June 21, 2007 | General

In-Vessel Composting: Rotating Drums

BioCycle June 2007, Vol. 48, No. 6, p. 28
The combination of pulping action and biological degradation can break down organic residuals in a few days to facilitate separation of inorganic contaminants. Part II
Robert L. Spencer

ROUNDING out (pun intended!) this two-part article – What’s New? In Vessel Composting – are rotating drums. Often referred to as rotary digesters, the term – rotating drum – more precisely describes the metallic barrels or drums widely used in aerobic composting. Since bacterial digestion takes place in both aerobic and anaerobic conditions, it is important to further define the following systems as aerobic rotating drums. Most drum systems include blowers to maintain aerobic conditions and minimize excessive temperatures, with one technology injecting oxygen.
Rotating drums are the most common in-vessel composting technology applied to mixed municipal solid waste in North America. The reason is that the combination of pulping action and biological degradation taking place in the drum breaks down the organic materials in just a few days to a rough compost that then can be separated from inorganic materials by a 1- to 2-inch screen opening. Even with the more gentle pulping action of a rotary drum (versus shredding of MSW), glass and plastic contaminants have plagued the compost produced at mixed waste, as well as source separated, composting facilities.
Of the six survey respondents, the largest number of applications of rotating drums is in agriculture and meat processing, with 43 Rotocom drums installed in Japan, and 15 BW Organics drums in the U.S. and other countries. Both companies report new projects with food waste applications. A-C Equipment has 17 drums in mixed waste applications. There is a trend toward smaller rotating drums compared to the much larger ones used for mixed MSW. This facilitates development of projects to process five to 20 tons/day in a unit that is four to eight feet in diameter and 16 to 50 feet long.
Table 1 summarizes details about each of the six rotating drum manufacturers included in Part II of this What’s New? In-Vessel Composting series. Part I (May 2007) covered enclosed aerated static piles and agitated beds and vessels.
One of North America’s pioneers in manufacturing rotating drums for large-scale MSW composting, A-C teamed with Bedminster Bioconversion in the early 1990s to supply equipment to the Sevierville, Tennessee, and Cobb County, Georgia facilities. Other A-C projects include Sumter County, Florida, Edmonton, Alberta, and Pinetop, Arizona. The company reports 17 of its drums currently in operation, and daily processing capacity for each ranging from 44 to 214 tons/day (tpd) assuming a 72-hour retention time.
The A-C composting drum, also called a BioMixer, is designed to accelerate breakdown of materials, primarily for separation of organics from inorganics. The drums are custom-designed based on client needs, using stock components. A new feature is heavier duty discharge doors that will function better given the physical and chemical forces present at the point of discharge. Materials drop out of the 10 to 16-foot diameter drum through one of four sliding discharge doors. The degree of opening of the doors is frequently adjusted by the operator in order to discharge at the fastest rate that does not overload the downstream trommel screen. The discharge doors are an integral part of rotating drum operations, and must be designed accordingly. A-C also reports development of a new rotary coupling to provide high and low pressure process and operating air.
Primarily installed at farms and meat processing facilities, BW Organic’s rotating drum in-vessel composter has been on the market for 15 years, with over 60 units in Louisiana, Florida, North Carolina, Texas, Kansas, Missouri, England, China, Mexico, and Puerto Rico. Daily capacity of the units ranges from one to 30 cubic yards/day. Types of materials processed include institutional food waste, slaughterhouse offal, poultry litter/mortalities, stable waste, dairy manure and hog mortalities. Temperature indicators and sample ports are provided for process monitoring.
One unique feature of the BW drums is their portability. Two of the models (408 and 616) are sold on a steel trailer frame. The Model 408 comes on a 5-foot by 7-foot frame; the one-quarter inch steel drum is 4 feet in diameter by 8-feet long. The next size is on a 6.5-foot by 15.5-foot double framed steel trailer. The company also sells stationary models that require a flat concrete floor for installation of equipment. Stationary units come with preset sloped trundle wheel stands.
While many of the examples of installations are farm-based, a BW Organics Green Drum was purchased in 2005 by Warren Wilson College in Asheville, North Carolina. The unit processes kitchen prep waste, unused prepared food and plate scrapings with woodchips, sawdust and shredded paper. After about five days in the drum, a raw compost is unloaded and moved to a curing site where it sits for about 30 to 45 days. Finished compost is used in the college’s garden, and as mulch/fertilizer by the landscaping crew.
In addition to its rotating drum, Conporec provides a MSW composting system that consists of an MSW receiving area, overhead crane/grapple for loading the drum, primary refining/separation of inorganics, post process sorting area for removal of nonorganic recyclables, biosolids feeding system, maturation/ curing of MSW compost, secondary refining of compost product, and curing/storage prior to compost sales.
The company’s most recent facility is owned by Delaware County, New York (see “Composting Mixed MSW and Biosolids To Extend Landfill Life,” November 2006), with one other North American plant in Sorel-Tracy, Quebec, and several more in Europe. Other types of wastes processed include liquids and dairy manure. Units can be sized between 80 and 125 tpd; a drum with 100-tpd capacity is 14 feet in diameter and 157 feet long.
Conporec teamed with Stearns & Wheler, LLC as a development partner for the Delaware County facility. Conporec has been marketing its system for 13 years, and is promoting a “stronger, more durable rotating drum.” The facility in Delaware County also uses the Siemens/IPS agitated bed technology, receiving material after it is processed through the drum.
The company’s Waste Wizard Aerobic Bioreactor differs from other drums on the market because of its series of injectors that deliver 95 percent pure oxygen into the vessel from an on-site oxygen generator. Three agricultural facilities utilizing the EPTC rotating drum include a 2,000 head dairy farm, a 1,000 head dairy farm, and a 3,300 head hog farm. Processing capacity of the drums ranges form 4.5 to 24 cubic yards/day. A continuous loading/unloading process cycles every six minutes. The company reports that the oxygen-rich environment helps minimize corrosion inside the drum. A new capability is remote monitoring of the drum, which includes failure alarms, and data collection to document the process.
An early version of the EPTC system was installed in 1998 at the Utah State University dairy, and operated for about five years. The installations at the three farms and two other research projects have led to development of a new “closed loop” system where all manure and flush water is captured. A portion of the waste is anaerobically digested to generate power required by the dairy, with the surplus sold back to the utility. The hot water from the engine water jacket provides energy to operate an adsorption chiller to cool the milk. Manure solids are composted and a portion is used as bedding for the cows, and the balance for soil enhancement. Water is recycled for reuse in the manure flush system. The recycling of water eliminates the need for a manure holding lagoon. At the North Carolina hog farm, the recycled water is fed back to the hogs.
Over the past 30 years, the Rotocom plug flow rotating drum composting system has been employed at 43 sites in Japan, primarily for processing animal wastes, including sludges, manures, and mortalities. NexGen, a subsidiary of ANDAR Holdings Ltd., has an exclusive license to manufacture and sell the technology outside of Japan. “We entered the United Kingdom market with our first large-scale facility incorporating the RC159 unit, waste storage systems, mixers, and conveyors,” notes Steve Kroening of NexGen. “The system was installed for Wyvern Waste Services in 2005 to process Category 3 animal by-products.”
In the Japanese plants, animal by-products are typically composted with sawdust and/or recycled material as bulking agent. The drum usually is fed using a bucket loader, and bulking agents using a conveyor although automated loading systems that meter materials from storage bins are also available. Discharge occurs whenever the drum is rotating; a conveyor running perpendicular to the drums collects discharged material and moves it to a point where it can be collected and formed into windrows. Machines are fed during the day, with a typical drum rotational speed of four revolutions/hour during the day and two revolutions/hour at night. Typical retention time is five days, with a range of three to 20 days. Depending on moisture content and bulk densities, meat by-products can be composted with green waste on a 1:1 basis. Airflow is counter-current to the direction of material in the drum, and is ducted to a biofilter and/or scrubber for treatment.
The vessel is manufactured from corrosion resistant stainless steel, as are the end. Six sizes of Rotocom units are available; the smallest is1.6 meters in diameter by 6 meters long, with total capacity of 12 m3 at 70 percent of total volume (30 percent of the space is recommended to be left open to allow mixing and aeration). The largest unit is 3 meters diameter by 22.5 meters long, with total capacity of 159 m3. The “plug and play” RC42 unit has the footprint of a standard 40-foot shipping container and can be transported.
Following a career in the farm equipment manufacturing industry, Glenn McConkey, President of X-ACT, set out to design and build a rotating drum to process agricultural residuals, food waste and municipal biosolids. He toured numerous facilities in North America, and teamed up with Norlen Systems Inc. to manufacture the rotary units. Following several years of pilot trials, the first full-scale unit was installed at Misty Hills farm in Troy, New York in the fall of 2006. The farm raises and trains horses, and was looking for an environmentally responsible means to manage the horse manure and sawdust bedding. Owner Herb Headwell also was looking for a way to recycle the manure for use on the farms hay fields, and decided on a 10-foot diameter by 32 feet long rotary drum.
The company is also marketing the rotating drums to the dairy industry, and has demonstrated it on a number of farms. For those applications, a water separator takes moisture out of the manure, with a target of 65 percent moisture; a conveyor moves the manure/ bedding mix from a feed hopper into the compost vessel. Typical retention time is five to seven days, with pathogen reduction temperatures achieved. The resulting compost can then be used as bedding for the cows.
McConkey is looking to new and existing solid waste composting facilities that want an in-vessel system, particularly for food waste. “After an initial shredding step, our drums can handle a mix of waxed cardboard and food waste delivered in 30 or 40 cubic yard compactors,” he says. “For programs collecting just food waste in Toters, we will add yard waste or other materials as a bulking agent. We also have been selected for a biosolids composting facility, and are talking to a number of meat processors as well.” A pilot was conducted at a turkey farm, recycling sludge and mortalities in the vessel.
The drums are manufactured with three-eighth inch thick rolled carbon steel in 8-foot long coupled sections, which enables capacity to be added to an existing unit in 16-foot increments. Support wheels are placed every 16 feet. Diameter of the digesters ranges from 4 to 12 feet, with lengths up to 176 feet. The interior of the vessel is lined with half-inch wear bars welded 6-inches apart. For the Misty Hills Farm installation, a 10 HP motor turns the 10-foot diameter by 32-foot long drum up to 10 revolutions per hour.
Robert Spencer is a Contributing Editor to BioCycle. An environmental planning consultant, he is based in Vernon, Vermont.
A-C Equipment Services
6737 W. Washington St.
Ste. 1400
Milwaukee, WI 53214
BW Organics, Inc.
475 County Rd. 2300
Sulphur Springs, TX 75482
Conporec, Inc.
3125 Joseph-Simard,
Sorel-Tracy, QC, Canada, J3P 5N3
Environmental Products & Technologies Corp. (EPTC)
2219 E. Thousand Oaks Blvd., Ste. 373
Thousand Oaks, CA 91362
NexGen Composting Ltd.
P.O. Box 435
Timaru 7910, New Zealand
X-ACT Systems, Inc.
Box 430, 340 Sidney St.
Trenton, ON, Canada, K8V SR6

Sign up