A filter unit includes a substantially cylindrical body portion having a proximal end and a distal end. First and second recessed engagement surfaces traverse at least part of the body portion. The first and second engagement surfaces include a first segment that extends substantially parallel with the longitudinal extent of the body portion. A second segment extends at an angle between 50 degrees and 60 degrees from the direction of the first segment. A third segment extends substantially orthogonal to the first segment. A laterally extending key member is disposed on the body portion between the first recessed engagement surface and the second recessed engagement surface. An engagement protrusion extends from the proximal end and has a sidewall with a water inlet and a concave engagement wall with a water outlet. The engagement wall includes a first portion that angles inward to the water outlet at a first angle. A second portion angles inward to the water outlet at a second angle different than the first angle. A periphery of the engagement wall at the first portion includes a first radius of curvature. The periphery of the engagement wall at the second portion includes a second radius of curvature that is larger than the first radius of curvature. First and second seals are disposed about the sidewall. The water inlet is disposed between the first and second seals.