A collection of particles held together by different interparticle forces might eventually give rise to the formation of activated sludge flocs. This process is known as bioflocculation and is crucial for both conventional activated sludge systems and membrane bioreactors. Since industrial wastewater treatment plants generally face varying reactor loading rates due to varying production schemes in the facility, this paper investigates the impact of reactor loading rates on activated sludge bioflocculation. For this purpose, two reactors were initially operated at a nominal reactor loading rate (RLR) and afterwards changed to a high and low RLR. Based on the obtained results, it can be observed that sludge under low RLR conditions is prone to floc fragmentation due to an increase in water-soluble extracellular polymeric substances (EPS). The reactor under high RLR indicated increased floc erosion as a result of increased biomass concentration, which might imply more collisions between sludge flocs, releasing small sludge particles from the floc. In the high RLR reactor, no significant increase in EPS was observed. A distinction between the different (de)flocculation phenomena was made based on sludge volume index, effluent suspended solids and EPS data supplemented with microscopic image analysis. (C) 2012 Elsevier Ltd. All rights reserved.