Page 80 - Materials Chemistry, Second Edition

62 LIFE CYCLE ASSESSMENT HANDBOOK

receiving water), or water that is withdrawn and not directly returned to a
receiving body (such as water embodied in a product or evaporated in a cool-
ing tower or drying operation). Consumptive use of water can be a very signif-
icant environmental concern, particularly in areas where fresh water is scarce.
There is less focus on consumptive use of saltwater.
Protocols are being established for categorizing and reporting the vari-
ous types of water use. There is an active UNEP/SETAC working group on
water use and consumption within LCA. As of June 2011, the International
Organization for Standardization initiated a working draft of a new standard,
ISO 14046 Life cycle assessment - Water footprint - Requirements and guide-
lines, to provide internationally harmonized metrics for water footprints [9].

3.4.6 Carbon Tracking Considerations

Biogenic carbon and fossil carbon are treated differently in life cycle meth-
odology. Biogenic carbon is carbon that is removed from the atmosphere
and incorporated into the physical mass of a plant or organism. The carbon
remains embodied in the biomass-derived product throughout its useful life.
At end of life, some or all of the carbon may be permanently sequestered (e.g.,
if the biomass-derived product is landfilled and some of the material does not
decompose), or carbon content may be returned to the atmosphere through
decomposition or combustion. Biogenic carbon released at end of life as carbon
dioxide returns to the atmosphere in the same form as which it was removed,
with no net increase in atmospheric carbon dioxide within the time frame of
natural biogenic carbon cycling. These biomass carbon dioxide emissions are
considered to be "carbon neutral."
Although fossil fuels such as petroleum and coal originated as biogenic car-
bon, the carbon uptake occurred millions of years ago. Fossil carbon resources
remain stored within the earth until they are extracted through human inter-
vention. Therefore, emissions associated with combustion of fossil fuels or
fossil-derived materials are treated as net contributions to atmospheric carbon
dioxide levels, and no carbon storage credit is given when materials such as
fossil fuel-derived plastics are landfilled [10].
There are additional considerations regarding decomposition of landfilled
biomass products. If a biomass-derived product decomposes aerobically, the
carbon dioxide released is considered carbon neutral. However, if the biomass
decomposes anaerobically, both carbon dioxide and methane will be produced.
For either type of decomposition, the carbon dioxide produced is considered
carbon neutral, but the methane is not. Since human intervention in the bio-
mass carbon cycle is responsible for some of the atmospheric carbon returning
to the atmosphere as methane, with a higher global warming potential than
the carbon dioxide initially taken up by the biomass, the methane releases are
not considered carbon neutral.
End of life carbon tracking calculations can become quite complicated when
considering the potential mix of fates of biomass products and the time frame
over which releases occur. As noted previously, biomass-derived products may