Process for treating water filled with dissolved or suspended colloidal impurities, in a treatment plant, including steps consisting of: - placing together in a flocculation zone, said water, at least one ballast constituted by at least one insoluble granular material that is heavier than water and at least one flocculating reagent so as to enable the floc to form; - introducing the mixture of water and floc thus formed into a settling zone; - separating the water treated in the top portion of said settling zone from a sludge and ballast mixture resulting from the settling of said floc; - extracting the mixture of sludge and ballast in the bottom portion of said settling zone and routing it to an agitated intermediate mixing zone; - extracting the mixture of sludge and ballast present in said intermediate mixing zone and subjecting it to a step of sludge/ballast separation by hydrocyclone separation; - recycling the underflow of the hydrocyclone separation step in said flocculation zone; - extracting a part of the sludge coming from the overflow of the hydrocyclone separation step and recirculating the other part of said sludge in said agitated intermediate mixing zone; characterised in that it includes: - a step consisting of continuously measuring at least one parameter representing the concentration of impurities in the water before or when it enters said flocculation zone; - a step consisting of using the results of said measurement thus performed so as to continuously deduce the amount of ballast that needs to be implemented in order to obtain treated water of a predetermined quality. - a step consisting of continuously measuring the ballast concentration in the mixture extracted from said settling zone or in the mixture present in said flocculation zone; - a step consisting of deducing, from the continuous measurement of the concentration of mixture extracted from said settling zone, the concentration of ballast actually present in said plant; - a step consisting of refilling the flocculation zone with ballast when said concentration of ballast actually present in said plant is lower than a predetermined threshold. - placing together in a flocculation zone, said water, at least one ballast constituted by at least one insoluble granular material that is heavier than water and at least one flocculating reagent so as to enable the floc to form; - introducing the mixture of water and floc thus formed into a settling zone; - separating the water treated in the top portion of said settling zone from a sludge and ballast mixture resulting from the settling of said floc; - extracting the mixture of sludge and ballast in the bottom portion of said settling zone and routing it to an agitated intermediate mixing zone; - extracting the mixture of sludge and ballast present in said intermediate mixing zone and subjecting it to a step of sludge/ballast separation by hydrocyclone separation; - recycling the underflow of the hydrocyclone separation step in said flocculation zone; - extracting a part of the sludge coming from the overflow of the hydrocyclone separation step and recirculating the other part of said sludge in said agitated intermediate mixing zone; - a step consisting of continuously measuring at least one parameter representing the concentration of impurities in the water before or when it enters said flocculation zone; - a step consisting of using the results of said measurement thus performed so as to continuously deduce the amount of ballast that needs to be implemented in order to obtain treated water of a predetermined quality. - a step consisting of continuously measuring the ballast concentration in the mixture extracted from said settling zone or in the mixture present in said flocculation zone; - a step consisting of deducing, from the continuous measurement of the concentration of mixture extracted from said settling zone, the concentration of ballast actually present in said plant; - a step consisting of refilling the flocculation zone with ballast when said concentration of ballast actually present in said plant is lower than a predetermined threshold.