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Disinfection• Objective
to understand the principles of chlorination, and the factors that influence its efficiency in the disinfection of water.
• Literature
Chemistry for Environmental Engineering - Sawyer et al
Water Supply - Twort et al
Water and Wastewater Engineering - Fair et al
Handbook of Chlorination - White
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DISINFECTION
“The removal of Pathogenic micro-organisms from Water”(-not necessarily removal of ALL micro-organisms)
• AIM: SAFE drinking wateri.e. < 1 Coliform/100 ml
• Standards: WHO Guidelines EEC Drinking Water Directives UK Water Regulations
PHYSICAL
(1)Boiling - Household use, temporary, expensive, emergency measure.
- Kills bacterial, viruses + other microorganisms.
(2)U-V light - effective for bacteria + viruses if Turbidity is low– (a) Simple storage in glass containers– (b) Tubular, jacketed, u-v lamps– (c) Impounding and storage Reservoirs
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CHEMICAL METHODS
Mostly Oxidising Agents
Large Scale: Chlorine(Municipal W.S.) Sodium / Calcium hypochlorite
ChloramineChlorine dioxideOzone
Small Scale: SilverIodinePotassium permanganateChlorine compounds
Used impregnated in ceramic filters or as tabletsFor household use, camping etc.
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Chlorination
(1) Free ChlorineChlorine Gas i.e. Cl2 + Pure water
(a) Hydrolysis Cl2 + H2O HOCl + HCl
(b) Ionisation HOCl H+ + OCl- (Free Chlorine Residuals)Hypochlorous Hypochlorite
Acid Ion
Form of Free Chlorine depends on pH
0
10
20
30
40
50
60
70
80
90
100
4 5 6 7 8 9 10
pH
% H
OC
l
0
10
20
30
40
50
60
70
80
90
100
% O
CL-
StrongDisinfectant
WeakDisinf.
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Chlorine Demand
Chlorine added to water is not necessarily available for disinfection.
Lowland surface waters – chlorine demand of 6 - 8 mg/l
• Chlorine Reacts with:– Ammonia
• breakpoint chlorination
– Organic Matter• Dissolved, colour• particulate
– Metal ions• pipe materials• from source water
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(2) Combined Chlorine
Cl2 + NH3 (1 - 50 PPM)
Sequential substitution of H in NH3
NH3
NH2 Cl (Monochloramine)
NHCl2 (Dichloramine)
NCl3 (Nitrogen trichloride) (Trichloramine)
Low pH NHCl2 and NCl3 become more High Cl:NH3 ratio abundant NHCl2 Good disinfectant but nasty to taste in water.NCl3 is particularly offensive
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High Cl:NH3 ratios also give increased rate of breakdown reactions
Wt. ratio Cl:NH3
< 5:1 HOCl + NH3 NH2Cl + H2O
< 10:1 HOCl + NH2Cl NHCl2 + H2O
> 10:1 HOCl + NHCl2 NCl3 + H2O
Ultimately: 2 NH3 + 3 Cl2 N2 + 6 HCl
Mole ratio 2 : 3 gives complete oxidation = Breakpointie. Wt. ratio 1 : 7.6 gives complete oxidation = Breakpoint
Other products of oxidation include:
- NO3- (Nitrate ion)
- Organo- chloramines (protein amino groups)
If NH3 concentration in water (including organic nitrogen) is knowncan calculate amount HOCL required for “breakpoint”
Theoretically Chlorine requirement = Wt. NH3-N x 7.6in practice (Margin of safety) = Wt. NH3-N x 10
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6
5
4
3
2
1 0 1 2 3 4 5 6 7 8 chlorine dose (mg/l)
chlo
rine
res
idua
l (m
g/l)
Breakpoint Chlorination
pH 7.0 30 min contact time0.5 mg/l ammonia
MarginalChlorination
BreakpointChlorination
Superchlorination(+ Dechlorination)
NH2Cl
BreakpointCl2
Total
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Chlorination Practice
Combined Residual(a) Simple, Marginal chlorination
Suitable for Upland waters
(b) Ammonia-chlorine treatment. (Add NH3, then HOCl)Suitable for groundwatersEnsures combined residuals in distribution.
Free Residual(a) Breakpoint chlorination
Suitable for Lowland surface waters.
(b) Superchlorination + Dechlorination (SO2, S2O32- or Act. Carbon. )
• For industrially polluted surface watersdestroys tastes + odours + colour
• Short contact time or pollution load variable (wells).
Desirable to have chlorine Residual in the Distribution System (in U.K.)
Combined chlorine preferable. Most persistent.
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Chlorine also reacts with H2S, Fe(II), Mn(II) (groundwaters or hypolimnetic water
H2S + 4 Cl2 + 4 H2O H2SO4 + 8 HCl
H2S + Cl2 S + 2HCl
2Fe(HCO3)2 + Cl2 + Ca(HCO3)2
2Fe(OH)3 (s) + CaCl2 + 6 CO2
(associated pH rise. Useful for: iron removal; coagulant production.)
MnSO4 + Cl2 + 4 NaOH MnO2 (s) + 2 NaCl + Na2SO4 + 2 H2O
(precipitate takes 2-4 hours to form, longer for complex Mn ions)
Where H2S, Mn or Fe present:previous practice used PRECHLORINATION + FILTRATIONBut T.H.M. problems, therefore now discouraged.
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Disinfection Problems
(1) pH influences effectiveness
(2) THM formation (CARCINOGEN)1 ug/l MAC (EC) and 100 ug/l MCL (USEPA) ug/l = ppb
Therefore Chlorination practice now modified
- Discourage PRECHLORINATION
- Aim to remove THM PRECURSORSusing O3 + GAC/PACbefore final chlorination
Alternative Strategy: replace Cl2 by other oxidants
or remove micro-organisms by more efficient clarification.
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Taste and Odour
(1) From Chlorine Residuals
Acceptable maximum levels of Chlorine and Chloramines
Residual Max Level (mg/l)
Free Chlorine 20
Monochloramine 5
Dichloramine 0.8
Nitrogen Trichloride 0.02
(2) From Chlorinated Organics
Chlorophenols
(3) From Natural Products
Fungal and algal metabolites
acceptable thresholdswill be lower for high purity water
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Superchlorination and Dechlorination
Where contact time must be short or pollution loads very variable.(Free chlorine levels so high, have to be removed before supply.)
SuperchlorinationAdvantages: Complete oxidation NH3
Correction of Tastes and OdoursRemoval 20-50% colourShort Contact Time
Disadvantage: THM
Dechlorination(a) Reducing chemicals e.g.
SO2 (Large supplies) or Na thiosulphate (Small supplies)
SO2 + Cl2 + 2H2O H2SO4 + 2 HCl
(b) Activated Carbon. As granular from (GAC), high rate filtration,
or As powder (PAC), added then removed by rapid sand filtration.
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Operational Factors Affecting Chlorination Practice
• Form of Chlorine– Storage and decomposition
• Mixing Efficiency– baffled mixing chambers
• Temperature– slower at low temps– seasonal variation significant
• pH• Concentration • Time