The HFF and SW membrane permeators were installed in parallel units of a reverse osmosis (RO) pilot plant. Brackish feed water to the two units was treated with identical scale control treatment consisting of 6 ppm of sodium-hexa-meta phosphate (SHMP) and about 46 ppm (98% basis) of sulfuric acid. The pH of the acidified feed was about 6.4. Unit #1, employing the HFF membrane, was operated at feed pressure of 27.6 bar (400 psig) and unit #2, employing the SW membrane, was operated at 15.2 bar (220 psig). The water recovery in both units was 70%. The pilot plant trial lasted for 7,500 h. The techno-economic evaluation showed that performance of the SW membrane was far superior; its product TDS in 7500 h was 86-98 ppm whereas that of the HFF membrane increased from 80 ppm to 468 ppm. Water output of both membranes declined appreciably, but output of the SW membrane declined 12% less. The average electrical energy consumed by high pressure pump of the SW membrane was nearly half the energy consumed by the HFF membrane (0.70 kWh/m(3) product vs. 1.51 for HFF). Variations in the pressure drops across the HFF and SW permeators were modest (5-10 psi) due to amorphous nature of the scale deposits found on fibers of both membranes and due to significant decrease in feed flows experienced by the permeators. The pretreatment, the scale control treatment cost and water recovery being the same for the two units, the SW membrane produced excellent quality water (86 ppm) at half the electrical energy cost compared with the HFF membrane which produced 468 ppm water. The SW membrane out-performed the HFF membrane.