United Slates	September
Environmental Protection	1986
Agency
*>EPA Vacuum
Assisted
Sludge
Dewatering
Beds
(VASDB)
An
Update

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Vacuum Assisted Sludge Dewatering Beds (VASDB) - An Update
Introduction
A study was undertaken to evaluate twelve operational
municipal treatment plants utilizing vacuum assisted
sludge dewatering bed (VASDB) processes. The
purpose of the study was to collect information on the
design, operation, performance, cost, and
improvements made to the VASDB process. The
purpose of this brochure is to update an earlier
brochure on VASDB with the improvements found
during the study. The twelve facilities represented
VASDB processes marketed by three manufacturers:
Infilco Degremont, Inc., SDS Company, and U.S.
Environmental Products, Inc.
While many smaller-sized conventional treatment plants
use sand drying beds for sludge dewatering, VASDB
has two advantages: (1) less land area is required, and
(2) greater operational control can be maintained.
VASDB has further advantages over other mechanical
dewatering processes by lowering costs and easing
operation.
Process Description
The basic component of the VASDB (Figure 1) consists
of a support structure upon which media plates are
laid. The media plates are underlaid with a filtrate
collection/drainage system which is connected to an
adjacent air tight filtrate sump. The media plates are
sealed to one another and to containment walls to
prevent solids seepage to the filtrate system.

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Filtrate Receiver
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Filtrate Collection Channel
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Figure 1. Cross-Section of VASDB System
The sludge is mixed with polymer prior to being
distributed on the media plates. Once the plates are
covered with sludge, filtrate valves are opened to allow
gravity dewatering of the sludge. Gravity dewatering
continues until the filtrate collection rate slows. The
vacuum cycle is then initiated. The vacuum sequence
generally proceeds in steps; the highest vacuum level
continuing until the sludge cake cracks and vacuum is
lost. Table 1 presents a typical vacuum sequence.
Stage
Vacuum
Duration
(in. Hg)
(hours)
Gravity dewatering
0
1 to 5
Low vacuum
1 to 4
1 to 2
Intermediate vacuum
5 to 8
1 to 2
High vacuum
10 to 15
1 to 2
Air drying
0
variable
Table 1. Typical Vacuum System Scheduling
The sludge cake is then air dried until the solids level is
high enough for the sludge cake to be considered
"liftable", meaning able to be removed mechanically
leaving only a small amount of residue behind. A small
tractor is most often used for cake removal operations.
The plates are then cleaned and prepared to receive
the next sludge application.
Design Considerations
One of the goals in studying operating VASDB systems
was to identify any design modifications made to
improve performance. In general, however, the basic
design features (Table 2) have remained unchanged.

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Bed Geometry and
Size
Bed Loading Factors
Feed Sludge Tank
Polymer Make Up &
Feed System
Polymer-Sludge
Mixing
Sludge Pumping &
Distribution
Standard sizes are 20' x 20'
or 20' x 40'; media plates
are generally 2' x 2' or 2' x
4'. The number of VASDBs
per facility range from one to
four depending on operation
flexibility and schedule
desired.
Manufacturers suggest 1-2 lb
dry solids/ft2 per cycle for
digester sludge; less for waste
activated sludge, and higher
for Imhoff tank sludge or lime
stabilized sludge. Facilities
have been able to increase
loading by decanting clear
supernatant from sludge bed,
then adding more sludge.
Since optimum polymer
dosage is a critical parameter
to sludge dewatering, it is
suggested that a feed sludge
tank be included in designs
where polymer mixing and
addition are provided.
A polymer feed pump which
can be adjusted during
operation is necessary since
sludge characteristics may
change during bed loading.
Mixing is provided in some
systems by air injection,
residence time, or a series of
90 degree or 180 degree
elbows.
A uniform sludge loading on
the plates must be
maintained, otherwise
premature loss of vacuum
may result. It was suggested
that bed flooding at the start
or use of dual discharge
headers may improve
uniformity.
Table 2. Design Considerations for VASDB
The typical bed size of the VASDB is 20' x 20' or
20' x 40' since media plates are generally 2' x 2' or
2' x 4'. The typical number of VASDBs per facility
ranges from one to four. A minimum of two beds, and
preferably three, is recommended. This will enable a
facility to operate two beds per 24-hour cycle (one idle),
five days a week, on a 21-day rotation.
The depth of sludge which can be applied varies and is
dependent on the solids concentration of the feed
sludge. At some of the facilities evaluated, the beds
were loaded with sludge to depths below the
recommended levels which underutilized the VASDB.
Other facilities have been able to increase loading by
decanting the clear supernatant from above the sludge
bed then applying additional sludge. This enables
solids loadings above those suggested by the
manufacturers to be achieved.
Chemical conditioning of the sludge is accomplished by
polymer addition to improve sludge dewaterability.
Polymer dosage is a critical parameter for sludge
dewatering. Operators typically judge proper dosage
visually during application. Since sludge characteristics
can change during bed application, an adjustable
polymer feed pump is necessary. Another way to
control this variability is to include a sludge feed tank in
the design in which polymer addition and mixing would
occur prior to application.
Another important design consideration is the uniformity
of sludge loading for prevention of a premature loss of
vacuum. Flooding the bed with one inch of clear water
is suggested by one manufacturer as a way to
enhance the uniformity of sludge application.
Operational Considerations
The most critical aspect of VASDB system operation is
the selection and control of the polymer dosage. This is
important from a cost standpoint as well as for the
potential effect that improper dosage has on the
VASDB system. Underdosage results in a wet, sloppy
cake which is difficult to remove; overdosing results in
unreacted polymer clogging the media plates. Jar tests
and filtration tests therefore are recommended for
selecting a polymer and proper feed rates.

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Decanting of the supernatant is an operational
consideration especially recommended for dewatering
dilute, aerobically digested sludge. In addition, gravity
drainage is recommended to continue for an extended
period of time, at least until 50 percent of the liquid
volume is removed.
Several installations have experienced plugging of the
media plates with oil and grease. Special maintenance
procedures may be needed to restore the plates to
their original high permeability. These procedures
include cleaning with a variety of agents such as high
pressure hot water, acids, alkalis, hypochlorite,
enzymes and lime, or repairing the plate surface.
Cost Comparisons: VASDB vs. Sludge Drying Beds
Costs were developed to compare covered and
uncovered VASDB systems to uncovered, roofed, and
totally enclosed sand drying beds for a wastewater
treatment plant generating 2000 Ib'day of aerobically
digested sludge solids. It is important to note that a
VASDB system is normally designed to yield only a
liftable dewatered sludge cake in contrast to the sand
drying bed which yields a much drier sludge cake.
Figure 2 presents sand drying bed estimated total
system costs as a function of sludge solids loading
rates. Also entered on the figure are the estimated totai
system costs for equivalent capacity uncovered and
covered VASDB systems. The intersections in Figure 2
indicate the following:
•	An uncovered VASDB system would be more cost
effective than an uncovered sand drying bed at
loading rates of less than 16-17 Ib/ft^'yr.
•	A covered VASDB system would be more cost
effective than a covered sand drying bed at loading
rates of less than 38-39 lb/ft2/yr.
•	A covered VASDB system would be more cost
effective than a totally enclosed sand drying bed at
loading rates of less than 62-63 lb/ft2'yr.
400
Uncovered Sand
Drying Bed
Covered Sand
Drying Bed
Enclosed Sand
Drying Bed
200
Covered
VASDB
Uncovered
VASDB
100
Drying Bed Loading Rate. lb. ft"' yr
Figure 2. Estimated Sand Drying Bed and VASDB
S/stem Total Costs as a Function of Solids
Loading Rate for Systems Processing 365
Tons of Sludge Solids Year
New Developments
Both the manufacturers and system users have
developed conceptual improvements for application to
the VASDB technology. New media plate designs are
being field tested and experimented with at various
locations. It has been reported that one of the new
designs can yield a drier cake when subjected to
higher than normal solids loading rates. Another
development is a device which is attached to the beds
to measure permeability. This provides a mechanism to
monitor media plate clogging or assess the
effectiveness of the cleaning operations. Various bed
geometries are also being investigated. One
manufacturer has suggested flooding the bed with
several inches of clear water to flood the oil and grease
and keep it away from the media plate while the bed is
being filled.
Recommendation
In summary, the VASDB process, under certain
conditions, can be more cost effectively constructed
than sand drying beds. The VASDB system is
designed to operate with substantially shorter cycle
times than sand drying beds and is not as severely
impacted by adverse climate conditions.

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In the design of a totally new municipal wastewater
treatment plant, there is the potential for additional cost
savings if a VASDB system is incorporated into the
initial design. These savings might arise because
minimal waste sludge solids storage facilities are
required for a VASDB system which is designed to
dewater sludge five days per week. Typical sand drying
bed systems cannot be operated at such a frequency
and thus must have an associated waste sludge solids
storage facility. Cost savings associated with this
aspect of a VASDB system should be thoroughly
evaluated by design engineers comparing the total
costs of VASDB and sludge drying bed systems.
Prepared by Environmental Resources Management, Inc.
For additional information contact:
EPA-OMPC(WH-595)
EPA-WERL (489)
401 M Street. SW
26 West St Clair Street
Washington DC 20460
Cincinnati OH 45268
(202)382-7368 7369
1513)569-7611
EPA Region 1
EPA Region 6
John F Kennedy Federal Building
1201 Elm Street
Boston. MA 02203
Dallas. TX 75270
EPA Region 2
EPA Region 7
26 Federal Plaza
726 Minnesota Avenue
New York, NY 10278
Kansas City. KS 66101
EPA Region 3
EPA Region 8
841 Chestnut Street
999 18th Street
Philadelphia. PA 19107
Denver. CO 80202
EPA Region 4
EPA Region 9
345 Courlland Street. NE
215 Fremont Street
Atlanta. GA 30365
San Francisco, CA 94105
EPA Region 5
EPA Region 10
230 South Dearborn Street
1200 6th Avenue
Chicago. IL 60604
Seattle. WA 98101

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