Aquaponics - Aeration

I'm planning on buying some trout in the next few days to grow in my aquaponics system, so I'm thinking a bit about my oxygen levels.

Toward the end of the trout growing period you have a lot of stuff acting against your trout. Your trout are big and make a lot of waste which depletes oxygen as it breaks down, your trout are big and need lots of oxygen, and your weather is getting warmer,  and warmer water doesn't hold as much gas as colder weather does.

You can test this by opening a warm can of beer, or waiting a few years and watching what happens when the oceans stop absorbing carbon dioxide as they warm up a bit.

Anyway...

I have a tube dumping water from my growbed to my sump.

This much water.

It's not under pressure, but is simply overflowing from the growbed at that height. ie this tube's height regulates the depth of the growbed.

no pressure

The tube stopped a few inches above the water and made some nice splashing and bubbling which I thought was adding some oxygen.

I'm probably quite correct in thinking that.

But then I added a tube all the way to the bottom of the tank, and pricked a few holes in the tube just after a narrowing where there was an elbow.

This sucks air into the tube via the Venturi effect.








Apparently, this is because the water experiences higher pressure at the restriction just below the elbow, but then lower pressure when the tube becomes wider again. The result of the low pressure is that it sucks in air.

Which is a little odd.

When you put holes in (or before) the bit that's actually restricted, the holes blow out water.

I know because I checked.

So the result is that oddly enough, air is drawn in and the flow of the water draws it down.

Even more oddly, is that the bubbles don't ever really come out. Some do when the water level is a little low, but these are very small, and not many.

The tube is full of bubbles all in chaos, churning around madly.

It looks like this in real life.

The bubbles enter at the top just where the clear tube starts, and they move all the way to the bottom.

Which is also odd given how gently the water is flowing.





At first I thought they were the same bubbles, but if you block the holes, they clear after a few minutes.

Interesting.

As far as I can tell the air is dissolving.

I have no idea if I'm probably quite correct in thinking this.

If gas really does dissolve in water that readily and in that quantity, it's no wonder fish can live in the stuff.

I think I just got re-interested in the world.

Perhaps it's because I stopped taking all those opiates for my hurty ribs.



120 Things in twenty years  thinks ribs make me less thinky.


[EDIT FROM THE FUTURE - I Thought I'd add a link to wikipedia's entry on Venturi and now that I've read it I don't think I have any idea what I'm talking about, but what I did clearly works. I'm just no longer sure why]

The shell grit seed raising seems to have worked.

There was some slight discolouration on some leaves that I think might be due to nutrient lockout because the pH is probably off the chart in that local area, but generally speaking, I think it works. The discolouration is not shown well on this pic. The true colour is closer to the lettuce leaf on the left.



When pH is at certain levels, various elements become less available. If your system sits at about pH 7.0 then everything is available.

I just lifted my first seedling out of the shell grit, and it had a 24cm root that came out in tact. I made no attempt to be gentle and just lifted it out. I tried another one and achieved the same result.

Anyway, no washing - not even a rinse, and three tiny bits of shell grit was all there was stuck to the roots.

I think I should have left it a little longer because there were hardly any side roots developed, but I planted it next to some existing established kale, so I'll have something to compare it with. The existing kale was planted around ten days earlier.

I add some seasol® from time to time, so I might add it directly into the shell grit once I see sprouts to make sure there are extra trace elements available next time. The slight discolouration might also be due to lower oxygen levels as the water probably moves quite slowly through the fine particles. I might sive the shell grit so I can get a slightly bigger particle size. I plan on reusing it for ever, so it's no big deal to sieve it.



120 Things in 20 years thinks raising seedlings in shell grit feels like a success