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Chloramination of Drinking Water in Scotland Nic Booker – Clean Water Process Technical Lead

Scottish Water

Contents

• What is chloramination

• Pros and Cons of chloramination

• How to chloraminate water

• Operating Guidance

Chloramination • Chloramination is used to:

– Limit disinfection by-product formation

– Maintain a disinfectant residual through long distribution systems.

• Chloramination is achieved by reacting free chlorine with ammonia.

• Chloramination is used at about 40 WTWs across Scotland – About 6 more chloramination schemes to be delivered in SR15

– About 30% of the population receives chloraminated water

• The process requires careful control at the treatment works.

• Safeguards are necessary to ensure the Total Ammonia PCV is not exceeded.

Advantages of Chloramination

• Less Reactive than Chlorine – Chloramines do not react as readily as free chlorine with

organics in the water – Much reduced risk of forming chlorinated disinfection by-

products ( eg THMs). • More Stable

– Chloramines are more stable and longer lasting than free chlorine

– Can limit bacterial regrowth in long distribution systems. – Potentially eliminates the need for secondary disinfection

• Fewer taste and odour issues – Potentially fewer customer complaints about “chlorine” or

“swimming pool” tastes and odours.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180

Tota

l TH

M C

on

cen

trat

ion

(m

g/l

)

Time (hours)

Chlorination Rep 1

Chlorination Rep 2

Chloram 30 mins

Chloram 60 mins

Source: Dr Christine Murray, SW , 2016

Advantages of Chloramination Filtered water THM formation rate test

Disadvantages of Chloramination

• Although it limits THM formation in distribution

– Masks potential problems with organics removal

– Nitrogenated DBP formation (eg NDMA) – If poorly controlled can lead to taste and odour

issues • Network management issues

– Potential for nitrification – Customer notification

• Dialysis patients • Aquarium owners

Scottish Water Fact Sheet 6

Provides some general information for:

• Why SW uses chloramination

• Kidney dialysis patients

• Aquarium owners

How to Chloraminate Water

• Chloramines are formed by the reaction of ammonia with free chlorine (i.e. HOCl)

NH3 + HOCl NH2Cl + H2O (mono-chloramine)

NH2Cl + HOCl NHCl2 + H2O (di-chloramine)

NHCl2 + HOCl NCl3 + H2O (nitrogen trichloride)

• The rate of these reactions is pH dependant

How to Chloraminate Water

• The aim is to form only monochloramine (NH2Cl)

– It is the most stable chloramine

– It is the least reactive chloramine

– Less disinfection by-product (eg THM) formation

– No taste and odour issues.

• Achieved by maintaining:

– The correct ratio of chlorine to ammonia

– pH in the range 7.5 to 8.5

How to Chloraminate Water • At most Scottish Water WTWs the chlorine is added as sodium

hypochlorite and the ammonia as ammonium sulphate

• Ammonia dosing should be applied after disinfection is complete

• Ammonia dose control is feed-forward

– Ammonia dose is calculated based on the measured free chlorine concentration in the disinfected water

– Dosed to maintain the required Cl2:NH4-N ratio

– SW target is currently 4.5:1

– Aiming for zero free chlorine and ammonia in chloraminated water

• pH correction is ideally achieved before ammonia dosing

– pH set-point typically between 7.5 and 8.5

Typical Chloramination Components (in blue)

Filtered Water

Sodium Hypochlorite Dosing (D/S)

CHLORINE CONTACTF

F1

S1 Cl2 (free)(D/S)

CHLORINATEDWATER

Alkali Dosing (D/S)

S2 Cl2 (free)(D/S)

S3

Ammonium Sulphate Dosing (D/S)

To DistributionNetwork

pH(D/S)

CWT

S4Cl2 (free)Cl2 (total)Ammonia

Mixer

S5

Cl2 (free)Cl2 (total)

FINAL WATER

WATERinto

SUPPLY

Mixer Mixer

Ammonium sulphate dose look-up table

3 3.5 4 4.5 5 5.5 6 6.5 7

(ppm)

0.5 0.79 0.67 0.59 0.52 0.47 0.43 0.39 0.36 0.34

0.6 0.94 0.81 0.71 0.63 0.57 0.51 0.47 0.44 0.40

0.7 1.10 0.94 0.83 0.73 0.66 0.60 0.55 0.51 0.47

0.8 1.26 1.08 0.94 0.84 0.75 0.69 0.63 0.58 0.54

0.9 1.41 1.21 1.06 0.94 0.85 0.77 0.71 0.65 0.61

1 1.57 1.35 1.18 1.05 0.94 0.86 0.79 0.73 0.67

1.1 1.73 1.48 1.30 1.15 1.04 0.94 0.86 0.80 0.74

1.2 1.89 1.62 1.41 1.26 1.13 1.03 0.94 0.87 0.81

1.3 2.04 1.75 1.53 1.36 1.23 1.11 1.02 0.94 0.88

1.4 2.20 1.89 1.65 1.47 1.32 1.20 1.10 1.02 0.94

1.5 2.36 2.02 1.77 1.57 1.41 1.29 1.18 1.09 1.01

Calculated ammonium sulphate dose rate (mg/l as ammonium sulphate)

Disinfected

Water Free

Chlorine

Required Chlorine to NH4-N Ratio

Ammonium Sulphate Dose (mg/l)

Potential to exceed total ammonium PCV (PCV = 0.5 mg/l NH4)

3 3.5 4 4.5 5 5.5 6 6.5 7

(ppm)

0.5 0.21 0.18 0.16 0.14 0.13 0.12 0.11 0.10 0.09

0.6 0.26 0.22 0.19 0.17 0.15 0.14 0.13 0.12 0.11

0.7 0.30 0.26 0.23 0.20 0.18 0.16 0.15 0.14 0.13

0.8 0.34 0.29 0.26 0.23 0.21 0.19 0.17 0.16 0.15

0.9 0.39 0.33 0.29 0.26 0.23 0.21 0.19 0.18 0.17

1 0.43 0.37 0.32 0.29 0.26 0.23 0.21 0.20 0.18

1.1 0.47 0.40 0.35 0.31 0.28 0.26 0.24 0.22 0.20

1.2 0.51 0.44 0.39 0.34 0.31 0.28 0.26 0.24 0.22

1.3 0.56 0.48 0.42 0.37 0.33 0.30 0.28 0.26 0.24

1.4 0.60 0.51 0.45 0.40 0.36 0.33 0.30 0.28 0.26

1.5 0.64 0.55 0.48 0.43 0.39 0.35 0.32 0.30 0.28

Total Ammonium Concentration (mg/l NH4)

Total ammonium concentration (mg/l as NH4)

Disinfected

Water Free

Chlorine

Required Chlorine to NH4-N Ratio

Operating Guidance Commissioning

• Notification of public and local health boards. – SW Spec requires 12 months notification

• Network Preparation – Flushing/swabbing

– WTW pH adjustment

• Operator training and familiarisation

• Management of chloraminated water flows through the network

• Network sampling and monitoring

Operating Guidance Health and Safety

• Ammonium sulphate solutions do not present any significant health and safety risks

– Review Material Safety Data Sheets and COSSH data.

– Handle in accordance with good industrial hygiene and safety practice.

– Wear appropriate personal protective equipment when handling,

• Do not store ammonium sulphate in the same area as sodium hypochlorite solutions.

Operating Guidance Environmental Issues

• Similar to chlorine, chloramines are toxic to fish and other aquatic life: – The toxicity of ammonia is primarily attributed to its un-ionized form

(NH3)

– As temperature and pH increase, the toxicity of ammonia increases

– Maintaining total Ammonia concentration <0.5mg/l limits environmental risk

• It is important to dechloraminate and control the rate of environmental discharges. – Dechlorination with thiosulphite will leave an ammonia residual

Operating Guidance Contingency Planning

• Nitrification and/or Taste and Odour complaints

– Adjustment of the chlorine to ammonia ratio

– Shock dosing of chlorine through the network

– Flushing and scouring,

• Emergency Tankering

• Ammonia is dosed before disinfection is complete

• Incorrect chlorine monitoring point used for control

• Ammonium sulphate batch strength not known

• Fixed dose of ammonia applied

• Single dosing pumps

• Lack of monitoring and control

• No alarms or telemetry connections

• Poor operator understanding of chloramination and/or control systems

Operating Guidance Common problems and issues