Effect of Centrate from Solid Bowl
Centrifuge on Activated Sludge Process
THOMAS J. KENNEDY, Research Chemist
GREGORY A. ETTELT, Chemical Engineer
Metropolitan Sanitary District of Greater Chicago
Chicago, Illinois
A. J. KAPLOVSKY, Chairman
Department of Environmental Science
Rutgers University
Rutgers, New Jersey
INTRODUCTION
Utilization of centrifugation for the dewatering of waste sludge has interested
investigators for some time (1,2,3,4,5). Experimentation involving the thickening of various types of waste solids has been more extensive in recent years (6,
7, 8, 9,10,11,12,13,14). Some of the studies resulted in plant scale installations
for concentrating solids as a prerequisite for disposal by trucking, barging, incineration, fluidized solids, and digestion. Generally, the trend has been toward the
use of centrifuges to satisfy the dewatering requirements greater than that provided
by gravity thickening. The wide use of centrifugation may be attributed to improvement in design, plus the desirability for greater densities of solids to save
hauling and/or land costs.
The solid-bowl centrifuge has received more interest in the water pollution
control field compared to the disc or nozzle type because of clogging problems
with the latter (5,10,11,13,14). The solid bowl centrifuge is designed more for
its thickening ability rather than clarification (15).
The situation is analogous to comparing concentrations and clarifier sedimentation tanks. Thus solids recoveries are rarely greater than 95 per cent, and frequently much lower, owing to the attractive economics gained by the greater
solids production at reduced recoveries. Consequently, at these increased rates the
solids remaining in the centrate (centrifuge effluent) becomes significant and_ perhaps limiting. The centrate content is such that it should not be discharged'to a
stream without pretreatment.
Recycling of the centrate solids or fines to the treatment plant facilities may
present a problem because of their poor settleability (3, 5,11,12,14,16). Centrate solids may pass unaltered to the final effluent and/or upset the performance
of the activated sludge process, and/or reduce the concentrating capability of a
waste sludge.   Information pertaining to these problems is lacking.
In a 147-day test with a disc type centrifuge, and operating at an average
solids recovery of 76. 3 per cent, Kraus & Longley (5) reported a higher sludge volume index in the aeration tanks receiving the centrate as compared to the other
aeration tanks.   However, no detrimental effects were evident on plant operation.
CENTRATE CHARACTERISTICS
During the operation of the centrifuge, an attempt was made to determine
the non-settable solids fraction of the centrate.   However, the results were incon-
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