A unified basis for design and operation of biological waste treatment systems employing suspensions of microorganisms is developed from microbial kinetic concepts and continuous culture of microorganisms theory. Biological solids retention time, the average time period a unit of biological mass is retained in the system, is suggested as an independent parameter for process design and control. This time is functionally related to process performance and is a readily controlled operational parameter. Steady-state kinetic models are presented for three process configurations. Coefficient values are substituted into the models to determine minimum values Biological Solids Retention Time, for each process. Minimum values of Biological Solids Rention Time are then compared with values for actual treatment systems to identify the safety factors implicit in current design practice. (Author Modified Abstract)