Abstract The results of a quantitative description of activated sludge flocs exposed to anionic surfactant sodium dodecyl sulphate (SDS) are presented. Image analysis procedures are applied to measure the morphological parameters of activated sludge flocs. Surfactant influenced sludge flocs properties even at low concentration from 2.5 to 25 mg L−1. Mean projected area of flocs decreased by about 30% at SDS concentrations from 2.5 to 25 mg L−1 and by about 60% at concentrations from 250 to 2500 mg L−1. The decrease of sludge flocs dimensions deteriorates their settleability. Additionally, a linear correlation between total suspended solids (TSS) and mean projected area was found within the duration of the biodegradation process.