Improving the permeate flux by unsteady operation of a RO desalination unit

Concentration polarization in reverse osmosis processes results in a lower production rate and a higher rate of fouling. In this paper a reverse osmosis water desalination unit having a small spiral-wound membrane was operated in a cyclic mode so as to reduce the effect of concentration polarization. Asymmetric square wave pressure pulses were employed in which the average pressure was always kept at a value of 25 bars. The effects of the time split (the duration of the higher operating pressure divided by the total period of the forcing wave) on the performance of the system was investigated. For the cases considered, cyclic operation lead to an increase in the permeate flux of about 6.5% over that obtained from steady-state operation. The maximum permeation rate was obtained at a time split gamma approximate to 0.6. As for the product quality, the concentration of the salt was not affected by the periodic mode of operation.