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Recycling – Household Waste
Recycling – Household Waste$ DB Spencer, wTe Corporation, Bedford, MA, USA r 2016 Elsevier Inc. All rights reserved.
1 1.1 1.2 2 3 3.1 3.2 4 5 5.1 5.2 6 7 References Further Reading
Ways to Recycle Typical Household Recycled Materials Various Collection and Processing Methods Focus on Recycled Materials from Household Waste Metals Ferrous Metals Nonferrous Metals Glass Plastics PET or PETE HDPE Paper Concluding Remarks
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According to the US EPA (2012), Americans generated about 251 million tons of trash. Much of that total tonnage is comprised of recyclable materials that could be either directly recycled for processing and reuse, or reclaimed from mixed waste streams disposed by the household. This article is all about recycling household wastes. The average composition of US waste has been analyzed by EPA and is shown in the pie chart below. As can be seen much of the material making up our trash or municipal solid waste (MSW) includes paper and paperboard, plastics, metals, and glass which should be recyclable. Food waste, yard trimmings, and wood that remain after direct recycling are amenable to composting as an alternative to disposal in a landfill. Materials that cannot be easily recycled can be combusted in a waste heat recovery unit, or energy recovery incinerator where combustible paper, food, wood, and yard trimmings are converted to steam, electricity, hot water, or chilled water leaving only residual ash for landfill disposal (Figure 1). From the year 2000 to 2012, waste generation for our nation has remained relatively flat – 244 million tons per year in 2000 versus 251 million tons per year in 2012. This is in sharp contrast to prior years where waste generation was increasing rapidly year over year. Historically, part of this growth in waste generation was due to population growth, and part was due to the fact that each person was disposing of more waste per capita per day. But as can be seen below in Figure 2, post 1990 waste generation per capita has also remained relatively flat to negative – being 4.57 pounds per person per day in 1990 and even less, 4.38 pounds per person per day in 2012. At the same time that waste generation is leveling out on a tons generated per year basis, and really leveling out on a per capita basis, recycling rates are climbing quite rapidly. This fact further reduces our reliance on landfills and incineration. During 2012, our nation recycled and composted roughly 86.6 million tons of its 250.9 million tons of MSW, which amounts to a 34.5% recycling rate. Looking at waste generation and recycling rates on an individual pounds per person basis, waste generation was 4.38 pounds per person per day in 2012 from which citizens recycled and composted 1.51 pounds. The rapid rise in total tons of materials recycled is readily apparent, as is the growth of recycling as a percentage of total waste generated in Figure 3. As can be seen, the recycling rate was quite flat until 1985 going from 6.4% in 1960 to less than 10.1% in 1985, a 3.7% increase over a 25 year period. Then after 1985 recycling rates began to rise rapidly. Over the next 22 years, recycling rose from the 1985 percentage recycling rate of 10.1 to 34.5% in 2012 – demonstrating a 24.4% increase over the next 22 years, almost four times the 6.4% increase achieved during the 25 years earlier. This is good news because it indicates that our nation is becoming less reliant on landfills and incineration. The need for new long-term methods to deal with our solid waste in an environmentally sound fashion has resulted in the following hierarchy of disposal alternatives established by the US EPA: 1. 2. 3. 4. ☆
reduction, recycling and composting, waste-to-energy (also called resource recovery), and disposal in landfills.
Change History: June 2015. D.B. Spencer added abstract and keywords, and updated the Introduction Section with new data on solid waste and recycling.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02237-2
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Figure 1 Total MSW generation (by material), 2012 and 251 million tons (before recycling).
Figure 2 MSW generation rates from 1960 to 2012.
On the top of the priority list is waste reduction which is followed by recycling and composting as alternatives to disposal. Following these, would come waste-to-energy, also called incineration with waste heat recovery. Lowest on the hierarchy is landfill (It should be remembered that landfills can also involve production of energy from the methane that is produced from
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Figure 3 MSW recycling rates from 1960 to 2012.
combustibles and compostable matter in the landfill when the waste decays). This gas can be recovered from the landfill and combusted either in heat recovery units to generate steam, or in gas engines to generate electricity. So landfill is not the end of opportunity for recovery and recycling. EPA encourages practices that reduce the amount of waste disposed, and encourages both waste reduction and recycling. Waste reduction, also termed source reduction, is about designing products in such a way that they will produce less waste when they reach the end of their useful life. Recycling is the recovery of useful materials, such as paper, glass, plastic, and metals from the MSW. Composting is, in a way, another form of recycling in that it is a way of keeping our yard waste out of a landfill. Composting involves collecting organic waste such as food scraps and yard trimmings to make soil additives such as mulch and the like. Compost does produce air pollutants when it is manufactured (methane, carbon dioxide, etc.) and also as a part of the natural biological degradation process when it is spread on the land. The compost can also add nutrients that can feed the soil for crops and vegetation. And surely composting is a means for diverting waste materials from landfills into more useful land applications. Organic materials make up the largest part of our MSW. According to EPA (2012), about 70% of our newspaper was recovered for recycling which amounted to about 5.9 million tons and about 58% of our yard waste and trimmings were recovered. These and the recycling rates of many other materials such as tires, steel cans, glass, and various plastics and metals are shown in Figure 4 below. The focus here will be on ‘recycling.’ There is some confusion regarding what actually constitutes household recycling. Recycling is an elusive concept about which everyone thinks they have a clear understanding until they begin to practice it. Many would argue that resource recovery facilities are a form of recycling because they burn garbage to produce electricity or steam. For example, a truckload of household waste can produce enough energy to replace over 20 barrels of oil. However, the focus will be on recycling of metals, plastics, glass, and paper and reuse of these materials in their current form rather than converting them into energy, oil, gas, or some other form. It is worthwhile to note that while some 25% of our waste is considered by the collecting communities to be recycled, the actual amount which is resold as finished products may be considerably less, the balance being processing wastes and residuals that go to landfills.
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Ways to Recycle
There are many ways to implement a recycling program. The program can either be voluntary or mandatory. A community can pick up the recyclables at the household as in the case of a curbside collection program. Recyclables can be sorted by the homeowner at curbside (Figure 5), or collected as a ‘commingled’ stream (Figure 6) for later sortation either by hand or in an automated processing system. Alternatively, the homeowner can deliver the recyclables to a drop off center, deposit location, or buyback center.
1.1
Typical Household Recycled Materials
The materials that are collected from household waste for recycling typically include the following: Paper: paper is normally collected in the form of
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newspaper (termed ONP or old newspaper),
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Figure 4 Recycling rates of selected products, 2012 (does not include combustion with energy recovery).
Figure 5 Recyclables sorted by the homeowner for curbside collection showing separate bins for metals, glass, plastic, and paper. Reproduced with permission from wTe Corporation.
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cardboard (termed OCC or old corrugated containers), or mixed paper (a low-grade frequently used for paperboard or insulation). Glass: glass can be collected either as mixed color or color sorted
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brown (termed amber), green (often termed emerald), and clear (also termed flint). Metal: metal food and beverage containers can be collected including
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tin cans (termed ferrous metals) and aluminum cans (termed nonferrous metals). Plastic: plastic is found in numerous forms including:
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PET soda bottles (♯1: polyethylene terephthalate sometimes called PETE), HDPE milk and water bottles and detergent bottles (♯2: high-density polyethylene), V or PVC bottles and sheet (♯3: vinyl or polyvinyl chloride),
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Figure 6 Recyclables commingled at curbside showing various plastics and metals contained in a single container to be sorted by hand or automatically at a materials recycling facility or by the collector at curbside. Paper may be set out separately and collected separately. Reproduced with permission from wTe Corporation.
• • • •
LDPE (♯4: low density polyethylene, typically a film plastic), PP (♯5: polypropylene for hot fill applications) PS (♯6: polystyrene, such as for foam cups but also packaging berries, etc.), and other (♯7: a resin other than the above six resins or a multilayer resin).
Each of these plastics typically has a resin identifier marked on the bottom of each container within the ‘chasing arrows’ recycling symbol. The resin identifier is a number from 1 to 7, referring to each of the above resins in the order listed.
1.2
Various Collection and Processing Methods
The various collection, consolidation, and processing approaches include: bottle bill returns including the use of reverse vending machines (Figure 7), drop boxes, drop off centers, or buyback centers for recyclables (Figure 8), curbside collection of homeowner-separated materials, curbside separation of homeowner-commingled recyclables, collection of commingled recyclables followed by hand or automated processing, sometimes called ‘single stream recycling,’ at a materials recycling facility (MRF) (Figure 9), 6. mixed waste processing of recyclables collected as part of the trash stream, often called a front-end processing plant (Figure 10), and 7. post-combustion processing of recyclables from incinerator ash after combustion at a mass-burning-type waste-to-energy plant (Figure 11).
1. 2. 3. 4. 5.
When one accounts for all the different types of materials that can be included in a recycling program, the various methods for segregation and the various means and methods of collection, as well as the types of processing and separation systems that can be used, seem enormous and confusing. These are all discussed in a more comprehensive article by the author contained in the Handbook of Solid Waste Management (National Solid Waste Management Association, 1989). Moreover, the composition and characteristics of each recyclable product can differ depending on the methods of collection and processing to which it is subjected prior to being sold for reuse. Many of these subtleties are also discussed in the referenced handbook.
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Focus on Recycled Materials from Household Waste
Rather than focusing on the various methods to collect and sort recyclable materials, the focus of this article will be on what materials are contained in household waste that can be recycled, and how these materials are actually recycled into end products
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Figure 7 A ‘reverse vending machine’ in which bottles from a deposit state are inserted, and the bottle is crushed or pulverized (e.g., canceled) and stored by material type and color. The depositor receives a cash payment for the container recycled and a database is maintained to track the bar code and transaction information. Reproduced with permission from wTe Corporation.
Figure 8 Drop off boxes where consumers voluntarily take back their containers and other recyclables for collection. Reproduced with permission from wTe Corporation.
and the special issues surrounding use of recycled feedstocks. In essence, rather than providing a treatment of alternative recycling management techniques, the focus will be on the materials that are typically produced by recycling technologies and how they differ from materials produced from nonrecycled (e.g., virgin) raw materials. The focus will particularly be on the recycling of metals, plastic, glass, and paper and the challenges that recyclable material create from a material engineering and design perspective.
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Metals
The two forms of metals typically recycled are ferrous (mostly iron and steel) and nonferrous (mostly aluminum).
3.1
Ferrous Metals
Three different forms of ferrous metals that are typically recycled are: 1. MRF ferrous. A photograph of a MRF was provided in Figure 9. Ferrous metals recovered from that facility are baled and typically resemble the material shown in Figure 12. At recycling facilities, magnetic can stock is often received washed and flattened often
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Figure 9 A full-scale automated materials recycling facility (MRF) process flow diagram at which commingled recyclables and paper are collected, sorted, and consolidated for shipment and resale to manufacturers to be mixed with virgin feedstocks for reuse. Reproduced with permission from wTe Corporation.
Figure 10 A photograph of a full-scale mixed waste processing facility where municipal solid waste is processed and complex processing systems remove recyclables from the solid waste prior to combustion, composting, or disposal. Reproduced with permission from wTe Corporation.
with labels removed by the homeowner prior to placing the material at curbside. This material is mostly made up of so-called tin cans and is very clean. It is frequently baled at the MRF prior to sale. The metal commands the highest prices in the marketplace despite the fact that the iron contains tin and other metal contamination. Typical markets include detinning and steel making facilities. However, because of the low value of tin cans, relative to aluminum, not many recycling programs actually collect ferrous metals. 2. RDF ferrous. Far greater quantities of recycled ferrous metals are recovered from refuse derived fuel (RDF)-type waste-to-energy plants (called pre-burn ferrous or RDF ferrous metals). At these facilities the trash is shredded and air-classified prior to magnetic separation. Metals are recovered prior to combustion using magnets. Coat hangers, sharp edges from shredded metals, and the like, hook significant quantities of paper, plastics, and rags that are carried along with the magnetic metals into the final product. The high level of contamination is evident in Figure 13. Even though the quantity of ferrous metals may be as much as 65–75% metal by weight, the feedstock looks much like trash due to the high degree of nonmetallic contamination. This product contains significant quantities of tin and is composed not only of can stock, but also many other types of ferrous metals such as from lawn mowers, bicycles, and other household goods. Typical markets for this material include feedstock for
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Figure 11 A photograph of a full-scale waste-to-energy plant that combusts all the waste prior to recovery of metals for recycling. The energy is recovered from the waste in the form of steam, chilled water, hot water, and/or electricity. Its own waste fired electrical generators power the entire facility. Reproduced with permission from wTe Corporation.
Figure 12 Baled ferrous metals (e.g., tin cans) from an MRF showing the cleanliness of the product. Reproduced with permission from wTe Corporation.
minimills. Figure 14 shows the condition of this RDF ferrous product after multiple stages of shredding, air classification, and magnetic separation prior to loading in railcars to ship to market. 3. Post-incinerated ferrous (PIF). Even greater quantities of ferrous metal are recovered from waste-to-energy plants after combustion where all the waste, including the metals, is burned before recovery. This ferrous metal, called post-burn ferrous or PIF contains large quantities of adhering combustion ash including glassy slag. For reasons not well understood, this material is not only high in tin, but also is high in copper. Many suspect that the copper is precipitated out on the iron by galvanic action when the ferrous metals and other ash are quenched in a cold-water quench tank as they fall from the furnace grate into the quench basin. The copper that is in solution precipitates out and is replaced by iron, which goes into solution. This is the same basic process used to recover copper from mining wastes. A photograph of PIF is provided in Figure 15. The metal content of this product can often be as low as 40% metal by weight. Some automobile shredder operators ‘blend’ this product into finished automobile shredder ferrous metals and sell the combined product to steel mills.
3.2
Nonferrous Metals
Aluminum is the principal nonferrous metal that is recycled. However, other nonferrous metals can also be recycled including copper, zinc, and stainless steel. Like ferrous metals, nonferrous metals are also recovered in three different forms: (a) MRF
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Figure 13 Ferrous metals from a mixed waste or RDF-type processing facility showing the high level of contamination of the metals. These materials look like trash but contain 70% metal. Obviously, they must be further processed prior to sale to a steel mill or minimill. Reproduced with permission from wTe Corporation.
Figure 14 Ferrous metals as shown in Figure 13 after further processing showing the higher degree of cleanliness needed for resale to smelters. Reproduced with permission from wTe Corporation.
Figure 15 Post-incinerated ferrous metals recovered from a waste-to-energy plant prior to further processing to remove ash and contaminants needed to sell it to conventional melting operation or steel mill. Reproduced with permission from wTe Corporation.
aluminum. MRF aluminum, like MRF ferrous metals at recycling facilities, is often received washed and flattened by the homeowner prior to placing the material at curbside for recycling. In addition much of this aluminum is received from deposit legislation states where the aluminum is recycled by the homeowner as part of a deposit or buyback program. Aluminum can stock from beverage containers is mostly made of series 5052 aluminum alloy which is a magnesium-rich alloy. The magnesium is added in order to
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provide excellent impact extrusion properties to form the can. The lids are made of a 3000 series aluminum alloy in order to provide the performance properties needed for a ‘snap-top’ opener. The combination of the lids and the cans are typically sold directly to an MRF and the supplier receives a payment for the aluminum, which is often sufficient to cover the entire collection, processing, and delivery cost. Clean, baled aluminum cans are sold to primary aluminum producers such as Alcoa and Alcan and typically have a value of $0.65–0.85 per pound ($1500 per short ton). This is five times the value of steel cans, which have a value of around $300 per ton. (b) RDF aluminum. Aluminum is recovered from RDF-type waste-to-energy facilities by utilizing ‘eddy current’ separators to sort the aluminum from other waste. Eddy current separators essentially induce an electrical field in conductors. Induced eddy current forces oppose the primary field forces and create a repelling force that can be used to achieve a separation of metals (conductors) from nonmetals (nonconductors). Strong conductors such as aluminum and copper are most affected. Weak conductors such as lead, stainless steel, and zinc experience only weak eddy current forces and thus the sorted product is made up mostly of aluminum. What is produced is a blend of aluminum alloys, often containing high amounts of copper. The result is an aluminumrich product that can only find a market with a secondary smelter. The price of this material is 20% lower in value than recycled aluminum cans. A photograph of RDF aluminum prior to loading into trucks or rail cars is provided on the left in Figure 16. (c) Post-burn mixed nonferrous metals. Eddy currents are also used to recover nonmagnetic metals after combustion at waste-toenergy plants. During the combustion process, much of the aluminum melts on the furnace combustion grate. Accordingly, the aluminum is frequently present as melted nuggets (Figure 17). The finished product can also be sold to aluminum smelters, but
Figure 16 Aluminum cans (left) and PET bales (right) recovered from a MRF prior to shipment to a primary or secondary smelter. Reproduced with permission from wTe Corporation.
Figure 17 Aluminum and other nonferrous metals recovered from a mass-burn type combustion facility where all the metal goes through the combustion chamber prior to sortation and recovery. Reproduced with permission from wTe Corporation.
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Figure 18 Glass cullet being loaded into trucks after processing at a MRF. Reproduced with permission from wTe Corporation.
due to the uncertain chemistry and high levels of contamination, this aluminum commands an even lower price, often only half that of the nonferrous metals from RDF-type waste-to-energy facilities.
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Glass
Old glass can be sorted by color and sold as glass cullet, which is recycled into new bottle glass. The grades of glass include clear, green, or brown, often also referred to as flint, emerald, and amber, respectively. A photograph of glass cullet in each of these three grades after size reduction in a granulator is shown in Figure 18. As a container material, glass has many wonderful properties. It is inert, attractive, formable into complex shapes, and provides excellent gas barrier properties that are especially important to the shelf-life of carbonated beverages such as soda and beer. However, it must be manufactured with thick walls due to its fragility and thus adds substantial weight to the package. Accordingly, glass has lost much of its appeal to the packaging industry and is rapidly being replaced by lighter and more impactresistant metals and plastic containers. One of the advantages of glass from a production standpoint is the low cost of the raw materials that go into it. However, strangely enough, this materials' cost advantage is actually a disadvantage from a recycling standpoint because the packaging material itself does not have much value after it has been used. This adds to the perception that the material is not very recyclable when in fact it is easily recyclable, but the costs to move the container to the recycling facility are high due to the fact that the shipping costs are high per container and the value per pound once delivered is very low. Glass recovered from bottle bill or deposit systems is very clean and pure, and highly recyclable. The situation can be quite different for glass recovered from curbside programs, especially commingled collection programs where compaction and processing result in significant breakage and contamination. Contamination is even much worse for glass recovered from mixed waste streams. The greatest problems in recycling MRF and glass recovered from mixed waste processing results from the fact that ceramic materials and refractory glass are often present. Moreover, stones from dirt, which frequently find their way into recycled containers, can also be a problem. These materials do not fuse or melt in the glass furnace batch melting process and produce ‘stones’ in the glass, which are inclusions or defects causing the glass to break when pressurized with carbonation. As the glass bottles have become thinner to reduce the weight of the container, the impact of these defects has become even more important. Stones can be removed from recycled glass cullet by processing the glass using a method called ‘froth flotation,’ which is a mineral beneficiation technique utilized in many mining operations, but this further adds to the cost of glass recycling and makes it less financially attractive. Recycled glass cullet is a low-melting eutectic requiring less energy to melt than the virgin raw materials that go into manufacture of new bottle glass. The value of glass is on the order of $30–50 per short ton, only a quarter or so of the ferrous metals and a small fraction of aluminum pricing. If the end user bottling plant is more than 50–100 miles from the recycling center, this entire market value is lost to shipping and handling costs. However, if the alternative disposal site is a waste-to-energy plant, the glass contributes nothing to the combustion process, and adds to the ash disposal costs, which can be significant. So a community, when considering the addition of glass to its recycling program, should not only look at its market value, but also the alternative disposal cost that it will incur for ash disposal if its glass is not recycled.
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Plastics
US plastic container demand in 2004 is projected by Fredonia to be about 13 billion pounds. Of that quantity, HDPE is estimated to make up 49% while PET will make up 39%. The balance is 6% PS, 3% PP, and 3% PVC, and others (The Fredonia Group,
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1989). While all of the plastics referenced earlier are ‘recyclable,’ there are really only two household plastics that have really demonstrated that are actually recycled in significant quantities. These are (1) PET (polyethylene terephthalate), which is the principal polymer used in soda bottles and many other packaging materials, and (2) HDPE (high-density polyethylene), which is used in milk bottles and detergent bottles. The plastic packaging containers made of PS, PP, and PVC, while recyclable, are not recycled commercially in significant volumes. Some other nonbeverage packaging materials such as plastic film materials, LDPE, are also recycled, but not much of this packaging film materials, such as laundry bags and shopping bags, finds its way into recycling centers in the US but is instead disposed of as waste.
5.1
PET or PETE
PET (sometimes called PETE) is the preferred resin for packaging soda because of its strong gas barrier properties. A photograph of various PET finished products is provided in Figure 19. PET has a value approaching that of aluminum. Even though PET bottles in baled form may only fetch about $0.10 per pound FOB recycling facility, the processed finished pellet has a value of $0.60–0.80 per pound. PET is difficult to form into bottles. Earlier designs required PE base cups, because the bottom of the package was round after blow molding. The base cup formed from PE was created with a flat base that would stand up on end. Liquor bottles, having a handle, are often clear and look like PET, but are frequently made of PVC, because PVC has much better formability, is clear in color like PET and less expensive per pound. PVC does not have the gas barrier properties of PET and cannot be used for soda. PVC is a very serious contaminant to PET. In fact, when recycling PET, just one PVC bottle in a million PET bottles, can negatively impact the ability to meet user specifications for bottles to bottles applications. PVC breaks the PET polymer chains reducing its intrinsic viscosity that is an indicator of strength and also in concentrations of 10 ppm or more can cause a color shift from clear to red. For this reason, PVC has been considered as highly nonrecyclable because it contaminates and impedes the recyclability of PET, not because it is not by itself recyclable. In the early stages of PET recycling, there were no practical methods to remove PVC from PET once the two polymers were combined. However, today, optical emission and infrared sorting techniques can be used to identify PVC bottles and PVC flakes in a PET stream, and the PVC may be removed. PET can be used in bottles to bottles applications including food applications, can be used to make polyester fiber as a substitute for cotton, and is used in various engineered resin applications. Recently beer has been packaged in PET containers, but the bottle design involves multilayer components (including either nylons, complex oxides, or even vapor-deposited glass) to provide longer shelf-life. While there are plans to increase packaging of beer in PET, consumer resistance, not technology, has limited the growth of this package at least in the short term.
Figure 19 PET flake and pellet after processing of the bottles at a plastics recycling facility showing the products produced for end use. Reproduced with permission from wTe Corporation.
Recycling – Household Waste 5.2
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HDPE
High-density polyethylene (HDPE) is the most common household packaging material due to its low resin cost and easy formability. Figure 20 is a photograph of various PE bottle designs and applications. HDPE is produced in two basic grades: one is translucent milk jugs, and the other is colored laundry and detergent bottles. The properties of these two types of PE are different. The milk jugs are the resin type of highest value and greatest mechanical properties. Unlike PET, PE is easy to recycle. It melts at relatively low temperatures, is not degraded much by long or multiple melt cycles, and it is easy to form. HDPE resin typically sells for $0.40 per pound, which is half the value of PET resin and this lower value negatively impacts recyclability.
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Paper
Of the 25% of household waste recycled, paper is the largest component by weight. There is no market for paper, as such. The demand is for a particular grade of paper. According to the Paper Stock Institute, there are 51 grades of paper, with some of the major recyclable categories being old newspapers (ONP), old corrugated containers (OCC), and brown Kraft grocery bags. Figure 21 provides a photograph of bales of ONP. If all of these grades are mixed together as ‘paper,’ the product will not be marketable even though there may be an unsaturated demand for the individual components. From this example, we may infer a recycling mixing principal. One part of recyclable
Figure 20 A photograph of various end use packages made from both virgin and recycled HDPE. Reproduced with permission from wTe Corporation.
Figure 21 A photograph of bales of ONP prior to shipping to market. Reproduced with permission from wTe Corporation.
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material no. 1 plus one part of recyclable material no. 2 produce two parts of lower-value recyclable material or alternatively equals municipal solid waste. It is the exception to the rule, and very unlikely, that the combined value of the mixture will equal the combined value of the parts. The key to securing strong, high-value markets for recyclable materials is quality, not quantity. Thus achieving unrealistically high recycling rates can actually destroy recycling, not contribute to it. It is necessary to segregate the materials into their various components or grades with minimal contamination and to produce products of consistent quality. Whenever industry representatives speak to citizens about obstacles to recycling, they constantly caution that it is necessary to keep supply and demand in balance and that the key to recycling is developing sufficient markets and demand for the finished products. However, they assume, perhaps incorrectly, that the product will meet existing material specifications designed around supply from virgin feedstock. These specifications assume a level of quality, which frequently is not achieved, in actual recycling practice. There must be, in place, sufficient end use demand and conversion capacity to absorb the raw materials and to provide the proper basic incentives to invest tens of millions of dollars in a high capacity project. There must also be dependable sources of quality raw materials available in appropriate quantities at the same time that the end use demand is sufficient to support conversion. But when we talk about demand for recycled materials, too often there is little or no discussion regarding the particular grade of product under consideration and the relationship between product purity and industry demand. However, this market differentiation is ‘the most critical factor’ in assessing the demand and value of the product. For example, the product specification for newsprint consists of baled newspapers containing less than 5% of other papers. Prohibitive materials may not exceed 1/2% and total outthrows may not exceed 2%. A special news bale should consist of sorted, fresh, dry newspaper, not sunburned and free from paper other than newspaper, containing not more than the percentage of rotogravure and colored sections normally contained in newspaper delivered to the household. In addition, each particular customer for the recycled paper product may have special contamination requirements that are a function of its particular requirements or processing system. The issue here is that there may be substantial demand for clean, segregated product, but there is little or no demand for unsegregated, dirty product. If all 51 grades were blended together, the paper would be unmarketable as paper. As can be seen mixing in McDonald's bags, cups with plastic coatings and liners, hamburger wrappers, and pizza boxes would make the entire bale of newsprint unacceptable for use or reuse. Few households' recyclers really understand this fact. Their enthusiasm to recycle results in disposing not only the added nonspecification materials but also makes the basic specification grade newsprint with which it is mixed, nonrecyclable.
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Concluding Remarks
In conclusion, while many materials may be theoretically recyclable, as a practical matter, these materials cannot be effectively recycled with the current recycling collection and processing systems. A recycling program, which seeks to achieve too high a recycling rate by including all grades of paper, could actually hurt the potential to recycle the larger part of our segregated waste paper, namely, old newspaper or old corrugated containers. This same principal is not only true for grades of paper, but also for the various grades of metal, plastic, and glass as well. A program that collects all of these materials, but does not adequately segregate them and keep the glass out of the paper and plastic, and the plastic out of the glass just creates a mess. While the mix may be picked up at curbside, and thus be considered by the household to have been ‘recycled’ it will not actually be recycled and will add to the volume of municipal solid waste that is generated each year.
References EPA, 2012. Last updated on 2/28/2014 for 2012 data. Available at: http://www.epa.gov/solid waste/nonhaz/municipal/ (accessed 25.11.15). National Solid Waste Management Association, 1989. Facts on File. Resource Recovery Round-up vol. 2, No. 2, May 1, 1989. The Fredonia Group, Inc, Cleveland, OH, April 1989.
Further Reading EPA: Municipal Solid Waste Generation, Recycling, and Disposals in the United States: Facts and Figures for 2011. Spencer, D.B., 1994. Recycling. In: Kreith, F. (Ed.), Handbook of Solid Waste Management. New York, NY: McGraw Hill, pp. 8.1–8.77. Chapter 9. The Paper Stock Institute, Institute of Scrap Recycling Industries (ISRI), Guidelines for Paper Stock.
1 1.1 1.2 2 3 3.1 3.2 4 5 5.1 5.2 6 7 References Further Reading
Ways to Recycle Typical Household Recycled Materials Various Collection and Processing Methods Focus on Recycled Materials from Household Waste Metals Ferrous Metals Nonferrous Metals Glass Plastics PET or PETE HDPE Paper Concluding Remarks
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According to the US EPA (2012), Americans generated about 251 million tons of trash. Much of that total tonnage is comprised of recyclable materials that could be either directly recycled for processing and reuse, or reclaimed from mixed waste streams disposed by the household. This article is all about recycling household wastes. The average composition of US waste has been analyzed by EPA and is shown in the pie chart below. As can be seen much of the material making up our trash or municipal solid waste (MSW) includes paper and paperboard, plastics, metals, and glass which should be recyclable. Food waste, yard trimmings, and wood that remain after direct recycling are amenable to composting as an alternative to disposal in a landfill. Materials that cannot be easily recycled can be combusted in a waste heat recovery unit, or energy recovery incinerator where combustible paper, food, wood, and yard trimmings are converted to steam, electricity, hot water, or chilled water leaving only residual ash for landfill disposal (Figure 1). From the year 2000 to 2012, waste generation for our nation has remained relatively flat – 244 million tons per year in 2000 versus 251 million tons per year in 2012. This is in sharp contrast to prior years where waste generation was increasing rapidly year over year. Historically, part of this growth in waste generation was due to population growth, and part was due to the fact that each person was disposing of more waste per capita per day. But as can be seen below in Figure 2, post 1990 waste generation per capita has also remained relatively flat to negative – being 4.57 pounds per person per day in 1990 and even less, 4.38 pounds per person per day in 2012. At the same time that waste generation is leveling out on a tons generated per year basis, and really leveling out on a per capita basis, recycling rates are climbing quite rapidly. This fact further reduces our reliance on landfills and incineration. During 2012, our nation recycled and composted roughly 86.6 million tons of its 250.9 million tons of MSW, which amounts to a 34.5% recycling rate. Looking at waste generation and recycling rates on an individual pounds per person basis, waste generation was 4.38 pounds per person per day in 2012 from which citizens recycled and composted 1.51 pounds. The rapid rise in total tons of materials recycled is readily apparent, as is the growth of recycling as a percentage of total waste generated in Figure 3. As can be seen, the recycling rate was quite flat until 1985 going from 6.4% in 1960 to less than 10.1% in 1985, a 3.7% increase over a 25 year period. Then after 1985 recycling rates began to rise rapidly. Over the next 22 years, recycling rose from the 1985 percentage recycling rate of 10.1 to 34.5% in 2012 – demonstrating a 24.4% increase over the next 22 years, almost four times the 6.4% increase achieved during the 25 years earlier. This is good news because it indicates that our nation is becoming less reliant on landfills and incineration. The need for new long-term methods to deal with our solid waste in an environmentally sound fashion has resulted in the following hierarchy of disposal alternatives established by the US EPA: 1. 2. 3. 4. ☆
reduction, recycling and composting, waste-to-energy (also called resource recovery), and disposal in landfills.
Change History: June 2015. D.B. Spencer added abstract and keywords, and updated the Introduction Section with new data on solid waste and recycling.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02237-2
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Recycling – Household Waste
Figure 1 Total MSW generation (by material), 2012 and 251 million tons (before recycling).
Figure 2 MSW generation rates from 1960 to 2012.
On the top of the priority list is waste reduction which is followed by recycling and composting as alternatives to disposal. Following these, would come waste-to-energy, also called incineration with waste heat recovery. Lowest on the hierarchy is landfill (It should be remembered that landfills can also involve production of energy from the methane that is produced from
Recycling – Household Waste
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Figure 3 MSW recycling rates from 1960 to 2012.
combustibles and compostable matter in the landfill when the waste decays). This gas can be recovered from the landfill and combusted either in heat recovery units to generate steam, or in gas engines to generate electricity. So landfill is not the end of opportunity for recovery and recycling. EPA encourages practices that reduce the amount of waste disposed, and encourages both waste reduction and recycling. Waste reduction, also termed source reduction, is about designing products in such a way that they will produce less waste when they reach the end of their useful life. Recycling is the recovery of useful materials, such as paper, glass, plastic, and metals from the MSW. Composting is, in a way, another form of recycling in that it is a way of keeping our yard waste out of a landfill. Composting involves collecting organic waste such as food scraps and yard trimmings to make soil additives such as mulch and the like. Compost does produce air pollutants when it is manufactured (methane, carbon dioxide, etc.) and also as a part of the natural biological degradation process when it is spread on the land. The compost can also add nutrients that can feed the soil for crops and vegetation. And surely composting is a means for diverting waste materials from landfills into more useful land applications. Organic materials make up the largest part of our MSW. According to EPA (2012), about 70% of our newspaper was recovered for recycling which amounted to about 5.9 million tons and about 58% of our yard waste and trimmings were recovered. These and the recycling rates of many other materials such as tires, steel cans, glass, and various plastics and metals are shown in Figure 4 below. The focus here will be on ‘recycling.’ There is some confusion regarding what actually constitutes household recycling. Recycling is an elusive concept about which everyone thinks they have a clear understanding until they begin to practice it. Many would argue that resource recovery facilities are a form of recycling because they burn garbage to produce electricity or steam. For example, a truckload of household waste can produce enough energy to replace over 20 barrels of oil. However, the focus will be on recycling of metals, plastics, glass, and paper and reuse of these materials in their current form rather than converting them into energy, oil, gas, or some other form. It is worthwhile to note that while some 25% of our waste is considered by the collecting communities to be recycled, the actual amount which is resold as finished products may be considerably less, the balance being processing wastes and residuals that go to landfills.
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Ways to Recycle
There are many ways to implement a recycling program. The program can either be voluntary or mandatory. A community can pick up the recyclables at the household as in the case of a curbside collection program. Recyclables can be sorted by the homeowner at curbside (Figure 5), or collected as a ‘commingled’ stream (Figure 6) for later sortation either by hand or in an automated processing system. Alternatively, the homeowner can deliver the recyclables to a drop off center, deposit location, or buyback center.
1.1
Typical Household Recycled Materials
The materials that are collected from household waste for recycling typically include the following: Paper: paper is normally collected in the form of
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newspaper (termed ONP or old newspaper),
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Figure 4 Recycling rates of selected products, 2012 (does not include combustion with energy recovery).
Figure 5 Recyclables sorted by the homeowner for curbside collection showing separate bins for metals, glass, plastic, and paper. Reproduced with permission from wTe Corporation.
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cardboard (termed OCC or old corrugated containers), or mixed paper (a low-grade frequently used for paperboard or insulation). Glass: glass can be collected either as mixed color or color sorted
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brown (termed amber), green (often termed emerald), and clear (also termed flint). Metal: metal food and beverage containers can be collected including
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tin cans (termed ferrous metals) and aluminum cans (termed nonferrous metals). Plastic: plastic is found in numerous forms including:
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PET soda bottles (♯1: polyethylene terephthalate sometimes called PETE), HDPE milk and water bottles and detergent bottles (♯2: high-density polyethylene), V or PVC bottles and sheet (♯3: vinyl or polyvinyl chloride),
Recycling – Household Waste
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Figure 6 Recyclables commingled at curbside showing various plastics and metals contained in a single container to be sorted by hand or automatically at a materials recycling facility or by the collector at curbside. Paper may be set out separately and collected separately. Reproduced with permission from wTe Corporation.
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LDPE (♯4: low density polyethylene, typically a film plastic), PP (♯5: polypropylene for hot fill applications) PS (♯6: polystyrene, such as for foam cups but also packaging berries, etc.), and other (♯7: a resin other than the above six resins or a multilayer resin).
Each of these plastics typically has a resin identifier marked on the bottom of each container within the ‘chasing arrows’ recycling symbol. The resin identifier is a number from 1 to 7, referring to each of the above resins in the order listed.
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Various Collection and Processing Methods
The various collection, consolidation, and processing approaches include: bottle bill returns including the use of reverse vending machines (Figure 7), drop boxes, drop off centers, or buyback centers for recyclables (Figure 8), curbside collection of homeowner-separated materials, curbside separation of homeowner-commingled recyclables, collection of commingled recyclables followed by hand or automated processing, sometimes called ‘single stream recycling,’ at a materials recycling facility (MRF) (Figure 9), 6. mixed waste processing of recyclables collected as part of the trash stream, often called a front-end processing plant (Figure 10), and 7. post-combustion processing of recyclables from incinerator ash after combustion at a mass-burning-type waste-to-energy plant (Figure 11).
1. 2. 3. 4. 5.
When one accounts for all the different types of materials that can be included in a recycling program, the various methods for segregation and the various means and methods of collection, as well as the types of processing and separation systems that can be used, seem enormous and confusing. These are all discussed in a more comprehensive article by the author contained in the Handbook of Solid Waste Management (National Solid Waste Management Association, 1989). Moreover, the composition and characteristics of each recyclable product can differ depending on the methods of collection and processing to which it is subjected prior to being sold for reuse. Many of these subtleties are also discussed in the referenced handbook.
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Focus on Recycled Materials from Household Waste
Rather than focusing on the various methods to collect and sort recyclable materials, the focus of this article will be on what materials are contained in household waste that can be recycled, and how these materials are actually recycled into end products
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Figure 7 A ‘reverse vending machine’ in which bottles from a deposit state are inserted, and the bottle is crushed or pulverized (e.g., canceled) and stored by material type and color. The depositor receives a cash payment for the container recycled and a database is maintained to track the bar code and transaction information. Reproduced with permission from wTe Corporation.
Figure 8 Drop off boxes where consumers voluntarily take back their containers and other recyclables for collection. Reproduced with permission from wTe Corporation.
and the special issues surrounding use of recycled feedstocks. In essence, rather than providing a treatment of alternative recycling management techniques, the focus will be on the materials that are typically produced by recycling technologies and how they differ from materials produced from nonrecycled (e.g., virgin) raw materials. The focus will particularly be on the recycling of metals, plastic, glass, and paper and the challenges that recyclable material create from a material engineering and design perspective.
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Metals
The two forms of metals typically recycled are ferrous (mostly iron and steel) and nonferrous (mostly aluminum).
3.1
Ferrous Metals
Three different forms of ferrous metals that are typically recycled are: 1. MRF ferrous. A photograph of a MRF was provided in Figure 9. Ferrous metals recovered from that facility are baled and typically resemble the material shown in Figure 12. At recycling facilities, magnetic can stock is often received washed and flattened often
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Figure 9 A full-scale automated materials recycling facility (MRF) process flow diagram at which commingled recyclables and paper are collected, sorted, and consolidated for shipment and resale to manufacturers to be mixed with virgin feedstocks for reuse. Reproduced with permission from wTe Corporation.
Figure 10 A photograph of a full-scale mixed waste processing facility where municipal solid waste is processed and complex processing systems remove recyclables from the solid waste prior to combustion, composting, or disposal. Reproduced with permission from wTe Corporation.
with labels removed by the homeowner prior to placing the material at curbside. This material is mostly made up of so-called tin cans and is very clean. It is frequently baled at the MRF prior to sale. The metal commands the highest prices in the marketplace despite the fact that the iron contains tin and other metal contamination. Typical markets include detinning and steel making facilities. However, because of the low value of tin cans, relative to aluminum, not many recycling programs actually collect ferrous metals. 2. RDF ferrous. Far greater quantities of recycled ferrous metals are recovered from refuse derived fuel (RDF)-type waste-to-energy plants (called pre-burn ferrous or RDF ferrous metals). At these facilities the trash is shredded and air-classified prior to magnetic separation. Metals are recovered prior to combustion using magnets. Coat hangers, sharp edges from shredded metals, and the like, hook significant quantities of paper, plastics, and rags that are carried along with the magnetic metals into the final product. The high level of contamination is evident in Figure 13. Even though the quantity of ferrous metals may be as much as 65–75% metal by weight, the feedstock looks much like trash due to the high degree of nonmetallic contamination. This product contains significant quantities of tin and is composed not only of can stock, but also many other types of ferrous metals such as from lawn mowers, bicycles, and other household goods. Typical markets for this material include feedstock for
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Figure 11 A photograph of a full-scale waste-to-energy plant that combusts all the waste prior to recovery of metals for recycling. The energy is recovered from the waste in the form of steam, chilled water, hot water, and/or electricity. Its own waste fired electrical generators power the entire facility. Reproduced with permission from wTe Corporation.
Figure 12 Baled ferrous metals (e.g., tin cans) from an MRF showing the cleanliness of the product. Reproduced with permission from wTe Corporation.
minimills. Figure 14 shows the condition of this RDF ferrous product after multiple stages of shredding, air classification, and magnetic separation prior to loading in railcars to ship to market. 3. Post-incinerated ferrous (PIF). Even greater quantities of ferrous metal are recovered from waste-to-energy plants after combustion where all the waste, including the metals, is burned before recovery. This ferrous metal, called post-burn ferrous or PIF contains large quantities of adhering combustion ash including glassy slag. For reasons not well understood, this material is not only high in tin, but also is high in copper. Many suspect that the copper is precipitated out on the iron by galvanic action when the ferrous metals and other ash are quenched in a cold-water quench tank as they fall from the furnace grate into the quench basin. The copper that is in solution precipitates out and is replaced by iron, which goes into solution. This is the same basic process used to recover copper from mining wastes. A photograph of PIF is provided in Figure 15. The metal content of this product can often be as low as 40% metal by weight. Some automobile shredder operators ‘blend’ this product into finished automobile shredder ferrous metals and sell the combined product to steel mills.
3.2
Nonferrous Metals
Aluminum is the principal nonferrous metal that is recycled. However, other nonferrous metals can also be recycled including copper, zinc, and stainless steel. Like ferrous metals, nonferrous metals are also recovered in three different forms: (a) MRF
Recycling – Household Waste
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Figure 13 Ferrous metals from a mixed waste or RDF-type processing facility showing the high level of contamination of the metals. These materials look like trash but contain 70% metal. Obviously, they must be further processed prior to sale to a steel mill or minimill. Reproduced with permission from wTe Corporation.
Figure 14 Ferrous metals as shown in Figure 13 after further processing showing the higher degree of cleanliness needed for resale to smelters. Reproduced with permission from wTe Corporation.
Figure 15 Post-incinerated ferrous metals recovered from a waste-to-energy plant prior to further processing to remove ash and contaminants needed to sell it to conventional melting operation or steel mill. Reproduced with permission from wTe Corporation.
aluminum. MRF aluminum, like MRF ferrous metals at recycling facilities, is often received washed and flattened by the homeowner prior to placing the material at curbside for recycling. In addition much of this aluminum is received from deposit legislation states where the aluminum is recycled by the homeowner as part of a deposit or buyback program. Aluminum can stock from beverage containers is mostly made of series 5052 aluminum alloy which is a magnesium-rich alloy. The magnesium is added in order to
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provide excellent impact extrusion properties to form the can. The lids are made of a 3000 series aluminum alloy in order to provide the performance properties needed for a ‘snap-top’ opener. The combination of the lids and the cans are typically sold directly to an MRF and the supplier receives a payment for the aluminum, which is often sufficient to cover the entire collection, processing, and delivery cost. Clean, baled aluminum cans are sold to primary aluminum producers such as Alcoa and Alcan and typically have a value of $0.65–0.85 per pound ($1500 per short ton). This is five times the value of steel cans, which have a value of around $300 per ton. (b) RDF aluminum. Aluminum is recovered from RDF-type waste-to-energy facilities by utilizing ‘eddy current’ separators to sort the aluminum from other waste. Eddy current separators essentially induce an electrical field in conductors. Induced eddy current forces oppose the primary field forces and create a repelling force that can be used to achieve a separation of metals (conductors) from nonmetals (nonconductors). Strong conductors such as aluminum and copper are most affected. Weak conductors such as lead, stainless steel, and zinc experience only weak eddy current forces and thus the sorted product is made up mostly of aluminum. What is produced is a blend of aluminum alloys, often containing high amounts of copper. The result is an aluminumrich product that can only find a market with a secondary smelter. The price of this material is 20% lower in value than recycled aluminum cans. A photograph of RDF aluminum prior to loading into trucks or rail cars is provided on the left in Figure 16. (c) Post-burn mixed nonferrous metals. Eddy currents are also used to recover nonmagnetic metals after combustion at waste-toenergy plants. During the combustion process, much of the aluminum melts on the furnace combustion grate. Accordingly, the aluminum is frequently present as melted nuggets (Figure 17). The finished product can also be sold to aluminum smelters, but
Figure 16 Aluminum cans (left) and PET bales (right) recovered from a MRF prior to shipment to a primary or secondary smelter. Reproduced with permission from wTe Corporation.
Figure 17 Aluminum and other nonferrous metals recovered from a mass-burn type combustion facility where all the metal goes through the combustion chamber prior to sortation and recovery. Reproduced with permission from wTe Corporation.
Recycling – Household Waste
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Figure 18 Glass cullet being loaded into trucks after processing at a MRF. Reproduced with permission from wTe Corporation.
due to the uncertain chemistry and high levels of contamination, this aluminum commands an even lower price, often only half that of the nonferrous metals from RDF-type waste-to-energy facilities.
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Glass
Old glass can be sorted by color and sold as glass cullet, which is recycled into new bottle glass. The grades of glass include clear, green, or brown, often also referred to as flint, emerald, and amber, respectively. A photograph of glass cullet in each of these three grades after size reduction in a granulator is shown in Figure 18. As a container material, glass has many wonderful properties. It is inert, attractive, formable into complex shapes, and provides excellent gas barrier properties that are especially important to the shelf-life of carbonated beverages such as soda and beer. However, it must be manufactured with thick walls due to its fragility and thus adds substantial weight to the package. Accordingly, glass has lost much of its appeal to the packaging industry and is rapidly being replaced by lighter and more impactresistant metals and plastic containers. One of the advantages of glass from a production standpoint is the low cost of the raw materials that go into it. However, strangely enough, this materials' cost advantage is actually a disadvantage from a recycling standpoint because the packaging material itself does not have much value after it has been used. This adds to the perception that the material is not very recyclable when in fact it is easily recyclable, but the costs to move the container to the recycling facility are high due to the fact that the shipping costs are high per container and the value per pound once delivered is very low. Glass recovered from bottle bill or deposit systems is very clean and pure, and highly recyclable. The situation can be quite different for glass recovered from curbside programs, especially commingled collection programs where compaction and processing result in significant breakage and contamination. Contamination is even much worse for glass recovered from mixed waste streams. The greatest problems in recycling MRF and glass recovered from mixed waste processing results from the fact that ceramic materials and refractory glass are often present. Moreover, stones from dirt, which frequently find their way into recycled containers, can also be a problem. These materials do not fuse or melt in the glass furnace batch melting process and produce ‘stones’ in the glass, which are inclusions or defects causing the glass to break when pressurized with carbonation. As the glass bottles have become thinner to reduce the weight of the container, the impact of these defects has become even more important. Stones can be removed from recycled glass cullet by processing the glass using a method called ‘froth flotation,’ which is a mineral beneficiation technique utilized in many mining operations, but this further adds to the cost of glass recycling and makes it less financially attractive. Recycled glass cullet is a low-melting eutectic requiring less energy to melt than the virgin raw materials that go into manufacture of new bottle glass. The value of glass is on the order of $30–50 per short ton, only a quarter or so of the ferrous metals and a small fraction of aluminum pricing. If the end user bottling plant is more than 50–100 miles from the recycling center, this entire market value is lost to shipping and handling costs. However, if the alternative disposal site is a waste-to-energy plant, the glass contributes nothing to the combustion process, and adds to the ash disposal costs, which can be significant. So a community, when considering the addition of glass to its recycling program, should not only look at its market value, but also the alternative disposal cost that it will incur for ash disposal if its glass is not recycled.
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Plastics
US plastic container demand in 2004 is projected by Fredonia to be about 13 billion pounds. Of that quantity, HDPE is estimated to make up 49% while PET will make up 39%. The balance is 6% PS, 3% PP, and 3% PVC, and others (The Fredonia Group,
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1989). While all of the plastics referenced earlier are ‘recyclable,’ there are really only two household plastics that have really demonstrated that are actually recycled in significant quantities. These are (1) PET (polyethylene terephthalate), which is the principal polymer used in soda bottles and many other packaging materials, and (2) HDPE (high-density polyethylene), which is used in milk bottles and detergent bottles. The plastic packaging containers made of PS, PP, and PVC, while recyclable, are not recycled commercially in significant volumes. Some other nonbeverage packaging materials such as plastic film materials, LDPE, are also recycled, but not much of this packaging film materials, such as laundry bags and shopping bags, finds its way into recycling centers in the US but is instead disposed of as waste.
5.1
PET or PETE
PET (sometimes called PETE) is the preferred resin for packaging soda because of its strong gas barrier properties. A photograph of various PET finished products is provided in Figure 19. PET has a value approaching that of aluminum. Even though PET bottles in baled form may only fetch about $0.10 per pound FOB recycling facility, the processed finished pellet has a value of $0.60–0.80 per pound. PET is difficult to form into bottles. Earlier designs required PE base cups, because the bottom of the package was round after blow molding. The base cup formed from PE was created with a flat base that would stand up on end. Liquor bottles, having a handle, are often clear and look like PET, but are frequently made of PVC, because PVC has much better formability, is clear in color like PET and less expensive per pound. PVC does not have the gas barrier properties of PET and cannot be used for soda. PVC is a very serious contaminant to PET. In fact, when recycling PET, just one PVC bottle in a million PET bottles, can negatively impact the ability to meet user specifications for bottles to bottles applications. PVC breaks the PET polymer chains reducing its intrinsic viscosity that is an indicator of strength and also in concentrations of 10 ppm or more can cause a color shift from clear to red. For this reason, PVC has been considered as highly nonrecyclable because it contaminates and impedes the recyclability of PET, not because it is not by itself recyclable. In the early stages of PET recycling, there were no practical methods to remove PVC from PET once the two polymers were combined. However, today, optical emission and infrared sorting techniques can be used to identify PVC bottles and PVC flakes in a PET stream, and the PVC may be removed. PET can be used in bottles to bottles applications including food applications, can be used to make polyester fiber as a substitute for cotton, and is used in various engineered resin applications. Recently beer has been packaged in PET containers, but the bottle design involves multilayer components (including either nylons, complex oxides, or even vapor-deposited glass) to provide longer shelf-life. While there are plans to increase packaging of beer in PET, consumer resistance, not technology, has limited the growth of this package at least in the short term.
Figure 19 PET flake and pellet after processing of the bottles at a plastics recycling facility showing the products produced for end use. Reproduced with permission from wTe Corporation.
Recycling – Household Waste 5.2
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HDPE
High-density polyethylene (HDPE) is the most common household packaging material due to its low resin cost and easy formability. Figure 20 is a photograph of various PE bottle designs and applications. HDPE is produced in two basic grades: one is translucent milk jugs, and the other is colored laundry and detergent bottles. The properties of these two types of PE are different. The milk jugs are the resin type of highest value and greatest mechanical properties. Unlike PET, PE is easy to recycle. It melts at relatively low temperatures, is not degraded much by long or multiple melt cycles, and it is easy to form. HDPE resin typically sells for $0.40 per pound, which is half the value of PET resin and this lower value negatively impacts recyclability.
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Paper
Of the 25% of household waste recycled, paper is the largest component by weight. There is no market for paper, as such. The demand is for a particular grade of paper. According to the Paper Stock Institute, there are 51 grades of paper, with some of the major recyclable categories being old newspapers (ONP), old corrugated containers (OCC), and brown Kraft grocery bags. Figure 21 provides a photograph of bales of ONP. If all of these grades are mixed together as ‘paper,’ the product will not be marketable even though there may be an unsaturated demand for the individual components. From this example, we may infer a recycling mixing principal. One part of recyclable
Figure 20 A photograph of various end use packages made from both virgin and recycled HDPE. Reproduced with permission from wTe Corporation.
Figure 21 A photograph of bales of ONP prior to shipping to market. Reproduced with permission from wTe Corporation.
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material no. 1 plus one part of recyclable material no. 2 produce two parts of lower-value recyclable material or alternatively equals municipal solid waste. It is the exception to the rule, and very unlikely, that the combined value of the mixture will equal the combined value of the parts. The key to securing strong, high-value markets for recyclable materials is quality, not quantity. Thus achieving unrealistically high recycling rates can actually destroy recycling, not contribute to it. It is necessary to segregate the materials into their various components or grades with minimal contamination and to produce products of consistent quality. Whenever industry representatives speak to citizens about obstacles to recycling, they constantly caution that it is necessary to keep supply and demand in balance and that the key to recycling is developing sufficient markets and demand for the finished products. However, they assume, perhaps incorrectly, that the product will meet existing material specifications designed around supply from virgin feedstock. These specifications assume a level of quality, which frequently is not achieved, in actual recycling practice. There must be, in place, sufficient end use demand and conversion capacity to absorb the raw materials and to provide the proper basic incentives to invest tens of millions of dollars in a high capacity project. There must also be dependable sources of quality raw materials available in appropriate quantities at the same time that the end use demand is sufficient to support conversion. But when we talk about demand for recycled materials, too often there is little or no discussion regarding the particular grade of product under consideration and the relationship between product purity and industry demand. However, this market differentiation is ‘the most critical factor’ in assessing the demand and value of the product. For example, the product specification for newsprint consists of baled newspapers containing less than 5% of other papers. Prohibitive materials may not exceed 1/2% and total outthrows may not exceed 2%. A special news bale should consist of sorted, fresh, dry newspaper, not sunburned and free from paper other than newspaper, containing not more than the percentage of rotogravure and colored sections normally contained in newspaper delivered to the household. In addition, each particular customer for the recycled paper product may have special contamination requirements that are a function of its particular requirements or processing system. The issue here is that there may be substantial demand for clean, segregated product, but there is little or no demand for unsegregated, dirty product. If all 51 grades were blended together, the paper would be unmarketable as paper. As can be seen mixing in McDonald's bags, cups with plastic coatings and liners, hamburger wrappers, and pizza boxes would make the entire bale of newsprint unacceptable for use or reuse. Few households' recyclers really understand this fact. Their enthusiasm to recycle results in disposing not only the added nonspecification materials but also makes the basic specification grade newsprint with which it is mixed, nonrecyclable.
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Concluding Remarks
In conclusion, while many materials may be theoretically recyclable, as a practical matter, these materials cannot be effectively recycled with the current recycling collection and processing systems. A recycling program, which seeks to achieve too high a recycling rate by including all grades of paper, could actually hurt the potential to recycle the larger part of our segregated waste paper, namely, old newspaper or old corrugated containers. This same principal is not only true for grades of paper, but also for the various grades of metal, plastic, and glass as well. A program that collects all of these materials, but does not adequately segregate them and keep the glass out of the paper and plastic, and the plastic out of the glass just creates a mess. While the mix may be picked up at curbside, and thus be considered by the household to have been ‘recycled’ it will not actually be recycled and will add to the volume of municipal solid waste that is generated each year.
References EPA, 2012. Last updated on 2/28/2014 for 2012 data. Available at: http://www.epa.gov/solid waste/nonhaz/municipal/ (accessed 25.11.15). National Solid Waste Management Association, 1989. Facts on File. Resource Recovery Round-up vol. 2, No. 2, May 1, 1989. The Fredonia Group, Inc, Cleveland, OH, April 1989.
Further Reading EPA: Municipal Solid Waste Generation, Recycling, and Disposals in the United States: Facts and Figures for 2011. Spencer, D.B., 1994. Recycling. In: Kreith, F. (Ed.), Handbook of Solid Waste Management. New York, NY: McGraw Hill, pp. 8.1–8.77. Chapter 9. The Paper Stock Institute, Institute of Scrap Recycling Industries (ISRI), Guidelines for Paper Stock.