Hydraulic and Process Aspects of Reactor
Design II - Response to Variations
JAMES C. McLELLAN, Senior Project Chemical Engineer
Amoco Chemicals Corporation
Whiting, Indiana
ARTHUR W. BUSCH, Professor
Laboratory of Environmental Science and Engineering
Rice University
Houston, Texas
INTRODUCTION
Current design procedure for the activated sludge biological waste treatment
process is based on steady state analysis. In most wastewater treatment plants, the
waste flow and influent concentration will vary with time and even if equalization
tanks are used ahead of the aeration tank the quality of the effluent produced may
also vary. It would be advantageous to be able to predict the effluent quality which
will be produced under any waste loading conditions and to investigate methods of
controlling wide variations in treatment effectiveness.
In this paper, only the treatment of soluble organic wastewaters is considered.
The term activated sludge "process" refers only to the biological oxidation step. The
term activated sludge "system" is reserved for the total treatment plant including
physical separation facilities for settling microorganisms and returning them to the
"process."
Process effluent quality is evaluated in terms of soluble organic carbon content.
The "system" performance reflects the capability to attain settling or separation of
the microorganisms from the "process" effluent. There is a distinct difference between maintaining a good effluent in terms of soluble organic carbon and in retaining
biota in the system.
The importance of the hydraulic regime of the activated sludge process has
been evaluated both experimentally and theoretically (1, 2, 3, 4). Although the
theoretical analyses lead to different conclusions, authors often state that from the
standpoint of operational stability, a completely mixed one-aeration-tank activated
sludge system has definite advantages. The most common advantage specified for
complete mixing is the ability to handle shock organic loads and to be less affected
by variations in hydraulic loads than plug flow reactors. Few attempts have been
made to quantify the limits of this ability.
Much has been written about biological waste treatment and in particular about
the activated sludge process, its many modifications, and its design and operation.
However, there are few data describing the effects of loading variations on process
efficiency and on the physiological condition of the sludge.
A concentrated research effort concerning substrate interactions during wastewater purification has been under way for the past few years at Oklahoma State
-493-