FILTER ELEMENT

An edge filter element, for use in wells of potable water, comprises a stack of rings defining filter gaps 6 between them, the size of gaps 6 using adjustable by changing the relative angular orientations of adjacent rings. In Fig. 1 each ring has nine sets of sockets 5 and projections 4 equally spaced round its circumference, said sockets and projections being of three different axial heights to give three possible gap sizes (Figs. 6 to 8). The peripheries of the rings may be extended by axial grooves (Fig. 9). In Figs 10 to 12 each ring has upper and lower faces of sinusoidal form, which themselves form sockets and projections to define various gaps. In Figs. 13 to 15 rings have sinusoidal upper and lower faces and sockets and projections of uniform size, but the relative orientations of the sockets and the waves vary to enable different gaps to be formed. Rings are metal or plastics and are clamped together by rods extending axially through holes which define the possible orientations.