I had a similar idea a while ago for efficient use of space in a round greenhouse. The main problem was the limbo dancing requirement to gain access to the plants.
What about an old trampoline frame.. 12-15ft in diameter with a
round tank/sump in the middle set a tad lower to get some gravity drainage going?
Use the frame to support the high end of the plant tubes and the single ring feedpipe or gutter distribution system.
For extra flexibility you could devise a way to move the plant tubes around the frame (and tank) to gain access as the plants get larger or perhaps even add/remove individual plant tubes as required.
Around here plastic sewer pipe is pretty expensive stuff considering and not that big a diameter either (110mm od).
With the circular layout it should be easy to make some (cheap) deep-as-you-like"gutters" using dark coloured tarp or poly supported on a couple of timber rails.. just hook them onto the trampoline frame.
Fix some black breathable weedbarrier (geotex etc) over the top of the "gutter" to reduce evaporation and block light to the rootzone.
The trampoline frame would make a pretty sturdy base for some form of canopy over the top too.
The possibilities are endless
Backyard Aquaponics
