&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 infeeddirectly 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 SystemSafety.
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
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