Process Optima in Activated Sludge
STEPHEN POLONCSIK, Graduate Student
ROBERT B. GRIEVES, Associate Professor
Illinois Institute of Technology
Chicago, Illinois
WESLEY O. PIPES, JR., Associate Professor
Northwestern University
Evanston, Illinois
INTRODUCTION
The hydraulic regime of a biological waste-treatment process may be thought
of as the overall pattern of the flow, mixing and distribution of liquid and of the
materials dissolved or suspended in the liquid, into and within all the components
of the process. The influence of the hydraulic regime is a fundamentally important consideration when improvement of a biological waste-treatment process is
sought.
During the past three years our group has presented a series of papers concerned
with a search for methods of increasing the efficiency of the activated sludge process. In this search, attention has been focused on the manner and extent to which
the hydraulic regime influences process behavior.
The search began with the development of a mathmatical model for the mixing regime of an activated sludge aeration tank (1). This model was based on the
theoretical division of the aeration tank into fractional volumes, with different
mixing patterns in each fraction. This permitted consideration of various combinations of plug flow, completely-mixed flow, feed short circuiting, and stagnant zones in the aeration tank. All factors of the hydraulic regime were included
with the introduction of the separator efficiency, sludge wastage flow and recycle
flow. The mixing model was combined with expressions describing the kinetics of
microbial growth of both the one-phase and two-phase types, these kinetic models
being currently favored by sanitary engineers, to produce overall system performance equations.
Briefly, the one-phase approach describes the entire growth cycle of sludge
micro-organisms in terms of a single growth rate equation which is at all times a
function of the concentrations of nutrient and active sludge organisms. The two-
phase approach postulates two growth phases, one in which the nutrient/active
sludge organism ratio is always greater than some threshold value, with the growth
rate of the sludge being considered as a function solely of the active organism
concentration; the other in which the nutrient/active sludge organism ratio is below the threshold value, and nutrient concentration is the growth rate governing
factor.
This model was programmed for a digital computer analysis and a variety of
mixing configurations were evaluated and compared. It was concluded that regardless of whether the one or two-phase kinetic approach is used, the effects of
mixing configurations on the system performance are similar. The presence of
stagnant zones and of short-circuiting of the feed decreases nutrient reduction and
organism growth. Parallel and series combinations of complete mixing and plug
flow produce results which lie between those of the extreme cases. Sludge recycle
has the influence of increasing the effective operating volume. Finally, a plug
flow process appears to be more efficient theoretically than one which is devised
on the complete-mixing pattern.
Actual conditions in aeration tanks lie somewhere between the extreme cases
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