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 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 ° C

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

Distribution between these species is important Killing effiency of HOCl is 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

Break Point 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 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.