Level of alkalinity and can it cause rough plaster?

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
lasawyer2
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Level of alkalinity and can it cause rough plaster?

Postby lasawyer2 » Sat 25 Aug, 2007 21:31

We have a salt water pool, which is around 5 months old. This week we noticed that the 3M plaster is like very rough sand paper. We were wondering if it was caused by our alkalinity. Also, could this be the reason that four the rough plaster? Here are the levels, which our pool guy says is fine:

Alkalinity: 150
Water PH: 8.0
Free Available Chlorine: 5

Thanks,
Lisa


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Postby chem geek » Sun 26 Aug, 2007 00:29

Lisa,

A combination of high pH, high Total Alkalinity, and/or high Calcium Hardness (CH) can cause scaling which is what it sounds like you are having. Your pH is indeed higher than normal. You have 8.0 when 7.5 is closer to ideal. Your TA is also higher than usual at 150 when 80-120 is more typical. But it really depends on your Calcium Hardness (CH) level which you did not report. You should also measure your Cyanuric Acid (CYA) level since only the carbonate portion of TA matters so one adjusts for that by removing the CYA component of TA.

If I assume a standard salt level of 3000 ppm and a CYA level of 70 ppm (most SWG manufacturers recommend 60-80 ppm for CYA) and put in a pH of 8.0 and a TA of 150, then at a CH level of 300 ppm which is normal you get a saturation index of +0.5 which indicates a tendency towards scaling, but usually you wouldn't see than until at least +0.7 to 1.0 so I suspect that your CH may be higher at 400 or more since at 500 the saturation index would be +0.7

By the way, an SWG pool usually has a strong tendency for the pH to rise and this is made worse with a high TA. If indeed your CH level is high, you can take advantage of that by lowering your TA level at least down to 80 (the procedure is described here) as this will significantly lower the tendency of the pH to rise as quickly. The pH rises because the SWG generates a lot of hydrogen gas bubbles and these aerate the water pulling out carbon dioxide from the pool into the air. You can see these bubbles most readily if you run your pump and SWG at night and turn on a pool light.

Perhaps others can tell you how this can be treated. I know you need to get your water chemistry in line, but don't know the best way to treat the existing problem.

Richard
fatybabe
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Postby fatybabe » Tue 28 Aug, 2007 05:45

Lisa:

A number of factors contribute to scaling and most probably than not it might be the case.

In addition to the posted reply try reducing your free chlorine level down to 2ppm, you might be over feeding your system which raises the TDS levels also another contributing factor to scaling. If in such case you are worried that by lowering your ppm will promote algae growth, try using a 60% quat another item available out there is phosphate removes.

It seems to me that your filter aint traping the particules, double check your filter, when was your last replacement, has it been cleaned lately? If ur using sand, best bet is the particule sizes are finer than 20 microns id recommend that you change this to DE, will help in clarity and in terms of prapping the deposits your plaster is experieincing.
Backglass
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Postby Backglass » Tue 28 Aug, 2007 12:21

fatybabe wrote:If ur using sand, best bet is the particule sizes are finer than 20 microns id recommend that you change this to DE, will help in clarity and in terms of prapping the deposits your plaster is experieincing.


Huh? You cant remove scale with a filter...or do I misunderstand you? A DE filter will have no effect if your Calcium Hardness is sky high.
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fatybabe
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Postby fatybabe » Tue 28 Aug, 2007 22:33

Backglass trust me on this, my daily work is to deal with waste water treatment programs.

By changing your filter to DE you are trapping particule sizes much finer than that of SAND (1) this will increase the clarity of water (2) High TDS levels area contributing factor towards scaling. (3) High TDS causes pH increases.


Further more Scaling are not only caused by calcium hardness but can also be affected by pH and TDS levels. As Richard has already mentioned her TA levels are a bit of scale on the High mark this does suggest high levels of carbonates in her water.

All in all changing the filter media will not fix all her problems however it does help a whole lot, better if she has good circulation in the pool water.

Another cheaper alternative to do so is to use water clarifiers this will allow dirt to be trapped in the sand filter which also reduces the micron size which you want happening, however downfall to this is cosntant backwashing and addition of chemicals.

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