This article analyzes the effect that the physicochemical parameters of the wastewater flowing into a treatment plant have on the activated sludge settleability. The statistical analysis shows that as far as the technological parameters are concerned, the activated sludge sedimentation capacity is mostly affected by the biomass concentration in the chamber, whereas as for the abiotic factors, settleability is significantly determined by the season of the year and thus the temperature. With regard to the wastewater quality-related parameters, biological oxygen demand has the greatest effect on settleability. The conducted analyzes involved the development of statistical models to predict the activated sludge sedimentation capacity on the basis of multiple linear regression and genetic programming.