Backyard Aquaponics
A place to discuss aquaponics

Your very welcome. Hope it works out for you. Remember the water running through the grow beds, and coming back to the fish tank all collect oxygen. Especially if you are using a siphon. The siphon makes the water move very fast and makes lot of bubbles in the water.

Considering most AP systems are either PIFT or SLO and the "rule" is to move the FT water once per hour I wold hardly consider stratification to be a problem.
Air introduction into the FT by an air pump is a redundancy backup for if the pump fails. While it will add some extra O2 to the system it is the most inefficient way of doing this. This is only an opinion.

It is I think. Mostly we tend to think of things in a macro-world fashion, but sometimes better understanding can come from trying to understand what is happening at the smaller scale. I think most people think of bubbles as little balloons in the water, but that cannot be how it really is - there is no 'skin' around the bubble, so the molecules have to interact as the bubble moves. 'Have to' because the air pressure is the result of molecules in motion, so for 'pressure' to be holding out the water, the air molecules must be constantly colliding with the water surface.

It might even be that the air molecules that finally reach the surface are not the same ones that started the journey, with an constant exchange happening at the edges of the bubble.

The more I try to picture it, the more I wonder if the explanations about surfacing bubbles actually describes what happens there at all. If the bubble burst straight away, the situation is like I described above - the 'new' surface is pushing up as the bottom of the bubble reaches the surface. And if the bubble stays as a dome for a while, there is no special conditions inside the bubble that weren't there while the bubble moved through the water.

I think, as best I can work it out, bubbles moving through the water must contribute more O2 than those at the surface - more surface area to use for the transfer in a full bubble than a half dome.

Water falling from the siphon will trap air and force it under the surface. I think maybe the downward rush of water would increase the exchange of molecules in the bubbles that form - it isn't just the pressure differential but an oncoming push of water molecules happening, so I think the effect would be increased.

I am not sure if agitating the surface would be as effective as the water from a siphon given there would be no oncoming 'current' hitting the bubbles and the bubbles are all near the surface to begin with so there is less time as compared to an air pump source.

But also I think agitating the surface would tend to break up stratification anyway - water is pretty much incompressible so any impact at the surface is going to generate shockwaves that travel through the medium quite efficiently. Whether the DO would carry along with the shockwaves is another matter. The molecules at the surface are unlikely to be on the wavefront as, at a molecular level, the shockwave is just each molecule bumping into it's neighbours.

Interesting to think about... I haven't yet read anything on the subject that looks at things from a physics PoV.

Try Googling 'Gas Transfer M B Timmons".

The wave maker is designed to do exactly that,they were used in marine tanks with anenomes,fan worms and other living reef items,the wave maker was to mimic the action of a natural reef and send pulses of water to wash the water back wards and forwards as it would in the natural reef. The algae Calorpa needs this to flourish its called light flashing I believe,where both sides of the leaves get flashed by the light as it rocks backwards and forwards.
As has been posted the air pump first,it's a lot more effective than the wave maker.

Hi journeyman,
I hadn't thought of the " simple" bubble in that way,but your explanation does make sense,this has been a very thoughtful thread.
I l

Dasboot

A wave pump is a modified prop pump, it is designed to move large volumes of water (no real head) at very low power input. The wave action is caused by the design of the marine tank or the wave pump controller.
Aside from problems on mounting etc I would imagine it is capable of providing far more gas exchange than a simple airstone watt for watt.
This is only an opinion as I have only played with them a little.

I agree after reading the post by journey man.

The wave pumps I have seen look like they are designed to run in water. Wouldn't that limit the air uptake to only what might occur if the swirl produced cause breaking waves at the surface? Hard to describe I guess, but what I mean is, the flow would cause a bulge in the water surface on the other side of the tank - only if that bulge got to breaking like a surf wave would there be air captured in any reasonable amount.

Mounting the wave maker near the surface might make that easier to achieve, but there would still be a problem (I think) with getting the DO down into the depths - you'd be relying on a vortex effect dragging the surface water down, and I am not sure that would happen with the wave maker sitting near the top constantly pulling in surface water.

I haven't seen wave pumps appropriate for moving air, and looking at most of them I think if you sat them at the water/air interface to try to make it happen, the fan would fail in short order due to being unbalanced. (one side in air, one in water)

I originally looked at wave makers for use with salmonids - all the ones I was interested in growing prefer running water and currents so I figured a nice low power wave maker pushing water around would be just the thing for good healthy fish.

You might be able to rig a venturi into the water flow to generate aeration.

My air pumps run at 300lph with 10 hour battery backup and draw 10W each. Most of the wave pumps I saw were around that figure but I have difficulty in seeing how a wave maker would produce the equivalent streams of bubbles I have rising from my FT floor. The air pumps also produce significant top-to-bottom circulation, which is obvious to see when I throw feed in.

I know it is top-to-bottom because when I move the air stones, the pattern of debris on the bottom of the tank changes - it tends to aggregate around the air stones. i.e. there's convection currents happening as water is drawn into the lower pressure areas where the columns of bubbles are.

Is Timmons the Recirculating Aquaculture guy?

Ah... it is him.

Something of interest... (maybe a Mod might move the general discussion to it's own thread? We seem to be drifting into deeper waters than the OP might have wished)

This is from Gas Transfer and Dissolved Gas Conditions

Why it is interesting is it implies an airstone will be more efficient at gas transfer than a paddle wheel - the bubbles are much smaller so there is a significantly larger surface area involved for a given amount of air. Also, if the surface of the water is where most of the transfer is meant to occur, the air from an airstone will be more efficient at depths due to a lesser amount of DO at a distance from the surface.

He also says further down in the PDF that the gas transfer happens both ways, depending on saturation of gas in the water - if there is high DO levels, the O2 can actually come out of the water into the bubbles. (ditto obviously with CO2 and N2)

Another way to increase DO is to increase the pressure - I'm wondering if that might apply to the drain from a siphon - the fall might be enough to significantly increase the pressure (the 'downward rush' I mention above) around the bubbles and so increase the transfer.

More to read - he talks about a 'liquid film' at the interface - he actually mentions that 'thinning' it will increase the transfer - and I don't see what that might be - at first blush he seems to be implying the water molecules somehow 'mesh' together into some kind of surface. I do know there are some fascinating structures that occur in water, including remnants of the ice form, to quite surprisingly high temps - maybe he is talking about some kind of flat lattice that forms at interfaces?

If so, I wonder about the bursting bubbles reaching the surface - the only place I can see the 'liquid film' being disrupted is around the edge of the circular depression that exists as the bubble bursts. The film across the indentation itself would still be whole.

Don't know which wave pumps you have been looking at. My cheap Chinese one moves 5000lph at 12 watts.

If I was going to do it I would use say a 300mm x 100mm wide pvc pipe with a hard foam circle with a hole cut in it glued on the top (perhaps a few baffles on it). Mount the wave pump about 150mm in the pipe pointing upwards. If you get the sg right the top of the foam should be the same as the top of the water.

Yes he is concerned with RAS and has written some very good papers for Cornell University.

Not sure where you draw the implication that "an airstone will be more efficient at gas transfer than a paddle wheel"...

Timmons specifically notes throughout the PDF... that airstones are very inefficient oxygen transfer devices...



If you look up works by Akmed/Boyd..... you'll find paddle wheels are rated the most efficient oxygen transfer devise... by a country mile...

Likewise airstones aren't really efficient at depths beyond 1 mtr... although pressurised diffusers are good for greater depths.... (also referenced in the PDF)

Paddles wheels are rated as the most efficient aerators in ponds.

The thing is the primary action of aeration in ponds is to circulate oxygen depleted water near the bottom of the pond to the top where it can absorb oxygen from the surface. Using air stones is a really poor way to do this relative to a paddle wheel.

In an environment where the water is already mixed, ie RAS or AP then the the aeration is increased by the increase in the total area of the water/air boundary. Whatever device is used is all about increasing this area with the least amount of energy. Generally this is done in a passive manner but some systems like trickle towers are fan forced to ensure a good flow of non depleted (in the case of oxygen) / saturated (in the case of CO2 and N2) air passing across and around the water/air boundary.

I don't know of any papers where the authors have tried to analyse the efficiency of aeration devices in vessels that do not have oxygen concentration gradients (ie are well mixed).

Although there are papers out there that have measured the ability of air bubbles to aerate water independent of there circulation and surface effect.

There are also micro pore air diffusers that can be used for pure oxygen that claim 100% transfer rates (ie no bubbles reach the surface) when used in water deeper than 2m.

Stuart

While I would go for a low/no extra power alternative in normal FT aeration, if I thought I was running into HSM I would use a f'ing big pump and smash the water before returning it to the FT.
The cost of running the pump for a short amount of time for HSM or simply preventative is not costly considering the amount of time and $ you have already spent on your investment. Only an opinion.

Thank you all for this lively discussion, I have some thoughts on what you all have being contributing to this topic.
I have two returns going into my ST and the water falls about 1.2m at the start of the return cycle of flood and drain so that obviousely traps air, my transfer pump from the ST the FT is moving about 4,000lt water/hr so that must help also, but the one thing that I did not realise was that the water at the bottom of the FT is hardly moving at all except for the SLO's so it looks like to me that the only way to move this bottom layer is to install an air pump and at least have all the water at the same level of DO.
Would there be any point in me angling the supply pipe from the ST so that the water is driven down somewhat instead of being horizontal to the FT water.
Ye are one bunch of knowledgable "DUDES"