Transcript DISINFECTION - Newcastle University

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
DISINFECTION
“The removal of Pathogenic micro-organisms from Water”
(-not necessarily removal of ALL micro-organisms)
• AIM:
SAFE drinking water
i.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
CHEMICAL METHODS
Mostly Oxidising Agents
Large Scale:
(Municipal W.S.)
Chlorine
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.
Chlorination
(1) Free Chlorine
Chlorine Gas
i.e. Cl2 + Pure water
(a) Hydrolysis
Cl2 + H2O
HOCl + HCl
(b) Ionisation
HOCl
H+ + OCl-
Hypochlorous
Acid
Hypochlorite
Ion
(Free Chlorine Residuals)
100
0
90
10
Strong
Disinfectant
80
70
30
40
50
50
40
60
-
60
Weak
Disinf.
30
20
10
70
80
90
0
100
4
5
6
7
pH
8
9
10
% OCL
% HOCl
Form of Free Chlorine
depends on pH
20
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
(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
High Cl:NH3 ratio
NHCl2 and NCl3 become more
abundant
NHCl2 Good disinfectant but nasty to taste in water.
NCl3 is particularly offensive
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
2 NH3 + 3 Cl2
N2 + 6 HCl
Ultimately:
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
chlorine residual (mg/l)
Breakpoint Chlorination
pH 7.0
30 min contact time
0.5 mg/l ammonia
Total
6
Cl2
Breakpoint
5
4
NH2Cl
3
2
1
0
1
2 3 4 5 6
chlorine dose (mg/l)
Marginal
Chlorination
Breakpoint
Chlorination
7
8
Superchlorination
(+ Dechlorination)
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, S2O32- 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.
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.
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.
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
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.)
Superchlorination
Advantages: Complete oxidation NH3
Correction of Tastes and Odours
Removal 20-50% colour
Short 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.
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