Abstract: An apparatus for extracting liquid from a sludge/conditioning agent mixture which comprises a first endless belt (10) of filter material; a second endless belt (12) of filter material; and rollers (20, 21, 22, 23, 24, 25, 26, 27, 13, 14, 15) for the belts (10, 12). That part of the belt (10) between the rollers (20, 23) is defined as a gravity de-watering zone; the part of the belts (10, 12) between the rollers (21, 13, 14, 15) is defined as a pressure de-watering zone. Filtrate is collected in a tray (17); liquid is collected in a tray (35). Monitoring means monitors a characteristic of the filtrate and the liquid to enable a rate of change in the characteristic of the filtrate/liquid to be determined. The apparatus functions at its optimum at a loading rate for the mixture which is just less than that which causes extrusion of the liquid in the pressure de-watering zone.