FC requirement when using peristaltic chlorine pump

[physicsrob]

Hi All,

So I've been pondering the difference between the recommended FC level for SWG/non-SWG pools at a given CYA. I'm curious if anyone has an explanation for why the FC requirement of a SWG would be less? I could see two possible theories:
1) Does the NaCl in some way make the FC more effective? Seems unlikely -- but I'm happy to hear other opinions (or better yet facts =] ).
2) Perhaps SWG result in a more consistent FC level, and thus a lower level is needed.

Thoughts?

Here's why I've been thinking about this: I just purchased a stenner pump that I'm going to be using to constantly inject liquid chlorine into my pool. In terms of dosing schedule it seems more closely aligned with a SWG. Do I follow the SWG guide for FC or the non-SWG guide? More importantly -- why? Thoughts?

 

[chem geek]

We're not sure why there is the difference, but we know that there is. It could be either of the reasons that you listed, but we've found that it seems to be required to keep the FC levels in automatically dosed non-SWG pools at the non-SWG levels in the Chlorine / CYA Chart. So while the automatic dosing may help keep a more consistent FC level, there does seem to be a bit of an inhibiting effect of the higher salt level to the common types of algae found in pools.

Since every pool differs in the level of algae nutrients, you can try the lower FC level for SWG pools and see if you notice any higher-than-normal chlorine demand which is usually the first sign of algae growth before it becomes visible algae or cloudy water. Probably best to start out at the non-SWG level to get a baseline daily chlorine usage (especially overnight) before doing any experiment lowering the FC level.

 

[physicsrob]

Thanks for the feedback. It seems like a really interesting conundrum. It amazes me how much original research you and others here have needed to do to understand pool chemistry.

Ultimately it seems like if one were to use a peristaltic pump, higher CYA levels, salt levels appropriate for a SWG, and the FC levels appropriate for a SWG system they would have essentially recreated a SWG-system without the SWG. I guess that's not the easiest experiment to do...

One last thought -- perhaps the electrolysis itself has something to do with it. Seems unlikely -- but who knows?

 

[chem geek]

Actually, you bring up a good point about the electrolysis and that would have some effect towards killing algae that gets circulated. Of course, it won't do anything for algae that clings to pool surfaces or otherwise doesn't get circulated, but some forms of algae especially during initial growth phases are free-floating. There is a difference between adding chlorine in an SWG vs. adding it manually or with a peristaltic pump because sodium hypochlorite is high in pH so as it dilutes it is limited in its active chlorine (HOCl) level. The following shows the hypochlorous acid (HOCl) concentration adding 12.5% sodium hypochlorite into a pool with 7.5 pH, 80 ppm TA and 50 ppm CYA at various dilution levels (assuming perfect mixing to those dilution volumes):

Dilution ... FC ...... pH . HOCl (ppm)
.. 1:1 .... 125,000 . 12.4 . 0.894
.. 1:10 .... 12,500 . 11.4 . 1.154
.. 1:100 .... 1,250 . 10.0 . 3.704
.. 1:1000 ..... 125 ... 9.3 . 1.377
.. 1:10000 ..... 13 ... 8.3 . 0.112

So you can see that because of the high pH of sodium hypochlorite, the active chlorine level is fairly low and then as the pH drops the binding to CYA takes effect lowering the HOCl further. So the active chlorine level never really gets that high. It's certainly much higher than normal levels, but it's over a limited pool water volume and the dilution occurs fairly rapidly in a pool with decent circulation.

In an SWG, however, the chlorine generated at the anode is at low pH due to the following reaction where the generated chlorine gas dissolves in water:

2Cl^- ---> Cl₂(g) + 2e^-
Cl₂(g) + H₂O ---> HOCl + H⁺ + Cl^-
Chlorine Gas + Water ---> Hypochlorous Acid + Hydrogen Ion + Chloride Ion

So as this dilutes we have the following where I use a similar quantity starting point similar to 12.5% chlorinating liquid (in practice near the anode plate the concentrations are even higher):

Dilution ... FC ...... pH . HOCl (ppm)
.. 1:1 .... 125,000 .. -0.1 . 124,917
.. 1:10 .... 12,500 ... 0.9 . 12,420
.. 1:100 .... 1,250 ... 1.9 ... 1,183
.. 1:1000 ..... 125 ... 3.6 ........ 81
.. 1:10000 ..... 13 ... 7.0 .......... 0.185

Actual FC levels measured by SWG manufacturers midway between the plates is at around 80 ppm, but at that point the pH is higher because of the pH rise from the hydrogen gas generated at the cathode:

2H₂O + 2e^- ---> H₂(g) + 2OH^-

where the net of the two is the same effect as adding hypochlorite:

Cl^- + 2H₂O ---> HOCl + H₂(g) + OH^-
or equivalently
Cl^- + H₂O ---> OCl^- + H₂(g)

So closer to the anode where chlorine is generated the active chlorine level is higher and essentially super-chlorinates at lower pH. Of course, this is very brief as the water mixes quickly, but it still could have some small incremental effect enough to make the SWG pools not need quite as much chlorine on average (at similar algae nutrient levels, temperature, sunlight, etc.).

 

[Patrick_B]

I think that makes the most sense. Water, algae spores or whatever passing through super concentrated FC levels in the generator stream has to be a big part of it.