Change of Oxygen solubility in water

[Jkaen]

Hi guys wonder if you can help me,

I have a client who wants 20mg/l oxygen dissolved into a fish lake. now in the pipe based on partial pressures I can easily acheive that with pure oxygen, however my worry is that when this rich stream is introduced into the lake (its not a great volume maybe pool would be a better word) I may loose some of that oxygen as the solubility of oxygen from air into water in nearer 10mg/l

Does anybody have any experience or knowledge on what wil happen to this oxygen rich stream once it reaches open atmospheric conditions?

Thanks in advance

Stephen

 

[btrueblood]

From my trusty old CRC handboook (40th ed.), the solubility of air in water is 29.18 cc per liter at 0 deg. C, with 34.91% of the air as O2. Doing the conversions yields 15 mg per liter dissolved O2.

At 10 C, the solubility drops to 11.2 mg/L.

This is what a pond of pure water exposed to air would reach "at equilibrium". So, you're pretty close to say it's around 10 mg per liter, depending on the temperature of the pond.

However, if you are continuously injecting the pond with oxygenated water, you can push the 02 content higher. It will slowly dissipate due to diffusion to the atmosphere, and probably due to microbial action too.

If you exceed the solubility of O2 that the water can support, though, (say by injecting the O2 into the water at high pressure, then dropping the pressure to atmospheric) you'll see tiny bubbles, just like uncorking a champagne bottle. The solubility of pure O2 in water is 69 mg/L at 0C, 55 mg/L at 10 C, and 44 mg/L at 20 C. These are at 760 mm pressure (1 atmosphere); so taking a cup of this water from your tank and settin it out into the atmosphere, you wouldn't see any bubbles, just a slow drop in O2 concentration as the O2 diffused out and N2 from the air diffused in.

Hope that helps.

Ben T.

 

[Jkaen]

Yes, it confirms what I thought would happen, the main question is the rate of the 'slow drop in concentration' i had no real handle on if it would be instantaneous or take days to drop to equilibrium

 

[LHA]

I'm not a chem eng, but saturation conc of O2 in water is 13 mg/L at freezing and down around 8 or 9 mg/L at STP. So, no matter how much 20 mg/L you pump in, if the water surface of the pond is exposed to atmosphere, won't it just keep deoxygenating. I don't see how you get a pond to maintain a conc. above saturation level.

 

[btrueblood]

Jkean, Iha:

Think about a soda pop can. Once you open the can, the soda is a super-saturated solution of water + dissolved CO2, yet it takes up to a day or so for the dissolved C02 to reach a final PRESSURE equilibrium with the atmoshpere (time for bubbles to stop forming). But, there will still be a 1 atm concentration of CO2 in the soda pop: it will take longer yet for the CO2 to diffuse out, and air (N2 + O2) to diffuse in, until the concentrations reach equilibrium (equilibria?). In one case, (pressure equilibria) we have bubbles forming and rising to the surface, thus have a fairly high mass transfer rate. In the other instance, (chemical concentration equilibrium b/n soda and air at the same pressures) the molecular diffusion across the surface of the pond determines the mass transfer. It's got to be slower than the bubble-transport mechanism, and it only starts to happen after the pressure equilibrium point is reached or approached.

BT

 

[Jkaen]

Ok so from what I understand we are looking at atleast a day for it to come to equilibrium, so as long as I keep introducing the oxygen rich stream into the pool at a certain (unknown) rate (which it seems shouldn't be too large given the pool/lake is less than 10m3) I should be able to maintain the DO level we are after?

 

[btrueblood]

Jkaen,

Yes, I think so. I'd still wonder a bit, and would ask the owner, why he wants such a high and apparently un-natural, O2 concentration?

BT