The effective management of compost contaminants such as plastics, foils, rocks, and metals is an important aspect in the efficient operation of a compost facility. Measures can be taken to source separate but the control systems available for most common input streams — yard, food and wood waste — are largely dependent upon organic waste generators. Input sources are typically generated through residential collection and it’s difficult to control contamination at the curb.
While traditional front-end options such as hand sorting provide benefits, critics say that they’re costly and can only be marginally effective. Back-end screening provides separation for the fine fraction, but there’s the risk of leaving oversized fraction laden with contamination. Over time this contaminated fraction can become burdensome to a facility mainly because of the space it consumes.
Traditionally the contaminated oversized fraction left over from screening is stockpiled and either reintroduced to the composting process or re-screened. Over time this circulation process leads to the incremental increase of contamination levels in the process. Eventually the contamination level may reach a point where the percentage of inert material is so significant that the material is difficult to compost must ultimately be landfilled.
Until recently there were few commercially available options to efficiently separate contaminants at compost facilities. But new air separation technologies are effective at containing plastics and other inert material.
One example is the Hurricane, a secondary-screening device manufactured by Komptech-Farwick in Austria. Since its debut in 2000 more than 100 units have been deployed throughout the United States and Europe.
The technology is used in tandem with a primary trommel, star or deck-type screening device. It uses a combination of vibration, air and suction to remove film plastic from oversized fraction. The unit is conveyor fed by the oversized fraction of the primary screen and is input into a covered vibratory input chute. The vibration from the stainless steel input chute promotes the liberation of the plastic from the organic matter.
The material falls from the input chute through an adjustable air stream. Because of the differences in density, the blower fan separates the light plastic fraction from the organic fraction. The separated plastic travels forward toward the adjustable suction fan while the organic fraction falls onto a conveyor belt. The suction fan extracts the plastic and transports it into a mesh tarp covered container.
The organic fraction then travels under the suction fan without extraction and over a head-pulley magnet for the removal of any ferrous metals. From this primary conveyor the plastic-free organics are dropped onto a high-speed angle adjustable conveyor belt. This conveyor (called a stone trap) removes rock and other inert material from the fraction. The stone trap discharges the clean organic fraction while neatly containing the plastic, ferrous metals and rock fractions. (See diagram.)
A critical element of this technology is adjustable features. Varied inputs, site conditions, weather and geography can cause a varied composition of input material, so it’s important to adapt. For instance the blower fan can be increased or decreased in conjunction with the raising or lowering of the suction fan.
The Hurricane does have its limits. It’s not totally effective at removing small plastic contaminant from the fine fraction of the primary screening device. Because material such as grass and leaves have a similar density to film plastic the unit cannot distinguish between the material. This causes a higher percentage of organic material to be extracted.
Also, when immature product is laden with cellulose, this stringy material may trap the plastic and reduce the efficiency of separation.
The unit is most effectively used at the back-end, final screening stage after the material has been shred, windrowed or placed in-vessel, turned, or after the aerobic or anaerobic digestion process. Optimum operation provides nearly 100 per cent removal of film plastic and less than 2 per cent organic in the light fraction. The unit retails for approximately US$110,000.
“The Hurricane makes good business for three primary reasons,” says Chris Valerian, a principal of Norton Environmental Equipment Co. in Independence, Ohio and the exclusive North American distributor.
“There’s a low acquisition cost, it increases revenue through a cleaner product and a larger clean organic fraction, and it lowers cost of disposal of the oversized fraction.”
Gary Kaufman of Kurtz Brothers in Valley View, Ohio acquired a unit for a composting facility. According to Mr. Kaufman, “The Hurricane has taken a costly waste stream and turned it into a high margin, sellable product.”
Connie Vitello is editor of this magazine.