|Fig. 1: MSW management by weight in teragrams (Tg) and emissions in CO2Eq. Sources: [1,2,7]|
What happens to trash, or municipal solid waste (MSW), in the U.S.? The rate at which trash is generated is nontrivial: in 2011, the U.S. generated 250 million tons of MSW, or approximately 4.4 pounds per person per day - up from 2.68 pounds in 1960.  This accounts for discarded materials like grass clippings, bottles, food waste, newspapers, appliances, and other typical garbage (excluded are construction and demolition materials, wastewater sludge, and industrial or hazardous waste).
What happens when MSW is discarded? MSW management falls into one of four categories: source reduction (waste prevention), recycling, combustion with energy recovery, and disposal via landfilling (see Fig. 1). The way in which MSW is managed affects many important factors, including greenhouse gases emitted and the amount of energy or resources that become recoverable. Additionally, because waste is regularly collected at public expense, it is clearly advantageous to make use of it wherever possible. Waste management and treatment activities account for 1.9% of total U.S. greenhouse gas emissions.  This report will review these options' advantages and shortcomings and discuss trends.
The EPA's most desired outcome for MSW, source reduction aims to reduce the amount or toxicity of trash created before it enters the MSW management system. Designing products to be more easily reused, require fewer materials or less toxic ones, or have longer life all fall into this category; packaging that reduces product damage or spoilage and on-site composting of nonproduct organic wastes similarly prevent waste.  Examples include the reduction in aluminum beverage can weight (0.462 oz in 2011 vs. 0.546 oz in 1996), replacing glass and steel with lighter plastics and aluminum in products, using electronic rather than printed information in the workplace, replacing nondurable goods with resuable ones, leaving grass clippings on the lawn rather than disposing of them, or purchasing resuable items rather than disposable ones. 
State or federal law has aided implementation of source reduction. For example, prohibiting the disposal of yard trimmings has caused a dramatic increase in the amount recovered from MSW (see Fig. 2). Paper, a highly discarded material, has also seen greatly increased recovery rates and slightly lowered wastes. This is largely attributable to increased use of electronic copies and documents. While undoubtedly the leader amongst the MSW management options in waste prevention and resource conservation, source reduction's application is limited: its impact is difficult to quantify, and without economic or law- enforced provocation, its implementation relies largely on social responsibility and consumer choices.
Recycling and off-site composting compose the second MSE management option. Recycling is the series of operations by which products like glass, metal, plastics, paper, wood, and yard trimmings are recovered from the waste stream for use as resources in the manufacture of new products.  In 2011, 66.2 million tons of MSW were recycled, or about 1.53 pounds per person per day, with an estimated recovery rate through recycling and composting of 34.7% (see Fig. 2). 
The advent of single-stream recycling in the 1990s enabled more efficient collection of recyclables by commingling them all in a single truck; material sorting for reuse occurs at a materials recovery facility (MRF).  The American Forest & Paper Association estimated that 65% of curbside recyclables collection programs were single-stream in 2010, and in 2011, there were 633 MRFs operating in the US with an estimated daily throughput of more than 88,900 tonnes per day.  Unfortunately, MRF sorting is imperfect, and small or flexible materials negatively impact efficiency and commodity quality when commingled.  The EPA has found that although the switch to single-stream generally decreases collection costs, the increased processing costs neutralize any profits due to system change alone.
|Fig. 2: Recyclable materials discarded and recovered. Sources: [1,2,7]|
Recycling has many environmental benefits, including energy savings, resource recovery, and reduced GHG emissions and pollutants.  The energy required to recycle (collect, sort, process, and transport) many materials is often less than that required to obtain (extract, refine, transport, and process) virgin natural resources, partly because the recycled materials have already undergone processing. This is particularly true for glass: recycled glass has a reduced melting point which allows energy savings of as much as 25% during processing and saves 14-20% in GHG emissions; an estimate by the EPA shows savings of 0.15 Tg of CO2 Equivalent (MTCO2E) per ton of glass during processing and transportation. [3,4] Metal recycling also has many benefits. The energy required to recycle it is much less than to produce it from ore, and far less waste is produced during recycling than extraction.  Plus, metals (especially lead and cadmium) that are not recycled tend to result in water and soil pollution when discarded in landfills.  Metals are more difficult, however, to recycle than glass; collection and impurity buildup are the main concerns.  Even so, estimates show savings of 1.7 MTCO2E/ton of steel and 13.5 MTCO2E/ton of aluminum during processing and transportation. 
Despite the benefits of recycling, the United States does not recycle as much as it could: given ample space for landfills, recycling is not always cost-effective; discarding waste into landfills or incinerators is often cheaper.  Critics often cite collection costs or processing costs as the profitability pitfalls of recycling; it is also unclear whether the products made from recycled products are cheaper or of equivalent quality.  Plastics, for example, poses challenges: because many plastics exist and cannot be commingled or have greatly inferior properties when recycled together, necessitating painstaking sorting to achieve the gains in MTCO2E (estimated 1.03 and 1.63 MTCO2E per ton for HDPE and PET, respectively ).  Meanwhile, benefits of paper recycling are dubious: while recycled newsprint has 'clear' gains with regards to fossil fuels, some argue that recycling office paper actually depletes more fossil fuels than when made from trees because of the pulping process, indicating incineration as the better option.  Downcycling, where high quality materials are combined with low-end ones, plagues both recycled plastics and paper or paperboard, causing the materials to lose value. Finally, many materials are not recycled because participants are unaware of their status; conversely, unrecyclable materials become contamination when commingled.  In short, successful recycling requires incentives for correct use, high participation from an informed community, disincentives for garbage production, and efficient collection infrastructure.
Composting has become more prevalent since the 1990s (see both figures above) with an increase of nearly 397% as of 2012.  Composting organic waste like food scraps or garden clippings has many advantages, including reduced waste volume and production of fertilizer or soil amendment. It is an aerobic process and converts much of the organic carbon in the waste material to CO2; most of the methane (CH4) is oxidized.  Composting is gaining traction but is still underutilized, particularly for food scraps; organic materials continue to be the largest component of generated MSW, and only 2.5% of food waste is recovered, leaving it the largest material in discards (21.3%). [1,7]
MSW combustion with energy recovery, or waste-to-energy (WTE), is becoming less popular: in 2000, 33.7 million tons were incinerated compared with only 29.3 million in 2011. WTE involves recovery of an energy product, typically steam or electricity, and reduces the amount of energy needed from other sources with significant energy benefits and fossil fuel offsets. 
Unfortunately, for communities with landfill space to spare, this method is often underutilized. Critics say it encourages waste, does not generate enough energy, and destroys those resources permanently; instead, communities should 'reduce, reuse, recycle'. Studies have found that recycling most materials from the waste stream saves between 3-5 times more energy than combustion, although the composition of the MSW certainly matters; some materials have higher energy density or emit more greenhouse gases than others.  Other disadvantages of trash incineration include its residue and other emitted pollutants. When MSW is combusted, about 25% by weight of the unprocessed MSW input results in residue - typically ash. Although this used to be landfilled, it is generally managed separately from MSW, and attempts are being made to reuse the ash. Waste incineration also results in greenhouse gas emissions, predominately N2O and methane; combustion of plastics accounts for approximately half of these emissions. 
However, incineration must be examined not simply as an alternative to recycling but as a complementary method to achieve lower landfill rates. Better to harvest energy now than allow waste to continue emitting methane for decades.
53.6% of MSW was landfilled in 2011, by far the most common MSW technique in the U.S. Landfilling entails deposition of waste in cells covered daily with soil (soil, clay, or sand) and monitored to collect leachate and landfill gas. [1,2] Current trends show that although more MSW is recovered, either via composting, recycling, or combustion with energy recovery, the amount landfilled has continued to increase.  Landfills remain the third largest contributor of any methane source in the U.S. (approximately 18.1%). 
The amount of waste landfilled, the characteristics of the landfill (size, climate, cover material), and the amount of CH4 that is recovered (flared or used for energy) or oxidized all affect the landfill's methane generation and emissions, primarily composed of methane and CO2.  A slowly decreasing trend in the net CH4 emissions has been observed over the past decade (see figure), likely because more decomposable materials are being recycled or composted and more landfill gas is collected and combusted.  However, this rate of increased landfill gas collection and combustion does not exceed the rate of additional CH4 generation from additional organic MSW landfilled.
Landfill methane emissions could be reduced by accelerating implementation of engineered gas recovery and encouraging alternative MSW management strategies, perferable in combination to best suit America's variable communities. 
© Natalie Burkhard. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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