United States
Environmental Protection
Agency
Biosolids Technology Fact Sheet
Use of Composting for Biosolids Management
DESCRIPTION
Composting is one of several methods for treating
biosolids to create a marketable end product that is
easy to handle, store, and use. The end product is
usually a Class A, humus-like material without
detectable levels of pathogens that can be applied as
a soil conditioner and fertilizer to gardens, food and
feed crops, and rangelands. This compost provides
large quantities of organic matter and nutrients
(such as nitrogen and potassium) to the soil,
improves soil texture, and elevates soil cation
exchange capacity (an indication of the soil's ability
to hold nutrients), all characteristics of a good
organic fertilizer. Biosolids compost is safe to use
and generally has a high degree of acceptability by
the public. Thus, it competes well with other bulk
and bagged products available to homeowners,
landscapers, farmers, and ranchers.
Three methods of composting wastewater residuals
into biosolids are common. Each method involves
mixing dewatered wastewater solids with a bulking
agent to provide carbon and increase porosity. The
resulting mixture is piled or placed in a vessel
where microbial activity causes the temperature of
the mixture to rise during the "active composting"
period. The specific temperatures that must be
achieved and maintained for successful composting
vary based on the method and use of the end
product. After active composting, the material is
cured and distributed. The three commonly
employed composting methods are described in the
following paragraphs. A fourth method (static
pile) is not recommended for composting
wastewater solids based on a lack of operational
control.
Aerated Static Pile - Dewatered cake is
mechanically mixed with a bulking agent and
stacked into long piles over a bed of pipes through
which air is transferred to the composting material.
After active composting, as the pile is starting to
cool down, the material is moved into a curing pile.
Yard Trimmings,
Source-separated organics,
or Mixed MSW
Blanket of
Finished Compost
6-12 inches
Finished
Compost
Perforated
Aeration Pipe
Blower
Odor Filter
Source: Hickman, 1999.
FIGURE 1 SCHEMATIC OF A STATIC-PILE FORCED-AIR COMPOSTING PROCESS
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The bulking agent is often reused in this composting
method and may be screened before or after curing
so that it can be reused.
Windrow - Dewatered wastewater solids are mixed
with bulking agent and piled in long rows. Because
there is no piping to supply air to the piles, they are
mechanically turned to increase the amount of
oxygen. This periodic mixing is essential to move
outer surfaces of material inward so they are
subjected to the higher temperatures deeper in the
pile. A number of turning devices are available,
including: (1) drums and belts powered by
agricultural equipment and pushed or pulled through
the composting pile; and (2) self-propelled models
that straddle the composting pile. As with aerated
static pile composting, the material is moved into
curing piles after active composting. Several rows
may be laced into a larger pile for curing. Figure 2
shows a typical windrow operation.
Source: Parsons, 2002.
FIGURE 2 WINDROW OPERATIONS ARE
TURNED TO PROVIDE ADEQUATE
AERATION FOR ACTIVE COMPOSTING
In-Vessel - A mixture of dewatered wastewater
solids and bulking agent is fed into a silo, tunnel,
channel, or vessel. Augers, conveyors, rams, or
other devices are used to aerate, mix, and move the
product through the vessel to the discharge point.
Air is generally blown into the mixture. After
active composting, the finished product is usually
stored in a pile for additional curing prior to
distribution. A typical composting vessel is shown
in Figure 3. This technology is discussed in greater
detail in the fact sheet entitled In-Vessel
Source: Parsons, 2002.
FIGURE 3 TYPICAL COMPOSTING
VESSEL
Composting ofBiosolids (EPA 832-F-00-061).
All three composting methods require the use of
bulking agents, but the type of agent varies. Wood
chips, saw dust, and shredded tires are commonly
used, but many other materials are suitable. The
U.S Composting Council lists the following
materials as suitable for use as bulking agents:
• Agricultural by-products, such as manure
and bedding from various animals, animal
mortalities, and crop residues.
• Yard trimmings, including grass clippings,
leaves, weeds, stumps, twigs, tree prunings,
Christmas trees, and other vegetative
matter from land clearing activities.
Food by-products, including damaged fruits
and vegetables, coffee grounds, peanut
hulls, egg shells, and fish residues.
Industrial by-products from wood
processing, forestry, brewery and
pharmaceutical operations. Paper goods,
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paper mill residues, and biodegradable
packaging materials are also used.
Municipal solid waste.
If municipal solid waste is used in compost, it is put
through a mechanical separation process prior to its
use to remove non-biodegradable items such as
glass, plastics and certain paper goods (USCC,
2000).
The length of time biosolids are composted at a
specific temperature is important in determining the
eventual use of the compost end product. 40 CFR
Part 503, Standards for the Use and Disposal of
Sewage Sludge (Part 503) defines time and
temperature requirements for both Class A and
Class B products (Table 1). The production of a
Class B product is not always economically justified
since the product cannot be used without restrictions
and the additional expense to reach Class A
requirements can be marginal.
TABLE 1 PART 503 TIME AND
TEMPERATURE REQUIREMENTS FOR
BIOSOLIDS COMPOSTING
Product
Regulatory Requirements
Class A Aerated static pile or in-vessel:
55 C for at least 3 days
Windrow: 55 C for at least 15
days with 5 turns
Class B 40 C or higher for five days during
which temperature exceed 55 C
for at least four hours
Source: 40 CFR Part 503.
If the compost process conforms with the time and
temperature requirements to produce a Class A
product and the maximum pollutant levels of Part
503 are met, the material is considered "Exceptional
Quality" (EQ) biosolids. If used in accordance with
sound agronomic and horticultural practices, an EQ
biosolids product can be sold in bags or bulk and
can be used in household gardens without additional
regulatory controls. Class A and EQ biosolids
typically have greater marketing success than Class
B biosolids. Control of industrial waste streams to
wastewater treatment plants (through pretreatment
programs) greatly reduces the presence of metals in
pre-processed wastewater residuals, enabling
compost to meet the stringent EQ standards of Part
503.
If the compost produced is Class B, it can be used
at agronomic sites with no public contact, with
additional site restrictions. Class A biosolids can
be used in home gardens with public contact and
no site restrictions. Consistent and predictable
product quality is a key factor affecting the
marketability of compost (U.S. EPA, 1994).
Successful marketing depends on a consistent
product quality.
Stability is an important characteristic of a good
quality compost. Stability is defined as the level of
biological activity in the compost and is measured
as oxygen uptake or carbon dioxide production.
Oxygen uptake rates of 50 to 80 mg/L are
indicative of a stable product with minimal
potential for self-heating, malodor generation, or
regrowth of pathogen populations. Stability is also
indicated by temperature decline, ammonia
concentrations, chemical oxygen demand (COD),
number of insect eggs, change in odor, and change
in redox potential (Haug, 1993).
Stable compost consumes little nitrogen and
oxygen and generates little carbon dioxide.
Unstable compost consumes nitrogen and oxygen
and generates heat, carbon dioxide, and water
vapor. Therefore, when unstable compost is
applied to soil, it removes nitrogen from the soil,
causing a nitrogen deficiency that can be
detrimental to plant growth and survival. In
addition, if not aerated and stored properly,
unstable compost can emit nuisance odors
(Epstein, 1998, Garcia, 1991).
APPLICABILITY
The physical characteristics of most biosolids
allow for their successful composting. However,
many characteristics (including moisture content,
volatile solids content, carbon content, nitrogen
content, and bulk density) will impact design
decisions for the composting method. Both
digested and raw solids can be composted, but
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some degree of digestion (or similar stabilization) is
desirable to reduce the potential for generation of
foul odors from the composting operation. This is
particularly important for aerated static pile and
windrow operations. Carbon and nitrogen content
of the wastewater solids must be balanced against
that of the bulking agent to achieve a suitable
carbon to nitrogen ratio of between 25 and 35 parts
carbon to one part nitrogen.
Site characteristics make composting more suitable
for some wastewater treatment plants than others.
An adequate buffer zone from neighboring residents
is desirable to reduce the potential for nuisance
complaints. In urban and suburban settings, in-
vessel technology may be more suitable than other
composting technologies because the in-vessel
method allows for containment and treatment of air
to remove odors before release. The requirement
for a relatively small amount of land also increases
the applicability of in-vessel composting in these
settings.
Another important consideration before selecting
the technology to be used for composting is the
availability of adequate and suitable manpower.
Composting is typically labor-intensive for the
following reasons:
Bulking agents must be added.
Turning, monitoring, or process control is
necessary.
• Feed and finished material(s) must be
moved with mechanical equipment.
Storage piles must be maintained for curing
and distribution.
• Bulking agents recovery adds another step.
Finally, proximity to the markets for the resulting
compost is desirable, although the usefulness of the
final product in home gardening and commercial
operations generally makes the material marketable
in urban as well as rural areas. This is especially
true for good quality material that does not emit foul
odors.
ADVANTAGES AND DISADVANTAGES
Biosolids composting has grown in popularity for
the following reasons (WEF, 1995):
Lack of availability of landfill space for
solids disposal.
• Composting economics are more favorable
when landfill tipping fees escalate.
Emphasis on beneficial reuse at federal,
state, and local levels.
• Ease of storage, handling, and use of
composted product.
Addition of biosolids compost to soil
increases the soil's phosphorus, potassium,
nitrogen, and organic carbon content.
Composted biosolids can also be used in various
land applications. Compost mixed with
appropriate additives creates a material useful in
wetland and mine land restoration. The high
organic matter content and low nitrogen content
common in compost provides a strong organic
substrate that mimics wetland soils, prevents
overloading of nitrogen, and adsorbs ammonium to
prevent transport to adjacent surface waters (Peot,
1998). Compost amended strip-mine spoils
produce a sustainable cover of appropriate grasses,
in contrast to inorganic-only amendments which
seldom provide such a good or sustainable cover
(Sopper, 1993).
Compost-enriched soil can also help suppress
diseases and ward off pests. These beneficial uses
of compost can help growers save money, reduce
use of pesticides, and conserve natural resources.
Compost also plays a role in bioremediation of
hazardous sites and pollution prevention. Compost
has proven effective in degrading or altering many
types of contaminants, such as wood-preservatives,
solvents, heavy metals, pesticides, petroleum
products, and explosives. Some municipalities are
using compost to filter stormwater runoff before it
is discharged to remove hazardous chemicals
picked up when stormwater flows over surfaces
such as roads, parking lots, and lawns. Additional
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uses for compost include soil mulch for erosion
control, silviculture crop establishment, and sod
production media (U.S. EPA, 1997a).
Limitations of biosolids composting may include:
• Odor production at the composting site.
• Survival and presence of primary pathogens
in the product.
Dispersion of secondary pathogens such as
Aspergillus Jumigatus, particulate matter,
other airborne allergens.
Lack of consistency in product quality with
reference to metals, stability, and maturity.
Odors from a composting operation can be a
nuisance and a potential irritant. Offensive odors
from composting sites are the primary source of
public opposition to composting and have led to the
closing of several otherwise well-operated
composting facilities. Although research shows that
biosolids odors may not pose a health threat, odors
from processing facilities have decreased public
support for biosolids recycling programs (Toffey,
1999). Many experts in the field of biosolids
recycling believe that biosolids generating and
processing facilities have an ethical responsibility to
control odors and protect nearby residents from
exposure to malodor.
Composting odors are caused by ammonia, amine,
sulfur-based compounds, fatty acids, aromatics, and
hydrocarbons (such as terpenes) from the wood
products used as bulking agents (Walker, 1992). A
properly designed composting plant, such as the one
shown in Figure 4, operated at a high positive redox
potential (highly aerobic) will reduce, but not
necessarily eliminate, odors and odor causing
compounds during the first 10 to 14 days of the
process (Epstein, 1998). Control of odors is
addressed in further detail in the fact sheet entitled
Odor Management in Biosolids Management (EPA
832-F-00-067).
In addition to odors, other bioaerosols, such as
pathogens, endotoxins, and various volatile organic
compounds, must also be controlled. Biofilters are
often used to control odors, but the biofilters
themselves can give off bioaerosols.
Pathogens, such as bacteria, viruses, and parasites
(helminth and protozoa), are present in untreated
wastewater residuals. These organisms can
potentially invade a normal, healthy human being
and produce illness or debilitation. Composting
reduces bacterial and viral pathogens to
non-detectable levels if the temperature of the
compost is maintained at greater than 55 C for 15
days or more. Additionally, it has been
demonstrated that viruses and helminth ova do not
regrow after thermal inactivation (Hay, 1996).
Regrowth of Salmonella sp. in composted biosolids
is a concern, although research shows that
salmonellae reach a quick peak during regrowth,
then die off. Composting is not a sterilization
process and a properly composted product
maintains an active population of beneficial
microorganisms that compete against the
pathogenic members. Under some conditions,
explosive regrowth of pathogenic microorganisms
is possible. A stabilized product with strict control
Source: Parsons, 2002.
FIGURE 4 ODOR CONTROL EQUIPMENT
CAN BE A SUBSTANTIAL PART OF
CAPITAL INVESTMENT
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of post-composting handling and addition of
amendments coupled with four to six weeks of
storage will mitigate Salmonella regrowth (Hay,
1996).
Compost workers may be exposed to a common
fungus known as Aspergillusfumigatus, endotoxins,
or other allergens. A. fumigatus is common in
decaying organic matter and soil. Inhalation of its
airborne spores causes skin rashes and burning eyes.
While healthy individuals may not be affected,
immunocompromised individuals may be at risk.
The spores of A. fumigatus are ubiquitous and the
low risk of exposure is not a significant health
concern. However, spore counts at composting
facilities are high, and the risk of operators and
persons handling composted biosolids being
exposed to these spores is also high (Epstein, 1998).
Inhalation of spores, particulates, and other matter
can be reduced or prevented by:
Wearing masks and other protective devices.
Equipping front end loaders with filters or
air conditioners.
• Thoroughly ventilating composting halls.
• Installing biofilters or other odor scrubbing
systems in composting halls (Epstein 1998).
Organic dust (such as pollen) is another nuisance
that must be controlled at composting operations.
These contaminants are primarily a concern to
workers at the composting facilities and are
generally not present in quantities that would cause
reactions in most individuals that are not exposed
outside of the facilities.
Environmental Impact
Potential environmental impacts may result from
both composting operations and use of the compost
product.
Composting Process
Dust and airborne particles from a composting
operation may affect air quality. The impact to
adjacent areas may need to be mitigated and
permitted.
To protect area ecology and water quality, run-off
from application sites must be controlled. The
potential nitrogen and phosphorus rich run-off (or
leachate) can cause algal growth in surface water
and render groundwater unfit for human
consumption.
Land Application of Compost Products
Excess nitrogen is detrimental to soil, plants, and
water, so care must be taken when choosing
application sites, selecting plant/crop types, and
calculating the agronomic rate for biosolids land
application. It should be noted that the most
plant-available form of nitrogen in biosolids
(ammonium ion (NH4+)) is converted to nitrate
(NO3") by the composting process. Improper use of
biosolids can result in the contamination of water
resources with leached nitrogen, because nitrate is
more mobile than ammonium, and is taken up less
easily by plants. However, applying compost in
accordance with the Part 503 Regulations poses
little risk to the environment or public health
(Fermante, 1997). In fact, the use of compost can
have a positive impact on the environment in
addition to the soil improving characteristics
previously discussed. Reduced dependence on
inorganic fertilizers can significantly decrease
nitrate contamination of ground and surface waters
often associated with use of inorganic fertilizers.
PERFORMANCE
Composting is a viable, beneficial option in
biosolids management. It is a proven method for
pathogen reduction and results in a valuable
product. According to a 1998 survey in Biocycle,
The Journal of Composting and Recycling, 274
biosolids composting facilities were operating in
the United States (Goldstein, 1999). Nearly 50
additional facilities were in various stages of
planning, design, and construction. A large
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number of these facilities (over 40 percent) use the
aerated static pile composting method.
Since 1984, EPA has encouraged the beneficial use
of wastewater residuals through formal policy
statements. The implementation of Part 503
enhanced the acceptance of biosolids as a resource
by standardizing metal and pathogen concentrations.
Moreover, Part 503 officially identifies composting
as a method to control pathogens and reduce vector
attraction.
Discussions of the specific performance factors of
the three primary composting methods are provided
below.
Aerated static pile systems are adaptable and
flexible to bulking agents and production rates.
Aerated static pile is mechanically simple, thus with
lower maintenance than other cost method.
Conversely, this configuration can be labor
intensive and may produce nuisance odors and dust.
Cover, negative aeration, chemically scrubbing, or
use of a well-maintained biofilter may be required
to minimize off-site odor migration. The popularity
of the aerated static pile method is based on the ease
of design and operation and lower capital costs
associated with facility construction. Selection of
an appropriate method requires an assessment of the
physical facility, process considerations, and
operation and maintenance costs (WEF, 1995).
Windrow composting is adaptable, flexible and
relatively mechanically simple. However, the
windrow configuration requires a large area and can
result in release of malodor, dust, and other airborne
particles to the environment during natural
processing, ventilation, and windrow turning.
In-vessel systems are less adaptable and flexible
compared with aerated static pile and windrow
systems. However, in-vessel composting requires a
smaller area. Because the reactor is completely
enclosed, the potential for odor and the need for
controls is increased. Due to the greater complexity
of in-vessel mechanical systems, trouble can be
encountered meeting peak flows, breakdowns are
more frequent, and repairs are more difficult and
costly. Failure of aeration devices, under- designed
aeration systems, or lack of a back-up aeration
method may cause large quantities of product to
become anaerobic, and therefore, unacceptable.
Often the compost residence time in in-vessel
composting systems is inadequate to produce a
stable product, particularly where the depth of the
composting mass is great, (e.g., more than 3 m [10
feet]) and mixing does not occur. In addition,
bridging sometimes occurs within these systems.
Finally, depending upon the configuration and
direction of air flow, the worker environment can
be very hostile. However, in-vessel composting
requires a smaller area and generates relatively
little dust outside the facility.
Table 2 compares the three methods and highlights
key features of each.
COSTS
The capital costs of aerated static pile or windrow
configuration may be lower than in-vessel
composting configurations, but costs increase
markedly when cover is required to control odors.
More highly mechanized in-vessel systems are
often more costly to construct, but tend to be less
labor intensive. On the other hand, in-vessel
systems tend to be less flexible in their ability to
adapt to changing properties of biosolids and
bulking agent feedstocks.
Capital costs of in-vessel systems range from
$33,000 to $83,000 per dry metric ton ($30,000 to
$75,000 per dry ton) per day processing capacity.
A typical aerated static pile facility costs
approximately $33,000 per dry metric ton ($30,000
per dry ton) per day of processing capacity
(Harkness, 1994; U.S. EPA, 1989).
Typical operation and maintenance (O&M) costs
for in-vessel systems range from $150 per dry ton
per day to greater than $200 per dry ton per day.
Aerated static pile O&M costs average $150 per
dry ton per day (Harkness, 1994; U.S. EPA, 1989).
Costs for windrow systems fall between the costs
for in-vessel and aerated static pile. The selling
price for compost ranges from $5 to $10 per cubic
yard or $10 to $20 per ton. Some facilities allow
landscapers and homeowners to pick up compost
for little or no charge.
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TABLE 2 COMPARISON OF COMPOSTING METHODS
Aerated Static Pile
Windrow
In-Vessel
Highly affected by weather (can
be lessened by covering, but at
increased cost)
Extensive operating history both
small and large scale
Large volume of bulking agent
required, leading to large volume
of material to handle at each
stage (including final distribution)
Adaptable to changes in biosolids
and bulking agent characteristics
Wide-ranging capital cost
Moderate labor requirements
Large land area required
Large volumes of air to be treated
for odor control
Moderately dependent on
mechanical equipment
Moderate energy requirement
Highly affected by weather (can
be lessened by covering, but at
increased cost)
Proven technology on small scale
Large volume of bulking agent
required, leading to large volume
of material to handle at each
stage (including final distribution)
Adaptable to changes in biosolids
and bulking agent characteristics
Low capital costs
Labor intensive
Large land area required
High potential for odor generation
during turning; difficult to
capture/contain air for treatment
Minimally dependent on
mechanical equipment
Low energy requirements
Only slightly affected by weather
Relatively short operating history
compared to other methods
High biosolids to bulking agent
ratio so less volume of material to
handle at each stage
Sensitive to changes in
characteristics of biosolids and
bulking agents
High capital costs
Not labor intensive
Small land area adequate
Small volume of process air that is
more easily captured for treatment
Highly dependent on mechanical
equipment
Moderate energy requirement
Source: Parsons, 2002.
REFERENCES
Other Related Fact Sheets
In-Vessel Composting of Biosolids
EPA 832-F-00-061
September 2000
Odor Management in Biosolids Management
EPA 832-F-00-067
September 2000
Centrifuge Thickening and Dewatering
EPA 832-F-00-053
September 2000
Belt Filter Press
EPA 832-F-00-057
September 2000
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owm/mtb/mtbfact.htm
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Standards for the Use and Disposal of
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3. Burkhardt, J.W., W.M. Miller, and M.
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ADDITIONAL INFORMATION
Chuck Murray
Chief of Plant Operations
Washington Suburban Sanitation Commission
14501 Sweitzer Lane
Laurel, Maryland 20707
R. Tim Haug
Deputy City Engineer, Wastewater
Dept. of Public Works, Bureau of Engineering
650 South Spring Street, Suite 200
Los Angeles, California 90014-1911
United States Composting Council
200 Parkway Drive South, Suite 310
Hauppauge, New York 11788
John Walker, PhD
U.S. EPA
Mail Code 4204
1200 Pennsylvania Avenue, NW
Washington, DC 20460
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-02-024
September 2002
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204M
1200 Pennsylvania Ave, NW
Washington, D.C. 20460
* 2002 *
THEYEAROF
CLEAN WATER
IMTB
Excellence in compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY
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