Spa Sanitisers

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Spa water can go 'off' much more quickly than swimming pool water. If you leave your spa without any sanitiser in it for several days the spa water will start to grow organisms very quickly. These organisms will not only be in the parts of the spa that you can see but also in all the pipework. They will form a layer on the inside of the pipes known as 'biofilm'. Biofilm allows all sorts of organisms to grow under it's protective layer. Once biofilm is established it is hard to get rid of it. The best thing is to establish a routine to prevent it getting established in the first place.

There are two types of sanitisers you can use in a spa:

Bromine is the most widely used sanitiser for hot tubs and spas as it is less volatile at the high temperatures which spas operate and does not form smelly by products. Bromine is normally supplied as white tablets, bromo-chloro-dimethyl hydantoin (BCDMH), which are placed in a floating dispenser. BCDMH should not be added directly to the spa or through a skimmer. The dispenser is adjusted to give a bromine level between 3 and 6 ppm.

Place 4 tablets in a floating dispenser and adjust the collar accordingly. They will take 10 to 14 days to dissolve.

Stabilised Chlorine
Stabilised Chlorine can be used in spas in the same way as pools. In order to stop chlorine being broken down by sunlight it is combined with a stabiliser. The stabiliser is cyanuric acid. Cyanuric acid can be added separately or more commonly it is supplied as stabilised chlorine granules or tablets. The problem with all stabilised chlorine products is that as well as stopping the sunlight breaking down the chlorine they also make the chlorine less effective. Therefore it is important not to get too much cyanuric acid in the water. This is unlikely to be a problem in a domestic spa or hot tub which should be drained every month.

The cyanuric acid level can be tested with a test kit. The only way of getting rid of cyanuric acid is by dumping water.

There are two forms of stabilised chlorine.

  1. Stabilised Chlorine Granules (sodium dichloroisocyanurate)
    This is a white granular solid. It adds chlorine to the water just like any other sanitiser but it also adds stabiliser at the same time. It has a pH close to neutral (7) so that it will have little effect on the pH of the spa water. It is added by dissolving the required quantity in warm water.

    To increase the free chlorine by 2.5 parts per million add 5 grams (1 teaspoon). Note: On average you should need approx. 1 teaspoon per person using the spa.

  2. Stabilised Chlorine Tablets (Trichloro isocyanuric acid)
    This is normally supplied as a 20 gram tablet. It adds chlorine to the water just like any other sanitiser but it also adds stabiliser at the same time. It is acidic so it will reduce the pH of the spa water. It may therefore be necessary to add an alkalinity builder to the water to prevent corrosion or the attack of concrete or tile grout. The tablets are slow dissolving. They are usually put in the skimmer or floating dispenser to dissolve over several days.

    Fill the floating dispenser up and adjust the collar accordingly. The tablets will take 10-14 days to dissolve.

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