overdose of CYA

[bph99]

I have an 80 m3 tiled pool in Portugal with a salt chlorinator.

During the winter, when we were not in Portugal, the pool water turned green and a well-meaning neighbour attempted to carry out what he thought was a chlorine shock treatment. Unfortunately, rather than using liquid chlorine, he appears to have added what must have been a massive dose of tri-chlor.

The chlorine level is now off the scale and has not shown any sign of reducing over 4 or 5 weeks with the chlorinator switched off. Even worse, the CYA level is also off the scale.

In order to get any meaningful readings with a Scuba II meter I have had to dilute the pool water considerably but current readings are approximately: Free Cl = 22 ppm, total CL = 40 ppm, CYA = 320

As I understand it, the only way to get rid of the excess of stabiliser is to drain the pool by about 2/3 and refill and I plan to do this on my next visit.

I assume that the stable and very high levels of both free and total chlorine mainly reflect the presence of CYA but I am slightly confused by the difference between the free and total levels. Is this explained by the high CYA level or does it indicate that I may also have a high level of bound chlorine / chloramines, which might require a shock treatment after the CYA has been diluted out ?

Any help and advice gratefully received !

[Denniswiseman]

HI
Welcome bph99
You are correct in what you assume in the difference between TC and FC
When you drain and refill for the CYA it will also reduce your chloramines so it won't be such a big job
You may have to dilute more than 2/3 to get your CYA down to a more acceptable level, say 3/4 (CYA 70-80) or rely on splashouts, backwashing, and rain to dilute it further
I take it that you are not in a high water table?