Consantly rising pH

[TreeFiter]

One of my pools has an issue with constant rising pH. It is a plaster pool with a SWG, built this season. I maintain this pool twice per week, Monday and Friday. It is a commercial pool so it cannot use CYA and must run at 100% to maintain FC between 3 and 5. I add about 1/2 gallon of muriatic acid almost every time, because the pH climbs to about 8.2. At first I was fighting relatively high TA, but I have since knocked the TA down to about 40. The TA has stabilized at 40, and doesn't seem to want to drop below that. Despite the stable TA, and the regular addition of acid, the pH continues to climb.

So how does the pH continue to climb with depleted TA? At some point the CO2 should run out, and the TA should drop.

Any ideas?

 

[JoyfulNoise]

Was the plaster started properly?

Your rising pH has nothing to do with your TA at this point and the pool has a very significant source of alkalinity in it - the plaster! The plaster was not likely properly cured (bicarbonate start up process) and you are now seeing calcium hydroxide leaching from a weak and porous plaster surface.

You can expect the pH to continue to rise for sometime after plastering (as long as 18 months). You need to raise the TA of the pool water to promote the formation of calcium carbonate in the plaster matrix and manage the pH. Large pools like this often need dedicated acid dosing systems as managing pH by hand is too labor intensive.

What are your options here for an acid dosing system?

 

[TreeFiter]

Was the plaster started properly?

Your rising pH has nothing to do with your TA at this point and the pool has a very significant source of alkalinity in it - the plaster! The plaster was not likely properly cured (bicarbonate start up process) and you are now seeing calcium hydroxide leaching from a weak and porous plaster surface.

You can expect the pH to continue to rise for sometime after plastering (as long as 18 months). You need to raise the TA of the pool water to promote the formation of calcium carbonate in the plaster matrix and manage the pH. Large pools like this often need dedicated acid dosing systems as managing pH by hand is too labor intensive.

What are your options here for an acid dosing system?

I'm not sure what my options would be at this point. The customer is pretty hands off with the pool, which is why I'm there twice a week. He has employees taking daily measurements, and brushing and vacuuming as necessary, but they are not capable or interested in adjusting the chemistry.

There has been some scaling (white staining) on the bottom at times, and with the addition of acid, it has gotten better. If the pH is allowed to come up, the staining comes back. Wouldn't raising the TA to promote formation of calcium carbonate cause more scaling? It is a black finish so any scaling stands out.

 

[JoyfulNoise]

Yeah, a black finish to the plaster is going to be a problem so forget about adding any TA to the water. I think you're simply just stuck adding acid to the pool until the plaster settles down. Can you add borates to the pool? If not, can you get some kind of acid injection pump set up? If so, you could simply set it to dose a little bit of acid each day. It might not completely alleviate the pH rise but it could help keep a lid on it so that your not walking into a high pH mess every time you visit.

 

[TreeFiter]

There is a Liquid Chlorine system in place. It was installed to chlorinate the pool for the first month before the SWG could be used, and as a backup if the SWG ever fell short. Would a system set up for liquid chlorine be able to tolerate muriatic acid? Its just a peristaltic pump and some tubing, so I'm thinking it would be fine. Might be worth trying. I'm not sure about having a big tub of Muriatic acid sitting around without a sealable container.

 

[JoyfulNoise]

Folks do Stenner based acid metering all the time. Just make sure you get all new tubing to avoid mix ups (liquid chlorine + muriatic acid = chlorine gas bomb). The major automated pH systems all use Stenner pumps.