Reverse osmosis (RO) membrane water permeability is measured under cyclic pressure loading conditions corresponding to batch and semi-batch operation modes. Due to the viscoelasticity of the membrane support layer, compaction effects carry forward across multiple pressurization cycles, with the intracycle behavior stabilizing after a large number of cycles. The average membrane permeability of semi-batch RO systems is lower than that of batch RO, since semi-batch RO spends a larger fraction of its cycle time at higher pressures. The instantaneous permeability in batch RO decreases during the cycle as pressure is increased, but at a lower rate than the steady-state permeability variation with applied pressure. Therefore, the batch permeability values are lower at low pressures and higher at high pressures than the corresponding steady-state values. Since more water is recovered in the initial low-pressure stages of a multi-stage RO system, compaction increases the specific energy consumption of batch RO more than that of staged systems. Other loss mechanisms such as channel and minor pressure losses and high-pressure motor+pump efficiency variation with load further result in batch RO becoming energetically inferior to two- or three-stage RO, especially for low-salinity, high recovery applications.
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