Operators say their number one contamination problem is plastic -
small, large and in-between - in the compost feedstock and thus in the end product. New study and equipment offer some excellent solutions.
PRESSURE in Europe to increase the diversion of organic residuals to composting led to a study that focused on the best ways to generate finished product with minimal plastic. The study, published by Entec UK Limited, specifically targeted municipal yard trimmings delivered in plastic bags as well as packaged food waste derived from the retail and manufacturing sectors. Under the Biowaste Directive of the European Union, the alternative to landfilling is that feedstocks be composted or anaerobically digested. The Entec study notes that a paradox arises when demand for convenience foods makes use of “ever more sophisticated and attractive packaging” that must be removed either manually or mechanically from the biowaste to be diverted from disposal.
In the Entec report, “Cleaning Plastic Contaminated Compost” (August 2004), a section on density separation explains that air separators are designed to convey heavy materials to a blower system, while the lighter fraction is collected from the primary stream. The separators include aspirators where the difference in density of plastic and organics should allow for effective separation, especially when compost particles have different flight characteristics than plastics. Vibrating feed aspirators can provide a more even distribution, increasing dispersion and surface area contact. Trommel air separators can push the lighter airborne plastic through the trommels to air collection components. It has been found that plastic separation is most effective when a high torque, low speed shredder has been used for size reduction of feedstocks.
The report described several vibrating feed aspirators that are used for plastics separation at compost facilities. These include the Screen-Aire unit from Forsbergs (Thief River Falls, Minnesota), which screens fine materials, with the plastic overs conveyed to an aspirator where lighter contaminants are lifted off; and the Fines Floater made by Triple/S Dynamics Dallas, Texas, a vibrating deck conveyor that is perforated to allow air to flow upwards through the material.
MANAGING PLASTIC WRAPPED INDUSTRIAL FOOD RESIDUALS
The Entec report includes a description of operations at the Alameda County, California green waste processing facility. The 475 tons/day project “often receives industrial food waste contained in plastic packaging,” notes the report. The packaged waste is ground with the green feedstock, then composted, and the contaminants are subsequently screened out through a trommel with 3/8-inch to 5/8-inch openings. “Plastic would 'float' through the trommel and be ejected with the rest of the overs onto a belt over which was placed an air knife,” describes the report. “The plastic would be 'sucked up' by the air knife, leaving the remaining overs clean enough to be used as a fuel or returned to the start of the composting process for use as a bulking agent.” The method works - verified when 23 tractor loads of ice cream were delivered in polycoated cardboard boxes and the plastic contamination was successfully screened.
The study findings are optimistic that plastic can be removed effectively from compost and screen overs - provided initial size reduction doesn't shred film and rigid plastics to bits. Notes the conclusion: “Initial shredding or particle size reduction needs to be designed to ensure the plastic film and rigid plastic remain a reasonable size while exposing the organic fraction sufficiently to the degradation process. Rather than using a system that cuts material into small pieces, the shredding option should focus on ripping technology, where material is pulled apart, thus maintaining large plastic and paper fractions to enable easier segregation at the post-composting phase.
REMOVING FILM WHEN COMPOSTING YARD TRIMMINGS
In Eugene, Oregon, Oren and Susan Posner operate Lane Forest Products, composting yard debris since 1994. The company processes over 70,000 cubic yards of green and wood waste annually. But removal of film plastic had been a steady challenge, until the company developed a unit - known as the Airlift Separator and made by the Hawker Corporation - that can be attached to the conveyor outfeed belt of a screen. The assembly sits on a 4-foot by 4-foot pallet-sized footprint and can remove about 95 percent of plastic in the compost or mulch on the first pass, according to Susan Posner.
“We blow a lot of mulch, so even the tiniest bit of plastic is quite apparent,” she explains. “We developed the unit to answer that problem, and it works on a variety of different screens as long as it's attached to a conveyor belt. It also takes out plastic bottles and aluminum. We agitate the conveyor belt, which loosens the plastic from the organic fraction and doesn't slow down production.”
The plastic removed can be blown into a drop box when large volumes of material are processed. For smaller amounts, a tube sock is attached to the fan assembly and catches the plastic as it is blown off. A facility can determine the length of the sock needed and cut it to that size (the end is tied). The flow of air compacts the plastic particles into the sock. When full, the bag is removed, emptied and can be reused. The actual amount of plastic removed depends on the moisture content of the compost and the processing speed of the screening plant.
Grover Landscape Services, based in Modesto, California, operates two composting facilities and a wood mulch yard. The composting sites receive over 1,000 tons/day of green waste; the mulch facility gets over 300 tons/day of wood waste. Plastic contamination in the incoming loads is a persistent challenge, and one that Grover Landscape has addressed through a variety of equipment purchases. Several years ago, it invested in two Farwick Hurricane air separation units that it runs in combination with Wildcat trommel screens to remove plastic from screened compost overs. More recently, Grover Landscape purchased two Airlift Separators, which are hooked up to the conveyor belts of two CEC shaker deck screens at the wood mulch yard. “We process construction debris, whole trees, chips from tree trimming companies, roof tear off and other wood waste,” says Jake Oasterman of Grover Landscape. “The Airlift units are able to remove plastic from the wood coming off of the screen.” Plastic particles are collected in drop boxes. “Right now, that is going to the landfill, but we just had a meeting today to explore recycling markets for plastic in China,” he adds. - J.G.
“Cleaning Plastic Contaminated Compost” (Entec UK Limited) can be accessed at the Entec website (www.entecuk.com). Information for equipment mentioned can be found at www.airliftseparator.com; www.forsbergs.com; and www.sssdynamics.com.