Precipitation of calcium carbonate in reverse osmosis retentate flow by means of seeded techniques — A tool to increase recovery

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that contains all rejected salts and impurities constitutes no more than 1% of initial feed water volume and can be withdrawn together with wet sludge as sludge moisture. © 2015 Elsevier B.V. All rights reserved.

Results of conducted studies could be used to introduce new water treatment techniques to directly treat natural water with high fouling and scaling potential using proposed “open-channel” spiral wound modules tailored by nanofiltration and reverse osmosis membranes. Elimination of the “dead areas” responsible for scale formation provides a new solution to increase recovery and utilize concentrate. A new technique is described that enables us to increase recoveries and utilize RO concentrate that consists of calcium carbonate precipitation on the “seed” crystal mass, withdrawal of excessive calcium carbonate and further blending of softened concentrate with RO product water. As a result, softened water is obtained with lower TDS and hardness than of the initial feed water. The developed techniques do not require chemicals and do not discharge concentrated brines. The increase of the seed mass increases Table 3 Main characteristics of feed water, product water and retentate on the first and second stage of membrane unit shown in Fig. 18. Characteristics Calcium, meq/l Alkalinity, meq/l TDS, ppm Flow rate, cubic meter per hour Feed water First stage product First stage retentate First stage flushing water Second stage product water (NF) Second stage concentrate (NF) First stage sludge Second stage sludge 3,1 0,05 6,0 0,06 12,2 12,2 6,9 – – 3,2 9,1 0,9 0,8 0,1 Fig. 19. A pilot double-stage membrane unit with 6 m3 per hour capacity, developed and built in conformity with flow diagram shown in Fig. 18.