From BioCycle Magazine
August 2000, Page 40

OPERATOR STRATEGIES
SORTING OUT THE PLASTIC
Experience is showing that with aggressive upfront sorting and optimizing the screening process, film plastic can be sorted successfully at composting sites.
Nora Goldstein and Dave Block

Plastics are, without a doubt, the organics recyclers’ Catch-22. They remain, for many types of generators, a convenient collection container, and for others, like grocery stores and restaurants, something too costly to sort out. So processors are faced with the tough choice of not accepting as much organics as might be available from a source, or dealing with the plastic at their recycling or composting facility. Use of plastic products made from biodegradable polymers is a good step in the right direction, especially for programs diverting organics from households, restaurants and institutional food service programs. In the case of grocery stores, however, the problem has more to do with compostable food prepackaged in plastic, such as precut vegetables, salads and breads.

Some processors, especially composters, will not touch plastics with a ten-foot pole, either because they don’t want to take a chance at compromising the quality of their finished product, aren’t able to invest in equipment to separate out plastic, and/or don’t want to deal with the litter and other nuisance factors. Other processors, especially ones who want to service generators or municipal programs that have plastic tied up in some way with their organics, are learning how to optimize recovery and minimize contamination and other negative effects.

SORTING IN SUMTER COUNTY

The Sumter County, Florida materials recovery and municipal solid waste composting facility receives almost all of its feedstocks — about 80 tons/day — in plastic bags (actually, often several layers of plastic bags, e.g., a bathroom bag inside a kitchen bag inside of a 30 gallon trash bag). “Film plastics, by weight, are six percent of what we get in,” says Terry Hurst, operator of the Sumter County facility. “Other plastics — not film but also not recyclable — account for three percent by weight.” The facility also gets in about three percent HDPE and PET, and one percent of the other grades of recyclable plastics.

The front-end of the plant has a materials recovery facility (MRF), where recyclables are sorted from the mixed waste stream. After sorting, remaining materials are loaded into a rotating drum to begin the composting process. Feedstocks move through the drum over a three-day period, then are passed through a trommel screen. The unders are composted, then go through a final screening.

There are three primary points in the overall processing layout where plastics are removed. The first is as part of the MRF. “We start dealing with plastic at the first manual sorting station, which is the oversized bulky waste sorting station,” explains Hurst. “Cardboard, noncompostables like carpeting, stringy plastic (e.g. hothouse plastic from growers) and other materials are pulled out. Immediately following that station, the garbage bags go into a Bulk Handling System bag breaker. It is essentially two drums with spikes that rotate. The spikes pull the bags apart and drop the contents onto a conveyor that goes to the main film sorting station.”

Hurst has found that the bag breaker is efficient opening the 30-gallon bags, but the smaller bags tied up and stuffed inside of the larger bags do not always get opened. “When those bags reach the film sorting station, workers have to open them by hand,” he says.

There are four to five people who work at the film sorting station, pulling only film plastic. They remove in excess of 1,000 lbs/hour. “This is the point where the majority of the film plastics get pulled out,” notes Hurst. “The plastics are baled in a vertical baler and disposed off-site.”

Past that station, material passes through an overhead rotating magnet, a disc screen, and then goes to the main sorting line. There are 14 sorting stations, seven on each side, and recyclables are sorted according to market demand. At this time, for example, the market is favoring sorted PET (#1) and HDPE (#2) plastics, and there isn’t a market for grades three through seven. Other times, #1 and #2 plastics can be commingled, along with the other grades.

Hurst estimates that 60 to 70 percent of the film plastic is pulled out between the film plastic and main sorting stations. “A fair amount still gets into the digester because we see it when that material is screened before the windrow composting phase.” Sumter County has a Powerscreen trommel with two-inch screens. Overs from the screen drop into a 100 cy walking floor trailer and are taken to off-site disposal. “We’re getting the bulk of the plastic out with that trommel,” he adds. “The tumbling action in the digester does cause some breakdown in particle size of the film plastic, but after screening, the amount left in the compostable fraction is pretty minute. It’s about one percent by weight going into the windrows.”

GOOD COMPOSTING EQUALS GOOD PLASTICS REMOVAL

The final plastics removal point is a trommel with a three-eighth-inch screen size, used for screening the finished compost. When it comes to removing plastic at this point — a critical “moment” in terms of compost marketing — Hurst has learned that the most important element is optimizing the composting process itself. It starts with getting the right mix of MSW, nitrogen source (biosolids are used primarily) and moisture content. “Everything is relative,” he says. “If we do a good job with the compost recipe — adding the right amount of nitrogen — and don’t overload the digester, we will get accelerated breakdown of the organic material, which improves separation after the first screening. If for some reason the recipe is mixed up, e.g. if there is too much moisture or not enough nitrogen source, there is a visible effect on the product coming out of the digester and thus the effectiveness of the screening. As such, we check the recipe daily in each of the three compartments of the digester. Samples are taken from each compartment and checked for moisture. Basically, our recipe is that for every two tons of sorted MSW, we put in one ton of biosolids or other nitrogen source to bring the C:N ratio down from 50:1 to 25 to 30:1. If we see the moisture level isn’t right, adjustments will be made to the recipe going into the digester.”

Moisture content also is closely monitored during the 15 days of windrow composting. “If material doesn’t go into the windrows at the right moisture content,” he adds, “we will see more organic matter coming out with the plastics on the three-eighth-inch screening. We also can pretty much tell the recipe is right if temperatures are where we need them for accelerated decomposition.”

Moisture content is measured prior to screening as well. If batches are too wet, operators have two options — run the Scarab turner through the windrow to dry it out, or feed the trommel screen more slowly. If the moisture content is too high, it will clog the screen, and more organic matter goes out with the overs, even with the clean-up brush running along the outside of the trommel. Screened compost is used for county public works and landscaping projects.

Ultimately, says Hurst, Sumter County would like to start a program that encourages use of paper bags instead of plastic. “Plastic is entirely incompatible with our system and paper would suit us ideally. For a start, we are looking into obtaining some funding to work with the main food vendors in the county to get stores to bag groceries in paper instead of plastic.”

TROMMELS DOING THE JOB

Across the country in California, composting consultant Matthew Cotton of Integrated Waste Management in Nevada City, California, has found that composting facilities servicing institutional, commercial and/or industrial generators need to be set up to deal with plastics, no matter how much source separation is worked into the diversion program.

“Educating generators and collectors is the first and best line of defense,” he says. “Still, you’re going to get some contaminants.” For example, although residents served by the City and County of San Francisco’s organics collection program are instructed to use paper bags or newspapers, workers still must go through material and pick out whatever plastic they find.

In Cotton’s experience, a trommel is an effective tool for plastics removal, especially if it has a cyclone — a pneumatic separator that vacuums off lighter material. “Even the most sophisticated composting sites don’t spend lots of money on advanced plastic removal technology,” adds Cotton. “They usually find that trommeling is enough. The only problem is that whatever you’re sifting out is removed by size, so you’ll get overs mixed with the piles of plastics. Most sites wind up landfilling that material, although some use it as alternative daily landfill cover.”

BIODEGRADABLES ROUTE

Another option for some composters with plastic bag woes, especially those handling food residuals and yard trimmings, is to wholeheartedly accept bags — as long as they’re biodegradable. Alan Chappell of Environmental Concepts and Designs in Battlefield, Missouri has tested and worked with biodegradable bags at a number of operations over the past three years (see “School District Supplies Organics To Commercial Composter,” page 57). For example, he foresees them having an expanded role for a composting client that will take large volumes of yard trimmings from Sedgewick County, Kansas when they are banned from disposal in October, 2001. “Hopefully, through education and being able to offer biodegradable bags on site, we’ll have a lot more control in keeping plastic out,” says Chappell.

He adds there haven’t been problems with biodegradable bags degrading before reaching the composting site, or not breaking down during composting. “They get put through without any change in materials handling,” he explains. “The turner breaks the bags open as part of normal operation; they don’t need to be separately cut, shredded or processed in any way. In 30 days, you virtually can’t find them.”

Certification according to the relatively new American Society of Testing Materials standards (see sidebar) is essential for Chappell to have confidence in a biodegradable bag. “I don’t have to rerun a test to ensure that we’re not bringing residue into someone’s facility,” he says. “If the product is certified, that means it breaks down and leaves nothing behind.”