Algae are commonly aquatic plants showing generally negatively charged. The fouling behavior of hollow-fiber ultrafiltration membrane by algal-rich water could be therefore influenced by various cations (K+, Ca2+, and Al3+) and their doses. Microcystis aeruginosa solution was used as the feed solution to study the fouling in detail. Constant flux experiments were performed with a laboratory-scale experiment. Increasing the concentrations of calcium and aluminum had a significant impact on alleviating membrane fouling and increasing flux recovery by backwashing, especially for calcium. However, kalium ion had little influence on the membrane filtration. Based on the measurement of MW distribution and zeta potential, charge neutralization was proposed to be the primary aggregation mechanism for calcium, in contrast to precipitate coverage and sweep flocculation for aluminum. It was demonstrated that the fouling layer became more loose and porous in the presence of Ca2+ and Al3+, respectively, which can be identified by scanning electron microscope (SEM). However, SEM also proved that the two additives damaged the cell walls and caused the intercellular organic matter released to different extents. (C) 2013 Elsevier Inc. All rights reserved.