A method and system for treating water or wastewater to remove both soluble BOD and suspended solids. The method entails directing first and second water or wastewater streams to a treatment system including an activated sludge system and a ballasted flocculation system. The first water or wastewater stream is directed to the activated sludge system which includes at least one reactor and a clarifier. The second water or wastewater stream is directed to a ballasted flocculation system. Mixed liquor or activated sludge is directed from the activated sludge system to a mixing chamber that is located upstream from the ballasted flocculation system. The water or wastewater of the second stream is mixed with the mixed liquor or activated sludge in the mixing chamber. The mixed liquor from the mixing chamber is directed to and through the ballasted flocculation system. In the end, the soluble BOD in the second water or wastewater stream is reduced and suspended solids in the water or wastewater are also reduced.