Abstract: A method for biologically purifiying waste water which produces substantially no excess waste sludge is provided. In this method, a waste water containing biodegradable substances is treated in a biological fixed film type reactor and aqueous water sludge discharged from the reactor is aerobically digested after being separated from the biologically treated waste water. The digested aqueous sludge is returned to the reactor or the upstream thereof. This method can be advantageously combined with any installed activated sludge type water purification process, to thereby substantially eliminate the production of excess waste sludge from the water purification system.

Abstract: A solid-liquid separation element which remarkably, improves the solid-liquid separation capacity of a solid-liquid separation device having the separation element therein is provided. In this separation element, guide members for accelerating the through-flow of the liquid are mounted on the back surface of a stationary inclined screen and, optionally, guide members for transporting solid materials are arranged on the surface of the screen so as to be substantially in parallel to each other and in a downward inclining direction from both edges of the screen. The solid-liquid separation apparatus having said separation element and a cleaning brush which is reciprocated in rubbing contact with the back surface of the screen is also provided. The range of the reciprocating motion of the brush is more than the width of the screen.