I'm not familiar with "H.B.C. Ring" media, but at 30 mm diameter in an aeration basin, it sounds like a moving bed bioreactor, similar to a Kaldnes (http://www.kaldnes.com/) media reactor I worked on. I think the Kaldnes media we used were closer to 15mm, but same principle.
If so, then the...
I have a section of small diameter (3/8") tubing that will be isolated between two closed valves, capturing hot water between the valves. When the water drops to ambient temperature, density changes will create a vacuum of an estimated 1 to 1.5 psia between the valves (Reverse of thermal...