As mentioned earlier (Chapter 3), the first law of thermodynamics (conservation of mass) implies that mass outputs from any process equal mass inputs. However, useful outputs are almost invariably a fraction of total inputs, sometimes a small fraction (as in the case of refining low grade ores).
In some cases, the output mass is entirely wasted, as with combustion processes. Thus wastes are an unavoidable by-product of physical production.
The law of mass conservation, on the other hand, is far from trivial. The so-called 'mass-balance principle' states that mass inputs must equal mass outputs for every chemical process (or process step), and that this must be true separately for each chemical element.17 All resources extracted from the environment must eventually become unwanted wastes and pollutants. Waste emissions are not exceptional phenomena that can be neglected or treated as exceptions. The standard multi-sector economic model of commodities produced from other commodities is misleading (Walras 1874; Sraffa 1960; von Neumann 1945 ).
It follows, too, that virtually all products are really joint products, except that wastes have no positive market value. On the contrary, they have, in most cases, a negative value. A producer of wastes will need a 'sink' for disposal. Options for free disposal are becoming rarer. Producers must, increasingly, pay to have waste residuals removed and treated, safely disposed of, or recycled. The implication that there exists a price-determined equilibrium between supply and demand (of commodities) must therefore be modified fundamentally (Ayres and Kneese 1969).
This means, among other things, that 'externalities' (market failures) associated with production and consumption of materials are actually pervasive and that they tend to grow in importance as the economy itself grows. Materials recycling can help (indeed, it must), but recycling is energy (exergy) intensive and (thanks to the second law) imperfect, so it cannot fully compensate for a declining natural resource base. Long-term sustainability must depend to a large extent upon dematerialization and 'decoupling' of economic welfare from the natural resource base (Ayres and Kneese 1989).
The mass-balance condition provides powerful tools for estimating process wastes and losses for industrial processes, or even whole industries, where these cannot be determined directly. Even where other data are available, the mass-balance condition offers a means of verification and interpolation, to fill in gaps (Ayres and Cummings-Saxton 1975; Ayres 1978; Ayres and Simonis 1999; Ayres 1995).
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