PURPOSE: To prevent the clogging of a screen and facilitate the carrying of a screened material by injecting a jet water flow toward a clay mass on a grizzly, and breaking and grinding it.

CONSTITUTION: A sludge is supplied onto a grizzly 12 by a backhoe. It is passed through the grizzly 12, further made fall in a hopper 14, and put on a vibrating screen 16. The sludge is screened by the vibrating screen 16, and after a foreign material and a large mass which can not pass the screen are removed, the sludge is sent to a sludge force feeding device. When a large clay mass which can not pass grizzly bars 12a is present, the arm 24 of a water injecting machine 20 is turned onto the grizzly 12 from a retreat position. A jet water flow is injected toward the clay mass through an injection nozzle by the drive of a speed increasing machine and the supply of high pressure water to an injection hose. The clay mass is ground and passed through the clearance of the grizzly bars 12a.