Characterization of foulants by autopsy of RO desalination membranes

A study was undertaken to identify various types of scales that were responsible for shortening the useful life span of the membrane permeators in a commercial reverse osmosis (RO) desalination plant. Compositions of the raw and treated feed water and of the reject brine were determined using the inductively coupled plasma (ICP) spectrometry and ion chromatography (IC). Various scaling index calculations showed that the feed and brine were non-scale forming with respect to CaCO₃ (calcite), SrSO₄, CaSO₄.2H₂O (gypsum), and silica (SiO₂). Two completely fouled membrane permeators, retired from stage 1 and stage 2 of a commercial plant, were subjected to membrane autopsy using scanning electron microscopy (SEM), X-ray diffractometry (XRD), optical microscopy (OM), and energy dispersive x-ray florescence (XRF). The deposits were predominantly amorphous in nature. The membrane autopsy showed that CaCO₃, SrSO₄, and CaSO₄.2H₂O (gypsum) scales did not constitute a serious problem in the plant. The advanced phosphonate+polyacrylate based scale inhibitor had itself formed Ca phosphonate sludge, but the amount was quite small. Though below saturation, silica is believed to have been precipitated due to the catalyzing effect of trivalent Al³⁺ and Fe³⁺ ions. Iron fouling was the major cause of reduced life span of the membranes and, to a lesser extent, calcium-alumino-silicates.