A transient study was conducted at pilot scale to assess the impact of Fe dosage on the dynamics of biological and membrane performance of an anaerobic membrane bioreactor (AnMBR) treating authentic municipal wastewater. A transient model of the AnMBR system was employed to assist with interpretation of the observed responses in the mixed liquor under different FeCl3 dosages. A high dosage (43 mg FeCl3/LSewage) resulted in a significant accumulation of fixed suspended solids and volatile suspended solids (VSS) and reduction of colloidal COD in the mixed liquor. The elevated dosages appeared to reduce the biodegradability of VSS that was present in the raw wastewater. Intermediate dosages of FeCl3 (21–12 mg/L) had less effect on these responses and did not appear to affect VSS biodegradation. Membrane performance was significantly affected by FeCl3 dosage as indicated by reversible resistance (RR) and physically irreversible resistance (IR). RR was closely related to the colloidal COD in the mixed liquor, thus responded quickly to Fe dosage. Physically, IR had a delayed response to changes in the colloidal COD concentrations in the mixed liquor and this was attributed to the effect of slow mass transfer of colloidal matter between the mixed liquor and the membrane.