The fouling characteristics of ultrafiltration (UF) hollow fibers are studied in dead-end mode with and without periodic backwashing by filtering dilute mixtures of humic acids (HA) and sodium alginate (SA) a polysaccharide in the presence and absence of calcium ions The fouling behaviour of the HA/SA mixtures (regarding reversibility by backwashing and flux recovery) is quite similar to that of polysaccharides alone even when the SA proportion is relatively small The organic species rejection in filtration of HA/SA mixtures is greater than that for separate HA or SA filtration increasing with increased polysaccharides concentration in solution In the presence of Ca2+ the cake filtration mechanism dominates some deviation at the start of filtration especially with HA-rich mixtures is attributed to membrane pore constriction possibly caused by small molecules not retained by the developing cake The specific cake resistances of HA/SA mixtures are relatively high and tend to be much closer to those of SA deposits than to HA layer resistances The reversibility of mixed fouling layers and the very high flux recovery by backwashing are attributed to their coherent gel-like structure which is a typical feature of polysaccharides (C) 2010 Elsevier BV All rights reserved