Backyard Aquaponics
A place to discuss aquaponics

have a sump tank (about) 500 Litres and a pump in that that will pump up about 1m into FT (IBC) this then uses the SLO to gravity feed into 3 grow beds that drain via siphon into the sump.

I understand a overflow pipe draining from FT into grow beds but how does it suck up the crap from bottom?

The SLO can only collect water from the bottom of the tank. So when it overflows, it takes the water from the bottom, any solids settling to the bottom are pulled up through the pipe, and into the growbeds.

However, you don't want a siphon in your fish tank, that could end badly, the top of the pipe needs to be open the air (t-pipe), so it can't create suction, otherwise you'll drain the tank.

Here's an picture of how it works in an old system I had. viewtopic.php?p=58997#p58997

OK thats what have, except my pipe is 90mm and when it goes out the side of IBC its leaking so now have to get a uniseal to fit....AAAHH!!

Another important concept that people can't stress enough: the SLO pipe diameter should be two or three times that of the inlet to the FT. Water is being pumped into the tank, and it needs to be able to drain freely through the SLO and outlet pipe. Otherwise, it's going to back up and overflow the fish tank.