Five years ago, this island town faced an expensive solid waste handling crisis. Today it has an 86% landfill diversion rate.

By Janice Kaspersen

Closing the Old Landfill
Recycling
The Heart of the Matter
How the Digester Works

Nantucket Island lies 30 mi. south of Cape Cod off the coast of Massachusetts. About 10,000 people live year-round in the historic maritime town, but summer tourists swell the population to more than 50,000. The island’s popularity has strained the infrastructure, especially its ability to handle solid waste. Its unlined landfill, created at the end of World War II, was by 1996 overflowing, plagued with seagulls, and under a state mandate to close.

Nantucket had long been searching for alternatives to improve its environmental and economic future. Expanding the landfill, even before Massachusetts ordered the closure of all unlined landfills, was never an option because the site is surrounded by fragile wetlands. Shipping the island’s waste to the mainland for disposal would have quadrupled islanders’ trash bills. State law also hampered some of the town’s early efforts: In the late 1980s it explored composting, but the high cost, along with Massachusetts Proposition 2½ (a state law that prevents municipal budgets from rising more than 2.5% per year), made it too expensive an option. At the time, recalls Nantucket Public Works Director Jeff Willett, "Our budget was $340,000, but the landfill required two to three times that to be run properly. Under Proposition 2½, our budget couldn’t go up 200%, so we were found in violation."

By the end of the 1980s the situation was critical, with material from the landfill leaching into the surrounding wetlands, threatening both wildlife and the island’s drinking-water aquifer. Finally, in 1996 the town opted to privatize waste-handling operations. Nantucket signed a 25-year agreement with the Warwick, RI - based Waste Options Inc. to build an integrated solid waste disposal system encompassing landfill cleanup, recycling, and composting.

One of the most ambitious parts of the plan was the building of a co-composting plant with a steel drum digester, based on proprietary technology developed by Swedish scientist Eric Eweson. This in-vessel composting system accepts organic wastes and dewatered biosolids, turning them into marketable compost.

Even before the digester went on-line in December 1999, the aggressive recycling and diversion program had increased the island’s recycling rate from 17% to 42%. Today the combination of composting and recycling results in only 14% of MSW generated on Nantucket going to the landfill.

Closing the Old Landfill

A new, double-lined cell accepts the small fraction of waste that cannot be recycled or diverted. This waste consists largely of plastic bags and such unrecyclable construction and demolition debris (C&D) as asphalt shingles. Waste Options is installing gas venting and burners in the old landfill, capping it, and topping it with compost from the island’s new composting facility. Landfill mining will take place in the winter months, with material taken from the old landfill going into the digester. "The state required that we mine the old landfill so there’s no net gain in materials that are being landfilled on the island," explains Waste Options’ Nelson Widell. "For every cubic yard of waste we put into the new lined cell, we take an equal amount out of the old cell and bring the height down over the next 25 years."

"Instead of just forgetting our past, we will clean that up as well," says Nantucket Selectman Tim Soverino. "All of this is for a contract price of $90 per ton, which is very competitive compared to other communities."

Recycling

Nantucket has a newly built materials recycling facility (MRF), which Waste Options agreed to run as part of the agreement. Commercial haulers and private citizens bring recyclables - cardboard, newspaper, magazines, mixed paper, mixed plastics, steel, and aluminum - to the MRF, where the materials are baled, stored, and shipped to a number of outlets on the mainland. Glass is collected separately at two designated areas (commercial and noncommercial) and used as beneficial daily cover for the landfill, a use Massachusetts counts as recycling.

Several additional areas accept almost any kind of waste discarded on the island. At the Take It or Leave It (TILI) exchange, residents can drop off or pick up used clothing, shoes, appliances, and other goods. Unclaimed items are sent to the proper venue for recycling. A hard-to-manage waste trailer accepts such bulky items as mattresses, box springs, rugs, and sofas. These items are shipped to a merchant in Hyannis who removes cotton, wood, and other components to be recycled and reused. Used tires collected at the tire staging area are sent to recyclers on the mainland; more than eight barge loads of tires have been shipped off the island so far. Yardwaste is delivered to a tree and brush chipper area. All metal items are sent off the island for recycling; a technician at the MRF first removes Freon from appliances. Even Styrofoam packaging peanuts are collected and distributed to local shippers for reuse.

A C&D processing building holds a typical week’s worth of C&D materials. Furniture, old boats, and the like also go this route. Usable wood is removed and offered to citizens at the TILI. Clean construction wood and pallets go to a chipper; the chipped wood later finds beneficial uses such as landscaping. Concrete, rubble, and stone are stockpiled for reuse; dirt and fill materials from construction sites are screened and the components sent to the proper areas for metals, wood, sand, soil, or bricks. Only nonrecyclable C&D wastes are sent to the landfill.

The Heart of the Matter

The principle behind the Eweson composter is simple: It speeds the decomposition of organic matter. Although metals, glass, and other recyclables can be removed from the wastestream, the organic matter usually sits in a landfill and slowly decomposes, releasing gases and leachate as it does. The Swedish scientist and inventor Eric Eweson (pronounced ai-vuh-son, 1897-1987) envisioned a method of removing organic matter from landfills and accelerating its conversion to compost. As trash and sewage sludge are fed into the microbe-rich environment inside the digester, the rotating drum increases their exposure to the microbes. "In the digester we create optimal conditions for decomposition, a situation that in nature rarely occurs," says Widell, who worked with Eweson and who, since Eweson’s death, has helped license the technology worldwide. "We are actually speeding up Mother Nature. We can’t do this better than her, but we can do it faster."

The composting facility was constructed by the Boston-based Stone & Webster Engineering Corporation. SECOR International Inc., an environmental engineering, consulting, and construction firm, helped with permitting for the project, and KeyBank of Portland, ME, provided financing.

The compost produced at the facility exceeds both Massachusetts and Environmental Protection Agency standards and even meets the stricter European standards for levels of heavy metals. There is a market for it as well: The volume of compost produced at Nantucket fills only about 20% of the demand for organic material on the island, with most of it used for landscaping.

How the Digester Works

The heart of the Nantucket Compost Plant is a multicompartment steel rotary digester, 12.5 ft. in diameter and 185 ft. long. The concept was developed by the Swedish scientist Eric Eweson; the technology is licensed from Bedminster Bioconversion Corporation in Bensalem, PA. The digester uses biological processes to turn MSW and dewatered biosolids, or sewage sludge, into compost. Nantucket haulers deliver MSW and sludge by truck to a tipping building, where plant operators visually inspect for unacceptable materials. Solid waste must be delivered in clear plastic bags to speed inspection. A conveyor transports both solid waste and sludge to the digester, and a hydraulic ram loads them into the digester drum, where they remain for three days. Air and water is added as needed to speed biological digestion as the drum completes one-half to one full rotation per minute. After three days, the mixture is discharged from the digester and conveyed to a trommel screen. Undersized material ("unders") less than 1 in. in diameter passes through the screen for processing as compost. Oversized material ("overs") is conveyed to a baler for landfill disposal. A front-end loader removes the compost from the trommel screen area to the floor of the aeration building. A computer-controlled system measures temperature and regulates airflow through the compost material, and a timer-based spray system maintains optimum moisture. The material remains on the aeration floor for approximately 30 days, turned daily with a front-end loader, before moving on to the final screening area. The compost is loaded into a hopper that feeds a belt conveyer, which discharges the material onto a vibrating screen. Here the overs (greater than 3/8 in.) are conveyed to a compactor; compacted material is transported to the landfill. Unders that have passed through the screen are conveyed to a destoner to remove glass, stone, and other heavy particles, which are either landfilled or put through the digester again. The light portion of the compost is mixed off-site with sand, peat, and bark and is used by nurseries, greenhouses, and landscapers. An essential function at the facility in this tourist-supported town is odor control. The tipping and aeration buildings are maintained at a slightly negative air pressure to prevent odors from escaping, and a biofilter, located in a separate building next to the aeration building, further controls odor.

Janice Kaspersen is the features editor for MSW Management.

MSW
Nov/Dec 2000