DEVELOPMENT AND PERFORMANCE OF
AIR-DRIVEN ROTATING BIOLOGICAL CONTACTORS
Robert J. Hynek, Manager
Process Verification & Pilot Plant Program
Charles Chi-Su Chou, Manager
New Products Development
Autotrol Corporation
Milwaukee, Wisconsin 53209
INTRODUCTION
The low-energy rotating disc method of treating wastewater was conceived in 1900, but
significant progress was not made untd the 1960s, when commercial installations began to
appear in Germany [1], and R&D began in the USA [2].   In 1970, Autotrol Corporation's
R&D led to the development of thin plastic media with a honeycomb internal construction,
made of polyethylene, in contrast to the polystyrene foam used for the flat disc.   This new
media was named Bio-Surf to contrast with the term Bio-Disc [3].
Through continuing research and development, it became apparent that this low energy
means of rotating a reactor could be accomplished in another way.   It was calculated that
rising air bubbles could exert the needed torque for rotation if the air bubbles were captured
efficiently.   Since part of the diffused air would dissolve into the wastewater, it was surmised
that higher oxygen transfer and more efficient BOD removal would result.   In addition,
individual drive motors need for the mechanical rotation would be eliminated in favor of
centrally located air blowers of well recognized capability and reliabdity.   The ultimate energy
needed for rotation could be transmitted through air manifold distributors rather than
electric wire.   Individual reactor rpm control could be easily accomplished by an engineer
wishing to optimize plant performance merely through regulating the amount of air going
to an individual reactor.
This Air Drive concept was formulated in January 1972, and development work since that
time has confirmed the advantages anticipated and has also identified additional advantages.
In July 1973 a side-by-side comparison was made with an identical reactor rotated by
conventional means.   Synthetic dairy wastewater was used as a BOD source.   Tests with
these two units confirmed that the Air Drive reactor removed slidghtly more BOD with less
biomass attached to the media.   Since approximately 20% of the air is deliberately allowed
to percolate upward through the media, the additional shear exerted on the surface thus
produces thinner biofUm.   This study also demonstrated that solids settleabdity was essentially the same for both drive systems [4].
In July 1975 a 3.2-m, 4-stage reactor capable of both air and mechanical drive was installed
at Chdd's Road Treatment Plant, St. Paul, Minnesota, and operated under the jurisdiction
of the Metropolitan Sewer Board of the Twin Cities.   Both primary and secondary clarifer
effluents were applied to the reactor at various loading rates.   The biofdm thickness was for
the first time field monitored with a load cell on the shaft, which allowed determination of
weight while the shaft was rotated.   It was recognized at the conclusion of this project that
the air drive process performance at 1.2 rpm matches that of mechanical drive at 1.6 rpm
[5].
The next field test took place from February to July 1976, at the Union Sanitary District in Newark, California, where a pdot study employing air and mechanically driven RBC
and a packed tower was conducted by the consulting firm of Jenks and Harrison.   The test
results again confirmed that suspended solids from the air driven RBC has the same settleabdity as that from the mechanical drive unit.   The air drive system showed an estimated
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