Flow Rates???

[Kaduda]

Just wondering if anybody has any figures upon the flow rate at a cross section within a system. I know that the deeper/wider the tanks the slower water will flow...Length wouldn't matter...

What is the flow rate needed in the raft portion of a system?

[arachdog]

There no simple answer to that question. But I think its fair to say it will usually be the fish portion of an aquaponics setup that will be the determining factor for flow rates. If you have in-tank oxygenation then it will usually be the ammonia build up that becomes a problem first. So you need to work out how much ammonia your stocking rate and feeding schedule produces. Measure the effectivness of your bio-filter\growbed in removing ammonia (a good bio-filter shouldl remove nearly 100% in a single pass), then do a mass balance calculation to workout the minimum flow rate required. But it's probably a lot easier to find someone with a similar system and find out what worked for them. As a rough guide, for a tank with a reasonablly low stocking density say 50kg/m^3 approximatley 2 tank volumes an hour should be adequate.

[mornings]

Kaduda,

In my new system with floating rafts, for the feeding of plants, I calculate the flow rate by doing a complete turnover of water in the troughs every 12 hours. Some of the hydroponics folks do it in as little as every 24 hours. However, if you are growing fish, the fish, I think, are the greater consideration. The UVI system turns over the water in their floating raft system in a little under 3 hours.(their troughs contain 17,235 gallons and their pump runs 100 gals/min). But I don't think this latter is necessary for plants. It has to do with the fish.

You can download the UVI pdf here and get a lot more detail on what they do and how they do it.

Hope this helps.

m

[GaryD]

Hi Kaduda,

Since the fish are the more sensitive organism in an aquaponics system, it's wise to drive flow rates (and all other factors that drive water quality) according to water tests.....particularly those for ammonia, nitrites and oxygen.

If your tests indicate the presence of ammonia or nitrite.....or low levels of oxygen.....you should increase the flow rate. Hopefully, you'll have sized your pump such that you have some ability to adjust flow upwards when required. If not, put another pump into the tank.

Arachdog......while you'd get away with it (and more) with Tilapia, 50kg of fish biomass per cubic metre is not what I'd describe as a "reasonably low stocking density" for aquaponics purposes......for Australian species. In fact, experience seems to suggest that 20 - 30kg per square metre might be more like the appropriate stocking density for your standard recirculating aquaponics system.

While I think that 50kg (and upwards for some species) is possible, it will require solids removal and plenty of aeration.

Gary

[arachdog]

Arachdog......while you'd get away with it (and more) with Tilapia, 50kg of fish biomass per cubic metre is not what I'd describe as a "reasonably low stocking density" for aquaponics purposes......for Australian species. In fact, experience seems to suggest that 20 - 30kg per square metre might be more like the appropriate stocking density for your standard recirculating aquaponics system.

While I think that 50kg (and upwards for some species) is possible, it will require solids removal and plenty of aeration.

Gary

Yeah fair point Gary, context is everything. In commercial intensive RAS, 50kg/m^3 is reasonably low. But your right in saying that good mechanical filtration and aeration would still be mandatory. I have worked at a Barramundi farm that operated at around 100kg/m^3 with good success, and I've read about systems using pure oxygen can achieve up to 150kg/m^3. But these kind of stocking rates do require very intensive management, so I take your point, in a backyard context, 50kg/m^3 is probably more towards the top end of recommended stocking densities.