Backyard Aquaponics
A place to discuss aquaponics

How big is your FT? I am assuming CHIFT PIST so your SLO is already pulling water from the bottom at whatever your pump is pumping from the sump.

Absolutely... and doing will also induce a swirl into the tank... which would "sweep" solids towards the centre of the tank where the pump can pick them up...

But if it's more the aeration you're trying to achieve... then pipe the sump return into the tank down a capped vertical pipe.. with lots of holes drilled in it... and add a venturi straw at the top of the downpipe....

You can always rotate the downpipe holes to also achieve a "swirl".... or cross flow...

So talking of air bubbles dissolving and pipes in fishtanks...
If i were to have a venturi at the top of the fish tank and it pumped out ALOT of bubbles, if i then addded a long pipe to send that water/air to the bottom of the tank and very few bubbles come out, but suction sounded similar in both cases, are the bubbles dissolving or does the extra pressure not allow the venturi to work as well?

I couldnt tell when i did it and since we are on the subject.....

Your venturi is not working, their is a pressure differential between the top and bottom of the tank.

Edit. Or if it is the air is moving back up the pipe, ? if it works maybe contraflow?

Not sure I get what you mean - are you talking a 100mm 'mushroom' effect with the wave maker driving the water up to the base of your foam circle so it spreads out? Or is the water running across the top of the foam once it exits the tube?

For wave makers and aeration it isn't a matter of how much water it moves - 100,000lph will not increase aeration unless there is an air source to mix with the water. By itself, all a wave maker can do is move the surface water down into the depths - unless it breaks the 'liquid film' Timmins talks about there is little increase in DO by this mixing.

Having said that, your idea (if I understand it correctly) would send water above the surface to splash back in - effectively a powered fall of water.

My reasoning about the paddle wheels is that they create large bubbles, meaning for a given volume of air, less surface area for the transfer to happen. Timmins is very clear that the 2 things influencing transfer are the surface area of the liquid/air boundary and the pressure differential of the gas-in-solution to the air.

The energy required to force a paddle wheel through water in a typical AP system has to be far more than driving air down a metre to bubble out an air stone, plus the air stone bubbles are smaller, so, again, for a given amount of air, the air pump will generate a larger surface area - Timmins says that means more air transfer.

@Vegipat - yes I think angling the ST flow downwards would help. It would also help move the particles towards the centre of the bottom of the FT, making your SLO more efficient.

So perhaps would redesigning you SLO - a good SLO will take enough water from the bottom of the FT to remove the crud and sludge and keep it clean.

This should solve the issue of the base level water not really circulating.

From what I have read, one of the issues people have with SLO's is they oversize their SLO. Another is too many holes - combine the 2 and you get water barely moving through the SLO. Paul from earthangroup has a 4000L system in which he uses a 40mm SLO and has no buildup of crud in the tank. Small holes spaced well apart will create a better flow than lots of them close together.

There's a balance between too little SLO and too much. If Paul can keep 4000L clean with 40mm, there is no need for a 75mm or even 62mm SLO in an IBC system. Start with just a few holes spaced along the SLO and see how you go.

FT size= 2,500lt CHIFT PIST with 2 X 40mm SLO'

Yea i wasnt sure, because i have stuck the venturi lower and it stopped due to pressure equalisation, but i thought when the venturi was still out of the water, it may have been different.

rupert got me thinking.

Just my opinion but I don't like venturi's for aeration. To get a significant amount of air drawn into flow because of the venturi requires a significant pressure differential before and after the venturi. This is created by the restriction in the venturi nozzle and seriously increases the total dynamic head and hence reduces your flow or increases your electricity use.

In some circumstances you can actually end up using more electricity and getting less transfer of oxygen into your water than if you had just left it alone with the inlet jetting into the FT.

I'd suggest people do some reading on Low head oxygenators (LHO). Not that I recommend them for backyard systems but rather once you understand how they work you will have a much better grasp of the principles involved.

http://bee.cornell.edu/cals/bee/outreac ... wnload.cfm

The best, sharpest and experienced minds have come up with the LHO as one of the best aeration options for RASs. Plus its all there for free to use as you will.

"Not sure I get what you mean - are you talking a 100mm 'mushroom' effect with the wave maker driving the water up to the base of your foam circle so it spreads out? Or is the water running across the top of the foam once it exits the tube?"

Basically the wave pump will pump lots at zero head, the purpose of the foam circle is twofold (perhaps three in terms of FT height variation), It allows, or makes the pvc sit vertically in the water and compensates for the high sg, its main purpose is to run water especially with baffles on a thin film return to the FT.

True enough Stuart.. with a true venturi design...

But you can get a significant increase in aeration from a water return by a "venturi effect"... without the overheads...

Like this... flow with venturi straws blocked...



Flow with venturi straws unblocked... you can see the air injected in the flow...

Mine is just like ruperts, simply 2 small tubes stuck into the last elbow that points into the tank.
I wouldnt want to restrict the flow ever, could always use another method for aeration with the extra flow instead.

Ok guys I just did what RupertofOZ suggested, I installed a capped vertical 25mm pipe which is all but touching the bottom of the tank with 4mm holes drilled and installed a 6mm venturi pipe at the top and hey presto loads and loads of small air bubbles rushing up to the surface.

If I added a second venturi would that increase the air being introduced or is there a formula for such things.

The beds are still flooding in the preset time of 17min so I have lost no pressure from sending most of the water to the bottom of the tank and now there seems to be bigger movement of water all over the FT "GREAT STUFF".

I only run my system every hour during daylight and 3 or 4 times during darkness 30min on 90 min off to conserve heat, should I still install an air pump for night time in order to keep the trout at optimum health, please also bear in mind that I will only have 75 fingerling rainbow trout in the tank (which hopefully I will be able to collect next week) because at present I have a 1 to 1 FT to GB ratio instead of the recomended 2 to 1 ratio so the fish have effectively twice the water that they need.
Anyway a great result from this discussion, a big thank you to one and all.

pat from Eire