An Investigation of
Sludge Dewatering Rates
JOHN H. NEBIKER, Assistant Professor
THOMAS G. SANDERS, Research Assistant
DONALD DEAN ADRIAN, Associate Professor
Department of Civil Engineering
University of Massachusetts
Amherst, Massachusetts
INTRODUCTION
Wastewater sludge disposal is well recognized as one of the most vexing
problems of water pollution control. The problem dimensions promise to increase
at a dramatic rate in the future, primarily due to an increase in the degree of
treatment and a decrease in availability of acceptable sites for sludge or sludge-
ash disposal. Presently some 25 to 65 per cent of total capital and operating
costs of primary and secondary treatment plants are expended on handling sludge,
whose volume is less than one per cent of the total plant influent. Some advanced
waste treatment processes will produce sludge volumes that approach 10 per cent
of the total inflow, thus greatly magnifying the sludge disposal problem. Increased emphasis must be placed on volume reduction of the sludge previous to
ultimate disposal.
Despite a variety of mechanical methods available for sludge volume reduction, some 72 per cent of treatment plants in the United States utilize sludge
drying beds (1). Irrigation or lagooning methods are also of current great interest, due in part to recent studies undertaken by the Metropolitan Sanitary District
of Greater Chicago (2). These studies suggest that the aDove mentioned gravity
dewatering and drying methods may be feasible and economically advantageous
for even large metropolitan areas.
The competitive position of these methods may be considerably enhanced by
the use of chemical coagulants. Reports on the acceleration of dewatering resulting from additions of alum date back as far as 1923 (3). Later ferric chloride
and lime were used as coagulants. However, such conditioning has not been popular, in part due to the deleterious effect of many of the chemicals to croplands
receiving the sludge, or to blinding of the dewatering bed. The use of polyelec-
trolytes as conditioners promises to quiet these objections.
Optimal gravity dewatering by, for example, the addition of polyelectro-
lytes, cannot be readily determined due to the lack of parameter formulation
with accompanying simple laboratory tests. This is in stark contrast to the situation with regard to optimization of vacuum filtration. No doubt the successful
application of the vacuum filter in many instances is due to the availability of
such readily determinable parameters, such as are found in the specific resistance
concept and the role of filter media on dewatering rates. Laboratory determinations of specific resistance as a function of the dosage of conditioner, allows direct computation of the change in vacuum filter yield, selection of the optimal
dosage, and accompanying cost justification. Similar parameter formulation and
laboratory tests are clearly needed for gravity dewatering.
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