When silica scales an RO membrane, aluminum is almost always mixed in. This study shows that even trace aluminum reacts with silica to trigger heavy scaling, and that only a couple of antiscalants can actually stop it.
Abstract
Silica scaling is very common in reverse osmosis systems that operate at high recoveries. Whenever silica fouling/scaling occurs, autopsy results almost always reveal the presence of aluminum. The problem is most prominent in membrane plants that use aluminum coagulants, but is also found in plants with non-detectable levels of feed water aluminum.
A review of the literature found only a single study about aluminum silicate fouling in Reverse Osmosis (RO) and Nanofiltration (NF) membrane systems. It theorized that aluminum silicates that can form under the ambient conditions of an RO system would be Kaolinite and Muscovite. These clays are both crystalline and have ratios of 1:1 Al:Si. However, aluminum can interact with silica in many different ways. Its hydroxide can act as a substrate for silica polymerization on the membrane surface and aluminum ions can also adsorb onto surface sites when silicic acid and silica oligomers polymerize.
Numerous laboratory experiments were performed using various dissolved aluminum concentrations with reject silica concentration maintained at 167 ppm. The pH of the solution was always in the range of 7.5-8.5; typical of concentrate pH in the tail element of RO or NF systems operating without acid dosing. The precipitates were filtered and analyzed by SEM/EDS and Prismatic Elemental Mapping (PEM) to determine their morphology and stoichiometry. The results were then compared against SEM/EDS analysis of a membrane foulant from a plant that had experienced co-fouling of silica and aluminum.
This paper investigates the types of silica fouling that would occur in the presence of aluminum carryover from coagulant dosing, and evaluates multiple types of antiscalants to determine if such fouling can be prevented.
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