Many wastewaters generated by industry contain a limited amount of very toxic and recalcitrant pollutants which can upset biological treatment systems. A new membrane biofilm reactor concept is studied to treat these wastewaters. A biofilm is grown on an ultrafiltration membrane supporting layer that separates wastewater from an additional nutrient stream. The biofilm bacteria are in close contact with the wastewater which allows an efficient degradation of organic pollutants. Meanwhile, the diffusion of specific nutrient compounds keeps the bacteria active. The study is focused on modelling and optimization of the reactor operation considering both mass transfer and degradation kinetics. The biofilm structure and characteristics are very important in this optimization study.