NOVELTY - Separation membrane (10), comprises a support layer (11) including a polymer (a1) comprising a structural unit of cellulose-ether ester compounds (I), and a polymer matrix layer (12).    USE - The separation membrane is useful: in a forward osmosis water treatment device (claimed); as a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, a reverse osmotic membrane or a forward osmotic membrane; and in water treatment (i.e. water purification), waste water treatment and reuse, sea water desalination, and power generator using osmotic pressures induced by the difference in salt concentrations.    ADVANTAGE - The separation membrane: exhibits improved salt rejection rate and permeation flow rate, and higher strength, higher porosity, and higher hydrophilicity; and is easily manufactured in cost effective manner.    DETAILED DESCRIPTION - Separation membrane (10), comprises a support layer (11) including a polymer (a1) comprising a structural unit of cellulose-ether ester compounds of formula (I), and a polymer matrix layer (12).    R1-R6 = 1-30C alkyl, 3-30C cycloalkyl, 2-30C heterocycloalkyl, 6-30C aryl, 2-30C heteroaryl, 7-30C alkylaryl, 7-30C arylalkyl (all optionally substituted), -COR7 or H;    R7 = 1-30C alkyl, 3-30C cycloalkyl, 2-30C heterocycloalkyl, 6-30C aryl, 2-30C heteroaryl, 7-30C alkylaryl or 7-30C arylalkyl (all optionally substituted);    L1-L6 = 1-30C alkylene, 3-30C cycloalkylene, 2-30C heterocycloalkylene, 6-30C arylene, 2-30C heteroarylene, 7-30C alkylarylene or 7-30C arylalkylene;    n, m = 0-150;    n+m = at least 1; and    s, p, q and r = 0-100.    Provided that at least one of R?1-R3 and at least one of R4-R6 are 1-30C alkylene, 3-30C cycloalkylene, 2-30C heterocycloalkylene, 6-30C arylene, 2-30C heteroarylene, 7-30C alkylarylene or 7-30C arylalkylene (all optionally substituted) or -COR7. INDEPENDENT CLAIMS are also included for:    (1) manufacturing a separation membrane, comprising preparing a polymer solution including a polymer and an organic solvent, where the polymer includes the structural unit (I), casting the polymer solution on a substrate, immersing the substrate casted with the polymer solution in a non-solvent to form a support layer including a skin layer (11a) and a porous layer (11b), and performing an interface polymerization reaction on the support layer to provide a polymer matrix layer; and    (2) a forward osmosis water treatment device, comprising a feed solution including impurities to be purified, an osmosis draw solution having a higher osmotic pressure than the feed solution, the separation membrane positioned so that one side contacts the feed solution and the other side contacts the osmosis draw solution, a recovery system configured to separate a draw solute from the osmosis draw solution, and a connector configured to reintroduce the draw solute of the osmosis draw solution separated by the recovery system back into the osmosis draw solution contacting the separation membrane.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of the separation membrane.    Separation membrane (10)    Support layer (11)    Skin layer (11a)    Porous layer (11b)    Polymer matrix layer (12)