How Much Air in the Pond?

[carlejo]

Hi Guys

I’m in the middle of a spin off thread to this which I started at work today but briefly you need some help here.
I mentioned or implied you can have to much air and that’s very true but only in so much as the unneeded air is redundant to saturation but that’s assuming the extra air is not doing or fulfilling an entirely different purpose to that of saturation EG making water move where you need it such as bottom drain sweeping , or agitating K1 in a fluidised system

The trouble with this is, this in itself is false because it assumes aeration cant do or perform two or more functions at once, in other words just because your air input is agitating your K1 does not mean its not saturating you water in fact air can do more than two jobs if you so desire in short what ever you have air doing dynamically for you within the water; it will still be saturating at the same time, be that fluidising pond sweeping flushing and a flow through system it will still be saturating water at the same time

However how well it does the latter (saturating) depends on how you’re applying the primary reason for aeration in that situation then the following applies

Its true oxygen water interface takes place commonly at the surface of the pond water but that does not mean it has to be bubbles bursting in fact the bubble bursting ends the process, there is no more to be extracted other than normal surface area air exchange. where as, as a bubble rises ( because a bubble generally does little else) the surface area of the bubble while its rising is where O2 is exchanging into the water is taking place. the longer that bubble is rising the longer its in contact with the water so the better the gas exchange
In other words the whole of the bubble surface area is the where O2 comes in contact with water and thus where saturation (gas exchange) take place several factors influence how well this takes place one is bubble size and the other is duration of bubble contact with water

So the smaller the bubble the better the exchange so the easier the saturation, next the longer the contact time of the bubble to water the better the exchange so the deeper the air is introduce the longer the bubble will be in contact so the better the transfer the shallower the air is input the less the interaction will be so this is the worse scenario for gas exchange

Look at marine guys they use wooden air stones this gives a massive amount of bubbles and with that comes a massive amount of surface area

Just something to think about those fine long air stones are great if that’s what you need in your system no other air stone will do it better

That's the airstone I have as additional air on top of the membrane of BD, it's fantastic TBH......masses and masses of fine bubbles

[madmole]

Us marine folks use wood airstone for fine bubbles in protein skimmers because they have more surface area for protein to stick to. BUT small bubbles are NOT better for gas exchange for two reasons, 1) Surface tension is much higher in small bubbles so its effectively harder for gas to pass through the skin and 2) far smaller volume of O2 in the bubble (its exhausted far faster)

In fact folks played with using pure O2 or ozone in protein skimers and found it didnt do much in the bubbles, the higher concentration of O2 or O3 immediatly above the water in the skimmer did the gas exchange better, Thats why modern skimmers have K1 like media in them to provide more void space for exchange rather than bubbkes

Water falls are a great source of saturation, as the water flows as a blade over the edge the surface tension is lowered which increases the gas exchange

Air in the pond works mainly by lifting the unsaturated deep water, allowing it to spread out across the surface (where the gas exchange takes place) before it falls along the edge of the pond. ie the better the cyclic movement the air sets up the more gas exchange, provided the waer isnt saturaed. I've seen resarch on this that if I remember correctly said that the bubbles did about 15% of the gas exchange, the rest being at the surface

Of course air has several more uses, in agitating media and the cyclic water movement can be used to move sediment to a bottom drain, and the fish seem to like it

[MikeWh]

Hi....My fish simply don't like to be in a bubble bath, Ive tried it & it causes them more stress than its worth. I have 380ltrs of air pumps on all the time, but only a trickle to the two the air discs for a bit of circulation....They get it on full tilt for about 20 mins a day to keep my bottom clean Then most of it goes back into k1 &/or protien skimmer if I'm using it.....Makes feeding time much easier for them & much more enjoyable for me.....Each to thier own though ...........Mike

[Duncan]

im looking up some reference stuff but again briefly the uk marine guys use wooden air stones too, they use it in the tank putting up a fine mist for aeration they do also use it in protien skimmers

but i can suusre you the size of the buuble and its surface tentsion has noting to do with the ability to collect protien

the way a protein skimmer works is simple the protein molecule is by polar and one end is hydrophobic the other hydrophilic so one end likes to be in water the other end hates to be in water so when a bubble comes along the hydrophobic end puts it head through the bubble surface into the air inside this satisfies that end of the molecule and the hydrophlic end stays the other side of the bubble in the water and that is happy it has absolutly nothing to do with the surface tension ther science behind this means what ever the tension this molecule will do this it just needs the bubble
the protien does not stick to the bubble its what holds the bubble together once it hits the surface if the protien was not there the bubble would burst the instant it hits the surface ut because the bubble now has thousdands of protein molecules holding it together they can be harvetsed off

[madmole]

Yep and thats why they like small bubbles, More surface area per unit volume

But us marine keepers also use our protein skimmers as an ozone introduction and off gassing point, hence newer skimmers use media and tumble the water down through it in an updraft of ozone for part of the skimmers length (kindof like a trickle filter). You still get protein froth for extraction, but also much better absorbtion of the O3

We tend not to use the airstones in the tank itself as the fine bubbles irritate the corals. Instead we use small powerheads on timers to simulate wave movement and stop dead spots

[Duncan]

In an attempt to draw a line under this because we never really started out discussing protein skimmers ro marine systems which are dynamically very different to a koi pond for instance yo8 can achieve the fine level of bubbles in fresh water like you can in salt water its just not possible we come close but no Cigar

So just to answer Geoff’s question about to much air.

If you look back I did say Some aeration does occur at the surface other wise many of our lakes would simply not support life but the idea that bubbles do not oxygenate other than to drag deeper unsaturated water to the surface is misleading sorry but it simply is.

You have to realise that as oxygenating methods go, our pumped atmospheric air via air stones is not really cutting edge in aquaculture terms, but it’s the best we have. Cutting edge would be direct oxygen injection and bubble reactors these would be much more efficient at gas transfer into the water because its injected at up to 22 atmospheres to create very fine bubbles . the bubble reactor is a container where bubbles are introduced at the bottom as they rise water is forced down from the top so they are constantly driven back down like I say these are modern aquaculture techniques costly and involved. ( yes I know salt water protein skimmer employ a form of bubble reactor ).

But the reality is bubbles also oxygenate water and of course will provide currents. If you have a bottom drain these currents are very important , because what they achieve is a sweeping action across the pond floor . As the bubbles rise they pull water behind them what comes up must go down so outside the rise you have a counter current going back down , these create a sweeping action across the pond floor as the down draft water gets pulled back towards to the centre to begin to rise again, this pulls debris to towards the drain where the pull of the filter takes it , but even without this current action gas exchange would be taking place at the surface, there is none of this current going on in an land locked lake yet they survive.

Timmons Recirculating Aquaculture “Gas Transfer” says on Diffusion air stones

We assume an absorption efficiency of 5 – 10% for typical air stones , Although some of the more recent fine bubblers bubbles of 100 – 500 micron) do perform well in deep tanks ( 50% oxygen transfer efficiency ) (these would be diffusers type ) but require a high pressure source.

where as Mueller : Aeration Principle and Practise puts this figure higher and talks of typical SOTE off air stones a being as high as 25% SOTE is the fraction (%) of the oxygen supplied which is actually transferred to the water (at standard temperature and pressure). So a SOTE of 25% means that 1/4 of the supplied oxygen was transferred to the water.

Don’t frown to heavily on the 5-10% figures quoted, this means of one bubble it worse case scenario it imparted just 5%-10% of its oxygen into the water not to shabby for a humble bubble considering we are making thousands per second with modern diffusers then again this figure would be higher with modern micro pore diffusers

If you read the page I am posting from Mueller you will see he quotes Barnhart observations where "25% of oxygen transferred was at bubble formation with fine pore diffusers and much less with larger bubbles"
Now the SO WHAT IS: Bubbles do in fact directly aerate water and they do it at the formation of the bubble which unless I’m mistaken as bubbles rise would be at the bottom, that’s not to say aeration is not also taking place at the surface but that’s not primarily the reason for putting in air stones in a pond and never will be.

Like I said I don’t believe in the oxygen stratification of a koi pond simple because there is to much environmental movement from pump to fish for this to happen and the laws of diffusion says pressures differentails will always try and equalise where there is no barrier to prevent this, the same would be of heat stratification in a very deep lake or ocean maybe, but In 6 ft deep agitated pond under the laws of diffusion well I leave you to make your own mind up I know what I think i'll try and reference some more scientific data but right now i have to do my sunday morning site back up

cheers guys

dunc

[Duncan]

some more from timmons this may explain diffusion and pressures

these two pages are 398 and 399 and there are around 50 pages total of the whole book just dedicated to aerating water the book itself being 070 pages , the whole book only covers recirculating fish systems nothing else its hevay reading