Starting in the mid-1980s, many wastewater utilities interested in composting solids turned to in-vessel composting
technologies. These technologies consisted of enclosed vessels with highly mechanized systems for mixing and
conveying materials, air handling, and odor control. Compared to traditional methods of composting, such as static-pile
and windrow techniques, in-vessel composting technologies were thought to provide several advantages. These
included small area requirements, fewer personnel for operation and maintenance, more effective odor control, and
more consistent product quality.
Nevertheless, many in-vessel composting facilities encountered serious problems with odors, moisture reduction,
reliable mechanical operation, and a marketable compost product that was consistent in quality. The U.S. Environmental
Protection Agency (EPA) commissioned CDM (Cambridge, Mass.) to conduct an in-depth survey of six in-vessel
composting technologies. In the time since the resulting report was published in 1989, more than 50 additional in-vessel
composting facilities have been constructed and operated. Of the most highly mechanized technologies, a large
percentage has either substantially reduced throughput compared to design capacity or curtailed operations.
This article summarizes the results of a follow-up survey of 25 highly mechanized composting facilities conducted in
2005. Many facilities reported problems with inconsistent product quality, reduced throughput, odors, unreliable
materials handling, and, most importantly for facilities that closed, high costs when compared with other methods of
biosolids management. The technologies studied all had common characteristics, including mixing of either dewatered
digested biosolids or dewatered undigested sludge with recycled product and an amendment. The amendments typically
were sawdust, wood chips, yard wastes, and other similar carbonaceous materials. Following mixing, the compost
was conveyed to a reactor. Some facilities were operated with two reactors in series. All reactors had a method of
supplying air to the compost mix to stabilize the biosolids and meet 40 CFR 503 requirements for pathogen and vector
attraction reduction by elevated temperature. Typical reactor detention times were 21 to 28 days. All technologies had
some type of odor control method to treat the air drawn from the reactors. Some facilities also had post-reactor
compost aeration or extended storage before product distribution.
Survey Methods
The most recent survey of in-vessel composting facilities was completed in 1999 by BioCycle: The Journal of
Composting and Organic Recycling. This information was supplemented by information from equipment suppliers
and contacts with federal, state, and local officials, resulting in a list of approximately 60 in-vessel composting
facilities constructed in the United States between 1984 and 2005 (see Table 1, right). The facilities were
designed primarily for municipal wastewater solids or biosolids. Some facilities were designed primarily for cocomposting
or solid waste composting odors. During the next few years, odor control and other modifications were made, such as
changing the aeration system to positive-pressure mode. By 1995,the facility was operating reliably with a
biofilter replacing a mulitstage chemical scrubber. All compost was transported offsite to a privately owned curing facility
before product utilization. Due to restrictive state compost distribution rules, the compost product was limited to use as a
potting soil mixture or as intermediate or final landfill cover material.
The 25 highly mechanized facilities were contacted by telephone or e-mail to determine operational experiences. Most
of the facilities’ owners or operators completed a 2-page questionnaire or made themselves available for telephone
interviews. The highly mechanized facilities were those six technologies originally reviewed in the late 1980s.
A Look at Facilities
Not surprisingly, the survey showed a mixture of success and failure of the highly mechanized in-vessel composting
technologies. Nine of the 25 highly mechanized in-vessel composting facilities have closed. Another six are considering
whether to remain in operation. The remaining 10 are operating successfully and intend to remain operational.
The following case studies illustrate the range of responses.
Schenectady, N.Y.
This 14-dry-tonne/d (15-dry-ton/d) invessel composting facility has been operational since 1987. It is operated by
Veolia Water North America (Houston). The American Bio Tech in-vessel aeration system was significantly modified
shortly after startup because the “air lance” aeration system was short-circuiting, creating anaerobic pockets in the
reactor. The problem was corrected by placing all air lances under positive pressure and constructing an airtight
enclosure over the reactors. This allowed even airflow throughout the composting material, achieving drying and
elevated temperature goals.
The facility treats anaerobically digested, centrifuge-dewatered biosolids at 25% solids. The digested biosolids
contribute to low odor emissions, which is important in the facility’s urban residential area. The facility has one of the
lowest operating costs of any surveyed, at $165/dry tonne ($150/dry ton). Approximately 85% of compost is sold,
and 15% is given away to city residents.
Akron, Ohio
The Akron facility has been operational since 1986. It is the largest in-vessel composting facility in the United
States, with a design capacity of 66 dry tonne/d (73 dry ton/d) and four 220-m long (720-ft-long) Paygro reactors. The
facility is operated by KB Compost Services Inc. (Independence, Ohio). Dewatered primary and waste activated
sludge average approximately 40 dry tonne/d (44 dry ton/d), or about 60% of the facility’s design capacity. In recent
years the facility has been substantially improved with new dewatering facilities and building protection. New belt filter
presses have increased cake solids from 22% to 30%. A change from a shredded bark amendment to sawdust, coupled
with the dewatering improvements, has reduced the initial compost feed volume by 50%. The number of reactors
normally needed has been reduced from four to two and decreased the airflow to odor control. These improvements
have resulted in simpler operation requiring less staff and reducing energy consumption by 50%.
All of Akron’s compost product is sold at the current rate of $3.25 per 1.2 ft3 bag, or $26/m3 ($20/yd3) bulk. This
yields $450,000 to $600,000 in annual product revenues. Annual composting facility operational costs, including the
product revenue credit and amortized capital costs, are approximately $5.2 million, or approximately $358/dry
tonne ($325/dry ton).
Cape May County, N.J.
This is the longest-running in-vessel composting facility in the United States, having commenced operation in 1985.
The facility has an original design capacity of 11 dry tonne/d (12 dry ton/d). The facility uses the Purac technology with
two reactors providing approximately a 15-day detention time, followed by a 30-day second-stage aerated static-pile
composting process (curing stage).
Originally, the facility operated reactors in series. However, when the aerated static-pile facility was constructed in
1987, the reactors could be operated in parallel mode, thus increasing capacity to 18 dry tonne/d (20 dry ton/d).
Ground-up and screened recycled wood pallets are used for amendment with undigested dewatered sludge.
The entire solids production ranges from approximately 5 dry tonne/d (6 dry ton/d) during winter to 18 dry tonne/d
(20 dry ton/d) during summer. Approximately 95% of sludge is converted to a Class A compost product at the in-vessel
facility, producing approximately 13,000 m3/yr (17,000 yd3/yr) of product. At certain times of the year, the authority takes
sludge from other wastewater treatment facilities for composting. Approximately 75% of the compost, which is marketed
as “Cape Organic,” is sold to users or brokers. The product is sold in bulk as either the “Original” version for
approximately $4/m3 ($3/yd3) or “Fine” version (screened to remove wood waste amendment) for $8/m3 ($6/yd3). The
remaining compost is utilized for demonstration projects or given away.
The facility has been successful because extensive modifications were made to control odors better, the second stage
aerated static-pile operation improved process performance, current operation costs are approximately equal
to other options, and the authority has an established, well-known quality compost product and marketing program.
Hartford, Conn., Metropolitan District
This 48-dry-tonne/d (53-dry-ton/d) facility utilizing American Bio Tech technology was placed in operation in 1990.
Initially, the facility operated intermittently and at low capacity due to unreliable materials handling systems, a short
21-day in-vessel detention time, and odors. During the next few years, odor control and other modifications were
made, such as changing the aeration system to positive-pressure mode. By 1995, the facility was operating reliably with
a biofilter replacing a mulitstage chemical scrubber. All compost was transported off site to a privately owned curing
facility before product utilization. Due to restrictive state compost distribution rules, the compost product was limited to
use as a potting soil mixture or as intermediate or final landfill cover material.
In 1999, with only a single off site curing facility and lack of product marketing opportunities, the district was storing
large quantities of cured and uncured compost at a local nursery. When the state regulatory agency discovered excess
nitrogen in nearby groundwater, it suspended all composting operations. The district, unable to transport the compost
from its facility, allowed the compost to become excessively dried with continuous aeration. That led to a fire that
destroyed the facility. The district chose not to rebuild the composting facility, since the state had no plans to allow
greater product utilization, and incinerator operating costs are significantly less than the costs of continued composting.
Portland, Ore.
Portland’s now-closed in-vessel composting facility was the first highly mechanized facility constructed in the
United States in 1984. The 54-drytonne/ d (60-dry-ton/d) facility utilized the Taulman-Weiss technology with six
reactors for a detention time of approximately 21 days. The feed was a mixture of anaerobically digested primary sludge,
dewatered to 25% solids, and sawdust. The facility was only able to achieve a throughput of approximately 14 dry
tonne/d (15 dry ton/d) for a variety of reasons, but generally because a longer total detention time was required.
Detention time was greater than 80 days, followed by approximately 3 months of onsite storage. However, the city sold
100% of its compost product, nearly all to topsoil blenders.
In 1999, the city decided to curtail composting operations because labor, maintenance, electricity, and amendment costs
were high, averaging approximately $386/dry tonne ($350/dry ton). The city diverted all anaerobically digested,
dewatered biosolids cake to a beneficial use dry-land pasture site. Although the site was approximately 320 km (200 mi)
from Portland, the land application program cost less than half of the in-vessel composting operation.
What Factors Determine Success
Three factors — experience with product marketing, system reliability, and cost of competing alternatives — seemed
to play a large role with the survey respondents.
Product Marketing
Many owners were promised that invessel systems would guarantee a product with significant demand. For example,
one facility’s engineer stated that the sale of compost could nearly offset the relatively low operating costs of in-vessel
composting, making the system relatively “inflation-proof.” At another facility, compost revenues were predicted to
amount to 102% of operating costs based on a price of $39/tonne ($35/ton).
Unfortunately, some in-vessel composting facilities produced inconsistent-quality compost that was high in moisture,
unstable, and odorous. These conditions were related to undersized reactors, nonuniform aeration, and lack of
adequate post-reactor curing and storage.
In general, those facilities that adjusted in-vessel detention times by derating the original design capacity or
adding second-stage composting improved product quality. Also, those facilities employing horizontal agitated
technologies, in which regular agitation breaks up compost and allows consistent temperatures and better drying, had
better products.
Many facilities reported that some or all product was landfilled, land-applied at an additional expense, or given away.
However, a significant number of facilities, both operational and closed, reported that all or nearly all compost
product was sold. Facilities with successful marketing programs reported that 75% to 100% of compost was sold.
Six facilities could charge prices high enough to yield revenues equal to or exceeding 5% of annual operating
costs.
At Newberg, Ore., for example, all compost is sold for $13/m3 ($10/yd3) during the “high demand” period (April
through October) and $9/m3 ($7/yd3) the rest of the time.
Compost produced by Clayton County, Ga., was of excellent quality, and customers continued to inquire about it 5 years
after the facility was shut down for cost reasons.
System Reliability
System reliability is critical to success or failure. Nearly all facilities currently operating have sufficient maintenance
budgets and personnel to sustain reliable system operation. Approximately one third of these facilities employ
contractors for operation and product marketing. Examples of successful facilities with contract operators include
Schenectady, Akron, and Baltimore. The remainder relies on municipal operating and maintenance staff with a mix of
municipal and contract marketing. Successful examples include Cape May County and Hamilton.
Among facilities that have closed, however, unreliable mechanical components, including mixers, conveyors, and
other materials-handling equipment, have been frequently cited as the problem. Frequent failure of the outfeed device
in Taulman-Weiss facilities was a major contributor to facility closure in Lancaster, Pa., and Reedy Creek, Fla.
Unreliable outfeed device performance also led to significantly reduced operation in Bristol, Tenn., where only 20% of
annual sludge production was composted in 2004.
Frequent mechanical problems were cited in the closure of other facilities employing a wide variety of in-vessel
technologies. These same problems also required facility operators to derate system capacity. For example, at the
Portland facility, only 25% of the rated capacity could be sustained. Camden County, N.J.; Henrico County, Va.; and
Hartford, Conn., were never able to process more than 50% of their rated capacity. These reliability problems
greatly influenced operation and maintenance costs, driving up per-ton costs for composting and sometimes requiring
alternative disposal for excess biosolids.
Costs
Operation and maintenance costs were reported for approximately 10 of the 25 in-vessel facilities. The average
cost was approximately $364/dry tonne ($330/dry ton) of dewatered sludge. In many cases, the high cost of composting
was the deciding factor in facility closure. Table 2 (p. 8) summarizes some of the reasons why facilities closed. In several
cases, in-vessel composting was replaced with methods costing substantially less.
At Henrico County, where a $23 million facility was operated for only a year, operators found that land application of
biosolids cost one-third less than composting. At Clayton County, alternative disposal costs were $303/dry tonne
($275/dry ton) compared to $661/dry tonne ($600/dry ton) for composting approximately 1.8 dry tonne/d (2 dry ton/d).
The reasons for the high cost of highly mechanized composting are many including maintenance and a higher than-
expected cost for amendment.
Other operations experienced higher than expected moisture content in sludge, amendment, or recycled compost.
If the moisture content of these ingredients was only a few percentage points higher than expected, considerably
more amendment was required. A secondary problem with high moisture content was that the higher volumes of
feed material decreased reactor detention time, sometimes causing unstable compost.
The Future of In-Vessel Composting
Successful facilities shared the following common characteristics:
• Owners and operators are committed to recycling. They focus on customer needs for high-quality compost.
• The facilities have adequate staff and financial commitment for continued operations.
• Odors are comprehensively controlled.
• The in-vessel composting process is operated to ensure adequate in-vessel detention time, aeration, curing, and
product storage.
Most of the equipment manufacturers of highly mechanized in-vessel composting facilities are no longer marketing
these technologies, so it is unlikely that new facilities will be constructed. However, in-vessel composting has its place
among biosolids management options. It is likely that most future systems will employ agitated bin technology, which has
a number of attributes to make it a good compromise between the highly mechanized technologies addressed in this
survey and more commonly used static-pile or windrow composting methods.
Among facilities that have closed, however, unreliable mechanical components, including mixers, conveyors, and
other materials-handling equipment, have been frequently cited as the problem. Frequent failure of the outfeed device
in Taulman-Weiss facilities was a major contributor to facility closure in Lancaster, Pa., and Reedy Creek, Fla.
Unreliable outfeed device performance also led to significantly reduced operation in Bristol, Tenn., where only 20% of
annual sludge production was composted in 2004.
Frequent mechanical problems were cited in the closure of other facilities employing a wide variety of in-vessel
technologies. These same problems also required facility operators to derate system capacity. For example, at the
Portland facility, only 25% of the rated capacity could be sustained. Camden County, N.J.; Henrico County, Va.; and
Hartford, Conn., were never able to process more than 50% of their rated capacity. These reliability problems
greatly influenced operation and maintenance costs, driving up per-ton costs for composting and sometimes requiring
alternative disposal for excess biosolids.
Portland, Ore, Clayton County, Ga., Hartford, Conn. Ft. Lauderdale, Fla. , Clinton County,N.Y.