bil,
That trick with the corrigated plastic sheets would be like an inclinded plate (aka inclined tube) sedimentation chamber. There is a commercially available product for use in inclined plate sedimentation which is made of fused sheets of corrigated plastic.
http://www.aquaticeco.com/index.cfm/.../5031/cid/1394
This stuff is also promoted for biofilter use, but the square feet of surface area per cubic foot of space is is pretty low.
I think the principal of the inclined plate is that as water moves through these inclined plates or tubes, a particle has to fall only a short distance straight down before it comes into contact with a solid surface. The vertical height within an tube inclined at a 45 degree angle is only the square root of two times the diameter squared. Hope this makes sense - a picture would be nice. Once the particle comes into contact with the solid surface inside the tube it sticks and, as crud accumulates, it may slide back down the inclined tube or plate. It is a way to increase the sedimentation rate within a vessel of given volume and flow. These things increase sedimentation well, but are not very good at self-cleaning and are often difficult to manuall clean. If the crud is not washed away on a regular basis, the spaces will clog.
I still like the idea of spinning debris to a center drain where it can be removed on a frequent basis with minimal effort - or even automatically via an actuator valve and timer. Not only does this reduce labor requirements, it gets crud out of the system before it can decompose further and release more nutrients to the water.
steve hopkins