I have been thinking about this one for a while, but the decision to add a sump for the aeroponic line I'm working on spurred me on to modify the old raft bed into a sand bed at the same time.  I took an old pool filter, cut the top aperture bigger and gutted out all the insides.  It was pressure washed several times and left with water in it until algae grew.  Then a second pressure wash and into place it went.

The fit is a bit tighter than what I would have liked, but space is at a premium and the sump at least adds about 120 liters of water volume.

The sump is connected to the bottom valve opening of the IBR tank (which was removed and replaced with 50 mm pipe).  Water feeds into the sump paccively, while the top pipe will be used to disguse the water lines leaving the sump.

The bin had its bottom drain modified with the addition of components recycled from the filter body.  As the sand is fine (1 - 5 mm) a bell or loop is out of the question.  I joined two of the filter's water pipes that used to be the inlet on the filter together, and added a 20 mm pipe sliced with a band saw in a few places inside the pool filter piping.  The idea is that the outer black perforated pipe acts as a guard pipe, and that the inner 20 mm pipe restricts flow from the bed to allow a gentle input to fill the bed.  Without the restriction, the water will never rise in the bed due to the draining ability of the black pipe.  I also have a ball valve in the 50 mm drain pipe under the bed to restrict things further if outflow is still too high.

The drain system is attached to the original centre drain standpipe via two short sections of clear 20 mm hose.  A standard 50 mm stand pipe will be added to the bed to allow the water height to be managed on the timed flood and drain cycle.  To be sure that the sand behaves, I will be covering the drain in pea gravel before adding the sand to the desired height, while the stand pipe will still be protected by a guard pipe as I do not intend to flood the bed to the top of the sand but to about 40 mm below the surface only.

Tomorrow comes the gravel if the leak check is favourable, followed by the sand ASAP.  Water will flow into the bed through a 40 mm figure-of-eight grid on the surface of the bed, and the initial flood cycle to be tested is 15 minutes on 1 hour off.