Transcript Chlorination

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
Protozoa
• Cryptosporidium
• Infectious Hepatisis ( viral infection )
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
Variable w /
temperature
Free available chlorine = [HOCl] + [OCl-]
• 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 trichloramine
4 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+ 2ClI2 + starch blue color
• Blue color  shows the presence of free
chlorine.