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What causes my pool to go green?

August 17th, 2015

What causes a green pool? The green colour is algae, which contains the green pigment chlorphyll. The more algae in the water the greener your pool. Algae growth is normally prevented by a sanitiser, most commonly chlorine. So when your pool goes green the problem is somehow connected to the chlorine, but as we’ll see the details aren’t quite so simple.

For the most part, salt water chlorinators and bleach pump pools both add chlorine to the water at a constant rate. Unless you’re changing the settings on the equipment the amount of chlorine being made available or added doesn’t change. So why does you pool go from crystal clear water to green on its own?

Load on the pool

A fixed dose of chlorine will kill a fixed volume of algae, if the pool water changes to increase the rate at which the algae grows it can exceed the rate at which the fixed amount of chlorine can kill it. Once the growth rate exceeds the kill rate the pool will go green.

Things that increase the rate of growth of algae:

  • Increased volume of organic material such as leaves or grass clippings in the water that provide food for the algae.
  • Increased use of the pool, which adds sweat, skin cells etc. which will consume chlorine
  • Phosphates (think fertiliser) may be entering the water – particularly if your pool is surrounded by grass/gardens. These again provide food for the algae.
  • Increase in water temperature and sunlight associated with warmer weather will allow algae to grow at a faster rate.

Insufficient chlorine in your pool

In a salt water pool salt (the compound “NaCl”, sodium chloride) is added to allow chlorine to be produced. When you add a bag of salt into the water sodium chloride dissolves and splits into two separate parts – sodium and chloride ions. Chloride won’t kill algae, it needs to be turned into chlorine to do this. This is the job of the chlorinator in your pool.

The chlorinator works by passing water past metallic plates and supplying a small electrical charge to convert the chloride into chlorine.

Achieving the right amount of chlorine in a salt water pool is hence dependent on a) having a sufficient amount of salt in the water b) operating the chlorinator for enough hours each day to produce the right amount of chlorine and c) having a chlorinator that is operating effectively.

In a bleach or chlorine pool, the chlorine is purchased in drums and added directly to the pool or using a pump on a time clock. Ensuring there is sufficient chlorine in the pool relies on ensuring the drums aren’t empty and that the time clocks are set to pump the right amount of chlorine in each day.

The “right amount” of chlorine changes based on the load on the pool per the previous point. In summer when the temperatures are higher you’ll need more. If you put a cover on your pool which both limits the waters exposure to UV and keeps leaves out you’ll need less. If you’re busier than usual and you don’t take the leaves out as often you’ll need more.

Chlorine needs the right pH to work

Chlorine is used as a sanitiser in swimming pools partly because it’s effective at a water pH that is comfortable for us to swim in. The effectiveness, or the ability for chlorine to kill the algae is highly highly (think exponentially) dependent on the pH. As the pH moves away from the optimum level the chlorine rapidly stops killing algae.

If your pH is sufficiently out of whack, even if it has a level of chlorine that is normally correct, your pool will go green.

A green pool can be triggered entirely by the pH moving away from the optimum range.

Too much cynauric acid / stabiliser

Stabliser is often added to pools to help maintain the right level of available chlorine. Chlorine is broken down by the UV rays in sunlight and so the idea of cynauric acid is that it forms a weak bond with some of the chlorine so that it doesn’t all breakdown when the sun is out.

In the right doses it works, but when too much is added too much of the chlorine is locked up and can’t do its job. Hence you have a pool with an adequate level of chlorine but it still goes green as it’s not available to do the job of killing algae.

Need help with your green pool?

We’ve covered the main causes of green pools. Our next post will be our guide to rapidly restoring a green pool (we’ve become experts at this as we onboard new customers) so make sure you subscribe to our mailing list to be notified when it’s published.

Alternately – contact us for a free site visit and to learn about our automated pool management system that will mean you never have to deal with a green pool again!



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