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It can happen, through forgetfulness or lack of maintenance, that the water in your pool turns green. However, all is not lost. It is possible to catch up with this color to rediscover the pleasure of bathing again. Here are the steps to catch up with your pool.

Step 1: Clean the basin

Before treating the water, you must prepare your pool. Start by cleaning your pool of impurities floating on the surface of the water using a dip net. Then pass the manual vacuum broom on the floor of your pool. It is better to proceed gently to avoid bringing dirt up to the surface. What has been sucked in must be evacuated through the sewers and must not pass through the filtering system. Also make sure the prefilter baskets, located in the skimmers, are emptied and clean.

The water level will certainly have dropped during this time, so do not hesitate to add fresh water to it so that the skimmers do not spin in the air. Finally, you can brush the walls and the bottom of your pool with a brush to remove the algae that will be attached to it.

Step 2: Chemical treatment of a green swimming pool

To catch up on green water, it must be subjected to a shock treatment. Before proceeding with this step, you will need to check the water balance and the filtration system:

  • First, check the pH of the water is between 7.2 and 7.6.
  • If your pool is usually maintained with stabilized chlorine, also analyze the stabilizer level. It should not be more than 70 mg per liter of water.
  • Finally, look at the status of the filter. It must be clean and in working order. To avoid passing very dirty water into the filtration, during this treatment step, you can equip your skimmers (for traditional swimming pools) with a pre-filter. The latter will recover most of the impurities.

    There are then two different types of treatment:

      • Treatment with shock chlorine and hydrogen peroxide (liquid active oxygen): it is more powerful and faster (a few hours). It is well suited for swimming pools already treated with chlorine. In the skimmer basket, add the shock chlorine tablets according to the volume of water (approximately 20 g / m3 of water). Leave it on for at least an hour, then check that your pond water contains at least 1 milligram of chlorine per liter. You can then incorporate the active oxygen in front of the delivery nozzles.
      • You can also make up for green water with hydrogen peroxide alone: ​​the process is slower (between 6 and 48 hours depending on the state of the water), but also less aggressive. It does not leave chlorine residue in your pool. It is enough to pour the substance into the basin, always in front of the outlets. It will take about 5 liters for a 50 m3 pool.

    Be careful! Liquid active oxygen is a product that should be handled with care to avoid splashing or inhalation. It is advisable to wear glasses and gloves.

    Step 3: Prevent the return of algae

    In the event that your pool water has turned completely green, it is advisable to use additional treatment two days after the end of the shock treatment. Use a curative algaecide to prevent further growth of algae. Some microscopic bits of residual algae may still be present in the treated water. For the dosage, it will be necessary to count half a liter anti-algae for 10 m3 of water, to be poured in front of the discharge outlets.

    Step 4: Clarification by flocculation

    Sometimes even after treatment, your pool water remains slightly opaque due to the presence of invisible particles too small to be recovered by the filtration system. It is then necessary to clarify the pond water by flocculation. This technique consists in facilitating the filtration of your pool using flocculants:

      • Or by refining the filtration capacities of the filter: this flocculant is in the form of a tablet or cartridge to be placed in the skimmer basket. You will then have to filter your pool water continuously for one or two days. ;
      • Or by agglomerating the impurities in the basin in order to facilitate their filtration (necessary when the bottom of the basin is not visible): the flocculant must be diluted in a watering can and distributed over the entire surface some water. Then let the filtration pump do its work for two hours in circulation mode.