Identification of scale deposits through membrane autopsy

Abstract

A polyacrylate and hydroxyethylidene diphosphonate (HEDP) based advanced anti-scalant was tested against the conventional H 2SO 4 and sodium hexa-meta-phosphate (SHMP) inhibitors in a pilot plant which had parallel RO units, each fitted with identical hollow fine fiber (HFF) permeators which received same brackish feed water. At the end of the 3000 h trial, product TDS of unit #2 operating on the conventional treatment was only 180 ppm vs 620 ppm for unit #1 operating on the advanced anti-scalant, and it produced an average of 20% more product water. To identify the scale deposits, autopsy of the first and second stage permeators was carried out using visual observation, scanning electron microscopy (SEM), x-ray diffractometry (XRD), x-ray fluorescence (XRF), inductively coupled plasma (ICP) spectroscopy, ion chromatography (IC), and coulometry techniques. Whereas thick deposits of soft and non-adherent types of scales were found on the permeator fibers of unit #1, no scale was evident on unit #2 permeator fibers and no samples could be collected from them. Analyses of the solids removed from the unit #1 fibers revealed that bulk of the deposited scale was amorphous in nature which the SEM and XRD techniques were unable to charaterize. The XRF, ICP, and IC results suggested that the amorphous sludge was predominantly alumino-silicate type. In addition, membrane fibers of stage 1 had calcium/magnesium phosphonate, formed by the advanced anti-scalant itself by reacting with Ca 2+ and Mg 2+ of the brine. No CaCO 3, SrCO 3, CaSO 4, SrSO 4, or iron-based scales were detected, indicating that the advanced anti-scalant was effective against these scales.