3 The Composting Process
THE COMPOSTING PROCESS
The major elements of the composting process are:
Preprocessing
1. Feedstock delivery and handling 2. Feedstock preparation
Composting
1. Composting phase or active composting 2. Curing phase
Postprocessing 1. Refining
2. Product preparation PreProCessing
Feedstock Delivery and Handling
The delivery and handling of feedstocks depend largely on the type and condi-tion of the feedstock upon arrival at the facility. Putrescent materials need to be handled differently. A great deal depends on location and type of system used. These feedstocks may need to be delivered to an enclosed building with an odor control system to contain odors. This may require a biofilter or other suit-able odor management system. For example, food waste, grass, municipal solid waste (MSW), manure, and biosolids delivered to a facility near residences or commercial enterprises may need to be delivered to an enclosed building. This building could be a component of the composting facility. Two illustrations of this type are:
• Davenport, Iowa: Biosolids delivered and deposited into hoppers located within the composting facility, as shown in Figure 3.1. The facility uses the aerated static pile system.
• Edmonton, Canada: MSW is delivered to the composting facility and depos-ited on a floor in the overall composting facility, as shown in Figure 3.2.
The facility uses the Bedminster drum system and an agitated bed system.
Curing is outdoors.
FIGURE 3.1 Davenport, Iowa, biosolids and yard waste composting facility. Yard waste to be used as a bulking agent is chipped on site. In the foreground, bags of the finished product are available for sale.
FIGURE 3.2 Edmonton, Canada, municipal solid waste and biosolids composting facility.
The rotating drums are in the front of the composting building.
For windrow systems, a separate materials handling building may be appropri-ate for feedstock delivery. Often, depending on conditions, the feedstock is deliv-ered and deposited directly into the windrow. Rapid mixing will significantly reduce odors. Several facilities have enclosed the feedstock delivery system because of odors to residences nearby. The LRI composting facility in the state of Washington is a totally enclosed windrow system. This is shown in Figure 3.3.
The system uses a turning machine with an elevated conveyer and a side chute for building the windrows. Notice the large amount of steam due to the heat. A good ventilation system is needed to remove the high amount of vapor, odors, and dust.
Feedstock Preparation
The objectives of feedstock preparation can be:
• Particle size reduction: The smaller the particle of the material to be com-posted, generally the faster the decomposition rate. Most of the microbial decomposition occurs on particle surfaces. The smaller the particles, the greater the surface area available for microbial decomposition. It must be kept in mind that particle size is also important from a structural point of view (Haug, 1993). Grinders and shredders (Chiumenti et al., 2005) can achieve this.
• Removal of undesirable material: With heterogeneous feedstocks such as MSW, it is often desirable to remove ferrous and nonferrous metals, plastics, rocks, and other material. Some technologies, such as DANO and Bedminster, prefer to remove material primarily after curing. Most systems conduct contamination removal prior to the composting process, and then further refine in the postcomposting phase. One of the issues FIGURE 3.3 The LRI indoor windrow composting facility in the state of Washington.
has been whether leaving contaminants such as metals in the system dur-ing compostdur-ing results in a higher level of contamination. As Chiumenti et al. (2005) point out, the separation of contaminants is more difficult when the feedstocks are wet, and therefore it is more desirable to remove contaminants after curing. However, incoming MSW is relatively dry and contaminants can be separated prior to composting. Numerous technolo-gies for separation of various materials are available. A partial list is indi-cated below:
• Screening: Removal of large plastics, cardboard, large paper, metal fragments.
• Hand sorting: Recyclables, plastics, cardboard, and miscellaneous items.
• Magnetic separation: Ferrous metals.
• Eddy current: Aluminum separated prior to composting.
• Air classification: Paper, plastic light material, glass, metals, miscel-laneous materials, both light and heavy.
• Wet separation: Metals, glass, stone, miscellaneous materials, both light and heavy.
• Ballistic separation: Light and heavy items such as plastics, metals, glass, gravel.
Earlier studies reported on the level of heavy metals as a result of various separation strategies (Richard and Woodbury, 1992).
However, there have been considerable changes in our waste streams.
For example, inks used in printing in the 1990s and earlier contained heavy metals. Cadmium ink gave paper its yellow color. Mercury, zinc, and nickel were important contributors to the U.S. MSW waste stream (Richard and Woodbury, 1992). Heavy metals are not used in inks today, and colors are predominantly from organic dyes.
There are changes in the use of mercury, and much less is discarded today.
The heavy metal content in sewage sludge and biosolids has dra-matically been reduced because of pretreatment (Chaney et al., 2001).
Furthermore, plastic pipes in home and commercial construction have replaced lead pipes as well as copper pipes. This has reduced these ele-ments in the water and in the resulting wastewater and sludge.
Mixing: Mixing is often necessary to obtain a homogeneous matrix for proper composting. This may occur when a bulking agent is added to a feedstock and nonturning technologies are used.
Mixing is also used if an amendment is added prior to composting.
As Chiumenti et al. (2005) point out, material collected at curbside in bags may need to be mixed with other types of biomass to produce a homogenous mixture. Anytime two or more materials are to be used in the composting process, it is often best to mix these materials prior to composting.
ComPosting
Composting Phase or Active Composting
The composting phase as indicated here is often referred to as active composting.
This is to distinguish this phase from curing. The objective of the composting pro-cess (composting and curing phases) is to produce a hygienic, usable product. In this way, the product will have an economic value and be accepted for marketing by the public. To meet this objective, the composting process must:
• Control human and animal pathogens
• Control plant pathogens
• Minimize vector attraction
• Destroy weed seeds
• Prevent the regrowth or reestablishment of pathogens
• Destroy volatile organic compounds (VOCs), which can produce odors The principal criterion needed to meet these objectives is time-temperature. The other variables, such as moisture, aeration, and carbon-to-nitrogen ratio, impact the process, and this in turn affects the temperature regime. Temperature affects the microbiological process, as well as the rate and extent of decomposition. This time-temperature relationship, as it affects pathogens, vector attraction, and odors, will be discussed in subsequent chapters.
Process configurations vary with the feedstock, its contamination, and the utili-zation of the final product. The more homogeneous the feedstock, e.g., biosolids or pharmaceutical wastes, the fewer the number of unit processes.
Figure 3.4 illustrates a generic process flow for most feedstocks. Feedstock prepa-ration involves the addition of bulking agent. For example, after delivery of biosolids to an outdoor composting facility by trucks, it can be deposited into a three-sided concrete bin. During feedstock preparation, a front-end loader (FEL) will pick up the biosolids and deposit the contents into a mixer, as illustrated in Figure 3.5. Wood chips or ground yard waste will be added to the mixer in an appropriate ratio. The mix will then be conveyed to an aeration system for composting.
In a windrow operation, the biosolids and bulking agent are placed in the wind-row, and the windrow machine does the mixing. This is not a preferred method, as it can result in significant odors. Whenever possible, especially with a putresable feedstock, mixing in a building is preferable.
In the case of biosolids, when a clean, uniform bulking agent, such as wood chips of yard wastes is used, the compost refining step usually consists of screening.
There are two options. In the first option, screening is done prior to curing. In this case, less material needs to be cured. In the second option, screening is done after curing. Curing is carried out with the bulking agent in the matrix. However, under this later option, the bulking agent will be more decomposed. If the intention is to recycle as much of the bulking agent as possible, it is best to remove it prior to cur-ing. If screening is done after curing, more space is needed. If the entire process is in a building, the building size to accommodate the curing is much bigger, and the odor control system is bigger.
Figure 3.6 illustrates a typical process flow for a within-vessel sludge or biosolids system. There could be several options within this process flow:
• The hopper and mixer could be a single unit or separate units.
• Curing could occur before or after screening. This author prefers curing after screening, as it reduces the amount of space needed for curing, and the bulking agent does not deteriorate during curing.
• Composting can be achieved by either static or agitated systems.
• Drying may be a necessity. Therefore, it is indicated as an option.
• Within this system, there are several options for material handling. These include front-end loaders and conveyors.
MSW, because of its heterogeneity and contamination, requires the most con-figurations. Figure 3.7 illustrates one type of MSW configuration. As indicated ear-lier, some drum systems do not do any extensive separation in the beginning except
Mixing or Other Feedstock Preparation
Compost Curing
Compost Refining
Finished Product
Composting Odor
Management Air and
Water
Bulk Handling Packaging
Shipping Feedstock
Delivery and Handling
Bulking Agent
FIGURE 3.4 Basic composting process flow.
FIGURE 3.5 Stationary mixer depositing biosolids and wood chips into a three-sided con-crete bin.
Hopper
Hopper Sludge/
Biosolids Delivery
WoodchipsNew
Recycled Woodchips
Mixer Composting
within Vessel
Hopper Screening
Curing
Distribution/
Marketing Bagging
StorageBulk
Drying (Optional)
FIGURE 3.6 Process flow illustration for sludge or biosolids composting within vessel.
removal of bulky items, which occurs on the tipping floor. Other systems, especially in the past, have resorted to hand separation alongside a conveyor as the MSW flows to the composting system. Today many systems (other than drum) conduct exten-sive separation prior to composting. This separation primarily attempts to remove ferrous metals, aluminum, and other nonmagnetic material, glass, and plastics. In
Reception Sorting
Large Items
Shredding, Grinding, Screening
Ferrous and Non Ferrous Separation
Products
By-Mixing Water and
Amendments
Composting Odor
Management
Curing
Refining:
Screening, Air Classification
Storage
Discards
Final Product;
Bulk or Packaging
FIGURE 3.7 Process flow for some MSW facilities.
Edmonton, Canada, the MSW is processed with biosolids in drums. The refining occurs after the composting process but prior to curing.
Another configuration of a multiple feedstock system is illustrated in Figure 3.8.
This system was designed in Hong Kong to provide the agency with the option of shifting to various feedstocks, such as animal waste or food waste. This is some-what similar to several other MSW composting facilities. In the Ngau Tam Mei horse manure composting plant in Hong Kong, the manure is brought in plastic bags. The manure is removed from the bags inside a receiving building and a bob-cat places the manure into a hopper. Using a conveyor system, the manure passes under a magnet to remove ferrous metals. Then it is conveyed through an eddy current system to remove aluminum and other nonmagnetic metals. It is then con-veyed onto a disc screen to remove plastics. Each one of the undesirable materials is deposited into containers. Some of these materials can be recycled. The clean manure is then conveyed into a mixer/shredder, as it contains straw. Water and amendments can be added to adjust the moisture content or the carbon-to-nitrogen ratio as needed.
The mixed and shredded material is then conveyed to a drum for initial compost-ing. The retention time in the drums depends on their capacity and the volume to be processed. Since the drums do not complete the composting process, the mate-rial from the drums is composted under aeration in covered bins. Similarly, curing occurs in aerated covered bins. When the compost reaches a desired stage in stability and maturity, it is then moved to a storage area where screening is done. Screen size depends on the market requirement. The final product can then be marketed in bags or in bulk.
Curing Phase
A major part of the composting process is curing. The major purpose of curing is product stabilization. Curing can be done before screening or after screening. If cur-ing is done after screencur-ing, as shown in Figure 3.6, the bulkcur-ing agent is removed.
This avoids deterioration of the bulking agent and more is recovered. Less space is needed for curing. Under these conditions forced aeration is best.
If the compost is screened after curing, more space is needed for curing. With the bulking agent still in the compost, convective air may be sufficient for oxygen and forced air may be unnecessary. The bulking agent will deteriorate, reducing its recy-cling potential.
During this stage the compost can achieve stability and maturity. Stability is a function of the process and denotes that sufficient biological decomposition has occurred. When a product is stable, oxygen consumption and carbon dioxide evolu-tion by the microbial community are low. The product will have an earthy odor and not be offensive. A mature product is one where fatty acids are low, and the product would not affect plant growth.
Curing can be done under forced aeration or under natural aeration. Forced aera-tion at very low airflows has been demonstrated to excel the process. In doing so, less time is required, and therefore less space.
Hopper
Magnet Ferrous
Metals
CurrentEddy
ScreenDisc
Mixer/
Shredder
ComposterDrum
Composting Bins Amendment
Water
Curing Bins
Storage
Screen
Final
Product Storage
Non-ferrous Metals
Plastics
FIGURE 3.8 Multiple-purpose composting system flow process for several different types of feedstocks.
PostProCessing
Refining
Postprocessing is the final phase in the overall composting process. The objective of this phase is to refine the product primarily from a physical perspective. The chemi-cal nature of the product will generally not change from the earlier curing phase. The exception to this statement is if the product is chemically or biologically enhanced to increase its marketability.
The major elements of this phase are usually screening and air classification.
During screening, particle size is reduced to from 3 to 9.5 mm (0.125 to 0.375 inch) unless the product is primarily going to be used as mulch. In that case, the particle size is usually larger. The very small particle size, 3 mm (0.125 inch) is often used for golf courses and turf. The 9.5 mm (3/8 inch) particle size is the most widely used.
Screening is best achieved when the compost has a moisture content of 40 to 45%.
At higher moisture contents, it is difficult to properly screen. At moisture contents below 40%, the material is dusty.
Another aspect of the postprocessing phase is refining. Refining is a density and size separation process. It typically removes glass, metals, wood, film plastic, hard plastic, and other physical contaminants. The term often used is man-made inerts.
Air classifiers can achieve separation along with magnets, eddy current classifiers, and other equipment. Moisture is again critical. Below 40%, excessive dust occurs.
Details of this process are in Chiumenti et al. (2005).
Product Preparation
Screening and refining produce a product for its ultimate use. The better the final product, the greater are its potential uses. Most compost products are the result of screening through a 9.5 mm (3/8 inch) screen. This product has a wide range of uses in horticulture, parks, public works, land reclamation, and other projects. If the product is to be used for turf cultivation, a finer product is needed. For use as mulch, a coarse product is desirable. More details are in Chapter 16.
Typically compost is shipped in bulk. The economical range of transportation is usually considered within 80 km (50 miles). Bagging the product increases the range and usually brings in greater remuneration for the product.
CONCLUSION
The composting process consists of three major elements:
• Preprocessing
• Composting including curing
• Postprocessing
The various process units within each of these elements will vary depending on the feedstock and its nature, the composting system, and the product desired.
Preprocessing primarily involves preparation of the feedstock in order to achieve maximum biological decomposition of the organic matter.
The objective of composting and curing is to obtain a hygienic, stable, and mature product, as well as a product that will enhance the soil physical properties for opti-mal plant growth.
Postprocessing consists of refining the product. It involves physical size prepara-tion and removal of man-made contaminants. The chemical nature of the product is primarily a function of the chemicals in the feedstock and any additional chemicals added during the process.
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