Analytical study of optimum operating conditions in semi-batch closed-circuit reverse osmosis (CCRO)

Energy-efficient desalination technologies are of prime importance due to the continuously increasing global demand for freshwater and its associated carbon footprint. Membrane-based desalination technology with energy recovery devices shows much better energy efficiency than any other desalination process. However, for brackish water desalination as well as for small-scale seawater desalination plants, the energy recovery devices are not recommended due to high capital cost and low return on investment. A novel method called closed-circuit reverse osmosis (CCRO) was proposed in 2002 to reduce the unutilized energy in these systems. However, the dependence of the SEC on operational parameters in the CCRO process was not fully understood until now. We have demonstrated an analytical model of CCRO setup for domestic water desalination. If operated with continuously increasing pressure in such a way that the rate of permeation remains constant at an optimum value, the corresponding SEC line with respect to recovery will be parallel and slightly higher than the ther-modynamic energy limit line, and the system will be more energy efficient.