In summer, it's not unusual for swimming pool water to turn green. There are several reasons for this. It can be the result of particular weather conditions (heatwave, rain/thunderstorms) that alter the balance of the water. Similarly, poor maintenance or filtration can lead to the development of green algae. In this article, you'll find practical advice on how to restore clear water through regular analysis and maintenance, and the right treatments, such as flocculant, which can be very useful in this situation.
If you're wondering why the water in your swimming pool turns green, you should know that it can be due to a number of different causes. There may be meteorological factors, technical causes linked to your equipment, or a human factor (natural pollution from swimming). It's also worth noting that the presence of large numbers of insects - or larvae - can alter the colour of the water.
When you restart your pool after wintering, it's very common for the water to have turned green, so there's nothing to worry about. In this context, a restarting your pool should be enough to solve the problem.
According to the Fédération des Professionnels de la piscine et du spa (French Federation of Pool and Spa Professionals), faulty filtration as a result of a pump problem or clogged filters is responsible for 80% of green water.
De plus, une durée de filtration trop courte modifie également l’équilibre de l’eau qui deviendra verte avant de développer des algues. C’est pourquoi il est important de savoir how long to filter your pool water !
Leaves, sand, pollen... the water in your swimming pool is full of dirt, especially if it's outdoors! So you need to clean it regularly.
Lors d’un épisode de canicule, l’eau de votre piscine augmente sans se refroidir la nuit. Dès que la température de l’eau dépasse 28°C, les bactéries se multiplient. Elle s’altère alors et verdit. Les parois de la piscine deviennent glissantes. Sans une action rapide, des algues vertes apparaitront. Il faut réagir dès que l’eau se trouble !
Rainwater is loaded with fine particles such as pollen, dust and sand. What's more, it has a pH between 5 and 6, whereas your pool water is balanced between 7 and 7.4.
The more bathers there are in a pool, the more the water deteriorates. Sunscreen, sweating...the water is rapidly polluted by the presence of bathers, who bring in organic matter. A quick rinse before diving in can help slow down changes in water quality.
When the water is green or cloudy, swimming is not recommended. This means that your pool water is polluted and full of bacteria. Ears, nose and eyes can become irritated or infected (ear infections, conjunctivitis), and this can trigger skin diseases.
As soon as the water in your pool starts to turn green, don't delay! The first step is to stop swimming until the water is healthy and clean again. The longer you wait, the more you'll need expensive chemical treatments that will deprive you of swimming.
If you notice that the walls are slippery or even slimy, algae growth has already begun.
It's possible that your pool water is turning crystal green, with no algae. This problem can arise after adding a lot of disinfectant, or from water that's too acidic, or too low in TH.
Start by checking the pH of the water every day to measure its balance. If the pH is too far outside the required range, it may be necessary to apply shock chlorine to restore clarity, or simply add a pH-correcting product (pH+ or pH-). Once the product has been added, activate filtration, respecting the following rule: water temperature divided by two = filtration time. If your pool water is still not perfect after filtration, you can use a flocculant while continuing to filter the water.
If your pool is very exposed to the sun, this problem is likely to recur throughout the season. That's why it's important to protect the water as much as possible with umbrellas and shade sails, to prevent its quality from deteriorating rapidly. This will also slow down the bacteria growth process.
After a heavy downpour, it is necessary to measure :
The water in your pool was green or cloudy, and you added flocculant and shock treatment. But the pool water quickly turned green again. It may be that your pool water is over-stabilised, making the shock treatment ineffective.
To confirm whether your water is over-stabilised, you can use a test strip to measure cyanuric acid (stabiliser). An acceptable level of stabiliser is between 25 and 50 ppm (or mg/l).
If the test you have carried out indicates a level above this threshold, you have no option but to empty 1/3 and renew the water in your pool.
Now that you know the factors responsible for a green pool, you can prevent the phenomenon. In high season, good filtration, water control and regular pool maintenance are essential. Don't forget that faulty or insufficient filtration is responsible for 80% of cases of green water. Follow our advices regular maintenance of your pool step by step to maintain optimum water quality all season long. So you can enjoy swimming all summer long!
Finally, you should know that there are ICO Poola connected water analyser for swimming pools that measures pH, temperature and disinfection.