Spa Chemistry - dichlor, ozone, and pH

OK - Janet said it was OK to post so here I go:

Have a new (2002 Sundance Optima) spa that I turned in service last week. I have rural well water and I am in WI so the water is both hard and alkaline. Using another forum I don't get quite the attention to detail that I like so I hope there are other spa folks who can give advice.

Like the way I care for my small AG pool, I think I am attempting to achieve perfect balance correctly but have some nuiances that I don't quite understand just yet.

I am dealing with a mere 480 gallons. TA was 400+ and I have been slowly adjusting down towards the ideal of 100 - 120 ppm. I am chlorinating with granular dichlor and have a brand new OEM ozonator (Corona Discharge) working perfectly well.

Bather load is quite light, the tub is covered with an insulating cover (no blanket), it is kept at 102 degrees, and the avg. air temp (day and night) is now in the mid 30s. Current readings at 4:45 pm today after a use and 1.5 tsp of dichlor last night were:

1.5 ppm FC (down from 3.0 this morning with no use)
0 CC
TA 100 ppm (down from 120 last night)
pH 7.6 (has been moving up towards 8.0+ and I have been adjusting it down with dry acid)

This is the lowest my TA has been. This morning I added 1 oz. of dry acid to get my 7.8+ pH down a bit. My TA fell sharply from 120 ppm to 100 ppm just with this morning's addition.

So, here are the nuggets -- Do I try to keep my FC at 3 - 4 ppm even with my ozonator or should I aim for a lower number? What may be keeping my pH rising with what seems to be very well balanced water and mostly neutral dichlor? Mind you, this is not a gunite or cement spa but rather the acrylic portable type.

Thanks to all who show interest.

AGE

The main issue is that Dichlor will continue to add to the Cyanuric Acid (CYA) level making the chlorine less and less effective. I would recommend only using Dichlor for about a week or two to get to around 20 ppm or so CYA and then switch to unscented bleach for sanitation. That way, you'll have enough chlorine to kill the bacteria that causes hot tub itch as it is harder to kill than most bacteria. I would recommend having a 4 ppm FC target when you add the chlorine after you get out and if you use it every day you would add chlorine every day (otherwise, just check the chlorine level and add as needed).

You will probably need to use non-chlorine shock, potassium monopersulfate (MPS), as hot tubs get a lot more organics for the small water volume and don't get sunlight that helps the chlorine break down those organics. MPS isn't needed for outdoor pools, but for indoor pools and spa/hot tubs it can be helpful. [EDIT] If you don't use the spa all the time, then you might be able to just shock with chlorine every now and then. You should probably just monitor the Combined Chlorine level and the water clarity to see how things go with a regular chlorine regimen, shocking with chlorine as needed. If that doesn't work, then you can use MPS (which unfortunately registers as Combined Chlorine though there is a Taylor K-2041 test kit that removes this interference). In my pool, for example, I recently used MPS to get rid of an oil film that wasn't going away with scum balls or enzymes and I didn't want to shock the pool with chlorine (which probably would have worked) because my wife was using the pool every day so I used MPS instead and it cleared up the oil film after only two applications.

If you get the TA even lower, to 70 ppm or even a bit lower (IF your CYA level is low -- if it's high, then a some of the TA comes from CYA), then you should find that the tendency for the pH to rise will be lessened. The procedure for lowering the TA, which uses a combination of acid addition and aeration at low pH, is here. The primary cause of pH rise is the aeration from the jets and even the use of Dichlor isn't enough to combat that -- plus the high TA makes the problem much worse which is why I recommend lowering the TA level.

As for the FC level and the ozonator, the problem is that the ozonator does not do anything to kill bacteria stuck to spa surfaces -- it only affects the water that is passing through the ozonator (plus some water downstream since the ozone does last briefly). So you still need a residual of FC in the main body of water to kill bacteria. The ozonator might help break down organics, lessening the need for the MPS or chlorine shocking that I described above, but you'll just have to see how it goes. The downside to the ozonator is that it can actually oxidize chlorine (into chlorate) thereby requiring more chlorine addition to keep up. If you have some sort of switch or option to turn off your ozonator, you might see if it changes your average daily chlorine usage.
[END-EDIT]

Richard

Richard -

Thanks. I have been following your posts for quite sometime.

I understand the affect of Dichlor and CYA. That is one reason that spas are "recommended" to have water changed every 3 - 6 months. I was concerned about the TA getting too low but if you think that 70 ppm is ok -- and that is where my point of pH stabilization occurs, then I am all for it. I thought that hitting around TA 100 would keep my pH stable as that is what I read. So, I will just keep and eye on things and see where I get the greatest stability.

As for the bleach -- because I could not find good details on maintaining a spa with bleach (other than a fight on the other forum and not contributors from this group), I went down the fairly inexpensive path that I mentioned. Nice to know that not much has changed in what is prescribed by this group.

Thanks again for your time and wonderful input. Hopefully others can learn from this lesson.

AGE

Just keep in mind that with Dichlor, for every 1 ppm FC it also adds 0.9 ppm CYA so if you were adding 4 ppm FC per day (which can happen if you are using it every day -- doesn't sound like your case though), then after 3 months that's over 400 ppm CYA. Even half that rate is still very high. That's why I mentioned switching to bleach after a week or two of Dichlor and you are right that pretty much no one in the spa world says to do that. They haven't gotten the word about chlorine and CYA the way that Ben spread the word here at The Pool Forum for pools, but there's nothing special about spas and hot tubs -- CYA has the same affect there as with pools in terms of reducing chlorine's effectiveness.

The main difference is that spas and hot tubs are generally not exposed to UV from sunlight since they are either not in the open or are covered (the exception begin "attached" spas that are really cycled water from pools). So algae is usually not a problem and in theory one could go with lower chlorine levels relative to CYA, BUT the bacteria that causes hot tub itch (Pseudomonas aeruginosa) is more resistant to chlorine than other bacteria so actually may need to have a slightly higher chlorine level than to keep away algae. I came up with the 4 ppm FC at 20 ppm CYA as a conservative number based on one published value of 30-50 for the CT (chlorine concentration in ppm times time in minutes) for this bacteria.

The other main difference is the one I mentioned about lots of organic buildup due to bather load since the water volume is so much smaller than pools. The same bather still sheds the same organics (probably even more in a hot tub due to sweating from the higher temperature), but it's concentrated in a smaller volume of water.

Richard

Richard, I always try to read your 'take' on these sort of questions, I don't always fully understand them - but I read them;) . In this case, would sodium percarbonate also work as the 'non-chlorine' shock? The reason I ask is because we have a supply of Pro-Team "system support" which is basicly the percarbonate and have some indoor pools/ spas that I TRY to take care of.

Thank you for the chemical expertise you provide to all the forums you are associated with, and for your willingness to share with those less knowledgible than yourself!!:)

Hope you and your pool are well:D -- again, thanks for all the info you've provided!!!!!!!!!!!

Ben never liked the percarbonates and thought they were more trouble than they were worth. See this link for more info (he doesn't mention percarbonates specifically, but refers to the other "per" oxidizers). Percarbonate, and any oxidizer that is stronger than chlorine (such as ozone), has the potential to oxidize chlorine to chlorate so this is why ProTeam System Support is only recommended for Baquacil pools. It's effective against white water mold which is a common problem in Baquacil after a couple of years. I wouldn't use it in a chlorine pool unless you add an algaecide, such as PolyQuat, then let the chlorine drop to near zero, then use the percarbonate, then after a day start up the chlorine regimen again and don't let anyone use the pool until the chlorine is back in the pool and stable -- sounds like a pain in the rear.

By the way, sodium percarbonate can be used to convert a Baquacil pool to a chlorine pool by getting rid of the Baquacil faster and without the colors associated with a conversion that shocks with chlorine -- higher levels of percarbonate are used to perform this conversion than for just treating white water mold. I'm not suggesting using percarbonate instead of chlorine for such a conversion -- I'm just saying it's an alternative that works. It's probably more expensive.

I think that indoor pools should use a small amount of CYA, around 10-20 ppm with an FC slightly higher at 2-3 ppm -- that should help reduce disinfection by-products (dichloramine and nitrogen trichloride). As for control of combined chlorine, that's harder but one could first try managing that with shocking with chlorine, but I suspect it will be difficult to control. That's where MPS can be helpful.

The only personal experience I have is that I recently had an oil film on the surface of the pool and tried scum balls and enzymes to remove it, but they didn't. I knew that shocking with chlorine would probably take care of it, but since my wife was using the pool every day I didn't want to do that (i.e. no time to wait for the chlorine levels to drop and there's little sun on the pool this time of year) so I used MPS in two doses and that did the trick -- after the first dose the oil slick broke up and after the second it was eliminated. It does seem to work -- it's just expensive and it adds a lot to sulfates though I'm not clear on what level when that would become an issue. Just remember that MPS will show up as Combined Chlorine unless you get the Taylor K-2041 kit to remove this interference to separately measure true Combined Chlorine vs. MPS. I think waterbear (Evan) said that the test strips for MPS were OK but I don't remember for sure -- I know that salt and borates were OK, but I'm not positive about MPS strips.

Richard