Electrodialytic boron removal from SWRO permeate

The electrodialytic removal of boron from seawater reverse osmosis permeate was examined in laboratory. Two model SWRO permeates were investigated. The composition of the first one (TDS 604 mg/L, B content 2.25 mg/L) was calculated using ROSA 6.0 software (DOW Chem.) assuming SW30HR-320 membrane, ocean seawater feed composition and 40% water recovery. The composition of the second one (TDS 400 mg/L, B content 1.3 mg/L) was calculated assuming higher membrane rejection. An ED unit, equipped with AMX and CMX Neosepta (Tokuyama Co.) membranes and 0.4 mm membrane-to-membrane distance, was applied. In both cases the ED processes were carried out to decrease the B content down to 0.4 mg/L. At the same time the RO permeate was almost completely deionized to diluate conductivity been ca. 0.7 μS/cm that is less than the distilled water conductivity. The boron current efficiency reached 10.4% and boron flux across membrane 15.2 μg/cm2 h in the first case and respectively: 4.6% and 11.3 μg/cm2 h in the second. The energy consumption was very low and equal to 0.237 kWh/m3 (0.186 kWh/m3) and the estimated cost of boron removal was found to be promising.

The possibility of electrodialytic boron removal form SWRO permeate down to the concentration of 0.4 mg/L was proved in the laboratory. This, however, requires high concentrate pH processing. The energy requirement and cost assessment showed that boron removal cost equals to $0.196/m3 at 1.3 initial boron content and $0.23/m3 at 2.25 mg B/L. This, however, high cost may be further decreased by applying cheap, but high in resistance membranes, since capital cost is the major component of the total boron removal cost. Unlike in our previous experiments, pH higher than 11.2 did not improve boron mass transfer much.