Indoor Gunite Pool Chemistry

[dmccarthy]

I have an indoor, heated (93 degree) pool in my fitness center. This is different from my residential pool that my signature indicates. I am considering switching my indoor pool to a SWCG but would like advice for making the switch. The indoor pool is 24,000 gallons. I just recently changed the water out to do annual maintenance. I currently have a hayward ecostar 1.85hp VS pump and a hayward pro-series 500lb high-rate sand filter. I currently sanitize with bromine (with an in-line brominator) but want to switch to salt (if possible) and would like to know how to go about it the best way.

The major issue in this pool is bather load and temperature. I need to keep the temperature at 92-94 degrees, also there are probably about 150 bathers/week in the pool spread out over 6 days/week with some days having upwards of 30-40 people spread out over 12 hours and some days a little lighter bather load.

Most of the pool companies in my area use the hayward aquaplus SWCG so I would probably stick with one of those systems, however I would like to know if one SWCG cell for up to 40,000 gallons would be enough? One of the pool companies had said that it might be a good idea to have a second SWCG so that I am not running the single-call generator at nearly full power all the time to keep up with the bather load. Would like some feedback on what to do and what would make the most sense.

Lastly, is it safe to make the switch to SWCG from bromine without switching the water out? I just changed the pool water about 3 weeks ago.

Thanks,

 

[chem geek]

You should of course check your state regulations which for Massachusetts is in this PDF file. Regarding disinfection equipment it says the following:

(4) Suitable automatic equipment shall be provided and so installed as to permit adequate disinfection of all the pool water. Hypochlorinators shall be dependable in operation and equipped with a calibrated controlling device capable of being finely adjusted to the required rates, and shall have a feed-rate capacity of at least three pounds of chlorine per 24 hours per 10,000 gallons of pool capacity for all outdoor pools, and at least one pound of chlorine per 24 hours per 15,000 gallons of pool capacity for all indoor pools.
Chlorine gas feeders and containers are prohibited.

For bather-load capacity, it says the following:

(1) For the purpose of computing bather load capacity, those portions of the swimming pool five feet or less in depth shall be designated as "non-swimmer" areas. Portions of the pool over five feet in depth shall be designated as the "swimming" area.
(2) In order to compute swimmer and bather capacity, swimming and wading pool areas shall be determined as follows:
(a) 15 square feet of pool water surface area shall be provided in the non-swimmer area for each non-swimmer expected at time of maximum load.
(b) 20 square feet shall be provided in the swimming area for each swimmer expected at time of maximum load.
(c) 300 square feet of pool water surface area shall be reserved around each diving board or diving platform and this area shall not be included in computing the area of the swimming area.
(d) 100 square feet of pool water surface area shall be reserved around each slide and this area shall not be included in computing the area of the swimming area.
(e) The bather load capacity shall be stated on the permit. (see 105 CMR 435.21(1)).
(3) The Board of Health may make additional allowance for bathers in cases of swimming pools with extensive deck areas used by patrons for lounging or sunbathing. These allowances shall be based on studies of actual swimming pool use in areas within the jurisdiction of the Board of Health.
(4) Ten square feet of water surface area per bather shall be used in computing the bather load capacity for special purpose pools.

There is a table showing that pH must be from 7.2-7.8, TA from 50-150, and FC from 1.0-3.0 (with CC from 0.0-0.2). Testing is required 4 times per day, at least once during peak bather load. Interestingly regarding CYA the code says:

(5) If cyanuric acid is used to stabilize the free available residual chlorine, or if one of the chlorinated isocyanurate compounds is used as the disinfecting chemical, the concentration of cyanuric acid in the water should be at least 30 mg/l, but shall not exceed 100 mg/l.

If you just changed the pool water about 3 weeks ago, have you since added more sodium bromide to it or used bromine tabs already? If not, then you can just go to an SWCG right away. If you've used bromine though, you'd have to change the water to turn it back into a chlorine pool. If you didn't add sodium bromide and just started using bromine tabs, then if you haven't added very much, you could use an SWCG and the bromine will over time outgas, possibly within a month or two if not much bromine was used (just a few ppm).

This pool sounds like a "special purpose pool" which "means a unit designed for recreational and therapeutic use which is shallow in depth and not meant for swimming or diving". In that case, you are allowed 10 square feet per bather. It sounds like you won't be anywhere near this maximum during peak.

As for sizing the SWCG, let's start with the code which requires "one pound of chlorine per 24 hours per 15,000 gallons of pool capacity for all indoor pools". For your 24,000 gallon pool that means (24000/15000)*1 = 1.6 pounds per 24 hours. The SWCG cells that claim to handle 40,000 gallons are usually 1.4 pounds per 24 hours so would not meet state code requirements. You need to use the 60,000 gallon sized cells that output 2.0 pounds per 24 hours or use more than one smaller cell. Let's just see what the state minimum supports in terms of bather load. Every person-hour in a pool requires roughly 4 grams of chlorine to oxidize the bather waste while in a hot (104ºF) spa it's roughly 9 grams. This therapy pool is a bit in between. There isn't swimming, but it's warmer. Let's use 6 grams to be a bit conservative even though it may only be 4 in reality. Over 12 hours, we have 1.6/2 = 0.8 pounds of chlorine which is 363 grams so would handle 363/6 = 60 person-hours (so 60/12 = 5 people for 12 hours). You indicated a peak of 30-40 people spread out over 12 hours, but you need to be more specific with "spread out" since I assume all 30-40 are not in the pool the entire 12 hours. Specifically, what is the maximum number of people in the pool at any point in time? If I assume the people are in the pool for 2 hours each, then that would be around 7 people at any one point in time which over 12 hours would be 84 person-hours. So this would require more than the state minimum for chlorine output.

Basically, the SWCG needs to be sized to handle the peak bather-load, not the total over 24 hours. Even though in reality much of the bather waste will be oxidized over many hours, from a worst-case it is sized to handle the peak as if all the waste needed to be handled immediately. This would then handle the situation of having that peak last throughout the day. For the example above, I came up with 7 people in the pool at the same time. So per hour that is 7 person-hours per hour so 7*6 = 42 grams per hour or 42*24 = 1008 grams per 24 hours or 2.22 pounds per 24 hours. Given our conservative assumptions, you should be fine with a 2 pound per 24 hour unit if the peak number of people were 7 in the pool.

The SWCG manufacturers, especially for commercial use, have spreadsheets they use to size the units and factor in more than bather load but also regular losses/usage, though usually with commercial/public pools the bather-load dominates especially for indoor pools. Since you are paying them for their systems, you should have them do these calculations for you, though the above will help you make sure they are doing them at least roughly right.

As for whether to use CYA in the pool, the state regs mean you could have 3 ppm FC with 30 ppm CYA but that may not be a high enough active chlorine level to handle bather waste well in an indoor pool though with the warmer water temperature it might be OK. It would have been better if the state would have allowed 4 ppm FC with 20 ppm CYA, but they don't. With no CYA, you could have 1.0 ppm FC and while this is higher in active chlorine, it will handle the bather waste faster. Since this is an indoor pool, do you have any supplemental systems such as UV or ozone? How are you managing to keep Combined Chlorine (CC) no higher than the 0.2 ppm that the state allows?

 

[dmccarthy]

First of all, thank you very much for all the information. Currently we test 4x/day and I keep the numbers as follows:
FC = 1-3 ppm (with the bromine registering at 5-6) as we use the taylor drop test kits. We use the brominating tablets through an in-line erosion-type feeder which is checked daily and bromine tabs are added as needed.
CC= 0-.2 and when it reaches .2 we typically use either liquid sodium hypochlorite or one of the powdered shocks using the the NSPS CPO course regulations and then I do the formulas for calculating breakpoint chlorination (CC x 10 - FC) and calculate how much product to use. We run our pump to get a turnover of 4-6 hours even though I think MA is 8 hours.

I think you were very close in your peak bather load calculations. I think the most we have in the pool at any one time is 10 bathers. So I think by your calculations that would bump the peak over 24 hours to 1440 grams or 3.2 pounds. Otherwise our numbers are as follows:

pH = 7.4-7.6
TA = 100-120
CH = 200-250ppm

Let me know what you think when you get a chance and again, thank you very much for all the information.

 

[chem geek]

Yes, your 3.2 pounds over 24 hours would be what is needed to handle the peak bather load. That's probably one 60,000 gallon cell at 2.0 pounds and one 40,000 gallon cell at 1.4 pounds or you could double up on two 60,000 gallons cells so each backs up the other if either fails. The economics are such that you save money because you don't run the cells as long so they last longer. Again, talk to the SWCG manufacturers to see what they say. They may "ride out" the peak bather load and have you catch up on the FC drop after that bather load subsides, but the state may want you to size for the peak.

I think when you switch to using chlorine you might need to target a lower TA level if you find your pH rising, but you can see how things go. Some of the shock that you are using (especially non-chlorine shock) is net acidic so currently keeping your pH down, but when you use a saltwater chlorine generator it will tend to have the pH rise mostly from the increased aeration from the hydrogen gas bubbles. By the way, the 10x rule isn't actually correct since it came from chlorine oxidation of ammonia where the ammonia is measured in ppm Nitrogen units, but CC is measured in the same ppm molecular chlorine Cl₂ units as FC so a factor of 5 difference goes away because of that. In addition, CC has at least one chlorine already attached to it. If you need to get rid of CC, exposure to UV as in sunlight or a UV system is the simplest way. If you cannot do that, then if you superchlorinate then do that at night and aerate the water and vent the room and then bring the chlorine level back down for the morning. Hopefully with your SWCG system you won't have your CC get too high, but it's difficult in an indoor pool with no UV especially with your higher bather loads.

 

[dmccarthy]

That's great advice. So it sounds like roughly I would be looking at 2 saltwater cells capable of handling up to 60,000 gallons each to handle the chlorine generation based on my peak bather load and a UV system to eliminate the need to have to shock to get rid of the CC? Like you said the company selling it should be able to calculate it for me, however, it is always tough to know if you are getting the best advice from the person selling you the product (I.e. the company that talked me into the nature 2 for my home pool) so I appreciate the advice. Right now I keep my target TA on the high side in the bromine/indoor pool because the bromine really drives down the ph and you are right I usually use the liquid shock with the hight ph to offset. If I am reading your information correctly it sounds like between the 2 salt cells and a UV system I could keep everything in check? I would love to eliminate (or nearly eliminate) the need to have to constantly be in contact with staff and direct them on how to go about shocking, adding chemicals, etc., all the time in my absence.

Also, I forgot to put it in the last response. You had asked whether I was using sodium bromide or not as to whether or not I could make the switch without having to change the water out. I have always used the bromine tablets through an in-line chlorinator/brominator? Not sure if that is the same thing or not. But the water is about 3-4 weeks old and ideally I would like to not have to spend the extra $$ to switch it out again, but if I have to and it is best I would. Thanks again.

 

[chem geek]

Usually the guys dealing with commercial/public pools tends to be a little better in the integrity department.

Yes, between the two salt cells and a UV system you should be in good shape. The SWG guys may say that you don't need the UV system and you could certainly try that to see if the SWG alone is sufficient, but I suspect you may need it so you can get out of the shocking to remove CC. There are other methods towards improving the water quality such as periodic use of HaloSource SeaKlear PRS Stage 1 and 2 which is a powerful coagulant combination that will remove virtually all organic chemicals into the filter where if you backwash/clean the filter you will remove organic precursor chemicals such as urea before they react with chlorine. That not only would reduce the production of CC but would reduce the need for as much water replacement (though your state doesn't seem to require that -- some require 7 gallons of water to be replaced per bather).

Your pH will tend to rise so you'll likely be targeting the lowest TA allowed by law which in that link I gave earlier allows for a TA from 50-150 ppm so is rather unusual in allowing such a low TA. You might only need to get to 60 or 70 ppm, but if necessary could go to 50 ppm if that reduces the amount of acid you need to add. You would also target a higher pH level where again the state allows 7.2-7.8 so you'd target 7.7 or 7.8 and the combination of lower TA and higher pH will reduce the rate of carbon dioxide outgassing and therefore the rate of pH rise and amount of acid addition to maintain pH. If your Calcite Saturation Index (CSI) is too low, you can increase Calcium Hardness (CH) to compensate.

Since you have used bromine tablets for a while, that would build up a sodium bromide bank because the bromine only goes away (from outgassing or water dilution) slowly especially when using bromine tabs that leaves 5,5-dimethylhydantoin (DMH) behind. Do you know exactly how many bromine tablets you've added by weight? Do you know if they are 1-Bromo-3-chloro-5,5-dimethylhydantoin (BCDMH) or 1,3-Dibromo-5,5-dimethylhydantoin (DBDMH)?

 

[dmccarthy]

The container says "Bromochloro-5, 5-Dimethylhydantion 98%, other ingredients 2%. It says "64.0% available bromine, 29% available chlorine. Since changing the water I would say we have added about 15lbs or so of tablets to the brominator over the past 3 weeks.

As far as the SeaKlear PRS Stage 1 and 2 is that something that I could use with my existing bromine-based system right now to help reduce the production of CC? I went to their website and I am a little unclear how the products work, are they heavy-duty clarifiers? Based on what you wrote above is this something that you would add prior to shocking and then backwash the filter once the product has had a chance to work? It sounds like a really good product as I would like the water as clear as possible and sometimes after shocking the water in the indoor pool never gets truly "sparkling" and "blue" again.

Thanks again

 

[chem geek]

So 15 pounds of BCDMH (which is what you used) in 27000 gallons 66.57 * 2 * 2*79.904 / 241.47 = is around 88 ppm bromine (equivalent to around 39 ppm FC) added to the pool and that's substantial and not something that will outgas quickly, especially not with the DMH in the water that partially binds to bromine. That's unfortunate and means you are stuck with a bromine pool until you next drain/refill.

Yes the SeaKlear PRS is a heavy-duty clarifier/flocculant. It's in two stages since stage one is like standard positively charged cationic polymeric clarifiers that remove negatively charged particles including most cells so bacteria, algae, and oocysts. The second stage is more like a flocculant in that it consolidates/combines the smaller clarified bundles and also deals with uncharged (and possibly some positively charged) particles, though it gets filtered out (not precipitated to the floor bottom). Note that you have to use stage 2 ideally 4-6 hours after stage 1. The difference is that this two stage approach is able to remove much smaller particles and a larger variety of them than single-stage clarifier products. It's most appropriately used in high bather-load situations such as in commercial/public pools.

 

[dmccarthy]

Maybe I could look into a UV system in the meantime to keep the CC in check and the next time I switch the water out would be the time to go to salt? Thanks for your help.

 

[chem geek]

Sure, a UV system will often help control the chloramines for an indoor pool. It needs to be a more powerful UV system, not just the minimum needed for disinfection.

 

[dmccarthy]

Thanks!