Composting and co-composting of MSW, organics, and biosolids is a convenient way of disposing of these wastes, but the process inevitably involves unpleasant odors that tend to limit composting in suburban and urban areas. Now a variety of in-vessel composting systems offer a solution to this dilemma.

By Charles D. Bader

Composting is by its very nature a small-scale local peration that does not readily lend itself to large-scale systems that can be sited far from urban centers. There is an advantage to having a composting system close to the source of its feedstock–in or near urban or suburban neighborhoods. Even if a composting system is originally sited outside of town, housing development eventually catches up with it. And when the decomposition inherent to composting creates unpleasant odors, the neighbors are likely to complain vociferously.

Although there are ways to minimize or control these odors, composting operations tend to have a bad reputation among the neighbors. One bad-odor day can cement that reputation for a long time, leading to difficulties in obtaining permits for composting systems and to lawsuits and regulatory action. For example, the Massachusetts Department of Environmental Protection's draft "Guidance and Policy for the Evaluation of Odors at Composting Facilities" set an emissions limit of five dilutions-to-threshold at the property line. Exactly what this limit will mean to a composting facility is unclear, but it does not appear to be easy–or cheap–to meet.

One increasingly popular approach to resolving these difficulties is the use of in-vessel composting. Debbie Linder, operations director for Ag-Bag Environmental in Warrenton, OR, cites the inherent advantages of in-vessel composting: "The ideal environment for the microbial growth that is the secret to better and more efficient composting is a wet, warm, and dark place. By having an enclosed vessel, there is less volatilization, which in turn leaves a higher-quality composted material. More active microbes are available for the soil and plant growth.

"This is different than turning the windrow every several days and having to rewater, which releases odors and slows down the whole process, as the microbes have to remultiply and rebuild. By enclosing the materials in a vessel, a higher fraction of nitrogen will be left in the compost as oxidation releases less nitrogen gases into the atmosphere. In effect, the material acts as its own biofilter. Thus, we are able to control the potential odors and deal with them while they are contained in the vessel rather than in the neighbors' yards," Linder concludes.

EcoPODs

Linder's "vessel" is actually a bag manufactured from low-density polyethylene recyclable plastic. Called an EcoPOD (POD stands for Preferred Organic Digester), Ag-Bag Environmental's bags are 200 ft. long and come in 5-, 10-, or 12-ft. diameters. Depending on the size selected, an EcoPOD will hold between 250 and 1,000 yd.³ of preground materials that are mixed and blended with a carbon-to-nitrogen ratio of 30:1 and contain an average of 55% moisture. When full, the bag is sealed on each end with a special sealing strip that prevents leachate from spilling onto the ground. The EcoPOD is supplied with strip-sealing equipment, aeration piping, controllable vents, temperature probes, and starter inoculant.

The mixed materials are pressed into the POD at the proper density to maintain porosity. As the POD is filled, aeration tubing is fed into the mix along the entire length. Specially sized and spaced slots in this tubing pressurize oxygen throughout the entire length of the POD. Then oxygen is introduced into the sealed POD using a high-pressure blower system controlled by timers that can be adjusted from seconds to hours, depending on the phase of the composting process. Initially, more oxygen is needed to feed the bacteria colonies, which rapidly expand and move through the material. Later, when less oxygen is needed, blower cycle times can be reduced to facilitate the microbial needs while finishing and drying the material.

As the material decomposes, it changes shape, creating new oxygen paths. The POD heats the material quickly and reaches the 130-140° F temperatures needed to kill pathogens and weed seeds. The POD acts as its own biosphere as condensation rehydrates the outside of the materials to sustain bacterial action there. This process eliminates the external drying that occurs in a windrow system.

"After eight to 12 weeks [depending on the feedstock makeup], the POD can be harvested and the material can be removed with a loader and moved to a static pile for curing and maturing for 30 to 60 days," Linder explains. "It is then screened, and any overs are reintroduced into another POD so that the new compost batch will have an additional inoculant." During this curing period outside the POD, there is a risk of odor emanating from the pile. Linder discounts this possibility, however. "Our EcoPOD was used in a biosolids project in Texas," she points out, "and we got a report back that there was no odor detected."

Odor is a tricky phenomenon, though; it seems to vary from application to application. For example, the City of Redding, CA, had a serious odor problem with its windrow composting system when it moved the operation from its remote landfill site to a new location closer to residents. Soon the facility was receiving many complaints from neighbors. "Then we learned that the city planned to build a sports complex right near us," recalls Redding's Dennis Carvalho, "so we knew we had to do something about the odor."

That something was to acquire an EcoPOD system from Ag-Bag in April 2001. "We're still experimenting with it," says Carvalho. "We keep the material in the bag for three months and then put it in windrows outside and treat it pretty much the way we do with windrows. The odor isn't as bad as when the material first goes into the bag, but it still stinks when it comes out of the bag, as one of my neighbors tells me every time. That's probably because we have so much grass in our feedstock. We had a 50% increase in resident participation when we automated our yardwaste collection this spring, and that increase seems like it was all in grass clippings. We don't have room to store a lot of wood, so our carbon-to-nitrogen ratio is probably much too low. We've addressed the problem by adding microbes to the mix using a product called Effective Microorganisms, which we get from EM Technology in Tucson, Arizona. It costs just $7 a gallon, and we use 25 gallons per bag, but it really helps. Not only does it ease our odor problem, but also it improves the quality of our compost."

Containerized Composting System

Ag-Bag CT-5 composter with EcoPOD attached. Each POD holds 76 tons.

Michael Bryan-Brown, president of Green Mountain Technologies (GMT), headquartered in Whitingham, VT, has a much different view of grass as an odor producer, claiming it "doesn't have putrescible compounds like fats and sugars." Perhaps Bryan-Brown discounts the odor potential of grass because most of his customers compost grass with biosolids and the odor comparison is odious, so to speak. Moreover, he claims that no GMT facility has ever been shut down because of odor.

GMT offers a turnkey containerized composting system built around enclosed stainless steel composting vessels, each 23 ft. long x 9 ft. high x 8 ft. wide and capable of supporting a load of 27 tons. Each vessel has a peaked aeration floor to ensure that the mixture is aerated uniformly from the top or the bottom. The vessel collects leachate or condensate in a space beneath the aeration floor, so it can be drained prior to unloading. It uses only noncorroding materials (plastic or stainless steel) in direct contact with the compost. "We build our vessels to last in this harsh environmental environment," Bryan-Brown explains. "For example, our use of stainless steel will provide 20 years of service. That's about double what you could expect from carbon steel."

The complete turnkey system includes integrated mixing, loading, and screening equipment and uses standard rolloff trucks for unloading and transport. After materials are blended in the compost mixer, they are loaded into the composting vessel through a gasketed door. When the vessel is full, the operator closes it, attaches aeration lines, and inserts temperature probes. A control unit automatically regulates blowers and dampers each vessel's aeration system; there can be as many as 50 vessels in a single facility. Temperature feedback determines whether each vessel should be heated or cooled by pressurized air delivery through the aeration system. A Windows operator interface allows the user to monitor the entire 10- to 24-day composting process of each individual vessel from a remote office off-site.

"We're seeing more composting biosolids in the past year," Bryan-Brown points out. "EPA has been campaigning for pretreatment of biosolids to remove metals from sludge, and this has had a major effect on the co-composting market. At the same time, we're seeing more and more source separating and subsequent composting of foodwastes. We have installed several facilities for this purpose because this kind of composting meets a strong municipality need."

One of these facilities is in the town of Hutchinson, MN, where it composts 30 tpd of yardwaste combined with organic wastes such as foodwaste, non-newsprint paper, junk mail, and light cardboard. Residents separate these organics from MSW and combine them in a bin for curbside pickup. "When we first started the program in June, we had a problem with residents contaminating the mix by also putting in nonorganic materials or recyclable newsprint," recalls Laurence Winter, the town's former resource recovery manager, "but already they're getting much more knowledgeable. To maintain quality control, we do a double inspection when the truck brings in a load. We check it on the floor, and the picking line checks it again before it goes into any of our 16 vessels.

"The accepted material then goes into a mixing drum where we add wood chips and water to get porosity and moisture up to our optimum levels. After 20 minutes in the drum, the now-mixed material is moved by a stacking conveyor to a vessel. The stacking conveyor extends, raises, lowers, and goes side to side so we can fill the vessel evenly as well as fully. Then the system keeps the material aerated and heated for two weeks, at which time we empty the vessel on a concrete pad, put the compost in windrows, and leave it there for six months, only turning it once a week.

"The residents in our town love the system," Winter continues. "Instead of participation dropping because residents have to source separate their organics now, participation among our 3,700 households went up to 85%. All we do is supply them with compostable bags and keep the odor down. There is no odor left when the compost comes out of the vessels, and the only odor emanating when the material is in the sealed vessel is the exhaust air that we direct through a biofilter consisting of wood chips and cured compost. When we have visitors to our facility, we make a point of meeting with them right next to the biofilter, thereby emphasizing the point that ours is not a smelly operation. And now the city is so confident that they are building a recreational area with trails winding right around the plant."

Bedminster Composting Systems

MSW is loaded onto a conveyor belt in the tipping building using a frontloader.

Employees make sure material discharged from the rotary composter is spread onto the conveyer belt.

As recently as five years ago, arguably the best-known name in in-vessel co-composting in the United States was Bedminster Bioconversion Corporation, which produced systems based on the invention of Swedish scientist Eric Eweson. Bedminster has since seen acquired by a Swedish firm that has not actively marketed the system in North America. A young company, Waste Options Atlantic of Warwick, RI, has filled the void with its option to market Eweson composting technology in the US. According to Director of Marketing Nelson Widell, Waste Options is now actively marketing a version of Bedminster systems, primarily in New England. Widell explains that this initial geographic concentration is a result of market conditions. Because recycling quota regulations are among the toughest in the nation here, he reasons, high-yield systems such as Waste Options's are likely to be in demand.

Perhaps the best known of the Waste Options installations is the system on Nantucket Island in Massachusetts. As reported in the November/December 2000 issue of MSW Management, Waste Options has been given an unprecedented 25-year contract to take over Nantucket's landfill, operate a newly built material recovery facility, and–most importantly–build and operate a co-composting plant for the town. This plant is built around a steel-drum digester, based on the proprietary technology developed by Eweson.

"A 25-year contract is unusual," Widell concedes, "but Nantucket was in a difficult situation. The island's strong growth in popularity among tourists had placed an increasing strain on its infrastructure, particularly as it related to the treatment of solid waste. The old town dump kept getting bigger and messier, and seagulls thronged it. Finally, the State of Massachusetts mandated that Nantucket close its landfill and that its waste be shipped off-island for disposal on the mainland. If this had occurred, the trash bill for all island residents would have quadrupled."

But the off-island shipping didn't occur. "Our program has taken the pressure off the landfill. Today, because of the combination of composting and recycling, only 14% of MSW generated in Nantucket goes to the landfill," Widell reports. "In addition, we have been able to initiate a landfill mining and reclamation project to reduce the amount of trash in the landfill. This mined trash will continue to be composted over the years, ultimately leading to the rebuilding of land now used for landfill.

"The key is the composter. Eweson's innovation was to transform each composting digester into an environment full of microbes. As trash and sewage sludge are fed inside, the cylinder rotates, exposing the waste to more and more microbes. [The microbes] actually accelerate the decomposition process to a point [at which] the household waste and sludge going in one end emerge as compost at the other end after only a few days. Ever since Nantucket's composter became operational in late 1999, that's exactly what has happened there."

Waste Options is not the only company involved in extending Bedminster operations. A-C Equipment of Milwaukee, WI, services most of the Bedminster systems in North America and also manufactures the digester drum and ancillary units. This year, Sumter County, FL, has contracted with A-C Equipment for a second digester: one with 25% greater capacity than the first Bedminster unit the county still operates. A-C Equipment is making some design modifications too. "Instead of baffles," explains A-C President John Vitas, "the new digester will be completely empty inside so that the material is mixed solely by the normal tumbling of the material. Another change is that the off-gas from the vessel will be fed into a biofilter for odor control."

Davenport's Compost Hall

Clearly, not all in-vessel systems use conventional vessels. There are bags, tunnels, fixed modules, and portable modules–all qualifying as vessels. The City of Davenport, IA, has expanded the definition of a vessel to include an entire building in its in-vessel composting system. It uses a 66,000-ft.² "Compost Hall" building on a 3-ac. site that also contains grinding, screening, and curing areas. This site amounts to a self-sufficient complex for co-composting yardwaste and biosolids.

"The Compost Hall is divided into east and west sides with a 40-foot-wide aisle down the middle," describes Scott Plett, the city's compost facility manager. "Each side has 12 individual piles, although because of our extended pile method, each side looks like a single mass. Each pile consists of 8 feet of mixed ingredients on top of fresh wood chips and is topped with 1 foot of finished compost that serves as an insulating blanket. There are four trenches per pile for aeration, and each pile has three 5-foot thermocouples. Temperature readings from the thermocouples are sent to a computer system where pile temperatures are monitored. There are four different levels of operation to aerate the piles, depending on the temperature. Off-gases are discharged from the hall to external biofilters; each side has a half-acre biofilter to deodorize these gases."

The material is processed for 21-28 days, after which it is removed from the hall, screened, and placed in an aerated curing area in 70-ft., thermocouple-monitored piles on corrugated air pipes through which air is blown with positive pressure to aerate the pile. The curing area is under-roof but open on two sides. "However," Plett notes, "the odor is pretty much nonexistent at this point.

"We are in the process of expanding the facility: adding on to the end of the hall and extending the biofilters to match. That will add 15% to 20% to our 31-tpd capacity," Plett points out. "Actually, we have needed to expand for some time now. Instead of the 20,000 to 25,000 yards of yardwaste per year we had expected, we have been getting 110,000 yards. To keep up, we have had to be quite creative. Today we first grind the yardwaste and allow it to decompose and lose some moisture; then we grind it a second time to further reduce its volume before we convey it to the mixing room and mix it with the biosolids prior to adding the mixture to the piles."

The facility currently produces 25,000 yd.³ of compost a year and sells all it can make, principally to contractors working on department of transportation projects. Moreover, Plett anticipates no difficulty in selling the additional compost after the expansion: "It's a good-quality product."

Composting Spells Relief for West Yellowstone

The West Yellowstone/Hebgen Basin Solid Waste District in Montana was seeking relief from high tipping fees and increasing trucking costs to distant landfills. In March 2001, the district issued a request for proposal (RFP) for composting the MSW generated from the district and from nearby Yellowstone National Park.

West Yellowstone wanted a single vendor to provide a turnkey MSW composting facility. The scope of work included furnishing a detailed process design, all process equipment, installation labor, and materials. The RFP included requirements commissioning the composting equipment, training the operators, and providing ongoing technical support at the West Yellowstone transfer station.

ECS of Seattle, WA, teamed up with Montana-based DA Construction to submit a proposal to supply an SV Composter system. After a thorough technical and economic review process, the ECS-led team was awarded the contract.

Practical Composting on a Larger Scale

West Yellowstone's transfer station receives 3,500 tpy of MSW, with significant peaks in the summer months. As a result, the facility is designed to handle 42 tpd.

Several constraints pushed the facility design. First, because of high snowfall amounts, all of the activities needed to be under one roof. Second, a limited area was available at the selected site adjacent the transfer station. Third, the waste district was intent on limiting the labor requirements.

The SV Composter provided an ideal starting point to meet these constraints. The tunnels share common walls, keeping the facility footprint efficient. The entire footprint of the facility is approximately 36,500 ft.² A conveyor system delivers the raw feedstocks from the wet mill and mixer directly to the composting tunnels. The tunnels are unloaded using ECS's self-propelled/self-steering Compost Miner. These two features greatly reduce the labor required for material handling.

The tunnel's aeration and control system is a direct extension of the technology used successfully for the last five years with ECS's CV Composter.

A wet mill located on the tip floor will homogenize the MSW and add water as needed. Biosolids and wood chips can be mixed in using a heavy-duty auger mixer. After the high-rate composting is completed in the tunnels, the compost is removed with the Compost Miner and placed in static windrows in the adjacent curing area. Once cured, it is moved with a loader to the product refining area for screening and destoning.

Valorga Anaerobic Digesters

Anaerobic digester located next to a Burger King in Freiberg, Germany

All of the systems described thus far are aerobic systems–that is, they use oxygen to grow and reproduce microorganisms and break down organic feedstocks, and they return carbon dioxide and ammonia to the atmosphere. Another approach to composting, widely used in Europe, is anaerobic digestion. Anaerobic bacterial processes operate in an enclosed, oxygen-free environment. In addition to producing compost, such processes generate methane and carbon dioxide in an enclosed container, thereby providing an opportunity to capture these gasses and use them productively, either for process heat, for steam, or for marketable excess electricity.

"Processes such as anaerobic digestion and composting offer the only biological route for recycling matter and nutrients from the organic fraction of MSW," contends Herman Miller, president of Environmental Developers Inc. of Stockton, CA. "However, composting is an energy-consuming process requiring 50 to 75 kilowatt-hours of electricity per ton of MSW input. Conversely, anaerobic digestion is a net energy-producing process, with around 75 to 150 kilowatt-hours of electricity created per ton of MSW input."

Anaerobic digestion in this country appears to have been limited largely to applications involving animal waste or organic industrial waste. There has been little or no use of anaerobic digestion to compost MSW. In Europe, however, the use of anaerobic digesters to process organic MSW is becoming relatively common. Not surprisingly, therefore, some European suppliers of anaerobic digestion systems have been eyeing the US market.

One such company is the German firm Steinmuller Valorga, which has systems in seven European countries. The advantage of the Valorga system appears to be its design for containing and capturing biogas (both methane and carbon dioxide). The methane has approximately 55-60% of the heating value of natural gas and is intended to be combusted in a dedicated boiler to produce steam. This steam is used for process heat in the digesters and to produce electricity that will both operate the plant and generate a significant excess that can be marketed. In the case of a facility designed to compost 500 tpd of MSW, the company calculates that the biogas generated will have sufficient energy content to operate the facility and produce about 2.5 MW of excess electricity per hour.

A US firm, Waste Recovery Systems Inc. (WRSI) of Newport Beach, CA, is marketing the Valorga system in North America. According to President Steve Morris, WRSI is promoting a program of solid waste management that includes composting MSW with sewage sludge using the Valorga process of anaerobic digestion followed by aerobic curing.

"By combining a sorting line to recover recyclable materials from mixed [not source-separated] MSW and composting the resultant MSW stream with a Valorga anaerobic digester, we will be able to recycle for beneficial use about 65% of the typical US mixed MSW stream," Morris says. "Moreover, it will supply a significant source of energy for the generation of steam/electricity or as fuel for motor vehicles. Of great importance to residents in the local community, the process does not emit odors to the atmosphere."

The MSW feedstock is mixed with the sludge and water to achieve a ratio of 35% solids to 65% water. At the mixer, steam is added to bring up the temperature of the mixture to the thermophyllic range to ensure that pathogens are killed. The heated mixture is then pumped to the digester. There are no moving parts in the digester, but a baffle system separates incoming from outgoing material. A 24-in. pipe carries the pumped slurry into the digester, and over the next 14 days the pressure of incoming material forces the slurry around the circular digester, past the opening in the baffle, and out a similar pipe at the other side of the digester. In the course of this two-week odyssey, jets of air keep the material mixed, and the biological decomposition continues. As waste is added each day, a comparable amount of compost exits the digester. When the biogas leaves the digester, it is routed to either (1) a biogas compressor and then to pressurized biogas storage or (2) a steam line to generate electricity.

When the slurry exits the digester, it passes through a press that squeezes out water to get the ratio to the correct level. The still-warm water is recycled to the mixing stage where it provides resident heat and some microorganisms to the new mixture. The wet compost goes into an aerated tunnel where it is cured for two to three weeks. The tunnel is enclosed so that no odors escape during this phase.

All in all, the Valorga system appears to have eliminated the odor problem completely. Indeed, the facility in Freiberg, Germany, towers over an immediately adjacent Burger King restaurant and provides a powerful visual symbol for this system in particular and for the in-vessel composting industry as a whole. In-vessel systems cannot compete on cost grounds with traditional open-turned windrow MSW composting. However, dust, health, sludge disposal, vermin, and odor concerns affect the formula significantly at urban or suburban sites. In many cases, these factors can make in-vessel systems preferable; occasionally they might well make in-vessel systems mandatory.

Charles D. Bader is with Dateline II Communications in Los Angeles, CA.