Comparison of the Conventional Cage Rotor and
Jet-Aero-Mix Systems in Oxidation
Ditch Operations
G.M. WONG CHONG, Research Specialist
A.C. ANTHONISEN, Research Associate
R.C. LOEHR, Professor
Department of Agricultural Engineering
Cornell University
Ithaca, New York 14850
INTRODUCTION
The oxidation ditch is an aerobic biological waste treatment system. The process is
essentially a closed-loop, open channel in which the liquid circulation and oxygen input are
maintained by a mechanical device, traditionally a cage-rotor. For convenience, this closed-
loop may take the shape of an oval shaped race track and within these physical confines the
following processes occur: 1) biological stabilization of the waste material; 2) aeration; and
3) mixing. The latter two processes, mixing and aeration, contribute to the effectiveness of
the biological stabilization process.
The oxidation ditch generally operates with a long detention time, and in treating
animal waste the system may also serve as a waste storage facility. Under the latter
condition, power for mixing becomes the major use of energy. The mixing is necessary to
achieve a minimum velocity to avoid sedimentation and subsequent anaerobic biological
reactions.
In aerobic systems, the input energy to satisfy the demand for adequate mixing is a
direct function of detention time while the energy demand for aeration, which is dictated by
the oxygen demand of the waste, is related to detention time in a curvilinear manner (Figure
1). For a single unit "aeration" device, the energy input for mixing generally will satisfy the
aeration requirement provided the detention time is long.
In aerobic biological units with long detention times, the largest portion of the energy
input is utilized to overcome the inertia of any rotor or surface aerator and to mix the
contents of the unit. The need for least cost alternatives exists for all waste treatment
operations and especially for small operations. This report discusses an alternative to a cage
rotor for an oxidation ditch and compares the performance of two aeration systems when
used for the treatment of high strength wastes.
The alternative to the cage rotor is the Jet-Aero-Mix system which achieves both
mixing and aeration by pumping the mixed liquor through nozzles. Mixing is maintained
by energy and momentum transfer from the jet streams to the bulk liquid and the
concomitant turbulence and aspirator capacity of the jet streams bring about the aeration.
The objectives of this investigation were:
1. to test the JAM system designed on the basis of the hydrodynamics of the oxidation
ditch;
2. a comparison of the JAM and cage rotor systems from the following standpoints:
a. oxygen transfer ability
b. economic evaluation
1. operating cost
2. capital cost
3. general maintenance
3. Performance of JAM system in treating mink waste.
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