Contained Composting Systems Rewiew

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Courtesy of BioCycle Magazine

Having gained success in recycling yard trimmings, composters and recycling coordinators are reaching deeper into the organic residuals pile to capture other feedstocks. In particular, their sights are set on source separated food materials, from institutions (e.g. schools, hospitals), grocery stores and produce markets, food processors and commercial food service facilities (e.g. restaurants). A niche has developed for systems that can compost food and other putrescible materials at a reasonable cost without nuisance. Many potential applications of food composting are in urban/suburban settings and involve relatively small volumes of material. Thus, an even more specialized niche exists for systems that can effectively and economically handle relatively small volumes of food in sensitive and space restricted environments.

Composting entrepreneurs have responded to this need with a variety of enclosed commercial systems that contain the processing environment, isolating it from the surroundings. These systems might be called in-vessel composting, but that exaggerates the scale. “Contained” composting systems seems to be a better label.

This two-part report reviews options for composting various volumes of food in contained systems. All of the methods discussed are feasible for on-site processing of food residuals. However, the systems clearly vary in their target scale of application. Part I of the article covers smaller systems that are intended for on-site processing of small volumes of materials but might also be used at a central site. Part II discusses methods that are primarily intended for central processing but can be adapted for on-site composting of small to moderate volumes of food. If it helps, consider the focus of Part I to be systems that handle generation rates of about one ton/day or less.

THE CHALLENGE AND NATURE OF ON-SITE COMPOSTING OF FOOD

Although centralized composting has advantages, there is a great deal of interest in the on-site approach because it avoids the high cost of collection and transportation. In addition, clean feedstocks are critical to producing usable compost. Feedstocks are more likely to be free of contaminants if the generator is also responsible for composting.

Composting food at the point of generation is a challenge. First, food is not an easy material to manage. It is moist, putrescible, easily odorous and attractive to pests. Some foods are particularly difficult, like fish because of odors, or postconsumer food scraps due to contamination and pathogen concerns. Dry amendments are required to absorb the abundant moisture and cover the food to keep out pests and keep in odors. The amount of dry amendments needed ranges from one-half to over twice the volume of food generated.

Second, at a commercial or institutional location, composting is restricted by several factors including lack of space, interference with other activities at the site, lack of time and composting expertise among staff, difficulty handling runoff and leachate, and aesthetic impacts on the site and the surroundings. A large generator, such as a food processing plant, may have enough volume and buffer space to accommodate windrows or aerated static piles. However, most generators of food residuals need a system that has a small footprint and totally isolates the composting environment and its aromas. On-site systems also need to deny flies and pests access to the materials, make the composting tasks easy for the staff (who have other work to do), and reach high temperatures to ensure pathogen destruction.

These needs are being addressed by composting systems that enclose the composting materials within a variety of containers. At a minimum, the systems provide aeration — forced aeration in most cases. They also supply some degree of process control to limit the composting time (and space), attain high temperatures, and minimize odor generation. A biofilter is often included in the package for odor control. To limit staff involvement, the systems are largely self-regulating or permit the composting process to regulate itself.

Each of the commercially available contained composting systems has its unique set of methods, equipment and components. Nevertheless, most can be grouped and described according to their composting approach. Generic categories of small-scale contained systems include passively aerated bins, aerated containers, agitated-aerated containers and rotating drums. Systems appropriate for larger scale applications include these categories plus agitated beds and tunnel units. In addition, there are a number of systems in various stages of development that challenge these generic categories such as aerobic digestion tanks (e.g. BioMate) and vertical cylinders (e.g. Celto Canadian, Verticom). These will be discussed in Part II. Table 1 summarizes the systems discussed in both Part I and II.

PASSIVELY-AERATED BINS

It is possible to use passively aerated bins for composting highly putrescible feedstocks, like food, on a small scale. Backyard composters do this routinely. Passive or natural aeration occurs by at least three routes: 1) Oxygen diffuses into material because there is more oxygen outside than within; 2) Heat causes thermal convection as warm gases rise out of the composting mass and cool fresh air enters; and 3) Wind blows air through the materials. For composting food at a school or restaurant, solid bins are often used to contain the composting materials. Solid containers block the normal routes of passive air movement so aeration aids are necessary. Even with aeration aids, diffusion and wind are constrained. Therefore the key to obtaining reliable passive air movement is generating heat to drive thermal convection.

The strength of the passively aerated bin approach is its simplicity. The bins are inexpensive. There are no moving parts, and there is no need for electricity. Success depends on the composting process working well. Therefore, operators must learn to be good at managing and reading the process.

An example of one commercial container intended for on-site composting of food residuals is the Hot Box, developed and patented by Open Road of New York, a nonprofit organization. Open Road licenses the Hot Box design, which is a solid one cubic yard bin, 3 feet by 3 feet by 3 feet in dimension. Its base and walls are made from wood or recycled plastic lumber. The bin is covered with planks or a hinged lid. One wall of the Hot Box is constructed with planks to facilitate unloading. Two rows of perforated PVC aeration pipes run across the lower and mid sections of the bin to enhance passive air movement. The pipes are inserted through holes in one wall and rest against a ridge on the inside of the opposite wall. One end of each pipe is open to the outside air.

After placing a base layer of wood chips, the bin can be filled gradually or in a single batch. Loading and unloading is accomplished manually. Open Road recommends a well mixed, one-to-one volume combination of food and wood chips. When the box is nearly full, it is capped with a layer of finished compost that serves as a passive biofilter. In very sensitive locations, the open end of the aeration pipes can be covered with a biofilter bag to filter odors further. The biofilter bag is a loose mesh bag filled with compost and woodchips. The number of bins required depends on the feedstock volume. With a 1:1 volume ratio of food and wood chips, each bin holds approximately one-half cubic yard of food. The feedstock is not agitated or mixed once in the bin. Compost is unloaded for curing within one month. When properly loaded, the Hot Box reaches temperatures above 130°F.

Passively aerated bins can take other forms and sizes. As another example, the slightly larger CM Pro container can be used as a passively aerated bin, although it is normally promoted as a forced aeration unit (see following section). Similarly, the Hot Box is aerated with fans in some applications. There also are the homemade varieties. For example, some schools and other institutions compost food in backyard-type wooden bins, enclosed within a building. They are managed much like backyard units, and like backyard composters, may not reliably reach high temperatures.

The Hot Box (and its passively aerated counterparts) represents the smallest scale operation. It is suitable for many low volume applications such as schools, individual restaurants and small food service facilities. It also is being used at an off-site location for composting materials from several small volume generators. As volume increases, however, the small size of the containers becomes inconvenient. Larger containers are difficult to aerate passively without sacrificing aerobic conditions and risking odors.

One technology, known as the TEG silo-cage system, attacks this difficulty by stacking feedstocks in tall narrow wire mesh cages. Several cages are arranged in series like slices in a loaf of bread. An air gap between adjacent cages provides a channel for passive aeration. As in other silo designs, materials move continuously and vertically through the system. Feedstocks are loaded at the top and compost is removed at the base. The currently available TEG equipment is suited to larger on-site applications and will be discussed in more detail in Part II. However, TEG-Environmental is developing a twin cage unit for small volume generators. It is recommended that the open cages be under roof or in a building.

AERATED CONTAINERS

Aerated containers are fully enclosed and covered aerated bins of various materials and dimensions. They are larger than most passively aerated bins and rely on fans for aeration. Many of the specific features vary among the available systems, including type of container, container size and configuration, access to the containers, air distribution, cost, and target applications. Examples of commercial aerated container systems include the CM Pro, a forced aeration hot box, Green Mountain Comptainer, NaturTech, Stinnes Enerco, and Ag-Bag. Of these, only the CM Pro system is targeted for small volume, on-site applications.

Aerated containers rely on forced aeration from fans to supply oxygen, and remove moisture and heat. In most cases, air is introduced at the base of the material and flows up through the composting mass into a headspace at the top. In other cases, air flows in the opposite direction, from the headspace to the plenum. Some systems have the ability to reverse the direction of airflow to even temperature and moisture gradients. Typically, the air from the plenum or head space is exhausted to a biofilter. Several containers can be aerated from a single fan by connecting individual containers to an air distribution header. Aeration may be controlled by time or temperature depending on the system. Leachate typically drains into the air distribution space at the base of the container where it either collects for later reuse or is directed to holding tanks. If too much leachate accumulates, it interferes with air distribution.

Each container is loaded with a mixture of feedstock and amendment and then composted as a batch. Additional containers are added as more feedstocks are generated. Two-container systems are feasible but more units can more efficiently and economically share a single aeration system and biofilter. Units are loaded by hand, a bucket loader, conveyor or special machinery, depending on the specific container. Most are loaded from the top, through a hinged or removable lid.

Aerated containers are essentially static systems. No agitation or turning takes place within the container. Therefore, feedstocks must be well mixed prior to loading. Many systems allow for the containers to be emptied so that materials can be examined, supplemented with water or more amendment, remixed, and reloaded for continued composting (or delivered to a second composting method like windrows). The emptying and reloading process improves moisture, mix uniformity, porosity and air distribution. It also provides an opportunity to add more feedstock or combine materials from several containers to make up for the shrinkage due to composting. However, the exercise of emptying and reloading containers obviously requires labor, time and expense so it is not practiced in many cases. Emptying also exposes the composting materials to the surroundings, potentially releasing odors and leachate — although after the initial period of composting, odor and leachate have been greatly reduced.

One version of the Hot Box, developed for restaurant applications, relies on forced aeration. A fan on the lid of the box draws air into the aeration pipes through box and up through the lid, exhausting the air into a biofilter. A typical two-box system, including a two cubic yard biofilter, occupies a three ft. by 12 ft. area.

The CM Pro system is another example of an aerated container that is sized, designed and intended for on-site composting at the point of generation. It also can be used at a central site handling relatively small volumes of materials. A single unit is 40 inches wide by 44 inches long by 44 inches high, holding slightly over one and a half cubic yards. 
  
After a bin is full, it is connected to the air distribution system, although natural aeration is an option. A total system is considered to be 16 bins plus two biofilters.

A bin can be handled by a fork truck or forks mounted on a small loader. It is emptied by tipping with a fork truck or stationary tub tipping equipment. The recommended minimum retention time is three weeks. One option promoted with this system is to incorporate a contained vermicomposting system as a second stage of composting, following seven days of precomposting in the CM Pro bin.

The larger aerated containers such as the NaturTech, Green Mountain Comptainer and Stinnes-Enerco units are similar to the CM Pro system in basic operation but grander in size and engineering. The smallest containers hold 16 to 40 cubic yards of material. They resemble solid waste roll-off units except they have provisions for aeration and process control. Conceptually, one or two containers can serve a modest on-site food composting system because the aeration system can operate while the container is being filled — once the air plenum is covered. New feedstock can be loaded into a second container while material in the first container is composting. Alternatively, it is possible to gradually load an aerated container while maintaining aeration until the container is full. Then, the unit can be placed on to a roll-off truck and delivered to a central composting site for further processing. Such a procedure has been proposed but not actually implemented (to our knowledge). While these large aerated containers are technically feasible for small, on-site applications, in general, their sophistication and cost are more easily justified with higher volume applications.

AGITATED-AERATED CONTAINERS

Agitated-aerated containers provide containment and controlled aeration plus the ability to agitate or turn materials within the unit. Agitation brings several advantages. Feedstock mixtures, and therefore the composts produced, are more uniform. Uniformity also improves because agitation breaks up air channels that form within the composting mass. Therefore, there is more flexibility in the type and amount of amendment needed. Usually less amendment can be used when agitation is provided. Added water is reasonably well distributed when agitation is provided. Without agitation, adding water is difficult, at best. In some cases, internal agitation can initially blend feedstocks and amendments, thereby eliminating a premixing step outside of the container. Methods to agitate materials within containers vary among composting systems.

Examples of commercial systems in this category include the Earth Tub by Green Mountain Technologies and the Wright Environmental Management, Inc. (WEMI) containers. The Earth Tub and most WEMI units are targeted for small-scale, on-site composting of challenging feedstocks like food. These two types of systems are unique and therefore do not fit a generic description. Horizontal agitated channel systems might be included in this category but since agitated beds are only applicable to larger systems, they are discussed in Part II.

Except for the agitation, the Earth Tub is comparable to the CM Pro system in scale, application, and operation. The Earth Tub is a circular tapered fully enclosed tub that includes a forced aeration system and an auger for mixing feedstocks. The diameters at the base and lid are 64 and 89 inches, respectively. The tub is 4 feet deep. A single tub holds 3 cubic yards of material. To increase capacity and approach continuous operation, multiple tubs are used, served by a common aeration and biofilter system. Feedstocks are periodically loaded through a hatch on the lid and mixed and shredded by the auger as they are loaded. The auger is mounted vertically (with a slight incline). It turns via an electric motor but is pushed around the tub manually, by rotating the tub lid through two revolutions. Compost is removed manually through a discharge door on the side. The aeration system is similar to that of the aerated containers. The air is drawn down through the tub to the air ducts in the tub floor and then exhausted into a biofilter. The floor chamber also drains leachate from the tub.

The WEMI composting system is continuous and involves more automation and mechanics than either the Earth Tub or CM Pro units. Materials compost and move through the system on stainless steel trays. Each tray holds one to two days of feedstock and has a perforated floor for aeration. An external hydraulic ram pushes an empty tray into the container or “tunnel.” In the tunnel, the tray is loaded with the feedstock mixture from an overhead hopper. When a new tray enters the tunnel, it nudges the preceding trays along and the last tray is discharged. At the discharge point, augers unload compost from the exiting tray. Within the tunnel, air is forced through the trays from a plenum below.

Air is recirculated and eventually exhausted to a biofilter, which is an integral part of the unit. Two aeration or temperature zones exist. Higher temperatures are maintained in the first zone for pathogen destruction (generally six days retention time in zone 1). Inside the tunnel, as a tray moves from zone 1 to zone 2, mechanical “spinners” agitate the compost. If necessary, water can be added during agitation. One self-contained unit incorporates the entire system — hopper, tunnel, aeration, agitation, augers and biofilter. Operators do not deal with the insides unless a problem develops. Wright Environmental Management has several standard sized units with throughputs ranging from 300 to 2000 lbs/day (at a retention time of 28 days). The 300 model measures roughly 7 feet wide by 19 feet long by 9 feet high. The 2000 model is approximately 10 feet wide by 37 feet long by 13 feet wide.

The WEMI system is intended for similar applications as the Earth Tub and CM Pro systems — composting of putrescent materials at the point of generation. One advantage promoted with the WEMI system is reduced need for amendment, as low as one-half the volume of food. However, a larger volume may be needed to justify the added technical features of a WEMI unit. Also, larger WEMI units surpass Earth Tub and CM Pro in scale. Like the large aerated boxes, WEMI units are also used for large-scale composting.

ROTATING DRUMS

Rotating drum composting digesters have been used both for large-scale facilities and backyard composting for many years. Recently, several versions of small commercial-scale rotating drums have emerged that are suitable to on-site composting of food. The vendors include BW Organics (Greendrum), Augspurger Engineering and Environmental Products & Technologies Corporation (EPTC).

Although the various drums differ in details and process management, they share the basic idea of promoting decomposition by tumbling material in an enclosed reactor. The typical small drum is 4 or 5 feet in diameter and 8 to 16 feet in length, but drums up to 10 feet in diameter and 50 feet long are available. Drums are oriented horizontally, sometimes at a slight incline. They slowly tumble material either continuously (Greendrum) or intermittently (Augspurger and EPTC). Feedstocks are loaded at one end and compost is removed at the opposite end. While various devices are used, loading with augers or conveyors and unloading by gravity are the norm. Inside the drum, the tumbling action mixes, agitates and generally moves material through the drum. In regard to the composting process, the key function of the rotation is to expose the material to air, add oxygen and release heat and gaseous products of decomposition.

Forced aeration is frequently but not always provided. In fact, the three commercial systems listed above follow different approaches to aeration. Most of the Greendrum applications do not use forced aeration. Passive air movement through the openings at the end delivers sufficient oxygen in most cases but a fan is sometimes used with longer drums. The Augspurger drums contain fans to move air through the drum. The EPTC system takes the unique approach of injecting the closed drum with an oxygen-rich atmosphere (80 percent plus) from an oxygen generator. The design of the drum and the loading and unloading devices create a closed system that allows the high oxygen concentrations to be maintained.

Rotating drum composting reactors always have been associated with very short retention times. In the past, drums have served as an intense first stage of composting followed by an extended curing period or additional composting in windrows or another secondary system. The Augspurger system takes this approach. A seven day retention time is suggested if the drum is used for initial decomposition followed by additional composting outside the drum. For complete composting within the drum, three weeks are recommended. However, the other two commercial systems promote very short retention times — three to five days for the Greendrum and only two days for the EPTC system. In the latter case, the short retention time is attributed to the high oxygen environment, special microbial inoculant, and close control of the process environment. Such abbreviated retention periods are cause for skepticism. The currently prevailing wisdom says that a minimum of two weeks is necessary to achieve a compost product that is mature enough for general horticultural use. Nevertheless, both BW Organics and EPTC are supporting their claims with research projects.

Small rotating drums are just beginning to find their niche. These small-scale systems have cut their teeth primarily on yard trimmings (in the case of Augspurger) and agricultural materials such as manure and animal mortalities (in the case of the Greendrum and EPTC). However, the containment and relatively small footprint that rotating drums offer make them suitable for on-site composting of food, biosolids and other difficult feedstocks. In fact, rotating drums are now starting to be tested and used for composting food residuals. For example, the Greendrum is being used for composting food at a prison in North Carolina. The EPTC drum is currently being piloted at Utah State University with food residuals plus a commercial scale drum is being developed for a food composting application in Hawaii. Drums also are suited to the strategy of initially composting material for a short period on-site, followed by additional processing at a central, off-site location.

SQUARE FEET VS. ACRES

Rotating drum systems highlight the difficulty in defining scale of applications. Depending on their size and design, rotating drums can be used for on-site composting of small volumes of food or tremendously large facilities composting mixed waste. However, while defining the boundaries between small and large scale is difficult, the systems discussed in Part I are reasonably well-suited to on-site composting by small volume generators. You can picture them in the back lot of a school, restaurant, or market. The amount of space required would be expressed in square feet.

Part II of this report takes a detailed look at contained composting options for larger volumes of food residuals. These systems are technically appropriate for on-site composting but only economical at moderate to large volumes. They might involve a land area worthy of the term “acre” and perhaps a separate building. Systems to be discussed include the larger drums and aerated containers, agitated beds, and tunnel systems. Part II also will review the performance of contained systems for composting food at the point of generation.

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