A movable deposition filter comprises a filter tank associated with two mixing tanks, all the tanks being connectable selectively through fixed connections, the filter tank being mounted upright near one end of a chassis and the mixing tanks near the other end, all controls and instrumentation being mounted at, and accessible from, this other end of the chassis. One of the mixing tanks is intended to supply filter aid for forming a layer on the filter elements prior to filtration, the other (with agitator) to supply filter aid continuously during filtration. Preferably the valves in the interconnecting piping are arranged in groups -an inlet group, a mixing group and an outlet group. In operation, liquid to fill all the tanks is drawn in through an inlet valve and pumped at (14), Fig. 6 (not shown), through a mixing valve to the filter tank (13) and thence, when full, by overflow to the mixing tanks (11, 12) which are thereafter charged with filter aid. Next, filter aid slurry is withdrawn from mixing tank (11) through an inlet valve by pump (14), discharged into the filter tank (13), and passed therefrom along the filtrate pipe back to the mixing tank (11) in a closed cycle. For the filtration step liquid is drawn in through an inlet valve and mixed in the mixing valve housing with filter aid slurry drawn by a metering pump (15) from tank (12); it then flows through the elements in filter tank (13) to one or two outlet valves. The next stage is backwashing to remove the cake, for which purpose liquid is drawn in through an inlet valve and pumped through a mixing valve communicating with the filtrate side of the elements in tank (13). The cake accumulates at the bottom of the tank. Then the filter tank (13) is partially emptied under pressure by closing all inlet and mixing valves, opening an outlet valve, and admitting gas under pressure through a separate valve to the "unfiltered" space in the tank (13). The liquid is forced up discharge pipes each open at the lower end and extending downward within a filter element nearly to the bottom. The lower part (31) of the filter tank (13) may then be lowered, swung sideways and tipped about pivots to dispose of the residues. Where filtering is not required, liquid may be pumped directly from one or two inlet valves through one of the mixing valves to the exterior, the filter being by-passed and all the outlet valves being closed. Preferably each filter element consists of a hollow perforate cylinder with a closed lower end surrounded by a filter sleeve. The discharge pipe (mentioned above) has an outwardly-directed flange contacting the upper rim of the element so that the filtrate must flow downwards in the element and then up the pipe to reach the filtrate reservoir (32) above the element-supporting plate (34) in the tank (13).