Results of a comparative study on the removal of anionic surfactants in the activated sludge flow system are presented. Three anionic surfactants: sodium dodecyl sulphate, sodium dodecylbenzene sulphonate and sodium alkyltrioxyethylene sulphate of different chemical structure were studied. All tested anionics strongly influenced the activated sludge flocs morphology and activity at the dilution rate above 0.05 h(-1). As a result of saponification processes the sludge flocs became smaller and more circular in comparison to flocs, which had not been exposed to anionic surfactants. Also dehydrogenase activity of the activated sludge decreased with an increase of the dilution rate in all surfactant runs. Among the tested anionics, sodium dodecylbenzene sulphonate, which belongs to linear alkylbenzene sulphonates, exerted the strongest saponificative effect on sludge flocs and dehydrogenase activity of microorganisms. (c) 2005 Elsevier Inc. All rights reserved.