1. Water Treatment Hypochlorite as a disinfectant for drinking water
Professor Nick Gray
Centre for the Environment
Trinity College
University of Dublin
© Tigroney Press

2. Water Treatment: Hypochlorite disinfection
Use of hypochlorite
Hypochlorite first form of disinfectant used with drinking water
Calcium hypochlorite (Ca(OCl)2) known as chloride of lime
Still used in tablet form or granules at smaller plants.
Sodium hypochlorite (NaOCl) familiar as bleach is also used as a liquid at smaller plants also generated on site at larger plants
Used in tablet form for household point of entry systems and distribution emergencies / backpacking and travelling

3. Water Treatment: Hypochlorite disinfection
Calcium hypochlorite
Powder contains 30-35% w/w of chlorine
Granules contain 65-70% w/w of chlorine
Transported and stored in sealed drums lined with rubber/plastic
Used by mixing known weight of solid to liquid in mixing tank…reacts to leave a concentrated chlorine solution as supernatant.
This added to finished water
3kg powder per 1000m3 of water gives a finished concentration of 1.0 mg Cl2 L-1.
1.5kg granules per 1000m3 of water gives a finished concentration of 1.0 mg Cl2 L-1.
Once a liquid decays rapidly (especially in sunlight) needs to be used within 24-48h and decays into chlorite (ClO2-) and chlorate (ClO3-) ions
As sodium hypochlorite ages it decomposes into chlorite and chlorate…leading to contamination potential when used…decomposition much slower in calcium hypochlorite . Where chlorine dioxide used then this decomposers rapidly in treated water into these two species the rate of conversion being higher in alkaline conditions.
Effects: Chlorite/chlorate- not carcinogenic but potential to cause anaemia and other changes with red blood cells, especially in very young..similar to methaglobinaemia and also neurological damage possible.

4. Water Treatment: Hypochlorite disinfection
Sodium hypochlorite
Used as liquid (easier to handle and allows more precise dosing)
Injected directly into water stream
Contains 14-15% chlorine so large volumes required making it more expensive to transport so overall more expensive than calcium hypochlorite
More stable, decaying at slower rate to form oxygen and chlorate (ClO3-). However, as decays releases H+ ions that increase pH accelerating rate of decay
Also decays into chlorite (ClO2-) and chlorate (ClO3-) ions as it ages before use which is a potential source of contamination of these chlorine species
Chlorite/chlorate - non carcinogenic - potential to cause anaemia and other changes with red blood cells, especially in very young and also neurological damage possible.

5. Water Treatment: Hypochlorite disinfection
Hypochlorite solutions are very alkaline (pH 11-12) which ensures the free chlorine is stored as the hypochlorite ion (OCl-).
These solutions are corrosive but relatively safe to handle, but if the pH is lowered by accidently adding an acidic chemical then the hypochlorite ion is converted to hypochlorous acid (HOCl) which can then be converted to chlorine gas :
OCl- + H+ → HOCl
H+ + HOCl + Cl- → Cl2 + H2O
In order to prevent the formation of chlorite and chlorate ions, care must be taken both in the preparation and storage of the hypochlorite solution, and also in the storage and containment of the hypochlorite parent compounds.
For this reason, hypochlorite should only be used by very experienced operators.

6. Water Treatment: Hypochlorite disinfection
Electrolysis of brine
Hypochlorite can be generated on-site for the disinfection of good quality upland water resources where only low doses of chlorine are required.
The main advantages are that it can be generated on-site without any of the transportation, storage and handling problems associated with chlorine gas, and is also cheaper.
Principle:
A solution of sodium chloride (brine solution) has a direct current passed through it generating chlorine at the anode and both hydrogen and hydroxide ions at the cathode
On mixing the remaining sodium ions react with chlorine and hydroxide ions to form sodium hypochlorite (Na+OCl-)

7. Water Treatment: Hypochlorite disinfection
Anode reaction
2Cl- - 2e → Cl2
Cathode reaction
2H2O + 2e → 2OH- + H2
Mixing
2Na+ + 2OH- + Cl2 → Na+OCl- + H2O

8. Water Treatment: Hypochlorite disinfection

9. Water Treatment: Hypochlorite disinfection
Process:
Saturated brine solution (358 g L-1) is made in a salt saturator by passing softened water through high purity salt (99.7% NaCl). The brine is then diluted to g L-1 to form the process water.
The water used for saturation and dilution must be as free from calcium and magnesium ions as possible with a hardness <35 mg CaCO3 L-1 and so may require to be softened before use.
A low-voltage direct current (≈40V) is passed through the process water in the electrolyser to produce a hypochlorite solution and hydrogen.
The temperature inside the electrolyser is raised by 20oC but must not be allowed to exceed 40oC overall to prevent chlorate formation.

10. Water Treatment: Hypochlorite disinfection
For every litres of process water that passes through the electrolyser 1kg of chlorine as hypochlorite is produced at a concentration of 6-9 g Cl2 L-1.
This is equivalent to 3.5 kg sodium chloride per kg of chlorine produced, with a conversion efficiency of 50% and at an energy usage of between 4 to 5 kWh per kg Cl2 produced.
The hydrogen gas is released from the hypochlorite solution very quickly and is forced vented from the headspace to the atmosphere.
The hypochlorite solution is then injected into the finished water using a positive displacement metering pump.
Unused sodium chloride remains in the water increasing the sodium concentration and also, in soft waters, its corrosivity.
Approximately 1.38 mg of sodium and 2.12 mg of chloride are released per 1.0 mg of chlorine produced which ends up in the finished water.

11. Water Treatment: Hypochlorite disinfection
Concentrated brine holding tanks
Electrolyser

12. Water Treatment: Hypochlorite disinfection
Problems:
Potential for byproduct formation during electrolysis, primarily chlorate (ClO3-) but also bromates.
Modern brine electrolysis is producing <0.1 mg of ClO3- per mg of chlorine produced, but with maximum limits in drinking water set provisionally by the WHO (2004) at 0.7 mg L-1 (i.e mg L-1 in the UK and 1.0 mg L-1 in the USA), the chlorate concentration in the finished water ultimately controls the upper dosage rate of hypochlorite that can be used.
Bromate formation is controlled by the use of high purity salt containing less than 0.015%w/w bromide, which results in concentrations of bromide of <1.0µg per 1.0 mg of chlorine produced. Bromate is a carcinogen. (EU limit 10 µg L-1)

13. Water Treatment: Hypochlorite disinfection
Chlorine tablets
Used for emergency use at plants also for backpacking
Large calcium chloride tablets contain 10g chlorine (added to distribution mains, storage tanks etc.)
Chlorine released over several hours but at an uncontrolled rate making them unsuitable for permanent disinfection operation
Larger tablets must be carefully stored as spontaneously combust at 175oC
This temperature will be achieved if calcium hypochlorite comes into contact with organic material (cardboard, paper, grease etc.)
Backpackers use 2.5 mg tablets with 1 tablet required per litre.

15. References:
Percival, S.L., Yates, M. Williams, D.W., Chalmers, R.M. and Gray, N.F Microbiology of Waterborne Diseases: Microbiological Aspects and Risks (2nd edn.) Academic Press, London. pp.695.