An analysis of five sludge‐settling characteristics data sets is presented, representing data from 51 separate suspended growth biological wastewater treatment processes and including 214 separate runs and over 1 500 individual data points. The results indicate that the data can be characterized using a single relationship between the initial sludge settling velocity (Vi), the initial sludge concentration (Ci), and the sludge volume index (SVI). The predictive capability of this relationship is independently verified. These results suggest that this single relationship can be used to accurately characterize the settling characteristics of an activated sludge based only on the measured SVI. A comparison of alternate sludge‐settling indexes indicates that the predictive capability of the relationship can be further improved by use of the stirred SVI3.5analysis rather than the SVI. More data are required to refine relationships developed based on the SSVI3.5and the diluted SVI. Secondary clarifier‐operating diagrams based on the developed relationships are presented.