Measures of industrial metabolism
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.
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)
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
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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