Investigations with three laboratory-model aerated lagoons proved that incompletely mixed (or aerobic–anaerobic) aerated lagoons can be rationally designed by using the Wehner–Wilhelm equation or Thirumurthi chart, which graphically depicts that equation for applications in biological wastewater treatment. Whereas the biological factors (such as temperature and wastewater quality) are delineated by a first-order biochemical oxygen demand (BOD) removal coefficient (K), the physical–hydraulic regimes (such as mixing pattern and depth–width ratio of lagoon) are represented by a dispersion index. The key to the design is the standard BOD removal coefficient (Ks), which is evaluated to be 0.29/day for municipal wastewater at a standard temperature (T) of 20 °C and organic load (L) of 20 g∙m−3∙day−1. When the actualTandLof a lagoon, however, are other than the standard values, appropriate corrections should be assigned toKsto obtainK. Two equations for such corrections are provided.Two companion papers of previous studies demonstrated the successful application of the same method for designing waste stabilization ponds and completely mixed (aerobic) aerated lagoons; the present study accomplishes the same for incompletely mixed lagoons. An example design is included; the technique, however, needs to be verified by investigating the actual, field lago