Abstract: A drier having a boxlike housing (12) for receiving a hopper (13) therein which contains a batch of wet sludge. The hopper (13) cooperates with the housing, when closed therein, to define a closed air recirculation system which supplies dry pressurized warm air into an air-receiving chamber (79) located at the bottom of the hopper. The air passes upwardly through a perforated bottom wall (78) into and through the sludge to remove moisture therefrom. The moist air passes upwardly through the open top of the hopper, through a filter and is supplied to a drying unit (34) located in the housing for removing moisture therefrom, which moisture is externally discharged. The drying unit resupplies the dry air back to the drying chamber formed in the hopper.