Municipal Solid Waste in Landfills The issue of plastics in the municipal solid waste (MSW) stream became particularly visible in the 1980s. The United States produces about 232 million tons of MSW annually (amounting to about 4.5 lb per day on a per capita basis) containing 10.7% by weight (or 25 million tons) of plastics . This rate is considerably higher than that of most western countries, Canada, or Japan. Since a fraction of the waste is recycled, only about 3.5 lb per person per day is discarded in the United States according to 1994 data. Most of the plastics discarded is postconsumer packaging materials such as bags and empty containers. About 57% of the MSW was landfilled and 16% incinerated mostly in waste-to-energy plants during 1995. In the United States landfilling is likely to remain the dominant solid waste disposal strategy in the near future. Once disposed in a modern landfill, very little deterioration of the waste is expected. Even the readily biodegradable materials such as paper and yard waste fractions persist for long periods of time in landfills due to the absence of enough moisture and oxygen needed to support a biotic environment within the fill . This is particularly true of food or yard waste contained in plastic bags. Not surprisingly, the plastics fraction does not break down significantly under these conditions. This lack of deterioration is not undesirable, as no polluting leachates or flammable gases are produced in any significant volumes, and the fill remains stable as the volume of compacted waste remains the same.42
The available landfill capacity in the United States is rapidly decreasing. An estimate in 1988 indicated that 40-45% of the then active landfills would reach capacity within 5-7 years. The generally perceived undesirability of plastics in the MSW stream stems from the argument that since plastics do not biodegrade in landfill environments they do not yield their occupied volume for additional disposal of waste [3, 49]. However, this expected additional fill volume had the plastics been biodegradable is far too small to provide any significant advantage. It is easy to confuse the relatively high volume fraction (about 18-25%) of plastics in the MSW stream with that in the landfill. Plastic packaging is compacted to a minimal volume under the weight of the fill above it and the volume of the plastics is likely to be quite small.43 In fact, biodegradability would have the undesirable consequence of generating leachate. The whole argument rests on the assumption of imminent shortage of fill volume in the United States. The total fill volume needed even for the United States for the rest of this century is small (18 mi2 area of land ) and the problem is one of siting and transportation of waste. Undoubtedly, this will slowly increase the cost of landfilling over time and promote the development of innovative alternatives.
The real issue relating to plastics waste in landfills is that it represents a valuable resource discarded well before the end of its useful lifetime (or even after only a single use). The true cost of the waste plastics can only be appreciated if all the natural resource and environmental costs invested in its production are correctly taken into account. Rather than burying these valuable raw materials, the landfills should perhaps be thought of as temporary repositories of durable plastics waste for future recycling or conversion into useful energy. Even regarding plastics in landfills as a future source of fuel is reasonable when one considers the high heat content of common plastics —18,687 Btu/lb for polyethylene and 16,419 Btu/lb for polystyrene, compared to only about 4800 Btu/lb for mixed MSW and 6800 Btu/lb for mixed paper.
Litter Problem Plastics waste in urban litter does pose a serious and real waste management problem. Unlike paper, plastics do not degrade in the outdoor environment at a fast enough rate compared to that of littering. The result is a serious aesthetic problem in highly populated urban areas in almost any part of
42 When a landfill is full, it is possible to cap it and use the surface real estate. The Santama landfill in Tokyo (Japan) 'for instance' plans to build a sports facility on the capped top of the fill after its useful life of 13 years. Other Japanese sites have made similar use of full landfills. The lack of biodegradation prevents any reduction in fill volume over time and is an important consideration when landfills are used in this manner. In fact biodegradable food or paper waste is not accepted at such landfills.
43 The volume fraction of plastics in the MSW stream is calculated assuming volume:weight ratios ranging from 2.4 to 3.9. These estimates are very unlikely to apply to plastics compacted at the bottom of a landfill where the ratio will be closer to unity.
the world. Litter commonly encountered range from cellulose acetate filter tips from cigarettes44 to polystyrene foam cups, and polyethylene plastic bags.
The situation is even worse in the case of the marine environment. Plastic waste, primarily packaging-related waste, finds its way into the worlds oceans from fishing, commercial, and naval vessels. The ships' waste dumped into the ocean includes plastic waste. For instance, a large passenger liner or an aircraft carrier may dispose of as many as several thousand plastic cups a day. Since the U.S. ratification of MARPOL Annex V (and compliance by the U.S. Navy), this practice has ceased. By 1999 naval surface ships were equipped with plastic waste processors for onboard melting and compaction of food-contaminated and other plastic waste. The compacted waste (a molded disk) is stored aboard the vessel for disposal at port. This is a major achievement as it prevents at-sea disposal of the 5.5 million pounds of plastic waste annually generated by the fleet. Fishing gear, however, is either routinely lost or are deliberately disposed of at sea. Commercial fishing nets such as trawls and midwater gill nets (usually made of nylon, negatively buoyant in seawater) are large and are comprised of many thousands of square meters of netting. Their loss or disposal at sea damages the fishery due to "ghost fishing." Gear made of polyolefin material (such as with trawl webbing) remains afloat for a time until heavy encrustation by foulant macroinvertebrates sink them. During this period, marine mammals, birds, fish, and turtles can be entangled or trapped by the discarded plastic waste, particularly in netting, strapping bands, and six-pack rings. Birds are even reported to feed on resin pellets spilt during transportation of resin by sea and then suffer impaired growth. Beach litter is also an important additional source of litter reaching the sea; in fact, a large majority of the plastics polluting the world's oceans include packaging materials that originate at beaches.
Littering is a behavioral problem as much as it is a technology-related issue. The standard approach of collection and recycling does not work well in the marine environment. While some efforts are made to clean beaches of plastics debris, the same is not feasible with plastics at sea. Technology can therefore hope to mitigate only a part of the problem. For example, the entrapment of animals in six-pack rings at sea might be avoided by using enhanced photodegradable six-pack rings made from (ethylene co-carbon monoxide) copolymer as opposed to low-density polyethylene (LDPE). (See Chapter 10) Degradable plastic bags might be used at sea as these disintegrate rapidly, avoiding ingestion by turtles (who apparently mistake the partially inflated bags for jelly fish). But, the enhanced-degradable polymer technology has not provided a generic answer to all problems associated with marine plastic waste. The ingestion of plastic pellets by birds or the introduction of microfragments of plastics into the feed of filter feeders at sea is not addressed by these technologies. The possibility of concentration of organic compounds in water in the plastic material via partitioning at
44 About 4.5 trillion cigarette butts made of cellulose acetate fiber is discarded as litter each year! In addition to the fire hazard associated with carelessly discarded buts, the possibility of their leaching out the concentrated trapped chemicals into soil water has been raised.
sea, and their possible introduction into the marine food chain via ingestion by organisms, have not been adequately addressed.
Release of Hazardous Emissions from Incineration Incineration of MSW allows the recovery of energy from the flue gases formed in the process. This energy might be used to generate process steam or converted into electricity. A state-of-the-art combustion plant operates at about 22% efficiency and produces 600 kWh of electricity per ton of waste (the efficiency can be much higher for combined heat and power generation plants). Compared to the burning of coal, for instance, burning of MSW produces less acidic gases and less carbon dioxide. Incineration, however, is not always a particularly clean process, and the plastics fraction in the waste has been blamed for compounding the air pollution problem. Specifically, the production of hydrogen chloride on burning PVC, the contribution of the chlorine from PVC to the formation of dioxins,45 and the influence of plastics on the metal content of the incinerator ash have been pointed out. The amount of data available to substantiate these claims, however, are quite small and somewhat inconsistent. For instance, incineration of feed waste containing increasing amounts of chlorine was found to have a minimal effect on the amounts of dioxin produced on incineration [50, 51]. Results such as these suggest that the chlorine content of the waste might not be the determining factor for dioxin production. Continuing studies will eventually quantify the extent to which plastics contribute to the issue.
Dehydrochlorination of PVC is a facile reaction and is expected to take place on incineration of the plastic waste [52, 53]. The corrosive fumes can potentially affect the incinerator structure itself and if allowed to escape into the atmosphere will contribute to the acidification of the environment. Analysis of flue gases from mixed MSW incinerators typically show very small amounts of HCl (<2000 mg/m3 of gas) . Either the amounts generated are quite low or the HCl formed in the incinerator undergoes reaction with other waste components or incineration products.
Particular emphasis has been placed on the emissions of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzo-furans (PCDF) during incineration of MSW. Depending on the design, operating temperature, and the composition of waste incinerated, most incinerators may emit small amounts (ranging from a few nanograms to a few thousand nanograms per cubic meter of gas) of these highly toxic compounds. The toxicity46 of these two classes of compounds, based on extensive animal studies, has been reviewed [55, 56] and is highly dependant on the number and position of the chlorine substituents in the molecule. Trace amounts of these already present in the MSW can be volatilized
45 This is a generic name for a family of about 210 related cyclic chlorinated hydrocarbons, some of which are believed to be very toxic. Toxicity studies based on animal studies, however, show different species to vary enormously in their susceptibility to dioxins.
46 The toxicity of these compounds is species dependent. The most toxic is 2,3,7,8-tetrachlorodibenzo-p -dioxin.
in the incinerator or these might be the result of de novo synthesis catalyzed by the fly ash at high temperatures. However, a third relevant mechanism is the possible formation of these compounds by in situ synthesis with the chlorine being derived from the hydrochlorination of PVC in the waste . While there appears to be no relationship between the amount of PVC in the waste and the emission of these toxic compounds during incineration [58, 59], the issue is still being investigated.
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