&EFA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-061
September 2000
Biosolids
Technology Fact Sheet
In-Vessel Composting of Biosolids
DESCRIPTION
Biosolids are primarily organic materials produced
during wastewater treatment which may be put to
beneficial use. Composting is the biological
degradation of organic materials under controlled
aerobic conditions. The process is used to stabilize
wastewater solids prior to their use as a soil
amendment or mulch in landscaping, horticulture,
and agriculture. Figure 1 shows an example of a
finished product of compost. Stabilization of
wastewater solids prior to their use serves to
destroy pathogens (disease causing organisms),
minimize odors, and reduce vector attraction
potential.
The Environmental Protection Agency's (EPA's) 40
CFR Part 503, Standards for the Use and Disposal
of Sewage Sludge, (the Part 503 Rule) defines two
types of biosolids with respect to pathogen
reduction: Class A and Class B. The difference is
defined by the degree of pathogen reduction on the
solids. When federal performance standards are
met, composting insures full destruction of
Source: U.S. EPA, 1986.
FIGURE 1 FINISHED COMPOST
PRODUCT
pathogens to non-detectable levels in the
wastewater solids (i.e., to Class A standards.) The
Part 503 Rule requires the composting process to
maintain a temperature of at least 55 degrees
Celsius for a minimum of three days to effectively
destroy pathogens and qualify as Class A.
In addition to performance standards for the
composting process, the Part 503 Rule established
maximum concentrations for nine metals which
cannot be exceeded in biosolids products, including
compost. These are known as ceiling
concentrations. The federal maximum allowable
metals concentrations are provided in Table 1. The
Part 503 Rule also established more stringent
pollutant concentrations. Biosolids products which
do not exceed pollutant concentrations, meet Class
A pathogen reduction requirements, and are
processed to reduce vector attraction potential are
often referred to as Exception Quality products.
Products meeting these requirements may be freely
distributed for a variety of uses.
There are three general methods of composting
biosolids: windrow, aerated static pile, and in-
vessel. Each method uses the same scientific
principals but varies in procedures and equipment
needs. This Fact Sheet addresses in-vessel
composting.
In-vessel composting occurs within a contained
vessel, enabling the operator to maintain closer
control over the process in comparison with other
composting methods. A typical flow diagram for in-
vessel composting is shown in Figure 2.
There are several types of in-vessel composting
reactors: vertical plug-flow, horizontal plug-flow,
and agitated bin. The primary difference involves
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TABLE 1 MAXIMUM METAL
CONCENTRATIONS
Metal Ceiling Pollutant
Concentration Concentrations
(mg/kg) (mg/kg)
Arsenic
Cadmium
Copper
Lead
Mercury
Molybdenum
Nickel
Selenium
Zinc
75
85
4,300
840
57
75
420
100
7,500
41
39
1,500
300
17
NL
420
100
2,800
NL = No established limit
Source: U.S. EPA, 1993 and 1994.
the aeration systems and loading/unloading
provisions. The first two systems operate as plug-
flow, which means that biosolids and bulking agent
are loaded on a periodic basis (typically daily or
weekly) while "finished" compost is discharged
from the opposite end of the system on roughly the
same schedule. The vessel is only completely
emptied for maintenance.
In vertical plug-flow systems, the biosolids and
bulking agent mixture is introduced into the top of
the reactor vessel and compost is discharged out
the bottom by a horizontally rotating screw auger.
Air is introduced in these systems either from the
bottom and travels up through the composting mass
where it is collected for treatment or through lances
hanging from the top of the reactor.
In horizontal plug-flow systems, the compost and
bulking agent mixture is loaded into one end of the
reactor. A steel ram pushes the mixture through the
reactor. Air is introduced and exhausted through
slots in the floor of the reactor. Compost is
discharged from the end of the reactor opposite the
ram.
The agitated bed reactors are typically open topped.
The biosolids and bulking agent mixture is loaded
from above. The composting mass is periodically
agitated using a mechanical device and air is
introduced through the floor of the reactors.
Agitated bed reactors can be operated as either plug
flow or batch operations. In batch operations, the
vessel is loaded with biosolids and bulking agent,
processing takes place, and the vessel is emptied.
As with other composting methods, the resulting
product is generally cured for at least 30 days after
EXHAUST TO
ATMOSPHERE
ENCLOSED
COMPOSTING
VESSEL
OPTIONAL
SCREENING
AND/OR
BAGGING
I
PRODUCT
DISTRIBUTION
Source: Modified from U.S. EPA, 1989.
FIGURE 2 FLOW DIAGRAM OF A TYPICAL IN-VESSEL COMPOSTING FACILITY
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Composting Basics
During composting, microorganisms break down
organic matter in wastewater solids into carbon
dioxide, water, heat, and compost. To ensure
optimal conditions for microbial growth, carbon and
nitrogen must be present in the proper balance in
the mixture being composted. The ideal
carbon-to-nitrogen ratio ranges from 25 to 35 parts
carbon for each one part of nitrogen by weight. A
lower ratio can result in ammonia odors. A higher
ratio will not create optimal conditions for microbial
growth causing degradation to occur at a slower
rate and temperatures to remain below levels
required for pathogen destruction. Wastewater
solids are primarily a source of nitrogen and must
be mixed with a higher carbon-containing material
such as wood chips, saw dust, newspaper or hulls.
In addition to supplying carbon to the composting
process, the bulking agent serves to increase the
porosity of the mixture. Porosity is important to
ensure that adequate oxygen reaches the
composting mass. Oxygen can be supplied to the
composting mass through active means such as
blowers and piping or through passive means such
as turning to allow more air into the mass. The
proper amount of air along with biosolids and
bulking agent is important. Haug (1980) provides
the basis for calculating the appropriate amounts of
these materials.
active composting and before use. A properly
operated facility produces a stable compost which
can be easily handled and safely stored. Compost
enhances soil properties, such as water holding
capacity, nutrient availability, and texture. In
Compost Engineering, R.T. Haug (1980) discusses
several ways to determine the degree of stability
achieved during composting including:
• Oxygen uptake rate.
• Low degree of reheating in curing piles.
• Organic content of the compost.
• Presence of nitrates and the absence of
ammonia and starch in the compost.
Because this process results in a usable material, an
important and often overlooked part of any
composting facility is product storage and
marketing. Unlike disposal-oriented technologies,
end users and markets for the product are seasonal
with peak demand in the spring and fall. Therefore,
provisions for storage of the final product until it is
sold are necessary. In addition, product marketing
efforts are essential to insure that end users
understand the material, recognize its value, and are
familiar with proper application techniques.
APPLICABILITY
The physical characteristics of most biosolids allow
for their successful composting. However, many
characteristics will impact design decisions. These
characteristics are discussed in the Design Criteria
section.
In-vessel technology is more suitable than other
composting technologies in suburban and urban
settings because the system allows for containment
and treatment of air to remove odors before release.
The requirement for a relatively small amount of
land also increases its applicability in these settings
over other types of composting. However, a
market for use of the resulting product will
generally be more readily available in suburban and
rural areas rather than urban settings.
ADVANTAGES AND DISADVANTAGES
Advantages
Composting offers advantages and disadvantages
that must be considered before selecting this option
for managing biosolids. First, composting produces
a reusable product as long as the feed materials are
suitable. Use of the product returns valuable
nutrients to the soil and enhances conditions for
vegetative growth. Compost can be handled more
easily than some other biosolids products such as
digested biosolids. It is very friable and has the
consistency of a peat soil. In addition, compost,
unlike other Class A products, is not subject to end
use restrictions. However, composting somewhat
increases the amount of material to be managed
through addition of bulking agent to improve
aeration in the composting mass. Typically, one
cubic yard of cake will produce three cubic yards of
compost. Some bulking agents can be screened out
and reused to minimize this disadvantage. This
"disadvantage" may also be an advantage because
the product can be sold.
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In comparison with other types of composting, the
in-vessel technology offers the following
conveniences:
• The composting process can be more closely
controlled.
• The effects of weather are diminished.
Less bulking agent may be required.
The quality of the resulting product is more
consistent.
• Less manpower is required to operate the
system and staff is less exposed to the
composting material.
• Process air can be more easily collected for
treatment to reduce odor emissions.
Less land area is required.
Public acceptance of the facility may be
better.
Disadvantages
There are also disadvantages associated with in-
vessel composting which must be considered before
selecting this technology for wastewater solids
management. In-vessel composting is generally
more costly than other composting methods,
particularly with respect to capital expenditures. In
addition, because it is more mechanized, more
equipment maintenance is necessary. A significant
drawback of composting that must be addressed
during facility design is the potential for fires. The
large amount of carbonaceous material stored and
used at composting facilities creates the potential for
fires in storage areas as well as in the active
composting mass. Sufficient aeration and moisture
are necessary to avoid fires.
Environmental Impacts
Several aspects of an in-vessel composting facility
can result in environmental impacts if the facility is
mismanaged. Proper design and operation can
reduce environmental impacts. Storage,
distribution, and use of the resulting product can
also result in environmental impacts if not
performed properly.
In-vessel composting facilities can impact air, water,
and soil. The primary impact to the air is nuisance
odors if process air is not properly treated before
emission to the atmosphere. Most in-vessel
composting facilities treat process air with either a
biofilter or chemical scrubbing system prior to
release to the atmosphere. Odors can result from
several possible constituents in the air exiting a
composting vessel. Much work has been done in
the last several years to characterize and control
odors from composting operations. Bioaerosols
(organisms or biological agents in air that affect
human health) are also a concern in compost
emissions. The most widely studied bioaerosol is
Aspergillus fumigatus, a fungal spore. Endotoxins
(non-living components of cell walls of gram-
negative bacteria) and organic dust (such as pollens)
are also bioaerosols. These contaminants are
primarily of concern to workers at the composting
facilities and are generally not present in quantities
that would cause reactions in most humans. Health
effects to compost facility workers have not been
readily apparent in studies conducted to identify
such effects (Epstein et a/., 1998.)
Impacts to surface water bodies resulting from in-
vessel composting are unlikely. The enclosed nature
of the technology greatly diminishes the potential
for impacts to surface water due to high nitrogen
concentrations in runoff. Buildings should be
designed with floor drains to sewers or holding
tanks. Any unenclosed portions of an in-vessel
composting operation, such as materials receiving
and mixing, product curing, and product storage
should be designed with leachate/runoff containment
and provisions for disposal or treatment to avoid
runoff potential.
The use of biosolids compost as a soil conditioner
results in the following:
Increases water holding capacity.
Increases aeration and drainage for clay
soils.
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• Provides organic nitrogen, phosphorus, and
potassium.
Provides essential plant micronutrients.
Can reduce the need for pesticides.
Other environmental benefits of producing and using
compost include the recycling of a valuable
resource, reduction of dependence on chemical
fertilizers, and offsetting the use of natural resources
such as trees or peat moss as mulch material.
DESIGN CRITERIA
The following biosolids characteristics must be
considered in designing an in-vessel composting
system:
Moisture content.
Volatile solids content.
Carbon content.
Nitrogen content.
Bulk density.
These factors are discussed in detail in Composting
Engineering (Haug, 1980.)
The following bulking agent characteristics must
also be considered:
Size.
Cost/availability.
Recoverability.
Carbon availability.
Preprocessing requirements.
Porosity.
Moisture content.
Metals content of the biosolids will affect the
usability of the final product and must be considered
during design to ensure a market for the final
product.
An odor control system is an inherent part of in-
vessel design. The cost of an odor control system
can account for up to 50 percent of both capital and
operation and maintenance costs. Composting
facilities usually use either wet scrubbers or
biofilters for odor control. The level of odor control
required is a function of the quality and quantity of
air to be treated, the results of air dispersion
modeling, and proximity to occupied dwellings.
The most important design feature of a composting
system is the ability to maintain uniform aerobic
conditions during composting. The air distribution
system may be controlled by cycle timers and/or
temperature feedback control. The design must
avoid compaction of the composting mass to
maintain sufficient pore space for aeration. In
addition, provisions for routine monitoring of
temperatures must be included.
Equipment should be designed to provide
maintenance staff with safe access. Equipment and
instrumentation should be able to be removed or
repaired without having to relocate composting
material.
Systems that minimize worker exposure to hot
exhaust process gases are preferable because
workers can maintain the system and control odors
with greater ease, including minimizing the volume
of process air that must be treated.
Many in-vessel systems include a water spray
system to add moisture to the composting mass, to
control temperatures, and for fire protection.
Detention times, which vary with system
configuration, will affect many design
considerations, including equipment sizing.
Horizontal agitated bed systems are designed for 21
days of aerated composting followed by curing.
Other in-vessel systems use 10 to 21 days of active
composting. Some state regulations dictate
detention times for composting systems. In general,
about 21 days is a good minimum time for adequate
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stabilization. Provisions to monitor the degree of
stabilization allow operators to determine when the
biosolids are adequately processed and ready for
removal to curing piles.
Features of the site on which the in-vessel
composting facility is to be located must be
considered during design, including size, relative
position to residential areas, availability of
wastewater treatment, drainage, and access.
Examples of optimum locations for in-vessel
composting include a large tract of land in an
industrial area or a site near a municipal solid waste
landfill. One needs to determine the meteorology of
a potential site so that odors can be adequately
treated, diluted and dispersed.
PERFORMANCE
According to a survey conducted by BioCycle,
Journal of Composting and Recycling, in January
1999, there were 54 in-vessel composting facilities
processing wastewater residuals across the United
States (Goldstein and Gray, 1999) and 11 more
facilities were in various stages of design or
construction. Since that survey, at least two
facilities (Portland, Oregon and Camden County,
New Jersey) have closed. The vendor systems used
at the facilities listed in this survey include:
• Davis Composting and Residuals
Management (formerly Taulman
Composting Systems.)
• Bedminster Bioconversion (co-composting
with municipal solid waste.)
US Filter/International Process Systems.
Longwood Manufacturing.
• American Biotech Systems.
Purac.
• Gicon Tunnels.
Resource Optimization Technology (ROT
Box.)
• Compost System Company Paygro.
• Green Mountain Technologies.
• Waste Solutions.
• Royer.
• Fairfield.
• Conporec.
• Compost System Company Dynatherm.
• Dano.
In addition to these, there are several aerated static
pile systems contained within a building that are
categorized as in-vessel systems.
The above list is not intended to be a comprehensive
list of vendors who offer in-vessel composting
facilities. There are also many facilities in operation
which use non-patented systems and components.
The number of operating in-vessel composting
facilities for biosolids in the United States has
steadily increased in the last two decades but has
leveled off in recent years. In spite of early
operational difficulties and challenges, many early
facilities have been upgraded and are successfully
operating today.
OPERATION AND MAINTENANCE
In-vessel composting systems can be relatively
complex but the skills required for successful
operation are common to wastewater treatment
plant personnel. Typical labor requirements include
heavy equipment operators, maintenance personnel,
and instrumentation/computer operators. A clear
understanding of biological systems is necessary.
Additional staff or consultants are needed to
manage end use and to market the compost.
In-vessel composting facilities can require
significant maintenance. Many early composting
facilities constructed in the United States
experienced a variety of operating problems. Odor
complaints from neighboring residents have caused
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facilities to operate at reduced capacity or to shut
down for extended periods of time for system
modification. For example, a horizontal plug-flow
system in Hickory, North Carolina, was shut down
for more than a year while an odor issue was
addressed. The system reopened after the addition
of air pollution control equipment. The lack of
available spare parts has also caused extensive
periods of downtime at some facilities. Design
configurations have caused some facilities, primarily
vertically oriented plug-flow systems, to experience
month-long periods of inoperation while routine
maintenance was performed. Difficulties in
emptying the vessels have been cited as a reason for
significant maintenance requirements (O'Brien,
1986.) A system in Lancaster, Pennsylvania, was
shut down when state regulators determined it did
not meet temperature requirements for Class A
pathogen reduction.
There are three basic compost market strategies.
The first is the use of compost areas used by the
public sector, such as parks, ball fields, landfill
cover, and urban reclamation projects. Second,
direct marketing to users maximizes revenue and
improves the public image of the producer. This
strategy could include distribution centers run by the
compost facility where customers, such as
homeowners, greenhouses, landscapers, and
nurseries, can come to pick up the compost. The
third strategy is to use a compost broker. This may
result in lower revenue but removes the
administrative burden of compost marketing. About
25 percent of composters employ a broker. It
should be noted that revenue from compost sales
will not cover production costs but should offset
market development costs. Sale prices range from
$5 to $60 per ton.
COSTS
Costs associated with in-vessel composting systems
vary considerably from facility to facility. Site
specific factors and the many configurations and
equipment choices make it difficult to provide
general costs for this technology. Annual operation
and maintenance costs as low as $61 and as high as
$534 per dry ton of biosolids composted were cited
in a 1989 survey (Alpert et. al., 1989.) A more
recent assessment estimated costs for composting
between $100 and $280 per dry ton of biosolids
processed. In-vessel systems generally representthe
high end of such cost ranges (O'Dette, 1996.)
The following items must be considered when
estimate costs for a specific in-vessel composting
facility:
• Land acquisition.
• Equipment procurement, including the
composting vessel, loading equipment,
conveyors, air supply equipment,
temperature monitoring equipment, and
odor control equipment.
• Operation and maintenance labor.
• Additives, such as bulking agents, to be
used in the specific vessel selected.
Energy (electricity and fuel for equipment).
Water and wastewater treatment.
Equipment maintenance and upkeep.
Product distribution expenses and marketing
revenues.
• Regulatory compliance expenses such as
permitting, product analysis, process
monitoring, record keeping and reporting.
• Preprocessing equipment for bulking agent.
REFERENCES
Other Related Fact Sheets
Odor Management in Biosolids Management
EPA 832-F-00-067
September 2000
Centrifugal Dewatering/Thickening
EPA 832-F-00-053
September 2000
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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/owmitnet/mtbfact.htm.
Alpert, I.E., White, D.O., and Williams,
T.O., 1989. The Realities of the Enclosed
System Sludge Composting. InProceedings
of the National Conference on Municipal
Treatment Plant Sludge Management.
Silver Spring: Hazardous Materials Control
Research Institute. 10.
The Composting Council Internet site
[http://www.compostingcouncil.org].
August 1998.
11.
Diaz, L.F., Savage, G.M., Eggerth, L.L.,
and Golueke, C.G., 1993. Composting and
Recycling Municipal Solid Waste. Boca
Raton: Lewis Publishers.
Epstein, E., Croteau, G., Wu, N., and 12.
Youngberg, C., 1998. Bioaerosols at a
Biosolids Composting Facility: Health
Implication to Workers. In Proceedings of
the 12th Annual Residuals and Biosolids
Management Conference. Alexandria:
Water Environment Federation.
13.
14.
Goldstein, N. and Block, D., 1997.
Biosolids Composting Holds Its Own.
BioCycle Journal of Composting and
Recycling 38:12: 64-74.
Goldstein, N. and Gray, K., 1999. Biosolids
Composting in the United States. BioCycle
Journal of Composting and Recycling
40:1:63+.
Haug, R.T., 1980. Compost Engineering.
Ann Arbor: Ann Arbor Science Publishers, 15.
Inc.
O'Brien, J. R., 1986. The Tunnel Reactor
The Flexible In-Vessel Composting System.
In Proceedings of the National Conference
on Municipal Treatment Plant Sludge
Management. Silver Spring: Hazardous
Materials Control Research Institute.
O'Dette, R.G., 1996. Determining The
Most Cost Effective Option for Biosolids
and Residuals Management. InProceedings
of the 10th Annual Residuals and Biosolids
Management Conference: 10 Years of
Progress and a Look Toward the Future.
Alexandria: Water Environment Federation.
U.S EPA, 1999. Environmental
Regulations and Technology: Control of
Pathogens and Vector Attraction in Sewage
Sludge. U.S. EPA, Washington, D.C.
U.S. Environmental Protection Agency,
1993. Standards for the Use or Disposal of
Sewage Sludge (40 Code of Federal
Regulations Part 503). Washington D.C.:
U.S. Environmental Protection Agency.
U. S. Environmental Protection Agency,
1989. Summary Report: In-Vessel
Composting of Municipal Wastewater
Sludge, Technology Transfer Document
EPA/625/8-89/016, Cincinnati: U.S.
Environmental Protection Agency.
U.S. Environmental Protection Agency,
1986. Sewage Sludge Management Primer,
Technology Transfer Series. Cincinnati:
U.S. Environmental Protection Agency.
Walker, J.M., Goldstein, N., Chen, B.,
1989. Evaluating The In-Vessel
Composting Option in The BioCycle Guide
to Composting Municipal Wastes., ed. Staff
of BioCycle Journal of Waste Recycling.
Emmaus: The JG Press.
Water Environment Federation, 1995.
Wastewater Residuals Stabilization, Manual
of Practice FD-9. Alexandria: Water
Environment Federation.
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ADDITIONAL INFORMATION
City of Davenport
Scott Plett
P.O. Box 2707
Davenport, Iowa 52808
E&A Environmental Consultants
Eliot Epstein
95 Washington Street; Suite 218
Canton, Massachusetts 02071
NC Division of Pollution Prevention
Craig Coker
1639 Mail Service Center
Raleigh, North Carolina 27699-1639
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection
Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Ave, N.W.
Washington, D.C. 20460
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