Effect of Bark Addition Upon the Dewatering
Properties of Biological Sludges
DUANE W. MARSHALL, Research Engineer
WILLIAM J. GILLESPIE, Regional Engineer
National Council of the Paper Industry for
Air and Stream Improvement, Inc.
Central-Lake States Regional Center
Western Michigan University
Kalamazoo, Michigan
INTRODUCTION
Since 1950, the National Council's research program has embraced: 1) basic research
into the fundamental nature of hydrous sludges, 2) applied study of novel disposal
approaches, and 3) evaluation and extension of disposal techniques already in limited use at
paper mills. It is in the latter context that this evaluation has evolved. Work conducted at
Southland Paper Mills, Inc., and reported by Bishop and Drew (1) has demonstrated the
successful dewatering of a hydrous primary-secondary sludge mixture with the use of
shredded bark. Bark additions in the range of 1.5 parts bark to 1 part sludge solids yielded
gross solids loading rates of 15 lbs/ sq ft/ hr with a resulting cake consistency of 22 percent.
Experience compiled in the literature cites such physical characteristics as particle size
and shape (2, 3,4,5,6, 7), size distribution (3, 5, 8 ), and addition level (4,6,7)ashavinga
significant effect upon the performance of a potential filter aid. Extrapolating theory and
experience with other filter aids to bark conditioned secondary sludge, it appeared I) that
successful vacuum filtration of biological sludge could be achieved by addition of uniformly
sized and shaped bark particles 2) that an optimum particle size exists within a range
defined by 20- and 60-mesh screens, and 3) that filter yield would be proportional to the
magnitude of bark addition.
Thus, to better define those variables which promote the successful dewatering of
bark-sludge mixtures, this investigation was accordingly developed to assess the following:
1 )The effect of added bark particle size distribution on dewatering properties of resultant
waste hydrous sludge-bark mixtures; 2) A comparison of the conditioning properties of
hardwood and softwood barks; and 3) The magnitude of bark addition required for
optimum dewatering results.
LABORATORY STUDIES
Basic Test Procedure
Vacuum filter leaf trials were conducted in accordance with procedures outlined in
NCASI Technical Bulletin No. 190 (9) and further described by Eckenfelder (10). Solids
loading rates were computed on the basis of form time only, unless noted otherwise, and
subsequently correlated graphically with form time on a logarithmic plot.
In each case, filtrate suspended solids were calculated by the difference between total
and dissolved filtrate solids, as determined by evaporating 25 ml samples to dryness at
103°C. Solids retention was computed as the proportion of original sludge solids, exclusive
of any bark, retained in the filter cake. Supplemental measurements included filtrate
volume and cake thickness, as well as subjective evaluation of cake-doctoring properties.
Phase I: Conditioning of Combined Sludges
To further develop the results reported by Bishop and Drew (1), evaluations were
conducted to: 1) Establish the benefit of utilizing shredded bark in a naturally occurring size
distribution as an aid to filtration of a combined sludge and its secondary sludge
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