An anaerobic thermophilic submerged membrane bioreactor was operated for 300 days, with a mixture of volatile fatty acids as substrate. Cake formation was identified as the key factor governing critical flux. Low levels of irreversible fouling were observed. During the first 60 days of operation a rapid decrease in critical flux was by observed, due to a reduction in sludge particle size. Critical flux at the end of the reactor operation was only 7 L/m(2).h, at a gas superficial velocity of 70 m/h. Based on this flux, membrane costs would be close to 0.5 (sic) per m(3) of permeate, assuming a membrane lifetime of 3 years and membrane price of 100 (sic)/m(2). Costs calculations show that a side-stream configuration would be more economic if fluxes 3 times higher can be attained.