Aeration does what?

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Chasville

Chasville

0
Jun 4, 2009
55
Coastal Plain, NC
Ok, I've read a bunch of stuff about aeration affecting pH and TA.
I don't get it.
Isn't aeration simply either passing air through the water, or passing the water through the air?

I can understand how aerating in this way can affect the desolved gas levels in the water, CO2, Chlorine, Oxygen and Nitrogen. I don't understand how that affects the level of carbonates and borates (TA) and pH (H- ions in balance with +ions (usually OH+) (I can never keep the cation and anion distinctions straight)).

So, how does this work?

One reason I'm curious is because my ozonator is not supposed to affect the pH levels at all, and there are tons of bubbles coming out my jets, lots of air.
 
Swimming pools are normally maintained with excess CO2 dissolved in the water. chem geek can explain the exact chemical sequence that creates the CO2, but essentially it comes from raising the TA level. Dissolved CO2 outgases when there is aeration. Since CO2 forms carbonic acid when it is dissolved in the water, a reduction in the dissolved CO2 raises the PH.
 
The primary relevant equations are the following:

CO2(aq) ---> CO2(g)
Aqueous Carbon Dioxide in Water ---> Carbon Dioxide Gas in Air
This is carbon dioxide outgassing which occurs faster with more aeration (more air/water surface area for mixing).

HCO3- ---> CO2(aq) + OH-
Bicarbonate Ion ---> Aqueous Carbon Dioxide + Hydroxyl Ion
This step occurs because the first equation lowers the amount of dissolved carbon dioxide in the water so the second equation occurs to make up for that such that a new equilibrium is achieved. In practice, at a pH of 7.5 the amount of bicarbonate ion is over 15 times higher than the amount of dissolved (aqueous) carbon dioxide so pretty much nearly one bicarbonate ion is removed for every carbon dioxide that is outgassed.

The net of these two equations is that the carbonate in bicarbonate ion is removed from the water as carbon dioxide gas. You can see from the second equation how the pH rises because of the production of hydroxyl ion (a base). The Total Alkalinity (TA) is not changed because both bicarbonate ion and hydroxyl ion count towards TA so though the form (composition) of TA changes, the TA amount itself does not.

The above processes occur faster at lower pH because at lower pH the second equation is already shifted more to the right. For example, at a pH of 7.0, the amount of bicarbonate ion is only around 5 times higher than the amount of dissolved (aqueous) carbon dioxide so at the same TA level (which mostly measures bicarbonate ion), there is a lot more aqueous carbon dioxide so it outgasses faster. In other words, the amount of dissolved carbon dioxide at a pH of 7.0 is roughly 1/5th the TA while at a pH of 7.5 it's roughly 1/15th the TA (by TA, I mean adjusted TA, removing the amount attributed to CYA).

Richard
 
Thanks WB, that link was great :goodjob:

It even answered more of the question of why Borates are beneficial for pool chemistry.

So, here's a question, and get Chem Geek in on this.
Is there something else that can perform the same kind of buffering as carbonates, but doesn't involve carbon?

Here's the rationale.
Algae is a plant.
Plants need CO2 + Water + Minerals + Nitrogen + ... to grow.
The carbonate buffering system seems to contribute to the amount of desolved CO2 in the pool water.
So, it seems that carbonates can/may help contribute towards algae growth.
If a different buffering agent were used that didn't contain carbon, wouldn't that be beneficial?
 
Chasville said:
If a different buffering agent were used that didn't contain carbon, wouldn't that be beneficial?
Click to expand...
Not really. First, if you have a plaster pool you really don't have any choice. If you brought the carbonate level in the water down to zero, the water would leach carbonate out of the plaster. But even if you have a vinyl pool, it is utterly impractical to eliminate all carbon sources in the water. Pollen, and other things that contain carbon, are constantly falling into the water.
 
And there are many different ways to kill algae or prevent its growth. As noted above, removing carbonates or carbon dioxide from the water is not practical nor desired. You could also keep the pool perfectly dark since algae need light to grow, but that's not practical either. You could remove algal nutrients such as phosphates and nitrates with the former being the basis for phosphate remover products, but like most solutions there are downsides to it. You can add algaecide, including ones with side effects like copper (staining) or others that require regular maintenance without skipping a dose (PolyQuat 60). The 50 ppm Borates are a decent compromise with benefits of pH buffering and mild algae prevention with the only apparent downside being increased toxicity of the water for drinking in significant quantities.
 
Please don't get silly/sarcastic/whatever with me -- "... keep the pool dark ..."

I am not aware that plaster contains carbonates.
Also, since I daily have to skim out levels, seed pods, insects, pine needles, etc. out of my pool, I know that there are other sources of organic materials (stuff containing carbon) that get into the water. Heck, the frog in my pool is breathing the oxygenated water and putting CO2 and wastes into the water.

Why can't I/we explore the possibility of other materials to use in dealing with pool chemistry?
I may be new to pool management, and won't live long enough to get anywhere near to WB's current level of experience, but that doesn't mean I'm stupid. I enjoy innovative thinking and discussions, more so than most people. Or, am I just wasting your time?
 

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Chasville said:
Please don't get silly/sarcastic/whatever with me -- "... keep the pool dark ..."
Click to expand...
I have known chem geek for a very long time now and I have to say that he was not being silly or sarcastic. If you kept the pool dark you would prevent algae growth but it is about as practical as trying to eliminate the carbonate buffer system.
Also, phosphate buffer systems are nothing new but they have some problems with calcium hardness, which is why they are not more common. I would not worry that much about the rise in phosphate levels since maintaining proper FC levels will keep any algae from forming (or in your case, keeping copper levels above .6 ppm).
 
Chasville said:
I had an aquarium for a number of years. I have an idea of how light affects algae growth.
Also, it is blantantly obvious that keeping the pool dark would retard algae growth and be impractical.
That's why I took that as an offhand sarcastic remark.
Click to expand...
Which is why I said what I did because you do not know Richard and he is never sarcastic but merely covering all bases as any good scientist would.
Also, it is not obvious to the average pool owner that has not kept aquariums that keeping a pool dark would retard the growth of algae and we do need to keep in mind when we are posting that often we need to include those that are not quite as savvy since they will be reading these posts also. ::epds:: (This is one of my favorite smilies! That's the only reason I put it in. Don't read anything into it! :-D )

BTW, I'm the one who is usually accused of being sarcastic or nasty or rude or argumentative but you know what? I know my stuff and people follow my advice and find out it works so I really don't care! :rockon:
 
Very interesting, although for the OP I'd be concerned that there is something wrong with his ozone setup that is causing big bubbles of ozone to be coming out of the returns.
Click to expand...

Not big, just lots of them.

My plumbing is rather old, and submerged. There is no practical way to change the pool plumbing without incurring seriously big 5-figure costs and ruining our backyard.

So, the ozonater is connected to a special venturi "mixer". There is a valve in the main line from the variflow valve before the return lines. The valve is shunted by the venturi mixer. If I open the valve fully, then essentially all the water flows through the valve and into the pool return lines, taking the path of least resistance. I put a special ball flow indicator onto a port on the Dell unit, and start closing the valve some. When the ball is bouncing equally above and below the indicator line, the air/ozone draw is supposed to be correct. I'll post a picture later. The bypass valve is in a straight line with the water flow, the venturi mixer is in parallel, but rejoins the main line. So, all the generated ozone has an opportunity to mix with all the water in the plumbing before it re-enters the pool. Once in the pool, most of the gas that has not desolved into the water bubbles out within less than 3' of the inlet jets.

The normal design for adding ozone, in a new build, is to have a separate line for the ozone, that enters the pool off the deep end's mid-pool down slope. This makes the entry deeper and farther from the edge. More water can be affected/treated by the ozone this way, and more gas can get dissolved into more water. I suppose I could rig up a complete ring of aquarium bubble curtains around the edge of my pool and use a huge air pump to pull the air through the ozonator and drive the bubble wands. That would be something wouldn't it? That might eliminate any wall clinging grundgies.
 
I would be very cusious as to the chemical composition of the bubbles. Is it primarily O3 or has it been converted to O2? If they are small bubbles then the venturi injector is doing its job and you should have primarily O2. The other half of the question is what is the chemical composition of the gas coming out of the ozonator? Primarily O3 or primarily N with an O3 component? Is there any CO2 that could affect the PH?

I love science!
 
This is a 4 year old thread so I'm not sure if all the people are around to respond to your question. Bubbles from an ozonator are air plus added ozone, so it contains mostly nitrogen gas and oxygen gas with some ozone gas (some of the oxygen has been converted to ozone via corona discharge or UV depending on the system). The amount of CO2 will be that in air and will be less than that in pool water so there will be some degree of mixing and aeration as with any air bubbles going through the water. The ozone will mostly dissolve into the water as an aqueous (dissolved) gas. The pH will rise more than it would if you had no ozonator, but it's not because of the ozone but rather because of the air bubble that not only pulls some carbon dioxide from the water, but more importantly physically disturbs the surface of the water where the bubbles escape. Such disturbance increases the rate of carbon dioxide outgassing.

The same principles hold for saltwater chlorine generators and their hydrogen gas bubbles except they have no carbon dioxide in them initially so have a somewhat greater effect on aeration.

High-end commercial ozonators have oxygen concentrators feeding nearly 100% oxygen gas to the ozonator to produce higher levels of ozone (6%+) and in this case the rest of the gas is oxygen.
 
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