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División de química básica

FAQ about water treatment

How do I make a shock chlorination?

To perform a shock chlorination, first, you must adjust the pH to a value between 7.2 and 7.6.

Then add between 0.5 Kg and 1 Kg of powdered or granular product (trichloro) per 100 m3 of water and algaecide 2 litres per 100 m3. If you use fast chlorine (granular or tablet dichloro), add between 0.8 Kg and 1.6 Kg per 100 m3.

The doses are indicative and should be adjusted in waters with exceptional features or with a large number of swimmers.

It is important that after you made a shock chlorination, swimming is not allowed until the free chlorine is less than 3 ppm. Approximately after about 48 hours.

What should I do if my pool water is green?

First thing to do is to check if the pH is properly adjusted, so you have to measure the pH with an analyser. The optimum value must be between 7.2 and 7.6. If it is below use a pH riser and if is above a pH reducer.

One of the most likely causes of green water in your pool is that there is a lack of free chlorine in the water. To remedy it, measure the free chlorine with the test-kit.

If is less than 0.5 ppm (pale pink or colourless) a shock chlorination must be performed. To do so, apply a fast chlorine disinfection, whose function is to destroy microorganisms (fungi, bacteria, germs,...) Follow the manufacturer's instructions for dosing and note the volume of the pool. The optimum chlorine level should be between 0.5 mg/l and 1.5 mg/l so, once applied, wait a few hours and analyse the water again with a pH and free chlorine analyser to verify that the levels are correct.

Water can also be green by the presence of algae (see question 4)

What should I do if my pool water is cloudy? I cannot see the bottom of the pool.

If the pool water is cloudy, it is likely that the pH is not adjusted, typically above 7.6. To correct it, measure the pH value and if the result is higher than 7.6 make additions of pH reducer in order to achieve the correct area (between 7.2 and 7.6).

Hard water may be another possible cause of turbidity (hardness causes formation excess of calcium and magnesium carbonate deposits). In that case, besides adjusting the pH, as was done in the previous point, you may need to add liquid flocculent to the skimmers. If it doesn’t work, leave the pool to stand overnight and the next day use the bottom cleaner.

I have algae in the pool, how do I eliminate?

If you already have algae in your pool, probably it won’t be sufficient only applying an algaecide; you may need to add an additional contribution of chlorine.

First of all, it is necessary to adjust the pH, as seen above.

Then apply a quick chlorine disinfection, whose function is to destroy microorganisms (fungi, bacteria, germs,...). Follow the manufacturer's instructions for dosing and note the volume of the pool.

Finally, add 2 litres of algaecide per 100 m3 of water to prevent reformation of algae.

Smells too much of chlorine and eyes sting me, what happens? How do I solve it?

 Excess chlorine odour may be caused by two reasons:

  • There is an excess of free chlorine and lower pH than the recommended value (less than 7.2). In this case you need to add a pH lifter and adjust to the optimum value (between 7.2 and 7.6). Too much chlorine can be harmful to eyes and skin.
  • There is an excess of chloramines and organic matter. In this case, it is necessary to adjust the pH to recommended value and if it is not enough, perform a shock chlorination. 

Some brown spots have appeared on the surface of the pool, what is the reason for? What is the solution for this?

Most likely is that in addition to a mismatch of pH, there is traces of iron, copper, manganese or other metals in the water. It depends on the quality of water that the pool is filled with.

To solve this problem, you must first make an adjustment of pH as seen above (between 7.2 and 7.6) and then add liquid flocculent to the skimmers. If no improvement is seen, leave the pool to stand overnight and the next day use the bottom cleaner.

Foam is formed on the surface of the pool.

The foam may be due to an excess of chloramines and organic matter, or the presence of iron, copper, manganese or other metals.

To remove it, you may need to renew part of the pool water (drain part of the pool and refill with fresh water). Then perform the recommended pH adjustment and shock chlorination (see the previous point).

I have made a water treatment and it has a light colour.

Probably is caused by the presence of metals in water (copper, iron, manganese, etc.). To remove the light colour, you may need to add liquid flocculent to the skimmers. It is recommended to leave the pool to stand overnight and the next day use the bottom cleaner. For an optimal flocculation effect, remember that the pH should be between 7.2 and 7.6.

I fail to lower the pH of the pool.

If the pH is above 7.6, you may add a pH reducer.

The dosage of the reducer should be done as recommended by the manufacturer and afterwards a re-measurement of pH must be done again until the water reaches the correct area (between 7.2 and 7.6).

Should I add more product when skimmer tablets are finished?

Dissolved tablets not necessarily indicate that the rate of chlorine in the water is depleted. There may be several reasons why the tablets dissolve and the rate of chlorine is high and, until the rate is low, we should not add more tablets.

You may always be guided by the rate analysis performed with a chlorine test kit, which should be between 0.5 and 2.0 mg /l.

Although the tablets in the skimmer have dissolved quickly, if the supply of chlorine is high and we keep adding tablets, the pool may reach a rate of excessive chlorine that may cause eye redness and skin irritation.

How often do I need to empty the water of my pool?

The pool water may be stored for several winters if it’s properly maintained (there are suitable products for this). You may only need to partially drain the pool when the level of cyanuric acid (stabilizer) is too high. It is considered too high and we should act if the cyanuric acid level exceeds 150 mg/l in water. This situation can occur after several seasons of treatment or inadequate water renewal.

How can I notice if the level of cyanuric acid in water is too high?

There are test analysis on the market, available in pool stores, large plumbing shops and online. However, the excessive amount of cyanuric acid in water is detected because chlorine is no longer effective, and no increase of chlorine level is observed even though more product is added.

What is the difference between treatment with chlorine tablets or granules and salt electrolysis?

In both cases the water disinfection is performed by chlorine. Upon treatment with tablets or granules, chlorine is formed by dissolving the product in the water. In the salt electrolysis, free chlorine for disinfection is formed in situ in the electrolyser. But in both cases, the product that enables the disinfection is chlorine.

The salt electrolysis system is free from chemicals?

The salt electrolysis provides the chlorine needed for disinfection. This chlorine is dissolved in water just like if added in another format. Furthermore, additional treatments needed to keep the water in optimal conditions (pH modifiers, algaecides, flocculants...) should be added separately.

What advantages are granulated or chlorine tablets (isocyanurate based products) versus other chlorine presentations?

The main advantages of granulated or chlorine tablets are: the stability in storage, the safe handling and the precise dosage necessary for the maintenance of the pool. You may also notice that this family of products is characterized by giving the water a stabilizing element of chlorine, the isocyanuric acid, which makes the disinfection of pool water more effective.

What advantages are granulated or chlorine tablets compared to liquid chlorine (bleach)?

Bleach is a contribution of direct and rapid chlorine, but is an unstable product. It doesn’t endure long periods of storage, especially in summer with high temperatures and exposure to sunlight. In these circumstances it degrades and loses properties (concentration) quickly.

Furthermore, it is an alkaline product that raises the pH of pool water, a situation that must be corrected afterwards. And, as a liquid product, its manipulation involves risk of splashing.

What controls are required for a product used in swimming pool water maintenance?

In order to achieve the Spanish Ministry of Health approval a product must present a series of data such as analyses accomplished by independent laboratories and internationally standardized efficiency tests, carried out also by independent laboratories. Also, to get the approval, the 100% of the product components must be declared in order to verify that none is harmful to health.

How can I know if the product I use to treat my pool is properly controlled by the authorities and is it safe?

Products for the treatment of swimming pools in Spain must be approved by the Ministry of Health. The Ministry has enabled a web (http://www.msssi.gob.es/ciudadanos/productos.do?tipo=piscinas) where citizens can check if the products they buy (all of them must carry an approval number) are duly registered and in force (the authorizations are valid for five years).

This situation is in the process of change, due to the entry into force of Royal Decree 742/2013, of 27 September, establishing the technical and health criteria of pools, which will replace the present authorizations of biocides records for treatment of swimming pool water.


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