how large can a media bed be? different drain tech?

[Trevor Johnson]

i would like to make a 4'x20' media bed with a bell siphon. i was reading around and some forums said that the bell siphon might stop before the whole bed is drained because it will suck the area around it dry and stop.  i dont think this is true due to the nature of gravity on water. maybe it would happen if the pore space between the media was too small.

Can the bed be longer? 40' even?

Is there other drain technology that can be utilized for sufficient draining?

what do you all think? Please incude pictures and links to pertinent information.

[TCLynx]

for really big beds, I think timed flood and drain would probably be a far more effective way to flood and drain.I used to have a "monster grow bed" that was about 28 feet long.  It actually had a pump on a float switch in the end of it and constant inflow to the bed at the far end.  The end where the water flowed in, was pretty much always flooded since the roots and the bio-slime and fish poo along with the gravel would slow the water flow through the bed.

Now the bed still worked, but it was kinda like two beds in one.  The inflow end was kinda like a constant flood bed while the pump end was more like the siphon flood and drain.  I just had to make sure any plants down at the inflow end where happy with constant flood.

If you really must do a giant bed, I would recommend timed flood and drain of some sort.

[Chris Smith]

I agree with TC that a timed flood and drain would be the best approach. Keep in mind that the water to fill a large media bed has to come from somewhere like a sump. The larger the media bed the larger sump needed to supply and absorb the floods and drains. A 20' bed could take 100 or more gallons to flood depending on media. I believe it is better to split the one larger bed into 4 smaller ones that get 15 minutes of flow each.Several smaller beds will not require as large a sump as one single bed since water from one bed will be draining as the next is filling and two will be dry.

I recently made the switch from auto siphons to timed F/D. My 4x6 media beds are working far better with a timed flow of 15 on and 45 off. I believe the longer dry cycle gives the worms more time to consume solids and organic bits. I no longer get the poo pavement or any buildup what so ever. The timed flow is working so well I plan to get rid of my solids settling all together and let the worms eat all the poo in my large system.