NOVELTY - The membrane separation tank has sedimentation unit (52), membrane separation unit (53) both connected by overflow opening (55), tank component (51) connected and installed in methane fermentation tank, membrane separator (56) immersed in unit (53), recycle sludge path (66) connected to upstream side of unit (52), and recirculates fermentation sludge from fermentation tank, sludge path (67) connected in the lower portion of unit (52), and a return sludge path (71) connected to downstream side of unit (53), and returns fermentation sludge to fermentation tank.    USE - For methane fermentation processing of organic refuse and sludge.    ADVANTAGE - The membrane separation tank is driven continuously without deposition of small impurities such as shellfish shell and bone.    DETAILED DESCRIPTION - The membrane separation tank has a sedimentation unit (52) of upstream side, a membrane separation unit (53) of downstream side connected by an overflow opening (55), a closed-type tank component (51) connected and installed in a methane fermentation tank, a membrane separator (56) immersed in the membrane-separation unit, a recycle sludge path (66) connected to the upstream side upper portion of sedimentation unit, and recirculates fermentation sludge from methane fermentation tank, a sludge path (67) connected in the lower portion of sedimentation unit, and a return sludge path (71) connected to downstream side lower portion of membrane separation unit, and returns fermentation sludge to methane fermentation tank. The fermentation sludge is circulated between methane fermentation tank and tank component through the recycle sludge path and return sludge path.    An INDEPENDENT CLAIM is included for operating method of the membrane separation tank.    DESCRIPTION OF DRAWING(S) - The figure shows the model of the membrane separation tank. (Drawing includes non-English language text)    Tank component (51)    Sedimentation unit (52)    Membrane separation unit (53)    Overflow opening (55)    Membrane separator (56)    Recycle sludge path (66)    Sludge path (67)    Return sludge path (71)