Turnover Myth

[Prickly]

posts split from Pool Math vs National Swimming Pool Foundation Range Variance
The only reason you would want to raise your CYA is if you're having trouble maintaining adequate chlorine levels due to UV loss. Otherwise, more is not better. CYA makes chlorine much less powerful and increases the amount of time it takes to kill germs. I keep mine at 30 ppm and that is more than enough for my pool. That being said, I live in Canada near a mountain and have lots of trees. Someone in the blazing sun down south will have a much different experience!

 

[AUSpool]

The only reason you would want to raise your CYA is if you're having trouble maintaining adequate chlorine levels due to UV loss. Otherwise, more is not better. CYA makes chlorine much less powerful and increases the amount of time it takes to kill germs. I keep mine at 30 ppm and that is more than enough for my pool. That being said, I live in Canada near a mountain and have lots of trees. Someone in the blazing sun down south will have a much different experience!

TFP recommends 40-60ppm CYA for liquid chlorine chlorinated pools and 70-80ppm CYA for SWCG chlorinated pools. 20-30ppm is only recomended for indoor pools. 30ppm is a bit low and can be a bit problematic for the turbidity disappearing dot test.

CYA protects the chlorine from UV loss but also increases bather comfort and increases a SWCG’s longevity. It doesn’t reduce chlorine’s effectiveness or pathogen kill rates if the correct ratio or FC/CYA Levels is followed.

 

[Prickly]

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. But like I said, with full sun exposure someone else may need a higher CYA level to avoid having to run the generator too much, decreasing its life span. Yes, bather comfort is increased with more CYA - because the chlorine is bound and not having its normal effect. A properly balanced indoor pool with no CYA would not cause any bather discomfort, so this shouldn't be a reason to increase CYA if it's not needed.

I understand that the recommendation to run a higher CYA/FC ratio is a SW pool is based on the observation that algae can be kept at bay with a lower level in a SW pool; it is not based on other pathogenic microorganisms of concern from a health perspective. This observed effect is likely due to the fact that as the water goes through the pipe to the SWG this small water volume is being superchlorinated after the cell so as the volume of the pool turns over any algae would be taken care of whereas a manually chlorinated pool doesn't get this superchlorination effect. While this will take care of visible microbes such as algae I personally wouldn't be comfortable with waiting for this turnover superchlorination effect to take care someone's fecal microbes at the cell because I'm running a lower FC/ CYA level and thus an increased microbial inactivation time in the pool basin.
My time as a Public Health Inspector has made me overly aware of all the microbes humans shed so I'm sure I worry more than most would, but that's my 2 cents.

 

[Donldson]

Just putting this out there: the NSPF (which no longer exists, by the way, they merged with the APSP and are now the Pool and Hot Tub Alliance) is just a non-profit organization that does a bunch of political advocacy. They are funded by the pool industry to lobby for the pool industry. I see no reason to place their recommended levels upon a pedestal unless you are a politician being treated to lunch... and dinner... in Orlando for a week.

 

[mknauss]

turnover superchlorination

Turnover is not applicable to pools. Your rendition of pool chemistry maintenance is not endorsed by TFPC. I apologize, but this forum is about TFPC, not other views.

 

[Prickly]

Turnover is actually applicable to the concept of being able to run a lower FC to CYA ratio in a saltwater pool versus a regular chlorine pool. While TFP might not get into the explanation behind it, that is how it functions. The only reason this lower ratio is achievable is because of the high chlorine concentration at the cell which is doing a superchlorination of that small volume of water in the pipe. This is similar to the way turnover would be applicable if you were using UV to supplement because the UV only works on the volume going through the pipe. That is science, not an opinion. The reason I am personally not a fan of the lower FC to CYA ratio for a SW pool is because I don't want to rely on turnover to ensure adequate disinfection based on the high chlorine level at the cell; I want an adequate ratio in the basin.

 

[mgtfp]

Disinfection (by which I understand deactivation of pathogens like bacteria or viruses) is entirely sufficient at the lower SWG FC-levels and not at all relying on the superchlorination within the cell.

Killing algae requires higher FC levels than necessary for pure disinfection (killing algae is indirectly also important for disinfection - allowing an algae bloom would create a significant chlorine demand which can result in FC dropping below levels required for disinfection).

The superchlorination within the cell of a SWG seems to allow maintenance of lower FC levels compared to non-SWG pools in regards to preventing algae from growing. I don't think that turnover is a very important parameter here, as long as the SWG is not being operated only sporadically. Superchlorinating some water regularly seems to be sufficient, it doesn't have to be the whole pool volume every day.

Yes, some of the TFP guidelines are based on experience.

Science explaines why the FC/CYA ratio is the determining parameter in pool water chemistry.

Which exact FC/CYA ratio (which in good approximation is proportional to the HOCl-concentration) is required, needs to be derived from experiments (which by the way is part of the scientific process).

That includes how much superchlorination within a SWG-cell is required to reduce the FC/CYA ratio without breeding algae (again, disinfection at the SWG FC/CYA-ratio does not rely on the superchlorination effect). The times usually required with typical SWGs to keep FC constant on a daily average have proven to be sufficient.

Of course, if you decided to buy a SWG that is ten times the required size and only run that for half an hour per day, then this is outside of TFP-experience, and you needed to run your own experiments to find out if the non-SWG levels can be reduced under these conditions.

There is an abundance of experiments on required HOCl levels for deactivation of various pathogens. All TFP-levels are well above that. There are not as many investigations on algae. That's where TFP builds on the experience of more than 250000 members.

No one forces you to use TFP's SWG levels. If you don't like them, then use whatever you want. But the TFP levels have proven to work in thousands of pools. If you have data that show that the recommended levels don't work for you, then please share that experience. Only the interaction with all the individual TFP members allows TFP to continually improve.


Regarding CYA:

There is another benefit in using higher CYA-levels with SWGs. CYA is also a very efficient buffer system. Replacing some of the Carbonate Alkalinity with CYA-Alkalinity (keeping Total Alkalinity constant) offers two advantages without compromising pH-buffering:

1) Less CO2 out-gassing, reducing the pH-drift

2) Reduced CSI, resulting in less scaling on the SWG-plates

 

[AUSpool]

Just putting this out there. I think super chlorination within a SWCG cell may be nothing more than an urban myth. If super chlorination was occurring wouldn’t we expect an elevated pH in the cell? But cell pH and bulk pool water pH are identical, the flow rate is too high to allow a difference. I questioned the rep for the newish Australian Insnrg range of products that have their pH port in the cell and after extensive testing they found no difference between the cell pH and bulk water pH. The SWCG pool’s disinfection stability may be more about automated constancy rather than elevated chlorine in the cell.

 

[mgtfp]

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.

 

[Newdude]

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

Most won’t be able to maintain the FC in a salt pool with a lower CYA unless the unit is massively oversized. In a liquid Chlorine pool, the gallon is added in one shot and spikes the FC instantly, where it drifts down until the next day. The SWG is for maintaining an already established FC and at any given moment, more would burn off with UV than is being produced. With a higher CYA more of that lesser produced FC lasts long enough to do its thing so you only need to produce enough to last the next while, and not a full 24 hours until the next liquid dosing time.

This is similar to the way turnover would be applicable if you were using UV to supplement because the UV only works on the volume going through the pipe. That is science, not an opinion.

Apples and oranges. The UV leaves no residual sanitizer in the water. You can ‘turn over’ the pool 20 times and never get every gallon, or even close to it. The return water mixes with the pool water and the next batch through the unit may be 50/50, leaving lots untreated.

The reason I am personally not a fan of the lower FC to CYA ratio for a SW pool is because I don't want to rely on turnover to ensure adequate disinfection based on the high chlorine level at the cell; I want an adequate ratio in the basin

You are not relying on turnover, you are relying on runtime to produce enough FC to mix well with your total gallons, effectively treating it whether or not it ever goes through the cell.

This observed effect is likely due to the fact that as the water goes through the pipe to the SWG this small water volume is being superchlorinated after the cell so as the volume of the pool turns over any algae would be taken care of whereas a manually chlorinated pool doesn't get this superchlorination effect

A liquid dosed pool will have moments of super-super chlorination as straight chlorine is dumped in and mixes around. Then it will be higher FC for the next 24 hours until it burns off. Again, with the SWG adding much less at any given moment, it needs more buffering from the CYA to keep it.

I live in Canada near a mountain and have lots of trees. Someone in the blazing sun down south will have a much different experience

No matter what, your much lower demand will give you that much more room for error. Don’t mistake how easy it is to maintain your demand up north for a regimen that will work world wide. TFP is based on what works for all. The lesser your specific demand is, the easier it gets.

 

[Prickly]

Most won’t be able to maintain the FC in a salt pool with a lower CYA unless the unit is massively oversized. In a liquid Chlorine pool, the gallon is added in one shot and spikes the FC instantly, where it drifts down until the next day. The SWG is for maintaining an already established FC and at any given moment, more would burn off with UV than is being produced. With a higher CYA more of that lesser produced FC lasts long enough to do its thing so you only need to produce enough to last the next while, and not a full 24 hours until the next liquid dosing time.

Apples and oranges. The UV leaves no residual sanitizer in the water. You can ‘turn over’ the pool 20 times and never get every gallon, or even close to it. The return water mixes with the pool water and the next batch through the unit may be 50/50, leaving lots untreated.

You are not relying on turnover, you are relying on runtime to produce enough FC to mix well with your total gallons, effectively treating it whether or not it ever goes through the cell.

A liquid dosed pool will have moments of super-super chlorination as straight chlorine is dumped in and mixes around. Then it will be higher FC for the next 24 hours until it burns off. Again, with the SWG adding much less at any given moment, it needs more buffering from the CYA to keep it.

No matter what, your much lower demand will give you that much more room for error. Don’t mistake how easy it is to maintain your demand up north for a regimen that will work world wide. TFP is based on what works for all. The lesser your specific demand is, the easier it gets.

I understand that comparing UV to the SWG is apples to oranges - I was just using it to illustrate the point I was making which is that it is the high chlorine level at the cell itself that allows a SWG pool to 'get away' with a lower FC to CYA ratio. The high level of chlorine in the spot that chlorine is manually added to a pool is not going to have the same effect as 'most'of the pool water traveling through the cell. Yes you are correct that there are dead spots and not all water will circulate through the cell. Pathogens will all be eventually taken care of in the basin due to the free chlorine residual - my point was that the speed of that is affected by running a higher CYA level relative to the FC level.

I have no problem with the recommendation to run a higher CYA level in a SWG pool. - what I'm saying is that the FC to CYA ratio should be the same as that which is recommended for a non SWG pool if you are like me and want to make sure pathogens are destroyed more quickly in the basin itself versus relying on water going through the cell to take care of what hasn't been destroyed in the basin. There is no question that a lower FC to CYA ratio will slow down pathogen destruction. CYA has a powerful effect. I have young children and kids tend to both shed more fecal microbes and swallow more water! People who just have adults or no guests using the pool don't have to be so concerned about that. Please do not confuse this with me promoting turnover as an important part of pool maintenance as that is not what I was trying to say.

As for me being able to get away with less CYA easily due to my personal operating conditions - I did acknowledge that someone with more sun exposure (and no cover) will have a much different experience than mine. I was simply explaining why I am able to run with 30 ppm CYA without any issues. Recommending a range might be more helpful as there are a lot of people with covered pools who won't need as much CYA.

Also, I am not trying to discredit TFP recommendations at all. I was simply explaining why some sources would not recommend such a high CYA level as this is what the OP was asking about.

 

[Newdude]

As for me being able to get away with less CYA easily due to my personal operating conditions - I did acknowledge that someone with more sun exposure (and no cover) will have a much different experience than mine

Yes, exactly. We point out tailored advice for geographical regions all the time. With many local members usually chiming in with what they see to help the OP. There will always be some level of ‘this is the range that works, chime in with your specifics to get it dialed in more’.

The guidelines aren’t rules set in stone. They are general ideas welocme to be fine tuned further as needed.

Personally, while I agree with the lower levels of the average SWG pool, I run mine at or slightly above the liquid chlorine guidelines. It gives me a huge safety net to not need to adjust anything 9 times out of 10. When I do occasionally need to adjust my FC, it’s because it drifted too high, or, not even ‘too high’ but higher than I needed to retain the extra safety net. I ‘waste’ some chlorine that burns off before being effective, and in doing so get slightly less life from my SWG from the unneeded runtime, but man oh man , the trouble free aspect and piece of mind is worth 10x what I waste.

 

[mgtfp]

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.

Have a look at this paper:

Assessing the Impact of Cyanuric Acid on Bather’s Risk of Gastrointestinal Illness at Swimming Pools

Current regulatory codes for swimming pool disinfection separately regulate free chlorine (FC) and cyanuric acid (CYA). It is well-known that CYA affects disinfection rates by reversibly binding to FC in aqueous solutions. However, limits for these regulated parameters have neither...

www.mdpi.com

The recommendation in this paper is to ensure that the CYA/FC ratio should be limited to 20 (equivalent to a minimum FC/CYA ratio of 5%). And this would be a huge improvement in the regulated public pool world in terms of sanitation.

This is exactly where the TFP minimum FC for SWG sits. And this is considered the absolute minimum in the TFP world - in agreement with scientific evidence and advice. TFP recommends to follow the target FC at 7.5% for SWGs, the min really is considered the emergency suspenders.

 

[mgtfp]

I think you are the first ever to complain that the TFP levels are not high enough. Usually the discussions around required FC/CYA ratios go differently

 

[Oly]

Personally, while I agree with the lower levels of the average SWG pool, I run mine at or slightly above the liquid chlorine guidelines.

Interesting so what CYA level do you maintain? When maintaining a FC/CYA target for a liquid chlorine in a SWG pool system what is your daily FC loss based upon your SWG model, % setting and hours run per day?

 

[Newdude]

Interesting so what CYA level do you maintain

70 which works fine in NY. Based on that my target was 8-10 (for the liquid part of the chart, which just so happened to be the way I always did it). I never once went under with 20/40/60/40/20 %s spread across the across the season. Ramped up/down as needed with the season. I ran 24/7 with an IC60 on 35k gallons.

I would dial it down at 13 or 14 FC. 11 and 12 were close enough.

 

[Prickly]

I think you are the first ever to complain that the TFP levels are not high enough. Usually the discussions around required FC/CYA ratios go differently

I will admit, I'm a bit of a germaphobe!

 

[Prickly]

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.

Have a look at this paper:

Assessing the Impact of Cyanuric Acid on Bather’s Risk of Gastrointestinal Illness at Swimming Pools

Current regulatory codes for swimming pool disinfection separately regulate free chlorine (FC) and cyanuric acid (CYA). It is well-known that CYA affects disinfection rates by reversibly binding to FC in aqueous solutions. However, limits for these regulated parameters have neither...

www.mdpi.com

The recommendation in this paper is to ensure that the CYA/FC ratio should be limited to 20 (equivalent to a minimum FC/CYA ratio of 5%). And this would be a huge improvement in the regulated public pool world in terms of sanitation.

This is exactly where the TFP minimum FC for SWG sits. And this is considered the absolute minimum in the TFP world - in agreement with scientific evidence and advice. TFP recommends to follow the target FC at 7.5% for SWGs, the min really is considered the emergency suspenders.

This paper illustrates the point I was trying to make which is that the higher your CYA to FC ratio goes, the higher the risk of infection gets.
I agree that the TFP levels are an improvement over health regs in most (but not all) jurisdictions. If they would adopt TFP levels in my current province perhaps I would actually use a public pool here!
The province where I obtained my degrees and inspected pools did have very stringent pool regs. The CYA/FC ratio was indirectly addressed by the ORP requirements. Anyone who has used ORP with CYA can tell you what a massive impact the CYA has on disinfection. Having an ORP requirement meant that I rarely came across pools using CYA at all. There was also no upper limit on chlorine levels so some pools were maintained at up to 10 ppm if needed.
My comments were in no way meant to discredit TFP recommendations. I am very glad that this community educates the public on these very important concepts. If not for this site, there would be no voice against the pool industry for the private pool world. My post was only an attempt to explain why SWG pools are able to get away with less chlorine ( because water is dosed with a high chlorine concentration when flowing through the cell) and also to explain why some may recommend a lower CYA leve in response to the OP's question.

 

[mgtfp]

Anyone who has used ORP with CYA can tell you what a massive impact the CYA has on disinfection.

A major problem with ORP-sensors is that they simply don't work with CYA in the water. As long as there is sufficient HOCl in the water, sanitation is sufficient. You can achieve that with any CYA level as long as FC is maintained accordingly. Of course, at some point it gets impractical to maintain very high FC. And Slamming becomes pretty much impossible at some point.

The best solution would be to directly measure HOCl. Unfortunately, that's not easy, particularly in private pools. Testing FC and CYA (and pH) and use chemical equilibrium equations to calculate HOCl based on these, is the next best option. That's what's is behind TFP's FC/CYA chart.

CYA is not affecting sanitation as long as the required FC/CYA ratio is maintained.

ORP is unfortunately not an option with CYA, but that doesn't mean that you shouldn't use CYA - it means don't use ORP with CYA. And without CYA you are actually fine with FC and pH alone. But ORP can be an add-on for an automation system.

Of you prefer lower CYA and have no issues maintaining FC, then that's fine. But as I said earlier, there are some benefits beyond UV-protection particularly with SWGs in higher CYA levels in regards to reduction of Carbonates.

 

[superuser]

Your last point is one I've been trying to wrap my head around, but I'm no chemistry major. How does a higher CYA level reduce carbonates? Is the desired benefit a reduction in the amount/frequency of scaling of the cell? I recently converted to salt, and my CYA is between 30 and 40 currently. I worked to get the level down last fall because it was off the chart high, and I didn't want to breed a swamp. Now with my SWG conversion I'm wanting to obviously protect the investment and not replace cells more frequently than necessary. It's only been a handful of days, but I'm still dialing it down to keep FC from continually rising. I'm within ranges on everything but CYA according to Pool Math.