process employing the technique of ballasted flocculation with a step of injection , into the liquid to be clarified , of a ballast consisting of a granular material and of a polymer , followed by a flocculation step which leads to the formation of a ballasted floc , and by an accelerated settling step , said ballast being recirculated into the flocculation step , either as it is or after cleaning , wherein said process includes two recirculation regimes : a high - energy regime ensuring draw - off of the ballast contained in the ballasted floc , removal of the floc in the form of sludge and recycling of the cleaned ballast into the flocculation unit and , a low - energy regime ensuring transfer of the ballasted floc into the flocculation unit as long as the ballast ratio in the flocculator is greater than a predetermined level , this low - energy regime remaining in service while the high - energy regime is being put into action , but then being shut down during the operation of the high - energy regime , the two systems being employed alternately so as to maintain the ballast ratio at a predetermined level .