PURPOSE: To obtain an apparatus, whereby problems concerning a separation equipment for anaerobic sludge using an anaerobic gas are settled, by separating anaerobic sludge in a supernatant from methane fermentor through flotation with an aerobic pressurized flotation equipment and then changing the condition of separated sludge to anaerobic at once in a tank for making sludge anaerobic.

CONSTITUTION: A supernatant from methane fermentor flows into a mixing tank 16 through a discharge pipe 15 and a connection pipe 28. On the other hand, a compressed air out of a compressor 22 is mixed with a pressurized water out of a pressurizing pump 18 by a mixing valve 20 and flows into the mixing tank 16 as a pressurized bubble water. Sludge floated up here is carried by a scraper 12 and sent through a discharge pipe 4 to a tank 5 for making the sludge anaerobic. In this case, the sludge flows down along a L-type partition plate 6, creeps through a bottom clearance hole 7, gets over a overflow dam 8 and goes into a sludge pit 34. As the height of dam 8 is higher than that of clearance hole 7, the sludge is always fully collected in a sludge receiver 33 so that the anaerobic condition is maintained by cutting off an intrusion of air from a pressurized flotation tank 11.