Engineers involved with the preliminary design of an activated sludge waste treatment system must efficiently evaluate the economic impact of many alternative process schemes. Optimization of aeration and final settling tank sizes requires that proper consideration be given to the thickening function of the final settler. Design graphs are presented to facilitate a rational determination of process unit sizes taking into account the performance interaction between the aeration and final settling tanks. The graphs for aeration tank performance are based upon mean cell residence time as the main biological process parameter, while those describing final settler operation incorporate the solids flux concept with the subsidence velocity expressed as a logarithmic function of the solids concentration. In order to aid the engineer in the use of the charts, values of the parameters characterizing microbial growth and sludge settling, as found in the literature for various industrial and municipal wastes, are presented in tabular form.