&EPA
     United States
     Environmental Protection
     Agency
Biosolids Technology Fact Sheet
Heat Drying
DESCRIPTION
Heat drying, in which heat from direct or indirect
dryers is used to evaporate water from wastewa-
ter solids,  is one of several methods that can be
used to reduce the volume and improve the qual-
ity of wastewater biosolids. A major advantage of
heat drying versus other biosolids improvement
methods, however, is that heat drying is ideal for
producing  Class A biosolids.

Class A biosolids, as defined in 40 CFR Part 503,
are biosolids that have met "the highest  quality"
pathogen  reduction requirements confirmed by
analytical  testing and/or the use of a Process to
Further  Reduce Pathogens (PFRP) as defined in
40 CFR Part 257. One advantage of Class A bio-
solids is that they are approved for unrestricted
use.  For example, Class  A  biosolids that also
meet appropriate metals limits and vector attrac-
tion reduction requirements can be sold or  given
away for residential use, such as for use on lawns
and home  gardens. They can also be land-applied
in public areas without restriction in  addition to
use as an agricultural amendment. The pellets
formed  from the heat-drying process have been
successfully   marketed  to  a  wide   range of
                 users for many years. They can be directly ap-
                 plied to agricultural fields, lawns, etc. or mixed
                 with other ingredients prior to application.

                 APPLICABILITY
                 Heat drying is an effective biosolids management
                 option for  many facilities that desire to reduce
                 biosolids volume while also producing  an  end-
                 product that can be beneficially reused.  For ex-
                 ample,  the Milwaukee Metropolitan  Sewage
                 District (MMSD) has been heat-drying wastewa-
                 ter solids and marketing the end-product  as a
                 fertilizer since the  1920s (USEPA 1979).  The
                 technology has gained popularity since the  mid-
                 1980s, as many large urban  wastewater solids
                 generators, especially on the  east  coast,  have
                 shifted from ocean disposal  to land-based, bene-
                 ficial  use  of  biosolids.  Most of  the  new
                 wastewater solids processing facilities use direct
                 rotary dryers. Table  1 presents a representative
                 list of facilities that heat-dry wastewater solids.

                                   Table 1.
                       Representative Wastewater Solids
                          Dryers in the United States
Used by permission of CH2M Hill, Inc.
Figure 1. Biosolids Dried Product Distribution
Center.
Location
Milwaukee, Wl
Baltimore, MD
(Patapsco)
North Andover,
MA
Newport, TN
Sacramento,
CA
Ocean County,
NJ
Waco, TX
New York City,
NY
Amsterdam, NY
Type of
Dryer
Direct, rotary
Direct, rotary
Direct, rotary
Indirect, rotary
chamber
Direct, rotary
Direct, rotary
Direct, rotary
Direct, rotary
Indirect, disc
Type of Biosolids
Dried
Blend of raw secondary
with digested primary
Blend of raw primary
with secondary
Anaerobically digested
Anaerobically digested
Anaerobically digested
Anaerobically digested
Anaerobically digested
Anaerobically digested
Anaerobically digested
                 Sources: Shimp et al. 2000; Pepperman 2005.

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Heat drying is applicable in both urban and sub-
urban  settings  because  it requires  a relatively
small amount of land and  facility design allows
process air to be captured for treatment. Markets
for dried products are generally more prevalent in
suburban and rural areas than  in urban settings.
However, because heat  drying reduces the vol-
ume of the solids to such a great extent, transport
of the end-product from urban areas to rural mar-
kets  is usually  economical. Heat  drying  is also
becoming more cost-effective even for small sys-
tems (< 20 dry tons/day), particularly with indirect
drying systems.  For example, recent changes in
the regulations in Texas over the past several years
have made it harder to find areas on which to land-
apply Class B biosolids. As urbanization spreads
outward from larger communities, close-in farms
where  Class B  biosolids can be land-applied are
being developed, leaving only  the farms  farther
out.  With the rising costs of fuel, communities
are turning to heat dryers  to produce  a Class A
biosolids  product  to   facilitate   transport and
enhance its value.

The  physical characteristics of most wastewater
solids allow for successful  drying. But the facili-
ties most likely to find heat  drying feasible include
those that have the following characteristics:

•  Produce 10 or more dry  tons of solids per day.
•  Dewater up  to 25 percent solids or greater.
•  Produce digested solids (heat drying  of raw
   wastewater  solids tends to produce a more
   odorous product, thus  reducing its market-
   ability).
•  Produce high-quality solids with  respect to
   metals content.
•  Are located  in an area where landfilling, incin-
   eration,  and  land   application  of  Class  B
   biosolids are expensive or not feasible.

Although these  characteristics  might make spe-
cific facilities better candidates for  heat drying,
some of these  characteristics also affect design
decisions  for  construction of the  heat-drying
operations.  These factors  are  discussed  in the
"Design Criteria" section addressed later.
ADVANTAGES AND DISADVANTAGES
There are both advantages and disadvantages to
using heat drying to stabilize wastewater solids.
Several of these advantages  and disadvantages
are summarized below.

Advantages
•  Requires  a  relatively small  footprint com-
   pared with other stabilization processes, such
   as composting,  alkaline  stabilization, and air
   drying/long term storage.
   Can be designed to accept a variety of feed
   material characteristics.
   Greatly reduces the volume of material that
   needs  to  be transported.  The  typical heat-
   dried product is at least  90 percent solids,
   compared  to 15 to 30  percent  solids com-
   monly  produced by mechanical  dewatering
   operations. This feature is particularly impor-
   tant for major urban  areas, where the end-
   product might need to be transported for con-
   siderable distances for use or marketing.
•  Reduces traffic into and  out of a facility. The
   number of trucks required to remove material
   is reduced because of the smaller volume of
   the final  biosolids product. In addition, no
   additives  or  amendments need to be  trans-
   ported into the facility.
   Generates a readily marketable product.

Disadvantages
•  Requires  a  substantial   capital  investment.
   Capital costs often  are  weighed against the
   long-term financial return that can be realized
   by the sale of the heat-dried pellets.
•  Requires  a  large amount of energy.  Heat-
   drying systems can require 1,400-1,700 Brit-
   ish  thermal  units  per  pound  of  water
   evaporated. This makes  heat  drying less en-
   ergy-efficient per pound  of final material than
   other beneficial  reuse  methods, such as com-
   posting and  land application. (Sapienza and
   Bauer 2005). In some cases, this can be at least
   partially offset through the use of on-site en-
   ergy sources. For example, some facilities use
   gas from their anaerobic digesters to fuel the
   heat-drying units. Wood chips have also been

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used as a fuel source to produce the hot gases
used in direct dryers. Recycling of these gases
also reduces fuel costs.
Generates dust that can affect plant workers
and  neighbors  in  the local community  and
must be controlled to avoid problems during
storage and transport of the product.  The
health  effects of the dust are similar to those
caused by exposure  to other sources of  dust
and primarily  affect lung function.  Controls
are available to address  dust concerns.  Dust
control is further discussed in the "System De-
sign Considerations" section below.
Creates an explosive hazard from dust gener-
ated in  the  drying process.  (Sieger  and
Burrowes (2006))  Dryer installations  have
experienced fires,  deflagrations,  and explo-
sions.  Much of the  recent  work in thermal
drying systems has been focused on enhanc-
ing their safety. (See discussions of thermal
drying safety  in the "Design Criteria"  and
"Performance" sections below.)
Requires systems that are relatively complex
in comparison with other  solids-processing
systems and need  skilled labor for operation
and maintenance.
Can produce nuisance odors that could nega-
tively  affect community  acceptance of the
process. Sapienza and Bauer (2005) note that
odor was "probably the single most detrimen-
tal impact from thermal  drying plants." For
example,  the   Morris Forman  Wastewater
Treatment  Plant  in  Louisville,   Kentucky,
struggled with odor control  in its  heat-drying
process for  a decade. However, in 2003 the
plant completed  an upgrade to  its solids-
handling  process  that   replaced   an  odor-
causing low-pressure oxidation system with a
system that  includes anaerobic digestion and
blending  of biosolids with  secondary  solids
prior  to  dewatering  and drying.  The   new
design not only significantly  reduced odors
emitted to the  atmosphere from  the heat-
drying process, but it also reduced the volume
of solid waste produced  at the plant and the
subsequent landfill charges that go along with
solid  waste disposal. In addition,  methane
produced in the anaerobic  digesters can be
   used to fuel the heat dryers, thereby reducing
   plant operating costs.
•  Results in an  end-product that  might have
   properties (such as offensive odor) that affect
   its  value and   marketability.  Sapienza  and
   Bauer (2005),  however, note  that the most
   current designs for heat-drying operations in-
   corporate recirculation  of  dryer  exhaust gas
   and the use of regenerative thermal oxidizers
   and other techniques to  reduce the odor of the
   final exhaust gas.  Therefore, the authors con-
   clude that odorous emissions are no longer a
   significant problem for heat drying facilities.
   (See discussion  on "End-Product Characteris-
   tics" below).

DESIGN CRITERIA
Operators  and  planners should consider three
basic questions  when selecting or  designing  a
heat-drying system:

 1. What characteristics are desirable in my end-
   product?

2. How could the heat-drying system be config-
   ured to achieve  my  desired  end-product,
   ensure  efficient operation,  and meet safety
   standards?

3. What  type of  dryer  is  best suited  for my
   specific system?

The  following  discussions  provide  background
information that should  enable treatment plant
operators and planners to answer these questions
and  identify  an appropriate heat-drying  system
for their needs.

End-Product Characteristics
Heat-drying systems are typically designed to pro-
duce  Class  A  biosolids.  Although  Class  B
biosolids can be produced using  a heat-drying
system, the lower market  value of a Class B prod-
uct typically does not justify the energy and cost
required to run  the system.  The  regulatory  re-
quirements  for  a  heat-drying process  to  be
considered a Process to  Significantly  Reduce
Pathogens for the production of Class A biosolids
are discussed later  in the "System Design Con-
siderations" section.

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Although federal regulations allow for Class A
biosolids that also meet the metal limits and vec-
tor  attraction  reduction  requirements  to  be
distributed to the public for unrestricted use, not
all Class A biosolids have the same market value
to consumers. The  following list describes sev-
eral biosolids end-product characteristics that can
be controlled to improve product marketability.

   Odors. It is preferable that the pellets be free
   of offensive odors. Undigested solids tend to
   create more odorous pellets than those made
   from  digested   or  waste-activated   solids
   (Dolak et al. 2001). Odors can increase if the
   pellets become wet, which  can  happen from
   condensation during cooling or through other
   mechanisms. The best way to reduce odors in
   the finished product is to continue  to  digest
   prior to dewatering and drying  (NBP 2005).
   In addition, the end-product must be properly
   stored to ensure that it is not exposed to mois-
   ture  before  use.  Exposure to  significant
   moisture presents  a potential  for anaerobic
   decomposition (leading to odors).
   Undigested biosolids led to odor problems at
   the Hagerstown, Maryland, pelletizing plant.
   The  plant  mixed  an  undigested  primary
   sludge (typically high in odor) with waste ac-
   tivated secondary  sludge prior to drying the
   material. Influent to the plant also contained
   waste from  local  dairy processors,  which
   added a pungent odor to the primary sludge.
   When the product  was first  dried, there was
   no odor to the pellets. However, after the pel-
   lets  cooled, they released a  strong offensive
   odor (R. Pepperman, personal  communica-
   tions, 2005). The facility eventually  added an
   odor-masking compound to make the pellets
   more marketable to the  agricultural commu-
   nity.  Further  information on the control  of
   odors in biosolids (related to more than heat
   drying)  can be obtained from the fact sheet
   Odor  Control  in  Biosolids   Management
   (USEPA 2002).

•  Nutrient content.  One  of the main reasons
   that heat-dried biosolids  can be sold and used
   as fertilizer is their nutrient content.  Heat-
   dried biosolids pellets contain up to 6 percent
   nitrogen, up to  5 percent phosphorus,  and a
   trace of potassium.  Sufficient nutrients must
be present in the biosolids to warrant the costs
associated  with  transporting and applying
them  as  fertilizer. A reliable sampling  pro-
gram  must  be established to determine the
nutrient content, and this information should
be provided to potential users (NBP 2005).
Mechanical  durability. It is important to
ensure that the product will maintain its form
through bagging, conveyance, handling, and
storage. Pellets that  are  not within the stan-
dard  range  for mechanical  durability may
crumble during handling; therefore, they may
not be acceptable even if they have sufficient
nutrient content.
Particle  size  distribution. Pellets produced
by heat-drying wastewater solids range in size
from  1 to 4 millimeters and are  angular in
shape. Screening and sizing abrade the pellets
into a more  spherical shape. Irregular particle
sizes  can result  in  larger particles  settling
faster than smaller ones.  Some users (such as
fertilizer blenders) must  ensure that products
remain well mixed  throughout  shipment to
their customers. End users may associate ir-
regular pellet sizes with an inferior product.
Moisture content. Too much moisture in the
pellets can  cause  odor problems  and might
also cause  the pellets  to smolder.  Adequate
cooling before the pellets are stored or trans-
ported will reduce the  potential for odor and
smoldering,  and therefore this step should be
included as part of the facility's biosolids
process (NBP 2005).
Dust content. Dust from pellets can be prob-
lematic for several reasons. First, dust can be
an  explosion  hazard.   Second,  dust  might
cause  human health problems.  And third,
some   potential  end-users  may  not accept
dusty  pellets;  since many potential users of
biosolids pellets find  excessive dust unac-
ceptable  or at least  characteristic of an
inferior product (NBP  2005). Dust  can be
generated because the pellets were not suffi-
ciently dried and hardened during heat-drying
or because  the pellets  were not  otherwise
processed to minimize their potential to cause
dust.  Sapienza and Bauer  (2005) note that,
typically, the  harder the heat-dried material,
the less potential  there  is  to generate dust.

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   Repeated handling  of some pellets  during
   storage and/or transport, however, can result
   in dust generation, which  may  be a concern
   for fertilizer blenders who must comply with
   air emission requirements.  Coating  pellets
   with vegetable oil or paraffin minimizes dust
   production.

System Design Considerations
Once the planners determine the desired charac-
teristics of the heat-dried biosolids end-product,
they must design a system that can produce that
end-product.  The  following  items  must  be ac-
counted for in the design process.

•  Characteristics of feed solids.  The moisture
   content of the feed solids partially dictates the
   required dryer capacity and affects  decisions
   on appropriate conveyance technologies and
   the amount of previously dried material to be
   mixed with the feed solids. Many experts rec-
   ommend  that biosolids be digested prior to
   heat drying to minimize odors produced at the
   processing  facility and in the final product.
   (See  the  discussion on odors  under "End-
   Product Characteristics" above.) Mixing pre-
   viously dried product into the feed solids will
   reduce the  moisture content of the mixture
   and help to prevent the solids from sticking in
   the   dryer.   There   are   several   options
   for mixing, including  pug mills  and  paddle
   mixers.
•  Process dust control. Dust control during the
   actual heat-drying process is important to pro-
   tect worker health and safety,  as well as to
   minimize the potential for  fire and explosion.
   (See "Safety Considerations" later in this sec-
   tion.) Dust can be controlled by  enclosing the
   drying system and using cyclone separators,
   wet scrubbers, or bag houses.  Site-specific  air
   modeling is recommended during the concep-
   tual  design  of  heat-drying  facilities  to
   determine the potential for  dust migration
   off-site.
   A process patented by Dutch company Gront-
   mij  Vandenbroek  International  has  several
   innovations to reduce the potential for  dust to
   become an explosive hazard. The process feed
   does not enter the dryer at the same location as
the dryer air, which keeps the  solids from
sticking and overdrying at the entrance to the
process.  The dryer uses the VADEB multi-
pass system, which  keeps  the material from
being over-dried in the dryer. Finally, the dried
particles  are entrained  in  exhaust  air, from
which they are separated by size. The under-
sized particles go back into the process to be
mixed with incoming solids, the oversized par-
ticles are correctly resized in a crusher, and the
correctly sized particles go to storage. Most of
the exhaust air is then recycled, while  some is
vented to the environment  through  an in-line
afterburner.

Storage  for feed solids  and the finished
product.  Control of dust and odor is neces-
sary  when  storing  both   feed  and  dried
biosolids. Feed solids can be stored in day
bins, which are common in solids-processing
facilities.  However,  special  considerations
must be made for storing the dried  biosolids.
High solids content can make the potential for
dust formation high. Nitrogen or some other
inert agent is usually injected  into storage si-
los to reduce the fire hazard.  Care  also must
be taken to ensure that the  dried biosolids are
stable, reducing the potential for odors. (See
"Odors" discussion above.)
Compliance monitoring. If Class  A biosol-
ids are to be produced, a  system to monitor
the heat-drying process must be incorporated
to ensure (1) that the moisture content is 10
percent or lower and (2) that the  temperature
of the biosolids particles or the wet  bulb tem-
perature   of  the  gas  in  contact  with  the
biosolids exceeds 176 °F (80 °C). In addition,
heat-dried biosolids  must be tested for fecal
coliform bacteria or Salmonella sp.  at the last
point before  being  used  or  disposed  of
(USEPA 1999).
Location of dewatering  and  drying  sys-
tems.  The  heat-drying system  should  be
located  near the  dewatering  system  to  cut
down on biosolids  handling  and  transport
within the facility.
System capacity. The heat-drying  system must
be sized to allow for required equipment main-
tenance.  If a single  system is implemented,

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use of standby grinders, fuel pumps,  an air
compressor  (if applicable),  and dual sludge
pumps should be  provided,  and this equip-
ment should be in good working condition. A
reasonable downtime for maintenance and re-
pair based on data from comparable facilities
is  typically  included in the  design.  A  good
rule of thumb is to provide storage or alterna-
tive handling for at least 3 days of peak solids
production.  Maximizing   storage  capacity
(based on available land area and economics)
increases program  flexibility. Additional stor-
age also enables a  facility to store its finished
product if market demand  fluctuates  or if
weather conditions make transporting pellets
off-site more hazardous.
Adequate space for  screening equipment.
Depending on the  type of dryer and intended
end use of the product, additional processing,
such as sizing, screening, coating, or pelleti-
zation,  might  be necessary.  Sizing  and
screening equipment is used to sort out parti-
cles  that  do  not  meet  an  end   user's
specifications or to recycle unacceptable ma-
terial back to the infeed—directly with small
particles  or  after further processing (such as
milling) for large  particles.  Adequate space
for this type of equipment should be factored
into any construction design.
Energy considerations. As  discussed above,
heat dryers require a large amount of energy,
and they are less energy-efficient per pound
of final material than  other  beneficial reuse
methods. Innovative designs, however, allow
newer dryers to operate at lower temperatures
than older dryers,  and thus they require less
energy. This has allowed some dryers to use
low-energy waste  streams as power sources.
Moss and Sapienza (2005) indicate that direct
dryers can use biogas,  landfill gas, gas turbine
exhausts,  and wood-fired  gasifiers as energy
sources, while indirect systems can use these
sources as well as  steam or hot water genera-
tor exhaust, or waste heat from water circuits.
For example, MMSD  uses waste heat from
turbine generators  to power its sludge dryers
(MMSD   2005).   New  England  Fertilizer
Company (NEFCO) designed, built,  and is
operating  the  Greater  Lawrence, Massachu-
    setts   Sanitary  District  Biosolids  Drying
    Facility, a direct rotary kiln dryer that  uses
    digester gas as a fuel source.  The  system,
    which  came  online  in 2002  at a  cost  of
    $13 million, has a capacity of 38 dry tons of
    Class A biosolids/day and is estimated to save
    the District an estimated $600,000 in opera-
    tions costs annually relative to other drying
    options because of the alternative fuel source
    (NEFCO  2006). A second NEFCO installa-
    tion in Palm Beach County, Florida, that can
    accommodate  600  wet  tons/day  will  use
    2,000  scfm of landfill gas as its fuel source
    and will  use  only  natural gas as a backup.
    Hillsborough County, Florida, uses the biogas
    generated from a local landfill  to operate the
    dryers.
•   Safety considerations. Because of their  high
    organic content, both  the heat-drying  end-
    product and the dust generated during produc-
    tion of the end-product are  flammable, and
    precautions must be taken to design the heat-
    drying process, equipment,  and storage  to
    minimize the potential  for explosion or fire.
    Various design modifications can be made to
    minimize the potential  for fire or explosion,
    including minimizing dust through the use of
    cyclone separators, wet scrubbers,  or  bag
    houses; minimizing  oxidation  potential by
    using  an  inert gas; and minimizing  combus-
    tion by cooling the end-product and ensuring
    that the end-product is not produced or stored
    near heat sources,  such as dewatering proc-
    esses.   Sieger  and  Burrowes  (2006)  also
    indicate that in addition to inertization, other
    safety considerations include isolation, explo-
    sion   suppression,  explosion   relief,  and
    venting and extinguishing. Designers  should
    work with the vendors to ensure that the vari-
    ous safety  considerations in  designing and
    implementing the system are well understood.

Types of Dryers
The most important feature of a heat-drying sys-
tem is the dryer. Typically, the rest of the facility
is  designed around this integral piece  of equip-
ment. Dryers can be classified as direct, indirect,
or other. Direct and indirect dryers typically have
been most successful for drying wastewater solids.

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Direct Dryers.  In  direct dryers, the wastewater
solids come  into contact with hot gases,  which
cause evaporation of moisture.

Direct dryers, which include rotary dryers  (the
most common dryers in use today, shown in Fig-
ure 2),  flash dryers, spray  dryers, the SWISS
COMBI ecoDry process, and toroidal dryers,  are
most  often the  technology of choice when  the
product is intended to be marketed as an agricul-
tural product.

Pellets from  direct  dryers are usually uniform in
texture, size, and durability,  and therefore they
rarely require additional processing to make them
marketable.  Generally, the plant must mix proc-
essed solids  (usually undersized fine  particles)
into the feed solids to raise the solids content of
the feed mixture and avoid a condition referred to
as the "sticky"  or "plastic"  phase.  This  phase
                      occurs in mixtures with between 40 and 60 per-
                      cent solids, and it renders the material difficult to
                      mix and move inside the dryer.

                      Indirect Dryers. In  indirect dryers, the solids
                      remain separated from the heating medium (usu-
                      ally thermal oil or steam) by metal walls, and the
                      solids never  come  into direct contact  with the
                      heating  medium. Moisture evaporates when the
                      wastewater  solids  contact  the  metal  surface
                      heated by the hot medium.  The heat transfer sur-
                      face  is  composed of a series of hollow metal
                      discs or paddles  mounted on a rotating  shaft,
                      through  which  the  heating medium flows. The
                      rotating  action  of the shaft  agitates  the solids,
                      improving heat transfer and  facilitating  the sol-
                      ids'  movement through the  dryer. Mixing  of
                      previously dried material with feed solids is re-
                      quired in some indirect drying systems.
                              Friction Seal
                                         Girt Gear
                                 (a)
                                                Knocker
                                            Breeching
                                              Seals
             Inlet Head
          (counterfiow only)
             Spiral Flights
             No. 1 Riding Ring

              Trunnion and
               Thrust Roll
                Assembly
  Drive
Assembly
               o
Lifting
Flights
No. 2 Riding
   Ring
                      Trunnion Roll
                       Assembly
                                                                             Discharge
                  Radial
                  Flights      (b)
        45-deg Lip
          Flights
        Source: WEF, 1992.
        Figure 2. Rotary Dryer: (a) Isometric View and (b) Alternative Flight Arrangements.

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Indirect dryers, which include steam dryers, hol-
low-flight  dryers (Figure  3), and tray dryers,
produce smaller quantities of noncondensable gas
than direct dryers, which means  that the process
produces less odor and requires less odor control
equipment. Indirect dryers usually have a higher
thermal efficiency and are more  suitable  when
pellets are to be used in energy  production or
combusted. Indirect dryers also produce less dust
during the drying process and have a lower risk
of explosion than direct dryers.  However,  the
end-product of indirect dryers (the pelletized ma-
terial) tends  to  be dustier  than  a dried product
from a direct dryer, and therefore it is not as mar-
ketable to some users. Finally,  indirect dryers
often produce oversized pellets, which are not as
desirable in  the  agricultural market (R. Pepper-
man,  personal communication, 2005). Additional
processing (such as granulation  or compaction)
might be required to  increase  the uniformity,
 consistency, and durability of the product.  Such
 processing  can improve the marketability of the
 pellets from indirect drying facilities, but it also
 increases costs.

 A comparison of direct and indirect dryers is pre-
 sented in Table 2.

                    Table 2.
         Comparison of Direct Versus
	Indirect Drying	
         Direct
        Indirect
 Dried solids recycling
 required.

 Many operating facilities in
 the United States.
Dried solids recycling
sometimes required.

Limited number of operating
facilities in the United
States; several successful
operations in Europe.	
 Source: Summarized by Parsons 2005.
Sludge Feed
Dryer Evaporation










Water .
or T =
Ai? Lh
1 TJ
»n /•
i/
\_
Steam or ,
Thermal Oil '
-v x-x X-N. S**^ s PI
w w w ^ II
-I
1 '
Condensate







Dry Sludge to -j 	 r
Disposal or Further / N^ .
Processing




	 1
	 	 *•
J
Cooling
m Or
I
I

| 	 _^.

v/ent Gases to Air Pollution Control E.quipmen



Fluid
Bed
Furnace

t



Dryer Moisture
Condenser


        Source: WEF, 1992.
        Figure 3. Flow Diagram of Hollow-Flight Dryer System.

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Other Types of Dryers. Other types of dryers
include those that use a combination of direct and
indirect drying or use  special carrier fluids. For
example, the Jones Island Wastewater Treatment
Plant  in  Milwaukee,  Wisconsin,  which has
been in operation  longer than any other facility
using heat drying in the United States, uses  a
combination    direct-indirect   rotary   system.
Carver-Greenefield has patented a dryer that uses
carrier oil. In this system,  wastewater solids are
mixed with the oil, and the  mixture flows through
a multi-effect evaporator, where moisture  is re-
moved. Although a number of Carver-Greenefield
biosolids  dryer facilities were constructed (in-
cluding facilities  for the  Los Angeles County
Sanitation District, the Ocean County [New Jer-
sey] Utility Authority,  and the Mercer County
[New Jersey] Improvement Authority),  none are
currently operated. This  system required consid-
erable maintenance  to operate reliably, and its
working capacity was smaller than that indicated
by the designer.

Microwave Dryers. Burch Biowave has devel-
oped a system that uses a high-efficiency, multi-
mode microwave specifically  designed to remove
moisture  and  destroy pathogens. The  process
does not affect the nutrient content  of the end-
product and  can produce  Class A biosolids.  A
Burch Biowave system in  Fredericktown,  Ohio,
began operations in 2004, and another is planned
for Zanesville, Ohio.

Annual buyer's guides published by trade organi-
zations   such   as  the   Water  Environment
Federation and the  Solid Waste  Association  of
North  America are  good  sources of additional
information on heat dryer manufacturers.

PERFORMANCE
Heat-drying technology is generally very reliable,
and few facilities experience significant periods of
unscheduled downtime. Nevertheless,  some instal-
lations have  experienced performance problems.
Spontaneous heating in storage areas  is a concern
because of the organic matter content of pellets
derived from wastewater  solids, and  improper
product storage procedures  and dust accumulation
have caused fires in some locations.
The volatile solids content and temperature of the
pellets  also  affect  their  explosion  potential.
Therefore, pellets must be cooled to avoid com-
bustion  in  storage  facilities.  Most  facilities
blanket the  pellets stored in  storage silos with
inert material  (such as nitrogen)  to lessen  the
explosion potential.  Facilities can  also monitor
the silos using  thermal sensors (to detect  in-
creases in  temperature)  or  carbon monoxide
monitors (to detect increases  in carbon monox-
ide), both of which  could indicate  potential fire
hazards (Sapienza and Bauer 2005).

The Occupational Safety and Health Administra-
tion (OSHA) issued a Hazard Information Bulletin
in December 1995 that described required safety
precautions for facilities that process, convey, or
store dried biosolids. OSHA has outlined  design
criteria that help minimize and control explosion
and fires connected  with the  organic dust from
heat-dried biosolids.  These criteria include vent-
ing systems to release any  buildup of pressure
within the drying vessels or storage areas, safely
releasing gas from drying  facilities, using non-
conductive materials in areas of drying or product
storage, reviewing all heat sources in and around
heat-drying processes and storage areas, and  en-
suring that workers  in these areas employ good
housekeeping practices (OSHA  1995). Sapienza
and Bauer (2005)  also note that maintaining  an
oxygen-deficient atmosphere in the process com-
ponents  (dryer,  solids separator,  recirculation
duct) can help to minimize this potential problem.
Used by permission of CH2M Hill, Inc.
Figure 4. Rotary Dryers, the Most Common
Type Used for Drying Wastewater Solids.

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Sieger and Burrowes (2006) also presented in-
formation on the safety and design of heat-drying
systems.

OPERATION AND MAINTENANCE
Heat-drying   systems   are   sometimes   highly
mechanized to maintain proper temperatures and
inflow/outflow.  Therefore,  operation  of such
systems  require  skilled  operators.  Preventive
maintenance  is a  necessary  part  of day-to-day
operations. Routine cleaning helps to avoid cor-
rosion caused by  the  properties  of the  solids.
Multiple units are often used to avoid disruption
to treatment works  operation when units are not
in service. All units should be in proper working
order so they can be used if needed.

Several  operating  heat-drying systems  report
common problems, including pitting of convey-
ance  equipment and dryer  drums  due  to the
abrasive nature  of the wastewater solids, and
scale formation on  dryers and  piping. Scale can
be  removed by  washing with acid or  high-
pressure water jets.  Mixing oil with the solids
also helps to prevent scale formation.

COST
Capital and O&M  costs for heat-drying facilities
are typically high relative to other solids alterna-
tives,  such as  land application and alkaline
stabilization (Sapienza  and Bauer 2005). It  is
difficult, however, to estimate the exact costs  of
heat-drying  wastewater  solids without  design
details such as the specific type  of dryer, fuel
source, and moisture content of the feed solids.
Santa  Barbara County, California,  (2004)  esti-
mated that heat drying  would  cost from  $51  to
$58 per wet ton, depending on  the availability  of
biogas or waste heat from co-generation facili-
ties.  These  costs  are  based  on  an average
biosolids solids content of 18 percent. Grace et  al.
(1994) compared the cost of direct versus indirect
drying of approximately  35  dry metric  tons  of
wastewater solids per day  and estimated $323 per
dry ton for indirect drying and $441 per dry ton
for direct drying. These figures included  capital
costs of $26.8 million for the indirect dryer ver-
sus $37 million for the direct drying system.
Sapienza and Bauer (2005) report that historical
costs for heat-drying equipment typically ranged
between $110,000 and $180,000 per dry ton/day
of solids processing capacity for facilities proc-
essing between 20 and 100 dry tons/day. Capital
costs for the entire heat-drying operation, includ-
ing buildings, site work, utilities, dewatered cake
conveyance, product  storage,  performance test-
ing,  and  so forth  can be  in the  $220,000-
$300,000  per ton per day range (Sapienza and
Bauer, 2005).  The city of Leesburg, Virginia,
installed  a direct rotary dryer  system with an
evaporative capacity of 2,000 kg/hr in 2001 as
part of a biosolids management upgrade project.
The project, which cost $11.5 million, also in-
cluded a  screening building and a  350,000-gal
sludge storage  tank. The  city chose an Andritz
system in which hot gases are routed directly into
the dryer instead  of an alternative  system  with  a
heat exchanger  because the Andritz system could
start up and shut down more quickly. This feature
was important because the city does not run the
system constantly (S.  Cawthron, City of  Lees-
burg, personal communication, 2006).

Items that must be considered when estimating
capital costs include

•   Dewatering feed solids
•   Feed solids  mixing
•   Dryer
    Conveyance to and from dryer
•   Air emission (including odor and dust) control
•   Product classification, screening, and/or pel-
    letizing
•   Product cooling prior to storage
•   Product storage, including provisions for ni-
    trogen blanketing

Sapienza and Bauer (2005) indicate that O&M
costs  for  heat-drying  facilities typically  range
from $180 to $300 per dry ton of material proc-
essed. These costs include  costs for fuel, power,
O&M labor, and  maintenance materials and sup-
plies.  Costs for fuel can be a  significant part of
these costs and can range from 25 percent to 55
percent of the total O&M costs.
10

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Typical O&M costs include

•  Labor
•  Auxiliary fuel
•  Air emission  control chemicals and mainte-
   nance
•  Equipment maintenance
•  Product transport
•  Product marketing

Another facet of costs related to drying is the sale
of the resulting product.  Biosolid  pellets  from
dryers are historically very marketable products.
The factors that influence the  price received for
the pellets are nutrient content, particle  size dis-
tribution, dust potential and mechanical durability
(which are closely  related),  bulk density, mois-
ture content, and odor.

Nutrient content usually has the greatest impact
on the price because most buyers base their pur-
chase on the amount of nitrogen in the pellets.
Many facilities sell dried biosolids to users with
the price based on the nitrogen  content of the
product. Current  prices are  typically around $9
per metric ton ($10 per ton) of material per per-
cent nitrogen. Sapienza and Bauer (2005) report a
range in value from $0 to  $36  per metric ton ($0
to $40 per ton). As with many types of products,
however, prices can fluctuate  with the  seasonal
demands of users and in response to supply. The
operation of several  large  dryers  has  recently
increased supply and led to  falling prices. Being
able  to store the products  until  supply is low
might  also help the bottom  line.  Producers that
can hold the  product until users are ready might
net a higher price than those who move the prod-
uct from the site every day regardless of price.

Although the sale of dried  biosolids provides a
welcome source of revenue  to wastewater treat-
ment plants to help offset O&M costs, it should
be noted that selling the end-product typically
does not completely offset heat-drying processing
costs.
REFERENCES
Other related fact sheets:

Odor Control in Biosolids Management
EPA 832-F-00-067
September 2002

Centrifugal Thickening andDewatering
EPA 832-F-00-053
September 2002

Belt Filter Press
EPA 832-F-00-057
September 2002

Other EPA fact sheets are available at the
following Web address:
http://www.epa.gov/owmitnet/mtbfact.htm

Cawthron, S., City of Leesburg Wastewater
   Treatment Plant. 2006. Personal communica-
   tion.

Dolak, I, S. Murthy,  and T. Bauer. 2001. Impact
   of Upstream Processes on Heat-drying Tech-
   nology. In Proceedings of the Water
   Environment Federation, American Water
   Works Association and California Water Envi-
   ronment Association Specialty Conference,
   Biosolids 2001: Building Public Support.  Ar-
   lington, VA: Water Environment Federation.

Feindler, K.S., and C.A. Holley, 1994. Method
   for Upgrading Thermally Dried Wastewater
   Solids into a Competitive Organic Fertilizer.
   In Proceedings for the Management of Water
   and Wastewater Solids for the 21st Century: A
   Global Perspective. Alexandria, VA: Water
   Environment Federation.

Foess, G.W., D. Fredericks, andF. Coulter. 1993.
   Evaluation of Class A Residuals Stabilization
   Technologies for South Broward County,
   Florida. In Proceedings of the Water Envi-
   ronment Federation 66th Annual Conference
   & Exposition, Sludge Management. Arlington,
   VA: Water Environment Federation.
                                                                                             11

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Grace, N., G. Carr, and J. Finley. 1994. Direct
   Versus Indirect Thermal Drying of Biosolids:
   A Comparative Evaluation. In Proceedings of
   the Water Environment Federation Specialty
   Conference, The Management of Water and
   Wastewater Solids for the 21st Century: A
   Global Perspective. Arlington, VA: Water
   Environment Federation.

Grontmij Vandenbroek.  2006. VADEB Thermal
   Kinetic Drying Technology.
   .  Accessed May 2006.

Komline-Sanderson Engineering Corporation.
   2000. Web site, .
   Accessed 2000.

MMSD (Milwaukee Metropolitan Sewer Dis-
   trict). 2005. 2020 Facilities Plan.
   . Accessed June
   2006.

Moss, L., 2006. Personal communication.

Moss, L., and F. Sapienza. 2005. Presented at
   Managing Biosolids:  A Toolbox for Texas,
   hosted by the Water Environment Association
   of Texas, Austin, TX, August 2005.

Murthy, S., H. Kim, C. Peot, L. McConnell, M.
   Strawn, T. Sadick, and I. Dolak. 2003. Evalua-
   tion of Odor Characteristics of Heat-Dried
   Biosolids Product. Water Environment Re-
   search Foundation 75(6): 523-31.

NBP (National Biosolids Partnership). 2005. Na-
   tional Manual of Good Practice for Biosolids.
   January 2005.

NEFCO (New England Fertilizer Company).
   2006. Web site.
   .
   Accessed May 2006.

OSHA (U.S. Department of Labor, Occupational
   Health and Safety Administration). 1995.
   OSHA Hazard Information Bulletin: Fire and
   Explosive Hazards Associated with Biosolids
   Derived Fuel (BDF) and Waste Water Treat-
   ment Plants. Washington, DC: Occupational
   Health and Safety Administration.
Pentecost, D.J. 2004. Biosolids 101: Understand-
   ing the Pathogen Classes. Pollution
   Engineering36(8): 20-23

Pepperman, R. 2005. Personal communication.

Pepperman, R. 2006. Personal communication.

Santa Barbara County, California. 2004. Strategic
   County-Wide Biosolids Master Plan. Prepared
   by CH2MHill.

Sapienza, F., and T. Bauer. 2005. Thermal Dry-
   ing of Wastewater Solids. Presented at
   WEFTEC 2005, Washington, DC.

Shimp, G.F., J.M. Rowan, J.S. Carr. 2000. Con-
   tinued Emergency of Heat-drying: A
   Technology Update. In Proceedings of the
   14th Annual Residuals and Biosolids Man-
   agement Conference. Arlington,  VA: Water
   Environment Federation.

Sieger, R.B., and P. Burrowes. 2006. The Key to
   a Successful Thermal Dryer System—Safety.
   Presented at Texas Water 2006, Austin, TX.

USEPA (U.S. Environmental Protection
   Agency). 1979. Process Design Manual for
   Sludge Treatment and Disposal.  Washington,
   DC: U.S. Environmental Protection Agency.

USEPA (U.S. Environmental Protection
   Agency). 1993. Standards for the Use or Dis-
   posal of Sewage Sludge (Title 40 Code of
   Federal Regulations Part 503). Washington,
   DC: U.S. Environmental Protection Agency.

USEPA (U.S. Environmental Protection
   Agency). 1999. Biosolids Generation,  Use and
   Disposal in the United States. Washington,
   DC: U.S. Environmental Protection Agency.

USEPA. (U.S. Environmental Protection Agency).
   1999. Environmental Regulations and Tech-
   nology: Control of Pathogens and Vector
   Attraction in Sewage Sludge. Washington,
   DC: U.S. Environmental Protection Agency.

WEF (Water Environment Federation). 2000.
   Milwaukee Metropolitan Sewerage District
   Continuing the Tradition ofMilorganiteR p.
   43-50. Biosolids Success Stories (CD). Alex-
   andria, VA: Water Environment  Federation.
12

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 WEF (Water Environment Federation). 1992.
   Design of Municipal Wastewater Treatment
   Plants. WEF Manual of Practice No. 8. Alex-
   andria, VA: Water Environment Federation.

 ADDITIONAL INFORMATION
 Synagro Corporation
 Karl von Lindenberg
 P.O. Box 9974
 Baltimore, MD 21224

 Milwaukee Metropolitan Sewerage
 Paul Schlect
 260 West Seeboth Street
 Milwaukee, WI 53204-1446

 New England Fertilizer Company
 Virginia Grace
 500 Victory Road
 North Quincy, MA 02171

 New York Organic Fertilizers Company
 Peter Scorziello
 1169 Oakpoint Avenue
 The Bronx, NY 10474

 New York Department of Environmental
  Protection
 Tom Murphy
 96-05 Horace Harding Expressway
 Corona, NY 11368
                                            The mention of trade names or commercial
                                            products does not constitute endorsement or
                                            recommendation for use by the U.S.
                                            Environmental Protection Agency.

                                            Office of Water
                                            EPA 832-F-06-029
                                            September 2006

                                            For more information contact:
                                            Municipal Technology Branch
                                            U.S. Environmental Protection Agency
                                            Mail Code 4204
                                            1200 Pennsylvania Avenue, NW
                                            Washington, DC 20460
MTB
I
Excellence In compliance through optimal technical solutions
MUNICIPAL  TECHNOLOGY
                                                                                     13

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