Enhancement of Productivity in Reverse Osmosis Desalination Processes

The steady-state and periodic performances of a seawater desalination unit based on a small-scale commercial spiral-wound membrane were studied. For the steady-state case, operating pressures ranging from 30 bar to 50 bar and temperatures ranging from 22°C to 28°C were investigated. As expected, increasing the pressure resulted in improvements in the water recovery, salt rejection and energy consumed by the feed pump per m of permeate produced. The improvements were larger for operation at low to moderate pressures than at high pressures. Both the permeation rate and salt rejection changed linearly with temperature; the permeate flux increased by 2.8%/°C whereas the salt rejection decreased by 0.007%/°C. For the case of unsteady-state operation, the operating pressure was varied according to a symmetric square wave function around an average pressure of 50 bar. The production rate increased as the period of the wave decreased. Such an improvement was obtained at the expense of a marginal increase in the total energy consumed. For a wave period of two minutes, the production rate increased by 8% over that obtained from steady-state operation while the energy consumed increased by a mere 0.15%. A simple theoretical analysis showed that a 12.3% improvement in the permeation rate would be obtained in the absence of concentration polarization. The salt rejection was not affected by this mode of operation.