II. THE COMPOSTING PROCESS IN BRIEF

byproducts such as organic acids, which can contribute to phytotoxicity when the

compost is ultimately used (Epstein, 1997).

In addition to O2, the organisms need moisture, a balance of nutrients, and

favorable temperatures and pH (Table 3.1). Abundant moisture is required for the

metabolic activities of the organisms. Water provides a medium for biochemical

reactions and transport of nutrients and organisms (Miller, 1991). However, excessive

water interferes with aeration by filling pore spaces and by increasing the weight of

the composting materials, causing them to compact and lose pore space (Das and

Keener, 1996). The ideal balance of moisture generally falls in the range of 50 to

60% (wet basis). Depending of the materials and composting method, moisture

contents of 40 to 70% are tolerable. Below 40%, the composting rate slows due to

lack of moisture. Above an upper limit of 60 to 70%, aeration is extremely difficult

(Haug, 1993). Proper moisture content is established initially by adding water to

dry feedstocks or by mixing dry feedstocks and wet feedstocks together. Mixing

diverse materials is common because it also improves the physical characteristics

of the composting substrate and its nutrient balance. Once the composting process

starts, high temperatures and air movement drive evaporation, which generally

reduces the compost moisture content over time. In outdoor systems, rainfall can

reduce or even reverse this drying process, as can metabolic water produced as a

byproduct of decomposition. During the composting process, moisture levels can

be maintained by adding water to materials that have dried excessively. If materials

become too wet from precipitation and metabolic water, they can be aerated or

agitated to promote drying.

Table 3.1 Preferred Conditions for Composting

Condition

Carbon to nitrogen (C:N) ratio

Moisture content

O2 concentrations

Particle size (diameter in mm)

pH

Temperature (°C)



Reasonable Rangez

20:1–40:1

40–65%y

Greater than 5%

3–13

5.5–9.0

43–66



Preferred Range

25:1–30:1

50–60%

Much greater than 5%

Variesy

6.5–8.0

54–60



z



Recommendations for rapid composting. Conditions outside these ranges can

also yield successful results.

y Depends on the specific materials, pile size, and weather conditions.

From Rynk et al., 1992. On-Farm Composting Handbook. NRAES, Ithaca, New

York. With permission.



Although most organic materials provide all of the nutrients needed by the

microorganisms, the nutrient content is often not balanced for optimal composting.

Ordinarily, nutrients are managed by providing balanced proportions of two primary

nutrients, C and N. In some cases other nutrients, such as phosphorus (P), have been

found to be limiting (Brown et al., 1998). Nevertheless, it is commonly assumed

that with a reasonable carbon to nitrogen (C:N) ratio, other required nutrients are

available in sufficient quantities (Rynk et al., 1992). An ideal C:N ratio is considered

to be in range of 25:1 to 30:1 (Epstein, 1997). With ratios above 30:1, composting



© 2001 by CRC Press LLC



may be limited by lack of N. With lower C:N ratios, excess N is converted to NH3,

which is subject to loss via volatilization and leaching. In practice, the C:N ratio of

the feedstocks is not as crucial as this implies. Composting takes place effectively

at C:N ratios from 20:1 to 50:1 and even higher (Horwath et al., 1995). Also, the

biochemical processes depend not only on the concentrations of nutrients available

but also on their biological availability, a factor that is more difficult to account for.

This is particularly true for C, which is often confined in biologically resistant organic

compounds (Epstein, 1997; Lynch, 1993).

Microorganisms must be able to access the nutrients in organic substrates for

composting to proceed. Composting organisms colonize the surface of solid particles

where water and O2 are present; here they can hydrolyze organic compounds into

more degradable soluble forms. With more substrate surface area, more organic

material is accessible to the organisms. Because smaller particles provide greater

surface area, decomposition increases with smaller particle size (Golueke, 1972).

However, very small particles reduce the size of the pores in the composting materials, which hinders aeration. Therefore, particle size is another factor to balance

during composting. An ideal particle size is impractical to specify because it depends

on the feedstocks, stage of the process, aeration system, and many other dynamic

factors. Generally, a mix of coarse and fine particles in the range of 3 to 50 mm

works well (Gray and Biddlestone, 1974; Rynk et al., 1992).

The heat produced during composting is a direct result of the biological activity

(Miller, 1991). Therefore, the composting temperature indicates the performance

and stage of the composting process (Golueke, 1972). Microbial activity early in

the process quickly raises the temperature to thermophilic levels, typically reaching

40 to 60°C. As the easily degradable organic materials are consumed, gradually the

activity slows and the temperature drops back to mesophilic levels. Near the end of

the process, only the more resistant organic materials remain. Microbial activity

slows to a constant level, with temperatures remaining near ambient. Thermophilic

temperatures are not necessary to composting. The process takes place readily at

lower mesophilic temperatures (10 to 40°C). In fact, some transformations are

thought to be carried out solely by mesophilic organisms (Rynk et al., 1992).

However, thermophilic temperatures provide the advantages of faster biochemical

reaction rates and more effective destruction of weed seeds and pathogens. Extremely

high temperatures, above 60°C, are detrimental to the composting organisms, retard

the process, and may impair the compost quality (Finstein and Hogan, 1993; Hoitink

et al., 1993).

Environmental and health regulations require certain feedstocks, especially biosolids, to be composted at thermophilic temperatures for a prescribed amount of

time (unless pathogen destruction is established by an alternative means). For example, the U.S. Environmental Protection Agency (U.S. EPA) regulations regard composting as a “Process to Further Reduce Pathogens” (PFRP) if temperatures are

maintained above 55°C for 3 consecutive days in a static pile or in-vessel composting

system (U.S. EPA, 1985). In a turned windrow system, 55°C must be maintained

for 15 days with a minimum of five turns in that time. A wider range of uses

are allowed for biosolids that meet PFRP conditions. In some jurisdictions, other



© 2001 by CRC Press LLC



feedstocks (e.g., yard trimmings) are required to be composted at PFRP conditions

if the compost is offered for sale or public use (CIWMB, 2000).



III. FUNCTIONS AND MECHANISMS OF AERATION

The composting method determines how aeration occurs. Aeration is a crucial

and inherent component of composting. It provides the O2 needed for aerobic

biochemical processes, and also removes heat, moisture, CO2, and other products

of decomposition. In fact, during most of the composting period, the amount of

aeration required for cooling greatly exceeds the amount required for removing

moisture or supplying O2 (Finstein et al., 1986; Haug, 1993; Kuter, 1995). Thus, the

need for aeration is more often determined by temperature than by O2 concentration.

Although there are many variations, aeration generally takes place either passively or by forced air movement. Passive aeration, often called natural aeration,

takes place by diffusion and natural air movement. Forced aeration relies on fans to

move air through the mass of composting materials. A possible third mode of aeration

being developed is a system that injects nearly pure O2 into a closed composting

reactor (Rynk, 2000c, 2000d).

A. Passive Aeration

Passive or natural aeration is driven by at least three mechanisms: molecular

diffusion, wind, and thermal convection. Oxygen diffuses into material because there

is more O2 outside than within. Similarly, CO2 diffuses outward. Although molecular

diffusion is constantly at work to correct concentration imbalances, the process is

slow and probably does not have a major effect on aerating piles (Haug, 1993; Miller,

1991). In exposed outdoor locations, wind can be a significant factor in O2 transfer.

Many composting site operators have observed gusts of wind causing puffs of steam

to spout from piles. The influence of wind on aeration of open piles has not been

widely documented.

Thermal convection is probably the greatest driving force for passive aeration

in most composting systems (Haug, 1993; Lynch and Cherry, 1996a; Randle and

Flegg, 1978). The heat generated during composting increases the temperature of

gases with the materials, which in turn decreases their density. The warm gases rise

out of the composting mass, create a vacuum, and cool fresh air enters. The aeration

rate is determined by the temperature difference between the interior gases and the

ambient air plus the air flow resistance of the composting media. Therefore, the keys

to obtaining reliable passive air movement are generating heat to drive thermal

convection and establishing a porous physical structure within the composting materials (Rynk et al., 1992). Porosity is particularly important. Dense, wet mixtures

such as are common in sludge composting can reduce or eliminate the potential for

convective O2 transfer (Finstein et al., 1980).

Most composting systems that rely on passive aeration commonly include periodic agitation or “turning” of the materials. Although turning charges the materials

with fresh air, the air introduced by turning is quickly consumed by the composting

© 2001 by CRC Press LLC



process (Epstein, 1997; Haug, 1993). The longer lasting effect of turning on aeration

may be to rebuild the pore spaces in the material, which are crucial to diffusion and

convection. However, there is evidence that this effect can be shorter lived as well

(McCartney and Chen, 1999; Michel et al., 1996). The functions and effects of

turning are discussed in more detail in the subsection covering the turned windrow

method of composting.

B. Forced Aeration

With forced aeration, air is supplied mechanically, via fans and associated ducts,

plenums and control devices — the aeration delivery and control system. There are

innumerable possible combinations of aeration and control strategies and equipment

configurations. Materials 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, pile sections, bins, or vessels. Air can be

provided by positive pressure, forcing air into the distribution network and up

through the materials (Stentiford, 1996), or by negative pressure, sucking air through

the materials from the exterior and into the distribution network. Air movement is

generally more efficient with positive pressure because pressure loss is favorable

(Finstein and Hogan, 1993; Kuter, 1995). However, negative pressure offers the

advantage of allowing the exhaust gases to be easily collected for odor treatment.

It is also possible to use both strategies, reversing the direction of air flow at different

stages of the composting process (Stentiford and Pereira Neto, 1985). Negative

pressure might be primarily used early in the process when it is more important to

capture gases for odor treatment. Periodically reversing the direction of air flow also

helps to correct moisture and temperature gradients that occur in static composting

systems (Stentiford and Pereira Neto, 1985). Some contained composting systems

recycle a portion of the exhaust air to retain moisture and heat within the composting

environment (Panter et al., 1996).

Depending upon the composting systems, forced aeration can be continuous and

then increased as needed, or intermittently turned on and off as needed. Continuous

aeration can reduce the required air flow rate. It also reduces the fluctuation of

temperature and O2 levels that occur over time. However, continuous aeration can

cause gradients within the composting environment, leading to excessive drying and

a permanently cool zone in the area where air enters (Citterio et al., 1987). This

may be a concern if PFRP is required. Forced aeration is typically controlled based

on the temperature within the composting materials. With many composting systems,

temperature activates the aeration devices directly, via a temperature feedback control system of sensors, electronic components, and computer programs. Aeration is

activated or increased when the process temperature surpasses a temperature set

point. In other systems, aeration is determined by a time cycle that is adjusted either

manually or automatically according to process temperature. Even with a direct

temperature feedback control system, a timer is often required to activate aeration

at regular intervals to maintain aerobic conditions when temperatures remain below

the set point, especially during the initial and final stages of composting (Finstein

et al., 1983). Aeration rates and intervals normally vary with the stage of composting

© 2001 by CRC Press LLC