The filterability and the associated structure of fouling layers formed from calcium alginate coexisting with ferric iron were investigated in this study. Filterability was assessed by using both specific resistance and material properties. The change of ferric iron from more "fresh" to more "aged" forms was considered. The ferric iron molar content was varied from 0% to 100% of calcium ions which were just sufficient for gel layer formation (2.53 mmol per grain alginate). Results showed that ferric content higher than similar to 30% could substantially increase the fouling layer filterability, although the influence by fresh ferric was much more significant than the aged due to the coagulating effect exerted by the former. The aged ferric iron was relatively inert and believed to have less interactions with the calcium alginate gel layer. When the ferric iron content was sufficiently low (i.e. no more than 15% of the calcium content), little influence on the fouling layer filterability was observed. The aging of ferric iron in the alginate dispersion had significant impact on fouling layer filterability in a time scale of days. The difference in filterability of various fouling layers was mostly attributed to the difference in physical structure. SEM observation of the freeze-dried specimen showed that the gel layer had a very porous structure with isolated cavities by thin films that may effectively obstruct water passage. In comparison, the cake layer was composed of inter-connected flakes which were favorable for water passing through during filtration. In many of the conditions tested, a combined fouling layer was formed consisting of both cake and gel substructures. (C) 2015 Elsevier B.V. All rights reserved.