Backyard Aquaponics
A place to discuss aquaponics

Hi,

What are the advantages of using an "auto-siphon" design vs. slow drain design (is there another name for this?).

In case its unclear, by "slow drain" I mean the gbs fill up (and overflow down a standpipe) and then the pump switches off and the gb slowly drains.


cheers
rob

Slow drain gives the bacteria more time to work. The auto siphon might put more o2 into the water.

Ok, cool. More 02 because of faster flower into tank?
I just read first part of Siphons for Aquaponians, perhaps I should have read that first before posting this.

In truth the difference is more in what methods you are more comfortable with. The stand pipe slow drain design lets you control the amount of time beds are flooded and the amount of time they are drained better. Siphons generally always have water in the beds, the level just fluctuates from flooded to lower but the fast drain siphon action often introduces more aeration into the water as it splashes back into the fish tank and the constant running pump can often be used for extra aeration to the fish tank as well.
For Siphons the pump usually runs all the time but can be a smaller pump. For timer/stand pipe operation the pump has to be large enough to move all the water needed in a fraction of the hour and you need a timer that can handle the pump size and do enough short cycles per day to be useful. During the time the pump is off, you might want additional aeration to the fish tank depending on your type of fish.

Cool, thanks TC.
I like the idea of more aeration due to pump running all the time. I'm a little hazey on why people say you can use a smaller pump with fast drain siphon beds
If a gb needs to be filled in 15 minutes, it doesn't matter whether its a slow drain gb (starts siphoning slowly when at correct level) or a fast drain auto siphon gb. The pump still has to be able to pump the same volume in 15minutes.

For example:
In a fast drain auto-siphon gb 1m3 (excluding gb media), where the cycle is 30 mins filling, few minutes emptying, the pump needs to be able to pump 2m3 per hour(excluding head loss).

In a slow drain gb 1m3(slow drain siphon that only starts when the water is at correct level) , where the cycle time is 30minutes filling, 30 emptying, the pump needs to be able to pump 2m3 per hour (ex head loss etc).

Are my drain times incorrect? That is, would it be "wrong" to have a gb that takes equal time filling and emptying e.g. 30minutes to fill and 30minutes to empty?
I assume the life of the pump will be extended in the fast drain example because its not starting and stopping all the time, BUT in slow drain you save on electricity (but maybe still need to aerate so its not a huge saving?).

Thanks
Rob

Slow drain needs greater inflow, it must start at the bottom of the GB or you will not drain properly.

With a siphon no water is lost during fill therefore 3000L per hour pump will take 15min to fill a GB with 750L of water once the siphon kicks in the GB drains quickly (maybe 5-10 min) therefore the pump can run continuously . With a slow drain water loss increases as the pressure increases so you have significant loss as the water flows in, the higher the level in the GB the higher the outflow rate. To reach maximum fill of the GB will take significantly longer with the same pump (maybe 1/3) so now fill times are more like 20-25min, drain time will be slower maybe 30-40min. Therefore you need to switch the pump off in order to let the GB drain completely. In fact if the inflow rate is not sufficient you reach a balance of inflow to outflow and the GB will never fill completely. If you continue to pump the bed will remain at this level constantly.

Siphons sometimes run into a similar problem that the inflow is not fast enough and the siphon drains the inflow with out ever reaching maximum and initiating the siphon to drain the bed.

Hi, thanks Novaris. I'm not sure my definition of a slow drain is correct but if the slow drain is also a siphon (i.e. it only starts draining the water when it reaches the correct level) I can see no difference in filling times thus there would be no difference in amount of water required to fill the tank, and no difference in pump size.

Is my definition of slow drain system correct?

Thanks rob

Here's the design I'm thinking of implementing for my new system. I haven't tested it, so not sure it works I realise that I could put a loop in the outlet pipe, but want to insure that the overflow works correctly and the system drains slowly. The black pipe(u bend at the top of the stand pipe) will be about 8-10mm diameter.

Thanks
Rob

For most people slow drain is not a siphon but a stand pipe with small holes in the bottom and flood and drain is achieved by use of a timer. So for slow drain the pump is only on for a portion of the hour while in a system with auto siphons, the pump is normally always on. Of course there are variations and exceptions to almost all rules here since we tend to make them up as we go along.



Now if you were to use too fast an inflow rate with a slow draining siphon, the bed would fill then drain and then pretty much stay drained with only a small amount of water flowing through the bottom. Unless of course the pump were way too fast in which case the bed would never drain or perhaps even overflow completely. Siphons are dependant on flow rates being within their range of operation. Too slow an infill and the bed will fill then trickle over without ever actually starting the siphon and you get a constantly flooded bed that never drains.

Great Tc, thanks for the clarification on the different drains(and their names!). I think I'll build a prototype of the slow drain siphon shown in my picture to see if it works. I'll use a time/float switch with my design. If it works correctly, it should only start siphoning (through the small pipe) when the water reaches the high level. If too much water comes in, it can just overflow down the standpipe. Would 30min flood, 30 min drain be OK, or should aim at 15mins fill 15mins drain?
Thanks
Rob