Method using separation membranes to treat a fluid containing matter in suspension and in solution

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 a membrane backwash fluid, comprising the steps of

• choosing a backwash additive that is capable of being neutralized by the feed additive,
• collecting in a tank as a discharged backwash solution the backwash fluid which contains the backwash additive and has washed-away unused feed additive,
• allowing the discharged backwash solution sufficient contact time in said tank to enable neutralization to take place in said discharged backwash solution, and
• separating the resulting product into a residual product that is concentrated in solid matter and a fluid that is recycled and added to the fluid to be treated.