ECONOMIC EVALUATION OF SEQUENCING
BATCH BIOLOGICAL REACTORS
Lloyd H. Ketchum, Jr., Assistant Professor
Ping-Chau Liao, Graduate Student
Robert L. Irvine, Associate Professor
Department of Civil Engineering
University of Notre Dame
Notre Dame, Indiana 46556
INTRODUCTION
Sequencing batch treatment of organic wastewater is emerging as a viable alternative
to conventional continuous flow systems.   Irvine and Davis [ 1 ]    reported results of an
early investigation of the use of sequencing batch reactors in 1971, and since then,
Irvine and others have reported on various aspects of its application to biological
treatment of organic wastes [2,3,4].
Operation of this treatment process has been described in detail in the above references.   In general, the process consists of two or more tanks, operated in parallel, with
both reaction and settling being accomplished in the same tank.   The tanks are operated
in a fill-and-draw mode.   One tank accepts the incoming wastewater while the others are
in reaction, settling, draw-down, or idle phases.   When the first tank is full, the incoming wastewater stream is diverted to a second tank which has been drawn down and is
in a standby phase ready to accept wastewater.   The first tank contains biological floe
and is aerated during the filling phase.   Aeration is continued after the tank is full
until the required organic carbon stabilization has been accomplished.   Aeration is then
discontinued and the solids allowed to settle before the clarified supernatant is drawn
off.   The settled solids are held for reaction in the next   cycle and wasted as needed to
maintain the   desired solids concentration.
During development of this process, it became apparant that a highly variable oxygen
demand is exerted on the system.   Typically, the required aeration rate increases from
that needed for endogenous respiration in the standby phase, when the substrate concentration and liquid volume are both low, to a peak at the end of the filling cycle.
During the reaction phase the required aeration rate drops to that needed for endogenous respiration and then is shut off completely to allow settling and draw down to
the minimum level needed to contain the settled solids.   To meet these very high peak
demands, extremely large aerator systems are needed.   Since these peak demand periods
occur for only relatively short periods, excessive energy is consumed during periods of
lower oxygen requirements, or relatively complex control systems are needed to match
more closely air supply with the highly variable oxygen requirements.
The approach developed to solve this problem was to limit the air supply to relatively low and constant levels, and to increase the tank size to allow reduction in aeration
system size and complexity of control.   The results of the theoretical analysis which
determined the required air supply rate and tank size, and provided the basis of design
for these systems, is reported herein under the section headed "Sequencing Batch System
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