Practice + Operations
Comparing PD Pump
Designs for Transferring
Dewatered Sludge Cake
Michael L. Dillon, seepex, Inc.
An explanation of various positive displacement pump designs, their
respective justification, and the compromises associated with each design.
By understanding these limitations, specifiers can better decide which
design best meets user expectations regarding performance, maintenance,
installations costs, operating costs and biosolids characteristics.
The concept of using a positive displacement pump to
transfer dewatered biosolids, commonly called sludge
cake, is not new. However, the designs of the pumps
used have evolved to increase their utility and reliability and
have fallen in and out of favor for a variety of reasons.
Justification for Pumping
Using pumps to convey sludge cake offers numerous advantages over using conveyors for the same purpose. Because
pumps can move cake through vertical sections of pipe, they
can consume much less volume in a building than a conveyor that is limited in its degree of incline. Enclosing cake
in a pipe is also much cleaner and eliminates all of the bearings and other wear points associated with a conveyor.
The tradeoff is that a typical pump is more technically
complicated to apply and maintain than a conveyor. Pump
speeds must also be matched to the output of the dewatering
device, which can either require some extra labor, increased
investment of an automated system, and/or increased maintenance skills.
Due to the viscosity, fibrous nature and high pressures
associated with pumping sludge cake, only positive displacement pumps can be considered. All positive displacement
pumps capture liquid in a defined cavity. Due to the high
apparent viscosity of sludge cake, two major considerations
are important in any pump design:
• The design must incorporate a mechanism to feed the
material into the pump cavity, because atmospheric pressure will not be sufficient to move the cake through the
restrictions that typically block the entrance to the cavity.
• The internal mechanisms in the pump must be relatively free of restrictions that increase backpressure when
moving such a viscous product.
The most common reciprocating design is a type of
piston pump adapted from designs typically used for pumping concrete. These pumps require some sort of screw feed
device and a non-restrictive check valve system to accommodate the two requirements listed above.
Of the rotary PD pumps, the most commonly used is
the progressive cavity (PC) design. Since PC pumps have
cavities that are sealed by a compression fit between the rotor
and stator, no valves exist to restrict the discharge. Various
feed mechanisms are used on the inlets of these pumps.
Feed Mechanisms
The feed screws used to feed both of these designs really
have two functions: to push the cake into the cavity and to
lower the apparent viscosity of the cake.
Sludge cakes, like most water-borne slurries and sludge,
are non-Newtonian liquids and are shear thinning. They are
generally considered to be “thixotropic,” meaning the viscosity thins with both the amount of the shearing force and
the length of time that the shearing force is applied.
