Valve for a fluid treatment apparatus and method and system for operating the valve

A valve for a fluid treatment apparatus, includes a body (23), having formed therein at least one first port (25) and at least two second ports (26,27), and a regulating device (32;55) positioned between the first and second ports (25,26,27) in flow sense. The regulating device (32;55) includes a first part (30;56) for occluding a path between the first port (25) and a first of the second ports (26,27) to an extent determined by a position along a range of positions of the first part (30;56) and a second part (31;57) for occluding a path between the first port (25) and a second of the second ports (26,27) to an extent determined by a position along a range of positions of the second part (31). The regulating device (32;55) includes a control member (33) for adjusting the position of the first part (30;56) in either of opposite directions along the range. The degrees of occlusion by each of the first and second parts (30,31) vary stepwise with their respective positions along the respective ranges. The second part (31;57) is journalled for relative movement with respect to the first part (30;56) along a range of relative positions. A catch mechanism is provided between the first and the second parts (30,31;56,57) for entraining the second part (31;57) at at least one end point of the range of relative positions.