A method and composition for processing natural water such as seawater, brackish, and estuarine water wherein the water is concentrated with or without heating comprising adding to the water (I) a water-soluble polymeric phosphonate and (II) a water-soluble polycarboxylate to control scale formation and deposition. Optionally a water -soluble poly[isopropenylphosphonic acid] can also be utilized as component (I) without the addition of component (II).