Chloramine Removal
Chloramines are formed by a reaction between hypochlorous acid and nitrogen based products from pool users. Chloramines can be reduced by a number of processes. Superchlorination/shock dosing, ozonation and dilution are three conventional methods of chloramine reduction.
Continuous dilution is the best way to minimise the build-up of combined chlorine, particularly the stable organic–nitrogen complexes formed from nitrogen-based compounds present in sweat and urine. The continuous maintenance of a free chlorine residual, which is at least 50 per cent (but preferably 75 per cent) of the total chlorine residual during normal pool operation, should control the accumulation of simple inorganic chloramines.
Superchlorination
Superchlorination is an industry term for a periodic maintenance procedure where the free chlorine residual is raised 2–4 times the normal operating level to prevent algae, remove colour and maintain clarity. Superchlorination also assists in reducing combined chlorine, in most circumstances. It also assists in keeping the pool water within bacteriological requirements during normal operation by periodically removing biofilms (bacterial harbourages) that resist normal chlorine levels.
– Superchlorination should be conducted when the pool is closed to bathers (for example, overnight). This will prevent the introduction of pollution that may hinder the superchlorination process.
– Superchlorination must be carried out with the pH between 7.6 and
7.8. If the pH drops below 7.5, nitrogen trichloride may be formed, which is a stable compound that causes chlorinous odours and irritates eyes.
– Under most circumstances superchlorination is achieved at chlorine levels around 6–8 ppm. This level is sufficient to remove chloramine and will return to normal operating levels by the next morning.
– It is generally recommended that superchlorination is conducted at weekly intervals. Some pools may require more frequent treatment, depending on their pollution profile.
Chloramine concentrations may also be increased if make-up water supplies contain chloramines. Periodic superchlorination is recommended in these circumstances as the best method of removing inorganic chloramines.
For pools using chlorinated isocyanurates as their regular disinfectant, superchlorination using these products may elevate cyanuric acid levels over time. Operators should consider using a hypochlorite for superchlorination purposes when cyanuric acid is at the preferred concentration.
Shock Dosing
Shock dosing is an industry term used to describe the process of superchlorination when it is specifically used to solve problems such as destroying algae blooms and treating colour and clarity problems. The chlorine dose is usually higher than that used for preventative superchlorination.
Shock dosing to 10–15 ppm, or around 5–7 times the normal free chlorine residual, may be used to help correct a serious problem, however dosing above this level would lead to excessive waste of chlorine.
Shock dosing immediately after refilling an empty pool from a town water supply that is heavily chloraminated will remove the chloramine present. In this situation, the free chlorine should not be raised by more than ten times the combined chlorine residual, otherwise nitrogen trichloride will form.
– pH should be maintained between 7.6 and 7.8 when shock dosing, for the same reasons as for superchlorination.
– Shock dosing good quality pool water will not change the water quality, and is simply a waste of chemical.
– More serious problems, such as persistent combined chlorine, can be solved by dilution through backwashing and by the introduction of fresh water.
– Consideration must always be given to the Regulations and bather comfort, as well as the levels of chemical in the water when the pool is open for use.
– If chlorine levels are too high to allow bathing even after allowing sufficient time for the process to work (for example, overnight), then dechlorination may be required prior to reopening the pool.
Dechlorination
Dechlorination can be achieved by adding sodium thiosulphate to the pool water if chlorine levels are above the regulatory limits. This will enable the operator to reduce chlorine levels and allow bathing.
Situations that require dechlorination should be avoided.
When using sodium thiosulphate, it is important to fully dissolve the crystals before adding them to the pool, as inadequate dissolution may prevent satisfactory chlorine neutralisation. Overdosing may result in a higher residual of sodium thiosulphate than required and this may prevent adequate chlorine levels being achieved. Approximately 10 g of sodium thiosulphate per 10,000 litres (10 m3) of pool water is required to lower chlorine levels by 1 ppm.
Ozone and UV
Ozone is an effective oxidant and ozonated pools have reduced need for superchlorination. Superchlorination is still periodically required to sanitise pool surfaces and prevent algal colonisation. UV light from both natural and unnatural sources has a positive benefit in chloramine reduction.
Oxygen Based Oxidisers
Oxygen based oxidisers are available, but they are difficult to control. Expert advice should be sought prior to application. They have no application in ozonated pools, as ozone works in a similar manner.
Cyanuric Acid
Cyanuric acid is a granular compound which, when dissolved in pool water, shields a percentage of chlorine from sunlight, thereby significantly reducing chlorine loss. It is an essential ingredient in outdoor pools, but has reduced benefit for indoor pools.
A slightly higher level of disinfection residual should be maintained within the pool water body because some studies have suggested that the speed of disinfection is slower when cyanuric acid is present. Oxidation is also impaired by the use of cyanuric acid as the oxidation-reduction potential is reduced, particularly in still water conditions. This can be demonstrated by measuring the oxidation potential of a chlorine solution of equal chlorine concentration and pH when cyanuric acid is absent or present.
At least 25 ppm of cyanuric acid is needed for it to work efficiently but there is no advantage increasing levels above 50 ppm except to allow for a drop-off in levels, due to backwashing and water losses on the pool deck.
Disinfectants that contain isocyanurate continue to add cyanuric acid through the swimming season, so there is no need to top up cyanuric acid levels. High levels of cyanuric acid may contribute to water cloudiness and are controlled by dilution with fresh make-up water. In these pools a start up dose of 25 ppm is recommended after refilling.
Cyanuric acid is extremely difficult to dissolve, and the gradual addition of chlorinated isocyanurate disinfectants to bring up the level may be the easiest method for some operators.
