Biological Sludge Dewatering Practices
In the Pulp and Paper Industry
REID A. MINER, Research Engineer
DUANE W. MARSHALL, Regional Engineer
Central-Lake States Center
National Council of the Paper Industry for Air and Stream Improvement
New York, New York 10016
INTRODUCTION
The removal of biologically degradable material from wastewaters by means of high
rate biological treatment systems produces excess biomass which requires disposal. The
handling and disposal of this gelatinous waste biomass, or secondary sludge, represents a
substantial fraction of the costs of operating a high rate treatment facility. The value of
documented experience to those responsible for exercising sound judgment in rational
selection of dewatering equipment from among the numerou s options available requires no
elaboration. As a consequence the National Council of the Paper Industry for Air and
Stream Improvement, in fulfilling an obligation as repository of paper industry experience
relevant to the capabilities of environmental control technology, has carried out a program
effort documenting the present sludge handling and disposal practices in the pulp and paper
industry. This paper contains some of the findings of that portion of the study devoted to
the dewatering of biological sludge solids.
ALTERNATIVE STRATEGIES
Approximately 60 percent of the papermill treatment facilities generating excess
secondary sludge utilize a mechanical dewatering stage in their sludge handling and
disposal systems. Among the most important factors involved in the growing interest in
sludge dewatering in the paper industry, is the trend toward utilizing environmentally
sound final disposal techniques that minimize land area requirements and nuisance
potential, such as landfill or incineration followed by landfill. Because the costs associated
with both of these disposal options are closely related to sludge water content, dewatering
has tended to be an integral part of paper industry sludge handling and disposal systems.
In the paper industry, the decision to dewater combined or individual primary and
secondary sludges has been based on: 1) the availability of adequate quantities of primary
solids for admixing; and 2) the likelihood of changes in sludge quantity or quality.
Adequate quantities of primary solids are usually available for admixing except in those
situations where: 1) it is possible for the mill to utilize its primary sludge as a raw material in
the production process; 2) a potential exists for by-product development from the primary
sludge; or 3) the mill effluent is characterized by high BOD load relative to suspended solids
losses, resulting in unsatisfactory primary to secondary solids ratios (I). Changes in primary
sludge quantity and quality usually result from the removal of portions of the fiber or clay
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