BioCycle September 2006, Vol. 47, No. 9, p. 27
Oregon green waste composter finds air separation effective in removing plastic from the overs fraction remaining after finished compost screening.

BUD DOW is a do-it-yourself compost entrepreneur. In 1980, he started Dow Brothers, Inc. of Cornelius, Oregon, doing land clearing and site preparation for construction jobs and road building. “We had acquired sufficient site work in the 1990s to justify two Diamond Z tub grinders,” Dow told BioCycle in 2001 (see “To Market, To Market … To Sell Certified Compost,” February 2001). “We were clearing our own projects while getting bids from other contractors to do logging, grinding and hauling wood debris.” Dow Brothers sold ground wood to a newsprint plant as hog fuel for electricity, but the price would fluctuate, leaving the company with excess materials.
To address the wood flow problem – and tap into a business opportunity – Dow and his wife Sharla purchased six acres of land near Cornelius, about 30 miles west of Portland, to open a composting facility. The new company was called Northwest Environmental and Recycling, Inc. (NER). It obtained a general composting permit from the Oregon Department of Environmental Quality, and a license from Metro, the regional solid waste authority. NER received curbside collected yard trimmings from several municipal programs, green waste from nurseries and landscapers, and horse bedding and manure from local stables. It had hoped to begin processing preconsumer vegetative source separated organics from area grocery stores as well. “That part didn’t pan out,” says Dow. “Going through the permitting to process food residuals is equivalent to permitting a landfill. Right now, we receive 100 percent green waste and no other feedstocks.”
It established a green waste collection system using 20- to 30-cubic yard drop boxes to service nurseries and landscapers. “It’s evolved into a pretty good business,” says Dow. “We have a roll-off truck that we used to transport the boxes.” While unsure about the exact volume of materials processed, he notes that throughput has roughly doubled in the past five years. “We have 3-acres that are pretty loaded.”
Materials are composted in large, 15- to 18-foot high windrows that are turned every seven to ten days with an excavator. After roughly six to eight months, the finished compost is screened through a 5/8th-inch trommel screen that Dow designed and built. “The drum is 10-feet by 20-feet, and puts out abut 100 tons/hour,” he says. “About 95 percent of the product comes out at half-inch.”
CONTAMINATED OVERS
While the compost fraction was clean, the overs from the trommel were contaminated with film plastic. As a result, NER was not able to reuse them and had to pay for their disposal. “The overs were far too dirty for boiler fuel and paying for disposal was costly,” says Dow. “That was a lot of material that was getting by us that could have been sold if it were clean enough.”
In his do-it-yourself style, Dow tried to build an air separation system to remove the plastic from the overs. After various attempts, he learned about the Airlift Separator that was introduced to the compost and mulch market by another Oregon organics processor, Lane Forest Products in Eugene. Lane Forest Products began composting yard trimmings in 1994. It processes over 70,000 cy of green and wood waste annually. The challenge of removing film plastic from the overs led the company to invent the Airlift Separator, an air classifer that attaches to the conveyor outfeed belt of a screen. The assembly sits on a 4-foot by 4-foot pallet-sized footprint. The unit can be powered by a hydraulic, diesel or electric engine. The outfeed belt vibrates to loosen plastic from the organic fraction. A vacuum pressure pulls plastic as well as other lightweight contaminants off the belt, and directs the discharge tube into a reusable filter sock that can be emptied as needed. For higher volume operations, the discharge tube can be directed into a covered container. The unit is marketed by Hawker Corporation, a separate division of Lane Forest.
“We installed the Airlift on the overs belt,” notes Dow. “It allows us to inspect the overs and see if it is working properly. We estimate that the air separator is removing about 90 percent of the plastic. Now that the overs are clean enough, we are reintroducing them into the compost mix to act as an inoculant. We just keep recycling them until they break down.”
PRINCIPLES OF AIR CLASSIFICATION
Air classification technology is used in a variety of industries to separate light and heavy fractions of materials. Essentially, an air current separates materials according to their density. The size and shape of the particles also are important factors. An article in BioCycle, “Air Separation Strategies Tackle Plastics Contamination” (September 2001), reviewed some fundamental principles of air classification. It noted that there are two separate applications of air classification technology being used for compost and mulch production. The first is strictly air separation, targeted at separating film plastic from a compost product. The second is destoning, which separates the light (film plastic) and heavy (glass, stones) fractions from the compost product through a combination of air flow and mechanical vibration of material. The same air classification and density principles are used in both cases.
The steps in an air separation process can be illustrated by a conventional vertical air separation system. Mixed materials are fed into a chute with an upward-flowing stream of air generated by a blower. The light materials are carried with the air; the heavy materials fall down into a bin or onto a conveyor belt. As the air stream continues with the light particles entrained, it enters into a cyclone, where the air velocity slows, causing the heavier particles to settle out. (A cyclone is not an absolute requirement to collect the light particles.)
Another variable is the direction and mode of airflow. The air can move as a result of a vacuum, with a fan above the feed, or by pressure, with the fan below the feed, or both. The air stream can be horizontal with the particles carried along a conveyor or screen. The split between light and heavy materials depends on the air velocity, which in turn depends on the air flow rate and the size of the air channel. Higher velocities are needed to remove heavier particles. However, if the velocity is too high, the air column would carry a high percentage of heavy particles along with the light fraction.
Air classification generally has not been effective to remove plastic from the finer screened compost product. These systems can have difficulty differentiating light plastic from light compost. For example, a study on an air classification system known as the Zincot Separation Process was conducted at a yard trimmings composting site in the Toronto, Ontario region operated by Miller Waste Systems. Trials were run on trommel screen overs, unders, and unscreened material straight from a windrow. With the unders fraction, the report states that the air separator removed both film plastic and fine compost particles as a result of their similar specific gravities. The fine compost particles burdened the effectiveness of the air separator and caused buildups on the ductwork and the fan assembly in the system. These buildups (even at high duct scour velocities) prevented the continuous operation of the system and quickly deteriorated its plastic removal efficiency. N.G.