Sources of FC Loss

[GearHead36]

I'm curious about the nature of FC loss. I know that algae will eat FC like popcorn. And if your CYA is zero, sunshine will, too. But what else affects FC? I've seen mention of "organic debris". I would assume this to be things like leaves, bugs, grass, etc. How about rain water? Tap Water? How do those affect it? Does FC drop faster at high levels vs low levels? For example, if you keep your FC on the high end of the range, will your bleach usage be higher than if you kept it closer to the lower end of the range?

ETA: Is there a resource that defines this somewhere? If so, please point me in the right direction.

 

[jblizzle]

FC is only lost to 2 things like you said: the sun and "organics"
"Organics" is anything in the water: algae, leaves, bugs, covers, toys, bacteria, viruses, sweat, urine, etc

The higher the FC/CYA ratio, the more ppm of FC you will lose to the sun. Think of it as the CYA protecting a % of the FC. The higher the CYA, the higher the % of FC that is protected from the sun each day. So if you keep the CYA the same and raise the FC level up a bunch, the % loss will be similar, but the number of ppm of FC lost will be higher.

 

[GearHead36]

Thanks, jblizzle.

FC is only lost to 2 things like you said: the sun and "organics"
"Organics" is anything in the water: algae, leaves, bugs, covers, toys, bacteria, viruses, sweat, urine, etc

OK. Now I see why many community pools have signs requiring everyone to shower before entering pool. Not that anyone does it.

The higher the FC/CYA ratio, the more ppm of FC you will lose to the sun. Think of it as the CYA protecting a % of the FC. The higher the CYA, the higher the % of FC that is protected from the sun each day. So if you keep the CYA the same and raise the FC level up a bunch, the % loss will be similar, but the number of ppm of FC lost will be higher.

OK. This is kind of what I thought. So... economically, it's cheaper to keep the FC on the low end of the range, but you then run the risk of letting it drop too low. I can see another appeal of chlorinators. I may have to investigate a Stenner pump.

So, back to economics.... What is the most economical levels for FC, CYA, TA, etc. My current levels are:
- FC:variable
- CC:0.5
- pH: variable
- CYA <30
- TA:90
- CH:225

As you can see, my CYA is low, so I can easily raise it to whatever I need. But, of course, I don't want to raise it too far since it's hard to lower. A CYA of 50 would protect the FC from so much loss due to sun, but it would also require a higher minimum FC level, which would encourage more bleach usage.

A pool is an expensive toy. I want to treat it right, but at the same time, I want to do so with as little waste as possible.

 

[chem geek]

For sunlight loss, the chlorine that is unbound to CYA comes in two flavors, hypochlorous acid and hypochlorite ion. The hypochlorous acid has a half-life in direct noontime sunlight of 2 hours and 10 minutes while the hypochlorite ion has a half-life of 35 minutes. At a pH near 7.5 with a 50/50 mix of these two, the overall half-life is 35 minutes. With CYA in the water, most of the chlorine is bound to CYA (97-99% of it depending on typical FC and CYA level). The chlorine bound to CYA has a much longer half-life on the order of many hours and there is some shielding effect of UV absorption by CYA (or chlorine bound to CYA) that may also be occurring.

In an uncovered pool with low bather-load as is the case with most residential outdoor pools, most chlorine loss is from sunlight. If you have a mostly opaque pool cover, the chlorine loss will be lower, but will still occur since chlorine will slowly oxidize the cover. This might be as much as 0.5 ppm FC per day (depends on temperature and cover material).

There is also a slow chlorine oxidation of CYA itself, but this may account for only around 0.2 FC per day.

A higher bather-load will consume chlorine, but one person-hour in your 20,000 gallon pool would only consume 0.05 ppm FC. So you would only notice this form of chlorine loss if you were to have a pool party with a lot of people in the pool for a while or if you had urinating kids since one cup of urine is roughly equivalent to 5 person-hours in a pool.

If you get blown-in pollen, leaves, and other organic debris, then that will consume chlorine. If you let the FC/CYA ratio get too low, then algae can grow and that will consume chlorine.

All of the chlorine demand except for that lost from sunlight is temperature dependent so the rate of loss is faster at higher water temperature.

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As you can see, my CYA is low, so I can easily raise it to whatever I need. But, of course, I don't want to raise it too far since it's hard to lower. A CYA of 50 would protect the FC from so much loss due to sun, but it would also require a higher minimum FC level, which would encourage more bleach usage.

You are missing a critical point and that is the CYA (or chlorine bound to CYA) shielding effect that is non-linear. This means that you actually lose less absolute amounts of chlorine at higher CYA levels even proportionally raising the FC level to keep the FC/CYA ratio constant. In other words, you will lose less chlorine with 5 ppm FC and 50 ppm CYA than you would with 3 ppm FC and 30 ppm CYA. The only reason you wouldn't want to go higher than 50 ppm CYA unless you had to (i.e. were in a very sunny area such as the desert or deep south with intense sun) is that you'd need a lot more chlorine to do a SLAM if that were ever to be needed.

 

[GearHead36]

Thanks, chem geek. This is pretty much exactly the info I was looking for, plus more.

A higher bather-load will consume chlorine, but one person-hour in your 20,000 gallon pool would only consume 0.05 ppm FC. So you would only notice this form of chlorine loss if you were to have a pool party with a lot of people in the pool for a while or if you had urinating kids since one cup of urine is roughly equivalent to 5 person-hours in a pool.

Wow! I will be enforcing a mandatory potty break for all bathers under the age of, oh, 12 or so.

You are missing a critical point and that is the CYA (or chlorine bound to CYA) shielding effect that is non-linear. This means that you actually lose less absolute amounts of chlorine at higher CYA levels even proportionally raising the FC level to keep the FC/CYA ratio constant. In other words, you will lose less chlorine with 5 ppm FC and 50 ppm CYA than you would with 3 ppm FC and 30 ppm CYA. The only reason you wouldn't want to go higher than 50 ppm CYA unless you had to (i.e. were in a very sunny area such as the desert or deep south with intense sun) is that you'd need a lot more chlorine to do a SLAM if that were ever to be needed.

Yep, this is what I was missing. And this is why I asked. So... the only reason NOT to let your CYA go stupid high is the difficulty in shocking it? OK. I'm going to keep mine low for now. We have an out-of-town trip coming up, and I'm going to use a tri-chlor puck while we're gone. When we get back, I'll set the CYA to 50.

 

[GearHead36]

If you let the FC/CYA ratio get too low, then algae can grow and that will consume chlorine.

What is too low? And how long does it take at this too-low level to allow algae to start growing?

 

[chem geek]

What is too low? And how long does it take at this too-low level to allow algae to start growing?

Lower than the "Minimum FC" level in the Chlorine / CYA Chart which for non-SWCG pools is roughly a 7.5% ratio while for SWCG pools it's roughly a 5% ratio.

Below these levels, whether green or black algae grows faster than chlorine can kill it depends on the level of algae nutrients (phosphates and nitrates) in the pool. The chart levels are such that no amount of algae nutrients will have the algae grow too fast because algae growth is ultimately limited by sunlight and optimum temperature. Under ideal conditions with no chlorine, maximum sunlight, ideal warm temperature, and plenty of nutrients, algae double in population every 3-8 hours depending on the specific species. That means that a pool will not go from being properly maintained to having algae in even one day, but after a couple of days you may see some algae or the water turning dull, after 3 days it will be more clearly visible or cloudy water possibly turning green and by 4 days it should be quite green.

So when you hear about a pool turning quickly into a swamp, that means that the chlorine level was already too low for the CYA level and algae growth had already started so what is being seen is what happens between days 2 and 3, not 0 to 1.

While algae are slower to grow, bacteria grow much more quickly where under ideal conditions they double in population in 15-60 minutes. So here it just takes hours before bacteria become a problem -- certainly half a day to a day can have a large number of bacteria not only growing but forming biofilms that are more resistant to chlorine. Fortunately it takes lower levels of chlorine to kill bacteria faster than they can reproduce, but if you let the chlorine get to near zero then you can have fast bacteria growth. This problem is more commonly seen in spas due to the hotter water temperature and much higher bather loads (small water volume).

 

[GearHead36]

If you get blown-in pollen, leaves, and other organic debris, then that will consume chlorine. If you let the FC/CYA ratio get too low, then algae can grow and that will consume chlorine.

How big is this effect? We get bugs and blown-in leaves. We also get a lot of evening and night thunderstorms, with nearby trees. And even with the trees, we typically have more problems with bugs. Is this effect big enough to warrant cleaning the surface more than once per day? Is it worth the effort to clean the debris at my first chance? Or just do it in early evening as I've been doing?

 

[ewkearns]

Thanks, jblizzle.
OK. Now I see why many community pools have signs requiring everyone to shower before entering pool. Not that anyone does it.

And another thing I think you'll see more of is frequent "everybody out of the pool" periods, so that people will pee anywhere other than the pool. (Couldn't hurt concessions, either, but that is another thread....)

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How big is this effect? We get bugs and blown-in leaves. We also get a lot of evening and night thunderstorms, with nearby trees. And even with the trees, we typically have more problems with bugs. Is this effect big enough to warrant cleaning the surface more than once per day? Is it worth the effort to clean the debris at my first chance? Or just do it in early evening as I've been doing?

In a word, yes! My pool is surrounded by all sorts of green stuff, from palm trees, to loquats, to roses.... etc. The quicker you get organic stuff out of the pool, the less chlorine you will lose due to attempted oxidation....

 

[GearHead36]

In a word, yes! My pool is surrounded by all sorts of green stuff, from palm trees, to loquats, to roses.... etc. The quicker you get organic stuff out of the pool, the less chlorine you will lose due to attempted oxidation....

Thanks. So I guess it's multiple cleanings per day, as needed. I've been doing that, but I've also been wondering if I was wasting my time. Glad (in a way) to know that I wasn't.

 

[chem geek]

How big is this effect? We get bugs and blown-in leaves. We also get a lot of evening and night thunderstorms, with nearby trees. And even with the trees, we typically have more problems with bugs. Is this effect big enough to warrant cleaning the surface more than once per day? Is it worth the effort to clean the debris at my first chance? Or just do it in early evening as I've been doing?

Leaves are generally not as much of an issue unless you get a LOT of them because they don't break down that quickly and being large that have a smaller surface-to-volume ratio. The chlorine usage for a given volume (weight, at a given density) of debris is proportional to the surface area, all else equal. So pollen or other small items consume chlorine more than leaves, for the same rough amount of total debris.

I'm not sure about bugs since they are different enough where some parts may get more readily oxidized while other parts may not. I'd guess they are worse than leaves but not as bad as pollen unless the bugs are very small (i.e. more like gnats than larger insects).