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