Assembly for treating fluids, comprising: a support (12, 72) having a first planar surface arranged for backing support of a semi permeable membrane (11, 71) and a second surface, the first and second surfaces being arranged oppositely one another, at least one first compartment interposed between the first and second surfaces, the first compartment being arranged for conveying fluid, a plurality of first fluid passages extending from the first surface and being in fluid communication with the at least one first compartment, a first semi-permeable membrane (11, 71) attached to the first surface and covering a membrane area of the first surface enclosing the first fluid passages, wherein the at least one first compartment is in fluid communication with the first membrane through the first fluid passages, a first duct attached or attachable to the support (12, 72) and configured to be in fluid communication with the at least one first compartment, at least one second compartment (125, 725, 925) being arranged for conveying fluid and being separate from the at least one first compartment, wherein the second compartment (125, 724) is delimited by a wall (1210, 1220) comprising apertures fluidly communicating with an external side of at least one of the first and second surfaces (121, 122), and a second duct attached or attachable to the support (12, 72) and configured to be in fluid communication with the second compartment (125, 724, 725, 925), characterised in that the second compartment is interposed between the first and second surfaces, and wherein the second duct is fluid tightly sealed from the at least one first compartment, and the first duct is fluid tightly sealed from the at least one second compartment. a support (12, 72) having a first planar surface arranged for backing support of a semi permeable membrane (11, 71) and a second surface, the first and second surfaces being arranged oppositely one another, at least one first compartment interposed between the first and second surfaces, the first compartment being arranged for conveying fluid, a plurality of first fluid passages extending from the first surface and being in fluid communication with the at least one first compartment, a first semi-permeable membrane (11, 71) attached to the first surface and covering a membrane area of the first surface enclosing the first fluid passages, wherein the at least one first compartment is in fluid communication with the first membrane through the first fluid passages, a first duct attached or attachable to the support (12, 72) and configured to be in fluid communication with the at least one first compartment, at least one second compartment (125, 725, 925) being arranged for conveying fluid and being separate from the at least one first compartment, wherein the second compartment (125, 724) is delimited by a wall (1210, 1220) comprising apertures fluidly communicating with an external side of at least one of the first and second surfaces (121, 122), and a second duct attached or attachable to the support (12, 72) and configured to be in fluid communication with the second compartment (125, 724, 725, 925), characterised in that the second compartment is interposed between the first and second surfaces, and wherein the second duct is fluid tightly sealed from the at least one first compartment, and the first duct is fluid tightly sealed from the at least one second compartment.