A valve for use in a vacuum drainage system, comprising: (i) a valve body having (a) an exterior wall comprising an inlet operatively connectable to an output from a suction device, and an outlet operatively connectable to a vacuum source, and (b) an interior chamber; (ii) a movable member movably secured within the interior chamber and comprising at least one movable member outlet aperture to selectively provide an operational open area constructed and arranged to allow fluid flow there through, and (a) being located to cooperate with the valve body outlet, such that movement of the movable member through a distance of travel through selected ones of a plurality of positions between a fully closed aperture position and a fully open aperture position presents at least a portion of the at least one aperture to the valve body outlet, to adjust the effective operational open area of the aperture presented to the valve body outlet and in operation to regulate a corresponding rate of flow through the at least one aperture and the valve body outlet; and (b) having a configuration such that during movement of the movable member to increase the effective operational area of the at least one aperture, a rate of change of the effective operational area increases non-linearly in relation to a linear increase in the distance of travel, characterised in that the rate of change of the effective operational area increases according to a polynomial equation y = ax 4 + bx 3 + cx 2 + dx + e, where y is the rate of change of the effective operational open area; x is the distance of travel of the movable member, expressed as a decimal ranging from a fully closed aperture position at 0.0, to a fully open aperture position at 1.0; a = 0.00008 ± 10 %; b = − 0.0013 ± 10 %; c = 0.0027 ± 10 %; d = − 0.0052 ± 10 %; and e = − 0.0009 ± 10 %. (i) a valve body having (a) an exterior wall comprising an inlet operatively connectable to an output from a suction device, and an outlet operatively connectable to a vacuum source, and (b) an interior chamber; (a) an exterior wall comprising an inlet operatively connectable to an output from a suction device, and an outlet operatively connectable to a vacuum source, and (b) an interior chamber; (ii) a movable member movably secured within the interior chamber and comprising at least one movable member outlet aperture to selectively provide an operational open area constructed and arranged to allow fluid flow there through, and (a) being located to cooperate with the valve body outlet, such that movement of the movable member through a distance of travel through selected ones of a plurality of positions between a fully closed aperture position and a fully open aperture position presents at least a portion of the at least one aperture to the valve body outlet, to adjust the effective operational open area of the aperture presented to the valve body outlet and in operation to regulate a corresponding rate of flow through the at least one aperture and the valve body outlet; and (b) having a configuration such that during movement of the movable member to increase the effective operational area of the at least one aperture, a rate of change of the effective operational area increases non-linearly in relation to a linear increase in the distance of travel, characterised in that the rate of change of the effective operational area increases according to a polynomial equation y = ax 4 + bx 3 + cx 2 + dx + e, where y is the rate of change of the effective operational open area; x is the distance of travel of the movable member, expressed as a decimal ranging from a fully closed aperture position at 0.0, to a fully open aperture position at 1.0; a = 0.00008 ± 10 %; b = − 0.0013 ± 10 %; c = 0.0027 ± 10 %; d = − 0.0052 ± 10 %; and e = − 0.0009 ± 10 %. (a) being located to cooperate with the valve body outlet, such that movement of the movable member through a distance of travel through selected ones of a plurality of positions between a fully closed aperture position and a fully open aperture position presents at least a portion of the at least one aperture to the valve body outlet, to adjust the effective operational open area of the aperture presented to the valve body outlet and in operation to regulate a corresponding rate of flow through the at least one aperture and the valve body outlet; and (b) having a configuration such that during movement of the movable member to increase the effective operational area of the at least one aperture, a rate of change of the effective operational area increases non-linearly in relation to a linear increase in the distance of travel, characterised in that the rate of change of the effective operational area increases according to a polynomial equation y = ax 4 + bx 3 + cx 2 + dx + e, where y is the rate of change of the effective operational open area; x is the distance of travel of the movable member, expressed as a decimal ranging from a fully closed aperture position at 0.0, to a fully open aperture position at 1.0; a = 0.00008 ± 10 %; b = − 0.0013 ± 10 %; c = 0.0027 ± 10 %; d = − 0.0052 ± 10 %; and e = − 0.0009 ± 10 %. y is the rate of change of the effective operational open area; x is the distance of travel of the movable member, expressed as a decimal ranging from a fully closed aperture position at 0.0, to a fully open aperture position at 1.0;