SWG's bubbles; contents and dissolution vs flow rate?

[JesseWV]

Today I cleaned the plates on the SWG cell. After getting it all hooked back up, I was enjoying a cold one while watching the increased bubbles coming out of the return.

This got me thinking. It has always been my observation that more bubbles make it to the surface when the pump is on low, and less bubbles make it when on high. I assume this mean more hydrogen is dissolving into the water on high. This brings me to a few questions:

1. Does the dissolution of hydrogen directly effect pH? Would allowing more bubbles to make out of the water and not dissolve effect pH less?

2. If hydrogen bubbles have already formed, does that mean that the chlorine is already in solution, or do they both exist in gas form until they dissolve into the water?

 

[JasonLion]

1) The bubble size affects the amount of aeration, which indirectly affects the PH. Dissolved hydrogen gas doesn't make any difference.
2) Chlorine starts as a gas right at the cell plate inside the SWG. The chlorine gas dissolves very quickly, while the hydrogen gas is much slower to dissolve, if it does at all.

 

[chem geek]

Hydrogen gas effectively doesn't dissolve in water (i.e. the water gets saturated with it very quickly) so at the higher flow rate the smaller bubbles are in fact getting to the surface and outgassing but they may be too small to notice. What happens is that carbon dioxide in the water will enter into those bubbles so the carbon dioxide will get removed to the air which tends to make the pH rise. Whether this happens more with bigger or smaller bubbles is hard to say because smaller bubbles would have more carbon dioxide enter into them while larger bubbles might disturb the water surface more allowing for faster outgassing away from the bubbles. My guess is that the smaller bubbles aerate more, but calculations I've done show that carbon dioxide into the bubble volume doesn't account enough for the pH rise that is usually seen.

In some SWG pools with short pipe runs some chlorine gas may not fully dissolve in the water and instead outgases. If this happens, then that is another source of pH rise.

Solubility of common gasses in water at 30ºC (86ºF) in milligrams per kilogram water (roughly a liter) so ppm is as follows:
Chlorine Gas ..... 5500 (this is when bubbles form, most chlorine gas becomes hypochlorous acid in pool water)
Carbon Dioxide . 1300 (this is when bubbles form, not equilibrium amount with air)
Oxygen ............... 37
Nitrogen .............. 17
Hydrogen .............. 1.5

So beyond 1.5 ppm hydrogen in the water the gas will no longer dissolve and will remain as bubbles.

 

[JesseWV]

I sure hope I'm not losing some chlorine to the air. The run from my SWG to the return is only 3 feet! Wouldn't the smell be noticeable? I've read it smells like a mix of pepper and pineapple.

 

[chem geek]

It's a small amount of chlorine so not clear whether you would notice it. The effect is from having this small amount escape over a longer period of time. 3 feet is a very short run. If you point the returns downwards (if more than one return, then the one closest to the SWG) then that may help give more time for the chlorine to dissolve. If you run the pump more slowly so that you see where the hydrogen gas bubbles come up, you can try sniffing around in that area to see if you notice a chlorine smell. Of course, just the aeration could outgas some hypochlorous acid which will also smell similarly. Perhaps putting a distilled or deionized water moistened pH test strip above the area might show a drop in pH if chlorine gas is present.

At 2 ppm FC per day, if 10% of the chlorine gas did not dissolved and were to outgas, then at 80 ppm TA (and no borates) that would result in a pH rise in one week from 7.5 to 7.62 while if 20% of the chlorine gas escaped that would raise the pH from 7.5 to 7.76.

 

[JesseWV]

Last season when borates were at 50 and TA at 60, I only had to add acid twice to go from 7.8 to 7.5. I think it was once in June and once in August. Each time it took about 20 oz 32 oz of 31.45% MA.

Would there be a downside to letting the pH stay at 7.8 since my CH is 60, I have a vinyl surface and don't need to worry to much about CSI?

 

[chem geek]

No problem having the pH at 7.8. The only issue with higher pH, assuming the CSI is not too high, is precipitating metals to produce metal stains. That's only an issue if you've got significant metal concentrations (iron, copper, manganese) in your water.

 

[JesseWV]

There shouldn't be any metals at all, so that shouldn't be a problem.

What about eye comfort? My kids don't listen and still open their eyes under water. I've read that human tears are between 6.5 and 7.6.

 

[chem geek]

Low pH, especially below 7.0, is worse for eye comfort than higher pH. The pH of human eyes varies somewhat and this paper is probably the one where you got the 6.5 to 7.6 range, but this paper gives an average of 7.5. This paper found that eye irritation did not occur in the pH range from 7 to 9. The primary source of eye irritation was chloramines (mostly nitrogen trichloride which is the most irritating) and a lower salt level due to osmotic pressure.

 

[JesseWV]

Indeed your first link was where I got the pH range from. Thank you for the links to the other two papers.

The third publication was very informative, despite being a little dated. It shows that having salt concentration at 5000 ppm reduces eye irritation greatly. This makes sense as I've read that human tears are 6000-9000 ppm salt. Too bad we can't keep our pools at 5000 ppm and still run the SWG without causing over current.

I suppose 3000 ppm is better than a few hundred.

 

[chem geek]

If one spends a lot of time in the water, especially in a non-SWG pool, then it's best to use goggles. That's what my wife does since she swims almost every day.

 

[JesseWV]

I can't stop thinking about how a SWG could do more if it's output was modified.

If the relatively high FC water stream could be held in temporary storage for a few seconds before returning to the pool volume, this would result in an almost continuous near shock situation in that chamber.

It also bothers me that the electricity is being used to make chlorine and hydrogen gas, but the hydrogen is going to waste.

I wonder what it would take to do both at the same time:

1. Delay super chlorinated water's reentry into the main pool volume long enough to essentially have a micro-SLAM environment. It would be kind of like an ozonator/UV but without the drawbacks of no lasting sanitizer.
2. Collect hydrogen bubbles to supplement gas pool water heater fuel. (something makes me think collecting/storing hydrogen in large quantities would be bad, but continuous a feed should reduce the risk of explosion or invisible infernos)

I'm glad I've posted this in The Deep End...

 

[chem geek]

There is already a very high 50+ ppm FC level near the chlorine generation plate (i.e. in the area in between two plates) though as you point out it doesn't last that long as it mixes with the rest of the water flowing around the cell and as it moves out of the cell and down the pipe (the chlorine level is only somewhat elevated coming out of the return). However, note that higher FC levels tend to produce more disinfection by-products so that isn't necessarily a good thing to have (i.e. high chlorine levels for too long a period of time).

As for increased oxidation capability, you get that when chlorine breaks down from sunlight as that produces hydroxyl radicals. You can, of course, increase that rate by using less CYA so having a higher active chlorine level, but that means needing to generate more chlorine faster. Both the higher active chlorine level and the hydroxyl radicals will be harsher on swimsuits, skin and hair. What's usually done instead is to generate those radicals inline such as from an ozonator or from electrolysis using boron-doped diamond electrodes.