Environmental Engineering Reference
In-Depth Information
Figure 6.10
A schematic diagram of biodegradation of a solid polymer
showing the two main stages of primary abiotic degradation to
embrittlement followed by biodegradation of fragmented residue.
Source: Modified and reproduced with permission from Krzan et al. (2006).
6.4.1 Terminology and Definitions
The terminology relating to biodegradation used in technical and trade
literature can be inconsistent and confusing. The consensus definitions of
the terms by testing organizations such as ASTM, DIN, or ISO only partially
address the inherent difficulties in developing nomenclature on
biodegradable products. Specifying a plastic or other material as being
“enhanced biodegradable” or “biodegradable” suggests, that the particular
plasticcanbedemonstratedtoenvironmentallybiodegradeatasignificantly
faster rate compared to conventional plastics. For brevity and convenience,
the term “biodegradable plastics” will be used here to mean “enhanced
biodegradable plastics.” In specifying a plastic material as being
biodegradable, several key issues need to be addressed:
1. As biodegradation of a substrate depends on it, the relevant
environment should also be specified when claiming a polymer as being
biodegradable (e.g., “chitosan is 90% biodegradable in coastal marine
sediment in 60 days” or “polycaprolactone is 100% biodegradable in
rhodochrous
and
C
. lab culture in 30 days” or “poly(lactic acid) is 90%
biodegradable under composting conditions in 1.5 months”).
Percentages here refer to the percent mineralization or carbon
conversion of the polymer. Generically, describing a plastic as being
“biodegradable” has little meaning.
2. Exposure environments are complex, being defined by the availability of
oxygen, temperature, availability of humidity, consortia of biomass,
pre-adaptation of the microbes, as well as other factors. A “garden soil”
environment or a sewage sludge inoculum used in tests, for instance, is
highly variable depending on the source and can yield variable test
results (Mezzanotte et al., 2005). Healthy garden soil will have diverse
consortia of bacteria and fungi ranging in 10
6
-10
8
cells/g (Martin and
Focht, 1977). Such a soil will also have active free enzymes liberated by
lysis of living cells that may contribute to biodegradation.
Table 6.3
summarizes the common environments where plastics wastes are likely
to end up in, and the mechanisms of degradation available in each case.
