Spiral membrane element

A spiral membrane element, comprising a plurality of membrane leaves in each of which a permeation-side flow-channel member is interposed between opposed separation membranes;

• a supply-side flow-channel member interposed between any two of the membrane leaves;
• a perforated central pipe on which the membrane leaves and the supply-side flow-channel member are wound;
• and a sealing part that prevents a supply-side flow-channel from being mixed with a permeation-side flow-channel;
• wherein the spiral membrane element has an efficiency index E of 0.024 to 0.10, the index E being calculated in accordance with the following expression:
• efficiency index:
• E=0.0024×X−0.2373+(Y/(D2×L))
• wherein X is a thickness [mil] of the supply-side flow-channel member, Y is an effective membrane area [ft2] of the separation membranes, D is an outer diameter [inch] of the membrane element, and L is a length [inch] of the membrane element, the L being defined with reference to a length of the perforated central pipe;
• the separation membranes independently have thicknesses from 80 to 100 μm;
• and
• the thickness of the supply-side flow-channel member is from 26 to 110 mil.