ABSTRACT
When a stream feeding a filter medium is moved by a centrifugal pump, the system rides the pump curve. As the filter medium loses permeability, more back pressure on the pump results in a decreased flow rate. When a sand bed is used to clarify water, the driving force is the column of water over the bed. Flow out the bottom of the bed is controlled by a valve so that, as the bed looses permeability, the valve opens more, trying to maintain a constant flow. But eventually as the bed plugs and the valve opens all the way, the flow rate falls to the point where the bed must be backwashed before operations can be resumed.
