Thank you Richard for another great explanation. I read the links and have the following dumb questions with probably more to follow.

1. If a pool had high CYA levels at the end of the season, it seems that the feasiblility of reducing those levels over the off season is reasonable if the chlorine levels are below breakpoint or 0 FC. This could occur without any significant clarity issues during the conversion of CYA to ammonia but would create additional chlorine demand to correct upon preparing the pool for opening or preventing an accelerated algea bloom, whichever came first? It wasn't phrased as a question but meant to be.

2. How significant is water temperature in question #1

3. Under what conditions can ozone deplete FC levels?

The problem with the approach in #1 is that you can't readily force it or control it. Many people close their pools over the winter and get to zero chlorine (mostly in freezing climates) and may open up a pool with some algae, but not with the CYA gone or ammonia produced. Even if we could bottle up the bacteria that did the conversion and fed it with nitrates and phosphates added to the pool (which algae also like, by the way), it takes at least 2.5 times the amount of CYA drop as FC (both measured in their respective ppm units) to get rid of the resulting ammonia. That's a LOT of chlorine so seems pretty impractical to me. Simple water dilution, especially with winter rains (if available) seems much easier.

As for the effect of water temperature on the bacterial conversion of CYA to ammonia, there is certainly a temperature dependence where warmer water has a faster conversion, but I don't know the degree of this dependence. I only know that in my own pool example the water wasn't fully heated yet, though it was around 75ºF which apparently is plenty warm for bacteria to grow fairly quickly.

As for ozone depletion of FC levels, it happens because ozone is a stronger oxidizer and probably oxidizes chlorine to chlorate. I'm not sure of the conditions that accelerate this except that clearly higher ozone concentrations would result in a proportionately faster FC drop. The thing with ozone is that it is also oxidizing bather waste. So in a high bather load situation, it can end up lowering FC consumption so FC will actually last longer because the chlorine won't get used up as much oxidizing bather waste (urea in particular). This is why ozone is more useful in a commercial/public pool, but probably not that useful in a residential pool since it is more likely to just increase the loss rate of chlorine. In a residential spa, however, ozone can be useful if one uses the spa every day; if they use it once a week or less, then ozone is more of a detriment.

Thanks again Richard. This is good information and will probably change the course of how we are sanitizing our pools.

We have been injecting Triclor, salt or both into the return line of the pool and feeding ozone into the normally spilling over spa. By doing this I thought we were using a sanitizer in the spa that was more stable at higher tempertures and counting on the residual chlorine levels as a backup. We use ozonators designed to treat 40,000 gal pools for this purpose. By separating the injection points I thought we were minimizing the potential for the ozone to breakdown the chlorine. The bather load is typcally far less than a commercial pool.

If I understand you correctly, the high levels of ozone we are injecting into roughly 1/2 of the flow during typical spillover operation is attacking the FC unless there is enough bather waste? Maybe it would be best if we had the ozonator come on only when its in spa mode? This would have the added benefit of prolonging the life of the ozonator.

In theory, yes. Of course, you can simply experiment for yourself and see if you notice a difference in chlorine demand based on having the ozonator on or not. If you come up with some specific numbers of chlorine usage each way, let us know since theory isn't worth much if it doesn't predict reality.