Includes bibliographical references (pages 143-149).

Over the past decade, research has been conducted to produce a general body of conceptual principles applicable to design and operation of activated sludge processes. The aid of the report is to present these biological concepts in terms readily understood by engineering professionals. Generalized concepts of BOD exertion, the use of the measure of biologically available organic matter in a waste sample as a design and operational tool, the stoichiometry and mass balance concepts of treatment, and kinetic equations for microbial growth are presented. Design models are discussed, and a model for completely mixed reactors holding recycle solids, X(R), constant is recommended. Some guidelines for accommodation of various types of shock loadings are included. Concepts of oxidative assimilation and the multiple effects of solids concentration, nitrogen concentration, and detention time are related; a new activated sludge process (continuous oxidative assimilation) for nitrogen-deficient wastes is presented. Data supporting the concept of total oxidation are presented. Some possible flow diagrams for complete aerobic treatment (purification and sludge disposal) of metabolizable organic wastes are presented.