Floc characteristics such as settling velocity, concentration factor and floc size were studied for five different flocculation modes for Chlorella: aluminum sulphate, electro-coagulation-flocculation, chitosan, cationic starch and pH induced flocculation. These floc characteristics were influenced by the flocculation mode, which depends on the coagulation mechanism: adsorption - charge neutralization, sweeping or bridging. Secondly, the influence of the presence of AOM was evaluated. The presence of AOM led to an increase between 1.5 and 5-fold in needed flocculant dosage for all flocculation modes. This resulted particularly in a comparable decrease of the concentration factor. The floc characteristics upon flocculation using cationic starch were least affected by the presence of AOM, while flocculation using chitosan was most affected. The impact on floc characteristics is an important parameter next to flocculation efficiency to consider in the assessment of flocculation-based harvesting of microalgae. (C) 2013 Elsevier Ltd. All rights reserved.