Precipitation is similar to noncompressible gel formation in the sense that many of the same foulants are involved. In this case, however, fouling occurs through formation ofscales that decrease membrane permeability. It is generally agreed that the causal mechanism is exceeding the solubility of the foulants in water. Solution becomes supersaturated with the dissolved salts at the membrane surface as the filtration proceeds and results in the precipitation of salts. The time required for the feedwater to reach supersaturation at the membrane surface is known as induction time and these times are short for filtration systems with high recovery. It follows that precipitation is worst in a high-recovery membrane system. Among the species commonly encountered in precipitants are Ca , Mg , CO2~, SO4_, silicas, and most forms of iron. Scaling is usually prevented by acidifying the feedwater to prevent the precipitation of carbonates and by the use of antiscalants to prevent the precipitation of sulfates of Ca2+, Mg2+, and Sr2+ . Precipitation of iron on membranes is generally associated with Fe2O3 or Fe(OH)3 formed by condensation reactions of hydrolyzable Fe3+ ions present in the feed solution. Reactions appear to occur in both bulk and concentration polarization regimes. Precipitation of this kind is generally not a problem when concentrations are approximately 4mg/L and recovery is below 80%. When the recovery limit becomes 80—90% despite the presence of antiscalants, the supersaturation ratio becomes too large (e.g., up to 400% of calcium sulfate). Cases have also been reported in which antiscalants themselves have contributed to the fouling of the membranes. In some cases, reversing the flow before reaching the induction time of the system replaces the supersaturated brine at the exit with unsaturated feed and thus ''zeroes the induction clock'' .
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