Impact of iron dosing of membrane bioreactors on membrane fouling

• Examination of impact of dosing of Fe(III) salt on resultant supernatant Fe(III) speciation.
• Assessment of implications of supernatant Fe(III) speciation on membrane fouling.
• Analysis of extent of Fe redox transformations following dosing of Fe(III) salt and assessment of impact on membrane fouling.
• Deductions concerning likely mechanism(s) of colloidal iron oxyhydroxide formation.

Flux decline in bench scale membrane bioreactors (MBRs) configured for chemical phosphorous removal using ferric chloride increased as the molar Fe to P ratio increased from 2 to 4 and was strongly dependent on the valence state of the low concentrations of filterable iron present in the supernatant. MBRs operating without pH control over the pH range 5.1–7.1 exhibited variable but generally higher concentrations of dissolved and colloidal (<0.8 μm filterable) iron in the supernatant of the anoxic zone (1.18–26.43 μM) than in the aerobic and membrane chambers (0.18–6.72 μM). At pH 5.1, 97% of the iron suspended in the anoxic chamber was present as Fe(II). In contrast, the concentration of iron was relatively uniform in the three chambers of the MBRs (3.2 ± 0.4 μM) when pH was controlled by caustic addition. Under pH-controlled conditions, 70–80% of the iron in the supernatant was in the form of soluble (<0.2 μm) Fe(II) rather than Fe(III). While dosing of the MBRs with a moderate concentration of ferric chloride (Fe/P molar ratio of 2) led to effective phosphorus removal and minimal enhancement in extent of fouling, a higher ferric chloride dose (Fe/P = 4) gave little improvement in P removal and led to severe membrane fouling. While Fe(III) dosing is an effective strategy for P removal provided sufficient Fe is added, the dangers of overdosing are severe with careful optimisation of dose clearly required in order to avoid debilitating membrane fouling.