A method of treating water including mixing the water with an inorganic powder reagent for reducing the content of organic matter. The water, after being mixed with the inorganic powder reagent, is subjected to a membrane separation step that produces blow off products that are then separated into two fractions: a first fraction containing a majority portion of the powder reagent in a first stream of water, and a second fraction containing a majority portion of inorganic matter not absorbed on the inorganic powder reagent rejected by the membrane separation step in a second stream of water.