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, CaSO4.2H20 (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 CaCO 3, SrSO4, and CaSO4.2H20 (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 A13+ and Fe3+ ions. Iron fouling was the major cause of reduced life span of the membranes and, to a lesser extent, calcium-alumino-silicates.

1. The scaling indexes, calculated from the analytical compositions of the raw feed water and reject brine, showed that, up to 75% water recovery, the feed water and brine were non-scale forming with respect to CaCO3, CaSO4.2H20 , SrSO4, and SiO 2. 2. Examination by scanning electron microscopy, x-ray diffraction, and optical microscopy showed the deposits to be of an overwhelmingly amorphous nature. 3. The elemental analysis by x-ray fluorescence showed that iron was by far the most predominant constituent of the deposits. The elements next in abundance were Si, Ca, AI, and P. 4. The source of phosphorus-based scale was the advanced phosphonate+polyacrylate based scale inhibitor which had itself formed highly insoluble Ca phosphonate sludge. However, the amount of the phosphonate sludge was quite small. 5. The scale due to silica, whose concentration in the reject brine was below saturation level, is believed to have been caused under the combined effect of aluminum and iron, resulting in highly insoluble calcium-alumino-silicates. 6. CaCO3, SrSO4, and CaSO4.2H20 (gypsum) did not constitute a serious scale problem in the RO plant. 7. Iron fouling was the main cause of the reduced life span of the permeators and, to a lesser extent, calcium-alumino-silicates.