B. In-Vessel or Contained Systems

Agitated bed composting systems contain the composting materials in a long

narrow horizontal bed formed within a channel created by concrete walls (Figure

3.7). The top of the bed is open, so the system is usually housed in a building. A

turning machine agitates and moves the materials in the bed. The turner moves down

the bed automatically (i.e., without an operator), usually guided by rails on the walls

of the channel. Agitated bed systems operate in continuous, plug-flow mode. Feedstocks are loaded in the front end of the channel. Compost is removed from the

opposite end. Starting at the compost discharge end, the turner moves toward the

front or loading end. With each pass, the turner displaces material a set distance

toward the back of the channel where the materials are discharged as compost.

Depending on the turner, material is shifted 2 to 4 m with each turning. This

displacement distance plus the length of the bed and the turning frequency determine

composting period in the bed (generally 10 to 28 days).



Figure 3.7



Horizontal agitated bed composting system with a single bed (courtesy of Karin

Grobe).



Turners function similarly in all of the commercial systems, although the turner

design differs among the systems. Most turners agitate with flails or paddles on a

rotating shaft and use a conveyor to displace the composting material backwards.

Another type of turner is a rotating cylinder that carries material backward as it

rotates down the bed. Turner dimensions match the size of the beds. Normally, the

depth of the material in the bed decreases gradually from the loading end to the

discharge end because the materials shrink in volume as they compost. With some

turners, the displacement can be adjusted to build up the height of the bed toward

the back end of the channel.



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The dimensions of individual beds vary among the commercial systems, with depths

ranging from 1 to 3 m and widths of 2 to 4 m. Bed lengths typically range from roughly

50 to 100 m. Most applications use multiple channels and one or two turning machines

that are transported among channels. Single-channel applications are occasionally used

for smaller scale applications, usually for composting manure on farms. Rather than

separate multiple channels, some commercial systems use a single bed, 7 to 13 m wide.

An overhead crane supports and moves the turner through the bed in strips. The turner

otherwise works in the same manner as other agitated bed turners.

In most applications, forced aeration is provided through the floor of the channel.

Air flow is controlled by temperature or a time sequence adjusted according to

temperature levels. Usually the channels are divided into three to seven aeration

zones along their length. Zones nearest the loading end require a higher rate of

aeration. Most aerated system use positive pressure, forcing air up through the bed.

If odor treatment is needed, air within the building is captured by an exhaust fan

and directed to a biofilter. There is some interest in switching to negative pressure

aeration for improved odor control.

2. Aerated Containers

Aerated containers are fully enclosed and covered containers of various materials

and dimensions (Figure 3.8). The types of containers used vary from solid-waste rolloff boxes to flexible polyethylene bags (Rynk, 2000c, 2000d). Aerated containers

typically compost feedstocks in batches. Therefore, they are modular. Additional containers are added to accommodate successive batches. Applications employ from 2 to

over 40 containers depending on the scale of operation and the container system.

Except for a few small passively aerated containers that are used for very lowvolume applications, aerated containers rely on fans to supply O2 and remove heat,

moisture, CO2, and other process gases. Several containers can be aerated from a

single fan system by connecting individual containers to an air distribution header.

In most cases, air is introduced at the base of the material from a pipe or plenum

in the floor and flows up through the composting mass into a headspace at the top

of the container. In other cases, air flows in the opposite direction, from the headspace

to the plenum. Some systems periodically reverse the airflow to correct temperature

and moisture gradients. Exhaust is usually collected and passed through a biofilter

for odor treatment. Aeration may be controlled by time or temperature, depending

on the system. Some systems employ sophisticated aeration controls that include

temperature feedback control, moisture and O2 monitoring, and air recirculation.

Aerated containers are essentially static systems. No agitation or turning takes

place within the container. Therefore, materials must be well mixed prior to loading

the containers. Most systems allow for the containers to be emptied partway through

the process. After emptying, the materials are mixed and adjusted externally, and

then reloaded into the container for continued composting (or delivered to a second

composting system like windrows). Emptying the container provides an opportunity

to add more feedstock or combine materials from several containers to compensate

for the shrinkage. Because the exercise of emptying and reloading containers requires

labor and expense, it is not practiced in many cases.

© 2001 by CRC Press LLC



Figure 3.8



Interior of an aerated-container composting reactor (courtesy of Jim McNelly).



Common examples of commercial-scale aerated containers include modified

steel solid waste roll-off boxes, long polyethylene bags, and specially constructed

tunnels. Several commercially available systems use corrosion-resistant roll-off containers modified with a tight fitting lid, an air distribution plenum in the floor, and

air vents in the headspace. Individual units range in capacity from 15 to over 50 m3.

They are typically 2 to 3 m high and wide and from 3 to 15 m long. These containers

are moveable and modular. They are loaded with conveyors or bucket loaders through

the lid and emptied from the end by tipping with a roll-off truck or tipping equipment.

The tipping system allows materials to be emptied and remixed externally as

described previously. As containers are filled, they are connected to the air delivery

manifold and materials compost as a batch. Valves govern the airflow to individual

containers. Aeration and process control strategies for these units can be highly

technical, involving computer control, monitoring of several process parameters, and

variation in airflow rate and direction.

Tunnel composting systems, commonly used by the mushroom industry, are

finding use for composting other feedstocks as well (Panter et al., 1996). Tunnels

are similar in scale and operation to the roll-off container systems, except they

usually are not intended to be moved. They may be constructed on site with concrete

or prefabricated with corrosion-resistant steel. They provide controlled forced aeration through a floor plenum. One characteristic of tunnel systems is their ability to

monitor and adjust internal air conditions by recirculating internal air with some



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outside air blended in. Tunnels are loaded from one end and operate in batch mode

after the tunnel is fully loaded. Multiple tunnels are used to attain a nearly continuous

operation. Unlike the top loaded roll-off containers, tunnels cannot be aerated until

they are fully loaded.

The polyethylene bag composting system is a different example of an aerated

container. It is essentially an aerated static pile in a bag. The bags used are similar

to those used on farms for making silage but, for composting, aeration pipe is inserted

to create an aerobic environment inside the bag. Bags range in size from 1.5 to 3

m in diameter and up to 65 m in length. Special equipment loads the bags with

premixed feedstock to a prescribed density. Aeration pipe is rolled out beneath the

feedstock mix as it is pushed into the bag. After the bag is loaded, the pipe is

connected to a positive-pressure blower which pushes air through the composting

pile. Ports in the side of the bag serve as exhaust vents and allow access for

temperature probes. A bag can be temporarily sealed and aerated before it is completed full. This system does not accommodate agitation or transport of an intact

container. When the compost is finished, or ready for the next stage of processing,

the bag is sliced open and the contents are removed with a bucket loader.

3. Aerated-Agitated Containers

Several commercially available composting systems combine forced aeration

and internal agitation. However, at present only one is applicable to the production

of compost on a large scale (Rynk, 2000c, 2000d).

In this system, the composting materials are supported on perforated stainless

steel trays that advance in sequence through an enclosed aeration chamber or tunnel.

An external hydraulic ram pushes an empty tray into the tunnel to be loaded with

the feedstock mixture from an overhead hopper. As the new tray enters the tunnel,

it nudges the preceding trays along and the last tray is discharged. At the discharge

point, augers unload compost from the exiting tray. Within the tunnel, air is forced

through the trays from a plenum below. Air is recirculated and eventually exhausted

to a biofilter, which is an integral part of the unit. Two aeration or temperature zones

exist. Higher temperatures are maintained in the first zone for pathogen destruction.

Inside the tunnel, as a tray moves from the first zone to the next, the compost is

agitated and, if necessary, water can be added during agitation. One self-contained

unit incorporates the entire system — hopper, tunnel, aeration, agitation, augurs,

and biofilter. Generally, system capacity is determined by the size and number of

the units used. Individual units range in throughput capacity from less than 100 kg

to over 30 Mg per day.

4. Silo or Tower Reactors

Silo type composting systems use one or more vertically oriented vessels, or

silos, in which composting materials move through the silo from top to bottom (U.S.

EPA, 1994). The movement starts when an auger or other unloading mechanism

removes a section of compost from the bottom of the silo. The materials above shift

downward and then a fresh mixture of feedstocks is added to the space created at

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the top of the silo. Although the materials within the silo move, they are not well

agitated. Therefore feedstocks must be well mixed before loading. Material typically

remains in the silo from 10 to 30 days, depending on the application. Often, material

removed from the silo moves to a second silo or a secondary composting system.

Nearly all silo systems are aerated with fans. Normally air is introduced at the

bottom and moved up through the materials in the silo. The air is collected at the

top and exhausted, usually to a biofilter. As the air moves up through the deep bed

of material it gathers heat, moisture and CO2. Therefore, the air loses its effectiveness

as a means of providing O2 and cooling, so it is difficult to maintain uniform

composting conditions across the entire depth. Attempting to overcome this deficiency, some systems use a set of narrow aeration ducts inserted down into the silo

to introduce fresh air at different depths within the silo.

One small commercial silo system relies on passive aeration (Rynk, 2000c,

2000d). This system contains the composting materials in silos formed by tall narrow

wire-mesh cages. The cages are arranged in series like slices in a loaf of bread. A

10-cm air space separates adjacent cages and provides a channel for passive air flow

and O2 diffusion. Cages are approximately 3 m high, 7 m long, and 1.2 m wide.

Therefore, the core of the composting mass is only about 0.5 m from the air space

that surrounds the cage. Materials move vertically through the system in a silo

fashion with feedstock loaded at the top of the cages and compost removed from

the bottom. Because the cages are made of wire mesh, the system is not totally

enclosed.

The main advantage of silo systems is the small surface area, or footprint,

afforded by the vertical orientation. However, vertical stacking of materials also

creates aeration challenges due to the greater depth of material in the silo and the

compaction that occurs when materials are stacked high.

5. Rotating Drum Reactors

Rotating drum composting digesters have been used for many years, primarily

for large-scale municipal solid waste (MSW) facilities (Richard, 1992) and, at a

much smaller scale, for backyard composting (Dickson et al., 1991). Recently,

several versions of commercial-scale rotating drums have emerged that are appropriate for applications between these extremes (Rynk, 2000c, 2000d). Although the

commercial drum systems differ in size, design details and process management,

they share the basic technique of promoting decomposition by tumbling material

inside an enclosed reactor.

Drums are mounted horizontally, usually at a slight incline (Figure 3.9). They

slowly rotate either continuously or intermittently, tumbling the material inside. The

tumbling action mixes, agitates, and generally moves material through the drum.

Feedstocks are loaded at one end of the drum and compost is removed at the opposite

end. Various loading and unloading devices are used, depending on the specific

system. Loading with conveyors and unloading by gravity are common. Some of

the large drums have internal partitions that separate the drum into compartments

and define distinct batches of materials. Doors in the partitions allow material to be

transferred from one compartment to the next.

© 2001 by CRC Press LLC



Figure 3.9



Small-scale rotating drum composting reactor.



In regard to the composting process, the key function of the rotation is to expose

the material to fresh air, to add O2, and to release heat and gaseous products of

decomposition. To deliver the fresh air and remove the gaseous products, forced

aeration is usually but not always provided. In some cases, the short drums can

obtain sufficient aeration by passive air exchange through the openings in the ends.

When forced aeration is used, air is directed from the compost or discharge end of

the drum to the opposite end, where the feedstocks are loaded.

The largest drum systems are used at facilities composting diverse feedstocks

like MSW. These drums are 3 m or more in diameter and over 50 m long. Several

drums may be used in parallel. Smaller scale drum systems are targeted for sourceseparate feedstocks like livestock manure, animal mortalities, yard trimmings, and

food residuals. These units range from 1.5 to 3 m in diameter and 3 to 15 m in length.

Rotating drum composting reactors have always been associated with very short

retention times, typically 3 to 5 days. In practice, drums have served primarily as a

first stage of composting. Material removed from the drums is usually finished in

windrows, aerated piles, or another secondary composting system. The drums start

the feedstocks composting quickly and evenly in a controlled high-temperature

environment. Drums are particularly effective at homogenizing heterogeneous mixtures like MSW. However, depending on the mixture and drum design, some systems

have experienced insufficient aeration leading to organic acid formation and a severe

drop in pH. This can be problematic, since in several applications, drums are the

only stage of active composting. In some cases the composting time is extended to

several weeks, which should allow aerobic degradation of any acids as aeration

demand drops over time. In other cases, the compost is cured after only 3 or 5 days

of composting in the drum. Such an abbreviated composting period deserves caution.



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The material discharged may be appropriate for some uses, such as direct land

application in the winter or autumn. However, analysis of compost maturity parameters suggests that a period of several weeks is necessary to achieve a compost

product that is mature enough for general horticultural use (Epstein, 1997; Golueke,

1972).



VI. ODOR MANAGEMENT

Odor is among the greatest threats to successful operation of composting facilities, and is a major contributor to many facility closures (Miller, 1993). Odor

problems are often misdiagnosed, and some purported solutions can actually make

the problem worse. A good understanding of the nature and types of odors associated

with composting is important to their effective control.

One of the difficulties with odor is that it is a subjective and quite variable trait.

Although significant advances in odor quantification continue to occur, there will

always be wide variation between two people’s perception of the same odor phenomenon. Not only are there differences in sensitivity among individuals, the same

odor that smells bad to one person may be tolerable or even pleasurable to another.

In addition to significant psychological effects (Schiffman et al., 1995), odors can

affect both cognitive performance and physiological response (Lorig, 1992; Ludvigson and Rottman, 1989). Accepting and addressing the differences in perception and

health impacts of odors is a critical part of responding to a community’s odor

concerns.

Anaerobic decomposition is the most common source of malodorous compounds

at composting sites. Despite all attempts to maintain aerobic conditions, during the

active phase of composting there are likely to be pockets of anaerobic activity within

large particles and in clumps where compaction or high moisture creates resistance

to airflow. Anaerobic odors include a wide range of volatile organic acids, Ncontaining compounds including NH3 and amines, ketones, phenols, terpines, alcohols, and S compounds (Eitzer, 1995; Epstein, 1997; Miller, 1993). Most of these

compounds are intermediate products of decomposition, and require an aerobic

metabolic pathway to complete their degradation.

Not all malodors are produced anaerobically. Ammonia is a common odor that

can be produced either aerobically or anaerobically. Protein degradation in a low

C:N mixture generates NH3 in excess of microbial growth requirements. Under low

pH conditions this NH3 remains in solution as the aqueous ammonium ion (NH4+),

but under high pH (>8.5) it is in the form of more volatile NH3 gas (Figure 3.10).

The partitioning into NH3 is also encouraged by high temperatures, and compounded

by high airflow rates that insure high concentration gradients at the gas/solute

interface. Ammonia gas is lighter than air and not as persistent as some of the strictly

anaerobic odors such as sulfides and organic acids, so although NH3 may be the

predominant odor in the immediate vicinity of low C:N composting, it rarely is a

significant problem off site.

Odors can be generated in almost any component of a composting system. In

facilities accepting readily degradable materials like food scraps and grass, the odors

© 2001 by CRC Press LLC



Figure 3.10



Effect of pH on equilibrium between gaseous ammonia (NH3) and ammonium

ion (NH4+).



may be arriving with the incoming materials. An enclosed tipping area with negative

air pressure is one approach to collecting these emissions. The initial stages of

composting are also likely to be a major odor source, especially when composting

highly degradable materials, a low C:N ratio mixture (< 25:1), or a mixture that

contains more than about 60% moisture. Quantifying these risk factors is imprecise

because of the wide range of composting feedstocks and mixture characteristics, but

if the O2 supply is inadequate to keep up with demand — or unable to thoroughly

penetrate the composting matrix — anaerobic odors are likely to result.

Odors can also be generated during the later stages of composting. Anaerobic

conditions may occur when material is moved from active composting to a curing

area, either because forced aeration is no longer being used, or because the pile size

has been increased beyond the ability of diffusion or convection to supply adequate

O2. Heavy precipitation in an outdoor curing pile can saturate the pores and reduce

air movement, also resulting in anaerobic conditions. Odor in a curing pile is

particularly problematic as it can affect both the site neighbors and potential customers. Aside from the obvious impact on the aesthetics of the final product, many

of the odorous organic acids can also be phytotoxic to plants.

Although it is important to minimize anaerobic conditions by maintaining good

process control, complete odor prevention is not always successful at even wellmanaged composting facilities. If odors do occur, there are a number of interventions

possible before those odors impact a sensitive neighbor. Many anaerobically produced odorous compounds still contain considerable amounts of energy, and if they

pass through an aerobic zone they can be further decomposed. Adsorption and

microbial degradation of odors is actively encouraged in biofilters (described later)

but also occurs in-situ near the surface of composting piles. In-situ biofiltration can

be enhanced by covering freshly decomposing materials with a layer of older,



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stabilized compost. Unmanaged and even unrecognized by most facility operators,

in-situ biofiltration is often adequate to reduce odors to tolerable levels, especially

in unturned piles.

Understanding the mechanisms of in-situ biofiltration can help explain the common misconception that more frequent turning reduces odor emissions. If one calculates the stoichiometric O2 demand at typical rates of biodegradation, many compost mixtures consume the amount of O2 introduced by turning within a few minutes

(Richard, 1997). If inadequate porosity or excessive pile size are constraining the

replenishment of O2 by wind, diffusion, and/or convection, turning a pile even daily

(the highest frequency generally possible) still provides more than 23 hours of

anaerobic odor production, which turning immediately releases. Unless the porosity

can be increased or pile size reduced, frequent turning can worsen odor emissions

compared to no turning at all.

For systems where exhaust air can be collected, treatment is possible by chemical,

thermal, or biological mechanisms. Chemical and thermal odor treatment methods

are well established for industrial applications, but to date their implementation at

composting facilities has been rare. For classical chemical treatment, the complexity

of composting odors would require multistage scrubbers with acid and alkali recovery systems (Dunson, 1993). Thermal treatment requires large energy inputs when

odor concentrations are relatively low, making it expensive to treat the large volumes

of mildly odorous air found at many composting facilities (Dunson, 1993).

Biological odor treatment via biofilters has been the most common approach

taken at composting facilities. A biofilter uses moist organic materials to adsorb and

then biologically degrade odorous compounds. Cooled and humidified compost

process air is typically injected through a grid of perforated pipes into a bed of

filtration media. The materials that have been used for biofilter construction include

compost, soil, peat, and chipped brush and bark, sometimes blended with a biologically inert material such as gravel to maintain adequate porosity. Biofilter bed depths

typically range from 1 to 1.5 m deep, with shallower beds subject to short circuiting

of gas flow and deeper beds more difficult to keep uniformly moist. Biofilters have

been shown to be effective at treating essentially all of the odors associated with

composting, including NH3 and a wide range of volatile organic compounds (including S compounds and amines).

The principal design criterion for biofilters is the airflow rate per unit surface

area of the biofilter. Literature values for biofilter airflows range from 0.005 to 0.0030

m·s–1 (1 to 6 cfm per ft2) (Lesson and Winer, 1991; Naylor, 1996) and are typically

0.015 to 0.02 m·s–1 (3 to 4 cfm per ft2). Over a 1 to 1.5 m path length, a typical

residence time for the gas is 45 to 60 s (Naylor, 1996). For the purpose of selecting

the biofilter blower, the backpressure expected across the biofilter at this airflow rate

is usually in the range of 20 to 120 mm H2O per m depth (0.22 to 0.9 inches H2O

per ft), although the pressure drop can be considerably higher through dense composts and soil (Shoda, 1991).

Moisture and porosity are also essential to maintaining an effective biofilter.

Moisture content should be between 40 and 60%, while air-filled porosity should

be in the range of 32 to 50% (Naylor, 1996). Unsaturated air coming out of the

compost building dries the biofilter, and rewetting from the surface is generally not

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Figure 3.11



Diagram of biofilter components with humidification.



uniform. A simple humidification scheme can help to maintain the correct moisture

and porosity as illustrated in Figure 3.11.

Several other approaches to odor mitigation and treatment can be used at composting sites. A number of facilities use odor masking agents, which attempt to cover

up malodors with a more desirable odor. Unfortunately, masking agents are rarely

as persistent as the original malodorous compounds, and thus may prove ineffective

downwind of the site. Chemical, bacterial, and enzymatic deodorants have a mixed

track record (Miner, 1995; Ritter, 1981), although some have recently been demonstrated as effective in reducing manure odor emissions (Bundy and Hoff, 1998).

Vegetative buffers can provide both a pleasing visual screen and a multitude of

surfaces for particulate odor filtration, gaseous odor adsorption, and then microbial

odor degradation (Miner, 1995). Trees also affect wind velocity and direction in

their immediate vicinity, which may be used to advantage if odor problems are

associated with a particular wind pattern.

If odors ultimately escape the composting facility they can travel significant

distances downwind. Many anaerobic odors are relatively persistent. Sulfur compounds eventually oxidize, while precipitation is the likely fate of highly soluble

compounds like NH3 and organic acids (Miller, 1993). The fate and transport of

odors is complex and subject to considerable local variation. When siting and

designing large facilities, or if a significant odor problem has already developed,

site-specific odor dispersion models can be used to evaluate alternative sites and

treatment options (Epstein, 1997; Haug, 1993; Miner, 1995).



VII. SITING AND ENVIRONMENTAL PROTECTION

Composting has long been viewed as an environmentally beneficial activity. To

maintain that positive reputation it is essential that compost facilities consider and

mitigate any adverse environmental impacts. Water quality protection can be accomplished at most composting facilities by proper attention to siting, ingredient mixtures, and compost pile management.

The results of several water quality monitoring studies indicate that outdoor

windrow composting can be practiced in an environmentally sound manner (Cole,

1994; Richard and Chadsey, 1994; Rymshaw et al., 1992). However, there are a few

aspects of this process that can potentially create problems. For leaf composting,



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the primary concerns are biochemical oxygen demand (BOD) and phenol concentrations found in water runoff and percolation (Lafrance et al., 1996; Richard and

Chadsey, 1994). BOD and phenols are both natural products of decomposition, but

the concentrated levels generated by large-scale composting dictate that runoff

should not be directly discharged into surface water. Additional potential concerns

when composting nutrient rich materials such as grass, manure, or biosolids include

N compounds such as nitrate (NO3) and NH3, and in some cases P as well. With

biosolids, manure, and even yard trimmings, there may also be pathogen concerns.

Although important, these concerns are readily managed, and can be mitigated

through careful facility design and operation.

A. Facility Design

Selecting the right site is critical to many aspects of a composting operation,

from materials transport and road access to neighborhood relations. From an environmental management perspective, the critical issues are soil type, slope, and the

nature of the buffer between the site and surface- or groundwater resources. Soils

can impact site design in a variety of ways. If the soils are impermeable, groundwater

is protected from NO3 pollution, but runoff is maximized, which must be managed

to prevent BOD, P, and pathogens from entering surface waters. On the other hand,

highly permeable soils reduce the runoff potential but may allow excessive NO3

infiltration to groundwater. Intermediate soil types may be best for sites that are

operated on the native soil. For some large facilities, or those handling challenging

feedstocks, a working surface of gravel, compacted sand, oiled stone, or even asphalt

or concrete may be appropriate. Such surfaces can improve trafficability during wet

seasons, but the surface or groundwater quality issues remain.

The buffer between the site and surface- or groundwater resources is the first

line of defense against water pollution. Deep soils, well above the seasonally high

water table, can filter solid particles and minimize NO3 migration. Horizontal buffers

can filter and absorb surface runoff, and can be enhanced by specially designed grass

filter strips.

Site design issues that may impact water quality include the selection of a

working surface (native soil or an improved surface), exclusion of run-on to the site

by surface diversions, possible drainage of wet sites, and the possible provision of

roofs over some or all of the composting area to divert precipitation and to keep

compost or waste materials dry. In all but fully roofed sites, surface runoff may need

to be managed as described later. Slope of the site and surface drainage to either

divert uphill water away from the site or collect site runoff for management should

be considered in the design process.

A number of factors combine to determine the quality of water running off

compost sites. One obvious factor which is often overlooked is the excess water

running onto the site from upslope. Diversion ditches and berms that divert water

around the site minimize the runoff that needs to be managed. Siting the facility on

a soil with moderate to high permeability also significantly reduces the runoff

generated on the site. For the runoff which remains, alternatives to surface discharge



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