Two sludge bed anaerobic reactors were operated for the treatment of a saline, completely acidified synthetic wastewater. One of the reactors was equipped with a microfiltration membrane, in order to achieve complete biomass retention. Reactors were operated at increasing organic loading rates and salinity. At a sodium concentration of 24g/L propionate accumulation was severe. When sodium concentration was reduced to 16g/L, propionate oxidation in the membrane based reactor recovered after 40 days. This was not the case for the sludge bed reactor, in which propionate oxidation rates remained very low. This shows that the enhanced biomass retention achieved by membrane filtration can have a positive effect in retaining active halotolerant bacteria. Fluxes in the range of 10-15 L/m(2) h were achieved during the membrane bioreactor operation, even though membrane was exposed to a solid concentration below 2 g/L. However, flux showed to be limited by cake formation and low levels of internal pore fouling were detected. (C) 2008 Elsevier B.V. All rights reserved.