Section 9. SLUDGE DEWATERING
THE ROLE OF CENTRIFUGES IN MINIMIZING/ELIMINATING
THE USE OF CHEMICAL ADDITIVES IN
DEWATERING AND THICKENING OF INDUSTRIAL WASTES
R. T. Moll, Manager, Technical Services
A. G. Letki, Project Engineer
Pennwalt Corporation
Sharpies-Stokes Division
Warmister, Pennsylvania 18974
INTRODUCTION
Centrifuges have been applied widely for years on waste streams from both municipal
and industrial plants. Their initial and principal purpose until recently has been for de-
watering sludges in order to produce a burnable or haulable cake. However, with sludge
disposal costs increasing and with internal plant sludge processing usually limited by
hydraulic rate, the need to thicken various sludge streams has occurred.
Excess or waste activated sludge has caused the most difficulty within plants, as it will
normally only settle and compact by gravity to 0.5-1.5% W/W suspended solids. These
flocculant particles when exposed to a centrifuge g-field settle readily, even though the
difference in specific gravity between the particle and mother liquor is less than 0.1.
Deliquefying or releasing the mother liquor from and around the particles occurs by
compaction rather than by bulk drainage through interstitial spaces as is the case with
primary sludge or crystalline solids. As would be expected, the higher the g-force, the
faster the settling rate and the greater the degree of compaction or cake dryness.
Until recently, only the solid bowl basket and the nozzle disc centrifuges could handle
waste activated sludge without the addition of polyelectrolyte. However, with new design
provisions which have been taken to eliminate the scrolling limitation, the Polymizer1™
BD design solid bowl conveyor centrifuge joins the other two centrifuge types. In addition
to operating without polyelectrolyte, the Polymizer provides many advantages to the user,
including continuous operation, high rates and operator flexibility to change cake dryness
at will. It also can be readily placed on or taken off line without cleaning or disassembly.
As with most centrifuges, the space requirements are low and the enclosed design confines
odors and promotes good housekeeping.
TYPES OF CENTRIFUGES
For the reader not acquainted with the centrifuges being mentioned, it would be
worthwhile to discuss the evolution of the various types of centrifuges as applied to waste
activated sludge, including their process capabilities, operating features and limitations.
Prior to this discussion, the reader should realize that except for the higher g-field, all
solid wall centrifuges simulate the action of and can be compared to a gravity settling
tank. The three solid wall centrifuges being considered have various sizes and shapes
yielding different surface areas and pool volumes which control liquid clarification and
cake compaction. Each of these three types utilize a different mechanism for removing
the solids from the bowl which results in differences in mode of operation, necessity for
pretreatment, and hydraulic feed capacity.
The solid bowl basket centrifuge, as pictured in Figure 1, is the simplest of all centrifuges. It best simulates what occurs in a bottle centrifuge tube or a gravity clarifier.
Incoming feed solids in the slurry are accelerated and these solids settle to the bowl wall.
The bowl volume is basically used for compaction since clarification occurs at the surface
of the pool. The percent of suspended solids recovered falls off quickly when the bed
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