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1. Industrial metabolism: Theory and policy
What is industrial metabolism?
The materials cycle
Measures of industrial metabolism
Policy implications of the industrial metabolism perspective
2. Ecosystem and the biosphere: Metaphors for human-incluced material flows
The ecosystem analogue
The environmental spheres analogue: Atmosphere, hydrosphere, lithosphere, and biosphere
Summary and conclusions
3. Industrial restructuring in industrial countries
Identifying indicators of environmentally relevant structural change
Structural change as environmental relief
Environmentally relevant structural change: Empirical analysis
Typology of environmentally relevant structural change
4. Industrial restructuring in developing countries: The case of India
Industrial metabolism and sustainable development
Industry and sustainable development
Energy efficiency: An overview
Energy use in Indian industry: A case-study
5. Evolution, sustainability, and inclustrial metabolism
Technical progress and reductionism
The mechanical paradigm
The evolution of ecological structure
Robert U. Ayres
The word metabolism, as used in its original biological context, connotes the internal processes of a living organism. The organism ingests energy-rich, low-entropy materials ("food") to provide for its own maintenance and functions, as well as a surplus to permit growth and/or reproduction. The process also necessarily involves the excretion or exhalation of waste outputs, consisting of degraded, high-entropy materials. There is a compelling analogy between biological organisms and industrial activities - indeed, the whole economic system - not only because both are materials-processing systems driven by a flow of free energy (Georgescu-Roegen, 1971), but because both are examples of self-organizing "dissipative systems" in a stable state, far from thermodynamic equilibrium (Ayres, 1988).
At the most abstract level of description, then, the metabolism of industry is the whole integrated collection of physical processes that convert raw materials and energy, plus labour, into finished products and wastes in a (more or less) steady-state condition (fig. 1). The production (supply) side, by itself, is not self-regulating. The stabilizing controls of the system are provided by its human component. This human role has two aspects: (1) direct, as labour input, and (2) indirect, as consumer of output (i.e. determinant of final demand). The system is stabilized, at least in its decentralized competitive market form, by balancing the supply of and demand for both products and labour through the price mechanism. Thus, the economic system is, in essence, the metabolic regulatory mechanism.
Fig. 1 The world of the market
Industrial metabolism can be identified and described at a number of levels below the broadest and most encompassing global one. Thus, the concept is applicable to nations or regions, especially "natural" ones such as watersheds or islands. The key to regional analysis is the existence of a well-defined geographical border or boundary across which physical flows of materials and energy can be monitored.
The concept of industrial metabolism is equally applicable to another kind of self-organizing entity, a manufacturing enterprise or firm. A firm is the economic analogue of a living organism in biology.' Some of the differences are interesting, however. In the first place, biological organisms reproduce themselves; firms produce products or services, not other firms (except by accident). In the second place, firms need not be specialized and can change from one product or business to another. By contrast, organisms are highly specialized and cannot change their behaviour except over a long (evolutionary) time period. In fact, the firm (rather than the individual) is generally regarded as the standard unit of analysis in economics. The economic system as a whole is essentially a collection of firms, together with regulatory institutions and worker-consumers, using a common currency and governed by a common political structure. A manufacturing firm converts material inputs, including fuels or electric energy, into marketable products and waste materials. It keeps financial accounts for all its external transactions; it is also relatively easy to track physical stocks and flows across the "boundary" of the firm and even between its divisions.
A third way in which the analogy between biological metabolism and industrial metabolism is useful is to focus attention on the "life cycle" of individual "nutrients."
The hydrological cycle, the carbon cycle, and the nitrogen cycle are familiar concepts to earth scientists. The major way in which the industrial metabolic system differs from the natural metabolism of the earth is that the natural cycles (of water, carbon/oxygen, nitrogen, other words, the industrial system does not generally recycle it trients. Rather, the industrial system starts with high-quality mat' (fossil fuels, ores) extracted from the earth, and returns them to nature in degraded form.
This point particularly deserves clarification. The materials c in general, can be visualized in terms of a system of compartments containing stocks of one or more nutrients, linked by certain flows. For instance, in the case of the hydrological cycle, the glaciers oceans, the fresh water lakes, and the groundwater are stocks, while rainfall and rivers are flows. A system is closed if there are no e nal sources or sinks. In this sense, the earth as a whole is essentially, closed system, except for the occasional meteorite.
A closed system becomes a closed cycle if the system is al steady state, i.e. the stocks in each compartment are constant an changing, at least on average. The materials balance condition plies that the material inputs to each compartment must be e, balanced (on average) by the outputs. If this condition is not m. a given compartment, then the stock in one or more compartments must be increasing, while the stocks in one or more other compartments meets must be decreasing.²
It is easy to see that a closed cycle of flows, in the above sense only be sustained indefinitely by a continuous flow of free en This follows immediately from the second law of thermodynamics, which states that global entropy increases in every irreversible process. Thus, a closed cycle of flows can be sustained as long external energy supply lasts. An open system, on the contrary, herently unstable and unsustainable. It must either stabilize or lapse to a thermal equilibrium state in which all flows, i.e. all physical and biological processes, cease.
It is sometimes convenient to define a generalized four-box model to describe materials flows. The biological version is shown in figure 2, while the analogous industrial version is shown in figure 3. Reverting to the point made at the beginning of this section, the nature tem is characterized by closed cycles, at least for the major nutrients (carbon, oxygen, nitrogen, sulphur) - in which biological processes play a major role in closing the cycle. By contrast, the industrial system is an open one in which "nutrients" are transformed "wastes," but not significantly recycled. The industrial system, exists today, is therefore ipso facto unsustainable.
Fig. 2 Four-box scheme for bio-geo-chemical cycles
At this stage, it should be noted that nothing can be said at least) with respect to any of the really critical questions. These are as follows:
It is fairly important to try to find answers to these questions.
Fig. 3 Four box scheme for industrial material cycles
Needless to say, we do not aspire to answer all these questions in the present volume.
It should also be pointed out that the bio-geosphere was not always a stable system of closed cycles. Far from it. The earliest living cells on earth obtained their nutrients, by fermentation, from nonliving organic molecules whose origin is still not completely understood. At that time the atmosphere contained no free oxygen or nitrogen; it probably consisted mostly of water vapour plus some hydrogen, and hydrogen-rich gases such as methane, hydrogen sulphide, and ammonia. The fermentation process yields ethanol and carbon dioxide. The system could only have continued until the fermentation organisms used up the original stock of "food" molecules or choked on the carbon dioxide buildup. The system stabilized temporarily when a new organism (blue-green algae, or cyano-bacteria) appeared that was capable of recycling carbon dioxide into sugars and cellulose, thus again closing the carbon cycle. This new process was anaerobic photosynthesis.
However, the photosynthesis process also had a waste product: namely, oxygen. For a long time (over a billion years) the oxygen generated by anaerobic photosynthesis was captured by dissolved ferrous iron molecules, and sequestered as insoluble ferric oxide or magnetite, with the help of another primitive organism, the Stromatolites. The resulting insoluble iron oxide was precipitated on the ocean bottoms. (The result is the large deposits of high-grade iron ore we exploit today.) The system was still unstable at this point. It was only the evolutionary invention of two more biological processes, aerobic respiration and aerobic photosynthesis, that closed the oxygen cycle as well. Still other biological processes - nitrification and denitrification, for instance - had to appear to close the nitrogen cycle and others.
Evidently, biological evolution responded to inherently unstable situations (open cycles) by "inventing" new processes (organisms) to stabilize the system by closing the cycles. This self-organizing capability is the essence of what has been called "Gaia." However, the instabilities in question were slow to develop, and the evolutionary responses were also slow to evolve. It took several billion years before the biosphere reached its present degree of stability.
In the case of the industrial system, the time scales have been drastically shortened. Human activity already dominates and excels natural processes in many respects. While cumulative anthropogenic changes to most natural nutrient stocks still remain fairly small in most cases, the rate of nutrient mobilization by human industrial activity is already comparable to the natural rate in many cases. Table 1 shows the natural and anthropogenic mobilization (flow) rates for the four major biological nutrients, carbon, nitrogen, phosphorus and sulphur. In all cases, with the possible exception of nitrogen, the anthropogenic contributions exceed the natural flows by a considerable margin. The same is true for most of the toxic heavy metals, as shown in table 2.
On the basis of relatively crude materials cycle analyses, at least, it would appear that industrialization has already drastically disturbed, and ipso facto destabilized, the natural system.
Table 1 Anthropogenic nutrient fluxes (teragams/year)
|To atmosphere, total||7,900||4||55.0||12.5||93||65.5||1.5||12.5|
|Fossil fuel combustion and smelting||6,400||45.0||92|
|Land clearing, deforestation||1,500||2.6||1||1.5|
|To soil, total||112.5||21||73.3||23.4||15||7.4|
|Anthropogenic acid deposition||30.0||48.3|
|Anthropogenic (NH3, NH4) deposition||10.0|
|To rivers and oceans, total||72.5||25||52.5||21||5||10.3|
|Anthropogenic acid deposition||55.0||22.5|
a. Assuming 10 per cent loss of synthetic ammonia-based fertilizers applied to land surface (75 tg/yr).
b. Total production (= use) less fertilizer use, allocated to landfill. The remainder is assumed to be disposed of via waterways.
Table 2 Worldwide atmospheric emissions of trace metals (1,000 tonnes per year)
tion by natural
Source: Nriagu, 1990.
There are only two possible long-run fates for waste materials: recycling and re-use or dissipative loss. (This is a straightforward implication of the law of conservation of mass.) The more materials are recycled, the less they will be dissipated into the environment, and vice versa. Dissipative losses must be made up by replacement from virgin sources.
A strong implication of the analysis sketched above is that a longterm (sustainable) steady-state industrial economy would necessarily be characterized by near-total recycling of intrinsically toxic or hazardous materials, as well as a significant degree of recycling of plastics, paper, and other materials whose disposal constitutes an environmental problem. Admittedly it is not possible to identify, in advance, all potentially hazardous materials, and it is quite likely that there will be (unpleasant) surprises from time to time. However, it is safe to say that heavy metals are among the materials that would have to be almost totally recycled to satisfy the sustainability criterion. The fraction of current metal supply needed to replace dissipative losses (i.e. production from virgin ores needed to maintain a stable level of consumption) is thus a useful, if partial, surrogate measure of "distance" from a steady-state condition, i.e. a condition of long-run sustainability.
Most economic analysis in regard to materials, in the past, has focused on availability. Data on several categories of reserves (economically recoverable, potential, etc.) are routinely gathered and published by the US Bureau of Mines, for example. However, as is well known, such figures are a very poor proxy for actual reserves. In most cases the actual reserves are greater than the amounts actually documented. The reason, simply, is that most such data are extrapolated from test borings by mining or drilling firms. There is a well-documented tendency for firms to stop searching for new ore bodies when their existing reserves exceed 20 to 25 years' supply. Even in the case of petroleum (which has been the subject of worldwide searches for many decades), it is not possible to place much reliance on published data of this kind.
However, a sustainable steady state is less a question of resource availability than of recycling/re-use efficiency. As commented earlier, a good measure of unsustainability is dissipative usage. This raises the distinction between inherently dissipative uses and uses where the material could be recycled or re-used in principle, but is not. The latter could be termed potentially recyclable. Thus, there are really three important cases:
Generally speaking, it is arguable that most structural metals and industrial catalysts are in the first category; other structural and packaging materials, as well as most refrigerants and solvents, fall into the second category. This leaves coatings, pigments, pesticides, herbicides, germicides, preservatives, flocculants, anti-freezes, explosives, propellants, fire retardants, reagents, detergents, fertilizers, fuels, and lubricants in the third category. In fact, it is easy to verify that most chemical products belong in the third category, except those physically embodied in plastics, synthetic rubber, or synthetic fibres.
From the standpoint of elements, if one traces the uses of materials from source to final sink, it can be seen that virtually all sulphur mined (or recovered from oil, gas, or metallurgical refineries) is ultimately dissipated in use - for example, as fertilizers or pigments or discarded as waste acid or as ferric or calcium sulphites or sulphates. (Some of these sulphate wastes are classed as hazardous.) Sulphur is mostly (75-80 per cent) used to produce sulphuric acid, which in turn is used for many purposes. But in every chemical reaction the sulphur must be accounted for - it must go somewhere. The laws of chemistry guarantee that reactions will tend to continue either until the most stable possible compound is formed or until an insoluble solid is formed. If the sulphur is not embodied in a "useful" product, it must end up in a waste stream.
There is only one long-lived structural material embodying sulphur: plaster of Paris (hydrated calcium sulphate), which is normally made directly from the natural mineral gypsum. In recent years, sulphur recovered from coalburning power plants in Germany has been converted into synthetic gypsum and used for construction. However, this potential recycling loop is currently inhibited by the very low price of natural gypsum. Apart from synthetic gypsum, there are no other durable materials in which sulphur is physically embodied. It follows from materials balance considerations that sulphur is entirely dissipated into the environment. Globally, about 61.5 million tonnes of sulfur qua sulphur - not including gypsum - were indicated schematically in figure 4. Very little is currently used for structural materials. Thus, most sulphur chemicals belong in class 3.
Fig. 4 Dissipative uses of sulphur, 1988 (millions of tonnes)
Following similar logic, it is easy to see that the same is true of most chemicals derived from ammonia (fertilizers, explosives, acrylic fibres), and phosphorus (fertilizers, pesticides, detergents, fire retardants). In the case of chlorine, there is a division between class 2 (solvents, plastics, etc.) and class 3 (hydrochloric acid, chlorine used in water treatment, etc.).
Chlorofluorocarbon refrigerants and solvents are long-lived and nonreactive. In fact, this is the reason they pose an environmental hazard. Given an appropriate system for recovering and reconditioning old refrigerators and air-conditioners, the bulk of the refrigerants now in use could be recovered, either for re-use or destruction. Hence, they belong in class 2. However, CFCs used for foam-blowing are not recoverable.
Table 3 Examples of dissipative use (global)
bleach, water treatment, (PVC)
solvents, pesticides, refrigerants
(H2SO4), bleach, chemicals,
|Ammonia||93.6||Fertilizers, detergents, chemicals|
nitric acid, chemicals
|NaOH||35.8||Bleach, soap, chemicals|
(CuSO4 - 5H2O)
|0.10||Fungicide, algicide, wood preservative, catalyst|
|Sodium bichromate||0.26||Chromic acid (for plating), tanning, algicide|
|Lead oxides||0.24||Pigment (glass)|
|Zinc oxides||0.42||Pigment (lyres)|
|Titanium oxide (TiO2)||1.90||Pigment|
|Arsenic||?||Wood preservative, herbicide|
Table 3 shows the world output of a number of materials - mostly chemicals - whose uses are, for the most part, inherently dissipative (class 3). (It would be possible, with some research, to devise measures of the inherently dissipative uses of each element, along the lines sketched above.) Sustainability, in the long run, would imply that such measures decline. Currently, they are almost certainly increasing.
With regard to materials that are potentially recyclable (classes 1 and 2), the fraction actually recycled is a useful measure of the approach toward (or away from) sustainability. A reasonable proxy for this, in the case of metals, is the ratio of secondary supply to total supply of final materials: see, for example, table 4. This table shows, incidentally, that the recycling ratio in the United States has been rising consistently in recent years only for lead and iron/steel. In the case of lead, the ban on using tetraethyl lead as a gasoline additive (an inherently dissipative use) is entirely responsible.
Table 4 Scrap use in the United States
(million short tons)
|% of total consump
tion in recycled scrap
Source: Institute of Scrap Recycling Industries, 1988.
Another useful measure of industrial metabolic efficiency is the economic output per unit of material input. This measure can be called materials productivity. It can be determined, in principle, not only for the economy as a whole, but for each sector. It can also be measured for each major "nutrient" element: carbon, oxygen, hydrogen, sulphur, chlorine, iron, phosphorus, etc. Measures of this kind for the economy as a whole are, however, not reliable indicators of increasing technological efficiency or progress toward long-term sustainability. The reason is that increasing efficiency - especially in rapidly developing countries - can be masked by structural changes,7 such as investment in heavy industry, which tend to increase the materials (and energy) intensiveness of economic activity. On the other hand, within a given sector, one would expect the efficiency of materials utilization or materials productivity - to increase in general.
Useful aggregate measures of the state of the environment vis-à-vis sustainability can be constructed from physical data that are already collected and compiled in many countries. To derive these aggregates and publish them regularly would provide policy makers with a valuable set of indicators at little cost.
It is clear that other interesting and useful measures based on physical data are also possible. Moreover, if similar data were collected and published at the sectoral level, it would be possible to undertake more ambitious engineering-economic systems analyses and forecasts - of the kind currently possible only for energy - in the entire domain of industrial metabolism
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