The only point to the bioballs is to provide surface area upon which bacteria may live. You could substitute LR rubble or almost anything else that you think will provide more surface area than bio-balls, which really do not provide that much surface area compared to LR or TUFA rock, etc. One option, the one I selected, is to spend $50 and purchase a 12" x 12" x 4" block of man-made, extremely porous, rock-like "stuff" (meaning I have no idea what it's made of), keep it above the water-line and run the draining water over it.
Just don't use lava rocks.
EDIT: just remember that the only function those bacteria serve is to convert ammonia to nitrite to nitrate.
Nitrate is the end result of the process. That's what you really need to worry about.
RE-EDIT: since the advent of bio-balls, we've discovered that a sufficient amount of LR in your DT obviates the need for any wet/dry media on which bacteria can live, b/c there's enough bacteria on the LR in the DT to do the job. IMHO, your sump should be dedicated to cheato (to feed off nitrate and thereby remove it) and a refugium to grow copepods or whatever else you might fancy.
RE-RE-EDIT: the only reason I got that 12" x 12" x 4" block suspended in the air was bad advice from my LFS. He told me that anaerobic bacteria would grow deep inside it, but only if I kept it in the air. He was wrong, I have since learned. Anearobic bacteria (oxygen-less) convert nitrate to gaseous nitrogen.
