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Technology and Environment 1989.
Pp. 23~9. Washington, DC
National Academy Press.
Industrial Metabolism
ROBERT U. AYRES
We may think of both the biosphere and the industrial economy as
systems for the transformation of materials. The biosphere as it now exists
is very nearly a perfect system for recycling materials. This was not the case
when life on earth began. The industrial system of today resembles the ear-
liest stage of biological evolution, when the most primitive living organisms
obtained their energy from a stock of organic molecules accumulated during
prebiotic times. It is increasingly urgent for us to learn from the biosphere
and modify our industrial metabolism, the energy- and value-yielding pro-
cess essential to economic development. Modifications are needed both to
increase reliance on regenerative (or sustainable) processes and to increase
efficiency both in production and in the use of by-products.
In this chapter, mass flows for key industrial materials of environ-
mental significance, and the waste emissions associated with them, are
reviewed along with the environmental impact of the waste residuals; eco-
nomic and technical forces driving the evolution of industrial processes;
long-range tendencies in the development of the industrial metabolism;
and some applications of "materials-balance" principles to provide more
reliable estimates of outputs of waste residuals. The Appendix contains
theoretical explorations of the biosphere and the industrial economy as
materials-transformation systems and lessons that might be learned from
their comparison.
Before presenting the main analytic framework, it is useful to begin
with some positive examples of how industrial metabolism can shift in the
direction of increased efficiency in materials flows and waste streams. It
23
OCR for page 24
24
ROBERT U LYRES
has been justly remarked that the history of the chemical industry is one of
finding new uses for what were formerly waste products (Multhauf, 1967~.
One of the most interesting early examples of such an innovation was the
Leblanc process (the predecessor of the Solvay process) for manufacturing
sodium carbonate circa 1800. As a raw material, it made use of sodium
sulfate, an unwanted by-product of the eighteenth-centu~y process for
manufacturing ammonium chloride (sal ammoniac). Sal ammoniac was
used for cleaning metals and as a convenient source of ammonia, but
sodium sulfate had no use. The Leblanc process reduced sodium sulfate to
sodium sulfide by heating it in a furnace with charcoal. This, in turn, was
heated with calcium carbonate (chalk), which induced a reaction producing
the desirable sodium carbonate and a new waste product, calcium sulfide
(Multhauf, 1967~.
Meanwhile, the market for ammonium chloride failed, so sodium
sulfate had to be produced in the same complex by reacting sulfuric acid
with common salt. This yielded hydrogen chloride (hydrochloric acid) as
another by-product. In this case, the hydrochloric acid quickly found a
practical use in the manufacture of chloride of lime as a bleach for the
rapidly growing textile industry. The other waste product, calcium sulfide,
was not successfully used until the 1880s, as a source of sulfur for the
manufacture of sulfuric acid.
One of the most important waste products of the early nineteenth
century was coal tar, which was generated in large amounts by gasworks
making "town gas" for illumination. It was a systematic search for useful by-
products, initiated by German chemists, that resulted in the creation of the
modern organic chemical industry. The synthetic aniline dyes introduced
after 1860 were all essentially derived from chemicals obtained from coal
tar. Phenolic resins, aspirin, and the sulfa drugs are later examples of
derivatives.
Until recently, even natural gas was regarded as a by-product of
petroleum production. Although it was used almost from the beginning as
a fuel for refinery processes and illumination in local areas, much of it was
wasted by "flaring" (indeed, this is still true in remote areas of the, world).
It had no significant chemical uses until World War II, when natural gas
became the feedstock for producing ethylene, and thence butadiene and
styrene, the major ingredients of synthetic rubber.
Chlorine is a final example. It is manufactured jointly with sodium
or sodium hydroxide (lye) by electrolysis of salt or brine. When this
process was introduced In the 1890s, it was the sodium hydroxide that was
wanted for a variety of purposes, including petroleum refining and soap
manufacturing. At the time, chlorine was a low-value by-product, which was
luckily available for use in municipal water treatment The development
of a number of valuable chlorine-based organic chemicals (e.g., the most
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INDUSTRIAL METABOLISM
TABLE 1 Mass of Active Materials Extracted Commercially in the United States,
1960-1975 (million tons)
Year
Material1960196519701975
Food and feed crops267295314403
(excluding hay)
Meat, fish, and dairy products82858584
Cotton, wool, hides, and tobacco5544
Timber (who moisture basis)256267271249
Fuels (coal, lignite, oil, gas)9901,1761,4581,392
Ores(Fe, Cu. Pb, Zn)400435528460
Nonmetallics2ooa24Oa266255
Total2,2~a2>ooa2,9262,847
25
-
aEstimated value.
NOTE: Vegetable material harvested directly by animals has been omitted for
lack of data, along with some obviously minor agricultural and horticultural
products. Figures for metal ores exclude mine tailings and gangue removed to
uncover ore bodies. Inert construction materials such as stone, sand, and
gravel have also been omitted. Inert materials account for enormous tonnages,
but undergo no chemical or physical change except to the extent that they are
incorporated in concrete or paved surfaces. The table also excludes soil and
subsoil shifted during construction projects or lost by erosion.
SOURCE: U.S. Bureau of the Census (1960 1975~.
common industrial solvents and refrigerants, pesticides, herbicides, and
plastics such as polyvinyl chloride) actually reversed the position. By the
l950s and 1960s, chlorine was the primary product and sodium hydroxide
was the less valuable by-product.
MASS FLOWS AND WASTE EMISSIONS
Our economic system depends on the extraction of large quantities
of matter from the environment. Extraction is followed by processing and
conversion into various forms, culminating in products for "consumption."
The U.S economy extracts more than 10 tons of "active" mass per person
(excluding atmospheric oxygen and fresh water) from U.S. territory each
year. Of the active mass processed each year, roughly 75 percent is mineral
and "nonrenewable," and 25 percent is, in principle, from renewable (i.e.,
biological) sources as shown in Bible 1. Of the latter, much is ultimately
discarded as waste, although much of it could (in principle) be used for
energy recovery.
It is difficult to estimate the fraction of the total mass of processed
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26
ROBERT U LYRES
active materials that is annually embodied in long-lived products and capital
goods (durables). None of the food or fuel is physically embodied in durable
goods. Most timber is burned as fuel or made into pulp and paper products.
At least 80 percent of the mass of "ores" consists of unwanted impurities
(more than 99 percent in the case of copper). Of the final products made
from metals, a large fraction is converted into "consumption goods," such
as bottles, cans, and chemical products, and "throwaway" products such
as batteries and light bulbs. Only in the case of nonmetallic minerals
(if inert materials are ignored, as before) is as much as 50 percent of
the mass embodied in durable goods (mainly Portland cement used for
concrete and clays used for bricks and ceramics). The annual accumulation
of active materials embodied in durables, after some allowance for discard
and demolition, is probably not more than 150 million tons, or 6 percent
of the total. The other 94 percent is converted into waste residuals as fast
as it is extracted.
Combustion and carbothe~mic reduction processes are the major
sources of atmospheric pollutants today but by no means the only important
ones; nor is the atmosphere the only vulnerable part of the environment.
From a broader environmental perspective, the production and dispersal
of thousands of synthetic chemicals many new to nature, and some highly
toxic, carcinogenic, or mutagenic- and the mobilization of large tonnages
of toxic heavy metals are of equally great concern. The complexity of the
problem Is too great to permit any kind of short summary.
However, two points are worthy of emphasis. First, as noted above,
most materials "pass through" the economic system rather quickly. That is
to say, the transformation from raw material to waste residual takes only
a few months to a few years in most cases. Long-lived structures are very
much the exception, and the more biologically "potent" materials are least
likely to be embodied in a long-lived form.
The second point is that many uses of materials are inherently dissipa-
tive (Ayres, 1978~. That is, the materials are degraded, dispersed, and lost
in the course of a single normal use. In addition to food and fuels (and
additives such as preservatives), other materials that fall into this category
include packaging materials, lubricants, solvents, flocculants, antifreezes,
detergents, soaps, bleaches and cleaning agents, dyes, paints and pigments,
most paper, cosmetics, pharmaceuticals, fertilizers, pesticides, herbicides,
and germicides. Many of the current consumptive uses of toxic heavy met-
als such as arsenic, cadmium, chromium, copper, lead, mercury, silver, and
zinc are dissipative in the above strict sense. Other uses are dissipative in
practice because of the difficulty of recycling such items as batteries and
electronic devices.
Bibles 2 and 3, which summarize estimates of emission coefficients for
various heavy metals by use category and annual emissions attributable to
OCR for page 27
INDUSTRIAL METABOLISM
27
dissipative uses, make clear the heterogeneity of the materials flows. In
some cases the dissipation is slow and almost invisible. For instance, paints
(often containing lead, zinc, or chromium) gradually crack, '~weather," and
turn to powder, which is washed or blown away. Tires, which contain
zinc (and cadmium) salts, are gradually worn away during use, leaving a
residue on the roads and highways. Similarly, shoe leather, containing up
to 2 percent chromium salts, is gradually worn away to powder during use.
Incinerator ash contains fairly high concentrations of heavy metals from a
variety of miscellaneous sources, ranging from used motor oil to plastics
and pigments.
On reflection, many dissipative uses (food and fuel again excepted)
are generally seen to be nonessential in the sense that technologies are the-
oretically available, or imaginable, that could eliminate the need for them.
1b take one example only: hydroponic agriculture in enclosed, atmospher-
ically controlled greenhouses, with genetically engineered pest controls,
would eliminate all losses of fertilizers and pesticides to watercourses by
way of surface runoff.
ENVIRONMENTAL IMPACT OF WASTE RESIDUALS
Materials do not disappear after they are used up in the economic
sense. They become waste residuals that can cause harm and must be
disposed of. In fact, it is not difficult to show that the tonnages of waste
residuals are actually greater than the tonnages of crops, timber, fuels, and
minerals recorded by economic statistics. Although usually unpriced and
unmeasured, both air and water are major inputs to industrial processes
and they contribute mass to the residuals-especially combustion products.
Residuals tend to disappear from the market domain, where everything
has a price, but not from the real world in which the economic system is
embedded.
Many services provided by the environment derive ultimately from
"common property," including the air, the oceans, the genetic pool of the
biosphere, and the sun itself. Distortions in the market (i.e., prices) are
unavoidably associated with the use of common property resources. Clearly,
environmental resources such as air and water have been unpriced or (at
best) significantly underpriced in the past. For this reason, such resources
have generally been overused.2
As noted, the total mass of waste residuals produced each year by
industrial metabolism far exceeds the mass of active inputs derived from
economic activities. This is because nearly half of the inputs other than air
and water are fossil fuels (hydrocarbons), which combine with atmospheric
oxygen and form carbon dioxide and water vapor. The carbon fraction
of hydrocarbons ranges from 75 percent in methane to about 90 percent
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28
TABLE2 Consumption-Related Emissions Factors: Heavy Metals
ROBERT U LYRES
Painting Paint Electron Other
Metallic and and Tubes and Electrical
Metal usea Coatingb PigmentsC Batteries d Equipment
Silver 0.001 0.02 0.5 0.01 0.01
Arsenic 0.001 0 05 0.01 N^.
Cadmium 0.001 0.15 05 0.02 N^.
Chromium 0.001 0.02 05 N^. N^.
Copper 0.005 0 1 N^. 0.10
Mercury 0.050 0.05 0.8 0.20 N^.
Lead 0.005 0 0.5 0.01 NA.
Zinc 0.001 0.02 05 0.01 N^.
N^. = Not applicable.
aAlloys or amalgams (in the case of mercury) not used in plating, electrical
equipment, catalysts, or dental work. Losses can be assumed to be due
primarily to corrosion, except for mercury, which volatilizes.
b Protective surfaces deposited by dip coating (e.g., galvanizing),
electroplating vacuum deposition, or chemical bath (e.g., chromic acid). The
processes in question generally resulted in significant waterborne wastes until
the 1970s. Cadmium-plating processes were particularly inefficient until
recently. Losses in use are mainly due to wear and abrasion (e.g.,
silve~plate) or to flaking (decorative chrome trim). In the case of
mercu~y-tin "silver" for mirrors, the loss is largely due to volatilization.
Paints and pigments are lost primarily by weathering (e.g., for
metal-protecting paints), wear, or disposal of painted dyes or pigmented
objects, such as magazines. (::opper- and mercury-based paints volatilize slowly
over time. A factor of 0.5 is assumed arbitrarily for all other paints and
pigments.
dIncludes all metals and chemicals (e.g., phosphorus) in tubes and primary and
secondary battenes, but excludes copper wire. Losses in manufacturing may be
significant. Mercury in mercury vapor lamps can escape to the air when tubes
are broken. In all other cases it is assumed that discarded equipment goes
mainly to landfills. Minor amounts are volatilized in fires or incinerators or
lost by corrosion. Lead-acid batteries are recycled.
Includes solders, contacts, semiconductors, and other special materials (but
not copper wire) used in electrical equipment control devices, instruments,
etc. Losses to the environment are primarily through discard of obsolete
equipment to 1andfi11s. Mercury used in instruments may be lost through
breakage and volatilization or spillage.
in anthracite. Petroleum is intermediate. Every ton of fuel carbon is
converted into 3.67 tons of carbon dioxide emitted to the atmosphere.3 It
is thought that only about half of this amount remains in the atmosphere,
but the carbon dioxide level of the earth's atmosphere has risen over the
past century from about 290 parts per million (ppm) to around 344 ppm at
present. Although the magnitude of the climatic warming (the "greenhouse
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INDUSTRIAL METABOLISM
29
Chemical Agri- Nonagri
Uses Not cultural cultural Medical, Miscella
Metal Embodiedf Embodiedg Uses h Uses i Dental3 neous k
Silver 1 0.40 N^. N^. 0.5 0.15
Arsenic N^. 0.05 0.5 0.8 0.8 0.15
Cadmium 1 0.15 NA. N^. N^. 0.15
Chromium 1 0.05 N^. 1 0.8 0.15
Copper 1 0.05 0.05 1 NA. 0.15
Mercury 1 NA. 0.80 0.9 0.2 0.50
Lead 1 0.75 0.05 0.1 NA. 0.15
Zinc 1 0.15 0.05 0.1 0.8 0.15
fuses not embodied in final products include catalysts, solvents, reagents, bleaches,
etc. In some cases a chemical is embodied but there are some losses in processing.
Losses in chemical manufacturing are included here. Major examples include copper and
mercury catalysts (especially in chlorine manufacturing); copper, zinc, and chromium as
mordants for dyes; mercury losses in felt manufacturing; chromium losses in tanning,; lead
in desulfunzation of gasoline; and zinc in rayon spinning. In some cases, annual
consumption is actually loss replacement and virtually all of the material is dissipated.
Detonators such as mercury fulminate and lead azide (and explosives) are included in this
category.
"Uses embodied in final products other than paints or batteries include fuel additives
(e.g., tetraethyl lead), anticorrosion agents (e.g., zinc dithiophosphate), initiators and
plasticizers for plastics (e.g., zinc oxide), wood preservatives, and chromium salts
embodied in leather. Losses to the environment occur when the embodying product is used,
for example, gasoline containing tetraethyl lead is burned and largely (lasso) dispersed
into the atmosphere. However, copper, chromium, and arsenic are used as wood
preservatives and dispersed only if the wood is later burned or incinerated. In the case
Of silver (photographic film), it is assumed that 60% is later recovered.
Includes agricultural pesticides, herbicides, and fungicides. Uses are dissipative,
but heavy metals are largely immobilized by soil. Arsenic and mercury are exceptions
because of their volatility.
i Biocides used in industrial, commercial, or residential applications. Loss rates are
higher in some cases.
j Includes primarily pharmaceuticals (including cosmetics), germicides, etc., as well as
dental filling matenal. Most uses are dissipated to the environment through wastewater.
Silver and mercury dental fillings are likely to be buried with cadavers.
k Miscellaneous emissions not counted elsewhere.
SOURCE: Ayres et al. (1988~.
effect") due to the rising level of carbon dioxide is still quantitatively
uncertain, the qualitative impact is likely to be adverse.
Carbon monoxide is, of course, toxic to humans and has been impli-
cated in health problems among urban populations. It is less well known
that carbon monoxide plays an active, and not necessarily benign, role in a
number of atmospheric chemical reactions before it is ultimately oxidized to
carbon diomde. Inefficient combustion processes convert up to 10 percent
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30
ROBERT U LYRES
TABLE 3 Emissions from Consumptive Uses: Heavy Metals, United States, 1980
(metnc tons)
Metallic
Uses Electrical
(E!xcept Protective Coverings Other
Coatings Plating Paints Batteries Electrical
and and and and Uses, Instru
Metal Electrical) Coating Pigments Equipment meets, etc.
Silver 0.83 a o.o 0.15 0.67
Arsenic 0.04 0.0 0.0 1.97 0.0
Cadmiumb 0.04 136.1 116 7.81 0.0
Chromium 151.80 155.7 6,490 0.0 0.0
Copper c 11,074.00 0.0 0.0 0.0 0.0
Mercury 0.0 0.0 0.0 195.91 1752
Leadc 1,249.00 0.0 48,S00 8,510.00 0.0
Zinc 514.00 8,778.0 77,750 63.00 0.0
bIncluded in first column.
cl979.
1977.
SOURCE: Ayres et al. (1988).
Of fuel carbon into carbon monoxide; carbothermic reduction of iron ore
and other metals generates even higher percentages. However, the average
percentage over all processes is much smaller. Actual emissions of carbon
monoxide to the atmosphere in the United States were about 110 million
tons in 1970 (mostly from automobiles and trucks), with a carbon content
of 47 million tons. Emission controls reduced the net output of carbon
monoxide in 1980 to about 85 million tons (U.S. Environmental Protection
Agency, 1986~.
The discovery of chlorofluorocarbons (CFCs) in the stratosphere has
raised an even more frightening prospect: ozone depletion (see Glas,
this volume). In apparent confirmation of this phenomenon, an "ozone
hole" has recently appeared in the stratosphere over Antarctica. This
"hole" has reappeared each spring for several years and seems to be
growing (Clark, 1987; Miller and Mintzer, 1986~. Chlorofluorocarbons
are chemically inert gases, discovered in 1928 and produced since the
1930s mainly as refrigerants and solvents and for "blowing" plastic foams
(see Friedlander, this volume). The major use is for refrigeration and air
conditioning. In most uses, CFCs are not released deliberately, but losses
and leakage are inevitable.
The problems revealed so far may be only the beginning. If (perhaps
it is better to say "when") the ozone level in the stratosphere is depleted,
the effect will be to let more of the sun's ultraviolet radiation through to
OCR for page 31
INDUSTRIAL METABOLISM
Chemical _Biocidal
Consumer Poison Uses
Industrial Uses, Nonagri
Catalysts, Additives, Agriculturalcultural Medical,
Reagents, Extenders, Pesticides,Pesticides Dental,
Explosives,Photography, Herbicides, (Except Pharma- Miscel
etc.etc. Fungicides Medical) ceutical laneousTotal
_
7.648 0 0 15; 0.4560
492.00 2,950 5,901 19.70 09,364
029 0 0 0 0.98290
1,297.03,890 0 1,038 0 2,141.0011,659
4,222.00 1,560 0 0 015,452
412.10 16 236 8.36 6.96893
OO O O 0 1,329.0056,900
2,508.018,622 188 251 1,003.00 0109,670
31
the earth's surface. One likely impact on humans is a sharp increase in the
incidence of skin cancer, especially among whites. The ecological impact
on vulnerable plant or animal species cannot be estimated, at present, but
could be severe.
Methane, oxides of nitrogen (NO=), and sulfur oxides (SO=) are other
residuals that have been seriously implicated in climatic or ecological effects.
All three are generated by fossil fuel combustion, as well as other industrial
processes. Like carbon monoxide, carbon dioxide, and the CFCs, they can
be considered as metabolic products of industry. Methane is released in
natural gas pipelines, petroleum drilling, coal mining, and several kinds of
intensive agriculture (especially rice cultivation or cattle and sheep farming).
Nitrogen oxides are also coproducts of combustion. In effect, at
high temperatures, some of the atmospheric nitrogen is literally "burned."
Nitrogen oxides are implicated in many atmospheric chemical reactions
(including those that create smog) and eventually oxidize to nitric acid,
which contributes to acid rain. In the stratosphere, where nitrogen oxides
are decomposed by ultraviolet radiation, atomic nitrogen also contributes
to the destruction of ozone. Finally, oxides of nitrogen are "greenhouse
gases" that contribute to climatic warming (Miller and Mintzer, 1986~.
Sulfur oxides are generated by the combustion of sulfur-containing
fuels-especially bituminous coal-and by the smelting of sulfide ores.
Most copper, lead, zinc, and nickel ores are of this kind. In principle,
sulfur can be recovered for use from all these activities, and the recovery
rate is rising. However, the costs of recovery, especially from coal-burning
electric power plants, are still far higher than the market value of the
potential products (e.g., dilute sulfuric acid). Hence, for the present,
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32
ROBERT U LYRES
calcium sulfites and sulfates-as well as SOc-constitute a waste residual
that must be disposed of. This is also true of fly ash.4
These examples show that although today's industrialized economic
system may be in rough equilibrium in terms of supply and demand rela-
tionships, it is far from equilibrium in thermodynamic terms. Enormous
quantities of fossil fuels and high~uality minerals are extracted each year
to drive the economic engine. The economic system is stable somewhat in
the way a bicycle and its rider are stable: if forward motion stops, the sys-
tem will collapse. Forward motion in the economic system is technological
progress.
EVOLUTION OF INDUSTRIAL PROCESSES
It is generally accepted by economists that the mechanism that normally
drives the evolution of industrial processes is technological innovation. The
primary incentive for taking risks appears to be economic. A new process
that saves one link in the chain between raw materials and finished ma-
terials or final goods can usually be justified through savings in materials
and energr inputs or capital requirements, if not both. Moreover, process
technology is inherently easier to protect from piracy than product tech-
nology. As noted, most chemical products are intermediates used in the
production of other chemicals. Thus, final products result from chains, or
sequences, of processes with an overall energy conversion efficiency that is
the arithmetic product of the conversion efficiencies at each stage.5 If the
typical chain has three steps, each of which has a conversion efficiency of
0.7, the overall conversion efficiency of the chain is about 0.34. A four-step
chain would have an overall efficiency of around 0.24. That is, the available
energy embodied in the final product might be somewhere between 25
and 35 percent of the available energy of the original feedstocks. Because
primary feedstocks are essentially indistinguishable from fuels, efficiency
improvements translate directly into cost savings.
Clearly, a powerful long-term strategy for improving overall effective-
ness in production is the development of new processes to shorten these
process chains, bypassing as many intermediates as possible. In other words,
one would like to be able to produce final products such as polyethylene or
synthetic rubber directly from first-tier intermediates or even from primary
feedstocks such as ethane and propane.6 Biological organisms differ from
industrial organizations in that they are able to build complex molecules
directly from elementary building blocks with relatively few intermediates.
Thus, biotechnology offers a long-term prospect of radically higher produc-
tion efficiencies and correspondingly lower costs (U.S. Office of Technology
Assessment, 1982~.
Another long-term strategy for increasing effectiveness is better use of
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INDUSTRIAL METABOLISM
33
by-products and wastes. When a process can be justified on the basis of the
market for its primely product, by-product sales can be highly profitable.
This positive motivation to seek new uses is compounded by the fact
that, because of some of the environmental problems already noted, waste
generation is being increasingly discouraged by environmental regulation.
Moreover, waste disposal is increasingly expensive, and the cost is
becoming more uncertain. Firms that buried toxic chemical wastes many
years ago in landfills (methods that were regarded as acceptable at the
time) are sometimes finding themselves saddled retroactively with heavy
costs of digging up the same wastes and disposing of them again in a safer
manner. The chemical waste dump used by Hooker Chemical Company at
Love Canal, near Niagara Palls, is one example. Meanwhile, suitable sites
for disposal of hazardous solid or liquid wastes are becoming scarce.7 Ma-
terials that are recovered and reused internally or embodied in marketable
products that can be readily and effectively recycled are less likely to cause
this kind of problem.8
The incentives for technological innovation in the area of waste re-
duction and disposal are not always operating, especially where there are
massive market failures. In practice, the "unpaid" environmental damage
costs have been deferred and many of them will have to be paid by later
generations. But as these costs become larger and more visible, there will
be growing political pressure to force the producers and users of fossil
fuels (and other materials such as heavy metals) to pay the costs of abat-
ing the resulting environmental damages.9 Notwithstanding resistance by
energy users, it seems inevitable that in the long run these costs will be
added to the prices of fuels and materials. This, in turn, will create sig-
nificant economic opportunities for innovations in the area of "low-waste"
technologies.
EVOLUTIONARY ECONOMICS AND GAIA
One view of evolution and probably the most common view among
scientists-can be characterized as "the myopic drunkard's walk" The
drunkard's walk is not exactly random, but it tends to follow the path of
least resistance in the short run. If a mutation or an innovation offers short-
term advantages, it will be adopted. In this view, there is no long-range
force or tendency to approach a distant goal. I,here is no Aristotelian "final
cause." Most scientists tend to regard more teleological theories, including
Lovelock's Gala hypothesis ~velock 1988), with skepticism, because of
their aroma of mysticism.
In the case of biological evolution, indeed, it was difficult for a long
time to suggest a likely mechanism whereby short-term advantages to the
individual could in some cases be overridden in favor of longer-term benefits
OCR for page 39
INDU512IAL METABOLISM
39
cycle are familiar examples. The concept is also applicable, of course, to
flows that are not truly cyclic, as in the case of arsenic (Figure 3~. This
concept has been widely used by geochemists, hydrologists, ecologists, and
environmental scientists to organize and systematize their work. Such a
presentation helps specify geographical scales of analysis. It also facilitates
such comparisons as the relative importance of natural and anthropogenic
sources. Finally, and potentially most important, it provides a starting point
for detailed analysis of the effect of anthropogenic emissions on natural
processes.
CONCLUSION
It appears that we have methods to describe our industrial metabolism
better, both qualitatively arid quantitatively. Initial analyses reveal several
important points, for example, that many materials uses are inherently
dissipative and thus pose difficulties for recycling. Analysis also shows
that although residuals do not disappear from the real world of human
health and environmental quality, they do tend to disappear from the
market domain. Thus, many environmental resources are underpriced and
overused. It is also clear that where the production and use of by-products
are concerned, many industrial processes involve multiple steps, resulting
in a low level of system efficiency, especially in comparison with biological
systems. The sum of the argument here suggests that we should not only
postulate, but indeed endorse, a long-run imperative favoring an industrial
metabolism that results in reduced extraction of virgin materials, reduced
loss of waste materials, and increased recycling of useful ones.
APPENDIX
The Biosphere as a Matenals-liansformation System
Three salient characteristics mark the difference-between the natu-
rally evolved biosphere and its human~esigned industrial counterpart, the
"synthesphere." The first is that the metabolic processes of biological or-
ganisms are derived (by photosynthesis) from a renewable source: sunlight.
The second characteristic is that the metabolism of living organisms (cells)
is executed by multistep regenerative chemical reactions in an aqueous
medium at ambient temperatures and pressures. Most process intermedi-
ates are regenerated within the cell. Reaction rates are controlled entirely
by catalysts (enzymes). The energy transport function is performed in all
living organisms by phosphate bonds, usually in the molecule adenosine
triphosphate (ATP).~3
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40
Hydroxide H2SO4 |
Hydroxide
. .
React with
with Sulfurous
A c i d fi n W e a k
2SO4 Ituric
~ //1
Sulfate Alum / / I
OKpi0,)~ ~
i:
ROBERT U AIRS
Melt in I
Reverberatory
Fumace
with Lime
and Soda or
_ Potash
~;~N
Iron 0(e
(Chromite)
`F6Cr2O~
~ \
~1 ~
Alkaline
Roast
, and Leach
~~ - ~ / /
Chromate ~
K2CrO4. J
Norma / /
~ / ~
/ 1\ / React ~
/ ~ / with Electro
/ IN Sulfuric Iytic
. / /; H2SO4 ~ Reduction
\~ / ~/_ ~ ~
,~tass~ ~\ ~ \
~ Bichromate Bichromate i ~\
\L K2Cr2O7 Na2Cr207 Jr\
~\\
~1
1
~ -
Stainless |
Steel and |
Cloy 1
Steel |
1 1 1
Ferro- Mordants Corrosion Lead Chromate
chrome (laxatives) Inhibitor Zinc Chromate
Ugno- for Dyeing (Primer)
sulfate
FIGURE 2 Chromium process-product flows, circa 1968. Except for the bottom row,
rectangles indicate processes; ellipses, products or reactions. SOURCE: Ayres et al. (1988~.
OCR for page 41
INDUSTRIAL METABOLISM
41
The third salient characteristic differentiating the biosphere from the
industrial synthesphere is that, although individual organisms do generate
process wastes- primarily oxygen in the case of plants and carbon dioxide
and urea in the case of animals the biosphere as a whole is extremely effi-
cient at recycling the elements essential to life. Specialized organisms have
evolved to capture nutrients in wastes (including dead organisms) and recy-
cle them. A significant exception to this rule in the present geological epoch
is the deposition of skeletal remains of zooplankton as sediments on the
deep ocean floor. These remains are largely, but not entirely, calciferous.~4
Over geological time periods, some of this sedimentary material is likely to
be recycled as chalk, limestone, or phosphate rock
The biosphere as it now exists is a nearly perfect materials-regycling
system, but this was not the case when life on earth began. The first
and most critical evolutionary "invention," from which all else follows,
was the process for replicating complex organic molecules. In effect, the
information describing the entire living structure is stored as sequences
of nucleic acids in the genetic substance known as deoxyribonucleic acid
(DNA). The mechanism for storing, coding, transferring, and decoding that
information apparently evolved some 4 billion years ago, before species
differentiation. Both the code and the mechanism are common to all
known living organisms.
The first cellular organisms, which appeared about 3.5 billion years
ago, were prokaryotes (i.e., cells without nuclei). They obtained the energy
needed to sustain the reproduction cycle from the anaerobic fermentation
of organic molecules previously created by natural geophysical processes in
an atmosphere containing no free oxygen. If the primitive atmosphere had
contained oxygen, organic molecules could not have survived long enough
to achieve the degree of complexity needed to construct self-reproducing
systemS.l5
In cellular fermentation a molecule of glucose is split into two mole-
cules of pyruvate. Energy is captured in the form of high-energy phosphate
bonds in ADP (adenosine diphosphate) and ATP. Fermentation of a glucose
molecule has a net yield of available energy in the form of two molecules
of h~gh-energy ATP, converted from the low-energy ADP form (Hinkle and
McCarty, 1978~. Further reactions in the cell convert the pyruvate to ethyl
alcohol, lactic acid, and carbon dioxide, all of which are excreted as wastes.
No oxygen is required. The fermentation-based forms of life could not
have been the foundation for a sustainable ecosystem, however, because
they were using up a finite stockpile of exploitable organic molecules.
The next great evolutionary innovation, about 3 billion years ago, was
anaerobic photosynthesis. Me first photosynthesizers were prokaryotic pho-
tobacteria. These organisms began to synthesize glucose from atmospheric
carbon dioxide and sunlight, thus replacing the depleted organic "soup" of
OCR for page 42
42
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OCR for page 43
INDUSDUAL METABOLISM
43
the primitive oceans. They also produced oxygen as a waste product. At
first, the free oxygen was quickly removed by stromatolites, organisms that
combined oxygen with iron dissolved in the oceans and precipitated as iron-
rich reefs. This seems to have been the origin of hematite deposits being
exploited as iron ores today. However, when the dissolved iron was used
up, about 1.8 billion years ago, the oxygen level in the atmosphere began to
rise, thereby increasing the rate of dissolution of all macromolecules. Thus,
the ecosystem was still unsustainable, because it could neither tolerate nor
recycle its own toxic wastes. The oxygen toleration problem was partially
solved by a new class of aerobic photosynthesizers, the cyanobacteria (or
blue-green algae). These appeared a little more than 2 billion years ago.
A third great evolutionary innovation the substitution of respiration
(using oxygen) for fermentation to obtain energy from organic molecules-
solved the recycling problem. The respiration process begins the same way
as fermentation, with the splitting of glucose into pyruvate (called glycoly-
sis). However, in respiration, glycolysis is followed by a longer sequence of
reactions, known collectively as the citric acid cycle. In effect, the pyruvates
are oxidized enzymatically to carbon dioxide, with the formation of many
more high-energy ATP molecules. In fact, each glucose molecule, when
fully oxidized, yields 36 molecules of ATP, whereas the glycolysis stage alone
yields only 2. The yield of available energy for further metabolic processes
is, therefore, 18 times that of the fermentation process.
Because the respiration process is far more efficient than its precursor,
aerobic respirators required much less organic material to sustain them.
Thus, anaerobic organisms could not effectively compete with aerobic or-
ganisms in the presence of oxygen. (They still fill an environmental niche
in sediments and deep oceans lacking free oxygen.)
With the "invention" of the citric acid circle, the biosphere became
sustainable within more self-defined system boundaries. Evolutionary de-
velopments since then-of which the most important were the develop-
ment of the eukaryotes (cells with true nuclei) and the advent of sexual
reproduction made the system more diverse and more efficient. It is in-
teresting to note that, despite the radical changes in energy metabolism
which occurred, the basic scheme of macromolecular reproduction seems
to have remained essentially unchanged for 4 billion years.
Industrial liar~sfonnation Processes
In contrast to modern biological processes, industrial processes are
almost exclusively energized by the combustion of fossil fuels, which (by
definition) are not regenerated within the system. In this sense, the indus-
tnal system of today resembles the earliest stage of biological evolution,
when the most primitive living organisms obtained their energy from a
OCR for page 44
44
ROBERT U LYRES
stock of organic molecules accumulated during prebiotic times. Instead of
regenerative cycles powered by solar (or nuclear) energy, industrial pro-
cesses are linear sequences of discrete, irreversible transformations (Figure
4~. The sequence begins with extraction of raw matenals, followed by phys-
ical separation and elimination of impunties, and subsequent reduction
or recombination into convenient "first-tier" intermediates. This category
includes prunary metals and other elements In pure form, cellulose, sodium
carbonate, ammonia, methane, ethane, propane, butane, benzene, xylene,
methanol, ethanol, acetylene, ethylene, propylene, and some others. These
materials, in turn, are subsequently recombined into desired chemical and
physical forms.
Almost all of the processes for reducing metals from ores or producing
first-tier intermediates are endothermic, that is, driven by externally sup-
plied heat. Processes often use catalysts, and rates and directions are fine
tuned by controlled variation of temperatures, pressures, and flow rates or
dwell times. There are five major categories of endothermic processes: (1)
dehydration; (2) calcination; (3) "reducing" processes for splitting metal
(or other) oxides into their constituents; (4) dehydrogenation processes,
of which the simplest is the splitting of the water molecule;~7 and (5)
processes for combining or synthesizing molecules that do not combine
spontaneously at ambient temperatures or pressures. Energy is obtained
initially from combustion (oxidation) and delivered either by process steam
or by direct contact with the oxidation products. Comparatively few indus-
trial processes are electrolytic: the production of aluminum, sodium, and
chlorine and the refining of blister copper are the primary examples.
The most familiar example of dehydration is the production of plaster
of parts from the mineral gypsum. Brick and ceramic manufacturing are
also based to some extent on this process. The major example of calcination
(and the origin of the name) is the production of calcium oxide (quicklime)
from calcium carbonate (limestone) by driving off carbon dioxide. This is
a major element in the manufacture of Portland cement. Both dehydration
and calcination are accomplished by the simple application of heat. The
so-called carbothermic reduction process by which iron ore is converted to
pig iron is a typical example of the third category. In this reaction, coke is
partially oxidized to carbon monoxide, which in turn reacts with the ore-at
appropriate temperatures to reduce the iron oxides, while converting the
carbon monoxide to carbon dioxide.~9 The temperature determines which
way the reaction goes. If the temperature rises too high (above 1300° C),
any pure iron that is present will reoxidize.
Ammonia production exemplifies both the fourth and the fifth cate-
gories. Dehydrogenation (water splitting) is an example of a process by
which synthesis gas for ammonia is produced. In a first stage of the reac-
tion, steam reacts with natural gas, in the presence of a catalyst, to produce
OCR for page 45
INDUS17UAL METABOLISM
Embodied Negentropy
(Increase in Information)
Separation of ore from crust
Chemical processes that increase
"availability" (or free energy)
Matching of specifications for trace
elements, crystal size, dislocations,
etc.
Matching of dimensions and shape
specifications
45
Materials Transformation Lost Negentropy
(Increase in Entropy)
Extraction
(Mining, Drilling, Harvesting, etc.)
.
mew Materia:3
Physical Separation and Refining |
(Beneficiation, Smelting, etc.)
termedia~
Pure )
Recombination |
(Alloying, Synthesis, Heat Treatment,
Hot/Cold Working, etc.)
~ Finished
\~ Materials J
Macro-Forming
(Casting/Molding, Machining, _
Fording, etc.)
(nished Pas)
Work done on environment
Heat loss to environment
Design | Fabrication and Construction 1 Informationlossas
· J finished products
1
`( Finished ~
Products J
Use
junk. Refuse
] Information loss with wear
Disposal ~Dispersal of materials
I I
FIGURE 4 Representation of the economic system as a multistage system for the
extraction, physical separation, recombination, formation, and consumption of materials. At
the end of the sequence, materials are returned to the environment in a degraded form as
waste. The process of creating a finished product (center column) concentrates information,
or negentropy, in the product at the expense of increased entropy in the environment.
OCR for page 46
46
ROBERT U AYRES
carbon monoxide and hydrogen. In a second stage, known as the water-gas
shift reaction, carbon monoxide reacts with added steam to yield more hy-
drogen and carbon dioxide, which must be removed by a scrubber such as
potassium carbonate. The hydrogen is then mixed with nitrogen gas (from
the air) in a 3:1 ratio.20 At very high pressures and temperatures, these in-
gredients combine endothermicall~again in the presence of a catalyst to
form ammonia gas, the basis of virtually all nitrogen-containing compounds
used by our industrial civilization.
Most of the reactions by which ammonia, chlorine, lime, sulfur,
methanol, ethanol, acetylene, ethylene, propylene, or other first-tier inter-
mediates are converted to other "downstream" compounds are exothermic
and in effect-self-energizing. For example, many second-tier intermedi-
ates are produced by controlled oxidation (e.g., sulfuric acid from sulfur,
nitric acid from ammonia, acetaldehyde from ethanol or propane, acetic
acid from acetaldebyde or butane, acetic anhydride from acetaldehyde,
ethylene oxide from ethylene, propylene oxide from propylene, and so
on). Most hydrogenation, chlorination, and hydrochlorination reactions
are also exothermic, as are most reactions between strong acids and metals
or hydroxides.
In effect, the first-tier intermediates are the energy carriers for subse-
quent reactions. They play a role somewhat analogous to that of ATP in
biochemical systems. However, whereas the ATP is cyclically regenerated
within the same cell, the first-tier intermediates are not regenerated but
are physically embodied in downstream products. This is another funda-
mental difference between industrial metabolism in its present form and its
biological analogue.
NOTES
The three major German firms Bayer, Hoechst, and BASE were merged in the
1920s into a single giant under the name I.G. Farl~enindustrie. Farben is the German
word for "color." The name reflected their common origin as synthetic dye (color)
manufacturers.
2. An important corollary is that the underpricing of environmental resources corresponds
to an underpricing of those exhaustible mineral resources whose subsequent disposal
as waste residuals causes harm to the environment. This is because of the lack of any
link between the market puce paid (for coal, oil, or whatever) and the subsequent
cost of waste disposal or more important-of uncompensated environmental or
health damages such as bronchitis, asthma, emphysema, cancer, soil acidification,
the greenhouse effect, and so on. Here the distinction between renewable and
nonrenewable resources is critical: although renewable resources can obviously create
pollution problems, such as sewage, they are almost invariably localized in nature and
can be abated at moderate cost. This is emphatically not the case for combustion
products of fossil fuels or dispersion of toxic heavy metals.
floe total world output of carbon dioxide from fuels and cement manufacturing has
been estimated to be 5.1 billion metric tons for 1982, of which 26.7 percent was
attributable to North America McFarland and Rotty, 1984~.
OCR for page 47
INDUSTIUAL METABOLISM
47
4. Fly ash is primarily a by-product of coal combustion. At present it is being recovered
fairly efficiently, from stack gases of large utility boilers and industrial furnaces, by
means of electrostatic precipitatom However, the ash itself has become a large-scale
nuisance because there exists no use or market for it. Ibe amounts are large:
over 50 million tons are generated annually in the United States alone. Several
possible remedies exist. Fly ash is a potential "ore" for several metals, especially iron,
aluminum, and silicon. These metals could probably be recovered commercially if,
for example, bauxite became unavailable (Ayres, 19823. Alternatively, fly ash could
be used as a substitute, or more likely as a supplement, for Portland cement in the
manufacture of concrete and concretelike products. fits major disadvantage in this
application is that concrete made with fly ash does not harden and set as rapidly as
the commercial variety. This has obvious economic costs, but so does the disposal
of fly ash in landfills.) Another use of fly ash- already demonstrated in France is
as a means for the permanent disposal of toxic liquid wastes in the folm of a hard,
impermeable rocklike material suitable for long-term storage.
The "chain" analogy is an oversimplification, of course, because many processes yield
more than one useful product (the chlor-alkali industry is an obvious example) and
many products also require two or more inputs. Thus, the structure of the system as
a whole is more like a "tree."
6. For example, the first process to manufacture acetylene proceeded by way of calcium
carbide production (from coke and limestone) but was displaced by direct dehydro
genation of hydrocarbon feedstocks. The first production of ac~y'onitrile involved a
reaction between ethylene oxide (itself the third step in a chain beginning with ethane)
and hydrogen cyanide (made from ammonia). This was replaced by an acetylene
cyanation process (acetylene being made from methane) and finally by a propylene
ammoxidation process (propylene reacting directly with ammonia). Similarly, the fimt
process in the manufacture of acetaldehyde started with acetylene (from calcium
carbide) or ethanol (from ethylene). A newer process made acetaldehyde directly
from ethylene. ~us, in the first example the calcium carbide stage was bypassed. In
the second example the oxidation of ethylene was avoided, and in the third example
the conversion of ethylene to alcohol was avoided.
7. The problem has been dramatized by several recent episodes in which cities have
attempted to dispose of solid wastes by using private contractors who, in turn,
thought to transport them to countries where disposal rules are nonexistent or wealcly
enforced. For example, the city of Philadelphia contracted with Joseph Paolino &
Sons to dispose of incinerator ash. Paolino, in turn, contracted with Amalgamated
Shipping, a Bahamian concern, to transport the ash to the Bahamas. However,
the Bahamian government bawd the dumping, and the ship carrying the ash was
subsequently turned away from ports in the Dominican Republic, Haiti (after it had
dumped 2,000 tons of ash), Honduras, Costa Rica, Guinea-Bissau, and the Cape
Verde Islands. The ship apparently succeeded in dumping its load of ash somewhere
in the Indian Ocean, after more than two years (New York Tones, November 10,
1988~.
8. Admittedly, it can still happen. Asbestos and polychlorinated biphenyls are two
examples of materials that were once thought to be safe but have subsequently come
to be regarded as hazardous and for which the major producers or users have had
to spend billions of dollars to collect and dispose of them safely. Nevertheless, there
would seem to be an economic opportunity for a "high-tech" resource-recove~y firm
to go into business reconverting fly ash and incinerator ash into its most valuable
components, light metals such as aluminum, iron, potash, and titanium (Ayres, 1982~.
OCR for page 48
48
ROBERT U LYRES
A mineralized glassy residue of sodium-silica and heavy metals would, of course,
remain for disposal (though it might also find uses as a construction material).
9. This is a straightforward implication of the widely accepted, but sporadically enforced,
"polluter must pay" principle.
A number of possible processes for large-scale thermal decomposition of water have
been suggested and studied in some detail, e.g., by the European Atomic Energy
Commission (EURATOM). See, for example, Marchetti (19733.
11. Ammonia, hydrazine, and other compounds have also been suggested, but there is no
clear leader at this time. A new study of the economic and technical feasibility of
non~arbon-based liquid fuels would be helpful in clarifying the choices.
1Z It is true that some organisms have evolved to function in the deep oceans under
conditions of high pressure and salinity, others have evolved to function in surface
waters, saline or fresh; still others have evolved to function in environments with
very little water. Nevertheless, the internal environment of every cell is aqueous and
the pressure inside each cell is essentially the same as the external pressure of the
environment in which the organism lives.
13. For a concise summary of the biochemistry of energy transport in cells, see Schopf
(1978~.
14. The so-called manganese nodules, which are accretions of iron, manganese, copper,
cobalt, and other transition elements, are evidently the result of some combination
(as yet imperfectly understood) of biological, chemical, and geological processes (see,
for example, Morgenstein, 1973~.
15. Even in the absence of oxygen, it would seem that dissolution of macromolecules
should proceed faster than synthesis (Wald, 1954~. The exact evolutionary mechanism
leading to self-reproduction is still obscure. Fixation of nitrogen must have been
accomplished during this early period, because the early atmosphere was nearly
transparent to ultraviolet radiation and any free ammonia in the atmosphere would
have been quickly destroyed.
16. The obvious exceptions are elements occurring naturally (such as sulfur) and hy-
drocarbons that can be obtained By physical separation from natural gas (methane,
ethane, propane, butane) or coal tar (benzene, xylene, toluene). Cellulose occurs
naturally in some very pure forms (e.g., cotton), but it is usually obtained from wood
pulp lay a chemical digestion process.
17. Other major examples of dehydrogenation include the splitting of methane, ethane,
propane, and butane to produce acetylene, ethylene, propylene, butylene, and butadi-
ene.
18. Pig iron is a solution of iron carbide in iron, with a typical carbon content of
approximately 6 percent. The conversion of pig iron to pure (wrought) iron or steel
requires removing this carbon and then adding any desired alloying elements.
19. Multistage reactions such as the reduction of iron ore and the synthesis of ammonia,
involving an intermediate (carbon monoxide) that is produced by the reaction and later
consumed, are quite common in industry. Less common are reactions involving an
intermediate that is not produced within the process but is recycled. One of the first
examples of such a process was the Solvay (ammonia-soda) process for manufacturing
synthetic sodium carbonate from sodium chloride and calcium carbonate. In this
process, ammonium hydroxide reacts with calcium carbonate to yield ammonium
carbonate and calcium hydroxide. Ammonium carbonate is converted to ammonium
bicarbonate. When this reacts with sodium chloride, sodium carbonate and ammonium
chloride are produced. Finally, calcium hydroxide and ammonium chloride are reacted
to recover ammonia (gas) for recycling and calcium chloride. Ibe latter is a low-value
by-product.
OCR for page 49
INDUSTRIAL METABOLISM
49
20. In principle, the nitrogen gas and steam could be produced together lay partial
oxidation of natural gas (or any hydrocarbon) in air to yield a mixture of nitrogen,
steam, and carbon monoxide. The steam in the hot combustion products could
then be reacted with additional natural gas to generate hydrogen and more carbon
monoxide to be used as feedstock for the shift reaction. However, the presence of
nitrogen before it is wanted complicates the engineering unreasonably.
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Ayres, R. U., L W. Ayres, J. A. Tarr, and R. C. W~dge~y. 1988. An historical reconstruction
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Clark, R., ed. 1987. The Ozone Layer [Series: UNEP/GEMS Environment Library], Vol.
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Representative terms from entire chapter:
carbon monoxide
