Differential Pressure (∆P) alone should not be used as an indicator of membrane scaling/fouling. Increases in differential pressure (∆P) across a stage indicate that a foulant has growth thick enough to interfere with flow through the feed channels.  However, not all foulants/scale form the thick deposits needed to obstruct the channel ; many form a consistent membrane coating that reduces productivity without significantly impacting ∆P. A typical scale only gets thick enough to causes an increase in ∆P when it has become very severe, making membrane cleaning much more challenging.  Some scales such as calcium sulfate even become completely irreversible if allowed to reach a certain thickness.

Always base your membrane cleaning on decreases in Normalized Permeate Flow.