Base Alkalinity problems

Don’t often post and now here have come two in quick succession.

An acquaintance rang today to say he has a problem with a pool’s alkalinity. Not sure about the rest of France but we are having a real problem this year maintaining the base alkalinity at a regular level on some pools. His is worse than any of mine.

He is repeatedly losing alkalinity and the pool goes green pretty much every 10 days. In the last ten days it has gone from 120mg/l to 7mg/l. He cannot see any reason for it. The pH has stayed pretty steady, fluctuating around the 7.3-7.4 mark. Normally he is disinfecting with hydrogen peroxide plus a uv unit and the pool has always been fine. He has switched over to chlorine to see if that would help solve it but the same is happening.

Any pointers as to what might be killing the alkalinity so quickly? None of the pools I look after use H202 as a disinfectant so not really sure of the potential chemical problems. I said I’d have a quick ask.


I’m by no means an expert, but if he is using a meter to measure the pH, I would check the calibration.

We have also had various pool issues this season which I put down to the hot weather, with one we can’t keep enough chlorine in the pool despite adding galets every three/four days.

Thanks for the suggestion. It has been checked by three means just to make sure. I am likely to pop over and retest myself just to quadruple check as I prefer the evidence of my own eyes even though I know he will be right.

One of the big problems we had this year was Veolia suddenly raising the alkaline content of the tap water to 340mg/l in a couple of villages and the pools that needed topping up slowly going off piste!
(I’m taking it by your profile pic we support the same team. Left it a bit late last night to get the win!).

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This is zero to do with alkalinity, it’s 100% to do with having sufficient active sanitiser.

You cannot just swap from hydrogen peroxide to chlorine and hence chemical levels go to pot but are they? Alterations means test reagents are suffering from interference from other chemicals.
All test strip testing is near useless and again any pool deemed commercial eg chamber de hote or gite has to be tested via DPD for chlorine phenol red for pH so may as well buy a decent test kit.

To actually loose the bicarbonate would be tricky.
Packaged chlorine galetts, lent, choc etc contain CYA stabiliser. Regular use of these packaged chlorine sources will make the normal chlorine levels ineffective and that will lead to green algae filled pools. Also high levels of phosphates which is food for algae.
Please post a full set of water test results including CYA stabiliser, the most important chemical in your pool.

Did they say why? My tap water is at a near constant 280mg/l but I keep my pool around 37-45 to keep the pH really stable. Most vinyl/fibre glass pools are over alkalinity it just causes pH to rise but that’s still not an issue unless there are metals in the water.

These are the figures he has supplied to me. The chlorine is very low and I have already told him to start adjusting it along with raising the CYA which is too low.

Free Cl : 0.07mg/l
Total CL : 0.76mg/l
pH : 7.32
Alkalinity 7mg/l
CYA : 7mg/l
Calcium Hardness : 90mg/l
TDS : 321
I don’t have a reading for phosphates.

Will the alkalinity stabilise as well when the cyanuric acid is within range or will that only affect the stability of the chlorine?
He allowed the hydrogen peroxide to go to nil before stating the chlorine which I think is correct although as I say this is not something I have used for regular sanitisation so no practical knowledge of how that conversion will go.
For me, I would also want the alkalinity and calcium levels up from where they are.

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How is he chlorinating, with low CYA using cheap galetts will build the CYA quite quickly. Cheap galetts are dichlor more CYA. More expensive galetts are trichlor more chlorine.

Not sure John. I will find out and advise accordingly.

I have seen phosphate test kits in BricoMarche, by the aquarium section.