Spa Chemistry - dichlor, ozone, and pH

[chem geek]

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

[waste]

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 -- again, thanks for all the info you've provided!!!!!!!!!!!

[chem geek]

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