As municipal waste management strategies evolve to incorporate more kitchen and yard organic materials, automated processing equipment is becoming more specialized. With wide acceptance of plastic bags for organics collection, de-bagging or polyethylene (PE) separation systems are in demand.
A recent survey by the Composting Council of Canada (1999) continues to illustrate that the use of clear plastic bags is the predominate method of curbside placement of residential organic material in Canada. Dave Douglas, environmental specialist with The Clorox Company of Canada Ltd., says, “As such programs continue to expand and evolve, so do the automated processing systems that compliment the process.”
In 1998, Toronto, Ontario-based First Brands (Canada) Corporation (now the Clorox Company of Canada Ltd.) entered into an investigative partnership with Markham, Ontario-based Miller Waste Systems to study North American separation practices for PE film from organic feedstock and compost material. The different approaches can be divided into two groups according to the size and characteristics of the plastic and organics: front-end and back-end.
A front-end separation system is designed to open, empty and remove the plastic collection bags prior to the decomposition process. Bag openers, bag bursters or bag slicers have been used in the recycling and municipal solid waste (MSW) industries.
An alternative bag opening technology that has been proposed and developed is a trommel debagger. Trommels were adapted as bag openers because the rotating motion of the trommel could allow hooks or blades to grab, tear and empty the collection bags while screening the organics from them.
Bag openers — Bag openers or bursters leave bags intact for secondary separation. Many of the manufacturers contacted for evaluation are listed in Figure 1.
Bag openers operate primarily to open bags, while emptying the bags depend largely on the weight and size of bag. All the bag openers are capable of puncturing and tearing the plastic bags, however, the sorters can tear and empty bags manually regardless of the amount puncturing or tearing. The speed and effectiveness of the plastic removal determines the amount of the sorters required, and therefore determines the cost-benefit of the bag opener. The study found that mechanical bag openers may replace the labour of several manual operators.
Trommel debaggers — A trommel is a cylindrical drum screener that conveys material. As the trommel rotates it lifts and exposes material through the screen. Trommels are large, high-powered machines that appeal to large field operations.
Trommels modified as debaggers have blades or hooks bolted to the inside of the drum screen. The blades inside the screen puncture and catch plastic bags as they roll down the trommel. The screen is typically perforated with five-inch diameter holes that allow smaller organics to pass through and separate from the plastic bags and oversized material. Trommel “overs” are large sticks mixed with the large pieces of plastic.
Several companies were contacted, including: Central Manufacturing Inc.; McCloskey Bros. Mfg.; Re-Tech; Scarab Manufacturing and leasing, Inc.; and Wildcat Manufacturing Co., Inc.
Central Manufacturing’s trommel debagger for opening plastic bags was designed for use in a New York paper recycling facility. The trommel is equipped with serrated blades on the inside surface of the screen.
Re-Tech has manufactured several trommel debaggers for an outdoor composting facility in New Jersey. (See photo.)
Essentially, the study found that the trommel system trades plastic separation efficiency for higher throughputs. (The larger perforated screening holes allow larger plastic pieces to fall through the screen holes.)
The two front-end separation systems have shown to have good to excellent plastic removal. The efficiency of the systems is offset by their throughput; higher throughputs result in lower separation efficiency. The need for manual sorters is a large part of the cost-benefits of the front-end systems. The bag openers require sorters to remove the plastic bags from the material, and the opening efficiency determine the number of sorters required, and thus the operating costs incurred.
A common difficulty seen in the front-end separating systems is that brush material can jam the feeders and interfere with the opening mechanisms. Brush enters yard waste facilities in lengthy bundles, and would be difficult to avoid. A primary separation may be required for effective front-end alternatives.
An efficient back-end separation system can distinguish, separate and collect plastic film from organic materials. The size and percentage of the plastic and organics vary depending on a facility’s operations. Conductivity, elasticity and density are properties that distinguish between the plastic and organics.
Back-end separation systems can be placed anywhere after the bagged organic material has been emptied. However, existing technologies have tended to place the separation system after the trommel. The advantage of this is that organics biodegradation reduces the volume of material and the trommel screens the material to create a more size-uniformed mixture. Grinding and turning processes are constantly in motion; the trommel is stationary.
An effective air-separation system depends on air contact, a function of the material dispersion when it enters the separation chamber. The ideal material stream would provide a one-particle-thick feed with no overlap.
Conductivity-electrostatic systems — An ideal electrostatic system charges the plastic film, then separates it by attracting the charged pieces to another charged source. The electrostatic generator should not affect the organic material as it passes through the system. These applications are commonly used in the plastics industry to separate HDPE plastic from dirt, dust or other material.
Manufacturers evaluated included: Carpco, Inc., Hamos USA and W.S. Wood Associates Limited. Representatives indicated that the material size and moisture could impede the separation of plastic film from organic compost, suggesting that the separation could prove more effective by drying and grounding material to a uniform particle size.
Elasticity-crushing/grinding systems — Crushing or grinding equipment is designed to use the force of a rotating hammer device to pulp material through screens. The crushing, in lieu of shredding, reduces the size of organics while leaving the more elastic plastic film in large pieces that are then screened from the rest of the material. A crushing separation system separates the overs material stream or (in a front-end application) opens the plastic bags.
Air density separation systems — An ideal density separation system fluidizes the material stream, using a blower to lift lighter plastic pieces away from heavier organic materials.
Two designs were evaluated: aspirators and vibratory-feed aspirators.
A number of aspirator manufacturers were contacted, including: Lewis M. Carter Manufacturing Co., Inc.; Rader Resource Recovery, Inc.; and Sterling Systems.
Lewis M. Carter Manufacturing Co., Inc.’s (LMC) aspirator successfully handled bulky organic-plastic material. The aspirator has a simple flow design with large openings to accommodate diverse material sizes. An air stream channels the material to a drop section where a vertical air stream blows upward and carries the lighter plastics with it. Heavy material falls through the drop section to a discharge opening.
The study found that an air separation system could experience difficulty with moisture and inconsistencies in the size of the organics. Processing prior to the separation was recommended such as drying and milling the material. The ideal aspirator feed is a one-particle-thick stream with no overlapping pieces. The isolated pieces can then be individually air-classified as light or heavy.
ing-feed aspirators (VFAs) are designed to increase the dispersion of the material stream and to increase the surface area contact of the separating air channels. The vibrating motion breaks up and disperses the materials and also begins a preliminary separation by bouncing the lighter materials to the surface. VFA applications are commonly used in agriculture to clean granular harvest products.
Manufacturers contacted included: Forsbergs Inc., General Kinematics Corporation and Triple/S Dynamics, Inc.
The Forsbergs Screen-Aire is a vibrating screen deck with an aspirator attachment. The plastic-contaminated stream of overs is conveyed into an aspirator where lighter plastic is lifted off. Screening and material dispersion on the vibrating deck allows effective air separation. (See diagram.)
The Forsbergs VFA removed most of the plastic film from both the overs and unders; varying the speed of the upward air streams controls which material type can be separated. However, the demonstration revealed some limitations: the unit is hand-fed; long sticks can clog openings; and, the separated plastic film may clog the blower that moves the light elements to the filter system.
Triple/S Dynamics, Inc.’s Fines Floater is a perforated vibrating-deck conveyer that allows an upward air channel through the material. The deck is enclosed and a blower draws air through the top of the enclosure. The Floater fluidizes material as it travels down the vibrating deck and the air channels lift the lighter materials from the stream to the air removal system. (This model was able to separate most of the plastic film from the overs.)
Trommel air separators — A trommel air separator system pushes lighter airborne plastic via an air stream. Lighter plastic film is exposed by the rotation of the trommel and is carried to the system’s air collection component.
Manufacturers contacted for evaluation included: Church & Throught Inc., Industrial Metal Fabricators (Chatham) Ltd., Minpro International Ltd., Sheridan Equipment and Transdynamics Engineering Limited.
Conceptual designs for the trommel include the attachment of a blower and an air collection system. The blower pushes an air stream through the trommel, picking up and carrying the lighter plastic film to the air collection system.
Organics-plastic separators — Two designs have been developed to remove plastic film from an organic material stream; both use an air density system to identify and separate the materials. Equipment adapted from other industries and applications may have features that don’t typically handle the bulk and moisture of organics-plastic material. Two companies that have developed equipment specifically for organics-plastics separation include Fuel Harvesters Equipment, Inc. and MGL Technologies Limited.
Fuel Harvesters Equipment (FHE) Inc.’s EarthSaver Levijet 2000 is an air-knife classifier and plastic separator developed to remove plastic bags, paper and styrofoam. The Levijet uses a metering device to feed material into an air-knife separation system that applies the same principles as an aspirator; a thin column of perpendicular air lifts the lighter material from the flowing stream of material. The air is then drawn into a blower and an air filter system.
The study found that the Levijet had moderate to good plastic separation, even though the model has not been designed to handle the throughput and erratic feeding of the trommel. However, the study found that Levijet did not have a feeding system and could not disperse the material entering into the air separator. Even at reduced throughput rates, clumps of material could tangle and trap plastic film from the air knife. As well, the blower that draws “lights” from the separation chamber could clog with larger pieces of plastic.
MGL Technologies Limited’s Compost Particle Separation Unit (CPSU) conveys material through a metering-dispersion system, and then into an air separation unit. The unit uses an air stream to lift lighter material and separate the heavy organics from the fines and plastic. A cyclone further separates the fines from the air and plastic while a drop box filters the plastic from the air stream.
Samples of overs, unders and pre-screened materials were tested. The CPSU was able to separate the plastic from the organics from all three materials.
Both organics-plastic separators were able to separate plastic film from the organics and each used a unique air separation system that relies on a consistent feed for effective separation. The difficulties observed with clumped and bulky material are identical to those of any air separation system.
According to Douglas, as equipment manufacturers continue to refine their designs, solutions will continue to materialize.
Connie Vitello is editor of this magazine. The information in the article was adapted from the report “Plastic Film Separation in Composting” prepared by Henry Wong for Miller Waste Systems.
|Bulk Handling Systems, Inc.
||Vertical feed. Inward rotating bladed drum and shaft with varied speeds.
||Vertical feed. Two rotating drums with grappling mechanisms.
||Horizontal feed. Auger mechanism to convey material
||through a restricted throat.
||Horizontal feed. Rotating drum with sensored fingers and guide plate.
|Muma Manufacturing, Inc.
||Horizontal feed. Inward rotating bladed drum and rubber guide shaft.
|Rader Resource Recovery,
||Horizontal feed. Bladed conveyer that squeezes material
||underneath a spring-mounted pressure plate.