MBR systems combine two complex processes — biological treatment and membrane separation — and faults in either affect the other. This guide addresses the principal failure modes with root-cause analysis and corrective actions.
Common questions on membrane-bioreactor design, flux, fouling, cleaning and operation — answered by our process engineers.
Read the MBR FAQSymptoms: Trans-membrane pressure increases faster than 0.5 kPa/hour at constant flux; recovery after relaxation/backwash insufficient; flux decline requiring reduction below design setpoint.
Root cause: Colloids and biopolymers (SMP, EPS) depositing on membrane surface; cake layer formation from excess fine TSS; oil or surfactant in feed above membrane tolerance (>50 mg/L); MLSS exceeding optimal range for membrane type (>15,000 mg/L).
Immediately reduce flux by 15–20% and observe TMP stabilisation. Perform enhanced backwash with 200–500 mg/L sodium hypochlorite. If TMP does not recover, perform maintenance CIP: NaOCl 500 mg/L for 60 min, followed by citric acid 2000 mg/L for 60 min. Review feed for oil/surfactant breakthrough. Check MLSS and waste sludge rates — ensure SRT is within design range. Increase coarse bubble scour air if below minimum spec.
Symptoms: Permeate turbidity rises above 1 NTU; permeate SS detectable (>0.5 mg/L); UV absorbance elevated in permeate.
Root cause: Membrane integrity breach — hole or crack in hollow-fibre bundle; damaged o-ring on flat-sheet cassette; failed header seal; permeate-side contamination from backwash system.
Perform integrity test immediately (pressure hold test or bubble-point test per manufacturer protocol). Identify breached bundle or cassette by pressurising individual elements. Plug or replace the identified element. If no single element identified, inspect all header o-rings. Check backwash pump for reverse contamination.
Symptoms: Thick, stable foam 100–500 mm deep on bioreactor surface; foam overflows into membrane tank or walkways; foam persists after aeration reduction.
Root cause: High surfactant load in feed (detergents from CIP, food-grade sanitisers); high Nocardia or Microthrix filament population (associated with long SRT >20 days and lipid-rich feed); sudden shock load of soluble BOD causing rapid mixed liquor activity.
Dose antifoam (silicone-based, food-grade) at 1–5 mg/L as immediate control. Reduce SRT by increasing sludge wasting 20% temporarily. Review feed for surfactant sources — check CIP discharge timing. If Nocardia confirmed (microscopy shows branching actinomycetes), reduce SRT to <15 days for 2–3 weeks. Install spray water bar above bioreactor as permanent foam knockdown.
Symptoms: MLSS reads normal (8,000–12,000 mg/L) but membrane air scour appears insufficient; flow resistance through membrane increased; sludge difficult to pump.
Root cause: EPS (extracellular polymeric substances) accumulation at high SRT (>30 days); filamentous bulking causing viscous MLSS; excess polymer dosing in upstream treatment contaminating bioreactor.
Check capillary suction time (CST) of mixed liquor — if >20 seconds, EPS is high. Increase sludge wasting to reduce SRT to 15–20 days. Review upstream polymer dosing — carryover from primary treatment raises MBR EPS significantly. Temporarily increase membrane scour air by 15%. If filaments confirmed (microscopy), use targeted chlorination: dose 0.5 mg/L Cl₂ to bioreactor for 30 min (check membrane compatibility first).
Symptoms: Permeate ammonia rising above consent; bioreactor ammonia >2 mg/L; DO in bioreactor appears adequate but nitrification failing.
Root cause: SRT dropped below minimum for nitrifier retention at current temperature (minimum SRT = 10 days at 15°C; 20 days at 10°C); inhibitory substances in feed (heavy metals, biocides, high NH₃-N loading); DO in bioreactor consistently <1.5 mg/L despite aeration.
Measure actual SRT from wasting records — if below minimum, immediately reduce waste sludge rate. Check feed for inhibitory substances using activated sludge respirometry. Ensure DO >2 mg/L throughout bioreactor (check for stratification by measuring at multiple depths). Increase aeration if DO limited. Allow 2–4 weeks for nitrifier population recovery after any inhibitory event.
Symptoms: Post-CIP TMP within 1 hour returns to pre-clean level; membrane exhibits permanent flux decline; NaOCl and citric acid CIP sequence does not restore performance.
Root cause: Cleaning concentrations below effective threshold; contact time too short; incorrect sequence (must do NaOCl before citric acid for protein+mineral fouling); biofouling deeper than surface wash can reach; scaling by calcium carbonate on membrane surface.
Increase NaOCl to 1,000–2,000 mg/L and extend contact to 3 hours at 25°C (do not exceed manufacturer maximum). Follow with citric acid 3,000 mg/L for 3 hours. Between cycles, rinse with warm permeate (30–35°C) — temperature significantly improves cleaning efficacy. If still ineffective, consider oxalic acid for mineral scale (500 mg/L, 60 min) as a one-off treatment. Check cleaning temperature — below 15°C, chemical cleaning efficiency drops significantly.
MBR Maintenance Tip: TMP trending is the single most valuable operational KPI for an MBR. Plot normalised TMP (corrected for MLSS viscosity and temperature) daily — a rising trend of >0.3 kPa/day warrants investigation before a cleaning event becomes mandatory. Our SCADA team can implement automated TMP trending and maintenance clean triggers.
Engineering comparison of the three main biological treatment technologies.
Technology ComparisonScheduled maintenance, remote diagnostics, and emergency callout for MBR systems.
Maintenance ServicesOur process engineers provide remote diagnostics, site visits, and optimisation services across our full equipment range.
Our expertise spans multiple industries with sector-specific water treatment solutions.