MSW Management | Gearing Up for the Customer’s Needs

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A basic fact of any operation, program, or task relying on voluntary participation is that it probably won’t be performed well (or at all) unless it is simple and easy for the participants.

Most current recycling operations are not simple for the participants. On the part of individual homeowners and businesses alike, they require source separation of paper, plastic, bottles, and cans.

These are then set out in separate bins for curbside collection, deposited into separate waste cans at public locations, or hauled off by the generator to local or regional recycling centers. In effect, the waste generators are creating two, three, or even more streams of recycled materials for ease of use by the recycler.

Needless to say, such a complicated and often burdensome procedure can often result in relatively low rates of participation and recycled tonnages that are less than the potential maximum.

Single-stream recycling seeks to replace this system with one that places the burden of recycling on the recycler and his materials recovery facility (MRF). The whole wastestream, or a significant proportion of the wastestream with multiple components, passes through the MRF in a single stream and is separated by mechanical means into recyclable components.

Gearing up a transfer facility to true MRF status—or increasing the capacity of an existing MRF—requires significant investment in processing equipment.


	Table 1

The Current State
So what does the wastestream consist of, and how much of it is recycled? According to the EPA’s latest data (2003), Americans throw away 236 million tons of waste each year. Table 1 indicates the components of this wastestream before recycling (actual percentages are rounded to the nearest tenth of a percent, and tonnages are rounded to the nearest thousand tons).


	Table 2

Current recycling efforts (along with composting) divert 72 million tons—approximately 30.5%—from this wastestream, leaving the components listed in Table 2 for landfill disposal (percentage figures in Table 2 are from 1995 and have not significantly changed).


	Table 3

Combining the two tables gives the amount of material recycled each year, as shown in Table 3.

Not surprisingly, paper leads the list as the most recycled material. (Yardwaste and foodwaste, primarily recycled via large-scale and small-scale composting operations and vermiculture activities, are therefore not included in this study of MRF equipment.) This leaves paper, plastic, and scrap metal as the leading recycled materials.

Single-Stream Recycling
What is single-stream recycling, how does it work, and how does it differ from traditional recycling methods? Single-stream recycling refers to the collection of fully commingled waste that has not been source separated into individual containers or bins. The recycled materials (newspaper, plastic, cardboard, and glass) are collected together, transported together, and processed together by vehicles and equipment designed to handle this commingled wastestream.

Single-stream is similar to double- or triple-stream recycling, where only a pair of commodities (newsprint and plastic bottles, for example) are collected in a single bin for processing.

The potential cost savings and increased recycling rates resulting from a single-stream recycling system are significant. Available evidence strongly indicates that single-source recycling results in much higher rates of recycling since it is user-friendly.

Additionally, the hauling trucks collecting the recyclables are much cheaper since they only need one holding compartment, not several for individual waste components. Further fleet cost reductions occur as a result of inherent operational flexibility and reduced fleet size.

Future changes in recycling loads and processes can be accomplished by equipment instead of people.

In fact, single-stream recycling’s emphasis on processing equipment instead of individual handling reflects a basic economic truth: It’s always more cost-effective to invest in capital than to invest in labor.

However, this cost savings doesn’t always show up on the books. The source-separation activities performed by individuals under multiple-stream recycling is “free” in the sense that nobody records the costs of these efforts (homeowners don’t get paid or even receive a tax break for their recycling efforts).

On the other hand, the cost of investing in the processing equipment needed to separate and sort the incoming single waste-stream can be a significant investment. All recycling programs require some up-front capital investment in equipment, containers ,and vehicles. The cost of a MRF that can handle single-stream recycling runs from $6.5 million to $7 million. A dual-stream system costs only between $4.5 million and $5 million, while a multiple-stream system costs from $2 million to $2.5 million. Given the inherent short-term volatility in the commodities markets, it can often be difficult to justify such expenses in terms of direct cost-benefit analysis.

But recycling in general and single-stream recycling in particular cannot be viewed in financial isolation. Every ton of waste that is recycled—even if its sale results in an immediate direct loss because of falling commodity markets and the increased cost of equipment—is one less ton going into a landfill.

Over the long term, less waste into a landfill results in lowered capital costs by avoiding the need to construct new landfills or to expand existing ones—or even by delaying the construction of the next disposal cell in an existing landfill.

Any delay in construction costs puts off the need for capital expenditure to a future date and thereby lowers the discounted present value or cost of the project. Any financial analysis of recycling needs to look at the big picture and overall cost savings to the community as a whole.

Over 80 single-stream recycling programs are operating in the United States, the majority located in high-population-density areas of the West Coast. Available data indicate that those communities with single-stream recycling have shown significant increases in participation rates. For example, Virginia Beach, VA, experienced an increase in participation rates from 50% to 95% as a result of shifting to single-stream recycling.

On the downside, the sorting processes at the MRF (as with any processes anywhere) are not 100% efficient or effective. Glass, for example, is a serious headache for single-stream MRFs. The sorting and separating processes can result in the breaking of glass and the production of shards, which can cause wear and tear of the equipment while reducing the quality of the recycled paper fibers. Clearly, the processing and separating equipment currently in use needs further refinement. We can assume that marketplace competition will ensure such advances occur as the number of communities utilizing single-stream recycling increases.

A Typical System
MRFs that manage single-stream recycling processes have the unfortunate nickname of “dirty MRFs,” as opposed to “clean MRFs” that process multiple recycling streams, or “wet MRFs” that use mechanical and biological treatment processes to separate materials by density as well as anaerobic digestion to biodegrade waste organics. Up to 90% of a typical municipal wastestream can be diverted to a dirty MRF, where the single-stream recycling is performed prior to landfilling. A dirty MRF can recover up to 50% of the incoming materials as recyclables, the remainder having to be landfilled.

Even fully mechanized MRFs usually begin with hand sorting. This will remain true until robots and artificial intelligence technologies have advanced to the point that they have enough “common sense” to distinguish between “two-dimensional” sheets of paper or cardboard and “three-dimensional” bottles and cans—and further differentiate between them and unusable residue. So manual workers take the incoming waste-stream on a sorting station and separate the waste into paper and paper products on the one hand, and plastic, metal, and glass on the other.

The plastic, metal, and glass first pass through a magnetic separator that pulls out the ferrous metals. Nonferrous metals, such as aluminum, follow the plastic and glass to the disc screen. The disc screen separates out glass shards that were broken during manual sorting or magnetic separation. The broken glass passes through a rotating trommel separator, which sorts large glass pieces for recycling as mixed glass and small pieces that end up as residue.

After the disc screen, the non-paper waste passes through an air separator that sends the rest of the glass to another trommel and the plastic and metals to an eddy current separator. After the trommel, the glass passes through a color sorter, where it is divided into various types (amber, green, clear, and mixed). Each type of glass finally passes through a glass crusher, in which it is pulverized in preparation for resale as glass cullet.

While the glass is being processed, plastic and nonferrous metals are going their separate ways after the eddy current separator. The nonferrous metals (mostly aluminum) pass through an air knife, which splits them into cans and foil. They are then compacted by a can flattener and a metal baler before being shipped out as scrap metal. Plastic, on the other hand, first goes through carton sorting and then bottle sorting. Cartons get compacted by a baler prior to shipping, and bottles get divided up by various types (HDPE, PVC, PET, etc.). Each plastic type passes through a granulator and baler for compaction and preparation for shipping.

While all the non-paper waste is being processed, paper is passed through another trommel separator that separates the mixed paper waste and unusable residue from the rest. The usable paper goes to another manual sorting station where it is divided into its various types (magazines, cardboard, and newsprint).

One of the problems facing single-stream recycling is the difficulty in separating different types of paper. Each type of paper is compacted and bound up in a paper baler for resale.


	Table 4

Table 4 provides a schematic of the process of mechanical recycling, not including residue removal. As the material (in clear cells) moves down the table, it passes through the various pieces of equipment (in shaded cells).

Equipment
The following is a short list and brief description of typical MRF processing
equipment.

Magnetic Separators—Ferrous metals are perhaps the easiest component to separate from the wastestream, thanks to their magnetic characteristics. Magnetic separators are designed to extract ferrous metals, leaving the rest of the wastestream behind. The belt separator, resembling a conveyor belt, is a popular magnetic separator design. The magnetized surface of the belt—or a strong magnet located just underneath the moving belt—attracts ferrous metals from the main wastestream. As the end of the belt turns under at the last roller, nonferrous waste falls into a bin or onto another conveyor belt below, while the ferrous metal is carried along the bottom of the belt to a point at which it is dislodged by a scraper blade and discharged into a storage bin of its own.

A similar design utilizes a magnetized drum or a drum whose interior is lined with powerful magnets. As the drum rotates, ferrous metals adhere to its outside and get carried around to the scraper blade for separation.

Another design calls for a rotating, perforated drum (or trommel) with a strong magnet in the form of a stainless steel tube welded to the inside end of the trommel. Ferrous metals are attracted to the magnetized tubing while nonferrous materials fall through the perforations as the drum rotates. The ferrous metals work their way down the tube to a point where the magnetic field weakens and they fall down a chute to a collection bin.

No matter what the design, the strength of the magnet must be balanced between the weight of the ferrous material being lifted out of the wastestream and the distance gap between the material and the magnet itself. The force of gravity on the nonferrous materials and the air gap keep other waste from contaminating the magnetic separator’s stream. The rate of attraction, the strength of the magnet, and the anticipated weight of the ferrous material determine the speed of the magnetized belt or the rotation of the magnetized drum.

Disc Screens—These mechanisms sort by shape and size. Though not very useful as primary sorters, disc screens are excellent polishers of wastestreams. Most disc screens are designed with an inflow bin. The inflow bin is fed by a variable-speed conveyor belt that controls the rate of movement and evens out the flow of waste material to prevent overflows or clogging. Disc screens can also adjust their opening sizes to manage different wastestreams. Disc screens are most effective when the smaller materials falling through the screen are also the heaviest materials.
Therefore, a disc screener would not be useful in situations where the larger materials are also the least dense (separating aluminum foil sheets from heavy cans, for example).

Once into the screener, the waste travels along a screen floor that contains multiple, intermeshing, and self-cleaning rotating discs arranged in two or more decks. The discs can be circular with different diameters. They can be star-shaped, with each tine of variable length, or oval-shaped with differing sizes.

The rotating action of the variable sized and shaped discs creates a wave action that carries larger particles from smaller ones. Each particle is accurately gauged by the size of the discs and the spaces between the discs. For example, disc screens used to separate different types of paper will raise corrugated cardboard for collection while lighter mixed paper will fall through the interstices between the discs.

As disc screens also sort by size, they are useful for removing small, broken glass particles for residue disposal.

Rotating Trommels—Trommels are rotating drums used in a variety of manufacturing, mining, and recycling operations. Sometimes the trommel’s axis of rotation is horizontal, but usually it is set at some angle to facilitate the discharge of material.

Trommels are usually lined on the inside with a series of parallel vanes set in their inside walls to promote gravity separation of various materials. The walls of the trommel are also perforated, with 2-inch-diameter holes being fairly typical. In addition to vanes, the inside of a trommel can be lined with flights that carry the material back up to the high end of the trommel for another pass through.

As material is fed into one end of the trommel, the rotating action causes the material to tumble and separate. The trommel’s diameter, angle, length, and speed of rotation depend on the material to be separated. What falls through the holes in the wall of the trommel are fines (soil, grit, broken glass, organic wastes, and other residue) that are not suitable for recycling. What are retained are large particles, such as whole glass bottles, plastic materials, cans, and large sheets. This makes trommel separation particularly useful for preparing glass for final sorting.

Air Classifiers—Air classification is a mature technology that was first used in the energy industry to prepare coal for combustion. Simply put, air classification is used to separate light materials from heavy objects by means of powerful, high-velocity air streams. The commingled waste is fed into the midpoint of what resembles a short smokestack. Also located just below this feed-in point is an air inlet regulated by a rotary airlock. At the top of the stack is a powerful blower that sucks air up the stack at high velocities. Objects, materials, and particles too heavy to be lifted by the air stream (metals, glass, and dense plastics) fall to the bottom of the stack for collection and separation by other means. Lighter materials (paper, most plastics, and foil) are sucked up by the blower and enter a cyclone separator. According to size and density, they lose their velocity in the cyclone.

Another version of this simple configuration utilizes a zigzag separator stack that can be used to separate low-density materials, such as plastic and paper, from glass. Instead of a continuous airflow and steady waste input, this system relies on pulsed airflows and a rotating blade drum-feeder to vary the waste input. The rotating blades inside of the feeder drum act as the rotary airlock, reducing both space and energy needs. In the zigzag stack, the materials pass in and out of the main airstreams repeatedly, resulting in more efficient separation.

Eddy Current Separators—This system works by separating electrically conductive but nonferrous metals from the rest of the wastestream by generating repulsive forces in the materials.

For the most part, the materials extracted by this method are aluminum cans and foils. Eddy currents are generated by rapidly spinning magnetic rotors with alternating polarity strips running the length of the rotors parallel to their axes. This generates alternating-current fields that in turn create electrical currents in the nonferrous metals passing over the drums. These currents then serve to create miniature magnetic fields of opposing polarity that repel the nonferrous metal from the rotating drum. As the various nonmetallic materials fall away from the drum, the metals themselves are propelled into adjacent collection bins.

Another version of this basic design involves the use of parallel rotating magnetic discs with a space between them for material flow input. The waste falls between these rotating discs. The discs create fluctuating magnetic fields in much the same way as the alternating polarity of the aforementioned single-drum separator. Nonferrous metals are deflected by the internally induced currents with a force proportional to the electrical conductivity of the material.

Glass Color Separators—This material-separating technology is based on light spectrophotometry (a technique developed by the chemical industry) and is used exclusively for the refined separation of different types of glass. The system makes this distinction by means of certain wavelengths of visible light reflected from the glass to photo sensors.

Of special importance is the removal of ceramic substances, which can contaminate glass cullet meant for resale. After removing the ceramic fraction of the glass stream, the glass can be divided into brown, green, clear, and amber types. This is a less advanced technology, offering lower removal efficiencies.

Air Knife—An air knife is a specialized type of air classifier used in a wide range of manufacturing and recycling processes to separate lighter and smaller objects from heavier and larger objects.

The “knife” in this case is a series of high-velocity sheet flows arranged in parallel layers of concentrated air to separate the lighter aluminum foil from heavier aluminum cans. The airflows are kept separate to prevent a swirling effect, which could disrupt the layers and remix the particles. In addition to separating nonferrous metals, air knifes are useful for sorting different grades of paper, such as lighter newsprint and heavier mixed or glossy paper.

Balers—Compaction of separated recycled materials is often necessary prior to shipping for resale. This is especially true for low-density materials, such as cardboard, paper, and aluminum cans. Normally powered by hydraulic systems and controls, balers come in a variety of sizes and applications.

The final density of the baled material will depend on several factors, including shipping cost and market price. It is not unusual for a MRF to have specialized compactors for each type of recycled waste (paper, cardboard, aluminum, and plastic). Balers often enclose their finished bundles with wire or steel straps to keep them contained for delivery.

Daniel P. Duffy, P.E., is an environmental engineer employed by URS Corp. in Akron, OH.

MSW - March/April 2007