Abstract: A method of treating a fluid containing matter in suspension and/or in solution by using separation membranes, in which method a "feed" additive for adsorption and/or filtration purposes is added to the fluid to be treated, and a "backwash" additive for the purpose of improving backwashing is added to the membrane backwash fluid, wherein a backwash additive is chosen that is capable of being neutralized by the feed additive, the backwash fluid which contains the backwash additive and which has washed away unused feed additive is collected as a backwash discharge, the backwash discharge is allowed sufficient contact time to enable neutralization to take place in said discharge, and the resulting product is separated into a residual product that is concentrated in solid matter and a fluid that is recycled to the inlet of the installation.