BioCycle November 2005, Vol. 46, No. 11, p. 19
Solid Waste Composting Trends — BIOCYCLE PROJECT UPDATE
Operating projects fall into two distinct categories – those composting a mixed waste stream and another group composting source separated household organics. This first installment provides updates on the 16 mixed MSW composting facilities in
Part I — Nora Goldstein
MUNICIPAL solid waste composting has taken a number of interesting twists and turns over the past 20 years in the United States. Early projects had sophisticated preprocessing lines with an assortment of screens, ferrous removal, picking lines and shredders. Most received mixed municipal solid waste, with vendors and project proponents advocating composting as a viable alternative to landfill disposal (which in the late 1980s and early 1990s was perceived to be a threatened MSW management option). Along the way, a handful of projects started up that were based on residential separation of the compostable stream from recyclables and trash, a model that today is quite widespread in Europe and increasingly, Canada.
The challenge of producing quality compost from mixed waste was blamed in part on shredding in the preprocessing phase. Therefore the next generation of plants began installing rotary drums ahead of composting. The tumbling action of the drum – versus mechanical shredding – was adequate to open bags of garbage, mix wet and dry feedstocks and initiate the composting process. Over time, many composting facilities processing a mixed waste stream, e.g., Rapid City, South Dakota, Pinetop-Lakeside, Arizona and Marlborough, Massachusetts, opted for rotary drums on the front end.
Today, a solid 20 years into this generation of municipal solid waste composting, some trends have become apparent. One is that mixed waste composting seems to have a specific niche, servicing rural areas and/or tourist destinations where the existing landfills have limited capacity and siting a new landfill isn’t environmentally or economically feasible. Recent examples of this situation are West Yellowstone, Montana, and Mariposa County, California. In both cases, nearby national parks result in seasonal peaks in the waste flows and siting new landfills would not be possible.
Another, which is still evolving, is a trend – typically in cities and suburban communities – to add source separated residential organics to existing yard trimmings collection programs. Composting facilities permitted to receive food residuals, e.g., from commercial organics generators, have the capability to handle the residential organics stream. This makes it feasible for municipalities to “upgrade” their existing organics collection program, taking advantage of already-distributed containers, established truck routes, automated cart collection, and in some cases, split body vehicles (enabling cocollection of recyclables and/or trash on alternating weeks with the organics stream).
A third appears to be improved quality of compost from mixed waste composting facilities.This is attributed to several factors, including a much better understanding of how to manage the composting process to optimize separation of contaminants from compost in the refinement step; and different configurations of screens, destoners and air separators to maximize contaminant removal.
What became clear since the last time BioCycle conducted a survey of municipal solid waste composting plants (January 2003) is that our definition of “what counts” as MSW composting may be a bit antiquated. Traditionally, we defined MSW composting as including residential organics in addition to yard trimmings. By that definition, completely source separated programs, where households are allowed to set out food scraps and soiled paper with yard trimmings (or without during certain seasons or collection configurations), fall under the same umbrella as projects that essentially start with the whole mixed bag. Brian Mathews, Senior Program Manager with the Alameda County (California) Waste Management Authority, has been overseeing the rollout of source separated organics collection and composting for several years. To date, 13 of the 17 jurisdictions that make up the Authority have source separated collection of food and soiled paper.
When I emailed Mathews to get the current numbers on the roll-out, I wrote that “to qualify for BioCycle’s list of MSW composting facilities, a project has to be diverting residential organics beyond yard trimmings. In the old days, this pretty much only involved mixed waste composting, but slowly municipalities are initiating programs to divert other household organics.”
Mathews response led us to some rethinking about the definition we created many years ago for MSW composting. He wrote back: “This may be semantics, but we would take exception to the label of ‘MSW’ composting. MSW is mixed (municipal) solid waste. The organics that are being diverted in Alameda County are source separated at the point of generation. So, on a purely technical basis, no Alameda County jurisdiction is MSW composting. However, almost all, with the exception of a few, are diverting residential food scraps, which is a portion of what is composted at an ‘MSW’ composting facility.”
Across the country, in Seville, Ohio, the Medina County Central Processing Facility receives 550 tons/day of MSW. In a recent article in the Medina County Community Advocate, William Strazinsky, the District Solid Waste Coordinator, explains that the job at the 12-year old plant (known in the industry as a “dirty MRF”) is to “process as much waste as we can and extract recyclables from the trash. Everybody recycles in Medina County whether they want to or not because their waste comes here and we pull the recyclables out. … If any new material becomes recyclable in the future, we don’t have to educate the public about recycling this new material, we just have to add a couple people to our sorting or picking rooms and we just tell them to start pulling this new material out of the trash.” The Medina plant includes a composting line, which is described below.
The difference between these two diversion approaches is stark. One maximizes household involvement, while the other minimizes it. After making calls around to update the operating “MSW” composting projects in the U.S., I think we can stick with the “household organics beyond trimmings” categorization. And while it may just be semantics, most communities with a source separation-based approach refer to the waste stream diverted as “residential SSO.” Conversely, those processing a mixed waste stream tend to categorize their operations as “MSW composting.” Semantics or not, BioCycle’s 2005 report of MSW composting projects illustrates the various directions being taken by municipalities and private companies, all of whom are committed to diversion over disposal. Part I focuses on mixed MSW composting plants; a total of 16 operating facilities were identified in this survey (see Table 1). Part II, to run in the December issue, covers residential source separated organics programs.
Pinetop-Lakeside, Arizona: Start-up of the cocomposting plant in Pinetop, Arizona was driven more by the need to find a biosolids management solution than a disposal option for MSW. “Our management challenge in the late 1980s arose when closing landfills presented our District with a biosolids, disposal dilemma,” recalls Phil Hayes with the Pinetop-Lakeside Sanitary District. “Hauling dewatered biosolids three times a day more than 80 miles was not financially viable, so we explored the possibility of in-vessel composting. Utilizing MSW to supply the carbon side of the composting equation is what led us to start our original plant.” An article in the August 2004 issue of BioCycle (“Expanding The Potential of In-Vessel Composting”) documents the evolution of the Pinetop-Lakeside plant, which started out with a used Eweson digester that was employed in chicken manure composting research in Arkansas. Initially, ten tons/day of mixed MSW were composted with five wet tons/day of sludge. In 2003, the District decided to expand the facility, and purchased a new digester from A-C Equipment Services in Milwaukee, Wisconsin. The digester is 125-feet long and 10-feet in diameter, and is designed to process 40 tons/day of feedstocks. A new metal building was constructed to house the digester. It was added to an existing building that houses the aerated static piles used for composting materials coming out of the digester.
“We are loading about 25 tons/day of MSW and 12 to 16 wet tons/day of biosolids into the digester,” says Hayes. “We have a new cinder company purchasing our compost for $8/ton. They produce a nice topsoil for sale in the local area. Since last year’s article, we haven’t made any major changes in our facility.”
Mariposa County, California: Mariposa County, at the gateway to Yosemite National Park, is getting ready to start its 50 tons/day mixed waste composting facility. A request for proposals recently was issued for an operator. The SV Composter stationary vessels, supplied by Engineered Compost Systems (ECS), make up the heart of the composting operation, which is located at the county landfill. The front-end preprocessing line was supplied by Bulk Handling Systems (BHS). “There isn’t any curbside recycling in the county, although there are some dropoff sites,” says Steve Diddy of ECS. “We teamed up with BHS to maximize the removal of recyclables and contaminants before the waste stream gets to the compost vessels.”
MSW is unloaded on the tipping floor in the preprocessing area, and sorted to remove oversized items. It then is loaded onto an incline conveyor, which passes through a manual picking station to remove materials inappropriate for composting. “Bags of garbage pass right through this station,” explains Diddy, “and go to a dedicated BHS bag breaker. The machine has different diameter drums that spin at various speeds. The drums have hooks in them, which the plastic bags get caught on. Essentially, the hooks grab the bags and elongate them as the drums spin, spilling out the contents.” Once or twice a day the operator performs a simple operation to remove the plastic “husk” that has built up on the drums. A second picking station removes bags that weren’t entrained by the hooks. The last piece of equipment on this line is a debris roll screen, which is similar to a disk screen. Fines that pass through the screen are conveyed to a vertical auger mixer located with the vessels in the enclosed composting area. Overs, which are expected to be fiber-rich, will be conveyed to an optional sorting station where additional recyclables can be removed. The process and room exhaust is humidified then scrubbed in a biofilter.
The mixer has knives on the auger, which will reduce the particle size somewhat without any shredding or grinding, adds Diddy. Reclaimed water will be added to adjust the moisture content prior to loading material into the vessels. The vessel aeration system is designed to minimize evaporative drying and fan power requirements, while maintaining desired oxygen and temperature levels. There is a 10-day retention time in the vessels, followed by remixing and moisture addition if necessary, and then another 10- days in the vessels. These retention times may be adjusted based on the feedstock mix going in. After the second period, the vessels are unloaded and material is put into aerated static piles. Initially, the finished compost will be screened with a trommel and used as alternative daily landfill cover. “Dirt is at a premium and so is the remaining landfill space, which were the drivers behind composting in Mariposa County,” Diddy notes.
Sumter County, Florida: The Sumter County composting plant is among the oldest mixed waste composting facilities operating in the U.S. Started in 1988, it was designed to process 75 tons/day of MSW and 25 wet tons/day of biosolids. The plant was designed with a sorting line to pick out recyclables, followed by composting in windrows. A Bedminster rotary drum digester was installed in the mid-1990s to facilitate mixing of biosolids and MSW, and to help remove contaminants ahead of composting. As a result, throughput capacity increased to 175 tons/day of MSW and 75 to 80 wet tons/day of biosolids. By this time, the materials recovery area was totally enclosed, with sorters on an elevated picking line ahead of the digester. Depending on markets for various paper streams, fibers were either picked out or left on the line to be composted. A Scarab turner was used to turn the windrows. Currently, the Bedminster digester is in need of repairs, and the county has not allocated the funds at this time. The digester is now permitted for about 90 tons/day of feedstock, including 50 tons/day of MSW, 25 tons/day of biosolids, and 13 tons/day of fresh compost, report Miriam Zimms of Kessler Consulting, a contractor on the Sumter County project.
In 2000, Sumter County was selected to house a research initiative called FORCE – the Florida Organics Recycling Center for Excellence. In addition to a 40-acre research farm on the Sumter County site, an in-vessel digester was installed last year (manufactured by A-C Equipment) specifically for FORCE-related activities. This includes testing a range of feedstocks and blends of finished compost. The FORCE digester is permitted for 121 tons/day, including 69 tons of MSW, 34.5 tons of biosolids and 17.5 tons of fresh compost, adds Zimms. (See “Research Projects Add Value to MSW Composting Facility,” August 2005, for background information on the FORCE facility in Sumter County.)
Cobb County, Georgia: Earlier this year (July 2005), BioCycle reported that the mixed waste composting facility in Cobb County, located in the Atlanta metropolitan region, was possibly slated to shut down, based on assessments of some of the county’s commissioners. Reasons cited for the possible closure were deterioration of the compost building, lack of markets for the compost and high operating costs. When we spoke recently with Joseph Accortt, Division Manager of the county’s solid waste agency, he reported that the composting plant is “still cranking away.” He added that in September, at the request of the county chairman, the facility was put up for bids. “There were two responses, one from a gasification company and the other from Bedminster, which originally supplied equipment and operated the plant,” says Accortt. The facility uses the Bedminster rotating drums, followed by composting in aerated windrows.
The plant had been processing 300 tons/day of MSW; about a year ago, the flow was reduced to 180 to 200 tons/day. “It was a cost-cutting measure,” he notes. “At 300-plus tons a day, we need to run two shifts. At 200 tons/day, it only takes one shift.” Finished compost is screened to one-eighth inch. Various Cobb County departments, such as parks, the Department of Transportation, solid waste and water, use much of the compost produced. Other users include developers and golf courses. There also is a giveaway program to residents.
Maintenance of the drums is a challenge, notes Accortt. The county has tried to alter the initial mix to reduce the acidity and thus corrosion of the interior of the drums. It also is testing various hard surface materials, such as coatings and a cement product. “Since we are not operating at capacity, we rotate the drums we use,” he says. “Right now, it takes three of the five drums installed to process the 200 tons/day.”
Buena Vista County, Iowa: Buena Vista County’s MSW composting plant, located in Storm Lake, processes about 14 to 16 tons/day of mixed MSW. Recyclables are picked from the incoming loads; remaining material is shredded and composted in windrows. Yard trimmings from nearby Cherokee County are composted as well. In return, residuals from the composting operation, as well as the fraction of MSW that isn’t recyclable or compostable, are landfilled in Cherokee County. Finished compost is used as landfill cover.
Marlborough, Massachusetts: In 1995, the city of Marlborough awarded a 20-year contract to Bedminster Bioconversion Corp. to permit, finance, build and operate a composting facility. The plant was designed to process 120 tons/day of MSW from the city, along with 60 wet tons/day of biosolids. The facility started operating in 1999; it has two Eweson rotary drums that feed into a hall where material is composted in aerated windrows. In 2002, Bedminster ran into financial challenges and began seeking a buyer. In the interim, the company asked permission from the city of Marlborough to operate as a transfer station, in order to fulfill its contractual obligations. In March 2003, WeCare Environmental LLC purchased the assets and made significant repairs. In July 2004, the composting facility started up again, with a plan to increase the flow over three phases. (See “An MSW Composting Journey,” August 2004 for the complete history of the Marlborough cocomposting plant.)
“We are currently in the second phase, with a feedstock flow of 75 to 80 tons/day, or about 1,500 tons/month,” says Chris Ravenscroft of WeCare. “That is about 50 tons/day of MSW and 30 wet tons/day of biosolids.” About 3,000 tons/year of leaves and yard trimmings from Marlborough are composted at the plant as well. In addition, WeCare is receiving source separated commercial organics from area grocery stores. (The annual permitted capacity is about 55,000 tons/year.) MSW and biosolids from Marlborough in excess of the 75 tons/day are transferred out of the facility. The WeCare companies include a hauling business as well as biosolids processing operations, both of which help to control the costs of waste transfer and residuals management. “As an integrated company, we can internalize some of those costs, which makes better economic sense,” adds Ravenscroft.
After three days in the drum, material is screened through a two-inch trommel. By volume, contamination is mostly plastic bags. WeCare has been working with Marlborough to improve the quality of what is being delivered to the plant. This includes increasing participation in the city’s curbside recycling program. “We’ve determined that every one percent increase in the city’s recycling rate saves them $15,000 per year in tip fees at the composting plant,” says Robert Spencer, compliance manager for WeCare.
Screened material from the drum is composted in an aeration hall. Last year, with a grant from the Massachusetts Department of Environmental Protection, WeCare purchased an Allu bucket to turn the compost. It also can be used to make topsoil blends. Compost is screened in a Wildcat trommel to one-quarter inch. It is wholesaled for $8/cy picked up at the plant, or $20/cy retail. WeCare has capacity at a facility in New York State to do additional curing and/or storage of compost as needed.
Nantucket, Massachusetts: The Island of Nantucket, Massachusetts has a mixed waste/biosolids cocomposting facility that is operated by Waste Options, Inc., based in Providence, Rhode Island. The site footprint includes a materials recovery facility. A rotary drum is used for mixing feedstocks, followed by aerated composting inside a Coverall building. The population on the island grows from 10,000 year-round to 60,000 during the summer months. The waste flow increases accordingly – from a low of 30 tons/day off-season to a high of 125 tons/day during peak season (tonnages include biosolids).
To build compost sales, Waste Options hired Ron Alexander of R. Alexander Associates, a marketing consultant. “Our work with Ron wasn’t so much to look at the mechanical side of our product refinement system but to help us understand our product and potential markets better,” says Whitney Hall of Waste Options. “We also learned how to communicate better with potential clients, to learn what their needs are and how to meet them. We hosted workshops with product end users to facilitate the learning and outreach process.”
Compost is sold directly from the plant and through distributors. The primary end uses are planting beds, topdressing and tilling compost into soils (essentially a manufactured soil). Compost sells for $15 to $35/cu yd.
Dodge County, Minnesota: The Dodge County Transfer and MSW Compost Facility in Mantorville, Minnesota began composting at its 50 tons/day solid waste transfer facility in 2004. The facility has six 40-yard Nature Tech in-vessel compost digesters, with a permitted capacity of 3,000 tons/year. The county is still refining its composting process, as well as what waste streams they will process. The main purpose of the operation is to compost the MSW residue left behind by trucks which carry waste to the waste-to-energy plant servicing both Dodge and Olmsted Counties. The leftover material is contaminated with broken glass and grit.
The county charges a lower tipping fee for organics ($45/ ton) compared to the county MSW transfer station tip fee ($70 a ton), which serves as an incentive for commercial haulers. A MRF at the transfer station sorts recyclables in two streams: bottles and cans and mixed paper. To market recyclables, Dodge County participates in a 10-county marketing cooperative, the Southeastern Minnesota Recyclers Exchange.
Once loaded, the organic fraction of the MSW stream stays in the digester for 21 days, with either positive or negative aeration, and is heated to at least 155°F for seven days to meet pathogen reduction standards. After 21 days, materials are unloaded from the vessels and placed in windrows 16 feet wide and about 10-feet tall, for three to six months of curing. Piles are turned about once a week, using a CAT front-end loader. After curing, the compost is screened by a private contractor. Compost still contaminated with broken glass, bottle caps and other small debris is used on-site as landfill cover. Clean compost is sold to area landscapers and farmers for $8 to $10 per yard. “Since we just started composting in 2004, 2005 has really been a stockpiling year,” says Mark Gamm, the county’s Environmental Quality Director. “We haven’t started moving a lot of material, yet. Much is still in the curing stage.”
Handling a variety of feedstocks, the facility staff is still working on “trying to get the right moisture and bulk” recipe, Gamm notes. For example, “we had sugar beets we thought would be more bioreactive; we had to add nitrogen to get the temperature up to the right level. We’re still playing with the recipe.”
To gradually increase the scale of its composting operation, the county is seeking “a set of customers who can provide regular feedstocks” – such as grocery stores, food processing plants, schools and public facilities.
Truman, Minnesota: The Prairieland Solid Waste Composting facility, in the southern Minnesota town of Truman, processes about 65 tons/day of mixed solid waste from the counties of Martin and Faribault, or about 15,500 tons/year. The approximately 25-acre facility opened in 1991. After sorting to remove recyclable materials (an electromagnet removes steel), the waste is ground and screened using a Heil 42-F hammermill and trommel screen.
The composting facility consists of 10 horizontal cement silos, which can each hold about 120 tons of material. Turning is done by a paddle wheel manufactured by OTVD, a French company. The material is cured in the forced-air aeration system for 28 days. “We measure daily temperature and oxygen levels, and add water as needed,” says Mark Bauman, director of the Prairieland Solid Waste Management Board. In the finishing building, a Bivitech vibratory deck screen does the final screening. The material is then cured for another 30 days, being turned periodically by a front-end loader. To control odors, the facility uses two water/chemical scrubbers made by Met Pro Duall.
Learning to develop a quality compost product involved a trial-and-error process in the early days of the operation, Bauman says. “We had some quality problems in the first few years, but now we have a really good product. Our compost meets the state’s Class I standards in all ways, except for lead, which therefore categorizes it as a Class II product. The limit for lead is 300 parts per million; we run 270 to 360 parts per million, so we’re right on the edge.”
Currently Prairieland sells about 2,500 tons of compost per year. Sales are gradually increasing, “but it’s a slow process,” Bauman notes. Compost is sold in bulk for around $3/ton to landscapers and a number of local farmers who use it for crop production. A local contractor does field application for farmers in the area. Bauman has been working to line up more bulk compost customers. “We’d like to find a way to work it into highway reconstruction projects.”
West Yellowstone, Montana: The West Yellowstone mixed waste composting plant services Yellowstone National Park as well as surrounding jurisdictions. Owned and operated by the West Yellowstone/Hebgen Basin Solid Waste District, the facility is designed to process 50 tons/day of mixed MSW along with several other organic streams. A detailed article on the facility ran in the July 2005 issue of BioCycle (see “Composting Finds Its Niche In Yellowstone National Park”). Operations began in July 2003.
The facility uses composting vessels supplied by Engineered Compost Systems. The preprocessing line includes a Seerdrum Waste Processing drum. Screened unders are mixed with woodchips, manure, biosolids from Yellowstone National Park and seasonal yard trimmings in a Luck/Now mixer. Vessels are loaded via conveyor. After a minimum of 14 days, the partially stabilized compost is removed from the vessels and placed in aerated static piles. Finished compost is refined through a Bivitec screen and a Forsberg destoner. End users include the national park, area hotels and developers and local gardeners.
West Wendover, Nevada: This month’s Compost Operators Forum features the mixed waste composting facility started by the small city of West Wendover. With flow coming mostly from casinos and hotels, and a choice between building a Class I landfill or some other MSW management alternative, cocomposting was selected. The enclosed facility houses two small rotating drums, biosolids dewatering beds, and an aerated composting floor. Overall MSW generation is 35 tons/day. The composting facility is sized to handle 10 tons/day of MSW, segregated food residuals and biosolids. For more details, see the Operators Forum on page 40.
Delaware County, New York: After many years of planning, design and construction, the MSW/biosolids cocomposting facility in Delaware County is ready to start its engines. A dedication was held in mid-September. The plant has been going through dry runs, equipment testing and operator training, and is expected to become fully operational in early 2006. “Since the early 1970s, Delaware County has had a policy commitment to managing all of the solid waste generated within the county in the county,” says Susan McIntyre, Solid Waste Director. One reason is that the landfill space the county has had since the 1970s is “extremely limited,” only 45 acres, she adds. “Current population is not that big – 48,000 – but we don’t have much landfill room left and can’t expand at that property. We felt that the capital cost for developing a new facility and landfill operation would be greater than the long-term cost of operating a composting facility. And, doing in-house composting and landfilling would be preferable to exporting waste, being able to manage long-term costs and liability.”
An additional incentive was the need to find a better solution to biosolids management. “Our waste stream has an extremely high percentage of biosolids, because we are located in the New York City watershed,” says McIntyre. “So we have a tremendous number of wastewater treatment plants that are very high-efficiency – the amount of solids they are removing is higher than the typical plant. Land application of biosolids is not an option, so we have historically landfilled them. The other matter is capturing the organics portion of the MSW as a bulking agent, with the biosolids. The type of facility we chose is designed for centralized separation of organic from inorganic material. So we use MSW as a base feedstock in our compost facility.”
The plant is designed to process 35,000 tons/year of MSW and 6,500 wet tons/year of biosolids. It is using the Conporec sorting system and bioreactor, followed by composting in an agitated bay IPS Composting System from Siemens Water Technologies. “MSW is fed into the bioreactor and biosolids get fed directly into the IPS maturation system,” says Jeff Heath of S&W Services, which formed a joint venture with Conporec to design and build the facility. The tip floor receiving pit has a capacity of 300 tons/day; there also is a 20,000 gallon biosolids storage tank. In addition, whey is taken from a local company that makes soy products for the food industry.
An overhead crane and grapple is used to remove bulky items and materials known to damage the processing equipment. “No one is on the tip floor where the waste receiving pit is located,” says Heath. “The operator of the crane and grapple is located in the control room. We don’t like to have employees dealing with raw garbage.”
A more detailed article on the Delaware County plant will appear in the next issue of BioCycle. In brief, the processing train is as follows: MSW is fed into the bioreactor; water is added as needed to ensure an initial moisture content of 53 percent. After three days in the bioreactor, material passes through a trommel with one-inch holes. Overs are conveyed to a sorting station where the county has the option of capturing recyclables. The one-inch minus is conveyed to a tripper car, and then the operator in the control room determines which bay to load. Biosolids are loaded on the conveyor that feeds the tripper car, and both feedstocks are mixed by the agitator. The IPS installation has 14 bays. The maturation phase is designed for 56 days, longer than typical for most in-vessel composting facilities. (Heath notes with a carbon-rich feedstock, the additional time lends itself to fully maturing the compost.) At the discharge end of the bays, the compost at about 35 percent moisture unloads onto a conveyor belt, which feeds a high speed incline conveyor and a magnet drum roller for removing small pieces of ferrous metal. The incline causes heavy pieces of glass, bottle caps, rocks, etc., to fall backwards, while the compost adheres to the belt. The compost then passes through a trommel with one-quarter-inch holes. The fines run through a pulverizer to crush any remaining pieces of glass.
The finished product will be stored in windrows. The entire composting facility is under one-roof, although it is subdivided into five different “buildings” that house the various pieces of the processing train. Air from the various operations is treated through a biofilter. The highly-automated facility will require “just a handful” of employees to run it, including two maintenance personnel, six operators and one plant manager.
Medina County, Ohio: The Medina County Central Processing Facility mentioned in the beginning of the article is operated under contract by Norton Environmental. After the first sorting phase for traditional recyclables such as newspaper, plastic, and cans, a system of conveyor belts feeds a trommel screen with 2-inch holes. The 2-inch minus fraction – about 45 tons of the total mixed MSW coming into the plant every day, is mixed with yard trimmings and wood and composted in windrows. The compost is used as daily cover at the landfill replacing the 6-inch layer of clay that would have to be used. “The Ohio EPA has approved that landfills can use the compost produced in our Class 1 facility as an alternate to the daily landfill cover,” says William Stravinsky, the county’s solid waste director.
Rapid City, South Dakota: Various elements of Rapid City’s integrated waste management facility – comprised of an MSW/biosolids cocomposting plant, materials recovery facility, an outdoor yard trimmings composting operation and landfill – have opened in phases. The MRF started operating in 1996. In 1997, the city installed two rotating Dano drums that had been used previously at a privately owned and operated mixed waste composting plant in Portland, Oregon. Until the composting portion of the facility was constructed and put into operation, the drums were used for volume reduction prior to landfilling. An IPS-Siemens agitated bay composting system was installed in a building adjacent to the MRF in 2002, with full-scale operations for the MSW fraction getting underway in 2003. There is a sorting line ahead of the drums, where noncompostables are picked off the line. Addition of biosolids to the drums began in 2004. Over 200 tons/day of MSW and biosolids are being processed.
After the drums, materials are screened and the unders are conveyed to the building housing the 9-bay IPS Composting System from Siemens Water Technologies. After about 30 days in the bays, compost is unloaded and moved to a curing building with an aerated floor. After a minimum of one month, the material is processed through a product refinement system that includes a Bivitec screen and a Triple S destoner. One of the markets Rapid City has been developing recently is use of its compost for erosion and sediment control and storm water management. A complete article on Rapid City’s integrated waste management facility appeared in the November 2003 issue of BioCycle (see “Rapid City Closes The Loop on MSW Management”). A more recent article (September 2005) provided updated information on the city’s yard trimmings composting operation (see “Latest Trends In Yard Trimmings Composting”).
Sevierville, Tennessee: Sevierville Solid Waste, Inc. (SSWI) operates a mixed MSW/biosolids cocomposting plant using the rotating drums followed by aerated static piles. Since the cocomposting plant was built in 1992, SSWI has been trying to get approval for the residual stream to be disposed in a Class 3/4 landfill (which covers construction and demolition debris, land clearing debris and animal waste). Instead, the residual stream had to be disposed in a Class I landfill, which SSWI owns and operates. “The last time we added landfill cells it cost $327,000/acre to build,” says Tom Leonard of SSWI. “The C&D landfill we constructed was about $50,000 for 14 acres. The reason we needed the Class I landfill was because it was where we were required to put our residuals. We needed another digester because there is a lot more waste being generated, but with the prospect of having to expand the Class I landfill, that wasn’t financially viable.”
In September 2005, SSWI received a permit amendment that allows them to use its Class 4 C&D landfill for residual disposal. “We spent the last five years going through testing procedures and doing analyses on the residuals,” notes Leonard. “In Tennessee, the definition of construction waste is anything involving construction or demolition of a building, which includes glass, metal and dry wall or any materials showing similar characteristics. Our residuals stream is primarily inerts, plastics and metals. The state was most concerned with the plastic. We tested a drying process that gets 98 percent of the compost off the plastic. Our permit amendment was to dry the plastics and shake them out so no compost is on them.”
The drying process entails a series of fans that blow in and on the primary trommel where material coming out of the rotating drums is being screened. The fans blow down as the material enters the trommel and while it is in the screen. “The primary trommel is 60-feet by 12-feet in diameter,” says Leonard. “We can get a lot of air in and around it.”
When the facility was built, it installed three used cement kilns, each 12-feet in diameter and 185-feet long. Originally, the plant was designed with capacity of 150 tons/day. It added a fourth drum in 1996, but the waste flow kept increasing. (SSWI services the tourist area around Gatlinburg, Tennessee, which is an entrance to the Great Smoky Mountains National Park.) With the recent amendment to its landfill permit, the Authority ordered two new digesters – one to replace one of the original drums, and a fifth digester to expand the plant’s processing capacity. The drums are supplied by A-C Equipment Services. It is expected the installation will be complete by the end of the year. Throughput is 240 tons/day of MSW and 50 wet tons/day of biosolids.
Capital costs for the digesters are about $2.5 million each, which includes the drums, concrete piers, rams, etc. Another recent upgrade was the purchase of a Backhus 1655 windrow turner to accelerate decomposition and improve pile aeration. “We decided to get away from using only forced aeration of the piles, and doing aeration via turning,” says Leonard. “Our building was designed to handle 150 tons/day of MSW and now we are trying to do 240 tons/day. This machine is capable of turning all the piles on the floor in one day. With a loader, it would take two weeks just to turn one side of the floor. Fortunately, we still have the aerated floor, and may end up using a combination of turning and forced air.” Finished compost is screened to one-quarter-inch minus. Most of the material is given to area farmers, although a soil blender has an arrangement to take material as needed.
Columbia County, Wisconsin: Columbia County was one of the first composting facilities to utilize rotating drums for initial materials processing. The facility is among the longest running, having started in 1992. It has two drums, each with 40 tons/day of capacity. “Our tonnage runs in the mid to high 70 tons/day of MSW,” says Bill Casey, who oversees the Columbia County Recycling and Waste Processing Facility. MSW is loaded into the drums five yards at a time via a hydraulic ram. Water is pumped in every fifth stroke. Spikes built into the drums tear open the bags of garbage. The drums are built at a three percent slope; retention time is five days. Material exiting the drums first passes through a screen with 2-inch holes, followed by a second screen with three-quarter-inch openings.
The screen unders are transferred to a building and windrowed. Piles are turned several times a week over the course of an eight week period. “The MSW compost is distributed to local farms at no cost (currently) to them,” adds Casey. “We also compost yard waste here – separate from the MSW – which we sell for $6/cy.”
BioCycle freelance writer Dan Emerson contributed to this article.
November 25, 2005 | General
MIXED MSW COMPOSTING FACILITIES IN THE U.S.
BioCycle November 2005, Vol. 46, No. 11, p. 19