Mixed Waste Composting Trends

BioCycle November 2008, Vol. 49, No. 11, p. 21

There are 13 facilities processing mixed municipal solid waste in the U.S. Eight use rotary drums for preprocessing, in some cases adding biosolids.

Robert Spencer and Rhodes Yepsen

IT'S been about 25 years since vendors started marketing systems to process mixed municipal solid waste via composting in the U.S. Today, there are 13 plants, most fulfilling their niche in the communities they serve - whether managing MSW in high tourism areas, or processing residual organics from materials recovery facilities.

Table 1 provides summary data on the 13 facilities. While we did not learn about any new plants on the drawing board during the survey process, there is still interest in the mixed waste composting approach, notes Jeff Heath of Stearns & Wheler, an environmental engineering and consulting company, which has a few prospective projects. “I think that the market for MSW composting continues to be driven solely by economics,” says Heath. “Where disposal costs are in excess of $65/ton it is a competitive option.”

Although some mixed MSW composting facilities are struggling financially, Jerry Wright, Rapid City, South Dakota's Superintendent of Solid Waste, says the city's solid waste operations are self-sustaining, based on a combination of tipping fees for MSW and biosolids, residential garbage fees, and sale of recyclables and compost. The facility's tipping fee was established by a rate study, and is currently $56.18/ton. This generates approximately $3,568,000 plus another $400,000/year from sale of recyclable materials, for total annual revenues of about $4 million - more than covering the total operating budget for 2008 of $3,639,362.

Key to facility success, notes Susan McIntyre, Solid Waste Director of the Delaware County, New York Department of Public Works, is committing funds to equipment maintenance and housekeeping. At the Fifth Annual Rotary In-Vessel Users Group Conference, held last month in Rapid City, she presented an update on the county's mixed MSW cocomposting facility. The three-year old plant has been steadily increasing its total percentage of MSW and biosolids processed each year, diverting materials from the adjacent lined landfill. The plant has three full-time maintenance staff. “And our total full-time staff is up to 12 persons who work a staggered schedule that allows us to have all of our employees at the plant one day a week for intensive housekeeping and maintenance,” she explains. The county also implemented an annual two-week maintenance shut down in the fall for the past two years.

Table 1 lists the 13 mixed MSW composting facilities in the U.S. Updates and a summary of plant operations are presented below.

Gilroy, California
Z-Best Composting Facility in Gilroy composts MSW with a CTI enclosed aerated static pile system. “This year we processed about 350 tons/day of mixed waste,” says Greg Ryan, Operations Manager of Z-Best Composting Facility.

The only major change at Z-Best is that material coming from the San Jose area now first goes through its MRF in San Jose, with organics-rich screenings sent to the composting facility. “The San Jose MRF has a more sophisticated sorting system, and is closer to end markets for recyclables,” explains Ryan. “Also, by sending the material there first, less has to be processed at the composting facility, making it more efficient.” Although preliminary data shows that about 30 percent of the initial tonnage entering the MRF goes to the composting facility, accurate diversion and contaminant removal rates have not been calculated yet.

The composting facility still uses its upfront sorting line - installed when the plant was permitted to receive MSW - to process organics-rich MSW from other sources. Finished compost is only sold wholesale to landscapers. Ryan notes that the future (and present state) of MSW composting is generally dictated by cities and county jurisdictions. “A trend towards source separated makes sense, because it's cheaper for residents and businesses, rather than paying for extensive processing systems for mixed waste,” Ryan says. “Also, cleaner material going in means more valuable compost coming out. I haven't heard of any plans for residential SSO collection in San Jose, but there is talk of commercial sectors switching to a wet/dry separation of organics and recyclables.”

Mariposa County, California
The mixed waste composting facility in Mariposa County continues to process 60 tons/day. The plant, which has been operating since 2006, takes residential and business wastes from Mariposa County, as well as material from Yosemite National Park. It uses SV Composter vessels from Engineered Compost Systems (ECS). The finished compost is used as alternative daily cover (ADC) at the county's landfill.

“The product has a very high level of inert materials and is only suitable for alternative daily cover (ADC),” reports Tim O'Neill of ECS. “The only reason composting MSW at Mariposa makes sense is that it extends the life of the county's landfill - it reduces the volume of the material being landfilled, and displaces the soil previously imported for daily cover. Dry recyclables are sorted and VOC reduction is achieved through composting.”

Cobb County, Georgia
The MSW composting facility in Cobb County continues to accept about 200 tons/day of material. However, in May/June 2008, a request for proposals (RFP) was sent out for operating the composting facility, as well as the county transfer station and the vegetative waste processing facility. According to the Atlanta Journal-Constitution (AJC), the county will negotiate contracts with companies to privatize these three operations, a move that could save the county $5 million/year in costs.

Jacksonville-based Advanced Disposal, in cooperation with Smurfit-Stone Container of Chicago, is currently the top choice for the composting facility, which it has proposed to turn into a recycling facility, halting the composting operation that has been in operation since 1996. The decision to privatize the facility, as reported in the AJC, was financial, with the county unable to charge a high enough tipping fee to break even. In addition, suitable markets could not be found for the finished compost, sold as Bio-Blend. Updates on the Cobb County facility will be covered in BioCycle.

Marlborough, Massachusetts
WeCare Environmental LLC owns and operates the composting facility in Marlborough. The facility is processing approximately 1,500 to 2,000 tons/month of source separated organics, biosolids and residential mixed MSW from the city's curbside trash collection program.

Chris Ravenscroft, President of WeCare Environmental, says that there have not been any major changes at the plant, and that they are “following operational and processing practices developed over the last five years since the company acquired the facility.” Approximately 20,000 cubic yards/year of compost are produced, and sold for as much as $8/cy. Approximately 20 to 25 percent of feedstocks entering the composting system are removed as residue.

Nantucket, Massachusetts
The Nantucket solid waste management complex includes a cocomposting plant, MRF, C&D processing facility, yard waste composting and a landfill. Due to the seasonal nature of the Nantucket Island population, the throughput of the compost facility, which is owned and operated by Waste Options, Inc., ranges from 100 tons/day in summer, to a low of 15 tons/day in winter. Biosolids are cocomposted with the MSW.

Nathan Widell, plant manager for Waste Options, reports that in 2008 the facility underwent a maintenance overhaul of major systems and is “running better than ever.” Approximately 15,000 tons/year of screened finished compost are produced, half of which is sold for $15/cy for on-island landscape and garden uses. The balance is used for landfill capping and facility landscaping. The final screening system includes a BiviTec vibrating deck, Forsberg destoners and an air classification system to remove light plastic.

Film plastics are the primary residual from the process. Widell reports that the Nantucket facility - including recycling, compost and reuse - has a diversion rate of approximately 85 percent. “Fortunately, the Nantucket Island waste stream is very high in organics due to the exceptional level of recycling participation by the citizenry,” he adds. “However, even more up front recycling, to reduce the amount of screenings, as well as mandatory use of biodegradable bags would be nice.”

When asked about the future of MSW composting, he answers: “The future of mixed waste composting is marginal at best. Except for situations such as Nantucket, which has a high rate of recycling, mixed waste composting is complex and expensive due to the requirements to screen the end product appropriately.”

Truman, Minnesota
Operations at Prairieland Compost Facility in Truman, which processes mixed MSW, remain the same as last year, with no plans for new equipment in 2009. Throughput is still about 65 tons/day, with 3,000 tons/year of finished compost produced, reports Mark Bauman of Prairieland. The compost is given away to residents, spread on agricultural land and increasingly used by pork producers as bedding for mortality composting.

The Prairieland facility has been operating for 17 years. In that time, Bauman notes, they've come to know what maintenance and equipment repairs to expect. “The main difficulty we face is public perception of the process and the product,” he says. “The common view is that garbage is garbage, even after composting.” The facility is ready to produce more refuse-derived fuel (RDF) with the residual fraction, but right now there isn't enough burn capacity in the area to warrant the expansion. “Alternative energy is a big topic now, so it looks like gasification and RDF are going to be the future,” he explains. “However, although there are many challenges with MSW composting, gasification is so new that we don't really know what all of those challenges will be.”

West Yellowstone, Montana
The West Yellowstone Composting Facility processes 40 tons/day of MSW in peak season (March to October) and 20 tons/day during winter months using an ECS in-vessel system. A detailed article on the facility starts on page 26 of this special report.

Delaware County, New York
The Delaware County Composting Facility expects to process 24,000 tons of MSW and 7,000 tons of biosolids in 2008, an increase from last year. “We project that 98 percent of the total MSW generated in the county will be processed through the facility in 2008, as well as 78 percent of the biosolids from our municipal wastewater treatment plants,” says Susan McIntyre. Last year the facility processed 55 percent of the total MSW generated in the county, and 81 percent of the biosolids. The facility also takes sawdust from a particleboard plant and liquids from several dairy products industries.

“We have gotten more selective about materials we process through the composting facility, and we distribute a list of unacceptable materials to the private haulers in the county in an attempt to minimize what we call structural and bulky debris from going through the composting plant,” she adds. The list of materials includes, but is not limited to: structural debris, siding, drywall, electrical wiring, insulation, carpeting and furniture.

The radiant floor heating system in the building was activated last winter, which extracts “free heat” from the maturation area and pumps 120° to 140°F air through the floors of the offices and the screening areas. Besides reducing heating costs, the incidence of condensation fog has been significantly reduced, improving visibility as well as function of the conveyors since there is less moisture on the belts.

The plant has a Conporec rotary drum for initial processing; IPS/Siemens (IPS) agitated bays are used for composting. A clogging issue with the drains under the IPS bays resulted in a saturated layer of compost. The drains were raised and the stone/wood media on top has been replaced with a stone/tire chip media, since the wood degraded and contributed to the clogging. Also, thermocouples in the walls of the IPS bays did not last and are being replaced with RTDs that send temperature readings to a separate data logger. “Our IPS/Siemens agitator is showing wear, and we have replaced the tines on the drum, as well as the hydraulic fittings, which had corroded,” adds McIntyre. “We are planning to replace the hydraulic fittings on an annual basis. Eventually we would like to have a second agitator so we can swap one out on a periodic basis while the other one is serviced.”

Finished compost has gone through several tests and is certified by the Seal of Testing Assurance (see sidebar). Most of the compost, branded as Oxbow Hollow Compost, is marketed by WeCare Organics. A net profit sharing agreement, says McIntyre, has brought the county between $10,000 and $20,000/year in sales. End uses include mulch, turf blends, potting mix, landscaping, top dressing, soil amendments, erosion control and filter berms. The county projects that 15,000 cy of finished compost will be produced in 2008.

Medina, Ohio
The Medina County Solid Waste Central Processing Facility (CPF) receives 550 tons/day of MSW, 200 of which are industrial and commercial material transferred directly to the landfill. The other 350 tons/day are residential and commercial MSW, which goes through various sorting procedures that include an electromagnet to capture steel cans, an Eddy current separator to pull out aluminum, and picking lines of 10 to12 people to sort material before and after the 40-foot trommel screen. The trommel screens material to a two-inch minus, which then passes by another magnet before entering the composting facility. An air classifier, referred to as an air knife, blows ultra light material onto a different conveyor belt for RDF. “Between 20 and 25 tons/day of this 'fluff' is sent to VEXOR Technology, which mixes it with nonhazardous waste to make engineered fuel,” says William Strazinsky, Medina County Solid Waste District Coordinator.

Approximately 33 tons/day of the preprocessed MSW gets composted. It is combined with about 66 tons/day of yard trimmings and wood chips delivered from the county's Class IV composting site (used as bulking material in a 2:1 ratio). “The total amount of material processed in our MSW composting system is therefore close to 100 tons/day,” explains Strazinsky. “If you take into consideration all of the waste generated in the county, including yard trimmings composted at the Class IV site, the county has a total diversion rate of 53 percent.”

Rapid City, South Dakota
The Rapid City composting facility started processing MSW and biosolids in 2003, utilizing two Dano drums for mixing MSW and liquid biosolids, followed by an IPS agitated bay composting system (see “Rapid City Closes The Loop on MSW Management,” BioCycle November 2003). Mike Oyler, Manager of plant operations, says the facility has become more selective about the type of materials acceptable for composting. It has cut back from 200 tons/day to approximately 160 ton/day by processing only residential MSW.

Rapid City has a blue bin program for curbside collection of recyclable containers; paper is composted with the MSW. Jerry Wright, Superintendent of the city's solid waste operations, says they tried cocollecting recyclables in blue bags along with the trash, but it didn't work. “We found our residents had the perception that they could put trash in the blue bags, plus the bags would break in the garbage trucks,” explains Wright.

Of the roughly 180 tons/day of MSW processed at the city's facility, quantities pulled out as rejects during the different stages include: 6 tons on the tip floor, 7 tons with the first sorting, 52 tons from the Dano drums, and another 29 tons with the final screening. Each Dano drum can receive approximately 18 tons/hour. About 90 tons/day are loaded in each drum, over 5 hours. The drums are turned approximately 6 to 8 hours to achieve particle breakdown and homogenization of the MSW and biosolids. The result is less reject off of the screens attached to the end of each Dano drum. “The drum discharge operator is key since he determines how much reject we get, with our loading rate,” says Wright.

A trommel screen, originally used to open bags of MSW prior to the picking line and Dano drums, was removed from the operation. Now the bags go into the Dano drums unopened, with workers on the MSW sorting line concentrating on pulling off bulky and problematic items such as garden hose, chain, cable, rope or long strings of carpet or plastic. These items tend to create wads of plastic that plug the Dano discharge system. Future changes may include removal of the secondary curing step and instead increasing the retention time to more than 30 days in the primary curing IPS compost bays. Material would then go from the bays directly to final screening. “This would reduce the odor associated with our secondary curing, which is an open bay building with only passive air and odor control,” says Wright.

Rapid City manufactures both yard trimmings and MSW compost. Due to small pieces of plastic in the MSW compost, it is mostly given away and is primarily mixed with soil for use in construction projects. The yard waste compost is higher quality and sells for $20/cy. Testing and application has shown both products to be “excellent compost.”

Sevierville, Tennessee
Sevier Solid Waste Inc., owner/operator of the Sevier County MSW composting facility in Sevierville, was the largest operating MSW composting facility in the U.S. until being destroyed by a fire in 2007 (see “Tennessee Composting Facility Rises From The Ashes,” BioCycle November 2007). Tom Leonard, Solid Waste Director, described the redesign of the composting facility at the Rapid City rotary drum conference last month. Construction is nearing completion, with operations expected to start in early 2009.

Columbia County, Wisconsin
The Columbia County Recycling and Waste Processing Facility continues to process 80 tons/day of MSW. The facility, which began operation in 1992, uses two rotary drums, which have spikes that tear open the bags. The drums are built at a 3 percent slope, and the MSW slowly moves from one end of the drum to the other (facilitated by lifting bars). The material remains in the drums for 5 days, where it reaches 140°F. It is then screened to 3/4-inch minus, with rejected materials transported via conveyor to a compactor, and then shipped to the landfill.

After five days, the screened compost is transferred to the compost storage building, where it is cured in windrows for eight weeks. Windrows are turned two to three times per week. About 3,000 tons/year of finished compost is produced, screened to 3/8-inch minus and given away to farmers and landscapers.

Copyright 2008, The JG Press, Inc.