July 25, 2005 | General

AGITATED BED COMPOSTING: PAST, PRESENT, FUTURE


BioCycle July 2005, Vol. 46, No. 7, p. 52
Survival of agitated bed systems is not only based on the number constructed, but also because early in the development of the new technology, design errors did not prevent these facilities from continuing to function.
Lewis M. Naylor and Geoffrey Kuter

A SURVEY conducted in 1999 – and reported on in the August 2000 issue of BioCycle – asked operators and managers of in-vessel composting facilities in service for more than five years why their compost plant had succeeded. Compost technologies included were agitated bed systems, vertical reactors, static horizontal reactors, and rotary drum facilities. Respondents contacted reported that their success was attributable to three major reasons: Manpower – an operations staff dedicated to making the facility successful; Maintenance – skilled operators performed regular maintenance; and Marketing – effective in-house or contract compost marketing program existed.
While even mechanically complex in-vessel systems succeeded if they achieved the three “Ms,” some technologies have been as a whole more successful, have been better accepted, and have continued to be marketed. Of the in-vessel composting technologies, the agitated bed’s continuing success is atypical of other mechanical, in-vessel systems. The majority of installations compost biosolids. Over 70 percent of all such facilities ever constructed continue to operate, some for as long as 17 years. The greatest number of facilities constructed occurred during the decade between 1990 and 2000. While growth in the number of new facilities constructed has not been maintained, the quantity of material composted continued to increase through 2004 as the capacity of individual facilities increased.
The agitated bed in-vessel composting technology incorporates composting in long parallel channels with concrete walls, and a fully automated compost turner traveling on the top of the walls. Most turners ride on steel rails mounted on the top of the walls that separate channels. Turners can move from one channel to the next on a transfer dolly at the completion of each cycle. The transfer dolly can be either at the front (loading) or the back (discharge) of the channel. The turner makes a pass through the channel typically five to seven times a week. With each pass, compost is discharged out the back as individual charges move more-or-less plug flow through the channel, leaving a space at the front of the channel where the next charge is placed. Individual charges of compostable material are loaded into the front of the bay typically using a wheel loader. Forced aeration is commonly practiced, and the aeration cycle is controlled either on a timer, a temperature feedback basis, or both. Temperature of the composting material in the channel can be monitored by thermocouples embedded at multiple locations in the concrete wall.
In-vessel agitated bed composting technologies marketed in the United States over the last 20 years include IPS/International Process Systems, Inc., owned variously by EarthGro, Wheelabrator/Waste Management, US Filter, and now Siemens; LMC/ Longwood Manufacturing Corp.; Royer Industries, Farmer Automatic, and most recently, TCS/Transform Compost Systems, Inc. While this list is not comprehensive, these systems best characterize the most widely used horizontal flow, agitated bed technologies.
This article looks back at the history of agitated bed technologies, discussing the process success, innovations, and its operational survival. While most agitated bed composting facilities process biosolids combined with an amendment, a few also process animal manures and organic residuals separated from solid waste, both with and without biosolids. While the intent here is to include all agitated bed composting facilities in operation in the United States through December, 2004, some may have been overlooked or be unknown.
DEVELOPMENT OF THE TECHNOLOGY
The development of the agitated bed technology and composting systems in general can be traced through BioCycle, the principal voice of the composting industry, and its predecessor, Compost Science. In the March 1977 issue of Compost Science, there was a list of composting plants in Germany, some of which processed biosolids. Twenty-four companies were reported marketing a composting technology or equipment used in composting. In-vessel technologies included vertical static reactors (e.g. BAV), horizontal rotary drums (Dano), and a vertical tower with multiple floors enabling material to be discharged downward from one floor to the next, providing a kind of agitation and mixing (Biocell). The same issue contained an advertisement for one American composting technology: agitated windrow (Cobey Composter, a mechanical compost turner).
Ten years later, a variety of in-vessel composting technologies were actively being promoted in the United States. A 1987 issue of BioCycle contained in-vessel composting technology advertisements for a vertical static reactor system (American BioTech, large rectangular tank with compost fed from the top and discharged out the bottom by gravity flow with aeration provided by vertical “air lances”), an agitated bed system (Fairfield Digester, large cylindrical tank agitated with multiple vertical augers mounted on an arm revolving about the diameter of the tank and aeration from the bottom of the tank), and a horizontal static reactor (Ashbrook-Simon-Hartley, horizontal tank with aeration from the bottom where compost is forced through the tank by a hydraulic ram). Mechanical compost windrow turners advertised included Brown Bear, Wildcat, Scarab and King of the Windrow (K-W).
By year’s end, BioCycle’s annual survey of biosolids composting projects reported 107 operational facilities, with 15 under construction, and another 39 in the planning, design, bid phase. Of the 14 operating in-vessel facilities on the list, eight were vertical reactors (American BioTech, Taulman, Purac, and Ladig), one horizontal reactor (Ashbrook-Simon-Hartley), three Fairfield digesters, and two semi-agitated reactors (PayGro, open top aerated long reactors agitated once during the composting cycle). While several agitated bed facilities (IPS) were reported to be in the planning stages, none had been constructed and were operational.
Moving forward another decade to January, 1997, not a single advertisement appeared in BioCycle (1997) for in-vessel composting. A possible exception would be Ag-Bag, in which a blended feedstock of amendment and biosolids (or other residuals) is composted inside a long plastic bag. The technology was developed for the agricultural industry for inexpensive storage of ensilage (a horizontal silo in which the bag keeps air away from the silage). For composting, a small blower provides aeration to the compost through a perforated plastic pipe along the bottom of the bag.
In the January, 2005, issue of BioCycle, advertisements for composting technologies included at least eight different manufacturers of windrow turners, but no in-vessel systems.
Looking at the January snapshots beginning in 1977, and every ten years thereafter, covering a period of nearly three decades, one might conclude that the in-vessel agitated bed technologies made a brief appearance on the composting scene and then disappeared as a genetic sport might arise and die off. However, nothing could be further from the truth. Just one year after the 1987 snapshot, advertisements for two in-vessel agitated bed technologies appeared in BioCycle: Royer (March, 1988) and IPS (May/June, 1988). Highly significant is that between 1987 and 1997, nearly 30 agitated bed composting facilities started up and were operating. Within another seven years, ten additional facilities came on line. During this period, two more agitated bed manufactures also entered the market: LMC (Longwood Manufacturing Corp.) and TCS (Transform Compost System).
CONTINUITY OF OPERATIONS
What is also significant is that most of the in-vessel agitated bed composting facilities constructed operated for at least ten years, including the first constructed in 1987 in Fairfield, Connecticut. Just over 70 percent of the individual facilities (82 percent of the capacity) continue in 2005 on a daily basis operating at full capacity. However, 30 percent have closed their doors. What are the reasons for both the success and failure?
The principal cause for discontinuing operations has been not mechanical or process failure, but rather economic pressures of substantially lower costs for landfilling biosolids. In the late 1980s to the early 1990s, landfill tipping fees advanced from less than $30/wet ton to over $90/wet ton in some areas, especially the Northeast. For example, tip fees in Onondaga County, New York, increased from $31/ton in 1989, to $47/ton in 1990, peaking at $99/ton in 1994. Tip fees began to drop about 1996, and bottomed out at $50/ton in 2000. Agitated bed facilities that could be constructed and operated at an estimated lifecycle cost of between $50 and $70/wet ton were competitive economically with biosolids management alternatives such as thermal drying and landfilling. However, this competitive price position did not continue.
Following the well-known supply-demand curve and its relation to price, landfill capacity increased in response to the rocketing tipping fees. As capacity increased, tipping fees dropped back into the $30 to $60/wet ton range. This left the agitated bed technologies in park for a few years. In the last five years, Class B biosolids options such as application to land have become controversial in some areas, and composting options are being given another look by potential customers.
Since 1998, less than a dozen agitated bed composting facilities have been constructed. While this suggests a downturn in the industry, facilities constructed in the years since 1998 have had larger bays and thus greater capacity per channel/bay. A transition begun in 1994 by LMC to build turners for wider and deeper channels/bays has increased the capacity of each charge loaded into a channel from about 6 wet tons/charge to as much as 16 wet tons/charge. This practice was followed by IPS in 1998, and continued by TCS. As a consequence of increases in charge capacity, the total quantity of materials composted in agitated bed composting facilities has continued to increase even though fewer facilities have been constructed (Figure 1).
TECHNOLOGY SUCCESS
While construction of new individual agitated bed composting facilities has not experienced the growth evident in the decade from 1990 to 2000, it is not zero, and the technology continues to be marketed. Many other in-vessel technologies continue to operate, but are no longer aggressively marketed. These technologies include the vertical flow, packed bed and the horizontal flow, static bed systems.
Survival of agitated bed systems is not only based on the number constructed, though this technology has certainly dominated the industry since 1985. A major contributing factor is that early in the development of the technology by IPS, Royer, LMC and TCS, design errors associated with the development of this new technology did not prevent these facilities from continuing to function. While other in-vessel technologies were rigid physically and operationally, the open channel agitated bed technology enabled operators and engineers to inspect the process and the mechanical systems at any point in the process. This ability not only guided the design of successive agitated bed facilities, but also enabled mechanical and process changes in operating facilities.
Other key factors contributing to the success of the agitated bed technology are as follows:
Process and mechanical accessibility. Totally enclosed in-vessel systems such as vertical flow, packed bed and the horizontal flow, packed bed are not readily accessible. Process mistakes cannot be corrected, or even detected, until too late, e.g., fires inside the vertical flow, packed bed reactor in Hartford, Connecticut (see “Fires Destroy Hartford In-Vessel Composting Facility,” January 2000). Short-circuiting is possible in such systems as the flow of compost inside the closed reactor, either vertically or horizontally, is not uniform. Temperature monitoring is uncertain. Probing the compost during the active composting stages is impossible. In contrast, compost in the agitated bed channels is turned and mixed on a regular basis, usually daily. Channels are open and thus accessible. Depth is limited to eight feet, and many are just six feet deep. The regular mixing and shallow bed depth work together to prevent fires as well as short-circuiting in the open top vessel (channel). Operators can probe the compost to monitor temperatures at any point, and at any location in the process. The compost turner can also be used as a compost excavator, enabling inspection at any point of the vessel.
Amendment choice and physical properties. Most well-designed in-vessel compost systems add various amounts of amendment (structural and carbon source) to adjust the porosity (bulking agent for structure) or the biodegradability (carbon or energy source) of a blend of compostable materials, especially those including biosolids. Static systems, including aerated static pile, require both structural and carbon source amendments. The agitated bed with its regular turning can use any dry, usually organic, material as an amendment to blend with biosolids. Amendments have included both kiln dried and green sawdust, wood shavings, shredded green waste, leaves, ground pallets, and, on an experimental basis, paper mill residuals, fly ash, shredded dry wall and woody construction materials, shredded newsprint and magazines, and rice hulls. The regular agitation provides the option to eliminate the structural amendment, and porosity and carbon source can be furnished by an amendment such as sawdust or even paper mill residuals.
Operational and process control flexibility. The open channel, agitated system enables variation in system capacity and active composting time. Since the capacity of the system is limited volumetrically by the number of charges per day, the open channel system can be agitated more than once a day for a limited period to double the composting capacity of the facility to accommodate heavy loading such as maximum day or maximum weekly loads. Because of the relatively small mass of compost within each channel or aeration zone of the channel, automated aeration control gives operators another tool to cause a relatively large change in aeration rate and temperature change to promote or reduce drying and oxygenation of the compost.
System can be adapted readily to add new design features. The proprietary components (machinery and controls) of the various agitated bed technologies is a relatively small part of the system, allowing engineers to participate in the design in a manner in common with wastewater equipment and wastewater treatment facilities. Individual turners can be replaced with no disruption in operation. Turners currently provided have been improved significantly over time such that one machine can now replace two that were required in the early facilities.
Changes in technology have been made in a progressive manner. For example, early in the development of the agitated bed technology, drying was observed to be excessive. Beyond a week to 10 days in the channel, the compost exceeded 65 percent solids, too dry for good biological activity. Operators have adjusted by adding simple irrigation devices and engineers designed an automated compost spray system that could add moisture to any zone of any channel that was concluded to be excessively dry. Compost sampling in the open channel system and testing of the compost enabled calculation of the quantity of water to be added to maintain optimum biological activity. Water addition was controlled by either a timer or automatically through a microprocessor.
A design feature added in response to a ventilation situation is vinyl strip curtains enclosing the most odorous, humid, and hot areas above the active compost areas. This area can receive preferential ventilation rates, and draw relatively clean air from the mixing/loading end of the building at the front, and from the discharge ends of the channels. While the active compost area may receive 10 or 12 air changes per hour, the areas in which petroleum fueled vehicles operate can receive as little as six air changes per hour.
In early facilities, fog was experienced in the loading and discharge areas during cold weather. Vinyl strip curtains not only reduced movement of humid air from above the channels into this working area, but the reduced ventilation rate also made economically possible modest heating of that occupied area.
Other new features are availability of deeper and wider channels. As described earlier, not only does this increase the quantity of material that can be composted on a given building footprint, it enabled loading the charges with a front-end loader (3 to 5 cy bucket), as contrasted with a skid steer loader (1 cy bucket). Two facilities are designed so that loading of each charge is fully automated, from mixing of materials to placing the materials into the charge zone of each channel. The power of the electric-hydraulically driven compost turner has increased from 25 HP to 75 HP, and a diesel powered unit is now available to enable operation of the system on a farm. In at least two facilities, the unloading has been automated with discharge of the compost onto a walking floor or a conveyor that delivers compost to the curing/storage area. Several facilities have suspended ceilings to reduce the volume of air required to achieve good ventilation. Some of these features are the engineer’s design, some based on operator suggestions, and others by the vendor/fabricator of the system.
Process control alternatives and monitoring. A number of controlled composting experiments have been conducted in agitated bed facilities. The multiple channels in which individual charges are loaded on a daily basis have been used to test amendment blend alternatives, unknown but proposed wastes to be composted, and impact of retention time on process and product quality.
Mechanically simple agitation equipment fabricated in North America. Many of the components of the heart of the process, the compost turner, can be purchased locally or procured through parts supply houses in North America. Repair and maintenance of the mechanical components of the compost agitator and other systems are complex, but within the capability of most skilled mechanics and electricians. The relative mechanical simplicity may be a major factor contributing to continuing operational success.
THE FUTURE
The technology, mechanical equipment, process control, and compost quality are all good to excellent for all the in-vessel agitated bed systems. The capital and operating and maintenance costs will continue to exceed the cost of application of Class B biosolids to land, and the cost of landfilling Class B dewatered biosolids. However, public acceptance of applying Class B biosolids to land is not increasing, and indications seem to point to greater determination for transition to a Class A biosolids product where public contact is possible. Landfilling of dewatered Class B biosolids at some percentage of the total quantity of waste seems likely to continue, especially as landfills are tapped as a source of biogas.
In-vessel agitated bed composting will continue to exceed the cost for turned or agitated windrows outside on a packed earth surface. Where a Class A product is preferred and the cost of amendment is not excessive, this practice is cost competitive and generates a quality product.
Every agitated bed system is a little different and operated a little differently. Every operator can run it in his or her own unique way and it manages to respond. Academicians may argue there may be “better operational strategies but the features of the agitated bed technology are robust, and the facilities continue to operate.
Lewis Naylor was with Black & Veatch Corp. in Gaithersburg, Maryland at the time this article was written, and has since retired and begun working as an independent consultant. Geoffrey Kuter is with Agresource Inc. in Amesbury, Massachusetts.


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