Use of Materials Balances to Estimate Aggregate Waste Generation in the United States

Robert U. Ayres and Leslie W. Ayres

One can view each industrial sector as a transformation process, where raw material inputs or purchased commodities from upstream sectors and "free goods" from the environment are converted into products for downstream sectors and wastes. This conversion process is subject to the materials-balance constraint not only in the aggregate, but also element by element. In other words, the sum of the weights of all inputs must exactly equal the sum of the weights of all outputs. When both inputs and outputs are known, it is possible to estimate wastes, making due allowance for processes utilizing the free goods (i.e., air, water, topsoil). Of course, in reality, there are often significant uncertainties with regard to either inputs or outputs, or both. These arise from statistical inconsistencies, stock adjustments, imports, and exports. In other work, we have attempted to take all of these into account.

It is important to explain what we mean by macrolevel in this context. In general, large-scale mass flows exceed by many orders of magnitude the flows of the most highly toxic pollutants, including trace elements. In our balancing efforts, we have attempted to construct input-output tables for major process stages. Thus, in the case of agriculture and forest products, we try to balance the flows of carbon, oxygen, hydrogen, and major nutrients. At this level of detail, it is not possible to account precisely for minor flows (e.g., of pesticide residues). Studies accounting for minor flows would have to use different methodology based much more on detailed chemical and metallurgical process data than on economic data.

To avoid unnecessary and distracting biological complications, we treat biomass as a produced good of the agricultural and forestry sectors, even though much of it is arguably free. Unfortunately, this leaves us with the problem of



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--> Use of Materials Balances to Estimate Aggregate Waste Generation in the United States Robert U. Ayres and Leslie W. Ayres One can view each industrial sector as a transformation process, where raw material inputs or purchased commodities from upstream sectors and "free goods" from the environment are converted into products for downstream sectors and wastes. This conversion process is subject to the materials-balance constraint not only in the aggregate, but also element by element. In other words, the sum of the weights of all inputs must exactly equal the sum of the weights of all outputs. When both inputs and outputs are known, it is possible to estimate wastes, making due allowance for processes utilizing the free goods (i.e., air, water, topsoil). Of course, in reality, there are often significant uncertainties with regard to either inputs or outputs, or both. These arise from statistical inconsistencies, stock adjustments, imports, and exports. In other work, we have attempted to take all of these into account. It is important to explain what we mean by macrolevel in this context. In general, large-scale mass flows exceed by many orders of magnitude the flows of the most highly toxic pollutants, including trace elements. In our balancing efforts, we have attempted to construct input-output tables for major process stages. Thus, in the case of agriculture and forest products, we try to balance the flows of carbon, oxygen, hydrogen, and major nutrients. At this level of detail, it is not possible to account precisely for minor flows (e.g., of pesticide residues). Studies accounting for minor flows would have to use different methodology based much more on detailed chemical and metallurgical process data than on economic data. To avoid unnecessary and distracting biological complications, we treat biomass as a produced good of the agricultural and forestry sectors, even though much of it is arguably free. Unfortunately, this leaves us with the problem of

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--> accounting for water, as both an input and output, which cannot be done with great precision. Fortunately, great precision is probably not necessary in this case. Labor and capital inputs, such as machinery, and fuel or electric power for operating the machinery, are not considered explicitly in this paper. However, it should be borne in mind that a considerable fraction of aggregate industrial output is actually capital (and operating) input to other sectors. Our immediate intention is to classify outputs as either economic commodities or missing mass. In subsequent work, the ''missing mass" will be further classified based on the level of waste treatment and final disposal medium (air, water, or soil). This means we need to be quite careful in accounting for the consumption of oxygen (from air) in oxidation processes and for the consumption or production of water in hydration, dehydration, dilution, dissolution, and so on. We selected 1988 as the year of reference for this study because it was the last year for which we had reasonably good international data at the time we began the work. Unfortunately, 1988 was a very atypical year for U.S. agriculture, as we note below. Agriculture1 Inputs to the agriculture sector consist of sunlight, water, carbon dioxide from the air, nitrogen fixation also from the air, topsoil, and some chemicals (e.g., fertilizers and pesticides). Commodity outputs are harvested crops. (Dairy products and meat are considered separately in the next section.) Missing mass, in the aggregate, consists mainly of crop wastes, runoff, evapotranspiration, and oxygen, a by-product of photosynthesis. Other losses include soil erosion, nitrogen (and phosphorus) carried away by water sources, and gaseous emissions. The production process in agriculture (and also forestry, considered below) can be estimated crudely from the following basic equation of photosynthesis: CO2 + H2O + photon ⇒ CH2O + O2. Plants fix carbon in daylight and release part of it (about half) at night. Water carries nutrients and metabolic products and provides evaporative cooling. There is a rough average proportionality between carbon fixation rate (gross photosynthesis) and evapotranspiration, but there is no fixed relationship between water content and metabolic process; some plant parts are very high in water content, others much less so. In general it seems reasonable to assume that raw biomass contains 50 percent water by weight on average, whereas refined or processed food or feed commodities are considerably drier. In cases where actual data are lacking, we assume 25 percent water content for processed "dry" commodities. Unfortunately, official statistics are not informative on this point. Raw products of U.S. agriculture include truck crops (fruits, vegetables), tree crops, and field crops (grain, oilseed, hay and alfalfa, sugar beets, sugar cane,

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--> TABLE 1 Agricultural Production in the United States, 1988 (million metric tons)   Production Consumption Commodity Raw Finished Exports Imports Raw Finished Beef and veal 17.82 10.88 0.31 1.09 11.62 11.20 Lamb and mutton 0.32 0.15 0.00 0.02 0.18 0.18 Pork 9.91 7.11 0.09 0.52 7.51 4.87 Poultrya 12.95         6.37 Eggsb 4.44         3.86 Dairy products 65.86 64.41       64.41 Subtotal 111.30   2.77 1.26   59.90 Food grainsc 56.55   44.26   35.40d 17.37 Feeds and fodders     11.37       Feed grains and productse 147.06   55.21       Oilseeds and productsf 49.16   26.90       Hay 114.31           Sugar cane 27.13 2.88   1.21   3.40 Sugar beets 22.51 2.97       3.40 Other field productsg 5.03   1.57     3.46 Corn syrup       7.64     Subtotal 421.75 139.31       35.27 Vegetables 25.14         22.27 Potatoes 16.17         14.77 Fruits 25.60         11.51 Nuts 0.55           Fruits, nuts, and vegetables 4.06 6.74     Coffee, cocoa     1.48     3.40 Subtotal 67.47   4.06 8.21   51.94 Fish 3.26   0.48 3.37 4.77 1.40 Vegetable oils   6.40 1.30 1.35   6.66 SUMMARY Animal products 111.30   2.77 1.26   90.87 Field products 421.75   139.31     35.27 Vegetable products 67.47   4.06 8.21   51.94 Fishery products 3.26   0.48 3.37   1.40 Vegetable oils 6.40   1.30 1.35 4.77 6.66 TOTAL 603.71   147.92 14.19   186.15 a Poultry conversion at $0.33/lb. b Egg conversion at 0.77 kg/dozen. c Food grains = wheat, rice, rye. d The difference in consumption of raw and finished food grains is used for beer and distilled spirits. e Feed grains and products = corn and sorghum for grain, barley, oats. f Oil seeds and products = soy beans, peanuts, cottonseed, flax seed. g Other field products = dry beans and peas, cotton lint, tobacco. NOTE: Table does not display stock changes, particularly large in 1988. SOURCES: Bureau of the Census (1990, 1991) and United States Department of Agriculture (1990, 1991).

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--> potatoes, cotton, tobacco). Harvested output of all field crops, including hay, in 1988 was 421.75 million metric tons (MMT).2 Truck crops totaled 67.5 MMT. Total weight of harvested crops was 489 MMT (Table 1).3 It should also be noted that corn plants harvested whole for silage, or "hogged" on the farm, are not included in the grain production figures. This material, which is fed to animals, is classed as "harvested roughage"; it amounted to 68 MMT in 1988. Total biomass harvested by humans and animals including grazing was 885 MMT. According to one estimate, the average ratio of above-ground crop residues remaining on the land to harvest weight is about 1.5 for cereals (straw), 1.0 for legumes (straw), 0.2 for tubers (tops) and sugar cane (bagasse), and 3.0 for cotton (stalks) (Smil, 1993). On this basis, total residues left above ground in 1988 would have been around 400 MMT. Total above-ground biomass production was about 1,285 MMT. In the United States, most of the crop residues are left on the land; a small fraction, about 20 percent, is burned for fuel or used for other purposes (Smil, 1993). (In China and India, by contrast, as much as two-thirds of crop residue production is burned as fuel in household cooking.) Biomass is a mixture of cellulosic fiber, carbohydrates, fats, proteins, and water, the latter accounting for about 50 percent by weight. Hence, we estimate that the dry weight of the biomass produced in 1988 was about 642 MMT. For each 100 units of dry output (CH2O basis), the photosynthetic process equation implies that 146.7 MMT CO2 (containing 40 units of carbon) were initially extracted from the air, 60 units of water were converted, and 106.7 units of oxygen were returned to the atmosphere. Overall, for 1988, water inputs—not including water required for evapotranspiration—were about 1,027 MMT, and carbon dioxide inputs were about 1,002 MMT. Oxygen produced by the photosynthesis process in agriculture would have been about 685 MMT. The overall flows for U.S. agriculture in 1988 are summarized graphically in Figure 1. One other air pollutant, methane, is worth mentioning. Most animals have anaerobic organisms in their guts that convert a small amount of the food they consume into methane, typically 1-2 percent on an energy basis. However, this percentage is larger for cattle and sheep, ranging from 5.5 to 7.5 percent, depending on the quality and quantity of feed. Taking these factors into account, Crutzen et al. (1986) have estimated annual methane output of 60 kg per head of cattle and 8 kg per head of sheep. The U.S. cattle population in 1988 was 99.6 million; the sheep population was 10.5 million. Methane emissions from these sources amounted to 0.68 MMT. It should be noted that the agricultural sector uses large amounts of fertilizers and pesticides. The nitrogen (N) content of ammonia used for fertilizer consumed domestically was 11.2 MMT in 1988, or 76 percent of all the synthetic ammonia produced in the United States that year. Large quantities of urea (about 0.33 MMT), a fertilizer material, are also used for animal feed supplements. Domestic agriculture consumed 33.5 MMT of phosphates containing 10.8 MMT P2O5. Many of these substances find their way directly or through animal excreta into surface water and groundwater. Much of the N content of animal feed ends up in

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--> Figure 1 Mass flow and carbon, hydrogen, and oxygen balances in U.S. agriculture, 1988 (million metric tons). Calculated by the authors from various sources, including the Bureau of the Census (1990) and United States Department of Agriculture (1992).

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--> urine, either on pastures or at feedlots, resulting in both air and water pollution. We do not have an accurate estimate of the quantities involved, but probably two-thirds of the urine is generated at feeding stations. Fertilizers and pesticides, direct chemical inputs to agriculture, are not counted explicitly as pollutants, although their use results in pollution. Animal wastes are a major pollution problem, especially in the vicinity of animal feedlots and large-scale poultry producers. Of 100 units of nitrogen in fertilizer, roughly 50 are taken up by harvested crops, of which 47 are subsequently consumed by animals, and 42 of these are eventually excreted as waste (Crutzen, 1976). Most of this waste is generated at feedlots because fertilizer is seldom used on grazing land, and the nitrogen uptake by grazing animals is largely left behind as manure or urine. About 24 units of nitrogen find their way to rivers, lakes, and groundwater, of which 10 units are direct runoff from the soil, 8 come from animal excreta at feeding stations, and 6 from human sewage. Thus, about 18 percent of nitrogen in agricultural fertilizer reappears within a few weeks or months as waterborne pollution, although only 10 percent is due to direct fertilizer loss. Because 11.5 MMT N were used in fertilizer and feed supplements in 1988, this implies an overall waterborne nitrogen-waste flow of 2.76 MMT. Schlesinger and Hartley (1992, table 4) estimated annual NH3 emissions per head at 15.5 kg from cattle and horses, 2.4 kg from sheep, 2.35 kg from pigs, and 0.21 kg from poultry. Based on 1988 populations of 99.6 million cattle, 55.5 million pigs, 10.9 million sheep and lambs, and 5.7 billion chickens and turkeys, this totals 2.91 MMT. Fertilizer itself is also a source of ammonia emissions; the emission factors for urea and ammonium sulfate spread on the soil surface are estimated at 0.2 and 0.1, respectively; for other fertilizers—including anhydrous ammonia injected directly into the soil—the emission rate is lower (around 3 percent) (Schlesinger and Hartley, 1992). In 1988, 2.49 MMT N in urea were used as fertilizer in the United States along with 0.340 MMT of nitrogen in ammonium sulfate and 6.84 MMT N from other sources. Altogether, animal metabolism and fertilizer use generated nitrogen emissions of 3.79 MMT (as ammonia). This represented nearly 33 percent of the 11.6 MMT (N content) of ammonia equivalent that was used as fertilizer. Of this, about 8 percent was a direct loss. The rest of the nitrogen unaccounted for in the applied fertilizer (about 32 percent) is embodied in root and stem material that is not harvested or is harvested directly by animals and remains with the soil (20-25 percent), or is reconverted to nitrogen gas and returned to the atmosphere by denitrifying bacteria in the soil (5-10 percent). For every 16 units of nitrogen emitted as N2, on average 1 unit is emitted as N2O, a potent greenhouse gas, but these emissions tend to be episodic. Recently, the use of nitrogenous fertilizer has come to be recognized as one of the major sources of N2O buildup. Worldwide, an estimated 0.7 MMT N2O are emitted annually from this source (Schlesinger, 1991). The United States was responsible for roughly one-eighth of worldwide nitrogenous fertilizer use in

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--> 1988 and probably a similar proportion of N2 Oemissions, together the equivalent of 0.055 MMT N content. The above estimates do not take into account the relatively small quantities of other chemical elements embodied in the crops, notably nitrogen, phosphorus, and other minerals taken up from the soil or, in the case of nitrogen, fixed by bacteria. It is of interest, however, that the three major chemical elements in dry plant tissue are carbon, hydrogen, and oxygen, which account for 95 percent of the total mass. Nitrogen accounts for another 2 percent, phosphorus for 0.5 percent, potassium for 1 percent, and sulfur for 0.4 percent. These are the major nutrients that are depleted by harvesting and must be replaced by the addition of fertilizers. The remaining 1 percent of plant mass consists of other mineral elements (Table 2) that are readily available from the soil. The flows of nutrients (nitrogen and phosphorus) in U.S. agriculture are summarized in Figure 2. In 1988, 133 MMT of grain, vegetables, and oilseeds (mostly soya beans) were exported, not including 11.4 MMT of "feeds and fodders," which are from the processing sector. The remainder was consumed directly or indirectly within the United States. Final consumption of all food products (not including beverages) for 1988 was 186 MMT, plus about 20 MMT for grain-based beverages, alcohol, cotton, wool, and other products. Indirect consumption (as animal feed) accounted for most of the difference between gross production and final consumption. According to the U.S. Department of Agriculture (USDA) (1992), U.S. livestock in 1988 were fed 119.4 MMT of feed grains and 3.7 MMT of food grains TABLE 2 Chemical Composition of Plants Element Percent by Weight Oxygen 45 Carbon 44 Hydrogen 6 Nitrogen 2 Potassium 1 Calcium 0.6 Phosphorus 0.5 Sulfur 0.4 Magnesium 0.3 Manganese 0.05 Iron 0.02 Chlorine 0.015 Zinc 0.01 Boron 0.005 Copper 0.001 Molybdenum 0.0001 Total 99.9011

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--> 103 Figure 2 Flows of nitrogen (N) and phosphorus (P) in U.S. agriculture, 1988 (million metric tons). Calculated by the authors from various sources, including Smil (1993).

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--> (mostly wheat). Other harvested animal feeds included 123 MMT of hay and alfalfa, 4.76 MMT of sorghum as silage, and approximately 68 MMT of harvested roughage (such as cornstalks) mixed with other feeds, for a total of 319 MMT of harvested feeds. By-products of the food processing industry such as grain mill by-products (e.g., gluten), oilseed meal, brewers and distillers dried grains, meat and fish meal, dried milk, dried beet pulp, and molasses accounted for an additional 33.4 MMT of animal feeds (United States Department of Agriculture, 1992, table 3).4 Assuming animal intake of pasturage (mainly by cattle) to be about 200 MMT, we can account for total animal feed consumption in 1988 of 552.4 MMT, not including water, salt, urea, or other minor inputs. Animal feed concentrates in the United States average 79 percent digestibility. From this, we can conclude that 21 percent of the mass of animal feed concentrates (156.3 MMT) fed to dairy cattle, beef cattle in feedlots, hogs, and poultry is passed through immediately in feces. Harvested roughage, or silage and hay (196 MMT), has lower digestibility, probably around 60 percent. This implies 40 percent passes through in feces. Therefore, annual manure output from on-farm and industrial animal feeding operations amounts to about 100 MMT. In addition, USDA (1992) estimates that animal intake from pastures is about 200 MMT. Assuming 60 percent digestibility, roughly 80 MMT of manure is probably left on pastures. This figure could be too low; the digestibility of pasturage may be as low as 40 percent.5 Of the total annual manure output of about 180 MMT, it appears that 100 MMT is generated in confinement, and of this, 75 percent (75 MMT) is probably recycled to croplands (Smil, 1993). The remainder of the manure from feedlots (25 MMT, about 50 percent water) is lost to runoff or in other ways. The 80 MMT of manure left on pastures is returned directly to the soil—but not to croplands per se—and does not constitute a waste. As to the outputs of the livestock sector, a total of 111.3 MMT can be accounted for as the gross weight of animal carcasses and dairy products produced for the market. (See below.) Adjusting for the "excess" water content of raw milk (87 percent water), we assume that the sector produces about 81 MMT of equivalent animal products having the same average moisture content as feeds (50 percent). As noted above, feed inputs equal 552 MMT and manure outputs (50 percent moisture basis) are 180 MMT. Simple arithmetic (552 - 180 - 81) reveals that 291 MMT are lost through metabolic (respiration) processes. An estimated 50 percent of this lost mass (approximately 145 MMT) is carbohydrate (CH2O) metabolized for energy. This implies that animals consume 155 MMT (1.067 x 145) of oxygen and produce 213 MMT (1.47 x 145) CO2. The oxidation process also generates 87 MMT (0.6 x 145) of water as vapor in addition to the 115 MMT of water contained in the feed and not otherwise accounted for. Most of this ends up in manure or urine. The water balance is more complicated, of course, because we have not allowed for the water consumed and re-excreted by the animals.

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--> The greatest mass movement from agriculture is the loss of topsoil due to wind and water erosion. A detailed study of topsoil loss in agriculture was carried out by the U.S. Soil Conservation Service in 1982 (Brown and Wolf, 1984). The study found that 44 percent of U.S. cropland was losing topsoil at an unsustainable rate (i.e., faster than the natural rate of soil formation). Topsoil loss in 1982 was estimated at 1,530 MMT. This amount of loss can be assumed to be roughly constant year to year, although optimists believe that the erosion-loss rate is declining as a consequence of increasing use of no-till methods of cultivation. Also, it must be pointed out that eroded material is not necessarily carried out to sea; it may be redeposited on the same field or in the bed of a nearby stream. To summarize, we estimate overall annual waste from U.S. agriculture as follows: topsoil erosion, 1,500 MMT; undigested and unrecycled feedstuffs (feces) from animals at feeding stations (not including grazing animals on pastures), 25 MMT (50 percent moisture) or 12.5 MMT (dry weight). The latter is mostly undigested cellulose, but includes about 4 percent (0.5 MMT) nitrogen and 1 percent (0.125 MMT) phosphorus. Urine apparently accounts for roughly 42 percent of the total nitrogen content of synthetic fertilizer, or about 4.8 MMT; but this is only the fertilizer contribution. The total must be about three times higher. About a quarter of this (1.2 MMT from fertilizer, 4 MMT total) is volatilized immediately as ammonia; around 2.8 MMT from fertilizer (9 MMT total) ends up in watercourses; the rest goes into groundwater or is recycled back to the land. Ammonia emissions to the atmosphere, direct from fertilizer use, seem to be about 1 MMT N, or 10 percent of inputs, but volatilization losses from manure and urine add another 4 MMT. Other sources (organic decay) add a further contribution. The total for U.S. agriculture is probably around 6.5 MMT per year. Methane emissions to the atmosphere from grazing animals in the United States were apparently about 0.68 MMT (Figure 1). A rough balance for nitrogen and phosphorus is shown in Figure 2. Food and Feed Processing6 The food and feed processing sectors entail a number of activities, including grain and oilseed milling, meat and dairy processing, cotton processing, oil products, sugar production, fermentation industries, baking, confectioneries, and canning and freezing. Unfortunately, USDA does not clearly separate these activities or identify their inputs and outputs. We estimate inputs to the food processing sector (361 MMT) as the gross agricultural production of harvested crops (489 MMT) less harvested crops fed to animals (123 MMT grains and 114 MMT hay) less exports (excluding exports from stockpiles, 133 MMT), plus animal products (111 MMT), fish (3.3 MMT), and net imports of foodstuffs (14 MMT). The consumption of domestic food products (flour, prepared cereals, pack-

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--> aged rice, etc.) from all grain mills in 1988 amounted to 17.37 MMT. This does not include grain consumed by the fermentation industries, which produce both alcoholic beverages and fuel alcohol. We estimate that about 19 MMT of grains, mostly corn, were used for fermentation products in 1988. In addition to grain products, 6.4 MMT of vegetable oils and 7.64 MMT of corn syrup were produced by grain and oilseed mills. The fermentation industries, in turn, produced 2.2 MMT of beverage alcohol and 5 MMT of ethanol for fuel, 1 MMT of animal feed concentrates, and an estimated 1 MMT of beverage carbohydrates. To make up the balance, we estimate outputs of 7 MMT CO2 and 3 MMT of water vapor. Cotton is a major agricultural product that contributes little to feed and nothing to food. In 1988, the United States produced a net of 9.2 MMT of raw cotton. This was ginned to yield 3.36 MMT of cotton fiber (lint), 5.5 MMT of cottonseed, and 0.27 MMT of linters. Linters are a fibrous material used for felting and cellulosic chemical manufacturing and so are not wasted. The cottonseed was allocated to mills and "other uses," including exports. Mills purchased 4.38 MMT of seeds, of which 3.38 MMT were actually crushed (United States Department of Agriculture, 1992, table 141). The mill product was 0.56 MMT of oil, 1.53 MMT of high-protein cottonseed oilcake, used for animal feed, and 1.29 MMT of milling waste. (The latter is included with overall milling waste in Figure 1.) Sugar cane weighing 27.13 MMT was reduced to 2.88 MMT of refined cane sugar; similarly, sugar beets weighing 22.51 MMT yielded 2.97 MMT of beet sugar. (About 0.69 MMT of lime was also used in the latter process.) Sugar refining also yielded about 0.59 MMT of molasses (equivalent dry weight), mostly fed to animals. The remainder of the sugar cane waste was mostly cellulosic bagasse. The sugar beet process produces large quantities of pulpy material; about 1 MMT (dry) of this was used for animal feed in 1988 (United States Department of Agriculture, 1992, table 73). The mass disappearance from these two processes alone amounted to about 42.1 MMT. At least half of this mass loss, perhaps as much as 60 percent, or 25 MMT, is water vapor from the various evaporation stages in sugar production. The remaining dry mass is probably burned for energy recovery, although some residues may be discharged into rivers by sewage plants. Truck crops (vegetables and berries) and tree crops (fruits and nuts) accounted for a harvest weight of 67.5 MMT. Exports took 4.06 MMT and imports added 8.21 MMT, for a total domestic supply of 71.6 MMT. Final consumption, on an as-purchased basis accounted for 51.9 MMT. The difference, 20 MMT, was presumably waste, at both food processing plants and retail stores. We estimate that 60 percent of this mass loss (12 MMT) was evaporative water loss from freeze-drying (e.g., of orange juice) and other processing. The bulk of the food process waste goes into waterways or municipal waste facilities. Some is recovered for other uses, and a small amount may be burned for energy. Animal products in the United States can be divided into red meats, poultry, and eggs and dairy. The live weight of animals slaughtered for red meat in 1988

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--> sions, primarily particulates and CO, is about 1 MMT (United States Environmental Protection Agency, 1991). We have no estimate of water use by the stone, clay, and glass sector. However, EPA estimated total wet wastes from the sector to be 560 MMT (United States Environmental Protection Agency, 1986). This seems quite high, given that most processes in the sector are dry. Fossil Fuel Consumption38 Combustion of fossil fuels produces a variety of wastes. This is particularly true for coal. On the average, U.S. coal has a sulfur content of 1.9 percent; coal burned by electric utilities averages 2.3 percent sulfur, whereas coking coal is 1 percent sulfur. The latter is mostly recovered as ammonium sulfate. Coal burned in the United States emits about 16 MMT of sulfur (32.1 MMT SO2). Most of this sulfur dioxide is released to the atmosphere. In 1988, 1.24 MMT of lime (CaO) and 1.035 MMT of limestone (CaCO3) were sold for use in removing sulfur from furnace stack gases. The limestone was equivalent to 0.495 MMT of lime. Because CaO has a molecular weight of 56 and SO2 has a molecular weight of 64, the total amount of limestone and lime used in scrubbers accounted for only 1.96 MMT of sulfur dioxide, or about 6 percent of the total emitted. None of the sulfur from coal burning was recovered for further use. (It is disposed of in landfills as a mixture of wet calcium sulfite CaSO3 and calcium sulfate CaSO4.) EPA estimated that flue gas desulfurization by utilities produced 14.4 MMT of solid wastes in 1984 (United States Environmental Protection Agency, 1988, 1991). The mineral content of these wastes, even in 1988, was evidently no more than 3.7 MMT. The remainder was presumably water of hydration. (The mineral gypsum has the formula CaSO4 · 2H2O.) If all the sulfur in U.S. coal were to be captured by wet scrubbers using lime, total U.S. lime production would triple to 26 MMT, which would require an additional 55 MMT of limestone to be quarried. All of it would, of course, be converted almost directly into a waste stream. Coal contains a small but significant percentage of fuel-bound nitrogen (about 1 unit per 68 units of carbon). Most of this is emitted as nitric oxide (NO) but some may be emitted as nitrous oxide (N2O), one of the greenhouse gases. However, experts disagree about the amount of nitrous oxide produced by this process. More important, coal combustion in high-temperature boilers, used to generate electric power, produces a significant quantity of NOx emissions, about 10 MMT/yr (United States Environmental Protection Agency, 1986). Virtually all anthropogenic NOx (about 20 MMT/yr in 1980 and probably a similar amount in 1988) is attributable to the burning of fossil fuel. Coal also contains significant quantities of mineral ash, equivalent to mineral shale. The average ash content of U.S. coal, as burned, is approximately 10 percent (Torrey, 1978). Actually, utilities alone seem to have collected and dis-

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--> posed of 62 MMT of ash in 1983. Assuming constant proportions of ash in coal used and complete ash recovery, the weight of disposed ash would have risen to 76 MMT by 1988, which would account for almost all of the ash in the utility coal. However, although the efficiency of recovery of fly ash from electrostatic precipitators is in the neighborhood of 99.8 percent for the most modern units, some utilities are not so well equipped. Fly ash not captured in 1988 probably amounted to at least 1 MMT. The ash content of coking coal, which is selected in part for its low ash content, ends up in metallurgical slag. The ash content of coal used as a fuel in the cement industry ends up as part of the cement itself. In fact, the cement industry also uses a small amount of fly ash as a raw material. Coal ash contains significant quantities of heavy metals. Although most fly ash is captured, the waste ash must be disposed of somehow. Moreover, the more volatile trace metals such as arsenic and mercury still escape as vapor and recondense downwind of the stack.39 Finally, the carbon in coal, along with the carbon in other fuels, is converted by combustion into CO2. The Carbon Dioxide Information Analysis Center (1990) at Oak Ridge National Laboratory estimated that the carbon content of these fuels was 1,288.6 MMT, or 84.7 percent. Of this, 493 MMT was from solid fuels (coal), generating 1,810 MMT CO2. This includes the CO2 from carbothermic reduction processes using coke. With the exception of electric power generation, most fuels are petroleum products or natural gas. Natural gas is mostly used for domestic purposes and space heating, although some is used in industry. Petroleum products are mainly used for transportation, although some heavy oils are used for industrial boilers or electric power production. The transportation system is of interest because there are so many complex mass flows involved, other than the straightforward consumption of fuel. We summarize this system, for private automobiles only, in Figure 11. The sum total of all fossil fuels consumed in the United States in 1988 was 1,521 MMT (Table 5). We assume that all of the fossil fuel carbon was converted to CO2 (4,726 MMT in 1988), not including calcination processes (lime and cement manufacturing), which are counted separately. Combustion processes also result in some releases of methane to the atmosphere, but more methane escapes to the atmosphere during production and transmission. One study allocates 11.86 MMT of methane releases in the United States in 1988 to all of these activities together (Subak et al., 1992). The study does not provide a breakdown for the United States among production, transportation, and combustion. However, for the world as a whole, the breakdown was coal mining (62 percent), oil and gas extraction (14.8 percent), gas distribution (17 percent), firewood combustion (4 percent), and other fossil fuel combustion (2.3 percent). For the United States, firewood combustion would be a negligible source of methane, coal would be less important than it is globally, and natural gas would be more important than it is globally.

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--> Figure 11 Mass flows associated with private automobiles in the United States, 1988 (million metric tons). Calculated by the authors from various sources, including International Energy Agency (1991), Bureau of Mines (1988), and Ecoplan International (1992). aBenzene-toluene-xylente. bThirty-three percent of total road/highway expenditure is for maintenance and repair. Materials use per dollar is equal for new construction and for maintenance and repair. Fifty percent of all road/highway depreciation is attributable to automobile use. cTotal estimated materials used for road/highway construction, repair, and maintenance: bitumen, 16 MMT: Portland cement, 10 MMT; steel, 35 MMT; slag, 15 MMT; sand and gravel, 600 MMT; and crushed stone, 840 MMT. Summary It may be interesting to summarize our results by waste category as well as by industry. Overburden moved by mining, mostly stripping, amounted to over 6,800 MMT in 1988. By contrast, topsoil loss in agriculture was on the order of 1,500 MMT. (In addition, the construction industry probably moves comparable amounts of topsoil.) Mineral concentration activities, mostly by froth flotation, produced waste (tailings) on the order of 900 MMT (600.6 MMT metals, 140.7 MMT nonmetals, 47 MMT coal cleaning, 57 MMT drilling wastes, 57.2 MMT

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--> phosphates and alumina) dry weight. In addition, about 3,600 MMT of water was used for flotation, most of which was evaporated in ponds, leaving semisolid sludges. Waste water discharged into rivers and streams by the mining industry amounted to 2,840 million gallons per day, or 3,900 MMT for the year. By contrast, the weight of solid wastes from metallurgical conversion and fossil fuel combustion processes, including metallurgical reduction (smelting), amounted to only about 146.4 MMT. We have included in this category 76 MMT of fly ash and bottom ash from thermal power generators and 14.4 MMT of flue gas desulfurization waste but excluded 14.4 MMT of iron/steel slag that have commercial uses. Organic pretreatment wastes are more difficult to account for. Crop residues, mostly recycled to land, amounted to about 360 MMT. Timber residues from logging operations amounted to 155 MMT, mostly burned or recycled to forest soils. (In some countries, both agricultural and timber residues are collected and burned as fuel, but this is relatively rare in the United States.) About 180 MMT (50 percent dry) of animal wastes—manure, urine, and dead animals—were produced, of which an estimated 155 MMT were probably recycled to cultivated land and the remainder lost in other unspecified ways. An additional 110 MMT of organic wastes (50 percent dry) were generated in the food processing sector and disposed of in various unspecified ways, including waterways. About 5 MMT were lost in the wood products and pulp and paper sectors. Gaseous combustion products constituted another very large waste stream. We estimate gross emissions of 5,046 MMT CO2, of which 4,726 MMT were from fossil fuel combustion. However, thanks to a take-up of 1,002 MMT by the agricultural sector and 368 MMT by the forestry sector, net emissions of CO2 were 3,759 MMT. (By the same token, industrial activities, mainly fuel combustion, consumed 5,393 MMT of oxygen, whereas agriculture and forestry produced 891 MMT of oxygen, for a net consumption of 4,732 MMT.) Other gaseous emissions included 32 MMT SO2 from coal and EPA's estimate of 20 MMT NOx from fossil fuel combustion. Our methodology is not well suited to estimating fugitive or particulate emissions. However, we note that petroleum refineries may have emitted as much as 4.3 MMT of hydrocarbons that are not accounted for anywhere else. Process water contaminated by acids or other wastes was also emitted in significant quantities by the petroleum refining and metallurgical sectors. The quantitative values discussed above are summarized in Table 7.

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--> TABLE 7 Summary of Estimated U.S. Dry-Waste Streams, Excluding Water, 1988 (million metric tons) Lost Mass Dry Organic Dry Inorganic Emission to Air Sector Combustible Noncombustible Soil Overburden Concentration Wastes Process Wastes (ash, slag, etc.) H2O CO2 O2 CH4, VOC N,S Agriculture +360a 25 1,500 -1,027 -1,002 -156 0.7 12(N) +155b +202 +213 +685 Food processing ? ? 66 6.8 (+19)c (+29.4)c (-21.3)c Forestry, pulp, paper 4.9 18.1 -545 -368 -74 +314 +132 +206 Mining 9625d 600.6d 275.7e 140.7e 5,600f 47f 9.8 6-7g 57g Petroleum refiningh (76.5)c (165)c (-120)c 2.0

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--> Lost Mass Dry Organic Dry Inorganic Emission to Air Sector Combustible Noncombustible Soil Overburden Concentration Wastes Process Wastes (ash, slag, etc.) H2O CO2 O2 CH4, VOC N,S Chemicals 57.2 22.9i 1.0 Primary metals 0.3j 0.3 (S) 3.0k Stone, clay, glass 10 51.3 Fossil fuel and 92 2,088 4.726 -5.393 6 10 (N) electric power 14 (S) TOTAL 520+? 25+? 8,346.7 902.5 146.3 1,098 3,759.1 -4,732 19.5 36.3 a Crop residues, normally recycled to land. b Animal manure, normally recycled to land. Not including emissions from free-ranging animals. c Values in parentheses are emissions or consumption ( - ) from energy recovery. These values are not included in totals. d Metal ore mining in the United States. e Nonmetallic mineral mining in United States, excluding alumina and phosphate concentration wastes. f Coal mining, not including coke-oven emissions. g Petroleum and gas drilling. h Including natural gas processing and transmission. H2O, CO2, and O2 emissions (consumption) based on combustion of process wastes for energy. i Process wastes of the organic chemical sector are partly combustible and partly incombustible. Detailed breakdown not available. j Iron and steel slag are by-products with many uses. concentration wastes are included with the mining sector. k CO2 emissions are included with fuel combustion NOTES: Plus ( + ) signs refer to production. Minus ( - ) signs refer to consumption. VOC = volatile organic chemical.

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--> Notes 1. Where specific citations are not given, basic data for this section are from Hoffman (1991), United States Department of Agriculture (1992), and Bureau of Census (1988). 2. For the sake of clarity, it should be noted that ''harvested output" of corn—by far the dominant grain—refers to shelled corn, not ears. The husks and ears are left behind on the farm, along with stalks. Similarly, wheat straw and chaff are separated from the wheat grains by the harvester and left behind. 3. To calculate the totals, it is necessary to sum up individual figures given by the United States Department of Agriculture (USDA) in a variety of different units, some volumetric and some in mass terms. Unfortunately, although the USDA does provide conversion coefficients, it does not calculate aggregated totals, except for grains. 4. It should be noted that although the totals of commercial high-protein feeds remain comparatively stable from year to year, the composition varies significantly. We are unable to account in detail for the exported "feeds and fodders" (11.4 MMT in 1988), which apparently originate in the processing sector (Bureau of the Census, 1991). 5. These estimates do not represent either the "fresh" weight of manure—which is relatively wet—or the "dry" weight of its solid content. Being based on inputs, the manure is assumed to have the same water content as the feeds (i.e., about 50 percent). In the case of cattle and pigs, actual fresh weight of animal manure is about four times greater than a similar volume of feed and at least in the case of other animals is at least twice as heavy. 6. Unless otherwise specified, production, export, and import data in this section are from Bureau of the Census (1988) tables 1148, 1149, 1156, 1163, 1166, 1167, 1168, 1173, 1175, and 1177. Data on per capita consumption of foods are given in tables 207 and 208. Beverages were not taken into account. 7. See United Nations Statistical Office (1988) tables on "Lard," ISIC 3111-31, and "Oils and fats of animals, unprocessed," ISIC 3115-07. 8. See United Nations Statistical Office (1988) table on "Hides, cattle and horses, undressed—total production," ISIC 3111-311. This refers to fresh weight, prior to tanning. 9. See United Nations Statistical Office (1988) tables on "Poultry, dressed, fresh (Total Production)," ISIC 3511-10, and "Poultry, dressed, fresh (Industrial Production)," ISIC 3511-101. For mysterious reasons, the latter figure is slightly larger. 10. Unless otherwise specified, data in this section are from Bureau of the Census (1991). 11. These data are essentially consistent with Ulrich (1990); however, Ulrich's table 51 appears inconsistent with Ulrich's table 7 as regards imports and exports of pulp. Table 7 includes pulp imputed to downstream paper and paperboard products. We account for imports and exports of downstream products separately. 12. Imports of "pulp products" shown in Ulrich (1990, table 4) apparently refer to pulp itself and to the pulp equivalent of paper and paper products, not pulpwood. The United States was a net exporter of pulp and a net importer of paper products. 13. Of this, 5.4 MMT were exported and 0.6 MMT was used for other purposes (Bureau of the Census, 1991, table 1194). 14. Actually, this is a lower limit, because it includes only inorganic materials (kaolin, alum, etc.) that we have been able to account for explicitly from published sources. 15. An attractive future possibility is to ferment or otherwise convert the hemicellulosics (sugars) to ethanol. Until now, all known fermenting agents produce an enzyme, lactate dehydrogenase, that breaks down the hemicellulosics into a mixture of ethanol and lactic acid. A new discovery at Imperial College, London, may change this situation. It is a mutant strain of the fermenting bacterium Bacillus stearothermophilus, which lacks the lactate enzyme and thus converts hemicellulosics directly to ethanol, without the usual mixture of lactic acid. Unfortunately, the mutation appears unstable and the organism reverts back to the original form, which produces

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--> the enzyme. Therefore, the current challenge is to bioengineer a strain that completely lacks the gene. 16. This does not take into account the bark, which constitutes about 11.5 percent of the raw weight of roundwood (United States Congress, Office of Technology Assessment, 1984, table 5). We have completely omitted bark from our calculations by assuming that a cord of roughwood is equivalent to 80 cubic feet of debarked (peeled) roundwood. This suggests that about 10 MMT of bark would be produced by debarking operations, which precede pulping proper. Based on 48 percent moisture content, 10 MMT raw weight might be consistent with 5 MMT dry weight. (Bark is burned as "hog fuel.") 17. U.S. production of sodium chlorate in just the third quarter of 1992 was 0.131 MMT, which implies an annual rate of over 0.5 MMT, twice the level of 1988. In the same quarter, apparent consumption was 0.203 MMT, of which 37 percent was imported (United States Department of Commerce, 1992). The explanation is that chlorine dioxide has been very rapidly substituting for elemental chlorine as a bleach for pulp. Most of the increase in U.S. demand since 1988 is for conversion to chlorine dioxide. 18. U.S. production of paper products in 1988 consisted of 5.364 MMT of bleached newsprint (made from mechanical pulp), 19.59 MMT of printing and writing paper (coated and bleached), and 44.57 MMT of "other machine made paper and paperboard," of which 36.056 MMT consisted of Kraft paper and paperboard (Bureau of the Census, 1991). 19. Data on materials handled are from Bureau of Mines (1989, Volume 1, tables 10, 11). Other data in this section on metals and minerals come from individual chapters in the same source. 20. In the case of iron, concentrates for blast furnaces (pellets and sinter) are treated differently. Pellets are produced at the mine, whereas sinter is included in the smelting sector rather than in the mining sector. For consistency, we adopt this convention. In the case of aluminum, the concentration stage is taken to be the chemical conversion of bauxite ore into pure aluminum oxide, or alumina. This process is conventionally included in the inorganic chemical industry. Phosphate rock concentration, yielding fertilizer-grade superphosphate, is included in the fertilizer industry. Phosphorus metal and phosphoric acid from phosphorus are both also included with inorganic chemicals. 21. Data on materials handled are from Bureau of Mines (1989, Volume 1, tables 10, 11). Other data in this section on metals and minerals come from individual chapters in this same source. 22. The quantity of refuse produced obviously depends on the intensity of the beneficiation (washing) process. For comparison, the only coal cleaning process described in an official report of the United States Department of Energy (1980) had only a 70 percent yield in mass terms and a 90 percent yield in energy terms. Specifically, 1,428 tons of "run-of-mine" coal produced 1,000 tons of "clean" coal. 23. All data in this section are extracted from International Energy Agency (1991, pp. 664-665). 24. Light ends are compounds with boiling points in the range of butane (about 0 °C) and below. Methane and the light alkanes (C2-C 4) fall into this category. 25. Benzene, ethylbenzene, toluene, and xylenes constitute, respectively, 0.1, 0.51, 0.19, and 0.88 percent of average crude oil by volume (Gaines and Wolsky, 1981). They constitute, of course, a much higher percentage of the volatiles. 26. Unless otherwise specified, the basic data for this section are from Bureau of Mines (1989). 27. Phosphorus pentoxide dissolved in water is phosphoric acid, the active ingredient in most phosphate fertilizers (e.g., superphosphates). It is not used, generally, in pure form. 28. The electrolytic process for chlorine production from brine yields 1.1 units of sodium hydroxide per unit of chlorine, with inputs of 1.75 units of sodium chloride. However, some chlorine is produced from magnesium chloride, and some is regenerated from hydrochloric acid, so the ratios are not exact. 29. Sodium chlorate used to bleach paper pulp is almost unique among chlorine chemicals. It is not manufactured from elemental chlorine but is made directly from sodium chloride (salt).

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--> 30. Feedstock data are from International Energy Agency (1991); consumption data for sulfuric acid, ammonia, fertilizer chemicals, and sodium carbonate (soda ash) are from Bureau of Mines (1988, 1989); data on shipments of inorganic chemicals from are Chemical and Engineering News (1997); and data on production and shipments of synthetic organic chemicals are from either Chemical and Engineering News (1997) or from United States International Trade Commission (ITC) (1989). The annual ITC reports, formerly a valuable source of data, unfortunately ceased publication in 1993. 31. Carbon black is used mainly in tires, of which it constitutes roughly 29 percent by weight (Ecoplan International, 1992). Total tire production in the United States in 1988 was roughly 2.2 MMT, accounting for 0.64 MMT of carbon, or 0.7 MMT of hydrocarbon feeds. There are other significant uses of carbon black, such as printing ink. However, most carbon black is made directly from natural gas. We do not include it as a chemical product. 32. Note that soluble cellulose is used to manufacture viscose rayon, cellulose acetate, and cellophane, among other products. Production in 1988 was 1.24 MMT, of which about 60 percent was used for rayon. Rayon is not counted as a product of the organic chemical industry. 33. Although about 0.2 MMT of phosphorus was used in detergents, most of it was inorganic: STPP and tetrasodium pyrophosphate. 34. Taking account of the availability of small amounts of other acids (HCl, HF, HBr, HNO3, P2O5), it might seem that the need for sulfuric acid would be reduced somewhat. However, to the extent that other acids were used, the neutralization products would be sodium or ammonium salts of chlorine, fluorine, etc. Since these elements are actually embodied in products, they cannot also be a major constituent of the waste stream. 35. Unless otherwise specified, data for this section are from Bureau of Mines (1989, 1991). 36. Assuming that the iron in ore is mostly in the form Fe2O3, the 57.5 MMT of iron content in ore (1988) would be combined with 25.55 MMT of oxygen. 37. Unless otherwise specified, data for this section were taken from United States Bureau of Mines (1989). 38. Unless otherwise specified, data for this section were taken from United States Bureau of Mines (1989). 39. U.S. coal is unusually low in ash, most of which is recovered. By contrast, most other countries burn coal that has a much higher ash content, 15-25 percent or more, very little of which is recovered. Therefore, the problem of heavy-metal pollution from coal burning will be far more serious in eastern Europe, the former Soviet Union, China, and India. References Brown, L. R., and E. C. Wolf. 1984. Soil Erosion: Quiet Crisis in the World Economy. Worldwatch Paper (60). Washington D.C.: Worldwatch Institute. Bureau of Mines. 1987. Minerals Yearbook. Washington, D.C.: U.S. Government Printing Office. Bureau of Mines. 1988. Minerals Yearbook. Washington, D.C.: U.S. Government Printing Office. Bureau of Mines. 1989. Minerals Yearbook. Washington, D.C.: U.S. Government Printing Office. Bureau of Mines. 1991. Minerals Yearbook. Washington, D.C.: U.S. Government Printing Office. Bureau of the Census. 1983. Census of Manufactures. Washington, D.C.: U.S. Government Printing Office. Bureau of the Census. 1988. Statistical Abstract of the United States: 1988, 108 ed. Washington, D.C.: U.S. Government Printing Office. Bureau of the Census. 1990. Statistical Abstract of the United States: 1990, 110 ed. Washington, D.C.: U.S. Government Printing Office. Bureau of the Census. 1991. Statistical Abstract of the United States: 1991, 111 ed. Washington, D.C.: U.S. Government Printing Office.

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--> Carbon Dioxide Information Analysis Center. 1990. Trends '90: A Compendium of Data on Global Change. Oak Ridge, Tenn.: Oak Ridge National Laboratory. Chemical and Engineering News. 1992. Chemical earnings. Volume 70. Chemical and Engineering News. 1997. Chemical earnings: Japanese chemical producers generally fared well in 1996, but petrochemicals ate into profits. Volume 75. Crutzen, P. J. 1976. The nitrogen cycle and stratospheric ozone. Paper presented at the Nitrogen Research Review Conference, National Academy of Sciences, Fort Collins, Colo., October 12-13. Crutzen, P. J., I. Aselman, and W. Seiler. 1986. Methane production by domestic animals, wild ruminants, other herbivorous fauna, and humans. Tellus 38B:271-284. Deevey, E. C. 1970. Mineral cycles. Scientific American 223:148-158. Ecoplan International. 1992. New Developments in Tire Technology: Technological Change and Intermaterials Competition in the 90s and Beyond. Multi-Client Industry Report. Paris: Ecoplan International. Gaines, L. L., and A. M. Wolsky. 1981. Energy and Materials Flows in Petroleum Refining. Technical Report (ANL/CNSV-10). Argonne, Ill.: Argonne National Laboratory. Hoffman, M. S., ed. 1991. The World Almanac and Book of Facts. New York: Scripps-Howard. Holmbom, B. 1991. Chlorine Bleaching of Pulp: Technology and Chemistry, Environmental and Health Effects, Regulations and Communication. Case study. April. Turku/Abo, Finland: Abo Akademi. International Bank for Reconstruction and Development (IBRD). 1980. Environmental Considerations in the Pulp and Paper Industry. Washington, D.C.: IBRD. International Energy Agency. 1991. Energy Statistics of OECD Countries 1980-1989. Paris: Organization for Economic Cooperation and Development. International Trade Commission. 1992. Synthetic Organic Chemicals 1992. Washington, D.C.: U.S. Government Printing Office. LeBel, P. G. 1982. Energy Economics and Technology. Baltimore, Md.: The Johns Hopkins University Press. Lowenheim, F. A., and M. K. Moran. 1975. Faith, Keyes, and Clark's "Industrial Chemicals," 4th ed. New York: Wiley-Interscience. Manzone, R. 1993. PVC: Life-Cycle and Perspectives. Urbino, Italy: Commett Advanced Course. Obernberger, I. 1994. Characterization and utilization of wood ashes. Technical paper. Institute of Chemical Engineering, Technical University, Graz, Austria. Roskill Ltd. 1991. Chromium. London: Roskill Ltd. Schlesinger, W. H. 1991. Biogeochemistry: An Analysis of Global Change. New York: Academic Press. Schlesinger, W.H., and A.E. Hartley. 1992. A global budget for atmospheric NH3. Biogeochemistry 15:191-211. Science Applications International Corporation (SAIC). 1985. Summary of Data on Industrial Nonhazardous Waste Disposal Practices. Washington, D.C.: SAIC. Smil, V. 1993. Nutrient flows in agriculture: Notes on the complexity of biogeochemical cycles and tools for modeling nitrogen d D. voand phosphorus flows in agroecosystems. Undated working paper. Subak, S., P. Raskin, ann Hippel. 1992. National Greenhouse Gas Accounts: Current Anthropogenic Sources and Sinks. Boston, Mass.: Stockholm Environmental Institute. Torrey, S., ed. 1978. Coal Ash Utilization: Fly Ash, Bottom Ash and Slag. Pollution Technology Review Series, No. 48. Park Ridge, N.J.: Noyes Data Corp. Ulrich, A. H. 1990. U.S. Timber Production, Trade, Consumption and Price Statistics 1960-88. Miscellaneous Publication (1486). U.S. Forest Service. Washington, D.C.: U.S. Forest Service, U.S. Department of Agriculture. United Nations Statistical Office. 1988. Industrial Statistics Yearbook: Commodity Production Statistics 1988 II. New York: United Nations.

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