United States
Environmental Protection Agency

  Distributed by the
  ERIC Clearinghouse for Science, Mathematics,
  and Environmental Education
  Columbus, OH
                        (614) 292-6717 (voice)
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       United States       October
       Environmental Protection   1987
&EPA Innovations
       In Sludge
       Drying Beds

       A Practical

                         In Sludge Drying            - A Practical  Technology
 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

 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-

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.

    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-
         _ (0.104) (S) 1(1-s; / s.- (1-s.) / sj + (100) (P) (A)

      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.
Total area, m2
Solids, kg/m2/yr
Labor, hr/yr
Capital costs
O & M costs $/yr
Present worth
$ 100,000
$ 2,500,000
$ 25,000
 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

 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-

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

 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.

 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.

 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

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

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.