EFFECT OF DISK ROTATIONAL SPEED ON RBC EFFICIENCY
A. A. Friedman, Associate Professor
Department of Civil Engineering
Syracuse University
Syracuse, New York 13210
L. E. Robbins, Engineer
R. C. Woods, Vice President
J. R. Wauford and Company
Consulting Engineers
Nashville, Tennessee 37214
INTRODUCTION
Rotating biological contactor (RBC) systems have become popular for treating
soluble organic wastewaters during the past decade because of low energy and man-
Power requirements.   An RBC system consists of a series of disks covered with an
active biomass film which is partially submerged in the wastewater so that rotation
alternately exposes the attached film of biomass to the substrate rich carrier fluid and
the atmosphere.   Substrate, measured as soluble BOD5 or COD, is oxidized and converted to new biomass, soluble metabolic by-products, and gaseous end products.
Sequential groups of disks, called stages, are designed to meet specific effluent requirements.   Under ideal conditions, the substrate is almost entirely removed from the bulk
liquid carrier stream.
However, the overall removal of substrate from the wastewater is a complex process.
A conceptual cross section of a single disk is shown in Figure 1 and any of the indicated transport or reaction mechanisms may control the overall substrate removal rate.
Either oxygen or substrate diffusion to and through the porous biomass layer can control the overall removal process, as suggested by Famularo, Mulligan and Muller [ 1 ].
Under ideal conditions substrate is entirely removed from bulk liquid and converted to
gases and readily settleable sloughed biomass leaving a surplus of dissolved oxygen in the
bulk liquid.   Under substrate rich and oxygen limiting conditions the biomass becomes
anaerobic, which results in decreased treatment efficiency and significant odor problems.
Overloaded conditions may foster dispersed growth in the bulk liquid, which results in
solids that are difficult to settle in the succeeding clarifier.   Waste products formed in
the biomass layer, both soluble and gaseous, must diffuse through the liquid layer and
then be dispersed in the carrier stream or exchanged with the atmosphere.   Mixing,
which is related to rotational speed and other physical and hydraulic characteristics of
the system, will affect the transport and removal of oxygen, substrate, and waste products in the system.   The accumulation of metabolic by-products in the biomass may
result in toxic effects.
Early experiments with municipal wastewaters, treated by a two stage, 1.75 meter
diameter disk pilot plant at Pewaukee, Wisconsin, in 1969 and 1970 resulted in the
recommendation that the peripheral velocity or tip speed for full scale units be designed for about 0.3 m/sec (60 ft/min) [2].   Based on this recommendation, which
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