What HOCl level is required to prevent algae?

[tomfrh]

I understand HOCl. is the form of chlorine that does the killing.

What levels of HOCl are actually required?

I'm reading different advice in various places. I've read you need minimum of 0.05ppm. But we also hear you need a minimum FC of 7.5% of CYA

Assuming CYA of 30 ppm, that gives 2.25ppm FC, of which ~97% will be bound, leaving 3% of 2.25 = 0.0675ppm of available FC, which results in less than 0.05ppm of HOCl for most pH levels

I'm reading this paper: http://www.lowrycg.com/wp-content/u...h-Free-Chlorine-Do-You-Really-Need-rev-03.pdf
which seems to offer contradictory advice:

You can see that algae are not killed (HOCl >0.05 ppm) with even 4.0 ppm FC when CYA is more than 40 ppm andthe pH is 7.5

If you have 50 ppm of CYA then you need 3.75 ppm FC minimum (50 × 0.075 = 3.75).... By having FC at 7.5% of CYA, the amount of HOCl will be more than the0.05 ppm HOCl needed for algae control.

Wouldn't you need more than 7.5% CYA to maintain HOCl above 0.05 in most situations? presumably many of us are running satisfactorily at lower than 0.05 HOCl?

 

[needsajet]

Chem Geek's thread, Pool Water Chemistry, is the best reference for the questions here and also for your other thread, including charts.

Also, try a search for "Chem Geek Pool Equations.xls" which is definitive in terms of formulas, and very handy.

For me, 0.02 ppm HOCl (as ppm Cl₂) is enough, but I keep my pool around 0.04

 

[JoyfulNoise]

There’s no exact answer here. There’s too many different species of “algae” to say for sure what the exact level is. For example, blue-green algae is easy to kill at HOCl levels less than 50ppb BUT yellow algae needs much higher levels. Typical environmental studies don’t really offer much advice as none of them use CYA in there protocols and you typically see numbers like 1ppm total chlorine is enough to kill algae.

Also, keep in mind that you have an equilibrium between HOCl and OCl- so that the total active chlorine levels are closer to 100ppb. While OCl- is the weaker of the two in terms of sanitation, it still does contribute to the overall kill rate. As well, monochloramine is biocidal and can kill both algae and bacteria.

The other idea to understand is this is not a situation of absolutes where X ppm of sanitizer kills but < X ppm does not. This is about competitive rates - the rate of kill needs to be greater than the rate of growth (colony doubling time) in order for a particular species to be held under control. Bacteria and viri are easy enough to kill even though their growth rates can be fast but algae is harder to kill and, luckily, slower in its growth.

 

[Donldson]

While Robert Lowry consulted with Richard Falk and is basing his advice off of the same data we are, he is not a part of TFP and his advice is his own. If his writing contradicts itself then that is something you would need to contact him about.

TFP has developed the recommended levels independently and are often being evaluated by volunteers curious about their own pool chemistry. As always, what others in the industry are suggesting is their own evaluation of the data they have. While Mr Lowry is basing his recommendation off of better data than most industry members they are still his own recommendations and do not alter those found at TFP.

 

[needsajet]

There's a link in Pool Water Chemistry... https://www.troublefreepool.com/~richardfalk/pool/HOCl.htm which provides a chart of the HOCl numbers at pH 7.5

The numbers all shift to the left at a more typical pool pH of 7.8, but that chart is still pretty handy. The blue numbers indicate the "safe" level, meaning "high enough to keep algae at bay". The green numbers indicate where most bacteria are kept in check, and the red numbers are too low.

 

[tomfrh]

Chem Geek's thread, Pool Water Chemistry, is the best reference for the questions here and also for your other thread, including charts.

Also, try a search for "Chem Geek Pool Equations.xls" which is definitive in terms of formulas, and very handy.

For me, 0.02 ppm HOCl (as ppm Cl₂) is enough, but I keep my pool around 0.04

Thanks. Interesting to hear you use 0.02.

The 0.05 number seems quite high.



I'll read chemgeeks page, and hopefulyl this will clear up my confusion!

 

[tomfrh]

There's a link in Pool Water Chemistry... https://www.troublefreepool.com/~richardfalk/pool/HOCl.htm which provides a chart of the HOCl numbers at pH 7.5

The numbers all shift to the left at a more typical pool pH of 7.8, but that chart is still pretty handy. The blue numbers indicate the "safe" level, meaning "high enough to keep algae at bay". The green numbers indicate where most bacteria are kept in check, and the red numbers are too low.

That chart is a good example of what I'm wondering about. It suggests you need 0.05ppm HOCl to be safe (the blue zone), but the blue zone is always above a FC level of 7.5% of CYA. I can't see how 7.5% corresponds to 0.05ppm.

The target non-SWG FCs in the TFP recommendations are about 12% of CYA, which seems to give the 0.05ppM of HOCl?

 

[JoyfulNoise]

Again, those numbers are not exact! There is no one specific threshold that will work. In my own pool, I have been well below 50ppb HOCl (about 15ppb) number and I have never gotten algae in my pool. Those numbers serve as a guide but they are not absolutes.

The "Target" values are set higher than 7.5% to account for normal daily usage. In other words, the conventional wisdom is your pool's floor value is 7.5% FC/CYA ratio and it should never go lower than that.

 

[tomfrh]

Ok, I’m starting to understand it.

-7.5% CYA is considered a bare minimum and will leave you with less than 0.05ppm HOCl

-0.05ppm is a conservative target

 

[JoyfulNoise]

Ok, I’m starting to understand it.

-7.5% CYA is considered a bare minimum and will leave you with less than 0.05ppm HOCl

-0.05ppm is a conservative target

That's right. Every pool is different and there are different techniques that could be employed to reduce that "floor" value. The TFP philosophy starts off with the assumption that chlorine is the only chemical that is needed for sanitation and oxidation and then further assumes that most pool owner wish never to have algae in the water. So, if those are your two assumptions, then a higher and more conservative chlorine target is where you will end up. Modifying those assumptions can allow for less chlorine use but then the risk profile changes.

 

[tomfrh]

While Robert Lowry consulted with Richard Falk and is basing his advice off of the same data we are, he is not a part of TFP and his advice is his own. If his writing contradicts itself then that is something you would need to contact him about.

TFP has developed the recommended levels independently and are often being evaluated by volunteers curious about their own pool chemistry. As always, what others in the industry are suggesting is their own evaluation of the data they have. While Mr Lowry is basing his recommendation off of better data than most industry members they are still his own recommendations and do not alter those found at TFP.

I emailed Lowry the other day asking for clarification on his statement that 7.5% CYA causes HOCl. to exceed 0.05ppm.

He wrote quite a long reply, but he didn’t really want to address that central point.