Huge Acid Demand - Help Please

[Steve]

So a little background. Pool is around 20 yrs old, current diamond brite around 12 years. This past 'winter' I kind of let the pool go and it had algae on/off most of the winter. Had some trouble cleaning it up as even after I shocked it algae would seem to come back pretty quick. Also it was looking a bit dingy. So... I decided to drain, acid wash and refill (2nd acid wash). Got it all filled and balanced and pool looks fantastic but now I have this huge acid demand. Pool never drank so much in the past, maybe a cup or two a week. Now EVERY DAY I get 2-4 drops (roughly 2-4 need cups) needed on acid demand test (I use a K-2006). Pool is roughly 11k gallon SWG w/Autopilot in South Fla (under screen).

Here are my current #s:
pH - Keep putting it down to 7.5 ish but it jumps up to > 8 daily
FC - 4, CC - 0
TA - 80
CYA - ~70ish
Cal - 240
Salt ~ 3400 ppm
Temp ~ 86

I understand that new plaster could affect pH rise/acid demand but this is > 10 years old and never had the problem, even after first acid wash. No additional added water except some rain since refill ~ 2 weeks ago. The pool sucking acid faster than a jet sucking fuel is driving me nuts.

Any ideas on what is going on and how I can get acid demand back to like a cup or two a week like it used to be?

 

[JasonLion]

In some cases an acid wash will expose a deeper layer of plaster that has not yet fully cured, giving a fresh plaster kind of effect on the PH (rapid rise for about a month, and slower for up to a year). However, I have never heard of that happening on a pool as old as yours. Plaster deeper down does cure eventually, and 12 years should be more than enough for it to cure all the way through.

In any case it is worth lowering TA to around 60, or letting it get there on it's own. That will help a little, though perhaps not all that much.

 

[laceygirl]

Would borates help to stop the fluctuation of the pH?

 

[JasonLion]

Adding borates will slow down the PH increase significantly, but it won't stop it, and it won't change the total amount of acid needed.

 

[Steve]

So this continues to frustrate me. I get the TA down to 60 or just below but it goes back to 70 or more in just a few days. I have to add around 2 cups DAILY just to keep it in normal range and more like 3 or more cups daily to keep the TA down. I'm going through jugs of acid way too fast (and too costly) and the thing that is puzzling me is this pool NEVER did this before, not after it was diamond brite'd and not after first acid wash. There just has to be something going on or something I can do to normalize this and get it back to the 1-2 cups per week it used to be. At this point I'm ready to fill it with concrete and it has never frustrated me that much in 20 years. Is there nothing else I can do, I just don't believe this isn't fixable.

 

[JasonLion]

There are very few things that can raise TA. One is acid etching the plaster, which raises PH, TA, and CH. Another is things which increase the PH, like baking soda, soda ash, borax, lye, etc. And the final one is plaster curing.

How old is your PH test reagent? Also, any chance the FC level has been above 10 this whole time? (I am starting to think about some really uniquely possibilities.)

 

[Steve]

Plaster should be cured, none of the other things ever added but yea, was acid washed with a 50% solution. I don't think FC was ever > 10 except maybe right after fill before I added stabilizer and balanced everything out. It's been running around 1.5-5 (while I dial in the SWG). My reagent is around 18 months old, stored indoors in the a/c in its dark container.