As part of a national project, “Aqua-Renaissance '90,” by the MITI, a pilot-scale evaluation of membrane-enhanced anaerobic fermentation, has progressed for the wastewater from a pulp and paper mill. A novel membrane filtration system was newly proposed with the aim of saving energy. That is, a gas-liquid two-phase cross-flow filtration which was generated with liquid circulation by an air-lift pump effect, was combined in the anaerobic bioreactor. It was confirmed that the membrane filtration not only offered very stable and large permeate flux, but enhanced the processing efficiency by retaining the microorganisms in the bioreactor. Furthermore, the power consumption per unit permeate volume in the membrane system of 1.78 kWh/m3was achieved. which was a very highperformance result from the viewpoint of saving energy, as compared with 3ndash;5 kWh/m3of conventional liquid single-phase cross-flow filtration.