Normalise before interpreting lost production
RO can produce less when feed temperature falls or concentration changes without a new deposit having formed. Normalisation compares performance against common reference conditions. Use a method or program suitable for the installed membranes, and retain the raw operating readings alongside the calculated results so that unexpected changes can be checked.
Record feed, permeate and concentrate flows; feed, concentrate and permeate pressures; temperature and conductivities. Measure differential pressure by stage where possible. Check calibration and water-balance consistency before turning an uncertain signal into a cleaning instruction.
Use manufacturer thresholds within their stated scope
For FilmTec elements specifically, DuPont’s cleaning document, revision 13 of February 2026, calls for cleaning when one or more of these changes occurs: normalised permeate flow falls 10 %, normalised salt passage rises 5–10 %, or normalised pressure drop rises 10–15 %.
These are relative changes against an appropriate reference. They are not percentage-point losses in rejection, and they are not universal limits for every membrane. The installed equipment’s procedures and warranty conditions govern the response. First exclude changes in recovery, pressure control or pretreatment that could explain the deviation without requiring cleaning.
| Cited FilmTec indicator | Reference relative change | What to check |
|---|---|---|
| Normalised permeate flow | 10 % decrease | Baseline, data and normalisation conditions |
| Normalised salt passage | 5–10 % increase | Conductivity, leaks and possible damage |
| Normalised pressure drop | 10–15 % increase | Affected stage, flows and obstruction |
Example: salt passage and rejection use different changes
Assume initial normalised production of 10.0 m³/h and current normalised production of 8.8 m³/h. The decline is (10.0 − 8.8) / 10.0 = 12 %. If the readings are reliable and comparable, this exceeds the cited FilmTec flow criterion. The same conclusion cannot be drawn from unnormalised production alone.
Salt passage rising from 1.00 % to 1.08 % is an 8 % relative increase. Rejection changes from 99.00 % to 98.92 %, a loss of 0.08 percentage points. Confusing them can delay intervention or cause unnecessary cleaning. This is an arithmetic illustration, not a Neptuno operating case.
Separate deposits, leaks and oxidant exposure
Increasing pressure drop can point towards channel blockage; declining normalised output can suggest material on the membrane surface. Stage distribution and chemical-dosing history help prioritise possible causes. These patterns guide investigation but cannot identify a particular deposit on their own. Compare them with feed analysis and upstream incidents.
Increased salt passage together with unusually high production also requires investigation of oxidation or mechanical issues. Check seals, connections, vessel results and exposure to oxidants. Cleaning removes deposited material; it cannot repair chemical damage to the membrane. Repeated, increasingly aggressive cleaning can consume time and chemicals while leaving the cause unresolved.
Where records do not distinguish possible causes, deposit analysis, element testing or an autopsy may provide more value than another cleaning recipe. Aim for stable operation with an explained cause.
Prepare a CIP procedure around the diagnosis
Specify compatible cleaner, sequence, concentration, flow, duration, pH and temperature for the particular element. Check pumps, seals and other cleaning-loop materials as well. Temperature and pH limits work together; separate maximum values must not be combined into an assumed permissible operating point.
DuPont and Hydranautics documentation provides their own cleaning and flushing requirements. Plant engineering must translate the applicable instructions into an executable procedure with usable solution volume, temperature control and a defined spent-solution outlet. Make-up and flushing water must also be compatible, including the absence of incompatible oxidants.
Measure the outcome and address the cause
After flushing and stabilisation, repeat measurements and compare normalised output, salt passage and pressure drop with the reference. Record recovered performance and remaining deviation. Successful cleaning does not justify leaving a pretreatment fault that will foul the system again.
If cleaning intervals shorten, review feed conditions, recovery, dosing and loading by stage. When purchasing or upgrading RO, include suitable instrumentation, access to historical records and the CIP circuit in the quotation scope. The ability to diagnose and clean the system properly contributes to the availability being purchased, as well as to membrane life and operating cost.
Common questions
Should RO be cleaned every month?
A calendar can organise maintenance, but CIP need should be tied to performance trends and equipment limits. Increasing frequency can reveal a design or operating problem.
Can higher pressure compensate for fouling?
It may temporarily raise production, but it can conceal declining performance while increasing energy use or operating load. Assess normalised trends before changing the setting.