The influence of feed pH on the performance of a reverse osmosis membrane during alginate fouling

The performance of a brackish water reverse osmosis membrane in the presence of a model polysaccharide foulant, sodium alginate, is investigated. The concentration of foulant, the feed pH and the presence of calcium are all shown to impact upon this performance. The flux decline is greatest at low feed pH, high calcium concentrations and high foulant concentrations. Sodium and chloride rejection falls as foulant concentration increases, due to cake-enhanced osmotic pressure. Effects are again greatest at low feed pH and high calcium concentrations. In part, this reflects a shift of the isoelectric point of the membrane from 4.3 to below 3.0 as alginate is added, reflecting the lower pKa of the alginate foulant. Conversely, calcium rejection improves in the presence of even small quantities of alginate foulant at all pH values. These increases in rejection are believed to relate to a direct interaction between the dissolved ions and the alginate which act to sequester the salt.