Environmental Engineering Reference
In-Depth Information
the genetic model organism, Arabidopsis, and within the hardier switchgrass. While technical
issues regarding both genetic engineering of the plants and separation of product polymers
from plant tissues persist, ongoing research efforts have provided steady progress in these
areas [11, 12].
5. Environmental Benefits of Green Plastics
Since 1993, the International Organization for Standardization (ISO) has been developing
LCA programs to analyze material and energy requirements of various products [13]. Life-
cycle Impact Assessment (LCIA), in turn, is the part of LCA in which the inventory of a
product's material flows is translated into environmental impacts and resource consumption
[14].
Although the LCIA of plastic products is still in the early stages of development, it will
soon be possible to compare the environmental impacts of various green plastics with one
another and with the conventional polyolefins that make up more than 90 percent of current
plastics production in a quantitative, reliable way.
Controversy does exist regarding the extent to which the production of bioplastics serves
the principles of environmental sustainability. Gerngross and Slater, for example, have argued
that the conversion of biomass to biopolymers is energy-intensive (involving fertilizer
production, farming, corn wet-milling, fermentation, and polymer purification, for example)
and results in significantly greater net CO2 emissions than the synthesis of petroleum-based
plastics [9]. While conceding that renewable energy sources (e.g., solar, wind, geothermal,
etc.) could be used to improve the environmental profiles of these processes, they argue that
greater environmental benefit could be obtained by using that energy to displace fossil fuels
in other applications. Gerngross and Slater also question the benefits of biodegradability, with
the rationale that non-degrading polymers produced from renewable resources could be used
to sequester atmospheric CO
2
.
The gaseous emission resulting from biodegradation of any biopolymers also represents
an important consideration, although few agree that the alternative, accumulation of plastic
debris, is desirable. In landfills, oxygen is limited, and organic matter is often degraded
anaerobically to yield a mixture of CO
2
and CH
4
rather than pure CO
2
.
Because CH
4
is a 23-
fold more potent greenhouse gas than CO
2
[15], anaerobic degradation of bioplastics is
potentially quite deleterious to the environment. In a composting environment, however, in
which regular mixing ensures aerobic degradation, only CO
2
is released, causing a process
supported by sustainable agriculture and composting to be effectively CO
2
-neutral [16]. In
addition, another promising disposal option for bioplastics may be waste incineration with
recovery of the energy generated, which then can be used to displace fossil fuel-derived
energy [17].
Gruber has noted that a life-cycle assessment performed by Gerngross considered only
the production of PHAs by microbial fermentation, and that other materials such as PLA hold
the promise of lower energy requirements for production. This perspective has, in fact, been
supported by other more recent life-cycle analyses [18]. A review of 20 such LCA studies of
biodegradable polymers indicates that starch, the major component of approximately 75
percent of green plastics production, offers important environmental benefits compared to
conventional polymers [19]. Concerning commercially produced biodegradable polymers, the