STAGING AERATION FOR HIGH-EFFICIENCY TREATMENT
OF AROMATIC ACIDS PLANT WASTEWATER
Chia M. Lau, Research Engineer
Research and Development Department
Amoco Chemicals Corporation
Naperville, Illinois 60540
INTRODUCTION
The activated sludge process is generally recognized as the most cost-effective option
for secondary treatment of wastewater. Growth dynamics of the activated sludge are sensitive to substrate availability and composition. Change in hydraulic flow and loading pattern
can significantly alter the specific growth and metabolic behavior of an activated sludge
mixed culture. Process improvement can be realized from these changes by devising a system which can optimally handle these variables. The ultimate goal in waste treatment is to
produce a high-quality effluent at all times. This requires a system that can withstand
loading transients and recover quickly from upsets.
The concept of using a staged configuration to achieve optimum environmental conditions for more effective treatment of specific wastewaters has led to the development of
several unique activated sludge systems:  step aeration, biosorption, contact stabilization,
extended aeration, the Unox process, the Kraus process, and the Zurn-Attisholz process.
Milburg, Pipes and Grieves [ 1 ] showed that optimal compartmentalization of the aeration
tank would lead to improved performance. Bischoff [2] presented a procedure for selecting
the optimum type of mixing for a fermentation process.  His suggestion was to design for
complete mixing in the first compartment and for plug flow in the remaining portion of
the aeration system. This is a sensible approach since a true plug-flow system in unsuitable
for biological processes due to the lack of in-process equalization capability. The front end
completely stirred tank reactor (CSTR) serves also as an equalization basin to dampening
possible slugs and the subsequent plug-flow reactor (PFR) takes advantage of the nonzero-
order reaction kinetics.  Erickson and Fan [3] compared optimal design of a single-stage
system with two- and three-stage systems without interstage clarification. They found that
two- and three-stage systems were economically superior to single-stage operation for equivalent organic removal efficiencies.
In the mass-culture, food-limited activated sludge system, the rate of substrate removal
in a given system is a function of both substrates and biological solids concentrations:
£-   f(S,M)
at
dS Y      v
— =   k(S)x(M)y
at
where       S is substrate concentration,
M is biological solids concentration,
t is time,
k is rate constant,
and x and y are constants.
In a reactor where biological solids concentration can be held constant through recycle and
wastage, substrate removal rate is apparently a function only of substrate concentration.
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