ENVE 201Environmental 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 dioxideOzone

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 Cl2

• Calcium Hypochlorite Ca(OCl)2

• Sodium hypochlorite NaOCl• Chlorine dioxide ClO2

(Cl- is not a disinfectant)

For small applications

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

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 + 2H2O ↔ 2HOCl + Ca(OH)2

NaOCl + H2O ↔ HOCl+NaOH

• 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

• Above pH 4 equilibrium (1) shifts to right.

• Cl2 decrease pH

• Hypochlorites increase pH

Rxns. with impurities in water:

• Cl2 and HOCl react with ammonia and humic material.

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

Dominant Species :

• Monochloramine (NH2Cl) and Dichloramine (NHCl2) combined available chlorine

• Chlorine readily reacts with reducing agents.• Fe2+ , Mn 2+ , H2S , organic matter : Chlorine is

reduced to Cl.H2S + Cl2 2HCl + S

• These substances increase chlorine demand.

Cl2 + Phenols Produce mono-, di-,Trichlorophenols produce taste , odor

• Cl2 also reacts with other halogens

Br- + HOCl HOBr + Cl-

• HOBr : Hypobromous acid

• Cl2 and HOBr reacts with humic substance

Halogenated organics. THMs Suspected human carcinogens.

• Maximum contaminant level 100 µg/L 80 µg/L

Alternative disinfectants ?

• Cl2 is the only disinfectant producing protective residual within the distribution systems.

Factors important in disinfection : Time to contact Concentration

Kill α Cn * t

Generalized curve obtained during breakpoint chlorination

Break Point Chlorination

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

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 trichloramine4 mole chlorine complete oxidation to nitrate

2NH3 +3Cl2 N2 +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.

Cl2 +2I- I2+ 2Cl-

I2 + starch blue color

• Blue color shows the presence of free chlorine.