An apparatus and method are provided that enable the control of a screening operation based on a calculated screen blockage percentage and the velocity throughput of the screen. The apparatus includes an upstream level detector, a downstream level detector and a flow determining device which determines the volumetric flow through the screen. The apparatus may further include a blockage determination unit which determines the percent screen blockage based on the flow level upstream of the screen, the flow level downstream of the screen and the volumetric flow.