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 Chlorination forms THMs
Alternative disinfectants : Chlorine dioxide Ozone Emergency chlorination w/hypochlorites (1850) Continuous chlorination of public water supplies 1904 ( Calcium Hypochloride)
Calcium Hypochloride instable during storage limited usage Development of gaseous chlorine feeding facilities increased use Continual decline of waterborne disease
Current increase in waterborne diseases: Giardiasis Cryptosporidium Infectious Hepatisis ( viral infection ) Protozoa
Chlorine Chemistry Chlorine compound used in disinfection Chlorine gas Cl 2 Calcium Hypochlorite Ca(OCl) 2 Sodium hypochlorite NaOCl Chlorine dioxide ClO 2 (Cl - is not a disinfectant) For small applications
Cl 2 when applied to water forms hypochlorous acid and hydrochloric acid Cl 2 + H 2 O ↔ HOCl + H + +Cl - (1) Stability constant for this rxn K = [HOCl][H + ] [Cl - ] / [Cl 2 ] = 4.5*10 -4 @25 ° C
Ionization : HOCl ↔ H + + OCl - (2) K = [H + ] [OCl - ] / [HOCl] = 2.9* 10 -8 @ 25 ° C Free available chlorine = [HOCl] + [OCl - ] Variable w / temperature
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
Percentage distribution of HOCl and OCl - : [HOCl]/ ([HOCl] + [OCl - ] = 1 / ( 1+ ([OCl - ] / [HOCl] )) = 1 / (1 + (Ki/ [H + ] Hypochlorite salts : Ca(OCl) 2 + 2H 2 O ↔ 2HOCl + Ca(OH) 2 NaOCl + H 2 O ↔ HOCl+NaOH
Rxn(1) is dominated by Cl 2. Obnoxious comp. NCl 3 may form requires high quality water For Chlorinator feed water use high quality water To avoid localized low pH flash mixing
Above pH 4 equilibrium (1) shifts to right. Cl 2 decrease pH Hypochlorites increase pH Rxns. with impurities in water: Cl 2 and HOCl react with ammonia and humic material.
Rxns with ammonia : Ammonium ion is in equilibrium with ammonia and hydrogen ion. NH 4 + ↔ NH 3 + H + NH 3 react with Cl 2 or HOCl (hypochlorous acid) Rxns are dependent on pH, temperature, contact time, and Cl 2 / NH 3 ratio
Dominant Species : Monochloramine (NH 2 Cl) and Dichloramine (NHCl 2 ) combined available chlorine Chlorine readily reacts with reducing agents. Fe 2+, Mn 2+, H 2 S, organic matter : Chlorine is reduced to Cl. H 2 S + Cl 2 2HCl + S
These substances increase chlorine demand. Cl 2 + Phenols Produce mono-, di-, Trichlorophenols produce taste, odor Cl 2 also reacts with other halogens Br - + HOCl HOBr + Cl - HOBr : Hypobromous acid
Cl 2 and HOBr reacts with humic substance Halogenated organics. THMs Suspected human carcinogens. Maximum contaminant level 100 µg/L 80 µg/L Alternative disinfectants ?
Cl 2 is the only disinfectant producing protective residual within the distribution systems. Factors important in disinfection : Time to contact Concentration Kill α C n * t
Generalized curve obtained during breakpoint chlorination
Cl 2 / NH 3 ratio 1:1 for the formation of mono, dichloroamines. Further increase in mole ratio trichloramine, oxidation of part of ammonia to N 2 or NO 3-. These rxns. are completed at mole ratio 1.5:1 Chloramine residuals maximum @1:1mol Then decline to a minimum till 1,5:1
Breakpoint Chlorination Chlorination of a water to the extent that all the ammonia is converted to N 2 or higher oxidation states. Theoretically 3 mole chlorine conversion to trichloramine 4 mole chlorine complete oxidation to nitrate 2NH 3 +3Cl 2 N 2 +6H + + 6Cl -
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.
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
Starch – Iodide Method : Oxidizing power of free and combined chlorine to convert iodide to iodine. Cl 2 +2I - I 2 + 2Cl - I 2 + starch blue color Blue color shows the presence of free chlorine.
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