Acid Demand

[piku]

Hey guys,

Many of you have been through my saga. Basically when I first started my Ph was very stable and my pool had an ammonia problem. I had no algae or problems but all the chlorine I put in my pool would be instantly converted to CC. I eventually gave up and put in roughly 2 qt 6% bleach a day and called it a day. After about a month, to my surprise, the pool started holding chlorine and has been "normal" ever since. My CYA was 0 so I was using trichlor to get my CYA up but I overdid it with stabilized granules and my CYA got to 70. I stopped using trichlor. All I ever really used was 4-5 pucks in a floater at a time and the floater sucked so the pucks would last a really long time...

Fast forward to now, I am using 12.5% chlorine from a local supplier. No trichlor. My pool consumes what I consider to be a large amount of chlorine - around 2 qt 12.5% per day with much of the consumption during the daylight hours. My CC is 0. Once I started down this path my pool started having a huge acid demand. It's consuming around 3 qt per week of muriatic acid to keep the Ph at 7.5. My CH is 325 and hasn't moved all year. My TA is at 130 and hasn't moved all year. Any idea what to do to quench the acid demand? My plaster is very old (25 years) so that shouldn't be causing it. I add chlorine all at once in the evenings. When nobody is swimming in the pool I might add every other day instead of every day.

I honestly don't mind the money or the high chlorine consumption but I really hate handling muriatic acid. I'm surprised my TA hasn't moved at all despite rmeoving and adding water, lots of swimming, etc.

If this thread needs moved to another group that's fine...

 

[chem geek]

2 quarts of 12.5% chlorinating liquid in 17,000 gallons would be 3.7 ppm FC and you are right that this is a lot of loss per day. What FC level do you raise the chlorine to that is 3.7 ppm FC higher than when you next add it? I assume you have measured the loss at night and find it to be very low (if so, how low), correct? It's important to distinguish the loss during the day vs. at night (i.e. average FC loss per hour during each). You said that you can add it every other day when no one is swimming in the pool, so what is the difference in chlorine demand with someone swimming vs. not or is the every other day demand 7.4 ppm FC for 2-days?

3 quarts of Muriatic Acid (31.45% Hydrochloric Acid -- make sure your acid isn't half-strength 15% or 16%) would lower the pH from 7.5 to 6.9 so having the pH remain stable would need carbon dioxide outgassing such that the TA would drop from 130 to 108 ppm. However, this implies that the pH would rise from 7.5 to 8.9 for the acid to lower it back to 7.5 so that doesn't sound right unless you see a rise and add acid more than once per week. I did these calculations assuming there were no Borates in the water -- if there are, let me know.

So you've got two very strange mysteries going on. It's possible that along with the ammonia, you had a lot of organic compounds in the water and that these very slowly oxidize, but if that were the case then the chlorine demand should be similar during the day and night. Water replacement over time may be the only solution to this assuming that the organics aren't in one simple place like stuck in the filter that can be cleaned.

As for the acid and the TA, see what the TA level is in your fill water. If the pH goes up and adding acid doesn't lower the TA over time, then that implies some sort of regular addition of a base to the water making the pH rise which doesn't make much sense, or there is addition of additional TA from the fill water after evaporation. If the pH rise was solely from outgassing, then the TA increase from fill water that had a TA of 100 ppm would need 108+100*f=130 or f=0.22 or 22% water replacement per week which doesn't make sense. Even fill water with a TA of 200 would need 11% water replacement. Evaporation in your area is 40" per year so even if this were mostly during 6-months of summertime, that's only 1.6" per week or about 3% of a pool's volume assuming an average 4.5 foot depth.

Richard

 

[piku]

Richard,

I need to buy more chlorine tonight but I am going to try to digest my consumption. To be honest I might have a slightly incorrect picture of things but when I test before adding the FC usually falls to 1-3ppm which is below my minimum at CYA of 70. This suggests algae could be ever so slightly taking hold and I need to shock but there is no CC. It seems like the chlorine I add will get used very quickly but once it falls to 1-3ppm FC it will sit there for days without further consumption. I'll do some further experiments and report back.

There was some organics, dirt, etc that I have been cleaning up little by little. The skimmer was filthy, etc.

 

[piku]

Richard, I tested CYA tonight for the first time in at least a month. It's down to 45 which might explain why I don't have algae letting it fall to 3. The TA of my fill water is 80. I tested FC and it's 3 right now. I'll see what it is tomorrow morning.