DEWATERING OF SLUDGES THAT PENETRATE THROUGH
FILTERING MEDIA
John T. Novak, Professor
Department of Civil Engineering
University of Missouri-Columbia
Columbia, Missouri 65211
Bj0rn-Erik Haugan, Research Sanitary Engineer
Norwegian Institute for Water Research
Oslo, Norway
William R. Knocke, Assistant Professor
Department of Civil Engineering
Virginia Polytechnic and State University
Blacksburg, Virginia 24060
INTRODUCTION
Most sludge characterization parameters consist of measurements of the rate of water discharge (specific resistance) and the moisture content of the dewatered solids (cake solids). A
parameter often overlooked in the characterization of sludges is the filtrate clarity.  Filtrate
solids measurements are usually considered important in centrifugal dewatering but frequently
ignored when considering other processes. However, experiences with two widely differing
sludges indicates that significant quantities of sludge solids can penetrate through filtering
media, causing poor solids capture and filtrate discharge problems.
A few references to sludge penetration problems may be found and most of these indicate
solids penetration is associated with chemical sludges. McVaugh and Wall [ 1 ] studied the de-
watering of heavy metal sludges and found that, under optimum pH and dewatering conditions,
the filtrate suspended solids level was still near 1500 mg/l.  Likewise, Pattengill [2] found
very serious problems of turbid filtrate quality when studying pure chromium hydroxide
sludges.  In PattengiU's study small doses of polyelectrolyte were found necessary to alleviate
the problem.  Novak and Montgomery [3] observed that water plant sludges with coefficients
of compressibility less than 0.6 would penetrate into or through a sand bed when applied for
drying.  Novak and Langford [4] also observed sand bed penetration with chemical sludges
and found that polyelectrolytes eliminated this problem.
With regard to sewage sludges, Ohara et al. [5] found that digested sludge would produce
high filtrate suspended solids when dewatered by vacuum filtration.  Polymer addition did
little to reduce the problem.  However, a combination of lime and iron greatly improved solids
retention. Cassell and Johnson [6] note that the filtrate from dewatering of activated sludge
by belt pressing "can cause recycle problems" due to excessive suspended solids.   Sawyer
et al. [7] also noted poor solids capture when using belt filter presses to dewater chemically
conditioned anaerobically digested sludge.  Solids capture as low as 43% was reported.
During the course of several separate sludge dewatering studies the problem of solids penetration through the filtering media was observed. One sludge, a chromium hydroxide sludge,
was observed to pass entirely through the filter paper while being dewatered by vacuum filtration in a laboratory apparatus. The other sludge, a waste activated sludge, was found to
pass through the filter cloth of a pilot scale filter press.
This study was conducted in order to better characterize the solids penetration phenomenon.
A variety of sludge characteristics were measured to determine if any of these could predict
solids penetration and methods of controlling the passage of solids through filtering media
were investigated.
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