Water Chemestry Testing

[jhalbin]

I have an indoor pool in Lenexa, KS, 53,550 gallons with sand filter, 3 sides of the pool are above ground and one side is not, painted cement interior finish, flow rate is 150. I should also mention our salt is 5,000 ppm per manufacters recommendation.

We are currently looking to expand our hours to 6:00 am to 8:00 pm at night. I am wondering how I should set my day up for water testing. I have about 50 users from 8:00 to 12:00 and then 50 useres from 2:00 to 4:00 and I am unsure what the afternoon will hold since we have not been open during that time. I am thinking I should close the pool from 12:00 to 2:00 to allow for any chemicals I would need to add and vacuum time each day. I am just unsure if I need to do an am, mid day, pm test -- is this overkill or will I need it to stay in balance. I am always fighting pH raising and I think I will need the middle of the day to add muratic acid to keep it in check. I also need to show my administration proof of what is recommended to keep the pool running safely and effectively.

I have looked in my CPO book and it really does not say much -- up to the operator.

Thanks for any help you can give me!

Jackie Halbin
Lenexa, Kansas

 

[JohnT]

Welcome to TFP Jackie!

If you have a SWCG, you should be able to set it to maintain chlorine through the day.

Your pH rise problem probably is a result of your SWCG. You may be able to lower your alkalinity which will reduce the tendency for the pH rise.

How much acid are you adding daily? What pH range do you see during a typical day? What is your TA?

 

[chem geek]

You are likely to have Kansas state and/or Johnson county requirements for commercial and public pools requiring a specific frequency and type of testing, keeping logs, etc. According to NSPF, Kansas has no state regulations for public pools, only RWI prevention "guidelines". The cities of Lawrence and Wichita are listed explicitly with their own codes. So perhaps you don't have specific codes you have to follow after all.

Just note that water chemistry levels, especially chlorine and pH, can change rapidly in high bather-load pools. Most codes have water chemistry checked every 2 hours during periods of high bather load.

 

[jhalbin]

I try to get the pool down to 7.2 at night and it is there when I leave. I add about a gallon to a gallon and a half a day of muratic acid a day. In the morning when I come it is usually is at 7.4 and then by the end of the day it is up to 7.6.

I run the alkalinity up to 110 and within 3 days it is back down to 70 or 80 because of the muratic acid.

I don't add sodium bicarb and muratic acid at the same time because it just seems to keep from dropping the pH, so I add the becarb on one day and then add acid.

 

[chem geek]

Do you find that the rate of pH rise and especially the amount of acid you need to add is lower when the TA is low at around 70 ppm? If so, then you should not be raising the TA to 110 and should keep it lower. A higher TA will usually result in faster carbon dioxide outgassing and that causes the pH to rise. You should also not try and lower the pH as much (i.e. not to 7.2).

1.5 gallons of full-strength Muriatic Acid (31.45% Hydrochloric Acid) would lower the pH from 7.6 to 7.0 and the TA from 110 to 96 ppm (i.e. 14 ppm per day). So after 3 days the TA would be 68 ppm which is about what you are seeing, but the pH doesn't get to 7.0 after you add acid probably because carbon dioxide gets outgassed much faster at such lower pH (so the pH doesn't stay at 7.0 for too long).

You could try just getting the TA to 80 ppm and see how much acid you need to add to get the pH down just to 7.4 and not below. See if the pH doesn't rise too much and add acid to lower it. If the pH rises to 7.8, that's OK. To minimize pH rise when adding baking soda, add it slowly over a return flow but don't get the TA above 80 and then at least 10 minutes later add the acid slowly over a return flow.

Unfortunately, part of the pH rise is due to the SWG itself, possibly from a combination of greater aeration and from undissolved chlorine gas outgassing. Having the TA be lower will help reduce the effects of aeration, but won't do anything about chlorine gas outgassing (if that's happening). Are your returns pointing straight out or upward? If so, you might try pointing them slightly downward, especially for the return(s) closest to the SWG.

Do you have any idea of the amount of your daily Free Chlorine (FC) consumption? Do you know the output level of your SWG and its on-time (pump run time and SWG % on-time, for example)? Also, I presume because this is an indoor pool you are not using any Cyanuric Acid (CYA), correct? A small amount (around 20 ppm) of CYA along with having a 4 ppm FC target can help reduce chlorine outgassing, but will slow down the rate of oxidation (and rate of creation of disinfection by-products). Unfortunately, adding CYA is not something easily undone.