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.
See also:
What does differential pressure or delta P (ΔP) mean when logging RO data?
What is the procedure for cleaning RO membranes? What CIP chemicals should be used?
How can we determine whether the membranes in our Reverse Osmosis system are damaged?
Under which conditions can antiscalants cause RO membrane fouling?