Although composting has been applied to the stabilization of organic matter for many years, it has only been during the last couple of decades that the process has been accepted and widely utilized in the waste management field.

By L.F. Diaz, A. Chiumenti, G. Savage, L. Eggerth and N. Goldstein

The management of solid wastes, particularly municipal solid waste (MSW), has undergone substantial changes over the last 30 years. Most of the changes have taken place in industrialized countries and address issues related to the protection of public health and of the environment as well as resource conservation.
The typical components of MSW include paper products, various types of plastics, metals, glass, biodegradable matter such as garden and yard trimmings, kitchen and food waste, and other miscellaneous components. Advances have been made in the storage, collection, and processing for most of these materials to divert them from disposal in landfills. One of the more important developments in the waste processing industry has taken place in the treatment of the biodegradable components.

Currently, there are close to 3,500 composting facilities dealing with yard waste in the US alone (Simmons, P.; Goldstein, N.; Kauffman, S.M.; Themelis, N. J.; and Thompson, J. “The State of Garbage in America”; BioCycle; April 2006). The growth in the use of composting throughout the world has led to the development and implementation of many innovations in the equipment and systems used to process and stabilize the materials. The challenges become more complex when the materials are highly putrescible (such as food wastes) and the processing facilities are located relatively close to human populations.

Because of the interest in the application of composting and due to the relatively large number of developments in the technical areas, the authors decided to publish a book (Modern Composting Technologies) on the latest developments in the equipment and systems being used primarily in Western Europe and in North America. This article provides a brief overview of the equipment and systems discussed in the book.

Processing Equipment
Various types of equipment are used for each of the key steps in a composting operation (preparation of the feedstock, composting process, and refining the finished product). Some of the more common unit operations used include size reduction and screening. Recently, air classification has become another important unit operation, particularly for the removal of contamination from finished compost.

Size Reduction
Size reduction is one of the first unit processes employed in most composting facilities. Particle sizes in the range of 1 to 3-inches are optimal. They provide sufficient surface area of the material to be composted and, therefore, facilitate contact between particles and microbes; and they assist in achieving an average particle size similar to that desired for the end product.

In general, the types of size-reduction units include grinders, shredders, and chippers. Another type of size-reduction unit, described in Modern Composting Technologies, is the Archimedean screw. This design originally was used in wagons to break up or mix animal feed. As indicated in the book: “The shattering/mixing trailers used for the composting process are characterized by having from two to four Archimedean screws.”

Screening
Most designs of solid waste processing facilities incorporate some type of screen. In general, screens are used for three main purposes: sizing of the input material for processing; sizing the finished product for marketing; and removal of contamination either before or after composting.

Screening is a unit operation that involves the separation of a mixture of materials with different particle size distributions into two or more fractions (each fraction having a discrete spectrum of particle sizes) by means of one or more sets of screen openings. The material that remains on the screening surface generally is known as the oversize (or plus) material. Material passing through the screen openings is generally referred to as the undersize (or minus) material. The screening surface may be fabricated using plastic, cloth, wire mesh, perforated steel plate, or steel bars.

In general, industrial screening processes can be conducted when the material is wet or dry. However, some materials are difficult to screen efficiently if they are too wet. There are several types of screens; the most common units used in the composting industry are deck (flat), disc, grizzly, orbital, star, and trommel screens.

Air Classification
This process relies on an air current to separate materials primarily according to density, although size and shape of the particles are also important factors. The air classification process can be illustrated by the operation of a conventional vertical air separation system. In this system, a mixture of materials is introduced vertically near the top of the column. The column can have a rectangular or circular cross-section. An air stream is forced upward from the bottom of the column. In this particular design, the lighter materials are carried away with the air while the heavier materials fall toward the bottom of the classifier. The light particles are then removed from the air stream by means of a cyclone unit. The relative division of light and heavy fractions of particles depends on several factors, including air velocity, particle size distribution, cross section of the air classifier, and feed rate.

Air classification has not been widely utilized in composting operations, particularly in North America. This situation has started to change as equipment specifically designed to process compost has entered the market.

Air classifiers generally are used for refining the finished product, in particular to remove contamination after size reduction or after screening. It is usually one of several steps used to remove plastics from compost. Hand sorting and screening still remain important components in producing compost that is nearly free of plastics.

In-Vessel Systems
The type and number of in-vessel systems have increased substantially in the past few years. This growth may be largely due to policies put into practice in some countries in Western Europe and in the European Union. In-vessel systems consist of enclosed, rigid structures or reactors used to contain the material being processed and in which the various critical process parameters can be controlled. The reactors generally are used for the accelerated biooxidation phase of the composting process. This is the phase that needs the most attention and also is the phase that, if mismanaged, can generate offensive odors. In-vessel systems are equipped with process control units that monitor the progress of the biological activity by means of sensors that measure the air temperature and the concentration of O₂ or CO₂. Monitoring the concentration of these gases in the gas discharged from the reactors enables an accurate determination of the status of the degradation process.

In-vessel systems can be divided into two main categories: vertical and horizontal bioreactors. Horizontal bioreactors can be further subdivided into horizontal reactors composed of channels, cells, containers, or tunnels. Another type of reactor is the rotating drum. The rotating drum generally is placed before the first phase of the composting process to “condition” the feedstocks (to size-reduce and mix the material) as well as initiate the composting process.

In-vessel systems can also be classified based on the general method of feeding the material into the system. As such, composting plants can be divided into continuous and discontinuous operations. As the term implies, in a continuous operation, the feedstock is loaded into the system on a continuous basis, generally over one or two shifts (8 or 16-hours every day). In the discontinuous operation, the feedstock is loaded only after the processed material is discharged from the reactor. Discontinuous systems also are known as batch systems.

An additional form of system classification, based on the presence or absence of movement of the material in the reactor, is static bioreactors and dynamic bioreactors. In systems using static reactors, the loaded material is not moved until it is discharged, while in those using dynamic reactors, the material is moved and turned periodically during the process.
In most cases, depending upon the type of material, the biooxidation phase performed in bioreactors is carried out between seven and 15 days, as opposed to the 20 to 30 days typical of windrow composting. However, to reach a level of intensive degradation during the biooxidation phase, the process must be optimized in terms of quality of the starting material and in terms of maintaining all of the parameters within the proper range of values that govern the biological activity. Generally, a curing phase follows the biooxidation phase.

Vertical Reactors
A vertical bioreactor consists of a cylindrical structure or container, manufactured from steel or concrete and thermally insulated. These units range in volumetric capacity from a few hundred to more than 2,600 cubic yards. In most designs, the material is loaded at the top and is removed from the bottom in a continuous cycle.

In the vertical plug-flow reactors, aeration is accomplished by a centrifugal blower, which forces air from the bottom of the reactor. The air flows in the opposite direction to the composting material (countercurrent). The air exiting from the reactor usually is processed through a biofilter. The composted material is removed from the bottom of the reactor by means of a screw conveyor.

Vertical reactors have experienced operational difficulties and have lost their attractiveness. Some of the difficulties include problems in achieving a homogeneous distribution of air throughout the reactor’s contents, compaction of the composting material at the base of the reactor, and difficulty in extracting the material from the reactor.

Horizontal Units
Horizontal reactors usually consist of stacks of composting materials contained by walls (much like trenches). The walls vary in height from 3 to 10 feet and are used to separate the various stacks (or windrows). In a horizontal reactor, the material to be composted generally is aerated through a combination of forced aeration and agitation. In most situations, the reactors are located inside a building and the building is kept under negative pressure. The air discharged from the building is forced through air treatment systems (usually biofilters).

The stacks or trenches can be loaded and unloaded using different equipment. Some designs rely on front-end loaders, while others use conveyor belts in combination with screw conveyors.

These reactors normally use stacks that are about 165 feet long and up to 20 feet wide. Loading and unloading can take place on a continuous or discontinuous basis. In a continuous cycle, the loading operations are performed almost daily. In a discontinuous operation, the loading is conducted as soon as the first phase of the process is completed, and after the processed material has been removed from the reactor.

Longitudinal Reactors
In this type of system, the composting material is continually moved from the loading end of the trench to the unloading end. These plants are classified as horizontal plug-flow. The shape of the track varies from straight to elliptical (much like a race track) to “U”. The rate of movement of the composting material depends on the type of turning machine used (generally movement of the material is on the order of 6 to 10 feet per turning). The material is kept in the trench for about four weeks during the first phase of the process. The detention time depends on the level of aeration afforded by the mechanical turning machine, the frequency of the turning operation, and other factors.

The book on which this article is based provides detailed information on several types of systems used in Western Europe, Japan, and North America. The types of units that are commonly applied in North America include those manufactured by Longwood Manufacturing Corp., Transform Compost Systems Ltd., and USFilter.

Lateral Movement Reactors
In this type of design, the material to be composted is moved sideways to the adjacent row each time the turning machine passes through the reactor. This type of plant offers the alternative of using a shuttle conveyor belt for loading operations. The OTV system, one of the more popular systems that rely on this design, uses rows that are 13 feet wide and about 6 feet high. The rows are filled every two to three days.

Horizontal Cylindrical Reactors
Horizontal cylindrical reactors, known in North America as rotating drums, generally fall into the continuous category and essentially consist of a rotating cylinder. The cylinder is slightly inclined on the horizontal plane, has a diameter of 6 to 14 feet and is about 150 feet long. The bioreactors rotate at about 0.2 to 2 rpm and are operated with forced air flowing in the opposite direction to the flow of the material to be composted. Some of the designs of the cylindrical reactors include sharp “teeth” inside the drum to force the material towards the exit and at the same time assist in tearing bags and other materials. Moisture can be added as needed. These units have been widely used to co-compost biosolids from wastewater treatment facilities with the organic fraction of MSW.

Generally the reactors are filled to about two-thirds of their capacity and the composting materials are kept at a detention time of about two to seven days for the intensive phase of composting. The material that is removed from this type of reactor usually is placed in windrows to complete the degradation process.

Biocells
Biocells consist of hermetically enclosed units in which composting is conducted under controlled conditions. These units are operated in the discontinuous mode. As such, the biocell is loaded with the material to be composted, the cell is closed, and the composting process begins. Generally, the intensive biooxidation process is conducted over a two-week period. Aeration is conducted under positive pressure. Air is supplied to the material through the floor of the unit by means of pipes or channels. The air that passes through the composting mass generally is removed through the top of the biocell. Some of the designs used in facility locations and climates that reach extreme temperatures incorporate air-to-air heat exchangers to control the temperature of the incoming air. Process air is recirculated or else treated through a biofilter.

Biocells have undergone a substantial degree of development during the last few years. Consequently, the systems now are very reliable.

These enclosures were developed based on similar units used for the preparation of organic material for growing mushrooms. Biocells can be divided based on method of loading and unloading, size, and degree of motion of material inside the cell (i.e., dynamic or static). In dynamic biocells, the material is “turned” inside the unit using an automatic system. In static biocells, the material is not moved during the entire process.
Biocells usually are rectangular in shape and have capacities on the order of 130 to 1,300 cubic yards. They can be more than 165 feet long and up to 20 feet wide. The enclosures usually are about 13 feet high, but the cells are only loaded up to 10 feet to avoid excessive compaction of the composting material. High compaction usually leads to channeling (i.e., short-circuiting) of air flow in the material and, therefore, poor air distribution and aeration of the composting mass.

Biocells can be cast in place onsite or can be prefabricated. Generally the cells are thoroughly insulated to keep heat loss due to conduction and radiation to a minimum.

The design of the floor of the cells plays an important role in the operation of the unit, because the floor typically is a component of the aeration system. The quality of the compost product and the efficiency of the system depend on the type and degree of aeration of the composting material. Biocells rely on different methods of aeration, including air expansion chambers, channels, and embedded nozzles. The designs of the floor also make allowances to collect and remove excess moisture (leachate).

Dynamic biocells use moving floors rather than fixed floors to transport material through the system. Some designs of static biocells also use moving floors. Moving floors offer advantages with respect to the methods of loading and unloading material.
The cells can also be loaded by means of screw conveyors placed inside the upper portion of the unit. Other designs allow for the use of front-end loaders or shuttle conveyors.
Biocells generally are unloaded by means of a front-end-loader. Some designs rely on moving floors combined with a rotor to perform the unloading.

Moisture is added to the material by means of a spraying system placed at the top of the biocell. The spraying unit can be operated either manually or automatically. Automatic operation relies on sensors located in the piping used for air discharge to determine the relative humidity of the air mass.

Reactors With Roofs That Open
This system is known as the Biodegma system and includes modules that consist of concrete walls with a metallic structure on top of the walls (in a triangular shape) that serves as a roof. The roof is covered with Gore-Tex membranes. These membranes keep precipitation from entering the windrow, but allow the CO₂ produced during the composting process to escape. The membrane also acts as a physical barrier against gaseous substances escaping from the composting material.

During the composting process, a fine film of condensation usually is formed on the inside of the cover. The condensation contributes to the suppression of odors and other gaseous substances. Condensation of the water vapor on the inside surface of the cover minimizes dehydration of the biomass, allowing a normal evolution of the composting process without the requirement for additional water. The shape of the entire structure is similar to that of a typical greenhouse. The roof of the unit can be opened. The doors to the structure open towards the outside and also are covered with Gore-Tex material.

Each unit can hold about 250 cubic yards of biodegradable materials. The units can include forced aeration. Generally, the units are loaded by means of front-end loaders or with a system of conveyors. After the loading process is completed, the doors and the roof are closed. The material is aerated for approximately three to four weeks. After the period of intensive composting is completed, the mass is transported to another set of modules. These modules are operated in much the same way as the others (closed and aerated). Moisture is added as needed. The composting mass is kept in the curing modules for an additional three to four weeks. As shown in Figure 3, the structure looks much like a traditional greenhouse, but the roof cover is made out of Gore-Tex.

Biocontainers
Biocontainers have some similarities to biocells; however biocontainers generally are smaller and are loaded in a different manner than biocells. Biocontainer systems are modular and generally consist of six to eight vessels. The throughput capacity of a module varies between 3,000 and 5,000 tons per year.
In the process, feedstock is loaded into the top of the container using a front-end-loader or a conveyor belt. The top of the container is then closed. The units incorporate a system for forced aeration using nozzles. Air discharged from the biocontainer is treated in biofilters. The vessels also are equipped with a system for adding moisture to the composting mass.

The active composting period lasts anywhere between 8 and 15 days. Upon completion of the active composting process, material is discharged by tipping the container. The container generally is tipped using a roll-off waste collection truck. Figure 4 shows a series of biocontainer systems, in particular the aeration system, and the cover in the lifted position.

Biotunnels
Biotunnels also are similar to biocells, with the main difference that biotunnels have specific areas for loading and unloading at each end of the reactor. Biotunnels are rectangular in shape, usually composed of brick or metal and generally are thermally insulated. The typical dimensions of biotunnels are up to 65 to 100 feet long, about 13 to 16 feet wide, and 10 to 13 feet high.
Biotunnels are loaded at one end of the reactor. The composting mass moves forward by means of a moving floor. The process has a detention time on the order of 15 days. Aeration of the material is provided by air forced through the floor. Similar to most other modern systems, the discharged gas is collected and treated. All operations are automatically controlled.

Luis F. Diaz is president, George M. Savage executive vice president, and Linda L. Eggerth, senior vice president of Cal Recovery Inc. at Concord, CA; Alessandro Chiumenti is a professor at Università degli Studi di Udine, Italy; and Nora Goldstein is editor of Biocycle at Emmaus, PA.

Further Reading
This article is based on the 2005 book entitled Modern Composting Technologies, by A. Chiumenti, R. Chiumenti, L.F. Diaz, G.M. Savage, L.L. Eggerth and N. Goldstein. The book consists of four chapters (“Basic Elements of Composting Technology,” “Equipment Used in Composting,” “Composting Systems,” and “Odor and Air Management”) and contains more than 100 pages including color photos and a comprehensive directory of equipment and system manufacturers in North America and in Europe. Modern Composting Technologies is available from The JG Press, Inc., Emmaus, PA (www.jgpress.com).

MSW - September/October 2006