Spa Chemistry - dichlor, ozone, and pH

[chem geek]

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