A method and system for treating water or wastewater to remove both soluble BOD and suspended solids. The method entails a ballasted flocculation process where a flocculant and insoluble granular material is added to the water or wastewater to be treated causing solids in the water or wastewater to aggregate around the granular material to form ballasted floc particles. In a settling zone, ballasted floc particles form settled sludge and the settled sludge is separated from clarified effluent. At some point in the process, activated sludge, in the form of return activated sludge or mixed liquor, is mixed with the water or wastewater being subjected to the ballasted flocculation process. The addition of activated sludge enables the process to remove soluble BOD.