We analyze the filterability of sodium alginate solutions in different ionic environments as a function of the operational flux. The alginates serve as a model component for polysaccharides in feed water. Next to filtration characteristics, the fouling reversibility was studied by employing strictly controlled filtration and backwash operation in a fully automated laboratory-scale filtration setup. In the absence of calcium ions, the filtration resistance is a strong function of the filtration flux showing higher values with increasing flux. The filtration resistance is fully reversible at low filtration flux, while slow irreversible fouling occurs at high flux. Increasing the ionic strength by the addition of KCl increases the filtration resistance: electro-static repulsion between the macromolecules decreases causing a denser gel to develop. With the addition of calcium, filtration resistance decreases, which is related to the formation of agglomerates and a permeable gel layer. However, the addition of calcium also leads to irreversible fouling and can, eventually, even plug the lumen of the capillaries.