ONE OF the biggest challenges in compost-ing municipal solid waste, yard trimmingsand food residuals is dealing with plastic.
To keep feedstocks and/or compost free of plas-tics, operators have resorted to several methods —source separation, hand sorting, screening, andair separation. Of these methods, air separation isprobably the least familiar to composting facilityoperators. Although it routinely is used in recy-cling and other industries, air separation is justbeginning to find use in compost production. Afew innovative operators have adapted air separa-tion machinery to serve their compost productionsystems. However, most are still waiting for equip-ment designed with compost in mind. Suchequipment has now emerged on the scene and isbeginning to find its niche.
Air Separation – How It WorksAir separation employs a stream of air to sepa-
rate materials according to their density (althoughshape is also a factor – see sidebar). The concept iswell illustrated by a conventional air separationsystem, commonly called air classification (Figure1). The mixed materials are fed into a chute withan 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 aconveyor. As the air stream continues with thelight particles entrained, it enters a cyclone sepa-rator where the air velocity slows, causing the par-ticles to settle out.
There are several variations of the concept. Forexample, a cyclone is not necessary to collect thelight particles. It is just particularly good at this, es-pecially for small particles like powders and dust.Other devices that slow the air velocity and thuscapture the light particles also are used. Compostershave used roll-off containers for this purpose.
In addition, the direction and mode of airflowcan vary. The air can move as a result of a vacuum,with a fan above the feed, or by pressure, with thefan below the feed, or both. The air stream can behorizontal with the heavy particles carried along aconveyor or screen (Figure 2). Additional equip-ment may be involved including air lock valves,chutes of various shapes and multiple conveyors.Several applications include a vibrating conveyorto shake light particles to the surface so the air cancontact it.
The split between light and heavy materials de-pends on the air velocity (which in turn dependson the air flow rate and the size of the air channel).Greater velocities are needed to remove heavierparticles. Different types of plastics, for instance
Air Separation StrategiesTackle PlasticsContamination
Robert Rynk
Feed
AirLock
Air withlight particles
Blower
Exhaustair
Cycloneseparator
Heavy particles
Light particles
Figure 1. Conventional air classification system
Heavy particles
Light particles Blower
Figure 2. Horizontal orientation with conveyor
AIR SEPARATION STRATEGIES TACKLE PLASTICS CONTAMINATION
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PVC versus polyethylene film, require different ve-locities for effective separation. Setting the veloci-ty too low reduces the separation efficiency butsetting the velocity too high blows a lot of heavyparticles in with the light fraction.
Air Separation in Composting SystemsIn composting situations, air separation is used
toward the end of the process, removing plastic af-ter grinding, composting and screening. It is usu-ally one of several steps that remove plastics fromcompost. Hand sorting and screening remain im-portant components in producing a nearly plas-tics-free compost. In fact, it is the screening stepthat separates the plastic from the finished com-post. Unfortunately, air separation cannot feasiblysegregate plastics from finished compost products,at least with the techniques presently available.The density and aerodynamic qualities of plasticparticles and compost are too similar for practicalseparation (see sidebar). Therefore, air separationis applied to the screen overs — the large fractionparticles collected by the screen. The overs are pri-marily comprised of pieces of wood, which havesubstantially different densities than the plastics,at least the film plastics.
There are advantages to taking the plastic out ofscreen overs. First, it reduces the amount of plasticthat gets returned with the overs into the com-posting process and that might eventually con-taminate the compost. Particles of plastic growsmaller with each recycling and eventually passthrough the screen. Second, overs piles are often solittered with plastic, they cannot be reused or evensold as mulch. In this case, air separation saves thecosts associated with storing and landfilling thepiles. It also recovers the organic particles formulch or compost products.
The primary scourge of composting facilities isfilm plastic, derived from bags, packaging, plasticmulch and shrink-wrap. However, other plasticitems also can taint the product including bottlecaps, straws, eating utensils, beverage containersand plant pots. Typically, air separation systemsare designed to extract the film plastic. Althoughmany of the other items are removed with the film,the heavier plastics tend to escape.
The fact that plastic film easily floats in movingair has not been lost on facility operators. A walkalong the perimeter fence provides proof enough.No doubt many operators have uttered in frustra-tion, "If I can only figure out a way to blow thebleepin’ plastic stuff out without creating a mess!"Well, a few operators have figured out a way. Morerecently, the concept has been developed into
commercial equipment being used by the com-post industry.
Palm Beach CountyIn 1995, operators at the Palm Beach County,
Florida composting facility found an opportunityto reduce the amount of plastics accumulating inthe composted screen overs. To control dust, thefacility already had enclosed the outlet of the trom-mel screen by a metal discharge chute that direct-ed the overs into a collection hopper of a stackingconveyor. The chute provided a means to channelair through the overs as they drop from the screen,removing plastic in the process (Figure 3). To cre-ate the airflow, a 110-amp variable speed exhaustfan was installed in the side of the chute below thescreen. Louvers for the fan inlet opened about fourinches into the chute so the overs actually fell ontothe louvers. This had the effect of mixing up theovers particles, exposing more of the plastic to theair. As the overs continued to drop past the fan in-let, the air moving toward the fan picked up plas-tic film and other light particles. From the fan, theexhaust was pushed through a metal duct and intoa roll-off container. The plastic settled out in therolloff while the air passed out through a mesh-covered opening in the container (2 ft. by 2 ft.).
According to Pat Byers, manager of the facility,the system worked well. It removed nearly all of theplastic from the screen overs without experiencingproblems, and at minimal expense. Nearly all ofthe components were fabricated or available onsite. The fan was the type typically used for venti-lating greenhouses. Because the exhaust air passedthrough the fan, plastic would gradually build upon the blades and housing and had to be clearedperiodically. Byers believes that a turbine type fanwould have eliminated this inconvenience.
Despite its effectiveness, the air separation sys-tem was only used for several weeks. At that time,the facility had started to change its methods forhandling and sorting feedstocks (yard trimmingsand biosolids). Emphasis was shifted to front-end
Heavy particles
Chute
Hose/pipe
Roll-offcontainer
FanLight particles
LouversTrommelscreen
Airexhaust
Light particlessettle
Figure 3. Air separation system at Palm Beach County,Florida
AIR SEPARATION STRATEGIES TACKLE PLASTICS CONTAMINATION
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separation so the need to pull plastic from thescreen overs was reduced greatly. The facility alsochanged its screening plant.
Gilton Resource RecoveryThe composting facility operated by Gilton Re-
source Recovery in Modesto, California compostsyard trimmings with a variety of food residualsfrom canneries and processing facilities. Many ofthe food residuals accepted are processed andpackaged products that, for a variety of reasons,cannot be sold. Truckloads of these materials areprocessed, packaging included. The facility dealswith the packaging materials via a series of opera-tions including hand picking, magnets, grinding,composting and screening, with some steps beingrepeated through the system. Plastic bags com-prise a major portion of the packaging. The plas-tics that survive the picking lines pass through thecomposting process and are largely removed by thetrommel screen.
In 1993, operators devised a system for remov-ing plastics from the screen overs. While the sys-tem is technically an air separation mechanism, itis perhaps better described as a vacuum cleaner.The equipment, fabricated at the facility, includesa custom-made vacuum nozzle, blower, air ductsand a debris collection box (Figure 4).
The system removes plastic from the inclinedbelt conveyor carrying overs away from the screen.The three-inch slot of the vacuum nozzles sitsthree to four inches above the conveyor surface.The nozzle spans the width of the conveyor andhas the same trough-shaped contour such that thegap between the nozzle and the conveyor is thesame across the width. The degree of separation isadjusted by changing the gap between the producton the conveyor and the nozzle. This is done by re-locating the pin that attaches the vacuum assem-
bly to the conveyor frame. A paddle-style blowerlocated in the vacuum duct draws air into the noz-zle, pulling up plastic from the belt. The air is thendirected through an eight-inch flexible pipe into adebris box that collects the plastic and exhausts theair. While the vacuum system was installed on theexisting conveyor, a vibration element could beadded to help float lighter fraction material to thesurface to improve separation if needed.
“The system removes 90 to 99 percent of the filmplastic from the overs,” says Brian Mathews, formermanager at the facility (currently program manag-er with Alameda County Waste Management Au-thority). It also collects items like straws and milkbottle caps but heavier items, like soda bottle caps,tend to slip through. Only a small amount of theorganic material in the screen overs ends up in thedebris (approximately ten percent of the light frac-tion collected). Mathews believes that the systemworked so well that it spawned copy-cats at othercomposting facilities in the state.
Commercial EquipmentBecause plastic has been such an annoyance to
the composting industry, it was only a matter oftime before equipment dedicated to the task ofseparating plastic from compost products wouldbe developed. For several years, equipment com-panies made attempts at adapting existing equip-ment to capture plastic during or after screening,frequently by retrofitting screens. However, noth-ing worked completely to the industry’s satisfac-tion. In 2000, the Farwick Company in Oelde, Ger-many began selling a plastics separating machinecalled the Hurricane (Hurrikan in Europe). It be-came available in North America in January 2001.As of this writing there are approximately 100 fa-cilities using the Hurricane separator worldwide.The cost of the separator is about $100,000.
The Hurricane removes plastic from coarsefeedstocks, like screen overs, and can also performother separation tasks. It incorporates severalmechanisms in its operation, with air separationbeing a key element (Figure 5). Following screen-ing, overs are fed by an external conveyor onto theinput chute — essentially a sloped stainless steeldeck about four feet wide that vibrates. As theovers vibrate toward the belt conveyor, the materi-al spreads out and plastics are exposed.
From the chute, the overs drop onto an inclinedbelt conveyor that is shrouded by a metal hood.The space between the chute discharge and beltconveyor serves as an inlet for the airflow. A blow-er fan, located between the base of the input chuteand the conveyor, pushes air through the overs as
D E T A I L
Trommelscreen
Light particlesto debris box
Blower
Taperedvacuum nozzle3”-4” Space
Heavy particles
Figure 4. Plastic separation system at GiltonResource Recovery
AIR SEPARATION STRATEGIES TACKLE PLASTICS CONTAMINATION
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they drop. The air picks up plastics and other lightparticles and then continues through the belt con-veyor hood to a suction fan near the top of the con-veyor. The suction fan gathers the plastics-ladenair from the hood and moves it to a discharge con-tainer where the plastics are captured. Various dis-charge devices can be used including mesh bagsand roll-off containers with a screened vent to ex-haust the air.
After passing the suction fan, the overs can bedischarged or delivered to another optional sort-ing step. In this step, the overs are deposited ontoa sloped high speed inclined conveyor that re-moves heavy objects, like rocks and golf balls. Therocks roll down the conveyor slope while the most-ly-organic fraction is conveyed up the slope anddischarged. Magnets can be placed along the con-veyor to remove ferrous metals.
The machine can be adjusted in several ways toproduce the desired degree of separation. Theheight of the suction inlet above the belt can bechanged to pull more or less material off the con-veyor. This is normally the first adjustment to bemade. The speed of the blower also is adjustable.In addition, the slope of the input chute can bechanged to alter the feed rate and initial segrega-tion of plastics.
According to Chris Valerian, vice-president ofNorton Environmental Equipment and the exclu-sive marketer of the Hurricane in North America,the machine is meeting expectations. "Althoughisolated pieces of plastic can escape if impaled ona stick of wood or trapped within a clod of woodchips, nearly 100 percent of the plastic will be re-moved from composted screen overs," says Valeri-an. "To accommodate the changes in materialmoisture content, the machine is adjustable." Headds that the optimum application is for screeningfinished product. "This minimizes the amount of
stringy material, typically found on immatureproduct, where plastic can be trapped."
Because this equipment is fairly new, it hasn’t yetestablished a substantial track record. However, ex-perience at composting facilities to date is encour-aging. For example, the large yard trimmings com-posting facility in Islip, New York is testing theequipment to remove the remains of plastic bagsfrom its screen overs. The facility will determinewhether to purchase the equipment based on es-tablished performance criteria. The equipment willbe evaluated according to its separation efficiencyat various moisture contents and levels of contam-ination in the overs. As of this writing, the tests arestill in progress but the preliminary results arepromising, according to Stuart Buckner, Directorof Environmental Services for the town of Islip.
One of the more experienced facilities with airseparation is County Conservation in Sewell, NewJersey. The facility, which composts a mix of yardtrimmings, had accumulated approximately20,000 cubic yards (cy) of plastics-contaminatedscreen overs. John Petrongolo, the site manager,says that they have tested several schemes to re-move the plastic from screen overs including mul-tiple screening and attaching an 18-inch vacuumhose to the deck of a star screen. While the vacu-um pulled off much of the plastic, its performancewas inconsistent and sensitive to the compost’smoisture content. In the end, the repeated screen-ing only increased the percentage of plastics in theremaining overs.
The facility started using the Hurricane in Jan-uary 2001. "It is the most effective method of re-moving plastics from our overs that I have seen sofar," Petrongolo says, adding that nearly all of theplastics are captured. Primarily, what remains inthe overs are a few heavy plastic items (e.g. plastictoys) and large pieces of film that get trapped with-
Figure 5. Separation mechanisms for Hurricane system
Head rollermagnet
Ferrous metals
Suction fan with hose extractslight particles to covered container
Oversfrom screen
Vibrating input chute
Compressed air stream
Optional high-speedsorting conveyor
Cleanend product
AIR SEPARATION STRATEGIES TACKLE PLASTICS CONTAMINATION
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in the overs particles. Since acquiring the machine,the overs pile has been reduced to 1,000 cy. The or-ganic material recovered from the overs has beenreground and sold as mulch.
One Weapon In The Plastics BattleAir separation is getting more attention in the
battle to remove plastics from compost products.It will likely become more useful as operators gainexperience and equipment manufacturers findnew applications and refine their techniques. Andthe composting industry appears to be eager togive the technology a chance. For example, two ofthe 16 recycling grants that the state of Mas-sachusetts awarded in 2001 were for air separationequipment.
However, air separation is just one weapon inthe arsenal to manage plastics contamination. As itcurrently works, air separation is limited to clean-ing up screen overs. While this certainly has value,it is only one phase of the production system. Tosubstantially eliminate plastics from compost,other management practices must be part of thesystem including source separation, bio-degrad-able bags and packaging, quality control in the tip-ping area, front-end sorting, proper size reductionand screening. Producing plastics-free compost isa major challenge. Although getting better than 95percent removal may seem impressive, it can stillleave the compost with an unacceptable level ofcontamination, especially if feed material initiallycontains a lot of plastic. �
AIR SEPARATION segregates light particles from heavyparticles according to how easily the particles become en-trained in a stream of air. The light particles go with theflow, the heavy particles don’t. It is often referred to asdensity separation because density is a primary factor de-termining whether a particular material becomes airborne. However, other particle characteristics also influ-ence separation, especially the shape. A particle that doesnot have an aerodynamic shape, like a flat sliver of wood,might fly away in the air while other, rounder, wood par-ticles slip through.
Actually, it is a quality known as the "terminal ve-locity" that matters. When a particle falls freelythrough a fluid, like air, the balance between gravityand the opposing friction causes the particle to fall ata constant speed — which is the terminal velocity. Ter-minal velocity is a quality of a particular particle, de-termined by its density and shape (and a few other fac-tors like surface roughness).
The point of separation between "light" and "heavy"fractions depends on the velocity of the air stream. If theair velocity exceeds a particle’s terminal velocity, the par-ticle is carried by the air. Since heavy (i.e. dense) materi-als have a large terminal velocity, greater velocities areneeded to remove heavier particles.
Part of the challenge with air separation is that a classof particles, like plastics, can have a wide range of termi-nal velocities due to changes in composition, moisture,shape and orientation. To capture all of the particles in thedesired light fraction, the air velocity must be set at thetop of the light fraction’s terminal velocity range. Particlesencompassing the heavy fraction — the stuff you want toleave behind — also have a range of terminal velocities.Making sure none of those particles fly away with the airmeans setting the air velocity below the low end of theirterminal velocity range. If the terminal velocities of thelight fraction and heavy fraction overlap, air separationbecomes difficult. You cannot get good separation. Thisis why separating plastics from finished compost is diffi-cult. In technical terms, the terminal velocities for compostparticles and plastic particles overlap too much.
However, screen overs present a different situation.The overs are largely composed of large particles ofwood, which have a substantially different density andshape than plastic. The difference in the terminal veloc-ities between the overs particles and plastic particles islarge enough to get good separation. Still, some heavyplastics may remain behind with the overs, perhaps aplastic fork, while a few fibers of wood may wind up inthe plastics bin.
Why Air Separation Works With Screen Overs But Not Compost
