Abstract: Blow down water from an evaporative cooling system, such as a cooling tower, is treated for re-use as cooling make up water. Water in or entering the cooling system is treated with chemicals, for example corrosion inhibitors, scale inhibitors, or biocides. These chemicals are preferably chosen to be compatible with ultrafiltration (UF) and reverse osmosis (RO) membrane systems. Blow down water is removed from the cooling systems and filtered first through a UF membrane unit. The UF membrane unit removes total suspended solids (TSS), microorganisms and organic contaminants, among other things, from the blow down water. Permeate from the UF membrane unit is then filtered through a reverse osmosis (RO) membrane unit to remove, for example, divalent ions and hardness from the water. The RO membrane unit is preferably operated at a high cross flow velocity to reduce fouling. Permeate from the RO system is returned to the cooling system as make up water.