The present work is aimed at development of a new approach to reduce RO concentrate flow and to increase recovery. To increase recovery values, a new process is developed that use continuous crystallization of calcium carbonate at a constant supersaturation level provided by a constant raise of retentate TDS while RO is operated in circulation mode. The described techniques enable us to remove calcium carbonate from RO concentrate by means of "seed" crystallization. These proposed techniques use "open channel" spiral wound membrane modules that can directly treat water with high scaling and fouling potentials without pretreatment. The experimental procedure is described and experimental plots are presented that describe precipitation kinetics. The test membrane unit was operated in circulation mode and recovery values reach 95% or higher. RO concentrate constantly passed through the precipitation reactor where seed crystals were contained. Seed crystal formation was initiated by injecting caustic solution to RO concentrate. The concentrated solution that contains all rejected salts and impurities constitutes no more than 1% of initial feed water volume and can be withdrawn together with wet sludge as sludge moisture. (C) 2015 Elsevier B.V. All rights reserved.