Vacuum Filtration Cost Optimization
EDWIN R. BENNETT, Associate Professor
K. DANIEL LINSTEDT, Assistant Professor
DAVID A. REIN, Graduate Student
Civil & Environmental Engineering Department
University of Colorado
Boulder, Colorado
INTRODUCTION
Solids processing constitutes one of the major capital and operating cost items in
conventional wastewater treatment. For a typical large-scale operation the solids
processing system is often comprised of vacuum filtration facilities for sludge dewatering
with associated incineration or landfilling capability for disposal ofthe dewatered sludge.
While the liquid stream portion of the treatment plant will usually have relatively fixed
operational costs, the solids processing costs are very sensitive to adequacy of design and
optimization of operation. This sensitivity develops in several areas ofthe sludge processing
system through: 1) the use of relatively large chemical doses for sludge conditioning; 2) the
effect of dewatered sludge characteristics on the capacity and auxiliary fuel requirements
for incineration; and, 3) moisture considerations in defining the conveyance capacity and
costs for land disposal.
At the heart of virtually all of the disposal cost considerations is the performance of the
vacuum filter. The cost variables directly controlling the economics of the vacuum filtration
operation are the chemical dose and yield. However, disposal costs are most directly
affected by the moisture content of the filter cake produced in the dewatering operation.
When vacuum filtration and sludge disposal are considered as an integrated solids
processing system, the chemical dose, yield, and cake moisture each have nearly equal
impact on the total system economics.
The principal objective of this study has been to investigate the relationship ofthe dose,
yield, and cake moisture variables in vacuum filter operation, with particular consideration
of the means of improving cake dryness. Most of the vacuum filtration literature has not
placed emphasis on evaluating the filtration operation from the standpoint of cake moisture
content.
VACUUM FILTRATION PROCESS DESCRIPTION
Vacuum dewatering with a drum type filter is a popular means of processing for plants
treating a flow of 5 MGD, or more. The process typically involves chemical conditioning to
release loosely held interstitial water and coagulate the fine sludge solids. The conditioned
sludge is introduced into a vat from which it is drawn onto the filter media in the "form"
portion of the filtration cycle (see Figure I). The solids are removed against the filter media
and form a cake. This cake produces a resistance to flow that increases as the cake thickness
grows during the form cycle. As the drum rotates, the cake leaves the vat and enters the
drying portion of the cycle. In the drying stage, most of the pore water in the cake is
removed. Following this drying stage, the cake is separated from the media for subsequent
disposal.
Most ofthe water is removed from the sludge in the first few seconds of the form cycle
and the early portion ofthe drying cycle. The filtrate flow rate is very high initially but drops
off rapidly as the flow of water increases the cake thickness and flow resistance in the form
cycle. This effect is shown in line (a) of Figure 2. The majority ofthe resistance is due to the
presence of fine particles packed against the filter media. The use of conditioning chemicals
agglomerates the small particles and tends to reduce this increased resistance. The resultant
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