Experimental Evaluation of Sub-Surface
Aeration Systems
J.L. STANTON, Manager
Research and Development
P.R. BRADLEY, Manager
Waste and Water Treatment Sales
Mixing Equipment Company, Inc.
Rochester, New York 14603
INTRODUCTION
More stringent effluent requirements combined with decreasing land availability (at an
ever increasing cost) presents a unique design problem to those engineering industrial and
municipal wastewater treatment plants. Where treatment plants exist but expansion is
required, land surrounding the existing treatment plant is either unavailable or carries a
prohibitively high price tag. For these reasons, biological treatment plants are being
constructed on smaller land areas utilizing deeper aeration basins.
As the available surface area decreases, the use of surface aerators may become
objectionable because the application of sufficient power at the surface to provide the
required oxygen transfer can yield an esthetically unpleasing plant due to surface
turbulence. Where the ratio of energy input to available surface area must be high in order
to provide the design oxygen transfer, the resultant high turbulence with surface aerators
produces a colloidal liquid particle termed "mist." This material can become objectionable
when it is blown by the wind outside the property boundary of the sewage treatment plant.
Additionally, closely confined aerators may create a noise problem by their pumping action
as they pump large quantities of liquid through the air.
Sub-surface aeration systems eliminate many of these problems in that high power
inputs beneath the surface of the aeration basin can be utilized with minimal surface
turbulence. This reduces the noise level substantially in the sewage treatment plant and
produces minimal colloidal "mist." Submerged turbine aeration devices provide additional
benefits in that 100% process flexibility is obtainable. Systems are designed to run with no
air supplied to the mixing turbines and at this design point total solids suspension is
obtainable with no oxygen transfer. As the air is turned from 0 to 100% of the design capacity of the system, oxygen transfer is increased up to the maximum design point. 100%
flexibility with constant mixing is obtained while simultaneously reducing the air quantities
that would otherwise be required with diffused air type systems. Reduction in air quantities
reduces mist production and provides minimal surface turbulence for the quietest possible
operation. Additionally, dissolved oxygen levels can be controlled during periods of low
loading thereby preventing problems with denitrification in the final clarifier. These
systems can be designed to handle any uptake rate and industrial biological treatment
systems with uptake rates substantially exceeding 200 mg/l/hr have been installed and
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