UF and MBR Membrane Fouling: Diagnosis and Cleaning

Abstract: Whether a UF or MBR membrane needs chemical cleaning comes down to one test: if backwash and air scouring bring the normalized specific flux back to the design value at the same temperature, the fouling is reversible; if they do not, adsorption and pore blocking have already formed and only chemistry will recover the flux. This article sorts the four fouling types by field evidence, routes each one to an alkaline, acidic, oxidizing or chelating sequence, and sets out the order criteria, the parameter limits, the material exclusions and the flux-recovery acceptance rules that close the job.

1. Flux Decline Has Only Three Sources

Concentration polarization and cake layer are physical: backwash and air scouring remove them. Adsorption, pore blocking and biofilm are chemical: backwash only strips the surface. The test is to normalize flux to one temperature and watch specific flux recovery — two or three backwash cycles that restore the design value mean no chemical is needed; a falling recovery that settles low means irreversible fouling has formed (industry experience value).

2. Identifying the Fouling Type

Three pieces of evidence do the work: the shape of the differential-pressure curve, the backwash recovery trend, and the ignition loss plus acid solubility of a membrane scrape.

The two usually overlap: identify the dominant one first.

3. Diagnose Before You Clean: Four Steps

Step one is normalization. Flux rises roughly 2% to 3% per degree Celsius (industry experience value), so data at different temperatures cannot be compared; use the manufacturer's correction factors. Step two is repeated backwash and air scouring with the recovery recorded each round — the round where recovery stops climbing is both the cleaning trigger and the baseline to beat.

Step three is the pretreatment check: if the raw water has not changed but the membranes collapsed, look at coagulation upsets, spent carbon, a breached cartridge filter or a dosing interruption that let oxidant through. Step four is membrane sampling: without opening the housing, take backwash drain water from the housing outlet for ignition loss and acid tests; strip a single element only when material damage has to be confirmed.

4. Four Chemical Routes by Fouling Type

Chemicals are recorded by system, not by trade name: LX-901 solvent-penetrating type (surfactant and penetrant blend), LX-902 alkaline-stripping type, LX-903 acidic passivating type (dissolves inorganic scale and oxides). Dose rates follow the membrane maker's limits and the cleaning procedure; no numbers are given here.

5. Cleaning Order and Operating Limits

Alkali first or acid first depends on how the layers stack: when organics, oil and biofilm sit on the outside, strip them with alkali and then dissolve scale with acid; when hard scale sits on the outside with foulant driven into the pores, acid first, then alkali to carry the organics away. Rinse to neutral between the two stages or the second stage is wasted.

Limits are given as ranges: alkaline cleaning at pH 10 to 12, acid cleaning at pH 1 to 3; temperature 25 to 40 degrees Celsius, not above 45 for polyamide composites and not above 35 for cellulose acetate; cleaning flow 1.0 to 1.5 times the operating flux, at a pressure below the operating pressure; alternate 20 to 30 minutes of circulation with 30 to 60 minutes of soaking.

Two red lines: UF transmembrane pressure stays below 0.15 MPa, while MBR maintenance cleaning is scheduled around 30 kPa and recovery cleaning around 50 kPa; and during backwash the permeate-side pressure must never exceed the feed side, or hollow fibres deform and debond at the pot, which is irreversible.

6. Membrane Damage Red Lines

Polyamide composites tolerate oxidants least and have a narrow pH window; PVDF, PS and PES tolerate oxidants but not long alkaline soaks or heat above 40 to 45 degrees Celsius; cellulose acetate dislikes high pH. Oxidant residue must be reduced (sodium bisulphite type) and verified before the train returns to service.

Three signs mean irreversible damage: flux still low after cleaning, falling salt rejection downstream, and a sudden rise in permeate turbidity.

7. Acceptance: Three Measured Comparisons

Acceptance compares measured values at one temperature before and after cleaning — normalized specific flux, transmembrane pressure, permeate turbidity and integrity testing; where RO follows the UF, add downstream salt rejection. Flux recovery typically reaches 85% to 95% of the original design flux (under typical operating conditions), bounded by the pressure baseline, fouling severity, membrane age and cleaning history. For the equipment itself, corrosion rate is judged against Table 2 of GB/T 25146-2010 with coupons retained, and waste containing oxidant or chelant is collected separately.

8. Common Mistakes

• Acid only, no alkali: biofilm untouched, pressure returns within days
• Alkali too hot or soaked overnight: fibres embrittle
• Oxidant left too long or not reduced: fibre breakage
• No rinse after acid: residual acid concentrates in the pores
• Air in the backwash water or flow above the limit: fibre fatigue
• Returning to service without an integrity test: leaks go unexplained

9. FAQ

After UF flux drops, should I backwash first or start chemical cleaning?

Backwash and air scour for two or three rounds and read the recovery. If specific flux returns to more than nine tenths of design, the cause is polarization and cake and no chemical is needed; if each round recovers less and settles low, pore blocking has formed and chemical cleaning is the only route.

Does MBR cleaning require an oxidant?

Only biofilm-dominated membranes need an oxidant, and the material must allow it. PVDF and PES hollow fibres usually do; polyamide composites and cellulose acetate must be avoided. Dose to the maker's limit, keep the soak short and reduce the residue before service.

Alkali first or acid first?

Look at whether the outer layer is organic or hard scale. Organics, oil and biofilm call for alkali then acid; hard scale blocking the pores calls for acid then alkali. Rinse to neutral in between.

At what transmembrane pressure must the membranes be cleaned?

Against the baseline after the previous chemical clean, not the maker's nominal figure. UF normally stays below 0.15 MPa; for MBR, schedule maintenance cleaning around 30 kPa and recovery cleaning around 50 kPa, and act once the machine is 30% above its own baseline with no backwash recovery.

What flux recovery counts as a pass?

Usually 85% to 95% of the original design flux, read with conditions attached. Membrane age, fouling severity and cleaning history all lower the ceiling, so a system older than five years reaching 80% is normal; acceptance rests on measured normalized specific flux and transmembrane pressure before and after cleaning.

Related Reading

• RO chemical cleaning steps and precautions — the neighbouring system's procedure
• Ship piping system cleaning and passivation — degreasing, acid cleaning, handover
• Piping & Membrane — all articles in this category
• Scaling Diagnosis Tool · Maintenance Cycle Calculator

Chemical cleaning acceptance follows GB/T 25146-2010, Code for acceptance of industrial equipment chemical cleaning quality (corrosion rate in Table 2, cleaning rate in Table 3); membrane-specific flux recovery and chemical compatibility follow the membrane maker's manual and the approved cleaning procedure.

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