Thoughts on FC Levels

[Sammy2]

The recommendation is to have FC / CyA = 0.075 and for a saltwater pool the CyA should be around 80ppm with FC = 6ppm. Any thoughts on going to FC / CyA = 0.10, or a FC of 8ppm at CyA of 80ppm? This puts it center range of a manually chlorinated pool.

Why wouldn't this be appropriate or sound? A heavy dose of Chemistry is okay for me.

 

[pooldv]

Yep, totally fine. We live in the woods and have lots of wildlife around our pool. I tend toward 9-10% FC to CYA ratios.

 

[JoyfulNoise]

7.5% is the min for manually chlorinated pools. For SWG pools, it's closer to 5%. Daily losses for an SWG pool with a CYA of 80ppm should be very low, around 1.5ppm/day. You should only need about 6ppm FC to be fine.

Better question - why do you want to have/use so much chlorine?

As you increase FC levels, your loss rate goes up in proportion to the amount of hypochlorite (OCl-) in the water. Running at higher FC/CYA ratios means more chlorine consumption over time at greater monetary cost.


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[Sammy2]

For whatever reason, the recommended level seems low because it is right at maintained killing concentration for bacteria. I think it should have more reserve than that but I have been wrong before.

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BTW, I know the goal is to reduce cost but at what point does that goal contradict with the primary goal of sanitary water?

 

[Sammy2]

Looking at the first logarithmic graph from the sticky above on Pool Chemistry one sees that the level required for a kill rate above the propagation rate of bacteria is a FC level of approximately 9ppm with CyA of 70ppm. this is greater than 10%.

 

[JoyfulNoise]

One thing to remember is that, just because the FC dips a little bit, doesn't necessarily mean that you pool will turn into a green swamp. It can takes days of clouded water before anything green ever shows up.

Here's a little personal experience - all last year (and planned for this year too), I ran my pool at 90ppm CYA and about 3ppm FC. My FC/CYA ratio was much less than the recommended levels. Guess what? Not a single day of clouded water or algae, ever.

Now, what I did not tell you is that I have borates in my pool water which acts as a mild algaestat (it inhibits algae growth) AND I treat for phosphates every season (my current PO4 levels are 0ppb). Both of those conditions make my water much less "reactive" to algae, that is to say, even if algae found it's way into my water the low nutrient conditions (low PO4) and presence of the algaestat (boron levels ~ 50ppm) mean that the algae can not efficiently grow. In real terms that means that algae replication rates are slower giving chlorine much more margin to act as a sanitizer.

Algae and pathogens (bacteria and viri) are not destroys in a binary sense - if you have X amount of FC, everything is dead, otherwise it lives. Sanitizers are defined by their CT Kill times (concentration multiplied by application time) - if you have very high sanitizer levels, then the application time can be short to achieve a 4-Log reduction (99.99% reduction). If the sanitizer concentration is low, then it take longer times to achieve the same 4-Log reduction. This is because you have two rate processes going on - you have the kill rate of the sanitizer versus the replication (reproduction) rate of the pathogen. So, there are two levers to play with - either ramp up the kill rate so you overwhelm the pathogen and destroy it OR reduce the reproduction rate of the pathogen so that a slower kill rate of the sanitizer is still effective.

TFP Recommended levels ignore the second part of that balance and just assume the reproduction rates are what they are. So the Recommended Levels are based on achieving the best possible CT kill times without compromising bather comfort. My pool water looks to optimize BOTH sides of the equation and thus allows me to operate at a much lower chlorine level, which I find more comfortable and "easier on the wallet"....

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Looking at the first logarithmic graph from the sticky above on Pool Chemistry one sees that the level required for a kill rate above the propagation rate of bacteria is a FC level of approximately 9ppm with CyA of 70ppm. this is greater than 10%.

The actual CDC minimum disinfection level for water, as measured by ORP, corresponds to a hypochlorous acid ([HOCl]) concentration of 11ppb (0.011ppm) of HOCl. That corresponds to a very low FC/CYA ratio. The levels you see in the chart are based on anecdotal observations of what "seems to work" in most pools. Think of it like a statistical quantity and that's your mean value. There's lots of pools on either side of the mean.

 

[Sammy2]

Okay. I'm still working on this whole concept. Back in the day, we're talking about 40 to 50 years back in the day, we chlorinated.. a LOT. We never had issues with algae and we nobody ever got sick or died. The pool never smelled of chloramines either. All that and we tested with a basic OTO kit. I'm trying to get used to the idea of running the pool so close to the line so to speak. If I have more in reserve I never have to worry about it. Right now my pool is a a FC of 14 with CyA at 70. I am working to bring FC down while also reducing pH from 7.8?? to 7.5 and take TA down below 100ppm, preferably around 80ppm. I'm not sure if I want to bring my FC down around 6ppm or shoot for 8ppm, which to me seems better than 6ppm. I'd rather kill pathogens before they even get a chance to reproduce and see this as proactive rather than reactive.

 

[JoyfulNoise]

Okay. I'm still working on this whole concept. Back in the day, we're talking about 40 to 50 years back in the day, we chlorinated.. a LOT. We never had issues with algae and we nobody ever got sick or died. The pool never smelled of chloramines either. All that and we tested with a basic OTO kit. I'm trying to get used to the idea of running the pool so close to the line so to speak. If I have more in reserve I never have to worry about it. Right now my pool is a a FC of 14 with CyA at 70. I am working to bring FC down while also reducing pH from 7.8?? to 7.5 and take TA down below 100ppm, preferably around 80ppm. I'm not sure if I want to bring my FC down around 6ppm or shoot for 8ppm, which to me seems better than 6ppm. I'd rather kill pathogens before they even get a chance to reproduce and see this as proactive rather than reactive.

That's entirely up to you and what you are comfortable with. I personally do not like a lot of FC in the water because I hate the chloramine smell that develops on my skin and bathing suit when I get out of the pool. I do not have a pool shower nearby so rinsing off immediately is not an option. I'll often get out of the pool, towel off a little and then work around the backyard as the kids play. When chlorinate water sits on your skin (even the amount left from toweling off), it will turn into chloramines as it reacts with sweat and skin. So running a lower FC pool has a benefit for me - I don't detect "that smell" that everyone associates with a chlorine pool....

...oh, and I save lots of money by not having to run my pumps so long to keep the FC at a higher level.....

 

[Sammy2]

When I took over from my pool guy two weeks ago the levels were as follows last week when I started testing:

FC 12 ppm
CC 0 ppm
pH >8.2 (deeper red than test kit gradiations)
TA 130
CH 600 (tap water here is 250ppm)
CyA 38
Salt 3200 ppm by test, 3600 ppm by meter on the Hayward generator Should be around 3800 ppm
Calculated CSI 1.09 (using TFP Calculator)

There was no chloramine smell or residue on the skin to later turn to chloramine either.

Now that I'm adjusting it to be in better balance it is as follows:

FC: 14
CC: 0
PH: 7.8
TA: 110
CH: 375
CYA: 70
Temp: 69
Salt: 3300
CSI: +0.1

That's the FC I'm bringing down. The SWG is off right now. FC rose due to CyA sequestration I believe but have not confirmed. I'm wondering if with a higher CyA, the water feel will change. I have no idea what it was before last week as I wasn't testing it but it felt fine. We'll see where it goes from here but I need to find a balance between killing pathogens and having water that feels good.

 

[JoyfulNoise]

Get your FC and TA down to more normal levels recommended by TFP and the water will feel fine. I know your past experience is shouting "chlorinate the heck out of that bad-boy, NOW!!!" but, believe me, if you try to operate at TFP recommended levels you will be amazed at how easy it is to take care of pool!

And, as a side not, CYA would not make the FC value change in any way...it's likely testing error. Are you using a SpeedStir with your test kit? They make testing A LOT easier....and you look like a major dork when you test your pool water, always a plus in my opinion....

 

[dirtworm_ato]

I'm interested in this discussion as well as I'm still learning about how to run my new pool. We're still in the break-in period and haven't yet started the SWG, so I've been more or less trying to follow the recommendations for a non-SWG pool for now. When I look at the FC/CYA chart, it recommends a minimum of 4 FC and target of 6-8 with CYA of 50. However, I'm scared to death of getting an algae bloom in my sparkling new pool so have been trying to keep it around 10. My FC usually takes a couple of days to drop down to around 6 at which point I've been adding more bleach to bring it back up. Am I understanding from this thread that by keeping my FC above the target, I'm actually causing it to fall faster than if I just kept it at target?

 

[JoyfulNoise]

I'm interested in this discussion as well as I'm still learning about how to run my new pool. We're still in the break-in period and haven't yet started the SWG, so I've been more or less trying to follow the recommendations for a non-SWG pool for now. When I look at the FC/CYA chart, it recommends a minimum of 4 FC and target of 6-8 with CYA of 50. However, I'm scared to death of getting an algae bloom in my sparkling new pool so have been trying to keep it around 10. My FC usually takes a couple of days to drop down to around 6 at which point I've been adding more bleach to bring it back up. Am I understanding from this thread that by keeping my FC above the target, I'm actually causing it to fall faster than if I just kept it at target?

Your best bet is to follow the recommended levels and to keep your FC as even as possible. Large fluctuations in chemistry are never a good approach. If you can get a feel for your FC consumption throughout a typical day and break up your chlorine additions to keep the levels more even, that's always the better approach.

There is no need to be "scared to death". Really, there's waaaay too much anxiety about algae. It takes quite some time for an algae bloom to get started and, being new and very sensitive to your pool, you will likely notice the water being off well before anything green starts to grown. Even if the water was suddenly cloudy from algae, fixing cloudy water is easy...much, much easier than opening a winterized pool and trying to resurrect a black/green swamp.

Just follow the recommended levels and you, your new plaster, and water will be just fine....

 

[Sammy2]

A couple of things.

Algae, while a nuisance is not a problem when keeping FC at a level to overcome bacteria reproduction rates. Recommended levels are high enough to just barely overcome bacteria growth rates which, for me, seems a little low.

The FC won't fall faster after it reaches the target rate, it will fall the same as if you just kept it there to begin with. The degradation rate from somewhere higher than the target and the target is faster though.

I could be wrong and someone should tell me if I am.

 

[Sammy2]

Get your FC and TA down to more normal levels recommended by TFP and the water will feel fine. I know your past experience is shouting "chlorinate the heck out of that bad-boy, NOW!!!" but, believe me, if you try to operate at TFP recommended levels you will be amazed at how easy it is to take care of pool!

And, as a side not, CYA would not make the FC value change in any way...it's likely testing error. Are you using a SpeedStir with your test kit? They make testing A LOT easier....and you look like a major dork when you test your pool water, always a plus in my opinion....

What I mean is that as I increased my CyA I sequestered more FC from my SWG generation and lost less at CyA of 70 vs CyA of 40 so even though I turned it down from 50% to 25% it went up because my losses went down, ie I kept more of it in the pool. I don't use the speed stir but can see that it could be advantageous to get one.

 

[JoyfulNoise]

Please read this post - Pool Water Chemistry

When you add chlorine to water with CYA in it, you get three things -

1. Chlorine bound to CYA
2. Hypochlorous Acid (HOCl)
3. Hypochlorite anion (OCl-)

More than 95% of the chlorine is in the form of item #1 on the list - chlorine bound to CYA. That form of chlorine is completely shielded from interaction with UV light, does not disinfect in any way and has a half-life of around 8.5 hours.

The remaining 5% or so of the chlorine you add splits up into HOCl/OCl-, the fractional amount of each is determined by pH and, at a pH of 7.5, the split is an even 50:50 split. The half life of hypochlorous acid is longer than hypochlorite anion but, since the mixture of the two is controlled by pH, it is roughly 35mins. But that is just the intrinsic half-life (HOCl/OCl- with no UV light). When UV light is present, the hypochlorite anion interacts strongly with the UV and breaks down in the following way -

2 OCl^- + UV_light -----> O₂ + 2 Cl^-

The above chemical reaction happens very rapidly and it is depth dependent in a pool where the top surface of the water will become fully depleted versus water at an 8 ft depth. As the OCl- gets used up, HOCl converts into OCl- by equilibrium chemistry to make up for the loss. So, overall, UV light starts to exhaust the supply of HOCl/OCl-. In response, the CYA begins to release more of it's chlorine to compensate because there is another equilibrium, between the amount of chlorine bound to CYA and the amount of hypochlorous acid in the water.

So, shining UV light on water is like pulling the plug on a bath tub - the OCl- is the plug that gets pulled and the reserve of chlorine (bath water) gets drained away. The size of the plugged hole is like the HOCl/OCl concentration - the smaller the hole (the concentration), the slower the reserve drains away.

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What I mean is that as I increased my CyA I sequestered more FC from my SWG generation and lost less at CyA of 70 vs CyA of 40 so even though I turned it down from 50% to 25% it went up because my losses went down, ie I kept more of it in the pool. I don't use the speed stir but can see that it could be advantageous to get one.

Gotcha.

 

[Sammy2]

I was reading that post actually. That is where I'm seeing raising the FC a smidge above 7.5% CyA in order to have it well above the bacteria kill line.

The question is this.. Does Chlorine bound to CyA show up in the FC test? If yes, that would explain my increase. I've been trying to find an answer to this question without luck. Do the reagents used for the FAS/DPD test pull FC from the CyA and report it?

Another thing is that back then it wasn't chlorinating out of reactionary practice but preventive practice. We move to CA from CT in 1961. There weren't many pools in CT in 1961 and we had almost the only one on the block in the mid-60's California as it is. It was like having a small public pool as the whole neighborhood would be in it all summer long as it wasn't as it is now where every other house in CA has a pool in their backyard.

My dad did what he could back then to make that pool sanitary.. He read up on it and he followed the advice of pool professionals. I don't think the pool store then resembles what it is now for sure. There were no teenagers working in a chain store just to make money to buy stupid stuff teenagers buy. These were men (sorry ladies, this was the 60's and I only ever saw men) who were making a living off of their own business selling pool supplies. There really wasn't all the chemical soup we have now. There was Sodium Hypochlorite and Muriatic Acid. We tested water with an OTO and added Chlorine and Acid out of returnable / recyclable bottles and we kept the chlorination levels high to combat the sun and the swimmer load, balancing pH for comfort. My dad, reading everything he could about stuff like this (I sorta inherited that trait), probably read the articles written in the 60's about CyA and FC lock so CyA wasn't for him. I specifically remember him saying something about other people add conditioners and such but we're sticking with what works Chlorine and Acid and a test kit.

I'm used to that mentality but can see that there is a better way but I'm trying to get a handle on chlorinating just above the kill rate for pathogens. That is all. I posted down here because this section is more geared towards theory than practice. Too me the theory still supports higher levels of FC so I'm asking..

 

[JoyfulNoise]

Yes, chlorine bound to CYA shows up on the DPD-FAS test. As HOCl/OCl reacts with the DPD indicator dye by oxidizing it to the pink-colored Wurster dye, chlorine is continuously released from the CYA until it is exhausted. So the test measures the total amount of active chlorine (HOCl/OCl) and reserve chlorine bound to CYA (HCy-Cl).

The true nature of how CYA interacted with chlorine was not fully understood until about 1974 when O'Brien, et al., released a detailed study of the chemistry of chlorinated cyanurates. Even then, most of the industry simply ignored the proper relationship between CYA and FC because there were not easy ways to measure most of this stuff independently and it was deemed "not necessary" or "overly complicated" to ask pool owners to try to understand it. And it likely cut into profit margins and was seen as an expensive alternative. OTO is a total chlorine (TC) test and I find it to be mostly useless for any kind of analytical/quantitative analysis as the yellow color is not all that precise. DPD is a much better chemistry but was mostly relegated to industrial labs and academic studies in the time frame of your father's pool care needs.

My earliest recollection of a pool (circa 1980's) was at my best friends house (his dad was an FBI agent and his mom was a public school teacher). They had the only pool anywhere in our neighborhood and all I remember is being yelled at about not fooling around on the diving board or jumping off the roof of the pool/equipment house...."well, if you didn't want kids jumping off the pool house into the pool, why the heck did you put it so close to the pool?!?!? Duh???"


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[Sammy2]

Yes, chlorine bound to CYA shows up on the DPD-FAS test. As HOCl/OCl reacts with the DPD indicator dye by oxidizing it to the pink-colored Wurster dye, chlorine is continuously released from the CYA until it is exhausted. So the test measures the total amount of active chlorine (HOCl/OCl) and reserve chlorine bound to CYA (HCy-Cl).

The true nature of how CYA interacted with chlorine was not fully understood until about 1974 when O'Brien, et al., released a detailed study of the chemistry of chlorinated cyanurates. Even then, most of the industry simply ignored the proper relationship between CYA and FC because there were not easy ways to measure most of this stuff independently and it was deemed "not necessary" or "overly complicated" to ask pool owners to try to understand it. And it likely cut into profit margins and was seen as an expensive alternative. OTO is a total chlorine (TC) test and I find it to be mostly useless for any kind of analytical/quantitative analysis as the yellow color is not all that precise. DPD is a much better chemistry but was mostly relegated to industrial labs and academic studies in the time frame of your father's pool care needs.

My earliest recollection of a pool (circa 1980's) was at my best friends house (his dad was an FBI agent and his mom was a public school teacher). They had the only pool anywhere in our neighborhood and all I remember is being yelled at about not fooling around on the diving board or jumping off the roof of the pool/equipment house...."well, if you didn't want kids jumping off the pool house into the pool, why the heck did you put it so close to the pool?!?!? Duh???"


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Right!? We used to jump off the roof of the single story house as it was only about 5 feet from the pool. Pretty sketchy though as the pool was only 6 feet deep.

Thank you for the clarification. That is what I thought, maybe even read, but now it is confirmed and supports my suspicion as to why, with my SWG settings unchanged, I saw an increase in FC after I started floating the CyA in the socks. I got to that to 70ppm measured 12 hours after pulling the socks with circulation all day and that is where it is my current target. My target FC is going to be 7ppm and we'll see how that goes for swimmer comfort and chloramine creation. If it is an issue, I'll bring it down. Thing is that my pool guy had it pretty high to start with as seen in my post above and it was pretty much perfect as far as feel and smell goes.

 

[JoyfulNoise]

As long as the pool receives adequate sunlight, your CC levels should be low. Last season I barely measured any CCs at all, mostly 0 or 0.2ppm as I use the 25mL water sample for maximum sensitivity (0.2ppm/drop). A low bather load, residential outdoor pool will have almost no CCs in it if it is properly chlorinated.

And here is another factoid you can search on and read about - CYA moderates the formation of CCs in a pool. Because CYA lowers the overall active chlorine levels, the CCs produced in a pool are often not even detectable. There are three primary CCs measured by the DPD-FAS test - monochloramine, dichloramine and nitrogen trichloride (there are also other chlorinated disinfection by-products created by chlorine but they are not all detected by the DPD and are usually quite low). The first two CCs, mono- and dichloramine, are easily destroyed by UV and/or outgas from pool water. Because low levels of active chlorine tend to favor the production of monochlormaine and dichloramine, most swimmers can't detect anything. However, if CYA levels are low OR you don't use any stabilizer in the pool, active chlorine levels will be very high and that will actually favor the formation of nitrogen trichloride. Nitrogen trichloride is extremely irritating and has a very low odor threshold. So, one other benefit of CYA is that it tends to reduce the formation of the more irritating CCs.

In your childhood days, pools without CYA and using high levels of total chlorine would experience the formation of some very irritating combined chlorine compounds and chlorourea compounds which would be very smelly. This is the "the smell" that everyone hates about chlorinated pools.

 

[Sammy2]

We didn't experience chloramines to any extent that I remember. I think my dad had the TC level dialed in pretty well and just by keeping on it all the time he kept "the smell" away. I remember doing the OTO tests but I have no idea what the targets we were shooting for. This was a lot of beers (among other things) ago.