Aerobic operation of an immersed hollow fibre membrane bioreactor, treating municipal wastewater supplemented with molasses solution, has been studied across mixed liquor suspended solids (MLSS) concentrations between 8 and 32 g L(-1), the higher concentrations being normally associated with thickening operations. Only a marginal loss in membrane permeability was noted between 8 and 18 g L(-1) when operation was conducted without clogging. The sustainable operational flux attainable above 18 g L(-1) was highly dependent upon both the MLSS concentration and the state of the membrane. A temperature-corrected flux of 28 L m(-2) h(-1) (LMH) was sustained for 18 h at an MLSS of 8 g L(-1) using membranes close to initial their virgin-state permeability. This value decreased to around 14 LMH at 20 g L(-1) and 5 LMH at 32 g L(-1) MLSS for an aged membrane whose permeability had been recovered following clogging. Below the threshold flux operation without significant clogging was possible, such that the membrane permeability could be recovered with a chemically enhanced backflush (CEB). Above this flux clogging took place at a rate of around 7-14 g solids per m(2) membrane per m(3) permeate volume passed irrespective of the MLSS concentration. The permeability of the unclogged membrane was depressed and could not be recovered using a standard CEB, indicative of irrecoverable pore clogging. The outcomes corroborated previously reported observations concerning the deleterious long-term impacts of clogging, and confirmed the critical importance of operation at a sustainable flux value.