca(OH)2 NaOCl + H2O ↔ HOCl+NaOH

  11. Rxn(1) is dominated by Cl2. Obnoxious comp. NCl3 may form requires high quality water • For Chlorinator feed water use high quality water • To avoid localized low pH  flash mixing

  12. AbovepH4  equilibrium (1) shiftstoright. • Cl2decreasepH • HypochloritesincreasepH Rxns. with impurities in water: • Cl2andHOClreactwithammoniaandhumicmaterial.

  13. Rxns with ammonia : • Ammonium ion is in equilibrium with ammonia and hydrogen ion. NH4+↔ NH3 + H+ • NH3 react with Cl2 or HOCl (hypochlorous acid) • Rxns are dependent on pH , temperature , contact time , and Cl2 /NH3 ratio

  14. Dominant Species : • Monochloramine (NH2Cl) and Dichloramine (NHCl2)  combined available chlorine • Chlorinereadilyreactswithreducingagents. • Fe2+ , Mn 2+ , H2S , organic matter : Chlorine is reduced to Cl. H2S + Cl2 2HCl + S

  15. Thesesubstancesincreasechlorinedemand. Cl2 + Phenols Produce mono-, di-, Trichlorophenols  producetaste , odor • Cl2alsoreactswithotherhalogens Br- + HOCl HOBr + Cl- • HOBr : Hypobromousacid

  16. Cl2andHOBrreactswithhumicsubstance  Halogenated organics. THMs Suspected human carcinogens. • Maximum contaminant level 100 µg/L  80µg/L Alternative disinfectants ?

  17. Cl2 is the only disinfectant producing protective residual within the distribution systems. Factors important in disinfection : • Time to contact • Concentration Kill α Cn * t

  18. Generalized curve obtained during breakpoint chlorination

  19. Break Point Chlorination

  20. Break Point Chlorination • Cl2 /NH3 ratio 1:1 for the formation of mono , dichloroamines. • Further increase in mole ratio  trichloramine, oxidation of part of ammonia to N2 or NO3-. • These rxns. are completed at mole ratio 1.5:1 • Chloramine residuals maximum @1:1mol • Then decline to a minimum till 1,5:1

  21. Breakpoint Chlorination • Chlorination of a water to the extent that all the ammonia is converted to N2 or higher oxidation states. Theoretically 3 mole chlorine  conversion to trichloramine 4 mole chlorine complete oxidation to nitrate 2NH3 +3Cl2N2 +6H++ 6Cl-

  22. Breakpoint chlorination  for better disinfection, required to obtain free chlorine residual , if ammonia is present. • Method of ammonia removal in ww • Combined chlorine residuals  Longer lasting ( final treatment with ammonia ) • Chlorine demand : Amount of chlorine that must be added to reach a desired level of residual.

  23. Chlorine Residual Determination • Old Methods  total chlorine • New Methods  free and combined chlorine Total Chlorine Residual • Measurement depend on measuring the oxidizing power • Other oxidizing agents present may interfere  manganese, nitrites

  24. Starch – Iodide Method : • Oxidizing power of free and combined chlorine to convert iodide to iodine. Cl2 +2I-I2+ 2Cl- I2 + starch blue color • Blue color  shows the presence of free chlorine.

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