Abstract: A filter unit comprises an outer casing defining a vertical-axis first chamber having a liquid inlet and a normally-closed discharge valve at its lower end and a second chamber defined by upper and lower end walls, a perforated cylindrical wall located within the first chamber and covered by a filter element and a non-perforated cylindrical wall having a lower portion coaxial with, and within the perforated wall and an upper portion projecting above the first chamber and having an outlet for liquid filtered through the filter element. A piston is slidable within the non-perforated wall to force liquid through the filter element to cleanse the latter when it becomes clogged, the piston being connected to open the discharge valve to discharge the particle-containing liquid from the first chamber. A non-return valve in a duct connected to the outlet allows air into the space above the piston to facilitate its descent.