Bicultural, Two-Stage, High-Rate Activated
Sludge Process
SHYAM N.S. DIXIT, Research Engineer
Dearborn Chemical
Lake Zurich. Illinois 60047
JAMES W. PATTERSON. Chairman
Department of Environmental Engineering
Illinois Institute of Technology
Chicago. Illinois 60616
INTRODUCTION
Activated Sludge Systems
Although aeration had been used by earlier investigators for the odor control or to
achieve partial treatment, recognition for discovering the phenomenon that eventually led
to the development of activated sludge, generally goes to Arden and Locket (I).
Over the years, better understanding of the activated sludge system has evolved due
primarily to an improved knowledge of the fundamental theory involved and to experience
accumulated in successful operation of the process. As a result, today, this process has
become the most versatile and economical biological treatment system available to the
engineer engaged in designing treatment facilities for removal of soluble biodegradable
organic matter present in the waste streams.
Historically, this process first gained popularity with larger cities where the ratio of
river assimilative capacity to waste load was small. However, due to more stringent
requirements of regulatory agencies (local, state and federal), in recent years, this process is
gradually being adopted by smaller communities as well as industries. Industrial and
community wastewaters may differ greatly with respect to unifoi mity of flow and the types
of waste matter contained therein. Because of this difference, in the last two decades, the
original concept of the activated sludge system has undergone several specific variations so
that some variants of the activated sludge systems are better suited than others in the
treatment of specific wastes.
Some examples of modified activated sludge systems are (2): I) Conventional; 2) Step
aeration: 3) Contact stabilization: 4) Complete mix or high-rate; 5) Modified aeration: 6)
Two-stage; and 7) Pure oxygen activated sludge.
Initially, the basic design criteria for conventional activated sludge plants as well as for
various modifications, were developed through trial and error procedures by plant
operators. In the majority of applications, these design and operational criteria are still in
existence, and adhered to by regulatory agencies. The basic parameters of concern in the
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