An Alternative Approach

Pbjective

‘Loading. Rates nds, oF. dy solids por

Bed'sive

Numibér of Beds

Ghemicar ‘conditioning’
Total-cyete' Time.
Percent Sollds,in Cake V

Cake Removal

Malor Cornpsnants:

‘Chinaite Protection

‘Solids Capture >

Table 1. Design Features of Vacuum-Assisted
Sludge Dewatering Beds

open sand drying beds with chemical addition. This
indicates the area required for drying beds could be
reduced by as much as 90 percent when utilizing
VASDB in place of open sand drying beds.

The’ performance of vacuum-assisted sludge drying
beds is influenced by the following factor

® Solid and liquid loading rates

@ Chemical pre-conditioning of solids
® Cycle time

Vacuum application characteristics

Data from several different treatment plants indicate
dry solids concentrations in the dewatered sludge
cake ranging from 8 percent to as high as 23
percent with cycle times of 8 to 48 hours. Typically,
the process produces an acceptable sludge cake
within 24 hours with a reduction of 80 to 90 percent
in the initial sludge volume. A typical dewatering
curve for an aerobically digested sludge is shown in
Figure 2. Analysis of the drying bed filtrate indicates
a solids capture rate of greater than 9S percent.



Mention of trade names or commercial products
does not constitute endorsement.

Prepared by Environmental Resources Management, Inc.

For additional information contact:

EPA-OWPO(WH-595) EPACMERL (489)

401 M Street, SW 26 West St. Clair Street
fashingion, DC 20460, Cincinnati, OH 45268

(202)982-7370/7369 (613)684-7611

EPA Region 1 | EPA Region 6
John F. Kennedy Federal Building 1201 Elm Street
Boston, MA 02203 Dallas, TX 75270

EPA Roglon 2 EPA Rogion 7
26 Federal Plaza 9324 East 11th Street
New York, NY 10278 Kansas City, MO 64106

EPA Region 3 EPA Rogion 8
6th & Walnut Streets 1860 Lincoln Street
Philadelphia, PA 19106 Denver, CO 80295

EPA Rogion 4 EPA Rogion 9
345 Courtland Street, NE 215 Fremont Street
‘San Francisco, CA 94105

EPA Region 10
1200 6th Avenue
Seattle, WA 98101



Large treatment facilities where a continuous
sludge flow system may be more appropriate

© Facilities requiring a consistently very dry sludge
cake (dry solids above 20 percent)

Experience with raw sludge applications to VASDB
is limited and the use of VASDB with raw sludges
should be carefully evaluated. In most cases,
digestion of the sludges prior to application to the
drying beds is practiced. Also, providing a means of
periodically cleaning the beds to prevent possible
problems due to the presence of greases in the
sludge should be considered. Beds are cleaned
after every use and at most VASDB installations,
chemical agents are used to wash the surface on a
regular basis.

Costs

The construction costs for vacuum-assisted sludge
dewatering beds indicate that this system offers the
potential for significant cost savings compared to
other dewatering systems. In Table 3, the actual
construction costs for VASDB systems are
compared to estimated costs for conventional open
sand drying beds, a rotary vacuum filter, and a belt
filter press system for wastewater flows of 1 and 5
million gallons per day. The comparison assumes
approximately 2,000 pounds of aerobically digested
primary and secondary dry solids per million gallons
of wastewater flow and return of the filtrate to the
plant influent. The actual cost of a VASDB system
is dependent upon site-specific treatment
conditions, The major factors which influence the
cost are sludge volume, dewatering time required,
and the sludge loading rate.

Design Flow
System 1MGD 5.MGD
Vacuum Assisted Dewater fs $158,000" _$1,068,000°
Open Sand Beds $330,

Rotary Vacuum Filters $382,500? $4,252,
Belt Filter Press $173,3007 $2,463.

Table 3. Construction Cost Comparison
(1984 Dollars)

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Annual operation and maintenance costs for
vacuum-assisted dewatering beds also exhibit a
potential for cost savings over some of the other
dewatering systems, particularly for smaller plants.
Table 4 compares the estimated annual O&M costs
for VASDB, open sand drying bed, rotary vacuum
filter, and belt filter press systems. As Table 4
illustrates, the annual O&M costs for mechanical
dewatering systems, as represented by the rotary
vacuum filter and belt filter press system, may
become less than the comparable costs for a
VASDB system at 5 MGD and higher flow rates
and sludge volumes. This is primarily due to a
significant reduction in the labor requirements per
unit volume of sludge dewatered for mechanical
systems as total sludge volume increases.

Design Flow
5.MGD
xd Dewatering Beds $25, $173,300'
Open Sand Beds 500 $131,100"
$182.4
$137,907
"stated fr actual 1984 projects
*Derived A De Municipal
rial 1962 (EPA 625:1-82-014). A

Table 4.Annua! O & M Cost Comparison

Actual construction and O&M costs for VASDB
facilities are lower than those estimated by the EPA
design manual. Actual construction costs have
been reported at $108 to $179 per square foot,
including a cost of $30 to $40 per square foot for
enclosures, when required.

Actual operating requirements have been reported
as 4 to 10 ibs. of polymer per ton of solids, 17 to
39 kwhrs. of electricity per ton and 4 to 10 man-hrs.
per ton. Total O&M costs have been reported at
$37 to $104 per ton, depending on the type of
sludge, the sludge feed concentration, polymer
dosage and cost, dryness of cake, type of removal
method, and local labor and electricity costs.

The cost comparison of vacuum-assisted sludge
dewatering beds with other dewatering systems will
vary with each treatment facility. A detailed life
cycle cost analysis which considers both capital
and O&M costs should be the basis for final cost
comparison.





ge Dewaitering Beds -

Sludge from the treatment processes is uniformly
applied to the bed to a depth of approximately 12
to 18 inches. During the sludge application, a
polymer is normally added to improve the sludge
dewatering characteristics. A gravity dewatering
phase, during which much of the free water may be
removed from the sludge, continues until the sludge
forms a relatively dense mat over the media plates.
At this point, the vacuum pump is started to create
a vacuum in the sump and under the media plates.
The vacuum draws additional water out of the
thickened sludge mat and continues until the sludge
mat begins to crack. Normally, the mat begins to
crack after 80 percent or more of the free water is
removed. After the sludge mat cracks, air drying of
the sludge continues until the dewatered sludge
cake achieves the desired dryness. The sludge
cake is then removed from the bed for further
treatment or disposal. Filtrate from the dewatering
process is normally returned to the treatment facility
by a float-actuated submersible pump located in the
filtrate sump.

The rigid porous media plates form an abrasion
resistant, load bearing surface which permits the
sludge cake to be easily removed manually or by a
tubber tired loader. The plates also eliminate the
need for periodic replacement of the filtering media
common with sand drying beds.

The total cycle time for dewatering of the sludge
depends upon many factors, including the sludge
dewatering characteristics, degree of sludge cake
dryness desired, and climatic conditions. Normal
total cycle times vary from 8 to 48 hours, with 24 to
48 hours representing typical total cycle times for
digested sludges. In comparison, drying times for
sand drying beds normally range from 1 to 2
weeks. Due to the reduced drying time and the
assistance provided by the vacuum system,
VASDB are generally less affected by inclement
weather than are sand drying beds.

Design and Performance

As noted in Table 1, vacuum-assisted sludge
dewatering beds are typically designed for a dry
solids loading rate of 0.75 to 1.5 pounds per square
foot per day, which is equivalent to an annual
loading of about 275 to 550 pounds per scuare
foot. This compares with typical annual design
loading rates of 40 to 60 pounds per square foot fer



Vacuum-Assisted Sl

Introduction

Nearly all wastewater treatment facilities require a
means of handling and disposing of sludges
generated by the treatment processes. Often, the
sludge handling process involves the dewatering of
the liquid sludge to reduce the sludge volume and
produce a relatively dry sludge cake for additional
treatment or less costly treatment and disposal.
Due to the relative simplicity of operation, sand
drying beds have been widely utilized for sludge
dewatering at many small and medium sized
treatment plants. Larger treatment plants often
utilize mechanical sludge dewatering systems
because the land area required for sand beds is
excessive.

Vacuum-assisted sludge dewatering beds (VASDB)
combine several features of both sand drying beds
and mechanical dewatering systems. Potential
advantages of VASDB in comparison to sand
drying beds include:

@ Reduced area needs

Greater operational control

Advantages over mechanical dewatering systems
include:

© Lower costs

@ Ease of operation

Due to their potential advantages, vacuum-assisted
sludge dewatering beds should be considered by
communities and their consultants when sludge
dewatering is proposed.

Description and Operation

Vacuum-assisted sludge dewatering beds closely
resemble sand drying beds in appearance. A cross
section of a typical VASDB is shown in Figure 1.

Polymer

System
Vacuum $98! Fig Porous
Media Plat

=> To Treatment Plant

Figure 1. Typical Section of Vacuum-Assisted
Sludge Dewatering Bed

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Sludge “Depth. (inches).
Percent-Solids:

$87 B
‘Time :(hours):

Figure 2.Dewatering Curve (Aerobically Digested
Sludge)

A list of typical facilities using vacuum-assisted
sludge drying beds is presented in Table 2.

‘Number.
Location Plant Size: ‘of Beds

0.9'migd.
Ef Lid-mgt,
Lutnberton, NC. 10;0'‘mad

Grand ‘Junction; CO:

‘Geneva: jk a
Wosdbridge, It. 4;0.mgd
Taylors, SC 7.5:mod
Hilfon Head, SC:

Union’ City, IN

Pittsfield} AL

Sullivan, aE

6
2
2,
8.
16;
4
es
2.
4
3
4

Table 2.Typical Vacuum-Assisted Sludge prying
Bed Installations

Design Considerations

Evaluation of the use of vacuum-assisted
dewatering beds must consider potential limitations
as well as benefits. The following conditions may
limit the applicability of the VASDB process:

© Treatment of highly viscous sludges

© Treatment of sludges which have a high
concentration of fine solids and/or grease