BACKGROUND: Technologies based on aerobic granular biomass are presented as a new alternative application to wastewater treatment due to its advantages in comparison with the conventional activated sludge processes. However, the properties of the aerobic granules can be influenced by the presence of residual amounts of coagulant-flocculant reagents, frequently used as pre-treatment before the biological process. In this work the effect of these compounds on aerobic granular biomass development was tested. RESULTS: The presence of coagulant-flocculant reagents led to a worse biomass retention capacity with a lower VSS concentration compared with a control reactor (4.5 vs. 7.9 g VSS L-1) and with a higher SVI (70 vs. 40 mL [g TSS]-1) and diameter (5.0 vs. 2.3 mm). These reagents also caused a decrease in the maximum oxygen consumption rate, but the removal efficiencies of organic matter (90%) and nitrogen (60%) achieved were similar to those in the control reactor. CONCLUSION: The continuous presence of residual levels of coagulant-flocculant reagents from the pre-treatment unit negatively affected the formation process and the physical properties of the aerobic granules; however, the removal of organic matter and nitrogen were not affected. Copyright (c) 2012 Society of Chemical Industry