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Assignment 1
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Sample responses
to problems
Problem 1:
Click here
for a material flow diagram for
mercury. The
key feature to note in the material flow is that the
inputs and outputs are distinctly out of balance. In
particular point out the discrepancy between inputs
and outputs in the chlor-alkali industry. Discuss
whether the mercury might be accumulated in
stockpiles in this sector and the uncertainties the
data imply about the potential environmental sources
of mercury. For a thorough discussion of the mercury
flows, see:
R. Ayres, "The Life Cycle of
Chlorine, Part I, Chlorine Production and the
Chlorine-Mercury Connection", Journal
of Industrial Ecology, 1
(1), 81-94 (1997)
R. U. Ayres and Leslie W. Ayres,
"Accounting for Resources, 1" Edward Elgar,
Cheltenham, UK (1998) (especially Chapter 8)
Problem 2:
Click here
for a material flow diagram for
arsenic. The
key feature to note in the material flow is that,
for the past 30 years, arsenic has been accumulated
in "pressure treated lumber". Discuss how
this arsenic might be collected as this lumber
reaches the end of its useful life and the
consequences of not collecting the lumber (leaching
of arsenic from landfills and release of arsenic to
the atmosphere during incineration).
Problem 3:
Click here
for a material flow diagram for
cadmium. The
key feature to note in the material flow is the
significance of battery use. Also discuss the fact
that cadmium is "mined" exclusively as a
by-product of zinc production. Point out that if
cadmium is prohibited from use in batteries but zinc
production continues to increase, cadmium may just
be displaced from product waste to mine waste. Also
note that a "hidden" cadmium flow (as a
trace contaminant in phosphate fertilizer)
overwhelms all other flows, yet is largely ignored.
Have the students attempt to determine an average
cadmium concentration in phosphate (100 ppm; USGS,
2003) and the total use of phosphate (15,000,000
tons per year in the U.S.; USGS, 2003) to estimate
this hidden flow.
Problem 4:
Click here
for the solution to this problem.
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