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Treating green water

1) What type of swimming pool is involved? (Is the pool fitted with a pump and (sand) filter?)
2) How much water is in the swimming pool? (Essential information to allow you to act correctly)
3) Measure the pH and chlorine levels!
4) What is the pH level?​​
​​​​​5) What is the chlorine level?


pH level

In the case of a pH level above 7.6, we recommend lowering the pH level first.
Recommendation: pH: 6.8. At this pH level, only a shock treatment will have an effect. To clarify:

Chlorine has two components: HOCl and OCl-. Of these two, HOCl is the active component, while OCl- has no effect. You may forget this information, but knowing it now should help you to better understand the pH table:

As an example, the table clearly shows that chlorine is only around 11% effective at a pH level of 8.5. At a pH level of 6.0, however, the effectiveness is 97.5%
The how and why is not particularly relevant – it’s important, however, to understand that it is a given fact that a shock will not be quite so effective if the pH level of the water is (far) too high.
In that case, why is it always said that the pH level should be kept at between 7.2 and 7.6?
If the water is of good quality, it is advisable to keep the pH level at around 7.2 as then, as the table
shows, the chlorine is still around 70% effective. That is usually sufficient for proper disinfection.
Here though, we’re talking about water that is NOT of good quality. In that case, to achieve favourable results quickly, we recommend lowering the pH level to 6.8.


Why not go as low as 6.0, wouldn’t it be even more effective then?

​​​​​​​» Start by reducing the pH level by adding pH Minus. Follow the dosage recommendation as stated on the packaging.
» Run the pump.
» Then apply a chlorine shock. You can raise the chlorine level to around 10 ppm (mg/l).

​​​​​​​» Run the pump.
» You should be able to see results within a few hours.

General dosages, as stated on our packaging:

pH Minus:
Add 30 grammes of product per 10 m³ of pool water. If the difference in the level is significantly out, this amount can be doubled. Dissolve in a bucket of (pool) water and distribute over the entire surface of the water. Check after 24 hours.

Chlorine shock (55/G) for small(er) swimming pools:
18 grammes of product raises the chlorine level by 1 ppm (mg/l) per 10 m³. In the case of green water, a chlorine shock is recommended. This shock is achieved by raising the chlorine level to around 10 ppm (mg/l).

Chlorine shock (sodium hypochlorite) for larger swimming pools:
​​​​​​​Liquid chlorine is often used to shock larger swimming pools. The benefit of sodium hypochlorite is that it contains no cyanuric acid. Consequently, it does not increase the level of cyanuric acid in the swimming pool either. When shocking, the pH level can firstly be reduced (to 6.8) as sodium hypochlorite will raise the pH level so that the water will later return to an acceptable level of between 7.2 and 7.4.

One minitab of 2.7 grammes raises the chlorine level by 1.5 ppm (mg/l) for every 1000 litres of water. At a desired chlorine level of perhaps 3, you would need to dose two tablets for every 1000 litres of water. So, if the pool has a capacity of 4.5 m³, you should use nine tablets. In abundant sunshine, UV radiation is extremely high, causing chlorine to be broken down ‘in place’.

Worth noting so that you can rule out causes:

Cyanuric acid:
» When was the water last (partially) changed? If the water has been in the pool for a long time, for several years or more, the cyanuric acid level may be too high. In that case, the chlorine will no longer be effective and your treatments won’t make a difference. See the table below.

It is not necessary to know what the cyanuric acid level actually is if the water was (mostly) changed just a few weeks ago, or at the beginning of the year. In that case, you can rule out this cause for the treatment not working.

Filter sand/glass:
» How long has the filter sand or glass been in the filter? If the filter material – sand or glass – has been in the filter for more than a few years, it could explain why the treatment isn’t working as it should. In that case, you’ll get a ‘bio-film’ in the filter, even with regular backwashing/backflushing. This is a slimy, dirty layer of bacteria. When you then add chlorine, it will be more concerned with attacking the bio-film than tackling the green in the pool water.

​​​​​​​If the sand or glass has been in the filter for a long time, it’s a good idea to change that too. If it’s been in the filter for no more than two years, you can rule this out as a cause as well.