Method for treating water by advanced oxidation and ballasted flocculation, and corresponding treatment plant

The invention relates to a method for treating water charged with colloidal impurities, either dissolved or suspended, in a treatment plant, wherein the method comprises: the step of contacting the water, in an advanced oxidation area (10), with hydrogen peroxide in the presence of at least one transition-metal salt; a flocculation step that comprises contacting the water, in a flocculation area (12), with at least one flocculation additive and with at least one ballast comprising at least one non-soluble granular material denser than water and used as a biomass carrier; the step of feeding the water and floc mixture thus obtained into a settling area (14); the step of separating the treated water at the upper portion of the settling area (14) from a mixture of sludge and ballast resulting from the settling of the flocs; the step of extracting the sludge and ballast mixture at the lower portion (23) of the settling area (24); and the step of recycling at least a portion of the sludge into the advanced oxidati