July 25, 2005 | General


BioCycle July 2005, Vol. 46, No. 7, p. 40
From the very large to the very small, from the very horizontal to the very tall, there are in-vessel composting units to meet the varied needs of composting operations.
Luis Diaz, George Savage and Alessandro Chiumenti

WHAT became clear when writing and editing our new book, Modern Composting Technologies, is that there are in-vessel systems on the market today to satisfy just about any processing need, whether based on scale, climate, cost or type of feedstock. The commonality between all systems is that these bioreactors are enclosed, rigid structures or vessels (reactors) used to contain the material undergoing biological processing. Such reactors generally are used for the first phase of the process (accelerated bio-oxidation), which needs more attention and control than the second phase. In fact, in most plants which use bioreactors, an air treatment unit is also included to limit the emissions of particulate and gaseous pollutants into the atmosphere. Independent from the type of construction, each plant of this type is equipped with process control systems that monitor the evolution of the biological activity by means of probes that measure the air temperature and the concentration of O2 and/or CO2. Monitoring the concentration of these gases in the bulk gas discharged from the windrows enables precise determination of the status of the degradation process.
Bioreactors can be divided into two primary categories: Vertical bioreactors and Horizontal bioreactors. The latter is subdivided into horizontal reactors using channels, cells, containers and tunnels. Another type of vessel is the rotating drum, which typically is used ahead of the first phase of composting to size reduce and mix feedstocks, and initiate the composting process.
In addition to the structural differences, composting plants can be divided into continuous and discontinuous operations or systems. In continuous operation, the feedstock is loaded on a continuous basis, usually throughout an 8-hour period every day. In the discontinuous operation, the loading of feedstock is performed only after the processed material is discharged from the unit. Discontinuous systems also are known as batch systems.
An additional 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 employing dynamic reactors, the material is subjected to periodic turnings during the process.
The biooxidation phase performed in bioreactors is completed in seven to 15 days, depending upon the type of material (versus the 20 to 30 days typical for windrow composting). However, to achieve this rapid biooxidation, the process must be optimized in terms of quality of the starting material and in terms of maintaining all of the parameters that influence the evolution of biological activity within the proper range of values. A curing phase follows the biooxidation phase.
A vertical bioreactor is a cylindrical structure or container, composed of concrete or steel, thermally insulated, and having a volume of a few hundred to more than 2,600 cy (2,000 m3). Typically, the material is loaded at the top and is extracted from the bottom in a continuous cycle.
In the vertical plug-flow type of system, aeration is carried out by forcing air from the bottom of the reactor by means of a centrifugal blower, counter-current to the flow of the composting material. The exhausted air generally is conveyed to a biofilter. Extraction of the material from the bottom of the reactor is carried out by means of an Archimedean screw, similar to those used in agricultural silos.
Among the other types of vertical reactors is the Peabody type. This unit is a multi-floor reactor in which the biomass proceeds from the top of the reactor to the bottom, with a detention time of one to two days in each floor of the tower. Passage of the material from one floor to another is induced by a rotating scraper and is conducted through an opening on the opposite side of the scraper. As the material falls to the next floor, oxygen is introduced to the composting mass by airflow originating from the lower portion of the reactor. This type of plant is built in modules that are 10-feet (3 m) high and can reach ten units, achieving a height of close to 100 feet (30 m).
Vertical reactors have experienced operational difficulties, and have lost their attractiveness. They have been almost entirely replaced by horizontal bioreactors. Some of the problems faced by the vertical reactors include difficulty in achieving a relatively 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.
The horizontal reactors differ from windrow composting plants (dynamic or static) due to the presence of walls, with a height ranging from 3-feet (1 m) to 10-feet (3 m), used to separate the different windrows. In this type of plant, the biomass is maintained at the necessary aerobic conditions by means of forced aeration, usually combined with mechanical turning. Generally, the ambient air in the building is under negative pressure, and the air discharged from the building is conveyed to an air treatment unit. In the United States, there are a number of horizontal bioreactors aerated in the positive mode, with process air pulled out of the building and treated in a biofilter. To reduce the amount of process air to be treated, the horizontal reactors can be “contained” within the building, e.g., with the use of plastic curtains.
Loading and unloading of the rows are carried out with a conveyor belt or with automatic systems using Archimedean screws. Alternatively, a front-end loader can perform the loading and unloading operations.
These plants are characterized by windrows of limited width, usually not over 20 feet (6 m), and a length that can exceed 165 feet (50 m). The working cycle can be continuous or discontinuous. In the case of a continuous cycle, the loading operations are performed almost daily. In the case of discontinuous operations, the loading is carried out only when the first phase of the process is completed, and after the product has been discharged from the plant. Plants using the continuous cycle are categorized as dynamic plants, and are further distinguished with longitudinal flow and those with lateral movement.
In plants with longitudinal flow, material moves gradually from the loading end of the windrow to the unloading end. Plants that use this operational scheme are defined as horizontal plug-flow. The rate of movement of the biomass depends on the type of turning machine employed, but typically is in the range of 6- to 10-feet (2 to 3 m) of movement per turning. Detention time for the biomass is about four weeks for the first phase of the process. The length of the detention period depends on the degree of aeration provided by the turning machine, frequency of the turning operation, and the presence or absence of an integrated aeration system. Apart from the different type of cycle, composting plants with channels can be characterized by the different shape of the tracks. The most common shapes of tracks are straight track, U-track, and elliptical (or “race”) track.
Modern Composting Technologies describes a number of proprietary systems that are used in other countries. These include the Okada and Sutco agitated bed plants that are characterized by the use of an inclined elevating conveyor belt (45° or 55°) combined with a front rotor, operating in channels (trenches) with walls of reinforced concrete, 6- to 10-feet (2 to 3 m) wide and 3- to 6-feet (1 to 2 m) high. The turning machine moves on rails installed on the top of the channel walls and is equipped with a lifting mechanism for the return phase. In a Backhus type plant, the turning machine is composed of a rotor operated by a 55 kW motor, moving on rails placed on the top of the walls, with a maximum height of 7.2-feet (2.2 m). The rotational speed of the rotor is about 100 to 250 rpm. The movement of the product is about 6-feet (2 m) per pass.
Another horizontal plug-flow design is the Kishi type plant, which has a loading hopper and operates on an elliptical track. The material is loaded in order to form different layers, one on top of the other, in the first half of the bin. The main advantage of using this technology is the possibility of processing materials with a moisture content exceeding the maximum values suggested for “typical” composting (i.e., over 65 percent). In fact, by loading the fresh material on top of the layers that have already achieved process temperatures, a rapid evaporation of excess moisture can be achieved. Loading of the hopper is carried out by means of a conveyor belt or an Archimedean screw connected to the external loading hopper. This type of plant is particularly suitable for composting municipal sludge mixed with other materials.
Systems more typically seen in North America include Longwood Manufacturing Co., Transform Compost Systems, and U.S. Filter. (An accompanying article in this In-Vessel section reviews the history and development of agitated bed composting systems.) In these types of plants, the turning machines are characterized by a whip rotor (having different shapes depending upon the material to be composted), with a diameter of 6- to 10-feet (2 to 3 m), that produces a uniform material advance. The unit is operated by electric motors of about 15 to 30 kW, a row width that varies from 10-feet to 20 feet (3 to 6 m), and a height of 3- to 5-feet (1.0 to 1.5 m). The turning machines move on rails and are lifted for the return. The machines can be moved from channel to channel using a special piece of equipment. Thus, a single turning machine can operate on several rows. Turnings usually are conducted every day. The advancement of the material in each row is about 3- to 10-feet (1 to 3 m), depending on the model.
Because of the characteristics of its turning apparatus, this type of machine primarily is used to treat animal wastes, sludge from municipal wastewater and food industry treatment plants, and in general for materials with a small particle size distribution. In North America, ground yard trimmings and wood waste often are used as the bulking agent when composting wastewater treatment plant sludge.
In plants with lateral movement, the material is moved sideways to the adjacent row at each passing of the turning machine. An advantage of this particular type of plant is the possibility of using a shuttle conveyor belt for easier loading operations; the material is conveyed to the row adjacent to the belt. One of the most common of these systems is the OTV, characterized by rows 13 feet (4 m) wide and 5.6 feet (1.7 m) high. Loading is conducted approximately every two to three days (which is the time required to fill the row).
The turning machine consists of a rotor with a diameter of 12 feet (3.6 m). As the rotor advances, it lifts the material from one row and transfers it to the next by means of a transfer apparatus equipped with two Archimedean screws. The biomass of the last row is loaded directly onto a second belt to be transferred to the area where the second phase of the composting process takes place. Upon its return, the turning machine is transferred from one row to another by means of a carriage. The processing capacity of the units is about 288 cy/h (220 m3/h). The installed power is 15 kW for the rotor and 15 kW for the two Archimedean screws (7.5 kW each).
Another example of a lateral movement reactor is the “Basin” plant, which features a bay with a single row, with a desired width (up to about 100 feet (30 m)) and a windrow height of 10-feet (3 m). Generally, the pile is defined as a rectangular windrow or extended pile. The material is moved by sectors, longitudinal or lateral, as if operating on different single rows. With the exception of those plants having self-propelled turners, in most plants the turners are carried by bridges that move the material from one sector to another. For these types of plants, some of the following turning machines can be used: inclined elevator belt, bucket wheel, and inclined or vertical Archimedean screws. The loading generally is carried out by means of conveyor belts, and the material is moved toward the discharge end with the successive turnings. During the last turning, the biomass is discharged onto a system of conveyor belts that transport the material to the maturation area. The flooring of this type of plant is equipped with systems for the aeration of the biomass.
A number of other types of lateral movement reactors are described in detail in Modern Composting Technologies. These include the Buehler, Weser, VKW type plants based on the use of a turning apparatus in the shape of a cup (or Pelton wheel) carried by a bridge; the EMMEPI type plant that uses a turning device composed of a carbon-steel structure and a horizontal shaft on which two discs are mounted; the Horstmann type plant, which employs the bucket wheel; and the De Nicola type plant, with an elevator belt turning apparatus.
Another design uses vertical Archimedean screws carried and powered by a bridge. This system is used in discontinuous cycle plants, since the product does not actually advance during turning. The diameter and length of the screws can be chosen based upon the particular needs of the facility. A length of 13 feet (4 m) is adequate to work on piles with a maximum height of 11 feet (3.3 m). A system that was marketed in the United States in the 1970s (Fairfield-Hardy) incorporated hollow screws with perforations at the edges. Air was forced through the perforations and into the composting mass. The installed power varies between 7 and 80 kW, and the processing capacity ranges from 130 to 325 cy/h (100 to 250 m3/h).
Horizontal cylindrical bioreactors, referred to 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-feet to 14 feet (2 to 4 m), and is about 150 feet (45 m) long. The bioreactors rotate slowly (0.2 to 2 rpm) and are operated with forced air counter-current to the flow of the material. In some designs, sharp protrusions are incorporated inside the bioreactor to move the material forward, while at the same time helping to tear plastic bags.
The reactor, filled to about two-thirds of its capacity, has a processing cycle of about one week for the first phase of composting. Feedstocks with high respiration index values may remain in the reactor for only two to three days. (The shorter retention time is typical of rotating drum facilities in the U.S. and Canada.) The second phase of composting generally is carried out in windrows.
Small-size bioreactors are also available in various capacities. The reactors basically consist of small rotating cylinders that are powered by electrical motors. Recently, these systems have been used for processing livestock mortalities and manures (e.g., BW Organics).
During the past decade, biocells have been the subject of nearly continuous technological development. One result has been a high degree of system reliability. Biocells are enclosed, completely sealed units, in which composting is carried out in a controlled environment. A characteristic of this type of plant is its aeration system, generally operated under pressure by means of forced air. The air is forced through the flooring, via channels or pipes. Generally, the exhaust gas is removed from the top of the enclosure.
Biocells operate in the discontinuous mode. The loading phase is followed by cell closure and the start of the process. The intensive composting process generally lasts about two weeks, but it can vary depending upon the type of material being processed.
These plants have basically evolved from similar designs used in beds to prepare organic material used for the cultivation of mushrooms. Biocells can be divided based on size, static versus dynamic, and method of loading and unloading. With static biocells, the material is not subject to transfer during the entire process. With dynamic biocells, the biomass is moved inside the cell by means of an automatic system.
In a biocell, the composting unit generally has a rectangular base and a volumetric capacity in the range of 130 cy to 1,300 cy (100 to 1000 m3). The length can be over 165 feet (50 m), and the width usually does not exceed 20 feet (6 m), and is typically related to achieving complete closure of the doors. The height generally is limited to 13 feet (4 m) or less, with a loading height of 10-feet (3 m). Greater heights result in excessive compaction of the biomass, resulting in poor air distribution through the material.
The main structure (cell) can be cast onsite or prefabricated. It is important to thermally insulate the structure to limit heat losses from the composting mass through radiation. Heat loss through convection due to forced aeration cannot be avoided. Most designs use external insulation.
The flooring is one of the more important components of the biocell as it generally becomes an integral part of the aeration system. In fact, both product quality and energy efficiency depend on the correct type and level of aeration of the biomass. Methods of aeration include channels, embedded nozzles, and an air expansion chamber below.
The flooring needs to be designed to drain leachate. A system of channels can be installed for the collection of leachate. There is also prefabricated flooring with holes installed. The quality of the flooring is important since it comes in contact with highly corrosive materials, such as leachate, and is exposed to loads of 0.15 to 0.18 kg/cm2, resulting from the mass in the cell.
In dynamic biocells, the common type of flooring is replaced by a moving one (walking floor) that also can be used in static biocells; this is a type of flooring normally used for mushroom beds. Walking floors allow flexibility in the method of loading and offers the convenience of self-unloading. The upper part of the module has a triangular shape to enclose an Archimedean screw loading device equipped with five discharge openings located on the longitudinal axis of the cell. This screw is connected with three other screws that, on the whole, form the loading/recirculating system. The movement periods are determined by the computer system that controls the plant. Generally, the recirculation system is operational for about 36 hours out of the two weeks of the process. Recirculation allows control of the moisture content of the material, since it is possible to grab samples from the Archimedean screw transport system.
Front-end loaders typically are used to load biocells, although some facilities are equipped with sophisticated loading systems that feature shuttle conveyor belts or Archimedean screw devices. The main advantages of a centralized loading system are less labor and less compaction of the material in the biocells. This latter advantage probably is the more important one. The main disadvantage is the initial capital investment.
The unloading process, in most cases, is performed with a front-end loader. Some of the first biocells built featured a system of unloading carried out by means of a moving floor coupled with a rotor discharge machine located on the floor of the biocell. This device was connected to a series of conveyor belts that transported the material to the area where the second phase of composting took place.
The type of blowers typically used in biocells deliver a maximum airflow of 13 to 16 cy/h per cy of biocell (10 to 12 m3/h per m3), at a maximum pressure of 5- to 6-inches (120 to 150 mm) of water. Discharge air can be directed to a biofilter or recirculated back into the cell. The choice of recirculating or discharging the process air is based on the data relayed to a computer by probes measuring temperature and concentration of CO2. Air that is discharged passes through a wet scrubber to eliminate particulate matter and soluble gaseous compounds, and then through a biofilter. The openings for air suction and the spraying devices are located on the top of the cell. Generally, air is drawn from the top of the biocell through openings located on the roof. In some cases, in extremely cold climates, incoming air also is passed through an air-to-air heat exchanger to preheat the air.
Generally, a spraying system is located at the top of a biocell to provide moisture to the biomass. The leachate eventually produced is collected and can be recirculated. The spraying system can be operated manually or automatically, using a system to monitor the moisture content of the air exiting the cell.
An interesting variation on the designs previously described is the Biodegma system, which consists of modules composed of concrete walls, with a metallic structure on the top that serves as a roof. The roof can be opened automatically. The structure looks much like a traditional greenhouse, but the roof cover is a Gore-Tex® material. The doors of the modules open outwardly and are also composed of Gore-Tex. The modules have a capacity of 325 cy (250 m3) each and incorporate forced aeration. Once loaded, the roof and the door are closed and the intensive composting period begins. During the three to four weeks of intensive composting, a controlled and aerated climate is maintained inside the composting modules.
After intensive composting, the material is placed into another set of modules, which also are kept closed and aerated. Depending on the nature of the material being composted, moisture can be added using the collected leachate or other suitable liquid. The material remains in the curing modules for another three to four weeks.
Biocontainers differ from biocells in terms of container volume and method of loading. Biocontainers generally are smaller than biocells, with volumes from 25 to 50 cy (20 to 40 m3). These systems are modular, being composed of six to eight reactors, with an installed power of 4.5 kW for each container. The throughput of a module is between about 3,000 and 5,000 tons/yr depending on the type of organic matter.
The loading process occurs at the top of the unit using a front-end loader or a conveyor belt, after opening the upper door of the container. Discharge of processed material is carried out by tipping the container, often with a roll-off waste collection truck.
Using forced aeration, air is distributed from the bottom of the container by means of nozzles; air leaving the reactor is treated in a biofilter system. Usually the process is controlled by a computer. Generally, each cell also is equipped with a system for adding moisture to the material. The system is capable of conveying up to 40 gallons (150 liters) of water per day. Leachate generated within the container is extracted by gravity through openings at the bottom of the cell. The process, similar to other cell type plants, lasts between eight and 15 days.
Biotunnels differ from the biocell type of reactor as a result of having different areas for loading and unloading at the ends of the reactor. Continuous plants equipped with forced aeration are included in this category. Biotunnels have a rectangular cross-section, thermal insulation, and dimensions similar to biocells, i.e., about 13 to 16 feet wide, 10- to 13 feet high and up to 65 to 100 feet long (4 to 5 m wide, 3 to 4 m high, and up to 20 to 30 m long). They are typically composed of brick or metal.
Aeration is carried out by forcing air through the flooring. The discharged gas is collected by means of suction pipes connected to the recirculation/discharge system, located on the roof of the tunnel. All of these operations are controlled by a computer system.
Loading is performed daily at one end of the chamber. The material advances forward due to the motion of the moving floor, similar to that used by the Gicom plants, or due to a moving wall pushed by hydraulic pistons (BAV type plant). Similar to biocells, the treatment process lasts about two weeks. A North American company, Engineered Compost Systems manufactures a stationary unit that consists primarily of insulated concrete tunnels of various sizes built onsite. The aeration systems typically include reversing aeration and recirculates process air up to 80 percent.
Generally, air compressors are preferred over the traditional centrifugal fans for air distribution. Compressors offer the advantage of allowing for air preheating. On the other hand, compressors generally are noisier than other options for moving air.
Modern Composting Technologies includes a directory of North American and European system manufacturers. The book also includes photos and diagrams of many of the elements discussed in this article.
Luis Diaz and George Savage are with CalRecovery, Inc. in Concord, California; Alessandro Chiumenti currently is with the University of Udine. Diaz, Savage and Chiumenti are coauthors of Modern Composting Technologies, to be published in summer 2005 by BioCycle/The JG Press, Inc.
WHEN the U.S. Environmental Protection Agency adopted its pathogen reduction standards for biosolids composting, horizontal agitated bed composting was categorized as windrow composting. This meant these in-vessel systems had to meet the 55°C consecutive day time/temperature requirement for 15 days. Other in-vessel technologies, such as vertical reactors, fell into the “three days at 55°C” time/temperature requirement.
At the time, this regulatory status led to a casual debate about whether agitated bed systems were “in-vessel” or windrows contained within concrete walls. Over time, it didn’t really matter as long as the systems performed as the operators and regulators expected.
A few years ago, a casual debate evolved at the BioCycle offices when working on our annual composting equipment and systems directory. An increasing number of composting projects were using aerated static piles enclosed in plastic bags (either Ag-Bag or CTI (now Versa Corp.) systems. The question was: should we characterize these systems as in-vessel as they completely contain the composting pile? Would that be confusing as typically composting “vessels” are rigid structures made from concrete or metal?
The questions became easier to answer when several states gave permits to sites composting food residuals in piles that had to be contained, i.e., they could not be on a bare surface and they also had to control vector attraction and leachate. In California, several fairly large composting facilities were permitted to take food residuals using the Ag-Bag or Versa systems. A site in Minnesota and another in Massachusetts received similar permits.
More recently, a new technology was introduced in North America using the Gore Cover composting system. This is essentially an enclosed aerated static pile. In this case, piles are constructed over concrete floors with aeration channels, then covered with a fabric made from Gore-Tex. In Washington State, a large-scale composter was permitted to take residential and commercial organics (it had been receiving the latter, but in fairly small volumes) as long as it installed a contained composting system. In this case, the company installed the Gore system.
The Gore installations in North America include several composting biosolids and several processing yard trimmings and source separated organics. All include forced aeration. Explains Modern Composting Technologies, “the Gore-Tex membranes keep precipitation from entering the windrow, but allow the CO2 produced during the composting process to escape. In addition, the membrane 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.”
Modern Composting Technologies also includes descriptions of the Versa and Ag-Bag systems: “A particular type of forced aeration system for windrow composting is based on the utilization of large plastic bags, with diameters of 8- to 12 feet (2.4 to 3.6 m) and lengths of up to 500 feet (150 m), into which the material is forced by means of a press. The bags are equipped with two perforated plastic pipes, connected to a blower for the supply of air. In addition, the bags have a pattern of openings in the upper portion to allow for the release of gases.”
“The pipes are inserted into the biomass with the same unit that is used to compress and spread the plastic sheet. The bag is formed during the progressive withdrawal of the press at the time that the material to be composted is loaded. The first and second phases of the process are conducted sequentially in the same bag and last about eight weeks.” – N.G.
AC Equipment Services
6737 West Washington Street
Suite 1400
Milwaukee, WI 53214
Ag Bag Environmental
92365 Riekkola Rd.
Hwy 101 Business
Astoria, OR 97103
BW Organics, Inc.
475 County Road 2300
Sulphur Springs, TX 75482
Engineered Compost Systems (ECS)
4211 24th Avenue West
Seattle, WA 98199
Gore Cover System
North America
C/O SEC Technologies
774 D Meadowlark Rd.
Lynden, WA 98264
U.S. Filter IPS
Composting Division
441 Main Street, P.O. Box 36
Sturbridge, MA 01566
Versa Compost Technologies
P.O. Box 747
Astoria, OR 97103

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