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 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

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.

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.

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 NHCl2 and NCl3 become more High Cl:NH3 ratio 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

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

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

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.

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.

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.

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

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.

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