A control valve (10) includes a valve body with a plurality of ports (A, B, C, D, E, F) and a plurality of annular flow passages (53, 55, 57). A piston (34) which includes a plurality of annular flow passages and a longitudinal flow passage is selectively movable within a bore (32) within the valve body through operation of a valve controller (70). The valve controller is selectively operative to control the position of the piston so as to enable liquid flow through a plurality of flow paths. The valve controller further includes a installable and removable valve controller housing (74) which is releasibly engageable with a valve base (72). The valve may include a changeable piston and changeable injector and plug components to adapt the valve to different flow and fluid mixing requirements.