Solids gravitate towards the centre, and sink to the bottom.
The first dump back to the grow beds is a hole in the stand pipe down the bottom, that creates a constant, gentle, downwards flow to further encourage solids to drop out of suspension.
The second dump device, is a bell siphon that periodically dumps water, creating a powerful suction, cleaning the filter of collected solids and returning them to the grow bed. This siphon is shortened by a breather pipe that means it only changes the height of water from between the "high tide" and the "low tide" marks rather than dumping all the water. If all the water is dumped, you loose the gentle entry of the water, and the swirl is broken down each time the device is empty.
A third dump intermittently exists at the top of the filter to act as a leaf skimmer. This would only flow at the top of each fill cycle, and would just be a trickle for a few seconds as the container is at its fullest, and would dump any floating particulates back to the grow bed.
Finally, water exits from the filter from a point just under the low tide mark. This would be the water that flows through the NFT tubes as it would be almost free of heavy solids, and would also not collect any floating solids.
Floating solids are sometimes sucked into the pump when fish disrupt them from the surface, when they get caught into a whirl pool and sucked down into the pump, or when the grow bed empties onto them, pushing them under water. Duckweed is at a very fine balance between floating and sinking when it's getting old, and can stay under water for a minute or so before slowly rising back to the surface. A fish swimming by can easily pull some under water where it might find it's way to the filter. It doesn't happen often, but I find 3 or four leaves a day go through the pump if I put a sieve under the grow bed outlet to check what the pump is lifting up.
There you have it, a self cleaning swirl filter.
Perhaps.
I'll make one to check.
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