A method and system for retrofitting an existing water treatment system having one or more biological reactor systems and one or more gravity clarifiers. To retrofit the existing water treatment system, the one or more gravity clarifiers of the existing water treatment system are converted to one or more biological reactors. In addition, one or more high rate clarifiers (HRCs) are added to the existing water treatment system. In some cases one or more high rate clarifiers are added downstream of the converted biological reactors, in other cases one or more HRCs is added upstream of the biological reactors, and in other cases a number of HRCs are added, upstream and downstream with respect to the biological reactors.