Turnover Myth

[Prickly]

If you are dealing with Guardia, then you will need very high CTs, that's correct. Here is a link to a summary presentation of Falk's paper, a bit faster to read and easier to refer to:

https://www.cmahc.org/documents/CMA..._on_Stabilizer_Use._WAHC_2017-10-16_FINAL.pdf

His recommended CYA/FC ratio of 20 provides a significantly reduced risk regarding Guardia in public pool environments. Of course, if you want to have the smallest possible risk for a Guardia infection, then you need way higher FC.

The TFP recommendstions provide a compromise between pathogen deactivation times and an efficient way to maintain a residential pool, that goes way beyond regulated levels for public pools with CYA in terms of min sanitation standards.

Public pools without CYA maintain way higher HOCl levels. That might make sense in a public pool with high bather load and a high risk for bather to bather transmission. But it also makes the water very unpleasant to swim in. No one stops you from going there. You can stick constantly to 50% FC/CYA, that's above SLAM, if that's what you want, but I don't.

If you generally want less CYA, then go for it. But it is a less efficient way to run an SWG pool.

I think we can agree that no CYA at all doesn't make sense in a residential outdoor pool, it is just not feasible to maintain required chlorine levels at all times. And once you accepted CYA in general, you can as well take most benefit out of it. Just maintain the required FC/CYA ratio. For SWGs, it has proven to be more efficient to have higher CYA and make benefit of being able to maintain lower FC. That will provide the most cost effective way to run the SWG (including lifetime considerations). By maintaining a target of 7.5% FC/CYA, we are well above current industry standards for pools with CYA, and still above the 5% that Falk et al. recommend as a minimum in their paper.

If even better sanitation has a higher priority to you, and lifetime of your SWG is less important to you, then go for it. Many here maintain higher FC levels with SWGs (myself included), usually for piece of mind to have a larger buffer towards the min-level, and to allow for less frequent testing. Nothing wrong with that, it's a personal decision. But I prefer to keep my CYA in the range recommended by TFP for SWG-pools.

There are automated systems (with ORP sensors) that allow to run an SWG on lower CYA, the automation is designed to keep ORP constant throughout the day. I have no experience how reliable these systems are.

AstralPool Australia recommends to run their ORP-SWGs w/o CYA or at max 20ppm. They argue with the reduced chlorine efficiency which is not true as long as you maintain the required FC/CYA. I suspect that they don't feel comfortable with recommending FC-levels above 3ppm to their customers. But the main reason is probably that ORP sensors are simply unreliable with CYA. Which leads us to the second point:



I have not looked deeper into ORP-sensors, I personally don't see any benefit in them. Why would I pay extra if my SWG just happily chugs along, maintaining my FC/CYA? I just see another expensive system that can fail.

The consensus seems to be that ORP-sensors can work to enable automation with low CYA levels, but because of the low CYA the systems have to cope with high UV-losses which the customer will pay for with reduced lifetime (on top of the already high costs to buy the system in the first place). Any hickups in the system will very quickly lead to insufficient chlorine levels.

Others have a better understanding how ORP sensors work, I trust their expertise. Here are some examples of the many threads on ORP:

The contradiction in those two graphs does make an interesting point. It seems correct to say that ORP sensors become useless at high CYA levels. Perhaps that's the rationale behind the 50 ppm max allowed CYA in the province where I used to work. The max really wouldn't be necessary for other reasons since there was no max on the FC allowed (and it wasn't uncommon for me to see levels of 10 ppm FC). Or perhaps they set the max believing the false statement that there is no amount of chlorine that can overcome a CYA of around 70 ppm.

 

[JoyfulNoise]

One thing to keep in mind about public pools in California and ORP is that public pools MAY NOT USE ORP as their standard for reporting sanitizer levels. They are free to use an ORP sensor to control a chemical pump but ALL RECORDS must detail pH, FC and fecal coliform levels (fecal coliform tests are slow so data always lags). CYA levels are strictly controlled by regulation and are usually low enough that they don't impede ORP sensors.

In residential pools, where CYA levels can be much higher through the use of solid chlorinating products, CYA directly affects the ORP sensor in 2 ways. It lowers the concentration of HOCl which is the primary oxidizer in pool water and thus greatly reduces the signal to noise ratio. Secondly, because ORP probes, like pH probes, use charge transfer across a glass membrane, the CYA can actually cause deposits to form on the glass probe tip that interfere with the voltage signal. It is often the case that the ORP membrane becomes fouled in high CYA pool water and small amount of light polishing (some manufacturers suggest using toothpaste) restores the signal. Because of the inherent limitation of ORP sensors, they can often cause more trouble in maintaining a pool.

 

[Prickly]

One thing to keep in mind about public pools in California and ORP is that public pools MAY NOT USE ORP as their standard for reporting sanitizer levels. They are free to use an ORP sensor to control a chemical pump but ALL RECORDS must detail pH, FC and fecal coliform levels (fecal coliform tests are slow so data always lags). CYA levels are strictly controlled by regulation and are usually low enough that they don't impede ORP sensors.

In residential pools, where CYA levels can be much higher through the use of solid chlorinating products, CYA directly affects the ORP sensor in 2 ways. It lowers the concentration of HOCl which is the primary oxidizer in pool water and thus greatly reduces the signal to noise ratio. Secondly, because ORP probes, like pH probes, use charge transfer across a glass membrane, the CYA can actually cause deposits to form on the glass probe tip that interfere with the voltage signal. It is often the case that the ORP membrane becomes fouled in high CYA pool water and small amount of light polishing (some manufacturers suggest using toothpaste) restores the signal. Because of the inherent limitation of ORP sensors, they can often cause more trouble in maintaining a pool.

One thing to keep in mind about public pools in California and ORP is that public pools MAY NOT USE ORP as their standard for reporting sanitizer levels. They are free to use an ORP sensor to control a chemical pump but ALL RECORDS must detail pH, FC and fecal coliform levels (fecal coliform tests are slow so data always lags). CYA levels are strictly controlled by regulation and are usually low enough that they don't impede ORP sensors.

In residential pools, where CYA levels can be much higher through the use of solid chlorinating products, CYA directly affects the ORP sensor in 2 ways. It lowers the concentration of HOCl which is the primary oxidizer in pool water and thus greatly reduces the signal to noise ratio. Secondly, because ORP probes, like pH probes, use charge transfer across a glass membrane, the CYA can actually cause deposits to form on the glass probe tip that interfere with the voltage signal. It is often the case that the ORP membrane becomes fouled in high CYA pool water and small amount of light polishing (some manufacturers suggest using toothpaste) restores the signal. Because of the inherent limitation of ORP sensors, they can often cause more trouble in maintaining a pool.

Operators are required to test manually alongside ORP here as well. If measuring ORP alone you could end up with some pretty wacky FC and pH levels. I think the ORP requirement is good to have for public pools though (even if it does have its weaknesses) because without it, I think it would be difficult to get operators to achieve the proper combination of FC/pH in an unstabilized basin and FC/CYA in a stabilized one since these parameters are all regulated separately (unfortunately).

 

[mgtfp]

I really thought I’d be able to detect a 1.4ppm difference out in the pool. My FC is a bit high at the moment so maybe that had something to do with it. I filled a new garbage bag from one of my spigots and took my output sample from that.

I managed to run some tests today. First measured pH and FC in the pool, and then in the return water. SWG was on 100%, pump speed in the middle range. For the FC test, I took the sample with a syringe directly from the return eye-ball. For the pH test I just poked my pH-meter straight into the eye-ball.

FC in the return water was about 3ppm higher than the pool water. Based on our estimation that would mean that my water turnover must be closer to 150 L/min rather than 300 L/min.

pH was 0.07 higher in the return water. Was pretty reproducible. I was putting my pH-meter in and out of the eye-ball, and the pH kept jumping back and forth, always the same value. Before I started the the pH-test, I kept the meter for maybe 10 minutes in the water to make sure that the probe had adjusted to the pool water temperature.

Afterwards I calculated with Chem Geek's sheet the expected pH-change by an FC-increase of 3ppm with an SWG to be about 0.08. So, the measured FC-difference of 3ppm and the pH-difference of 0.07 seem to be pretty consistent.

I don't run my SWG very often on 100%, usually more around 40%, so the FC-difference between pool and return-water would be smaller, back to the 1-1.5ppm range that we initially estimated (my SWG adjusts the current when reducing the output, it doesn't operate on a duty-cycle).

Many US-SWGs operate on duty cycles, so will always produce max chlorine while in the on-cycle. And following TFP-recommendations, many SWGs are at least double the size. Plus lower pump speed that many with robots (I have a suction cleaner) will be able to run. I could imagine that FC within the return line might be more elevated in these cases. And finally, there could still be a higher concentration closer the the anode before the waters flowing past anode and cathode mix again.

So maybe there can be FC-levels in the cell and the return line that deserve the name "super-chlorination". But probably not under all circumstances, depending on SWG-size (on the other hand, doubling the chlorine output will result in halving the run time, keeping CT constant ), flow rate (that might have a double effect on CT by reaching higher FC and spending a longer time in the return line with slower flow rates), and duty-cycle vs. plate-current regulation (not sure if that plays a role, the average CT is probably the same in both cases).