A membrane filter unit comprising: (a) a filter case for housing a membrane; (b) an inner tube housing a wastewater chamber and a hollow core tube, a middle tube housing a pure water chamber and an outer tube housing a feed water chamber, wherein the inner tube, the middle tube and the outer tube are coaxially arranged and formed inside the filter case between the inner wall of the filter case and other components positioned within the filter case and inner tube is formed by at least a part of the central hollow tube and at least a part of the hollow core tube sealingly fitted with each other; (c) the hollow core tube having a top opening at a top end and a bottom opening at a bottom end, and having a water collecting channel on its outer wall and positioned coaxially inside the case, wherein the top opening is adapted to open into the wastewater chamber; and (d) a membrane having an inner peripheral surface, a top outer peripheral surface, a bottom outer peripheral surface and a side outer peripheral surface; and arranged around the core tube such that the inner peripheral surface of the membrane is spirally wound around the core tube and the side outer peripheral surface is capable of permeation of raw water through its surface; wherein the membrane has a bottom cover and a top cover, the top cover sealingly engaged with the top outer peripheral surface of the membrane and comprising a through central hole adapted for and sealingly fitted to the core tube to facilitate passage of waste water through the core tube into the waste water chamber; and the bottom cover engages with the bottom outer peripheral surface of the membrane having a bottom cover central hole spanning beyond the bottom opening of the core tube to allow passage of water from the outer tube through the membrane and the bottom cover central hole of the core tube and wherein the bottom cover of the membrane is capped by a bottom cap which sealingly engages with the bottom cover and the top cap of the filter case comprises an inner wall and an outer wall and wherein from the inner wall of the top cap two hollow concentric tubes descend downwardly, a central hollow tube and a peripheral hollow tube each having respective inner walls and outer walls. (a) a filter case for housing a membrane; (b) an inner tube housing a wastewater chamber and a hollow core tube, a middle tube housing a pure water chamber and an outer tube housing a feed water chamber, wherein the inner tube, the middle tube and the outer tube are coaxially arranged and formed inside the filter case between the inner wall of the filter case and other components positioned within the filter case and inner tube is formed by at least a part of the central hollow tube and at least a part of the hollow core tube sealingly fitted with each other; (c) the hollow core tube having a top opening at a top end and a bottom opening at a bottom end, and having a water collecting channel on its outer wall and positioned coaxially inside the case, wherein the top opening is adapted to open into the wastewater chamber; and (d) a membrane having an inner peripheral surface, a top outer peripheral surface, a bottom outer peripheral surface and a side outer peripheral surface; and arranged around the core tube such that the inner peripheral surface of the membrane is spirally wound around the core tube and the side outer peripheral surface is capable of permeation of raw water through its surface; wherein the membrane has a bottom cover and a top cover, the top cover sealingly engaged with the top outer peripheral surface of the membrane and comprising a through central hole adapted for and sealingly fitted to the core tube to facilitate passage of waste water through the core tube into the waste water chamber; and the bottom cover engages with the bottom outer peripheral surface of the membrane having a bottom cover central hole spanning beyond the bottom opening of the core tube to allow passage of water from the outer tube through the membrane and the bottom cover central hole of the core tube and wherein the bottom cover of the membrane is capped by a bottom cap which sealingly engages with the bottom cover and the top cap of the filter case comprises an inner wall and an outer wall and wherein from the inner wall of the top cap two hollow concentric tubes descend downwardly, a central hollow tube and a peripheral hollow tube each having respective inner walls and outer walls.