TL;DR
A rising normalized differential pressure across the RO train means feed channel fouling. Perform a low-pH citric acid CIP to dissolve inorganic or biological deposits. If that does not work, plan for element replacement.
What you might see
- pressure drop from feed to concentrate above 60 psi per pressure vessel
- feed pump operating at higher pressure than normal for the same flow
- normalized differential pressure trending upward over weeks
- feed spacer area visibly fouled at element autopsy
Likely causes
Colloidal or biological fouling accumulating in the feed spacer channel
Inorganic calcium carbonate or iron scaling in the feed channel reducing the open area
Pre-treatment failure allowing high silt density index (SDI) feed to reach the membrane
Inadequate antiscalant dosing for the feed water chemistry
Required tools
- Pressure gauges at feed, permeate, and concentrate
- Thermometer for temperature correction
- CIP pump and tank (sized for the system volume)
- Citric acid and NaOH for CIP chemicals
- Chemical-resistant PPE
Safety first
- RO CIP chemicals are corrosive acids and caustics. Wear face shield, chemical gloves, and work in a ventilated area.
- Do not exceed the OEM maximum allowable feed pressure during normal operation or CIP. Over-pressurization can telescope elements out of the pressure vessel.
Procedure
- 1
Calculate the normalized differential pressure (NDP) across the first and last pressure vessels using feed pressure, concentrate pressure, and temperature correction per standard membrane normalization practice.[1]
- 2
Review the SDI or turbidity readings at the RO feed. A failing cartridge filter or pre-treatment system allows colloidal fouling to accelerate.
- 3
Check the antiscalant dosing pump output against the design dosing rate for the current recovery ratio and feed chemistry.
Warning: RO CIP solutions are either acidic (citric acid pH 2-3) or caustic (NaOH pH 11-12). Wear chemical-resistant gloves and face shield when preparing or circulating CIP chemicals. Do not pressurize the system above 60 psig during CIP. - 4
Prepare a low-pH citric acid CIP solution at pH 2.0-2.5 with clean permeate water. Circulate through the pressure vessels at a low cross-flow velocity per the element manufacturer's CIP procedure.
- 5
After the low-pH step, flush with permeate until the pH returns to within 1 unit of the feed water pH, then if biological fouling is suspected, follow with a high-pH caustic CIP at pH 11-11.5.
- 6
Return the train to service, re-normalize, and compare NDP to the pre-CIP baseline. If improvement is less than 50%, plan for element replacement.
Sources
Veolia (Suez) FilmTec BW30 / SW30 elements Reverse Osmosis (RO) System general technical documentation, Veolia (Suez)
RO membrane CIP procedures, normalized differential pressure calculation, general water treatment references (general)
More guides for Veolia (Suez) FilmTec BW30 / SW30 elements
How to diagnose low permeate flow and recovery decline on a FilmTec BW30 / SW30 RO system
Normalized permeate flow declining 10% or more below baseline means membrane scaling or biofouling. Review antiscalant dosing, check SDI, and plan a high-pH CIP to recover flux.
How to investigate salt rejection decline and high permeate conductivity on a FilmTec BW30 / SW30 RO system
Rising permeate conductivity means a membrane element has been physically damaged, oxidized, or has a sealing defect. Sample each pressure vessel's permeate individually to locate the problem vessel.
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