Black Mould in Swimming Pools
The black mould that is often found growing in swimming pool tile joints is not actually black but very dark green. If you scrape it out with a fingernail or toothpick it leaves a bright green cloud. Black mould should not be confused with metal staining, which occurs when the pH and/or total alkalinity are too low. The colour depends on the actual metal(s) and can vary from brown (iron) to turquoise (copper) and shades of both. The difference is easy to tell because, with the staining, there is nothing actually growing in the joints. The joints may come out in a cloud, if the situation is severe, but the cloud is basically white and not bright green. This should be carefully checked first because the cure for black mould will make copper staining worse. Furthermore very low TA can cause permanent damage to the pool structure, purification plant and heater. TA defects should be rectified as quickly as possible. Prevention of Black Mould – My personal belief is that the mould spores are present in garden soil and get walked, first onto the surrounding terraces (where they flourish and grow) and then into the pool. Wherever there is black mound in the pool there is usually some black discolouration on the terraces and/or pool coronation. Preventing the spores from getting onto the terrace is simply a matter of stopping bathers from walking from earth or lawns directly onto the terrace or into the pool. Curing Black Mould – Removing the mould from inside the pool is simply a question or killing, scrubbing then more killing and scrubbing - but it is a hard, long job! Adding copper-based algaecide is a good start but this should not be overdosed as more staining could occur. Follow the manufacturers instruction. Even if the sanitization system is Bromine (which controls normal algae without copper), if you have black mould, it is still a good idea to also treat with a copper-based algaecide.
The pH should also be adjusted to between 7.2 and 7.5 and the Total Alkalinity to 100 - 120 (in Costa Blanca).
The next step is to super-chlorinate the pool to at least 10 ppm in the evening, then add ½ kilo of TriChlor (Lento) in granular form by sprinkling it directly onto the affected areas and leave the pool overnight with the pump OFF. The following morning the joints must be scrubbed and, an hour or so later when the debris has settled, the pool should be carefully hovered TO WASTE. This will probably not remove every trace and the whole process has to be repeated every day until all signs of the mould spores are gone. The mould on the walls is far more difficult to remove because the strong (killing) chlorinated water layer stays at the bottom. Once the bottom is clear it is then a question of getting into the pool with a snorkel and rubbing pieces of a broken TriChlor Tablet into the joints where on the wall where spores are still present. If there are spore stains on the terrace or border then these areas should be cleaned up by spraying with Liquid Chlor (Sodium Hypochlorite). Once the liquid Chlor has dried, the dead spores can be easily brushed off and later these areas should be treated by spraying with Algaecide (10:1 solution). NB: Whilst the pool is being treated for black mould, no swimming should be allowed until the Chlorine drops below 4 ppm or 9 ppm in the case of Bromine. Be aware, that if your Cyanuric Acid is high - you will probably never be able to remove black mould effectively because your Chlorine simple will not work properly. See the article
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