Method for the treatment of water by ballasted flocculation and gravity separation with variable function modes

A method of treating water comprising at least three different operation modes including a first mode of treating the water with a ballasted flocculation process and including injecting into the water granular ballasting material denser than the water, a coagulant, and a flocculant to yield treated water, separating flocs from the treated water by a gravity separation step and separating granular material from the flocs leaving sludge that is directed to a sludge treatment process;

• changing modes of operation by changing from the ballasted flocculation process to a second mode where contaminants in the water are separated by a gravity separation process without the use of coagulation and flocculation reagents;
• and changing modes of operation from the second mode to a third mode wherein the water is treated with at least a chemical coagulation or flocculation reagent, but without granular ballasting material;
• the changeover from one operating mode to another being determined based on at least one item of information related to the water at any step in the method of treatment and before the treatment.