Synthetic surfactants are believed to act on the structure of activated sludge flocs and some of them are toxic towards aquatic organisms. Nevertheless, the number of studies concerning their effect on the activity of sludge biomass and flocs morphology is limited. In this work, the removal of two different nonionic surfactants: alcohol ethoxylate and alkyl phenol ethoxylate in the activated sludge wastewater treatment processes was investigated. In contrast to our previous study, in which the influence of anionic surfactants on flocs morphology was the main aim, in this paper also the chosen aspects of the kinetics of the tested processes were included. Moreover, the influence of two most common groups of synthetic surfactants: anionics and nonionics, on the activated sludge flocs and biomass is compared and discussed. The presence of nonionic surfactants in wastewater at the concentration of 50 mg L-1 in the influent causes a decrease of flocs size when the dilution rate is equal to or higher than 0.102 h(-1). The removal rate of organic pollutants for both nonionics tested is lower than in the control run for this range of dilution rates and so is the degree of COD removal, which then does not exceed 80%. The affinity of substrate to the biomass, expressed as a COD half-saturation constant, decreases in the presence of synthetic surfactants, as this constant is elevated in all surfactant runs. The strongest effect was exerted by the anionic surfactants: sodium alkyl benzene sulphonate, while nonionics acted moderately on the affinity of the substrate. (c) 2006 Elsevier Inc. All rights reserved.