Swimming pool flocculant: usefulness and operation

A pool flocculant gathers particles that are too fine to be caught by the filter and turns them into larger clumps, which are then trapped by the filter or settle to the bottom to be vacuumed up. It helps restore crystal-clear water when it remains cloudy despite proper filtration and disinfection. It does not replace the filter or the disinfectant.

Your filter is running, the chlorine level is correct, and yet the water remains dull? This is often a sign of microparticles passing through the filter media. The flocculant is therefore the right tool. Experts in aquatic environments since 1998, we explain how it works and how to use it correctly.

What is a flocculant used for?

A sand or glass filter catches the vast majority of impurities, but some particles are so fine that they pass through the filter media. Dust, pollen, organic residues or traces of sunscreen can give the water a slightly cloudy or milky appearance.

This is precisely where flocculant comes in. It improves filtration fineness by clumping these microparticles together. It is used:

• curatively, when the water is cloudy despite proper filtration and disinfection;
• preventatively, in low doses, during periods of heavy use or high pollen counts;
• when reopening the pool in spring, to quickly restore clear water.

How does it work?

Suspended microparticles carry small electrical charges that keep them dispersed. The flocculant neutralizes these charges: the particles can then stick together and form larger and larger clumps. Once large enough, these clumps are either caught by the filter or become too heavy to remain suspended and sink to the bottom.

The result depends heavily on the water balance. A pH outside the range recommended by the manufacturer reduces the effectiveness of both the flocculant and the disinfectant. Therefore, check the pH and chlorine before treating.

Liquid, gel or cartridge: which format to choose?

• Liquid flocculant is the most versatile. According to the CTX-41 data sheet, it is used with the filtration system running and when no swimmers are present: pour in the product, leave it to work, then vacuum up the sediment with the valve in the waste position. The indicated dosage is 1 L per 100 m³ for cloudy water and 0.5 L per 100 m³ preventatively.
• Single-dose gel is placed in the skimmer basket and dissolves slowly. For CTX gel, the data sheet indicates a dose for 50 to 60 m³: wash and rinse the filter, place the tablet in the skimmer, then wash the filter again after 24 hours.
• Cartridges work on the same principle, in the skimmer, to clear slightly cloudy water.
• Natural flocculant, made from chitosan, is used weekly. The CTX data sheet indicates 8 ml per 10 m³ per week, with filtration running for at least six hours after application.

Using a flocculant step by step

1. Check the water: pH and disinfectant within the recommended ranges, filter clean.
2. Read the product label and calculate the dose according to the actual volume of your pool.
3. Prohibit swimming for the entire duration of the treatment.
4. Apply the product according to the intended method: in the skimmer for a gel or cartridge, or spread in the water for a liquid.
5. Leave to work for the indicated time, with filtration running or stopped depending on the chosen method.
6. Finish with a filter backwash, or by slowly vacuuming the sediment to waste.

Flocculant and filter type

Flocculant works hand in hand with the filter. Granular media, such as sand or glass filter media, are those for which most flocculants are designed. The CTX gel data sheet states it is compatible with sand and other filter media such as glass or zeolite.

With a cartridge filter, the clumps risk quickly clogging the cartridge. Check the label to ensure the product is compatible, and if necessary, opt for the settling method followed by vacuuming.

Flocculant throughout the season

Flocculant needs are not the same throughout the year. Here is how to incorporate it into your routine without overdoing it:

• When reopening: after winter, the water is often full of fine particles. Once filtration has restarted, the pH adjusted and disinfection restored, a flocculant treatment helps quickly restore clarity.
• In spring: pollen settles on the surface and partially passes through the filter. Low-dose preventative use can be useful during the busiest weeks.
• In the middle of summer: attendance increases, with more sunscreen and organic matter. Natural flocculants for weekly use are designed for this period.
• After a thunderstorm: rain and wind bring dust and debris. First remove the debris, check the water balance, then treat if the water remains cloudy.

In all cases, remember to measure before treating. A test kit for chlorine and pH allows you to check in a few minutes whether the problem is indeed due to suspended particles, and not an imbalance in disinfection.

Also note the filtration time: in summer, filtration that is too short allows particles to accumulate. Adjusting the pump's operating time to the water temperature limits the need for flocculant.

Errors to avoid

• Overdosing thinking you will work faster: the water can become even cloudier and the sediment harder to remove.
• Treating without checking the pH and disinfectant: flocculant does not correct a chemical imbalance.
• Vacuuming up the sediment in the filtration position: the clumps will return to the filter or the pool.
• Forgetting the backwash after a filter treatment.
• Swimming during treatment: always follow the instructions on the label.
• Mixing several products without checking their compatibility.

When flocculant is not enough

If the water remains cloudy after a properly carried out treatment, look for the cause elsewhere: filtration that is too short, worn filter media, insufficient disinfection, unbalanced pH. Green water, for example, first reveals an imbalance in disinfection. Flocculant then helps restore clarity once the water has been rebalanced.

If in doubt, have your water analyzed or ask for advice before stacking products: multiplying treatments without a diagnosis is expensive and rarely yields good results.

Also remember to check the condition of the filter media: compacted or dirty sand allows more particles to pass through. Replacing it with glass filter media can improve filtration fineness.

Frequently asked questions

What is a flocculant used for in a pool?

It gathers particles that are too fine for the filter into larger clumps, which are then caught by the filter or settle to the bottom to be vacuumed up.

Can you swim during a flocculant treatment?

No. The CTX-41 data sheet specifies using it when no swimmers are present. Follow the instructions on the label of the product used.

What dose of liquid flocculant should be used?

For CTX-41, the data sheet indicates 1 L per 100 m³ for cloudy water and 0.5 L per 100 m³ preventatively. For other products, follow the label.

Is flocculant suitable for a cartridge filter?

It can quickly clog the cartridge. Check the compatibility on the label and if necessary, opt for settling followed by vacuuming.

Does flocculant replace chlorine?

No. It improves the clarity of the water but does not disinfect. It complements filtration and disinfection.

How long does it take for flocculant to work?

This depends on the product and the method. For CTX gel, the data sheet calls for a filter wash after 24 hours; for natural flocculant, at least six hours of filtration after application. Follow the label.

Also read

• Removing flocculant from a pool: vacuum up the sediment and rinse the filter
• Sediment at the bottom of the pool: identify each type of sediment and remove it
• How to choose a sand filter for a pool?