EPA
United States
Environmental Protection Agency
R-030
INNOVATIONS IN SLUDGE
DRYING BEDS
A PRACTICAL TECHNOLOGY
CSMEE
1-800-276-0462
Distributed by the
ERIC Clearinghouse for Science, Mathematics,
and Environmental Education
Columbus, OH
(614) 292-6717 (voice)
(614) 292-0263 (fax)
ericse@osu.edu
http://www.ericse.org
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United States October
Environmental Protection 1987
Agency
&EPA Innovations
In Sludge
Drying Beds
A Practical
Technology
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In Sludge Drying - A Practical Technology
Background
Sand drying beds are still the most common sludge
dewatering process at small to moderate sized
wastewater treatment facilities in the United States.
When suitable land is available, the conventional
sand bed can still offer low costs and energy savings
but does require significant operational labor. The
new EPA Process Design Manual (EPA 625/1-87-
014) Dewatering Municipal Wastewater Sludges
(available in October 1987) offers updated criteria on
sand beds, a rational design procedure, information
on the use of polymers and winter time freezing to
improve performance, as well as current case studies
and design examples.
Among the concepts also discussed in the new
manual are Paved Beds and Reed Beds, both of
which show promise as alternatives to conventional
sand beds.
Paved Beds
Until recently, paved beds used an asphalt or con-
crete pavement on top of a porous gravel subbase.
Unpaved areas, constructed as sand drains, were
placed around the perimeter or down the center of the
bed to collect and convey drainage water. The main
advantage was the capability to use heavy equipment
for sludge removal. Experience showed that drainage
was inhibited by the pavement so the total bed area
had to be greater than a conventional sand bed to
achieve the same dewatering results.
A tractor-mounted horizontal auger, or other device
may be used to regularly mix and aerate the sludge.
This mixing and aeration breaks up the surface crust,
which inhibits evaporation, and therefore allows more
rapid dewatering than conventional sand beds. Some
tractor units now used for this purpose were originally
developed for rapid backfilling of trenches or for
composting operations and serve well for paved bed
dewatering.
Underdrained beds are still used in some locations
and in these cases the free water is allowed to drain
and then the mixing (auger/aerator) unit is used to
accelerate evaporation of the remaining water. In
suitable climates, low cost impermeable paved beds
which depend on decanting of the supernatant and
mixing for enhanced evaporation are used. Figure 1
illustrates a typical cross section of a paved bed using
soil cement as the construction material. While paved
beds have been constructed with both concrete and
asphalt pavements, the most economical approach
has usually been soil cement. These completely
paved beds have an advantage since the mixer will
mix sand with the sludge if operating on a conven-
tional underdrained sand bed. The length and width
of the bed can be similar to those used for conven-
tional sand beds. Other features include draw-off
pipes for decanting the supernatant in each of the
bed corners and a sludge inflow pipe at the center
of the bed.
Slope 02 0.3%
Figure 1. Cross Section of a Paved Bed
If the sludge has good settling characteristics, it may
be possible to draw off 20 to 30 percent of the water
by decantation. If the sludge has particularly good
settling characteristics or if polymers are used, it may
be possible to use several fill and decant cycles prior
to the evaporation stage. The initial decantation
phase might require two or three days for sludge
settling and another one to two days to decant
supernatant for each sludge layer added. The final
evaporative drying period will depend on climatic
conditions and on the regular use of the mixing
equipment. Solids in the range of 40 to 50 percent
can be achieved in 30 to 40 days in an arid climate,
for a 30 cm (12 in) sludge layer, depending on the
time of the year and on the effectiveness of decanta-
tion.
Paved beds can be used in any location, but since
evaporation is the major pathway for water loss, the
concept is most advantageous in warm, arid and
semi-arid climates. Assuming the same degree of
effort is expended with the mixer, the design solids
loading will be directly related to the potential evapo-
ration for the local area. For example, the design
loading rate for a system in Roswell, NM was 244 kg/
m2/year while the loading for a pilot test in Wichita, KS
was 127 kg/m2/year.
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Table 1 presents the design equation for determining
the total bottom area for a paved bed system. This
design area should be divided into at least three beds
for all but the smallest facilities to provide operational
flexibility. The equation as presented in Table 1 is
based on annual averages. The optimum number of
beds required can be determined during final design
via a month by month analysis of weather records and
expected sludge production rates. The system
designed for Roswell, NM for example has a total of
seven beds, six of which need to be used in Decem-
ber, but only three are required in June due to in-
creased evaporation and decreased sludge produc-
tion.
_ (0.104) (S) 1(1-s; / s.- (1-s.) / sj + (100) (P) (A)
A=
Where'
A = bottom area of paved bed, m2
S * annual sludge production, dry solids, Kg.
33 m percent dry solids in the sludge after decantaJion, % as a decimal.
se = perceni dry solids required for final sludge disposal, % as a decimal.
P •* annual precipitation, m.
ke = reduction factor for evaporation from sludge versus a free water surface
use 0.6 for preliminary estimate, pilot test to determine for final design.
E p = free water pan evaporation rate, cm/yr.
Table 1. Design Equation for Sizing Paved Beds
The major operational tasks are sludge application,
decanting, mixing, and sludge removal. Depending
on the time of the year and the size of the operation,
the sludge on the bed should be mixed several times
a week to maintain optimum conditions for evapora-
tion. Labor requirements at the Roswell, NM system
are estimated to be about 0.3 hours per year per
metric ton of dry solids processed.
The capital costs for a paved bed system are depend-
ent on the cost of land at the project site. Other cost
factors include the containing dikes, the pavement,
piping for sludge application and water decantation,
the mixing vehicle, and sludge removal equipment.
Table 2 compares the costs of a paved bed operation
to conventional sand beds for the same location in an
I arid climate.
Item
Number
Total area, m2
Solids, kg/m2/yr
Labor, hr/yr
Capital costs
O & M costs $/yr
Present worth
Sand
Bed
16
60,600
108
8,580
$1,465,000
$ 100,000
$ 2,500,000
Paved
Bed
7
26,200
243
1,700
$520,000
$ 25,000
$780,000
Table 2. Cost Comparison - Paved vs. Sand Bed
The new EPA Process Design Manual provides a
case study of sludge dewatering using paved beds at
the Village Creek Wastewater Treatment Plant
serving Fort Worth, TX. In this case, the existing
beds were converted to this method. The average
wastewater flow is 3.9m3/s (88 mgd) and the sludge
beds cover 78 ha (193 ac), demonstrating that the
concept can also be effective in large scale
operations.
Reed Beds
This concept combines the elements of an under-
drained sand bed and a dense stand of vegetation in
the sludge dewatering process. Most of the beds in
current operation have been planted with the common
reed Phragmites; other emergent vegetation has also
been used successfully in European systems.
New beds are typically constructed as a deep trench,
lined to prevent exfiltration. About 25 cm of gravel
covers the underdrain piping and the gravel is over-
lain by about 10 cm of sand. The root stock is planted
on about 30 cm centers and the bed is flooded with
water for several weeks to encourage plant develop-
ment. The freeboard above the sand layer is at least
one meter to provide for long-term sludge storage.
Sludge is not applied until the plants are well estab-
lished.
The vegetation is an essential component in the
dewatering process. The root system absorbs water
which is transpired to the atmosphere. More impor-
tantly, the penetration of the plant stems and the root
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system maintains a permanent pathway for drainage
of water from the sludge layers. Reeds and similar
plants have the capacity to transmit oxygen from the
leaves to the roots so there are aerobic microsites
adjacent to the roots which assist in stabilization and
mineralization of the sludge.
Stabilized, thickened sludge, at about three to four
percent solids is applied in 10 cm layers. The propo-
nents of the concept indicate that a layer can be
applied about every 10 days during warm weather
conditions. The dewatered sludge remains on the
bed and the new layer is applied directly on top of it
as compared to conventional sand beds where each
layer must be removed prior to the next application.
An operational system in Washington Township, NJ
was designed for an annual solids loading of 100 kg/
m2/yr (3.5 m depth of aerobically digested sludge at 3
percent solids). The average loading on 16 operating
systems in New Jersey, New York, and North Caro-
lina, is about 81 kg/m2/yr which is at the low end of
the range for conventional sand beds. These sys-
tems have been successfully operated in New Jersey
on a year-round basis, with only 20 to 30 days down-
time for adverse weather conditions. Since the
benefits of the process will be minimal during the
dormant season and during prolonged freezing
weather, it is likely that a longer downtime may be
required for locations with severe winters. An alterna-
tive, in cold climates, may be to use the same bed for
winter freeze dewatering as described in the new EPA
manual (reference 1).
A ten-year operational cycle has been planned for the
several systems in New Jersey. At the end of this
time, the accumulated sludge and the sand layer are
removed. A new layer of sand is installed and new
vegetation is planted. The residual dried sludge from
a typical one-year period is estimated to be about 10
cm deep, so there is sufficient freeboard for a 10-year
cycle. An annual harvest or controlled burning of the
vegetation is recommended when the plant is dor-
mant but before the leaves are shed. This is neces-
sary to avoid clumps of dead vegetation which would
interfere with sludge distribution on the bed.
Multiple beds are required for every installation to
allow one bed to be out of service each year and one
for emergencies. When a bed is to be cleaned,
sludge applications are stopped on that bed in early
spring, the vegetation harvested in early fall, and the
sludge residue and sand removed by early winter.
The total bed area required will be equivalent to
conventional sand beds, or larger, based on the
loading rates cited above. The main advantage is the
infrequent need for sludge removal and bed cleaning
which are some of the most time consuming tasks
facing plant operators. Instead of cleaning a sand
bed on a frequent schedule every year, the need is
extended several years. The major disadvantage is
the need for vegetation harvest. This material can be
burned, land filled, or composted. The total volume of
harvested vegetation and sludge residue on a ten-
year operational cycle should be less than the sludge
cake volume requiring disposal if the same amount of
sludge were dewatered conventionally on a sand bed.
The reed bed concept appears to be best suited for
small facilities. It might be ideal for managing the
waste sludge at remote extended aeration systems
where routine sludge wasting is required.
Conclusions
Conventional sand drying beds can still be a cost-
effective process for sludge dewatering at small to
moderate sized facilities and at large systems where
land costs and the climate are favorable. The infor-
mation provided in the new EPA Process Design
Manual on the use of polymers and freeze dewatering
in cold climates can be used to improve the perform-
ance and efficiency of these sand bed systems. The
paved bed with a mixing vehicle offers an effective
alternative to conventional sand beds. The reed beds
described above seem to offer a low maintenance
alternative for the smaller sized systems.
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References
1. U.S. EPA Process Design Manual for Dewatering
of Municipal Sludges, EPA 625/1-87-014, U.S.
EPA CERI, Cincinnati, OH, October 1987.
2. U.S. EPA Region VI, Project Report
C-35-1052-01, Innovative Sludge Drying
Study, U.S. EPA Region VI, Dallas, TX, 1985.
3. Banks, L, S.F. Davis. Wastewater and Sludge
Treatment by Rooted Aquatic Plants in Sand and
Gravel Basins, In: Proceedings of a Workshop on
Low Cost Wastewater Treatment, Clemson
University, Clemson, SC, pp205-218, 1983.
For More Information Contact:
EPA Region 1
John F. Kennedy Federal Building
Boston, MA 02203
EPA Region 2
26 Federal Plaza
New York. NY 10278
EPA Region 3
841 Chestnut Street
Philadelphia, PA 19107
EPA Region 4
345 Courtland Street, NE
Atlanta, GA 30365
EPA Region 5
230 South Dearborn Street
Chicago, IL 60604
EPA-OMPC(WH-595)
401 M Street
Washington, DC 20460
(201) 382-7286
EPA Region 6
1445 Ross Avenue
Dallas. TX 75202
EPA Region 7
726 Minnesota Avenue
Kansas City. KS 66101
EPA Region 8
999 18th Street
Denver, CO 80202
EPA Region 9
215 Fremont Street
San Francisco, CA S4105
EPA Region 10
1200 6th Avenue
Seattle, WA 98101
EPA-WERL
26 West St. Clar Street
Cincinnati, OH 45268
(513) 569-7611
Text prepared by Sherwood C Reea USAGE - CRREL. Hanover.
NH. under EPA/IAG No. DW 96361.
Final preparation, editing and format by
Environmental Resources Management, inc.
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