Maintenance Free Mixing
DANIEL A. YOUNG, Senior Chemical Engineer
Ecodyne Corporation
Graver Water Conditioning Company
Union, New Jersey 07083
INTRODUCTION
This is a paper about the use of hydraulic eductors in wastewater treatment; it deals
especially with a specific application. The application involves the recirculation of sludge in
a solids contact clarifier. Some clarifiers are in operation using this concept at the present
time. A Graver Reactivator containing an eductor two feet in diameter is working in
clarification service and a five foot eductor is in service in a water softening operation. I
would like to discuss some of the general characteristics of large scale eductor type mixers
which make them satisfactory for mixing and blending applications in industrial waste
treatment.
WHY AN EDUCTOR MIXER?
An eductor mixer has no moving parts. The only maintenance necessary is prevention
of corrosion or abrasion caused by the working fluid. Hence, an eductor truly approaches
being a maintenance free device.
The original material cost of an eductor mixer is only the cost of the construction
material in the throat and nozzles. This is approximately one fourth the cost of a mechanical
type mixer.
Finally, the eductor is ideally suited to use in a high rate, solids contact, internal sludge
recirculation clarifier such as the Graver Reactivator.
WHAT IS AN EDUCTOR?
Basically an eductor is a tube in which an axially directed nozzle is placed. This is
shown diagrammatically in Figure 1.
Water injected into the tube through the nozzle induces turbulent flow in the pipe
which entrains surrounding water. This induces flow across the entire cross sectional area of
the tube. Hence, a relatively small amount of driving water can induce a relatively large
amount of water to flow through the tube.
BASIC PHYSICAL PRINCIPLES
For an explanation of the reason why an eductor works, it is best to review Bernoulli's
principle which describes the various forms of energy in a fluid piping system. This principle
is usually expressed as the following equation:
— Pj + Hj + l/2Vj2= 1p2 + H2 + 1/2V22+ W + Ev
Where the symbols represent the following:
P       = density of fluid
P       =   pressure
H       =   elevation
V       =   velocity
W      =   work performed on liquid
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