Volumes of sump tanks and circulation rates

[Louise]

I'm busy building my first "real" ap-system. I can't get the theory and practice to work together. According to Sylvias book, I should size the pump/flow rates to circulate my entire volume of water every hour. And if I use a timer, as she suggests, which is on for 15 minutes per hour, I should be able to circulate my whole volume in 15 minutes. Fine. So, I add my volumes up, and come to the conclusion that I have approximately 1000 litres.

The sump tank is by the same book recommended to be large enough to hold all the water from the grow bed at once, plus some, to ensure the pump doesn't run dry. My grow bed's volume is 300 litres, so, to oversize it a lot, not counting that the media takes up some space, let's say we choose a sump tank volume of 350 litres.

This means that in 15 minutes, the pump needs to empty the sump tank almost three times. This assumes that the water returns fast enough through the grow bed that the pump doesn't run dry before it's 15 minutes are up.

But in a flood and drain system where a standpipe with little holes at the bottom is what empties the grow bed, the emptying has to go slower than the filling, at least up to a certain point, otherwise it'll never fill. And with the media slowing down the flow of water through the grow bed, I'm uncertain that the return water will be able to give enough water back fast enough to the sump tank to keep the pump going with it's aim to circulate 1000 litres per 15 minutes.

Here's a diagram of my system, but in the picture I added an "extra sump" that I'm now considering, since I noticed that the 200 litre barrel I've got is not enough. Also the Deep Water bed on top isn't added yet in reality, but will contain water and duckweed.So, my questions are:

1 How wide do the different pipes and overflows need to be in relation to each other?

2 Is the recommendation to circulate your entire volume every hour a utopia to aim for, but not to fret over if you don't achieve?

[Vlad Jovanovic]

Hey Louise...I've not read Sylvia's book so I'm not sure if you meant to write "turn the entire volume of your FISH TANK over once an hour"...or the entire system volume..? At any rate, neither one are an absolute necessity in every situation depending on how you proportioned and designed your system. Turning your FT over once per hour is kinda a good general rule of thumb for many people in particular situations, but again...hardly necessary. The two main reasons you'd want to do that are oxygenation, and waste removal. I've often seen that type of info promoted in intensive RAS (re-circulating aquaculture) literature/studies...where it would totally be good practice. In a relatively low density system with plenty of places where water free falls and has a chance to splash around and oxygenate, I wouldn't really worry about it a whole lot IMO.

You may (or may not) want to reconsider growing duckweed in a tank that is directly connected to your AP system...It might be wise (judging from some of the horror stories) to grow the duckweed separately in another tank. But that is not anything that I have direct experience with, so check with someone who does...

How was your trip to NY? Did you get a chance to check anything out AP-wise?

[Louise]

Thanks! I started thinking this might be the case. Looking again, the book says "choose a pump that can, at a minimum, cycle the entire volume of your tank in an hour". Sounds more like it means fish tank, than system volume.

Well, I need to go pump-shopping again, the one I first got is way too strong.

New York, yes, I had fun! I met with Jonathan Kadish, and saw his system, and learned some things from his plans for his upcoming system, that I can use for my next project (after this one).

I also went to SAVE farms at The Point and met Chris Toole and Anya Pozdeeva and saw some of their projects, not so much ap, but nice anyway. Teryl Chapel was another acquaintance I made who I can learn something from.

[Vlad Jovanovic]

Well good that you got to see/learn some things and have some fun :)

Make sure you look at pump curves that the manufacturers provide, and not just the rated litres per hour on the front of the box. If the pump is 'advertised' at 1000 litres per hour, it can usually pump that amount only horizontally...so measure the vertical distance from where your pump sits in your sump tank to the point it needs to pump up to. The same 1000 litre pump, may be only able to pump half that volume of water if it needs to move it 1.5 meters vertically...So check out the pump curves (every halfway decent manufacturer provides a pump curve graph somewhere...often on the back or side of the box)...and definitely automatically figure that the pump can move about 10-15% less than what it says on the pump curve chart...

If you place a PVC "T" adapter and a ball valve above your pump you can use it to regulate the flow (if the pump is stronger than needs be)...that way you can run the pump continuously (which will prolong the life of the pump) as well as have the excess water fall back into the sump tank and help to add more oxygen to the water....