Advice re. high TDS and CH

[amadeus]

Hello, everyone!

First and foremost, this is a spectacularly educational site! I have learned so much from all the posts. Thank you everyone for generously sharing your time and knowledge.

I need your opinion and advice about a possible RO treatment of the pool (or leaving things as is for the time-being -- for how long? What are the risks?).

The pool is in-ground gunite/plaster with tile waterline plus an overflow spa.
Mostly 10% bleach as chlorine source, muriatic acid for pH control, occasional correction of TA with sodium bicarbonate. No other chemicals to speak of except for boric acid loading about a year ago to introduce the borate.The current water parameters are below (reliable tests, LaMotte colorimeter or lab-quality pH/conductivity/TDS meters):

FC 5
TC 5
pH 7.2-7.3 (staying on the lower end because of CH)
TA 90-100
CH 490
TDS 2,300
CYA 50-60
Borate ca. 30

There are no serious issues with the pool yet: no algae to speak of; usual brushing takes care of it easily, and the robotic cleaner does its job pretty well too; no visible scaling, although the pool cleaner started to pick up much more grayish "dust" which was not the case before about 2 months ago. The water starts to taste a little "salty" but not a big deal.

Two things I noticed are (1) pretty sudden drop of CYA (was over 140 just 6 months ago, and is now less than half of that. Some minor water changes when filter was back-washed, but not enough to cause such a big drop), and (2) much harder time keeping FC/TC levels up; the other day the overnight drop was from 10 to 3 ppm (acid corrected as usual, then added bleach). Thoughts?

Do not want to drain an refill (and pretty pointless too, the supply water is about 370 CH anyway). Is it time to bite the bullet and do the RO treatment? I do not like the idea of precipitating, flocculating and vacuuming calcium.

All thoughts and experiences are welcome and much appreciated. Thank you!

 

[mas985]

Welcome to the forum.

I have chronically high CH as well because of my fill water and I just live with it up to about 800 ppm (double fill water). As long as you keep your CSI between -0.5 and -0.25 by managing PH, you should be ok. If you have high TA water as well like I do, you might want to consider an automatic acid dosing system to keep PH in check.

 

[amadeus]

Thank you for responding! Wow, you are staying in the really negative CSI range, I did not go that low (yet). The TA is manageable, plus I am topping off with softened water now, which is not to say that an automatic acid dosing pump would not be a good thing to have! Thanks again.

Welcome to the forum.

I have chronically high CH as well because of my fill water and I just live with it up to about 800 ppm (double fill water). As long as you keep your CSI between -0.5 and -0.25 by managing PH, you should be ok. If you have high TA water as well like I do, you might want to consider an automatic acid dosing system to keep PH in check.

 

[mas985]

-0.25 is not really that low. It is within the recommend range for those with swgs to prevent scaling. Below -1 I would consider low and where the water can start to get aggressive.

 

[duraleigh]

Welcome to the forum.

You have a very good grasp of the TFP concepts. Two things jump out...


1. Most importantly, a loss of 7 ppm overnight is a serious problem. You should SLAM that pool without hesitation if that testing is accurate.

2. Your CYA loss over the last six months would indicate a testing error.....it simply doesn't drop like that.

There is no need for any draining as your tds and CH are quire manageable as mas985 points out. Additionally, if you are refilling with softened water, CH and TDS will be in constant decline.....the numbers will get better and better.

What does your water look like? Tell us more about that overnight FC loss.....if it is correct, you need to take some action now.

 

[amadeus]

Thank you for the response, Dave! The water looks crystal clear, virtually no turbidity at all (I can barely see a few particles even at night when the lights are on ... Or point a laser light into the water at night. Anything insoluble really lights up). There is pretty much no (visible) algae -- every now and then there is a mustard spot here and there, but that happens only when the chlorine level drops too low and is easily brushed off. No sediment on the bottom, but the pool cleaner is picking up more than usual grayish "dust").

FC loss is what alarmed me the most too and triggered more rigorous testing of CYA too. I have different test kits (K-2006, LaMotte's ColorQ, Pentair's black dot), and all of them converged on ca 50 ppm. They all were off scale in the winter, at 140-150 (I actually had to dilute the samples to get a reading). I do not think there is a leak: CH and TA did not drop and I did not really add much more water (no autofill, so I just open the fill valve and time it). There is, of course, some splashing, and I back-washed the filter too, but that would not account for a 50%+ water change (I realize the CYA tests are not that accurate, but still).

I will repeat the overnight FC loss test. The way I did it last time was pretty straight forward: adjusted the pH from 7.7 to 7.3 (just over half a gallon of 31% acid, poured slowly by a return jet on the deep end with the pump running full speed), then put in two gallons of 10% bleach. Only did a paper strip test, looked good, but I know that this amount usually increases FC by 8.5-9 ppm if the bleach is fresh. Next morning (about 11, so probably about 2 hrs of sun exposure) used K-2006 and got 2.5 ppm FC and zero CC (well, maybe, maybe there was a very faint pink tint on the CC test, but it went away with one drop). Repeated with the ColorQ; same thing. So, whatever the bleach "burned" in the water overnight was not algae or some other complex organic matter, right? The bleach could have decomposed, I guess, if the acid was not mixed well when I poured the bleach, but I have been doing this times and over again without any problems.

I still am puzzled both by the loss of FC and y the significant drop of CYA. I understand that it does not just disappear -- but it is not there now.

Welcome to the forum.

You have a very good grasp of the TFP concepts. Two things jump out...


1. Most importantly, a loss of 7 ppm overnight is a serious problem. You should SLAM that pool without hesitation if that testing is accurate.

2. Your CYA loss over the last six months would indicate a testing error.....it simply doesn't drop like that.

There is no need for any draining as your tds and CH are quire manageable as mas985 points out. Additionally, if you are refilling with softened water, CH and TDS will be in constant decline.....the numbers will get better and better.

What does your water look like? Tell us more about that overnight FC loss.....if it is correct, you need to take some action now.