Odor generation leads the list of problems facing composters in the United States, followed by product consistency and the development of viable markets.

By Katherine Holden

Butting Heads With Carbone
Working With Biosolids
Composting in Europe
What's Up With Digesters

In order to combat the problem of odors, many facilities contacted for this article currently rely on in-vessel procedures. One who doesn't is Robert Gillespie, owner and operator of DK Recycling Systems in Lake Bluff, IL.

Gillespie owns two sites and operates three others owned by municipalities. All of his feedstock is yardwaste, more than 125,000 yd./yr., which translates into 40,000 tons of compost. Some of the feedstock involves curbside pickup, which he only accepts in paper bags. Gillespie says, "I learned 10 years ago, You'll pay me up front or out the back.’ Paper bags compost the same as leaves. I won't accept plastic bags. I built the market for compost in this area over the last 11 years. I get twice the amount of money for my compost as my competitors do because mine is clean."

Diversion is the purpose for DK's sites. "Illinois was the first state to ban yardwaste from landfills in July '89. There are no alternatives for yardwaste other than compost. Landfills can be fined $10,000 for accepting yardwaste."

With a few years experience in the business, Gillespie learned he had a "black gold" product. "Landscape firms buy everything we produce. Contractors bring in leaves, grass, and brush and come back for compost."

Two of DK's sites have homes valued at a million-plus dollars within 800 ft. "Where I’m operating, the public tolerance for odor is zero," he notes. DK uses a windrow system. "The volumes we handle need a lot of room. We do 7,000 cubic yards per year per acre. We get 50,000 to 60,000 cubic yards of leaves when the leaves are down between October and early December."

For DK, windrows are the only financially efficient way to compost. "Tipping fees around here are $13 to $14 a yard. Most of our end product we sell at $12 per yard because it's sold in bulk. We work with volume and speed, and it costs us $5 per yard to produce the product," Gillespie explains. With seasonal surges, such as the fall when 40% of the material comes in over six weeks, DK must be able to create a stable product quickly. "I can do that within 100 to 120 days. I must, or I'd be constipated. There wouldn't be any flow-through."

DK mixes all incoming materials with a bulking agent. In the spring, it mixes wood chips and partially decomposed leaves with grass clippings. Dry fall leaves must be mixed with water and bulked up with grass. DK shreds all material to about 3 in. through the Jenz 300 shredder, and a custom-built Jenz UM100 turner goes back and forth between the various sites, along with a trommel screen from Farwick Max.

The Jenz shredder also forms the windrows. The piles sit undisturbed for 45 days and then are either reshredded to reduce wood particles, aerated (and any necessary water is added), or processed through the turner. Once again the material sits for 45 days.

Gillespie feels that controlling odors is much more of an art than a science. He says the science of composting is basic microbial decomposition of organic material. What one needs is a basic understanding of the sequence of how material breaks down in the process. "For example, in the early stages of composting, hydrogen sulfide smells like sewer gas. Twenty to 30 days later, you get an odorless hydrogen sulfide."

He uses cooking as his metaphor. "Anybody can learn to cook. With basic ingredients like flour, sugar, and salt you can make a cake. Yet some people have the touch to make a wonderful cake with those same ingredients. Some people are willing to learn from nature what works and what doesn't work, and they come out on the right side. There are many arrogant people who came in to play this game and were broken."

He gives an example of a company in Michigan that tried to handle 100,000 yd. of yardwaste. "By the time they began to handle it, it smelled to high heaven. Neighbors complained, the state closed them down, and the company lost three-quarters of a million dollars by the time they finished with the cleanup."

Gillespie has the art down perfectly. He is also adamant about how to go about setting up a new composting facility. "Spend two grand going around the country and interviewing successful operators, like Jeff Gage, who knows his business inside and out."

Many people interviewed commented on Jeff Gage's expertise and success. He is the director of recycling services for Land Recovery Inc. in Pullyalup, WA. He oversees three sites at two locations. One site, the Pierce County Compost Facility (PCCF) in Purdy, WA, opened in April 1992. About 45 minutes away, in Hidden Valley, alongside a landfill now closed, Gage directs two different composting sites side by side as well as a solid waste transfer station and a large receiving area, where all materials enter no matter their final destination. Four haulers bring in the bulk of the feedstock from curbside collectors. Commercial landscapers and self-haul residential homeowners also deliver feedstock. All grinding also occurs at this location.

An agitator mixes and moves the material along so there is constant mixing and aeration.

In 1997 at Hidden Valley, LRI opened a 35-bin in-vessel composting program put together by NaturTech. In March 1999, LRI opened its Compost Factory adjacent to the multibin steel-box site. Both are covered under the Compost Factory's permitting system to handle any clean source-separated organics. "How we’re using the bins is what’s interesting," Gage says. "We bought them to expand our capacity and to handle nastier materials like foodwaste." Gage also uses them when he receives preliminary feedstock batches that he hasn't handled before. "We start off in the bins, then later handle that product in the Compost Factory once we know we can work with it. It gives us useful scale trials."

To ameliorate fears, LRI invited neighbors to look at the in-vessel bins when Land Recovery first made its proposal for composting at Hidden Valley. It's a sensitive site with houses, schools, restaurants, and a golf course close by. Although the adjacent landfill still exists, it quit taking garbage in January 1999 and is now being capped.

Fifteen of the vessels are each 50 yd.³ Each is portable and set up to connect hose lines to push air in and pull stinky air out via two blowers into four biofilters. "Ninety-eight percent odor removal is the lowest number we get," Gage states. For most of the compounds tested, odor removal ranges from 98 to 100%. LRI unloads material, waters it in a mixer, then reloads it into the enclosed vessels.

There are 20 bins for curing, each 40 yd.³ in size. Materials stay in the first 15 bins for about two weeks, then move to the second bins for another 30 days for curing. Gage uses water in the first set of bins once, when the material arrives; during the first week, when the material is turned; and when the material is taken out. "In the curing bins we use a tarp made by William Gore and Associates. It's a Gore-Tex type fabric with somewhat larger pores. Water vapor can pass, but water droplets and odor molecules can't."

The bins handle 20,000 tpy, including five months a year of making green mulch, which is a five-day process using grass-rich yardwaste. Local farmers spread it on lands that were once nourished by cattle manure. This green mulch accounts for 14,400 tpy. The rest of the year, the bins handle regular composting material that can include any source-separated organic waste, and the annual yield is 5,700 tons.

Fortunately the bins can handle more than 100 tpd of grass-rich yardwaste. When neighbors complained about odors at Northwest Cascade's site, the plant closed and LRI took its greenwaste.

LRI's newest project is the 3.5-ac. steel-structure Compost Factory. It is a fully enclosed site with processed automation. "In truth, it’s a factory manufacturing a product out the other end," Gage beams. The Compost Factory has the same type of air floor and turner as the PCCF facility. Where PCCF houses a 30,000-ft.² concrete pad, however, Compost Factory sports 152,000 ft.² Each site is equipped with an underground aeration piping system to provide continuous aeration of the material for active composting. There are 6-in. (8-in. at Compost Factory) pipes every 6 ft., 2-in. pipe risers come to the surface, and all pipes drain so LRI can flush them out.

"Most aerated systems use static piles, so there’s no turning. Ours are very active materials, and we must rewater and turn for porosity. We rewater to maintain moisture and push and pull air to keep temperatures consistent in piles from top to bottom, and we do it without concrete trenches. We do it on an open floor."

The Compost Factory boasts important improvements compared to PCCF, including process automation using temperature probes that send radio signals to computers to adjust the volume of air and the direction of airflow. Green Mountain Tech handled the process automation, computer controls, and blower controls. "We brought them in to make sure we could manage this thing consistently," Gage notes. "There's a lot of flexibility in their design. There's 24-hour continuous monitoring: If a problem arises, a 'critical faults' computer will call us. We can also call in from our laptops at home."

Another difference from PCCF includes walls. Gage says, "PCCF can only handle yard and garden waste and we wanted any clean source-separated organics at the new facility, so we had to enclose. A second reason for enclosure stems from odor problems in the region."

To take advantage of the landfill, LRI built a gas-generation plant to burn the landfill's methane. "We expect methane over the next 12 to 15 years. It's dumped onto the power grid so when we don’t use it all, it goes out to the local community. We get a good deal on the power we generate. We still pay for it but pay less. We only need 3 megawatts of power at the Compost Factory."

Tipping fees for yardwaste at the whole system run $35/ton. The feedstock is in and out in 45 days. "It's pretty well stabilized at that time. We could but don't bag and sell to two distributors," Gage states.

His outfit can grind 470 tpd. An excavator loads the all-ground material into a feed hopper and sends the material across a BHS Debris Roll. The bins can hold 27-100 tpd. The PCCF site can handle 80 tpd, while the Compost Factory handles an average of 200 tpd. LRI had the Compost Factory designed to handle yard and garden waste, clean woodwaste, foodwaste from grocery stores and kitchens (preconsumer as well as postconsumer plate scraps, both commercial and residential), manures, food-processing wastes such as chicken feathers, clean biosolids, and paper products. In short, anything organic intended for composting that can undergo quality control.

In its first seven months, LRI handled 42,000 tons at the Compost Factory, including 6,000 tons in greenwaste. "We expect 47,000 to 50,000 tons per year, but it's designed so we can handle 91,000 tons per year if we’re pushed," Gage admits. "We finance ourselves. With tipping fees and selling the finished product, he says they will potentially make a profit.

The three sites were built to keep materials from ending up in landfills and to create a product. Gage remarks, "We start not with garbage but with organic resources we desperately need in the soils. It never was a waste, only ignored and abused." Hence, the name Land Recovery fits perfectly.

Butting Heads With Carbone

In Burlington County, NJ, the Board of Chosen Freeholders oversees the solid waste management program, which also uses the IPS system. Mary Pat Robbie is the management specialist. The composting facility opened in May 1998. A major resource-recovery complex, it encompasses a 550-ac. tract where the county wants to locate as many solid waste and recycling facilities as possible.

Robbie says, "Burlington County didn’t go in the direction of incineration back in 1975. We proposed MSW processing from residents in the county through mechanical separation to be able to create compost." The facility can now process about 45 dry tpd of biosolids. Although the facility strives to be self-sufficient with the solid waste tipping fees, there are serious flow-control issues. Across the Delaware River from Burlington County are two Pennsylvania landfills owned by Waste Management. Robbie admits, "About 40% of the waste we once received now goes out of the county. All private-business contracts for solid waste now take material to Pennsylvania. That’s a $3 million annual loss for us, which is one-fourth of our revenues." This loss has put a halt to plans for a solid waste facility. "So we use wood chips as amendments, including demolition wood, wood from new construction projects, and tree limbs. We’re trying to pursue solid waste diversion, but not as fast as we once hoped. We won’t build a big solid waste facility where we would mechanically separate out until we solve the flow-control and revenue issues."

Flow control has been a major problem around the country since the Supreme Court handed down its Carbone decision, stating one cannot regulate interstate commerce. For example, citizens in Wright County, MI, were billed for a small MRF composting site costing $18 million. Next door, Iowa opened a landfill. Although Wright County cried foul, the people have to pay the MRF costs plus the landfill hauling costs.

Robbie's feedstock now includes MSW and biosolids from the cities of Burlington County. "Each year our contracts with municipalities are up for renewal. People have a choice: here or Pennsylvania. But we provide curbside recyclables paid for by the solid waste tipping fees. Under New Jersey laws, municipalities are responsible for collecting recyclables."

Burlington County chose the IPS Composting System because of its agitated bin system. "Agitation mixes daily and moves the material along, so there’s constant mixing and aeration. That was the key feature in that selection of system. We wanted to move into other types of solid/organic wastes, and this system would be more effective and speed up the composting. We're seeking state approval to compost coffee grounds and paper recycling sludge residue. We're slowly moving in that direction," Robbie states.

The biosolids and amendment are dumped into the facility receiving area and mixed by a typical agricultural-type mixture. Every truckload is tested as soon as possible to find the proper moisture content for the material in the bins. The mixture stays in the bins a minimum of 14 days. When the compost reaches the end of the bin, it goes to a curing pile within the building. This is an entirely enclosed system. Under New Jersey regulations, it must be kept in curing piles another 30 days to be certain it's completely stabilized before going to market.

Robbie admits that costs and energy requirements "certainly are the downside of composting."

Burlington County contracted with Wheelabrator Water Technology for approximately $37 million. "It's certainly costly compared to land application and landfill, but composting is more environmentally friendly."

Recent electricity costs were $30,000/month to pay for the 300,000-320,000 kWh of electricity. "We pay 11 cents per kilowatt hour. We’re obligated by state regulations to run the blowers all the time, 24 hours a day. We can never vent directly. In some other states, you can." By the end of 2000, Robbie expected to cut the electricity bill in half, thanks to capturing methane gas from the nearby landfill.

The county offers a two-tiered tipping fee structure at the facility: $35/ton or $17/ton for better-quality material, for example from pallet companies. Tipping fees at the landfill run $50-$83/ton.

Currently the site produces an average of 6,667 yd.³ of compost each month. "Folks tell us it takes at least two years to develop a good market in your area. Local nurseries are comfortable with our product. I've even taken it out to street fairs to show it. We now have bagged compost we can sell via All Gro."

Robbie feels the facility has performed well beyond original expectations. Rust Engineers and Construction (later taken over by Raytheon) designed the composting facility. The key equipment, the agitator, is by IPS. Burlington County uses an Erin Screen. For front-end loaders, it chose Volvo. "So we'd always be able to find another one to minimize the downtime due to equipment malfunction," says Robbie. The site has two 45,000-ft.² biofilters. "They're very effective in removing odors from the atmosphere in the building." Designed by Rust Engineers, a mulchlike material filters over the pipes transmitting air out of the building, and microorganisms break down the ammonia (a major component of odor-causing compounds). The biofilters maintain the proper pH in the media as well as the proper moisture content. Robbie handles all this with a staff of eight.

Working With Biosolids

In Santa Rosa, CA, Mike Reynolds, biosolids coordinator, directs the Laguna Sub-Regional Compost Facility. Although it is owned by the City of Santa Rosa, it serves a larger area. Designed for a 9-ton capacity, the facility routinely exceeds that by 25%.

Reynolds says, "We use about four volumes to one of other organics along with biosolids. Curbside-collected greenwaste is our primary bulking agent." Not only does the facility use all the tree waste generated within the city, it also receives sawdust and mill ends from sources without a tipping fee. "We need dry sources of bulking agents in the winter because we have pretty wet winters here."

Diversion is the primary reason for the compost facility. Before this project, one-third of the material was for direct land application and the rest to landfill. Now one-third becomes compost, one-third goes for land application, and one-third goes for landfill. Reynolds says that with new storage, the city will be out of landfill entirely within six months.

The composting system involves an in-vessel agitated bed manufactured by Longwood Manufacturing. It includes a forced air system with 12 bins, each 10 ft. wide, 6 ft. deep, and 210 ft. long. Air is forced up through the bins’ gravel floors and compost. A computer controls the air cycle, which supplies the oxygen to keep the piles aerobic and to control the temperature.

The biofilter is the size of 1 ac. Reynolds says, "By the time the air leaves the biofilter, you can stand on the compost and not smell a thing."

What led the city to Longwood? It wanted an in-vessel system because of the small, 7-ac. site. For windrows to work, it would have needed 30-40 rows requiring far more space. Reynolds says, "The boss went traveling and decided on an agitated-bed system. Longwood was the first US manufacturer of this technology and was building units for IPS. The two companies divorced, and only Longwood offered 10-ft.-wide bins at that time. It was sole-sourced to Longwood when built."

Biosolids first go through a treatment plant that produces Class B biosolids (and those can go directly to land application). The biosolids headed for composting are dewatered via four Winkle belt presses, then trucked across the street for mixing with two SSI mixers. The vehicles are articulated electrostatically driven, each with an 18-yd. capacity. They are loaded in the loading area with about four volumes of bulking agent per one volume of biosolid.

For regrinding of yardwaste, the staff uses a Bandit Beast 3680 Recycler. "We mix with a goal of achieving 40% total solids. We use a strain press in the dewatering process so that no plastics or paper or anything else remains in the biosolids."

The mixing vehicles come through the building and discharge directly into the bins. Each loaded bin is agitated five times a week, making detention time about 22 days. During active composting, the criteria the staff must meet is the 41 CFR Part 503. "This is the section of the Federal Register regarding biosolids processing, so we have to achieve the pathogen reduction they require. This includes a temperature of at least 40º Celsius for 14 days. Three consecutive days must be at least 55º Celsius. That's active composting," Reynolds says.

Prior to traveling via conveyor to windrows where it will stabilize for 30 days, the compost is screened to 3/8 in. with a Power Screen 620 trommel. About half the material is oversized and goes back through the process. "We like to have a lot of porosity in our compost pile," Reynolds explains.

He is proud that the city sells finished Class A compost. In the first nine months of 1999, totals included 13,630 yd.³ "We donate about 10% to a creekside-restoration project and school playing-field restoration, city parks, roadway medians, and nonprofits. There's more demand now than product," Reynolds sells to bagging operations, landscape supply houses, and other composters. "Other composters use our product to enrich theirs because composted greenwaste doesn't have many nutrients."

Reynolds admits it’s a really expensive facility. It had a $14 million price tag. "We do look at avoided costs. Right now it costs $24 per wet ton to land-apply biosolids. It costs us about $40 per wet ton to landfill biosolids. Our direct production expenses for compost are $30 per ton. Since we can’t land apply in winter months, compare the composting costs to the landfill expense. So not only are we out of landfill because we have to be, it’s also less expensive to compost."

Reynolds feels the compost system is a good fit for the city, considering the regulatory issues and environmental concerns in his county. "For sure it’s appropriate for everyone to recycle biosolids." In addition, five California counties have outlawed the land application of biosolids, and additional counties are considering severe restrictions. "We have other options in case we lose the ability to do land application here through restrictions or higher costs."

Composting in Europe

Walking-floor conveyors collect compost for stabilization

Composting ranks high among waste management practices in many European countries. Perhaps it is a perceived scarcity of land that leads to a more conservationist approach to materials management, but better collection and separation procedures and an interdisciplinary approach to their various wastestreams are important elements as well.

Oley Sheremeta, a composting operations and management consultant with the TransAlta Energy Marketing Corporation in Calgary, AB, thinks highly of Sorain Cecchini Techno (SCT) of Italy, which composts 1 million to 5 million tpy of MSW. "Its turning system has a huge advantage over all others. It allows a more uniform temperature gradient regardless of the turning and adds less energy into the pile so it restores and monitors pile porosity."

Gillespie of DK Recycling brought us up to date about composting in Baden-Baden, Germany, a country with few landfills and therefore used to composting. Franz Vogel directs composting operations at two sites about 7 mi. apart from each other. Vogel has approximately 2.5 million Deutsche Mark (DM) invested. The capital is his, and the loans are secured by his long-term contract with the City of Baden-Baden. As part of the land-lease contract, Vogel accepts materials from the city at no charge. Others (and there are many) are charged about $17/ton for horse manure and brush, $60/ton for stumps, $35/ton for hay and straw, and up to $100/ton for unclean (rough) material. Tipping fees at the neighboring household-waste plant run in the $120/ton range.

Feedstock includes about 90,000 tons comprising 75% green yardwaste (mostly brush and leaves, very little mown grass); 10% hay, straw, and manure; and 15% residential kitchen waste from the nearby municipal biosolids process plant. That plant houses one of the newest and most sophisticated digesters in Europe and came with a $2 million price tag. This first-stage processing plant separates all foreign material, such as metal and glass, from the raw waste, then washes the remaining material in very hot water to remove soluble fats and sugars. A large centrifuge removes excess water, and the "clean" MSW arrives via truck to Vogel’s plants, where he incorporates it with the other feedstock in windrows.

Concrete walls surround the actual composting part of each site, acting as push walls as well as some measure of security. Channels underlie the surface of the composting area and carry air, provided by blowers mounted on the outside of the concrete walls, to the composting material, which is in a trapezoidal block or "brick" approximately 120 ft. long, 10 ft. high, and 25 ft. wide. Each brick touches its neighbor to fully utilize the area. A Willibald turner turns each brick only every 30 days, always moving in the same direction to ensure that the oldest material reaches the final composting area in 180 days. There it is screened to 10-, 20-, and 30-mm sizes and set aside for outdoor curing.

Even though the sites sit in a region world-famous for its health spas, odor has never been a problem. The asphalt surface is graded for drainage, with all surface water directed to a center line drain and then carried to a collection basin for reuse.

Gillespie says, "The air control system is quite precise, controlled by a computer that monitors every block of material for temperature and oxygen-water content on a constant basis." Computers maintain oxygen levels at 18% and water at 55% and hold the temperature at 160º. When oxygen levels fall below 18%, the computer turns on the appropriate blower(s) until it measures 18% once again. This system operates 24 hours a day, seven days a week, all year long.

Vogel creates about 40,000 yd.³ of product annually, and it all sells at prices ranging from about $12 for 1 in. screened to $26 for 3/8 in. screened per yard. Forty-pound bags sell for about $6 in compostable paper sacks. Vogel also sells many mixes at prices ranging from $22 to $60/yd. All prices then have the 16% value-added tax tacked on.

Gillespie points out that Vogel has worked as a consultant in Bremen, Milan, Tel Aviv, and Saudi Arabia, among other places. "Maybe he and I will do one here in the United States," Gillespie says. "The closest thing I know here to his technology is what Jeff Gage has in Washington."

SCT’s Perugia, Italy, plant, now 20 years old, takes up to 600 tpd. In this ancient town, SCT markets compost to vineyards and golf courses. "We have had 20 years to develop a market," says Alberto Carrera, the company’s managing director. "MSW compost is harder to sell to the public because of concerns over heavy metals."

SCT includes MSW paper in its dynamic composting process (which is low in contaminants as a result of the widespread use of environmentally friendly inks). The paper and fibers are flaked by the augers into very small pieces, and they end up with a reduced heavy-metal content. "Although paper lowers contaminant levels, like heavy metals, the compost is still not the same quality as product made from source-separated waste." Again, Carrera emphasizes that even though the municipalities want a good product, their first goal is landfill diversion, which reduces leachate and methane-gas generation as well as rodent and bird issues.

SCT currently has three sites in Rome and another is under construction to handle source-separated food materials. Carrera says, "Rome has 5,000 tons per day of waste. By 2001, we hope to have the capacity to handle all of Rome’s waste."

Currently Rome has 400 wet tpd of biosolids that now goes into landfills. "Our target is to mix it and be able to handle it in Rome as in Edmonton, Canada."

Presently 70% of SCT’s Roman compost ends up as landfill cover, 30% as compost for use in vineyards. Tipping fees at plants in Italy range from $35 to $70/ton.

Europe is ahead of the US in prohibiting raw MSW from landfills. "In 2000 in Italy, only presorted MSW could go into landfills. Every government in Europe is headed in the same direction," Carrera says.

What's Up With Digesters

A forced air system with 12 bins - each 6ft. high, 10ft. wide, and 210 ft. long. Note temperature sensors in bin walls.

Europe offers expertise and insight on how to approach composting from a completely different angle - in-vessel anaerobic composting. Although anaerobic trial sites are underway in America, including Delaware Solid Waste Authority CEO N.C. Vasuki’s two test cells now in their 10th year in Delaware landfills, the tests do not include in-vessel digesters. Aerobic composting requires air. With anaerobic composting, air must be removed. Nature creates anaerobic conditions on its own. Consider the bottom of sediment lakes and ponds, swamps, peat bogs, or the forgotten bag of lettuce in plastic at the back of a refrigerator. Such "natural" processes are slow. Modern in-vessel anaerobic processes are lightning-fast in comparison.

Steinmuller Valorga (SV) of Germany developed its proprietary anaerobic digestion process and in 1982 began test plant operation in France. With four European plants now under its belt and four additional facilities under design and construction, SV has solid experience with anaerobic composting. SV’s newest plant in Freiburg, Germany, is located next to an industrial park and immediately adjacent to a new restaurant (see Editor’s Comments in the September/October 2000 issue of MSW Management for a description of the facility).

SV was involved in the design, permitting, and construction of the plant and the training of the operating staff. The company designed the facility to process source-separated household organic waste (biowaste). The site represents the state of the art of the SV process and incorporates air locks for all trucks entering the facility to dump waste or pick up compost. This is SV's first facility to operate its anaerobic digester in the thermophilic temperature range, guaranteeing to meet all protocols for the destruction of harmful pathogens.

Capital cost of the facility reached $10 million. The plant equipment includes (1) a biowaste-preparation unit: receives waste, removes metals by magnetic separation, and handles size reduction; (2) an anaerobic digestion unit: dilution and mixing of biowaste, pumping into 1- x 4,000-m³ digester, biogas buffer storage, compression and stirring system, digested matter extracted by gravity. Digestion performance occurs under thermophilic conditions, 50-55ºC; (3) digested material treatment: mechanical dewatering of digested matter, suspended solids removal of resulting sludge by flocculation-filtration. A part is used for dilution of the incoming waste. Excess process water is decanted and treated to reduce both chemical and biological content before it is sent to the local sewage treatment plant; (4) an aerobic postcomposting unit: digested material is matured aerobically in a 1- x 300-m³ tunnel by circulating air for two days to produce fresh compost for local farmers. This compost is screened to remove remaining stones and plastic; and (5) an air-treatment unit including: extraction of foul air, ammonia absorption unit, and biofilter.

The facility, which opened in April 1999, produces about 15,000 metric tpy of fresh compost used by local farmers directly on its fields and about 3 million Nm³/yr. of biogas, which is delivered to a gas-powered electricity-generating station located next to the facility. Annual throughput capacity ranges from 30,000 metric tpy (126 tpd) to 36,000 metric tpy (153 tpd).

The facility owner, who is also a waste-hauling contractor operating in the City of Freiburg, built the plant to conform to German laws that require that by 2003, any waste going into landfills cannot contain more than 5% biodegradable organic matter. Tipping fees charged at the plant by the owner for his own trucks and for waste delivered by other hauling contractors cover the overall cost of collection, transportation, and processing of the source-separated material and the ultimate disposal of inert materials remaining after passing through the entire process. We were unable to obtain exact tipping fees.

Residence time in the plant includes 20-30 days for the biowaste and 15-25 days for mixed waste. Compost aeration takes 12-24 hours. Fresh compost is stored for two to five days.

Currently there is no profit on the compost, which is free to farmers and others. The owner intends to charge 5-10 DM/ton at a later date. A subcontractor receives the biogas and operates the generators under his own responsibility. Profit is calculated per Nm³ transferred to the generators and is paid directly to the owner of the Freiburg plant, BKF Biogas und Kompostbetrieb. The sole owner and shareholder of BKF is Company Meier-Entsorgung (which collects waste for the City of Freiburg and the surrounding area). It is a privately owned, midsize company with about 25-30 million DM turnover per year.

Steve Morris of Waste Recovery Systems, SV's US affiliate, says, "The Freiburg plant represents the state of the art of the SV anaerobic digestion process." He explains how it is impossible for the enclosed arrival trucks to emit odors, dust, or litter and how they enter a delivery pit through a double door air lock. Once the trucks dump their waste, the inside door closes and the air is evacuated for three minutes. This exemplifies the lengths taken to keep odors from reaching even one neighbor's nostril.

Ten-horsepower aeration blowers. Each bin has five aeration zones; each zone has a blower.

Why is it taking so long for successful anaerobic compost digestion to reach American shores? Flow control, guaranteed waste source, and costs remain issues. Morris says municipalities that own their own MSW even after it’s picked up (thus offering a guaranteed waste source) could specify tipping fees in any contract with private entrepreneurs like Waste Recovery Systems. He adds that it’s less costly to acquire anaerobic composting equipment in the US, so tipping fees here could be far less than in Europe - perhaps around $45-$50/ton.

"Laws in Germany are very strict regarding waste disposal. Whatever it costs will be paid, and that leads to processing by composting." With Germany's 2003 landfill law right around the corner, "Cities have to find ways to dispose of the vast majority of their biodegradable waste.

Morris says 55% of the biogas produced inside the enclosed, anaerobic vessels is methane and 45% is CO₂. If the gas is used to produce electricity or steam, there’s no need to take out the CO₂. If it’s to be used as compressed natural gas (CNG) for auto fuel, the CO₂ must come out. "In Los Angeles and Brooklyn, New York, there are some public vehicles on CNG," Morris adds.

Clearly in the last decade, science, engineering, and individual innovations have made it more and more plausible to turn what most call "waste" into "black gold." Science can surmount odor issues, and municipalities might want to exert more muscle to handle flow-control issues. As the richest nation on earth ever to exist, perhaps we need to rethink the last hurdle: costs. Instead of creating landfill boils, we have the know-how and, increasingly, the environmental incentives to nourish the earth. Looking into the future, some things are worth every penny.

Katherine Holden writes on composting and anaerobic digestion for MSW Management.

MSW
January/February 2001