disinfection theory

1. 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

2. DISINFECTION
“The removal of Pathogenic micro-organisms from Water”
(-not necessarily removal of ALL micro-organisms)
AIM: to produce SAFE drinking water
i.e. < 1 Coliform/100 ml
Standards: 1984 & 1993 WHO Guidelines
1980 EEC Drinking Water Directives
1989 UK Water Regulations
Treated water ENTERING Distribution system must conform
Treated water IN distribution system should have
< 3 coliforms /100 ml
but NEVER any E.coli /100 ml
Therefore must maintain RESIDUAL disinfectant in
Distribution System to control growth or contaminant bacteria.

3. Disinfection Methods
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 - effective but not very practical
(b) Tubular, jacketed, u-v lamps
- Need power supply
- Used in operating theatres + isolated communities.
(c) Impounding and storage Reservoirs

4. CHEMICAL METHODS
Mostly Oxidising Agents
Large Scale: Chlorine
(Municipal W.S.) Sodium / Calcium hypochlorite
Chloramine
Chlorine dioxide
Ozone
Small Scale: Silver
Iodine
Potassium permanganate
Chlorine compounds
Used impregnated in ceramic filters or as tablets
For household use, camping etc.

5. Chlorination
(1) Free Chlorine
Chlorine 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
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0
1 0 0
4 5 6 7 8 9 1 0
pH
%
HOCl
0
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0
1 0 0
%
OCL
-
Strong
Disinfectant
Weak
Disinf.

6. 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

7. (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

8. 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 = Breakpoint
ie. 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 known
can calculate amount HOCL required for “breakpoint”
Theoretically Chlorine requirement = Wt. NH3-N x 7.6
in practice (Margin of safety) = Wt. NH3-N x 10

9. 6
5
4
3
2
1 0 1 2 3 4 5 6 7 8
chlorine dose (mg/l)
chlorine
residual
(mg/l)
Breakpoint Chlorination
pH 7.0
30 min contact time
0.5 mg/l ammonia
Marginal
Chlorination
Breakpoint
Chlorination
Superchlorination
(+ Dechlorination)
NH2Cl
Breakpoint
Cl2
Total

10. Chlorination Practice
Combined Residual
(a) Simple, Marginal chlorination
Suitable for Upland waters
(b) Ammonia-chlorine treatment. (Add NH3, then HOCl)
Suitable for groundwaters
Ensures combined residuals in distribution.
Free Residual
(a) Breakpoint chlorination
Suitable for Lowland surface waters.
(b) Superchlorination + Dechlorination (SO2, S2O3
2- or Act. Carbon. )
• For industrially polluted surface waters
destroys 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.

11. 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 + FILTRATION
But T.H.M. problems, therefore now discouraged.

12. 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 PRECURSORS
using O3 + GAC/PAC
before final chlorination
Alternative Strategy: replace Cl2 by other oxidants
or remove micro-organisms by more efficient clarification.

13. 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 thresholds
will be lower for high purity
water

14. 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

15. Kinetics of Disinfection
Ideally: All cells equally mixed with disinfectant
All cells equally susceptible to disinfectant.
Disinfectant concentration unchanged in contact tank.
No interfering substances present
Then: Disinfection is a function of:
(1) Time of Contact
(2) Concentration of Disinfectant
(3) Temperature of Water

16. (1) Time of Contact
Chicks Law “The number of organisms destroyed in unit time is
proportional to the number remaining
Rate of Kill where:
k = the reaction rate constant
N = number of viable organisms
Integrate, gives: where:
N0 = number of organisms at time = 0
Nt = number of organisms at time = t
K = Death Rate Constant
i.e. rate of disinfection is Logarithmic
kN
dt
dN


Kt
N
Nt








0
ln
time
ln
(n/no)
straight line

17. Minimum Bactericidal Chlorine Residuals
Based on Coliform Removal at 20-25oC
pH Minimum
Free Chlorine
(mg/l)
after 10 min
Minimum
Combined Chlorine
(mg/l)
after 60 min
6.0 0.2 1.0
7.0 0.2 1.5
8.0 0.4 1.8
9.0 0.8 > 3.0
10.0 0.8 > 3.0
For Virus and Protozoan Cyst disinfection,
greater residuals required.
 Taste problems

18. USAAlternative Strategy:
Aim for oxidative disinfection of
Coxsackie virus A2
Use ‘K’ values in CT = K relationship for design of disinfection process.
‘CT’ i.e. ‘K’ values under different conditions are:
pH 0-5oC 10oC
7-7.5 12 8 best kill
higher temp
8 20 15 neutral pH
8.5 30 20
9 35 22
Poorest kill
low temp
high pH

19. Practical Disinfection
Can Control: Type of disinfectant
Concentration of disinfectant
Time of contact (pref. 10 - 60 min)
o Mixing
pH.
Cannot Control: Temperature
Organics / NH3 / Interfering subs.
Chlorine demand
measure Free Available Chlorine after set period of time
Primary requirements: Adequate contact time before
distribution
Adequate mixing / turbulence
(Difficult to achieve, especially
in small systems)

20. Summary:
Factors which influence disinfection:
(1)Number and nature of pathogens
(2)Type and Concentration of Disinfectant.
(3)Temperature (High temps. Increase kill rate)
(4)Contact Time (Longer contact  better kill)
(5)Presence of organic particulates, H2S, Reduced Fe + Mn 
“Chlorine demand”
(6)pH
(7)Mixing
(8)NH3  “Chlorine demand”