The study presents a mathematical model developed to better understand and control membrane fouling in a single-staged, anaerobic fluidized bed membrane bioreactor (AFMBR) using polyethylene terephthalate ( PET) beads as scouring media. The model was based on combining the anaerobic biological model AM2b and a fouling model applied in membrane filtration. The presented model was validated using experimental data obtained by a laboratory scaled AFMBR reactor run during 250 d under various operational conditions. The combined AM2b and fouling model was able to simulate volatile suspended solids, soluble COD concentration, soluble microbial products concentrations and the methane production rate at steady-state condition with R-2 of 95% as well as the trans-membrane pressure with R-2 of 99%. The model was able to predict dominant fouling mechanism by assessing fouling resistances caused by cake formation and pore blocking separately.