V. CHARACTERIZATION OF COMPOSTING METHODS

discussed within these broad categories. Table 3.3 summarizes the methods covered

in this chapter.

Table 3.3 Summary of Selected Commercial-Scale Composting Methods

Method and Description



Open Methods

Turned windrows: Long narrow piles that are regularly turned and aerated passively.

Passively aerated static piles: Freestanding piles that are turned infrequently or not at all

and aerate passively without aeration aids.

Static piles and windrows with assisted passive aeration (e.g., Passively Aerated

Windrow System — PAWS; and Naturally Aerated Static Piles — NASP): Static windrows

and piles with passive aeration aids such as perforated pipe and aeration plenums.

Aerated static piles and bins: Freestanding piles or simple bins with forced aeration and

no turning.

Aerated and turned piles, windrows, and bins: Freestanding piles or windrows, or simple

bins with forced aeration system. Materials are turned regularly or occasionally.

In-Vessel or Contained Methods

Horizontal agitated beds: Materials are composted in long narrow beds with regular turning,

usually forced aeration, and continuous movement.

Aerated containers: Materials are contained in variety of containers with forced aeration.

Aerated-agitated containers: Commercial containers that provide forced aeration, agitation,

and continuous movement of materials.

Silo or tower reactors: Vertically oriented forced aerated systems with top to bottom

continuous movement of materials.

Rotating drums: Slowly rotating horizontal drums that constantly or intermittently tumble

materials and move them through the system.



A. Open Composting Methods

With methods that would be considered open, the materials are composted in

freestanding piles or windrows. In some cases, materials are placed in simple twoor three-sided bins. Composting may take place outdoors or under the cover of a

building. The defining feature of open composting methods is that they do not control

the environment surrounding the composting materials. Examples of open composting methods include turned windrows, passively aerated static piles, and forced

aerated static piles and bins.

1. Turned Windrow Method

Turned windrow composting may be the most common method practiced (Haug,

1993; Rynk et al., 1992). Since this method typifies large-scale composting, it is

frequently the standard by which other methods are compared (Dougherty, 1999;

Golueke, 1972; Hay and Kuchenrither, 1990; Kuter, 1995; Rynk et al., 1992).

© 2001 by CRC Press LLC



Figure 3.2



Turned windrow composting facility.



Turned windrow composting forms materials into long narrow piles or windrows

(Figure 3.2). Windrows essentially aerate by passive means — diffusion, wind, and

convection. To supplement passive aeration, windrows are turned on a regular basis.

Turning is simply a thorough agitation of the materials. It is accomplished with

bucket loaders or special windrow-turning machines. In practice, the number of

turnings and time between turnings varies greatly, ranging from 3 or 4 turnings over

6 to 12 months to 40 turnings in a 2-month period.

Turning mixes and blends feedstocks; homogenizes materials in the windrow;

releases trapped gases and heat; distributes water, nutrients, and microorganisms

throughout the windrow; and exchanges material from the cool oxygenated environment at the surface of the windrow with material from the warmer O2-poor areas

near the core. Depending on the feedstocks and aggressiveness of turning equipment,

turning also reduces particle size.

It is often said that turning aerates the windrow. This is true, but only to a limited

extent. Although turning does add fresh air and O2, microorganisms consume the

O2 within hours (Epstein, 1997). Between turnings, windrows must aerate passively

to remain aerobic. Another belief is that turning fluffs materials in the windrow,

increasing porosity, reducing density, and making passive aeration more effective.

Michel et al. (1996) showed that this is not necessarily the case. In experiments with

windrow turning of yard trimmings, the bulk density of the window materials

increased after turning — the windrow became more compacted after turning. Most

likely the effect of turning on bulk density depends on the feedstocks and stage of

composting. With loose brittle material like leaves, turning decreases particle size

and increases bulk density. With material that is already dense, like manure or nearly

finished compost, turning does reduce the bulk density. However, the effect may last

for only several days as the turned material again settles and becomes compacted

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(McCartney and Chen, 1999). Therefore, without very frequent turning, the effect

of turning on aeration may be small. The turning equipment is also a factor. Some

compost turners aggressively shred materials in the windrow while other turners

have more of a tossing effect. Turning with bucket loaders is also more likely to

decrease bulk density. Although its effect on aeration may be limited, turning still

advances the composting process. It charges the windrow with fresh air and generally

invigorates the process (Michel et al., 1996; Rynk et al., 1992).

Windrows are managed according to the goals and preferences of facility operators and managers (Golueke, 1972; Rynk et al., 1992). Thus, in some systems

windrows are turned infrequently when operators have time and the weather is good.

At the other extreme, turnings take place almost daily based on measurement of

CO2 concentrations and temperature. A common strategy is to turn windrows based

on temperature patterns. For example, a windrow may be turned when temperature

falls or rises to prescribed levels or if temperatures consistently decrease over a

certain number of consecutive days (Figure 3.3). This management strategy typically

results in turnings occurring daily to every 2 days during the first 2 or 3 weeks of

composting, followed by weekly turnings for another 6 to 8 weeks. Again, windrow

management varies greatly among facilities because goals, resources, feedstocks,

and management philosophies also vary greatly among facilities.



Figure 3.3



Typical relationship of windrow temperatures with turning. Turning events are

indicated by vertical arrows.



Many facilities turn windrows with a bucket loader because loaders are already

available. Bucket loaders turn by repeatedly lifting and dropping materials while

rebuilding the windrow. Turning with a loader can be time consuming. Normally,

slightly less than 1 min is required to turn one bucket-load of material (Rynk, 1994).

Thus a loader with a bucket that holds 1 m3 can process roughly 100 m3 per hour.

In contrast, special windrow turning machines handle several thousand m3 per hour.

Windrow turners use a variety of mechanical devices to agitate and rebuild windrows,

including flails or paddles mounted on a rotating shaft, augers, or inclined conveyors.

Nearly all turners have a housing that shapes the windrow. Some turners are selfpropelled, while others require a tractor for power and/or travel. A few include

nozzles and a water tank so water can be added during turning.

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Windrow dimensions vary with the materials being composted, the turning equipment, and the characteristics of the site. Generally, windrows range from 1 to 5 m

high and 4 to 7 m wide (Rynk et al., 1992). Windrows higher than 5 m are discouraged because they are difficult to aerate and also increase the risk of fire from

spontaneous combustion. Windrows can be of any length allowed by the site layout

and movement of materials. Usually, turning equipment dictates the windrow crosssectional dimensions. When a bucket loader is used for turning, the reach of the

loader determines the maximum windrow height and width. Most bucket loaders

can build windrows 3 to 4 m high. Windrow turning machines create windrows that

are much smaller. The largest windrow turners can handle windrows up to 3 m high

and 7 m wide. However, most turners form windrows that range from 1 to 2 m high

and 4 to 5 m wide.

The composting feedstocks also affect windrow size. Large windrows are difficult to aerate, so materials that are dense and resist air penetration should be

composted in smaller windrows, less than 2 m high. Bulky materials, like deciduous

leaves or bark, may be composted in larger windrows, up to 4 m high. Larger piles

are sometimes used, but increase the risk of odor as well as spontaneous combustion

from excessively high temperatures (Rynk, 2000a, 2000b). When windrows are

turned with a loader, the size can be adjusted to suit composting conditions. For

example, windrow size may be increased during the winter to retain more heat.

The turned windrow method of composting is a simple and flexible approach to

composting. It easily accommodates a wide range of feedstocks, scales of operation,

financial resources, equipment, and management strategies. It is a proven, successful

method of composting. One of the disadvantages is that windrows and space for

turning activities occupy a large area and, therefore, are expensive to enclose within

a building. A second disadvantage is that aerobic conditions are not always maintained within windrows. Occasionally the windrows are anaerobic and odorous in

the interior. Because odors are released during turning, windrows may not be acceptable for some locations.

2. Passively Aerated Static Piles

The passively aerated static pile method of composting is a low management

approach used for slowly decomposing feedstocks like deciduous leaves, brush, bark,

wood chips, and some farm residues. It is described by a number of guidelines and

manuals concerning the recycling of these particular feedstocks (BioCycle, 1989;

Massachusetts Department of Environmental Protection, 1988; Richman and Rynk,

1994; Rynk et al., 1992; Strom and Finstein, 1985).

The passively aerated static pile method takes a patient approach to composting,

relying on little more than passive aeration, natural decomposition, and time to

produce compost piles. Little manipulation of the process takes place. Several

feedstocks may be combined and mixed to adjust moisture, porosity, bulk density,

and/or C:N ratio, but once a pile is formed it is left undisturbed for months. Typically

piles are combined after they shrink and then they are turned occasionally with a

bucket loader (e.g., every 1 to 3 months). Often, turning is performed only when

piles are moved within the site. Each turning reduces the time required to produce

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compost. Piles need to be turned at least once or twice during the composting cycle.

Otherwise, the composted material is inconsistent and contains visible remains of

the feedstocks. If necessary, the consistency can be further improved by shredding

or screening the compost.

Most static piles are simply freestanding piles. They tend to be relatively large,

usually ranging from 2 to 5 m high. The equipment forming the piles determines

the maximum height. Normally the width of a freestanding pile is slightly less than

twice its height. Pile length does not affect the process. It is determined by materials

handling preferences and the constraints of the site. The large pile size conserves

heat and moisture, allowing decomposition to continue steadily but slowly. Piles

may be covered with a layer of finished compost to filter odors or a tarp to exclude

precipitation. A few operations build extremely large extended piles, exceeding 10

m in height and 30 m in width. Piles of this size are the exception. The extreme size

not only greatly lengthens the composting period but also risks the development of

fires from spontaneous combustion (Rynk, 2000a, 2000b). In smaller volume applications, piles are sometimes enclosed in bins to more neatly contain the materials,

segregate batches, or allow materials to be stacked higher in a narrow space. A

common configuration is a series of three-sided bins housed within an open-sided

building (Figure 3.4).



Figure 3.4



Passively aerated material contained in covered composting bins.



Because static piles depend on passive air movement only, maintaining aerobic

conditions is a challenge. Oxygen diffusion and air movement are restricted by the

large mass of compacted materials. Therefore, O2 concentrations within the piles

tend to be very low (Lopez-Real and Baptisa, 1996). Normally, aerobic conditions

exist only within 1 to 2 m from the pile surface and perhaps along air channels that

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form within the pile. As the porosity of composting feedstocks increases, aerobic

conditions penetrate deeper. Also, O2 distribution improves if the feedstocks are

uniform (e.g., well-mixed). Nevertheless, with most large static piles, much of the

decomposition occurs at low O2 concentrations. This lengthens the time required for

composting and increases the chance of odors.

In large piles, passive aeration is practical only with porous feedstocks that

decompose at a slow to moderate rate. Therefore, static piles are most often used

for coarse feedstocks like dry leaves, shredded brush, bark, and wood chips. Intense

odors are not normally a problem with these materials because of their slow rate of

decomposition. However, odors can be strong when piles are disturbed.

Unless the composting site is isolated from neighbors or the piles can be kept

small and porous enough to maintain aerobic conditions, static piles are not appropriate for materials which tend to decompose quickly, like grass clippings, biosolids,

food residues, and manure. Under anaerobic conditions, these materials produce foul

odors that can travel considerable distances, especially if the pile is disturbed.

Manure is sometimes allowed to compost anaerobically in passive piles because

odors are acceptable in some agricultural settings. However, this situation is changing. For many farms, manure cannot be composted in static piles without odor

problems (see odor management section of this chapter).

The key factor in composting by the passively aerated static pile method is time.

Composting occurs slowly in static piles because conditions are largely anaerobic

and because the materials rarely receive the benefits of turning. Large passive piles

turned several times require a year to produce mature compost. With extremely large

piles and no turning, 3 years or more may be needed. Because of the long composting

period, a second key factor is space. The site must have a large enough area to hold

1 to 3 years of partially composted feedstock, although the area requirement is

reduced somewhat by the large pile size.

In general, the static pile method is a potentially economical and successful

approach to composting if time and space are available, and if odors are not critical.

A 1-year composting cycle is practical for many feedstocks, especially those that

are generated almost entirely at a particular time of year, such as deciduous tree

leaves. In this case, the material can remain composting on the site for a year. Then,

after a year, the compost can be moved off-site to make room for the next lot of

incoming feedstocks.

3. Methods Using Assisted Passive Aeration of Static Piles

To overcome the limitations of passive aeration, techniques have been developed

that assist air movement through static windrows and piles. The most effective way

to promote passive aeration is to create a composting medium that is porous. However, a porous medium is not always possible to achieve. Also good porosity, by

itself, may not be enough, especially with highly degradable feedstocks like grass

clippings, food, or manure. Therefore, to increase air flow and improve O2 distribution, some composting methods employ aeration aids such as pipes and air plenums.

Prominent within this category is a method known as the Passively Aerated

Windrow System (PAWS). Researchers with Agriculture Canada developed the

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PAWS method in the late 1980s for composting fish processing residuals and farm

manures. Several modifications and variations of the original PAWS technique have

since evolved, but the basic concept remains the same (Lynch and Cherry, 1996a,

1996b; Mathur, 1995, 1997; Mathur and Richards, 1999; Mathur et al., 1988, 1990).

The PAWS method (and its variations) involves relatively short (1 to 3 m high)

windrows that are not turned but have some deliberate means of delivering air without

using fans. There are four basic features: (1) a homogenous and relatively porous

mixture of composting feedstocks; (2) a delivery system for passive air flow; (3) a

base layer of stable absorbent material like straw or compost; (4) and an exterior

layer (approximately 14 cm thick) of stable coarse material that retains heat, moisture, odors and NH3. These features are common to most PAWS variations, although

the base and cover layers may not be used in all cases. Other modifications include

box-like containers, variations in the air delivery system (discussed later), fabric

covers, and varying windrow dimensions.

The original PAWS method delivers air to the windrows through a series of

perforated PVC pipes running parallel to one another across the width of the windrow

(Figure 3.5). The pipes are the same as those used for septic systems. Each pipe is

10 cm in diameter and, along its length, it contains two rows of holes, 1.25 cm in

diameter spaced 7.6 cm apart. One row of holes is situated at the 10 o’clock position,

the other at 2 o’clock.



Figure 3.5



Passively aerated windrow system (PAWS) with perforated PVC pipes (courtesy

of Sukhu Mathur).



A variation of the original method replaces the pipes with a concrete platform

which has a hollow core. The core serves as an air channel or plenum. Holes or slits

in the top of the concrete platform direct air from the plenum into the windrow

above. The permanent concrete platform eliminates the inconvenience of handling

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pipes when constructing and breaking down windrows. It is strong enough to support

materials handling equipment such as a small bucket loader (Patni, 2000).

The most recent modification of the PAWS approach is called the Natural Aeration Static Pile (NASP) method (Mathur and Richards, 1999). The NASP method

eliminates the pipes and concrete platform and relies solely on a layer of porous

base material as an air plenum. The thickness of the base layer is approximately 45

cm. The base can be any coarse material which facilitates air movement such as

wood chips, bark, or straw. Windrows used in the NASP variation tend to be higher,

2 to 3 m high, compared to the 1 to 2 m windrows typical of the PAWS method.

The key to all variations of the PAWS method is generating high temperatures

to drive thermal convection. This method depends on the heat of composting to

cause warm gases to rise out of the top of the windrow and cool O2-rich air to replace

it at the base. The purpose of the pipes, concrete platform, and porous plenum is to

direct and distribute the incoming air. Research examining the PAWS method, and

more recently NASP, demonstrates that these passive aeration techniques are sufficient to produce and maintain thermophilic temperatures with a wide variety of

feedstocks including fish, several types of livestock manure, sawmill and paper pulp,

food residues, and yard trimmings (Mathur, 1997). The required composting time

varies with the feedstock but generally ranges from 3 to 6 months (Mathur and

Richards, 1999; Rynk et al., 1992).

4. Aerated Static Piles

The aerated static pile method of composting was developed in the 1970s by

researchers with the U.S. Department of Agriculture (USDA) for composting biosolids from wastewater treatment facilities (Willson, 1980). Forced aeration was

employed to improve aeration, reduce the processing time, and reduce the odors

associated with composting of biosolids. It has become the archetypal method for

composting biosolids. It is used with biosolids more than any other type of feedstock

(Goldstein and Gray, 1999). The method has generated numerous variations, particularly in terms of process control, but the basic approach remains the same (Finstein

et al., 1985, 1987a, 1987b, 1987c; Haug, 1993; Kuter, 1995; Rynk et al., 1992;

Singley et al., 1982).

The aerated static pile method relies on fans to aerate and ventilate the composting materials. No turning or agitation takes place except for what occurs incidentally

when materials are moved. Piles are constructed on top of a system of aeration vents

that supplies and distributes air through the composting materials. Forced aeration

provides O2, cools the pile; and removes water vapor, CO2, and other products of

decomposition.

The components of an aerated static pile include the air distribution network, a

coarse porous base layer, the composting materials, an outer layer of stable material,

and the air delivery and control system. The air distribution network underlies the

pile. Several air distribution techniques have been used. The most common is perforated PVC or polyethylene pipe that rests on the composting pad. The pipe has

holes along its length for delivering air to, or collecting air from, the pile above.

Because the pipe interferes with materials handling and is often damaged when the

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pile is taken apart, many facilities have constructed durable air vents embedded in

the composting pad. One approach is to cut air channels in the concrete pad. The

channels can contain the aeration pipe or serve as an air plenum with slotted or

spaced covers to distribute the air to the material above. Some facilities use air ducts

buried beneath the pad. Spaced along the duct, vertical risers or orifices extend up

to the pad surface. Aeration pipe and embedded channels are normally covered with

a mound of wood chips or some other coarse material. The porous material helps

prevent the air holes from clogging and further spreads the air exiting or entering

the pipe or channel. Facilities that use vertical orifices typically do not use the base

covering because high pressure at the orifices keeps the holes clear and distributes

the air.

The mixture of composting feedstock is stacked over the air distribution and

porous base. The feedstock mixture is capped with a layer (18 to 30 cm thick) of

stable material, usually compost. This exterior layer insulates the pile; separates flies

and other pests from feedstocks; and helps retain odors, NH3, and water.

Air is supplied to the aerated static pile via fans and associated piping and control

devices — the aeration delivery and control system. There are innumerable possible

combinations of aeration and control strategies and equipment configurations. Piles

can be aerated by individual fans each turning on and off independently, or by a

group of fans feeding a common manifold with valves controlling air flow to

individual piles or pile sections. Air can be provided by positive or negative pressure,

continuously or intermittently. The process variables that control the amount of

airflow can also vary widely. The original USDA system was based on O2 levels,

but today aerated static pile aeration systems are commonly controlled by temperature or by a timer that is adjusted according to temperature levels.

Aerated static piles range in height from 2 to 4 m. The height is limited by

equipment forming the piles and by the weight of the materials compacting the lower

portion. Pile lengths commonly range from 40 to 80 m. Length is limited by the

variation in air distribution along the aeration pipe and ducts. Otherwise, aerated

static piles vary in size from small individual piles 3 to 6 or 10 m wide to very large

extended piles that may exceed 50 m in width (Figure 3.6). Single piles have a

typical triangular or parabolic shaped cross-section. As new feedstock is generated,

it is added to the end. One or two rows of aeration pipe run the length of the pile.

Extended aerated stated piles are large and roughly rectangular in shape. New

feedstock is added to the side in batches or cells. New cells are constructed by

stacking feedstocks against the side of the previous cell. With extended piles, a row

of aeration pipe is placed at the base of each cell, or ducts are spaced at regular

intervals in the pad. Air valves are opened as the rows are covered with new

feedstock. Extended piles are commonly used because they use space efficiently.

Frequently they are enclosed within a building. The aerated static pile method of

composting can also be accomplished in bins rather than freestanding piles. It is

common for materials to be stacked on aeration pipes between partitions in a

building.

Because the materials in aerated static piles are not turned, feedstocks must be

well mixed before being placed in the pile. The mix also must be relatively porous

and have good structure to resist compaction and settling. For composting biosolids,

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Figure 3.6



Extended aerated static pile composting method.



good structure is usually achieved by adding wood chips as an amendment or bulking

agent. Other bulking agents used include deciduous leaves, mixed yard trimmings

(ground), sawdust, and finished compost (Naylor, 1996). A rigid bulking agent like

wood chips not only supports the pile physically, but also creates channels for air

to penetrate the materials as they compost. However, the formation of air channels

can also be a drawback of this method. Excessive channeling leads to short circuits

in the air flow. This leaves some sections of the pile without sufficient O2 and air

movement, while others sections become dried out. As a result, materials within the

pile compost inconsistently. A good combination of feedstocks, thorough blending

of feedstocks, proper design of the aeration system, and occasional agitation minimize channeling.

With a mixture of biosolids and wood chips, composting by the aerated static

pile method typically takes 3 to 6 weeks, followed by a curing period of 1 to 2

months (Kuter, 1995; Rynk et al., 1992). The composting time should be similar for

manure and grass clippings. Less degradable feedstocks like mixed yard trimmings

require a longer period. Without the physical agitation of turning, wood chips and

other large woody bulking agents remain almost intact in the finished compost. In

most cases, the compost is screened to improve its consistency and to recover wood

chips for reuse as bulking material.

The aerated static pile is a well practiced and proven approach to composting.

Like the windrow method, it is technically simple, but it requires less area and

accomplishes aeration with more certainty than passively aerated methods, including

turned windrows. Because it is more space-efficient, extended aerated piles can be

enclosed within a building. The primary shortcoming is the fact that it is a static

system. Thus, it requires special attention to the type and amount of amendments

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used, and it can suffer from compaction, short circuiting of air, and inconsistent

decomposition within a batch of compost.

5. Methods that Combine Turning and Forced Aeration of Open Piles

and Windrows

Several composting facilities employ methods that combine attributes of turned

windrows and aerated static piles. The intent is to gain the advantages of forced

aeration while avoiding the disadvantages of a static system. Three methods in

particular exemplify this strategy. Although they lack generally recognized names,

these methods can be referred to as forced-aerated windrows, turned extended aerated

piles, and turned aerated bins.

Forced-aerated windrows are built like typical windrows but over an aeration

channel or plenum recessed in the composting pad (Hay and Kuchenrither, 1990).

As with the standard windrow method, the windrows are turned periodically with a

conventional windrow turner. Between turnings, fans push or pull air through the

windrow via the plenum. A windrow must be kept centered over the plenum to

maintain even air flow on both sides of the windrow. Because the aeration system

provides O2 and temperature control, turning is less frequent than required for

standard windrows.

The turned extended aerated pile technique resembles the previously described

extended aerated static pile in all respects except that the pile is regularly turned

(Schoenecker and McConnell, 1993). With each turning, the pile is shifted to one

side in successive sections by a turner with a side-discharging conveyor. Compost

is always removed from the pile on the same side and fresh feedstocks are always

added to the opposite side. Starting at the side containing the oldest material, or the

compost, the turner moves along the edge of the pile and picks up the compost in

a slice equivalent to the width of the turner. The side-discharge conveyor places the

compost directly into a truck bed. Next, the turner moves to the adjacent, newly

exposed edge of the pile, picks up a slice, and deposits it over the strip of floor space

previously occupied by the harvested compost. The turner progresses through the

remaining slices, shifting each one over toward the harvested side of the pile. Each

slice roughly covers the floor area vacated by the previous slice. Eventually, the

entire pile is moved over by a distance equivalent to the width of the harvested

compost. Finally, new feedstock is placed along the side of the pile, in the space

left vacant when the last slice shifted over. The air distribution system is recessed

in the floor beneath the pile. It operates in the same manner as aeration for an

extended aerated static pile. Facilities that employ this technique have used specially

designed turners and conventional elevating face turners fitted with side–discharging

conveyors.

The turned aerated bin approach is a minor variation of the aerated static pile

method. A series of three-sided bins are used for composting, each with aeration

plenums recessed in the floor (Alix, 1998). A batch of mixed feedstocks is placed

in the first bin. It is aerated and managed in the same manner as a conventional

aerated static pile. After 1 to 2 weeks, a bucket loader or conveyor moves the batch

from the first bin into the adjacent bin. The batch aerates in the second bin for several

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