IV. COMPOSTING AS A SYSTEM OF OPERATIONS

Figure 3.1



Flow chart of common operations in a typical composting system.



from raw feedstocks (e.g., removing soil from yard trimmings prior to grinding).

The most common types of screens used at composting facilities are trommel screens

(Rynk, 1994). Shaking, vibrating, and rotating disc screens are also used for size

classification. If plastic or similar contaminants need to be separated from the



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compost, an air classification or ballistic separation may also be used as a compost

refining technique (Richard, 1992).

Due to concerns about the impact of odors generated at composting facilities,

many composting facilities include an odor treatment operation in the composting

system. Odor treatment captures exhaust air from the composting aeration system

and/or air from buildings on the site where materials are handled, processed, or

stored. The odor-laden air is then passed through physical-chemical or biological

treatment units (Dunson, 1993; Haug, 1993; Williams and Miller, 1993). Chemical

units expose the air to reactants that remove specific odorous compounds. For

example, sulfuric acid is used to remove NH3. Biological units are large beds, or

“biofilters,” containing relatively porous organic material such as wood, compost,

peat moss, sawdust, shredded yard trimmings, or combinations of these materials.

As air passes through the biofilter, volatile odorous compounds are adsorbed on the

organic particles. Organisms that inhabit the biofilter decompose the adsorbed compounds. A biofilter is essentially another stage of composting and therefore it must

be managed as such, maintaining adequate moisture, C substrate, and O2 levels.

Additional information on odor management is included later in this chapter.

Curing is a latter phase of composting in which the rate of decomposition

declines to a slow steady pace and the compost matures at low mesophilic (<40°C)

temperatures (Haug, 1993; Rynk et al., 1992). Curing does not have to be a separate

operation, but can be an integral part of the primary composting method. However,

there are advantages in separating the curing stage from the active composting stage.

As compost matures, the heat generation and O2 demand decrease substantially. This

generally permits a lower level of management. By separating the curing step, it is

possible to reduce time or simplify management involved in monitoring and manipulating the process.

Curing typically involves stacking partially finished compost in piles that aerate

passively. Increasingly, curing piles are aerated by fans to further guard against anaerobic conditions, odors, and excessive temperatures. It is difficult to draw the boundary

between active composting and curing. The point at which active composting is

terminated and curing begins is typically determined by either temperature or time. In

continuous composting systems, the curing operation may begin simply because the

compost must be removed from the pile or vessel to make room for incoming materials.

Some systems accomplish composting in two stages, with each stage employing

a different method (Haug, 1993; Rynk, 2000c, 2000d). The first stage is a more

intensive and expensive method of composting, usually contained within a vessel

or building. It is followed by a less expensive method with less containment like

static piles, windrows, and aerated piles. The first stage serves to get the process

started quickly and intensely. Because odors are greatest during the early phase of

composting, the first stage isolates the materials from the surroundings and captures

odors. When the partially composted materials are delivered to the next stage, the

potential for odor and the attraction to pests has been greatly reduced.

For more information about composting systems and operations that support

compost production, refer to Dougherty, 1999; Haug, 1993; Kuter, 1995; Richard,

1992; Rynk, 1994; Rynk et al., 1992; The Compost Council, 1994; and Willson et

al., 1979.

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V. CHARACTERIZATION OF COMPOSTING METHODS

The composting method provides the conditions for microorganisms to convert

raw feedstocks into compost. The method dictates how the composting materials

are aerated, contained, and moved through the system. There are numerous ways to

characterize composting methods including the degree of containment (open vs. invessel), mode of aeration (passive vs. forced), use of agitation (static vs. turned),

and the physical progression of materials through the composting process (batch vs.

continuous). Table 3.2 defines these and other terms that are used to classify and

identify composting methods.

Table 3.2 Terms Used to Classify Composting Methods and Systems

Open: Materials are composted in freestanding piles or windrows (i.e., long narrow piles).

Materials may be stacked in simple bins that are not fully enclosed. Systems may be

enclosed within a building but the composting environment is otherwise not controlled.

In-vessel or contained: Materials are composted within reactors or vessels. Most methods

employ forced aeration and some means of agitation. Environment surrounding the

composting materials is controlled. Examples of reactors include aerated steel containers,

large polyethylene tubes, vertically oriented rectangular and cylindrical reactors, and various

enclosed bin configurations. Horizontal agitated bins are usually considered within this

category.

Passive or natural aeration: Relies only on natural air movement as means of aeration.

Driving mechanisms include diffusion, wind, and thermal convection.

Forced aeration: Employs fans or blowers and an air distribution network of vents or pipes

to deliver air to composting materials.

Static: Materials are composted without regular agitation or turning. Some infrequent and

irregular turning when piles are moved or combined.

Agitated or turned: Materials are agitated or turned regularly at intervals ranging from

every day to every 2 months. A variety of agitation mechanisms may be used.

Batch: Feedstocks are composted in identifiable batches, usually with little or no change

in physical location through the process. After a batch is formed and starts to compost, no

new material is added to the composting unit.

Continuous: Feedstocks physically move through the compost system in a nearly

continuous fashion. Movement corresponds with progressive stages of decomposition.

Compost is removed and new feedstock is added regularly and frequently.

Modular: Materials are composted in multiple, often relatively small units or modules. Each

module may represent an individual batch of material. Modules can be freestanding piles

or windrows, bins, or enclosed reactors. As the number of modules increases, the system

approaches continuous operation.



Due to differences in cost and management, composting methods are broadly

categorized by whether or not the composting materials are physically contained.

Two categories are typically used: (1) “open” methods that provide little or no

containment and (2) “in-vessel” methods that contain composting materials in a

reactor or vessel. The distinction between open and in-vessel composting is not

sharp. Several methods can be considered in either category. Nevertheless, these

categories provide a reasonable framework for describing the various commercial

composting methods that are used. Therefore, individual composting methods are

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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).

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