RO plant permeate flow dropping: fouling, scaling or something else?

Normalise before you conclude

RO permeate flow changes with feed temperature — roughly 3 % per °C. A drop in winter may be normal. Always compare normalised permeate flow, differential pressure and salt passage against the start-up baseline. A 10–15 % change in any of them is the usual trigger for cleaning.

Read the symptoms

• Flow down, differential pressure up on the first stage: particulate or biological fouling
• Flow down, differential pressure up on the last stage, salt passage up: scaling (calcium carbonate, sulphate, silica)
• Salt passage up suddenly with flow up: O-ring leak, damaged membrane or chlorine attack
• SDI of feed above 3–4: pre-treatment is not protecting the membranes

Corrective action

Low-pH cleaning removes scale; high-pH cleaning removes organic and biological fouling. Clean in the right order for the foulant identified. Then fix the root cause: check antiscalant dosing, multimedia filter backwash, cartridge filter changes and that sodium bisulfite removes all chlorine before the membranes.