The use of oxygen in the activated-sludge process has grown over the past decade from a position of relative obscurity and limited academic interest to a position today of occupying a prominent role as a secondary wastewater treatment process. The purpose of this two-volume series is to present a consolidated and comprehensive reference on oxygen-activated sludge technology. The subject matter is treated in considerable breadth and depth. The fundamental advantages of oxygen use and operation of the activated sludge process at high mixed-liquor dissolved oxygen levels is covered in detail. Many basic concepts of activated sludge system process design, equally applicable to both air and oxygen systems, are also included. The present status of the oxygen-activated sludge process is due in large measure to the development and commercialization of the UNOX System. This series is devoted almost exclusively to the UNOX oxygen-activated sludge system. Volume I of the series is divided into two parts which includes: (1) historical and background material relating to the use of oxygen and the development of the UNOX System, and (2) the basic process design considerations involved in oxygen-activated sludge system design. Volume II of the series is also divided into two parts consisting of (3) overall oxygenation system design considerations and additional applications, and (4) oxygen supply.