Dynamics of an osmotic backwash cycle

Recent studies have provided preliminary indication that an osmotic-induced backwash of RO membranes may offer an innovative, effective and potentially chemical-free cleaning method. In this paper, we present numerical simulations of the two-dimensional, transient concentration field during an osmotic backwash event. Presented results illustrate the dynamics of the de-polarization process, permeation rate and characteristic time-scales, as influenced by various transport mechanisms present. Different possible configurations for inducing the osmotic backwash are considered, illustrating possible advantages and shortcomings. For a backwash cycle initiated by reduction of the trans-membrane pressure, it is shown that during short times, the backwash process is only weakly affected by the presence of a crossflow velocity, whereas it is this axial advection mechanism which strongly influences the permeation rate at longer times. For an osmotic backwash induced by injection of a high concentration ‘draw’ solution, it is shown that the pulse duration should be longer than the residence time for a maximum achievable cycle-averaged permeation rate. A shorter pulse is significantly diluted, particularly on the membrane surface, to the point where its concentration may drop below that required for inducing osmotic flow. Consequently, the pulse concentration and duration must be carefully optimized if efficient osmotic cleaning is to be achieved throughout the full length of a membrane train.

Research highlights▶ Osmotic permeation rates are generally comparable or higher than corresponding RO permeation rates. ▶ At short times, the permeation rate is weakly affected by the presence of crossflow. ▶ The presence of crossflow strongly affects long time osmotic permeation rates. ▶ When inducing an osmotic backwash through an injected ‘draw’ solution, a longer pulse is preferable in order to ensure its effectiveness along the full length of the membrane train.