Abstract: A sludge press wherein a piston is reciprocable along a horizontal path between a rectracted position in the interior of a hopper and an extended position in which extending forwardly from and communicating with the hopper so that the piston thereby expels a slug of compacted sludge through the open front end of the barrel. The front end of the barrel can be sealed by the knife-like valve member of a gate valve during movement of the piston to retracted position. The front end face of the piston is surrounded by a cutting edge which cooperates with a complementary annular cutting edge at the rear end of the barrel to sever solid ingredients of sludge during penetration of the piston into the barrel. The cutting edge of the valve member cooperates with a complementary cutting edge surrounding an opening in the rear section of the housing of the valve.