Flocculation: What Is It?



Why Flocculate? How to Flocculate?


As a pool owner, you know that perfectly> balanced water provides an ideal swimming experience. However, from time to time you may find yourself with cloudy water even though your water analysis appears correct. Turbidity is due to the presence of small suspended particles. In this case, the use of a flocculating agent is essential to regain crystal-clear water.

Why Flocculate?

Pollutants in swimming pool water can be classified according to particle size or consistency. The larger a particle is, the easier it is to filter. However, small particles will be able to pass through the filter. Thus, a filter with a low filtration fineness will be able to filter out small particles. Sand filters have a filtration fineness of 20 to 50 µm. This means that particles smaller than 20µm will not be retained by this type of filter. These particles are almost always organic matter, and when they are not stopped by the filter, they can slow down the disinfection process. It is therefore necessary to increase their size in order to trap them in the filter: this is achieved by flocculation. Flocculants destabilise the electrical charges of the small particles and make it possible to bring them together. By agglomerating with each other, their size increases so that they can be trapped in the filter.

Metal ions, colloidal pollutants, and microorganisms can be effectively removed from the pool water. The result will be clearer water, making it easier to clean the pool and saving on disinfectant costs. Note: Some kinds of micro-organisms are resistant to disinfectants. The only way to get rid of them is to combine flocculation and filtration. It is for this reason that a flocculant should be used in public swimming pools equipped with sand filters.

How to Flocculate?

In order to obtain a high quality of filtration and flocculation, it is crucial to pay attention to certain parameters.


Recommended values


7,0 – 7,4

Calcium hardness (TH)

> 50 mg/l CaCO3

Alkalinity (TAC)

> 80 mg/l CaCO3

Recommended parameter values using flocculant

Automated use (recommended for use in public swimming pools)

Due to the large volumes of the pools and filter elements, it is recommended to use an automated installation to guarantee optimum comfort for swimmers. This type of system effectively combats the large number of particles brought in by swimmers. With this in mind, we have developed ACTI FLOC, a concentrated product specially designed for use in public pools. Thanks to its high concentration, ACTI FLOC ensures crystal-clear water at all times. Its packaging (in 1.5 or 20L containers) allows a high level of autonomy, thus avoiding repeated handling. It is therefore the ideal product for public swimming pools using sand filters.

Instructions for use

The flocculant is added continuously by dosing pump, after the pump pre-filter and before the sand filter, directly from the canister. The permanent addition of flocculant will vary according to the number of visitors and the environment of the pond.

Basin in conformity

100 m3

400 m3

2500 m3

>Filtration per day

1400 m3/d

4000 m3/d

20,000 m3/d

1 l/day

3 l/day

7 l/day

Outdoor swimming pool

2 l/day

5 l/day

12 l/day

Quantity of pure liquid flocculant used per day in routine application.

Note 1: The flocculant injection point must be placed after the pump pre-filter and before the sand filter. Due to the speed of the flocculation process, it is necessary to ensure that there is sufficient mixing between flocculant and water at the injection point. This can be improved by increasing the diameter of the pipe at the injection point.

Note 2: Flocculation must be applied according to:

  • the filtration speed
  • the height of the filter layer

Cleaning the filter

Despite excellent flocculation, the water in your pool can remain cloudy. This can be caused by your sand filter. It is therefore important to maintain your filter carefully. To ensure optimum operation :

  • Do a backwash once every 10 to 15 days (depending on the internal pressure of the filter, position the 6-way valve to "WASH" for about 2 minutes. Then, set the 6-way valve to "RINSE" for 30 seconds.
  • Clean / descale regularly and thoroughly at the opening of the pool and at the end of the season. If the water is very hard (TH> 35°f), carry out this operation once every 6 weeks.

For this we recommend a filter cleaner whose formulation is specially designed for all filtering elements.

For sand filters: destruction of scale deposits, rust and, in general, all mineral deposits present in the filter media.

Instructions for use:

  • 1 litre for 75kg of sand for family pools ("fast" filtration)
  • 1 litre per 100kg of sand for public swimming pools ("slow" filtration)
  • Introduce the product upstream of the filter (skimmer, chute or pre-filter) and start the filtration system for a few seconds in the "Drain Rinse" position to disperse the product in the filter until the colour appears in the control light.
  • Leave on for at least half a day or even overnight.
  • Backwash and rinse the filter thoroughly until the colouring has completely disappeared.

Note: Although more constraining, the best method, however, is to pour the product directly into the open filter, on the sand, after having taken the precaution of bringing the water level to sand level and having closed all the valves.

Why change the filter sand?

Filter sand inevitably wears out. When it is new, the sand has grains with a well-defined size and contour. During use and with each backwash, the grains are polished by the circulation of water and become smaller and smaller. It is therefore imperative to change the filter mass every 5 years or so.


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