beastieboy; according to the video gravity takes the algae-filled water from the holding tank above the level of the tube (the white tank) and the water flows through the system slowly, then it is either pumped or drained into a holding tank in the basement. from there it runs through some different-sized pipes to remove oxygen, the pumped back into the white holding tank. I could see an extra pump being used when the pig is put in to move it through for cleaning purposes a couple times a day for a few minutes... IMO