This paper discusses the possibility of applying the three “black-box” methods to sludge settleability predictions. Additionally, the impact of the load of biogenic compounds and parameters of activated sludge on predictive abilities of the devised mathematical models is analysed in the paper. To conduct analyses we relied on the results of measurements of wastewater quantity and quality, and of the bioreactor operational parameters, taken on continuous basis at the Sitkówka-Nowiny treatment plant in 2012-16. The analyses conducted for the study indicate that the lowest values of errors in activated sludge settleability predictions for the wastewater treatment plant of concern were obtained for input data on the load of biogenic compounds at the inflow, microorganism culture environment, and activated sludge concentration.