Chemical Engineering and Processing, Vol.131, 84-91, 2018
Intensification and energy minimization of seawater reverse osmosis desalination through high-pH operation: Temperature dependency and second pass implications
Operation of seawater reverse-osmosis (RO) desalination using high flux membranes at pH > 9 was shown to be an energy-efficient approach for single-pass boron removal. This operational approach was previously studied only at 25 degrees C, however since RO desalination is highly temperature-dependent, the current work tested the high pH approach throughout the typical temperature range of the Mediterranean Sea. Since total dissolved solids (TDS) removal in a high-flux-membranes single-pass is limited, the effect of applying a 2nd RO pass was also examined. Finally, the process cost was assessed at varying operational conditions. Results showed 1st pass permeate TDS to be similar to 440, 290 and 200 mg/l at 54% recovery-ratio and 31, 25 and 15 degrees C, respectively. Boron removal was adequate (i.e. < 0.4 mgB/l in permeate) at pH 9.5 throughout the temperature range. However, at 31 degrees C antiscalant had to be added to prevent Mg(OH)(2) scaling. TDS and boron concentrations in the 2nd-pass permeate met the threshold limit (30 mg/l and 0.5 mgB/l) at 90% recovery ratio. The cost of applying the single pass alternative process was lower by about $0.03/m(3) permeate than the conventional alternative, for temperatures lower than 28 degrees C. The addition of a second pass increased the operational cost merely by a similar to$0.03/m(3), making the described two-pass process competitive.