The Effect of
Waste Activated Sludge Addition on
Vacuum Filtration of
Primary Clarifier Sludges
ANDRE L. CARON, Regional Engineer
National Council of the Paper Industry
Air and Stream Improvement, Inc.
CIRO A. MAZZOLA, Development Engineer
Department of Paper Technology
Western Michigan University
Kalamazoo, Michigan
INTRODUCTION
Disposal of the hydrous sludges produced by biological treatment processes
represent a serious problem since they cannot generally be dewatered solely by
mechanical means. Their gelatinous properties can sometimes be altered by admixture with the underflow from primary clarification of waste waters to a point
where satisfactory dewatering by vacuum filtration or centrifugation can be achieved.
Whether this can be accomplished, and the degree to which admixture will be effective, depends upon the relative quantities of the two sludges, their consistencies,
the composition of the primary sludge and their individual dewatering characteristics.
Excess biological cell solids are generated during activated sludge treatment in
quantities ranging from 0.5 to 1 lb dry solids/lb BOD removed (1). This material can
be gravity thickened to from one to four per cent consistency, depending on its sludge
volume index (SVI) and ash content, yielding from 1.5 to 12 gal of highly active biological slurry to be disposed of per lb BOD removed. Waste activated sludges hold
water tenaciously and their presence in relatively small percentages has a marked adverse effect on the dewaterability of primary clarifier underflow sludge.
In Table I projections as to quantities of clarifier and waste biological sludge
solids generated by the activated sludge process are presented on a production basis.
These are estimates based on average values obtained from well operated mills. With
the exception of sulfite pulp mills and some fine paper mills, the waste biological
solids generated by the activated sludge process will comprise 25 - 45 per cent of the
entire sludge load to be dewatered.
Initial laboratory work on primary-secondary sludge mixtures of boardmill
origin indicated the vacuum filter loading rate decreased progressively as the activated
sludge : primary sludge ratio was increased (2).
Difficulty was experienced in removing cake from the filter media at volumetric
biological solids addition rates of 30 per cent, reducing loading rates from 4.2 to 3.2
Ib/sq ft/hr. Aerobic digestion of excess cell solids obtained from the secondary treatment of deinking and boardmill effluent decreased slurry filterability and increased
chemical conditioning requirements (3). Pilot work with a Kraft sludge indicated
successful vacuum filter operation at primary-secondary sludge mixtures of two to one
(4). Chemical conditioning was not necessary since primary underflow was rich in
fiber and lime.
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