The salinity effect on a biofilm membrane bioreactor (biofilm-MBR) was investigated to treat shipboard wastewater. The membrane permeability decreased and poorer permeate quality was obtained when salt shocks (influent salt concentration: 30 g/L NaCl for 3 h, 6 h, and 9 h respectively) took place. On long duration step tests, a better membrane performance with lower fouling rate was observed when smaller salt concentration increments occurred, with acclimation of the biofilm taking place with time. Results indicate that the membrane fouling mechanism is quite complicated with fluctuation of feed water salinity, where SMPs concentrations resulting from the microorganisms' response and/or salinity variation play a main role in membrane fouling. However, surface charge effects on particles and the membrane surface starts to dominate the membrane fouling with the increasing salt concentration. (C) 2010 Elsevier B.V. All rights reserved.