It is believed that the presence of calcium ions causes more severe organic fouling during nanofiltration, since they serve as bridges near the membrane surface between foulants and the surface through complexation. However, complexation can also occur in the solution (bulk complexation), leading to the formation of aggregates. This study used alginate as a model foulant to investigate the potential for bulk complexation to lower organic fouling. Results demonstrated that two regimes existed with regard to initial fouling, i.e., flux decline. As the concentration of calcium increases from zero, the initial rate of fouling increased until a critical calcium concentration was reached. Increasing the calcium concentration above this point decreased the initial rate of fouling. Analysis of the size distribution in the alginate solution revealed that at very high calcium concentrations, significant formation of aggregates occurred and reduced the amount of dissolved organic carbon. Thus, effective transportation of alginate toward the membrane and the fouling it causes occur more slowly. The accumulative total organic carbon at the membrane surface was similar to the initial fouling with respect to the concentration of calcium. This strengthens the slower effective transportation of alginate at very high calcium concentrations. Moreover, the stabilized flux and specific resistance of the fouling layer varied monotonically with calcium concentrations over all concentrations tested. Both depend on intermolecular forces between foulants, which is enhanced at increased calcium concentration. (C) 2011 Elsevier B.V. All rights reserved.