Cylindrical tank with obtuse conical base around which raking gear moves slowly, pushing settled sludge to a central discharge. In the body of the tank hang radially mounted tube frames covered with filter cloths (socks). These are connected with a central valve and timing mechanism, so set that vacuum is applied for 1 to 10 min to 1340 remove filtrate, after which the gathered solids are displaced by a brief flushback so that they fall to the raking zone.