Hollow fine fiber vs. spiral-wound reverse osmosis desalination membranes part 2: Membrane autopsy

A comparative evaluation of a holloe fine fiber (HFF) and a spiral-wound (SW) membrane was carried out by operating them in parallel units of an RO pilot plant. The two units received the same brackish feed water, identical scale control treatment (H₂O+SHMP) and operated at the same recovery (70%). Unit #1, using the HFF membrane, was operated at a feed pressure of 27.6 bar (400 psig); unit #2, using the SW membrane, was operated at 15.2 bar (220 psig). The trial lasted 7500 h. The SW membrane produced water of excellent quality (86 ppm) vs. 470 ppm by the HFF membrane, and it consumed only half as much pumping energy, thereby outperforming the HFF membrane. Following the 7500 h trial, an autopsy of the two membranes was carried out to identify the scale deposits. Visual observation, scanning electron microscopy (SEM), X-ray diffractomery (XRD), inductively coupled plasma (ICP) spectroscopy, coulometry, photometric and gravimetric techniques were used. Both membranes were found in fouled condition. However, the HFF membrane was much more fouled. SEM and XRD analyses showed that 90-100% of the deposits were amorphous in nature. Results of the coulometric and photometric methods suggested that sodium hexa-meta-phosphate (SHMP) was effective in controlling the CaCO₃ scale but was itself responsible for the fouling of both membranes by reversion to orthophosphate (PO^3-₄). The ICP results showed that iron contributed to the fouling. Among the inorganic scales, calcium alumino silicate clay was the major scale; but the great bulk of the deposit on both membranes was organic matter or biomass.