51    CONDITIONING OF PULP AND PAPER SLUDGE USING
DIRECT SLURRY FREEZING
M. Z. AH Khan, Assistant Professor
Department of Civil Engineering
King Abdul Aziz University
Jeddah
Saudi Arabia - 21413
INTRODUCTION
Surveys [1] of the pulp and paper mill industry indicate that the wastes generated from different
processes involved in producing pulp and paper are generally quite high in sulfates, sulfites, solids,
etc. The BOD tends to be high especially with wastes containing sulfites. These side streams are
usually treated physically, chemically or biologically [2-6] to produce effluents which meet water
quality standards for discharge into the receiving water body. This treatment results in a residue called
"sludge," which contains a lot of chemicals, especially sulfates and sulfites, in colloidal suspension
and dissolved forms, making this sludge less amenable to settling, thickening, conditioning, dewatering, etc. before final discharge. For final discharge of the sludge, it is very important to reduce its
volume by removing the associated water by physical or mechanical means. Lower volumes mean
lower disposal costs.
Most of the sludge treatment processes [6-22,47-55,57-60,62-64] presently used have their difficulties and their drawbacks, particularly with regard to certain types of sludges. Many investigators
[23-46,56,61] had previously considered natural and indirect freeze-thaw processes on laboratory or
large size pilot plant scale experiments as an alternative in conditioning and dewatering of domestic
sludges. Most of the studies, as mentioned above, indicated that both the natural and indirect
processes were quite effective in improving the sludge drainage characteristics but were not considered
cost-effective due to higher costs, poor quality of filtrate, and other operational problems. To overcome or reduce these problems a direct slurry freezing process [67,68,69] was developed for conditioning and disposal of the waste activated sludge from sewage treatment works serving residential areas.
The process was evaluated as quite economical [69]; the filtrate was much better than the other
freezing processes and either similar or better than the conventional processes. The direct slurry
freezing process was not tested for the industrial sludges, especially the pulp and paper waste sludge.
History and development of the sludge freezing processes is presented in Table I.
It is the purpose of this research to attempt to evaluate the feasibility of the direct slurry freezing
process for conditioning, dewatering and disposal of sludge obtained from the treatment of pulp and
paper waste streams.
SAMPLING AND TESTING
The sludge samples were collected from the outlet of the return sludge line from the secondary
clarifier of the activated sludge process utilized for treating the side-streams of the Wesvaco Pulp and
Paper Mill at Covington, Virginia. The samples were transported directly to the laboratory, located at
Blacksburg, Virginia, and stored by refrigeration for a few hours before testing and analysis. The
analysis, before and after direct slurry freezing, was conducted according to Standard Methods [66],
The Beckman 215 analyzer was used for total carbon measurements and the pH was measured using a
Leeds and Northrup probe.
FREEZING PROCESS DESCRIPTION
The experimental sludge at a certain solids concentration is allowed to flow from a sludge tank "ST"
(surrounded by ice) through a flow regulator "FR" into the freezer "F" where liquid butane, flowing
through the heat exchanger "H" and flow regulator "R," comes in contact with the sludge (intimate
contact is achieved by mechanical mixing). After heat exchange between butane and sludge, the slurry
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