Turnover Myth

[AndyTN]

As I have tried to explain before: TFP's recommendations are based on levels required to control algae. These levels are higher than those required to deactivate pathogens.

The above is the main point for residential pools which normally only have 2-6 people in them at a time. The CYA also keeps some of the FC in reserve for a more consistent effect on sanitation throughout the sunlight hours. If your pool has a bather load like the below gif, then I would probably increase my FC to CYA ratio.

 

[mgtfp]

How does a higher CYA level reduce carbonates?

CYA adds alkalinity to your pool, it is a good buffer system at pool pH. Of you have e.g. a TA of 80 without any CYA (and Borates), then all of that TA will be more or less Carbonate Alkalinity, and contribute to the CSI and to CO2 outgassing.

With TA 80 and CYA 80, CYA-Alkalinity will be about 1/3 x 80, roughly 25ppm (exact value is slightly pH dependant). Carbonate Alkalinity will be reduced to 55ppm. That reduces CSI and CO2 out-gassing.

 

[mgtfp]

The above is the main point for residential pools which normally only have 2-6 people in them at a time. The CYA also keeps some of the FC in reserve for a more consistent effect on sanitation throughout the sunlight hours. If your pool has a bather load like the below gif, then I would probably increase my FC to CYA ratio.

They are all wearing the super protective Hazmat donut , so they should be fine.

 

[AndyTN]

They are all wearing the super protective Hazmat donut , so they should be fine.

I found the video of the GIF I shared. This is from a pool in China. I think @Prickly was correct after watching this video...

 

[AUSpool]

I do believe that there is some sort of elevated chlorine level in the cell and probably in the pipes leading to the pool. Whether that's high enough to be called "super chlorination" is another question. But it certainly won't hurt in giving algae an extra knock. The water behind the cell must have a higher FC compared to the incoming water - how would it otherwise be possible to increase the overall pool-FC. But yes, super chlorination might be an exaggeration. Don't know.

Regarding pH, you have to consider the local pH distribution within the cell. Right at the cathode, pH will be elevated, with each H₂ two OH^- will be created. At the anode, or shortly after, the reaction of the generated Cl₂ gas with H₂O (forming the precious HOCl) will release one H⁺, reducing the pH around the anode. Scale building around the cathode, and switching polarity from time to time helping to minimise scale built-up (because existing scale will be exposed to a more acidic environment) is evidence for these local pH zones.

The H⁺ from the anode will compensate some of the pH-increase from the cathode once these waters mix again. But the overall pH behind the cell should be elevated to a certain degree compared to the main pool water. Because of the constant fresh water supply, the overall pH in the cell is probably not much higher. Maybe not enough to see a direct difference to the pool pH, but there must be a difference - that's just what the chemistry of creating chlorine with a SWG does. Chem Geek described it in his water chemistry thread as "This process is partly basic, but not strongly so due to the HOCl weak acid."

I assume that Insrg have chosen a location within the cell for their pH-probe far enough away from the electrodes to show the bulk-cell-pH rather than the pH directly next to the plates.

I once found a paper that shows the pH-distribution within an electrolysis cell. It's a slightly different situation, but I believe that the general effect is transferable. This is the relevant diagram in the paper.


I wholeheartedly agree with you (@AUSpool) in regards to the constant and reliable chlorination with a SWG playing a role in allowing lower FC-levels. I actually was about to mention that in my previous thread, but decided that I should better start cooking dinner rather than making an already too long post even longer.

It would be interesting to see if pools with a Stenner system will also allow lower chlorine levels. Don't know if we have reliable field data on that.

Just for interest I ran the numbers for my SWG and pump output. 25gms/hour and ~300litres/minute on medium. Thats 25gms (Cl2 gas?) in 18,000,000ml of water which I think is roughly 1.4ppm of Cl2. Comparing my bulk water to my filter effluent with the SWG set to max I could not detect any difference between the two. For superchlorination to occur in the cell or pipe after the cell I would expect a substantial difference in FC.

Concentrations at the cell plates may be another story but given the disinfection mode for bleach and the high flow rates I think this may be limited. As I understand it, bleach passes through the cell membrane and kills the cell from the inside, the high flow rate would limit the ability of bleach to pass through the membrane. Strong acids and bases will react directly with the cell membrane.

 

[mgtfp]

Just for interest I ran the numbers for my SWG and pump output. 25gms/hour and ~300litres/minute on medium. Thats 25gms (Cl2 gas?) in 18,000,000ml of water which I think is roughly 1.4ppm of Cl2. Comparing my bulk water to my filter effluent with the SWG set to max I could not detect any difference between the two. For superchlorination to occur in the cell or pipe after the cell I would expect a substantial difference in FC.

Concentrations at the cell plates may be another story but given the disinfection mode for bleach and the high flow rates I think this may be limited. As I understand it, bleach passes through the cell membrane and kills the cell from the inside, the high flow rate would limit the ability of bleach to pass through the membrane. Strong acids and bases will react directly with the cell membrane.

Thanks for doing that, had that on my agenda, too. I think you put the milli during the calculation at the wrong spot, ppm is mg per litre, not g per mL (ppm is a bit sloppy, what we mean is mg Cl₂ per litre of water, which, because of water's density being pretty close to 1 kg/L, is roughly the same as mg Cl₂ per kg of water and mg/kg = 10^-3g / 10³g = 10^-6 = ppm).

But the end result of 1.4ppm is correct again.

I tackled my calculation a bit differently. My SWG cell has a volume of about 1L (if I take the whole thing out, leaving the plates in, and turn it around, I can fill about 1L of water in until it overflows). With a flow rate of 300L/min, the water inside the SWG cavity (1L) gets turned over every 1/5 seconds. In that time the SWG adds about 25000mg/3600/5 = 1.4mg. Since that was 1L, the SWG constantly adds about 1.4ppm to the water volume inside the cell. Same as your calculation for the total water volume. I think that should be measurable when taking a sample straight after the cell.

But superchlorination does sound exaggerated

An elevated level of 1.4ppm in the few seconds in the return pipe might show some effect on required contact time to kill algae.

There certainly should be a higher local FC near the SWG anode. Whether the flow makes it difficult for HOCl to get into biological cells to kill them, I don't know. I would imagine that the HOCl gets accelerated pretty quickly to the water speed. Don't know whether that happens faster or slower than the mixing of water and FC dilution.

I guess the ultimate test would be to pass contaminated water through an SWG and test how many pathogens have been removed with the SWG on and off. Or some other stuff that will get oxidized by chlorine.

Maybe it really only is the constant and reliable chlorination with an SWG that allows running a lower FC/CYA ratio. Who knows.

 

[cptkirk]

This is an interesting discussion. I think what mgtfp means is that if you increase CYA and your TA is kept constant, it means that your carbonate alkalinity is lower, and therefore tends to make your CSI more negative, reducing tendency to scale. At the same time, lowering you carbonate alkalinity will tend to stabilize your pH at a somewhat lower value.

I think the bottom line is this as I understand it. If you FC/CYA ratio is 7.5% or higher, you are good from a sanitation point of view. If you are able to keep your FC at your desired target while running your SWG at a reasonable percentage with lower CYA, there is nothing inherently wrong with that. You just need to pay attention to your CSI to keep it in balance to prevent scaling.

 

[AUSpool]

I guess the ultimate test would be to pass contaminated water through an SWG and test how many pathogens have been removed with the SWG on and off. Or some other stuff that will get oxidized by chlorine.

We did that for microbial ecology I think, although we sampled before and after a large bank of UV sterilisers on an aquaculture system (Barra, I think). Interesting but inconclusive due to old bulbs.

Love those numbers and thanks for checking my rational and math. Its great to put a value to it, is it happening and by how much. 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.

 

[mgtfp]

I really thought I’d be able to detect a 1.4ppm difference out in the pool

I might give it a go on the weekend. Always wanted to do that, check FC and pH straight from the return. I should be able to poke a syringe into the return line and take a sample like that. At the moment it's pouring down with rain here.

Just fiddled round a bit with Chem Geek's spreadsheet. Adding 1.4ppm of FC should increase the pH of the return water by about 0.04 compared to the bulk water, should be able to pick that up with my PH60. Not sure how close to your 300L/min my pump is, I might try a few different pump speeds.

 

[mgtfp]

I think what mgtfp means is that if you increase CYA and your TA is kept constant

Yep. I was a bit sloppy in saying that carbonates get reduced. What I meant is that at the same TA, Carbonate Alkalinity will be lower with CYA in the water.

When you add dry CYA granules, that's actually exactly what happens: Carbonate Alkalinity gets replaced with CYA Alkalinity, keeping TA constant. Different when adding CYA via Trichlor or liquid stabiliser.

 

[superuser]

So what I'm hearing is if otherwise balanced, I don't necessarily need to worry about a "lower than recommended" CYA value from the TGT app?

(tested at sundown)
FC: 6.0
pH: 7.8
TA: 80
CH: 340
CYA: 40
Salt: 4000
Temp: 79F / 26C
CSI: 0.01
SWG: 50% output, ~4 hours runtime on high, ~3 hours runtime on low speed

 

[Prickly]

I found the video of the GIF I shared. This is from a pool in China. I think @Prickly was correct after watching this video...

That video is horrifying!

 

[Prickly]

To say that sanitation is sufficient if algae is not present is a bit of an oversimplification. I don't know what the CT value is for algae (since it's not a microbe of public health importance I'm not aware of it being studied) but the CT value for Giardia for example is 62. If you are running with a FC / CYA ratio of 10% you have the equivalent of 0.09 ppm chlorine. This means that it will take 688 minutes to inactivate any giardia introduced into the pool. As Richard Falk indicated in one of his earlier papers on the subject, you can still have bacteria present in a pool sample with 5 ppm FC and no CYA. That's 55 times as much chlorine as you would have in a pool with a FC /CYA ratio of 10%.

I have dealt with many pools and spas that have had positive psedumonas of fecal coliform tests while running a much higher effective chlorine concentration than that with no signs of algae present. When you sample a pool the bottle contains sodium thiosulphate so that you can capture a snap shot of the conditions in the basin rather than giving the chlorine more time to work on the way to the lab.
In the end, it really all comes down to how quickly you want to inactivate pathogens.

I'm also not convinced that ORP doesn't work with CYA. Public pools required to use ORP aren't exempt if using CYA. I can see why some users may feel it doesn't work - CYA plummets the power of chlorine so substantially that you need a 50% CYA to FC ratio to achieve the equivalent of 1 ppm of unstabilized chlorine. Here is a chart showing the equivalents with different ratios.

 

[iamnos]

I really don't see a need to have a higher CYA level in a salt water pool versus non salt water pool IF you can easily maintain your FC level. I run my pump for 4 hours a day with my SWG set to 6% and my chlorine level is never below recommended.

Forgive me, but is that a typo?
I have a Hayward T9 as well, and happen to live in the same area as you. I'm generally running my pump for about 6-8 hours at 60-75% to generate around 2ppm/day of FC in my pool, at least going from the calculator here: SWG Run Time Calculator

 

[Prickly]

Forgive me, but is that a typo?
I have a Hayward T9 as well, and happen to live in the same area as you. I'm generally running my pump for about 6-8 hours at 60-75% to generate around 2ppm/day of FC in my pool, at least going from the calculator here: SWG Run Time Calculator

No, not a typo. Is your pool covered? Mine is covered when not in use. I have low sun exposure with pine trees and my yard backing onto a hill. In the summer I will increase it to 15 or 20% with a longer run time but that's where I'm at now.

 

[iamnos]

No, not a typo. Is your pool covered? Mine is covered when not in use. I have low sun exposure with pine trees and my yard backing onto a hill. In the summer I will increase it to 15 or 20% with a longer run time but that's where I'm at now.

Definitely covered the majority of the time when not in use.
Even now, coming off the opening SLAM, I'm running around 50% for 6 hours/day and my FC is dropping about 1ppm per day, which is right about what I'd expect. Now, the cover is a "solar cover" that partially translucent, so there's certainly some UV getting in there and of course there's always various organics using up chlorine, so my real loss per day is probably right around 2ppm.

 

[Prickly]

Definitely covered the majority of the time when not in use.
Even now, coming off the opening SLAM, I'm running around 50% for 6 hours/day and my FC is dropping about 1ppm per day, which is right about what I'd expect. Now, the cover is a "solar cover" that partially translucent, so there's certainly some UV getting in there and of course there's always various organics using up chlorine, so my real loss per day is probably right around 2ppm.

I just have a solar cover as well but I find it reduces the chlorine loss a lot when it's on. My pool is pretty small (just 8400 gallons). It is also fiberglass, which may account for a lower chlorine demand.

 

[iamnos]

I just have a solar cover as well but I find it reduces the chlorine loss a lot when it's on. My pool is pretty small (just 8400 gallons). It is also fiberglass, which may account for a lower chlorine demand.

Going by the calculator, to replace just 1ppm at 6% in a 8700 gallon pool, you'd need to run your pump for 72.6 hours... per day.
I can't even set my SWCG to 6% on my Prologic panel. I get 1% increments from 1-5, then by 5% from 5-100.

 

[Prickly]

Going by the calculator, to replace just 1ppm at 6% in a 8700 gallon pool, you'd need to run your pump for 72.6 hours... per day.
I can't even set my SWCG to 6% on my Prologic panel. I get 1% increments from 1-5, then by 5% from 5-100.

My panel allows for 1% increments above 5. I'm definitely not going through anywhere close to 1 ppm a day. I can leave the generator off for a day right now and not see a measurable drop in FC. My hot tub other hand goes through at least 1 ppm a day...

 

[mgtfp]

To say that sanitation is sufficient if algae is not present is a bit of an oversimplification. I don't know what the CT value is for algae (since it's not a microbe of public health importance I'm not aware of it being studied) but the CT value for Giardia for example is 62. If you are running with a FC / CYA ratio of 10% you have the equivalent of 0.09 ppm chlorine. This means that it will take 688 minutes to inactivate any giardia introduced into the pool. As Richard Falk indicated in one of his earlier papers on the subject, you can still have bacteria present in a pool sample with 5 ppm FC and no CYA. That's 55 times as much chlorine as you would have in a pool with a FC /CYA ratio of 10%.

I have dealt with many pools and spas that have had positive psedumonas of fecal coliform tests while running a much higher effective chlorine concentration than that with no signs of algae present. When you sample a pool the bottle contains sodium thiosulphate so that you can capture a snap shot of the conditions in the basin rather than giving the chlorine more time to work on the way to the lab.
In the end, it really all comes down to how quickly you want to inactivate pathogens.

I'm also not convinced that ORP doesn't work with CYA. Public pools required to use ORP aren't exempt if using CYA. I can see why some users may feel it doesn't work - CYA plummets the power of chlorine so substantially that you need a 50% CYA to FC ratio to achieve the equivalent of 1 ppm of unstabilized chlorine. Here is a chart showing the equivalents with different ratios.

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'm also not convinced that ORP doesn't work with CYA. Public pools required to use ORP aren't exempt if using CYA.

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:

ORP measures the the oxidation potential (in mV) of water which is affected by many factors - oxidizer level, pH, alkalinity, temperature, UV light, etc, etc. It does not measure the concentration of hypochlorous acid directly. As well, because cyanuric acid stabilizer reduces the overall level of hypochlorous acid to parts per billion levels while holding the vast majority of chlorine in reserve, the electrode is left to try to measure a low level voltage signal in a highly noisy environment. As an engineer you should be able to appreciate the concept of signal-to-noise ratio. Finally, CYA tends to foul up and deposit onto the delicate gold covered glass frit that separates the internal solution of the probe from the pool water. That build up causes even more signal degradation.

ORP sensors are great in principal but fail in practice to live up to the marketing hype.

You two understand the FC/CYA relationship and that there is no such thing as chlorine lock where at some CYA level no amount of FC above that is effective. You also understand that you lose less total chlorine at higher CYA levels even with proportionally higher FC levels. You know to decide how much chlorine savings you want vs. the risk of having the higher CYA that would need a much higher FC to SLAM if something went wrong. So this thread is not for you.

What this thread is about is fighting incorrect statements made by some in the industry and those who copy such information as if they were facts rather than looking at the core science and observations to learn the truth. Let's look at each of the graphs in the Kent Williams PPOA article Cyanurics - Benefactor or bomb? article to see both inaccuracies and deception. Let's start with Figure 1:

Notice how at 70 ppm CYA the ORP is the same regardless of the FC level. This is an artifact of the ORP system used and has nothing to do with reality in terms of actual active chlorine (hypochlorous acid) level or its effects such as disinfection rate. This is made obvious in Figure 3 which comes from a scientific paper:

where miracle of miracles look at what happens at 70 ppm CYA. The kill time for 0.1 ppm FC is about 4 times longer than that at 0.5 ppm FC, which is close to what one would expect (it would normally be about a factor of 5 difference since it is roughly proportional to concentration). Let these two graphs and the claims made from each sink in as their points contradict each other. The only explanation is that ORP is in fact useless as an absolute standard, at least for the ORP sensors used for the PPOA paper and for the higher CYA levels. So you can't look at Figure 1 as meaning anything regarding actual disinfection or oxidation rates at higher CYA levels. And as I noted in an earlier post in this thread, if you look at a constant active chlorine level with an FC/CYA ratio of even 5%, then for the 0.5 ppm FC in Figure 1 this would be a CYA of 10 ppm and a reasonably fast 99.9% kill time (from other studies of around 3 minutes but looking like 2 minutes in the graph). The PPOA article is extraordinarily deceitful when it states:

Stated simply, as CYA exceeds 70 ppm, virtually any level of chlorine will result in no more than about .2 ppm equivalent effectiveness. Longer lasting, yes. Better working, No.

when the truth is what the article later states which is:

At much over 50 ppm CYA, controllers are a bad investment too; and as the water approaches 70 ppm CYA, controllers of ORP (the principle behind virtually every pool-chlorine automation device on the market today) simply quit - dead.

It's the ORP sensors that are the problem and fail to operate properly at higher CYA levels. This has NOTHING to do with "equivalent effectiveness" or not being "better working". Lies, lies, and yet more lies. Also, the equivalent effectiveness scale is wrong since 4 ppm FC with 40 ppm CYA and 1.5 ppm FC with 15 ppm CYA are closer to 0.1 ppm FC with no CYA. Also, 700 mV should be closer to 0.1 ppm FC with no CYA equivalent (at pH 7.5), not 0.3 ppm as shown in the graph. Other ORP sensors (e.g. Chemtrol) at least get that equivalence right though this does show that ORP can and does vary by sensor manufacturer.

Now let's look at the Chlorine Staying Power (Figure 2 though not labeled as such):

This graph is absolutely positively not true and not at all what is seen in thousands upon thousands of outdoor residential pools. The graph implies significant diminishing returns at higher CYA levels but the degree of such diminishment is not at all what we see, even when we have the FC raised proportional to the CYA level which for the above graph was probably not done (so the graph is even MORE inconsistent with reality). Again, look at Mark's experiments that showed significant increases in chlorine retention at higher CYA levels. Anyone can tell you that there is no way at 30 ppm CYA that you get (as shown in the graph) 90% of the chlorine remaining after a day of full sunlight. At 30 ppm, one typically loses more than half, nearly two-thirds, of their FC in a day while at 50 ppm this drops to less than half at around 40% loss and at 80 ppm drops to around 20-25% loss and this is all with maintaining a constant FC/CYA ratio for the same active chlorine level. Typical non-SWG pools on TFP at 50 ppm FC use 2-3 ppm FC per day going from 6 or 7 ppm down to 4. That is NOT 90% staying power.

CYA narrows the range of the signal that an ORP sensor picks up. Without any CYA the ORP level can vary from about 550 to almost 900. As you add CYA the range narrows. At typical CYA levels it is often down to perhaps 550 to 700. Eventually the range gets so small that the natural noise in the ORP sensor is larger than the signal you are trying to read.

The exact CYA cutoff at which the sensor becomes unreliable varies from brand to brand, but all of them are somewhere a little above 50. Keeping the CYA level below 50 insures that the ORP sensor will be able to reliably detect variations in the FC level.

It is important to keep in mind that ORP sensors are affected by a number of factors other than just the FC level. For reliable automation, you want FC to have the largest effect on the reading and for all of the other variations to be treated as noise. Keeping the CYA level low keeps the sensors sensitivity to FC high and the system behaves as expected.