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

Transcription

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

2 Contents What is chloramination Pros and Cons of chloramination How to chloraminate water Operating Guidance

3 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.

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5 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 byproducts ( 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.

6 Total THM Concentration (mg/l) Advantages of Chloramination Filtered water THM formation rate test Chlorination Rep 1 Chlorination Rep 2 Chloram 30 mins Chloram 60 mins Time (hours) Source: Dr Christine Murray, SW, 2016

7 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

8 Scottish Water Fact Sheet 6 Provides some general information for: Why SW uses chloramination Kidney dialysis patients Aquarium owners

9 How to Chloraminate Water Chloramines are formed by the reaction of ammonia with free chlorine (i.e. HOCl) NH 3 + HOCl NH 2 Cl + H 2 O (mono-chloramine) NH 2 Cl + HOCl NHCl 2 + H 2 O (di-chloramine) NHCl 2 + HOCl NCl 3 + H 2 O (nitrogen trichloride) The rate of these reactions is ph dependant

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11 How to Chloraminate Water The aim is to form only monochloramine (NH 2 Cl) 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

12 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 Cl 2 :NH 4 -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

13 Typical Chloramination Components (in blue) Alkali Dosing (D/S) Filtered Water F1 F Mixer CHLORINATED WATER S1 Cl 2 (free) (D/S) CHLORINE CONTACT S2 Mixer Cl 2 (free) (D/S) S3 ph (D/S) Mixer S4 Cl 2 (free) Cl 2 (total) Ammonia FINAL WATER CWT Sodium Hypochlorite Dosing (D/S) Ammonium Sulphate Dosing (D/S) WATER into SUPPLY To Distribution Network S5 Cl 2 (free) Cl 2 (total)

14 Ammonium sulphate dose look-up table Disinfected Water Free Chlorine (ppm) Calculated ammonium sulphate dose rate (mg/l as ammonium sulphate) Required Chlorine to NH 4 -N Ratio Ammonium Sulphate Dose (mg/l)

15 Potential to exceed total ammonium PCV (PCV = 0.5 mg/l NH 4 ) Disinfected Water Free Chlorine (ppm) Total ammonium concentration (mg/l as NH 4 ) Required Chlorine to NH 4 -N Ratio Total Ammonium Concentration (mg/l NH 4 )

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17 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

18 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.

19 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 (NH 3 ) 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

20 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

21 Operating Guidance Common problems and issues 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