Environmental Engineering Reference
In-Depth Information
Chapter 5
Transport with Decay and Degradation
5.1 Decay and Degradation
Organic matter and organic substances are subject of degradation . The degradation
processes are of biochemical nature as they are mediated by bacteria. The details of
these processes are usually quite complex. For their activity, the different bacteria
cultures depend strongly not only on the biogeochemical environment but also on
temperature and pressure conditions. One crucial condition, for example, is the
availability of oxygen. In an aerobic environment bacteria dominate that consume
oxygen aside from organic matter. These components are transformed into various
products which always include carbon dioxide. In an anaerobic environment, when
the available oxygen is consumed, other bacteria take over the role of major
contributors to organic matter degradation. Other electron acceptors become more
important, as manganese and iron in the solid phase, or nitrogen and sulphate in the
fluid phase.
As an example the decay of the herbicide Linuron in soil, taken fromWalker and
Brown ( 1983 ) is given in Fig. 5.1
The term decay generally is used for physical or chemical processes that cause a
loss of substance. The term is well known in connection with radioactive decay for
the transformation of radionuclides into daughter products. Uranium U 238 decays
with a half-life of 4.5
10 6 a, i.e. after that time half of the initial mass is still present
while the other half is transformed into Th 234 (if no other processes are involved).
As the daughter product Th 234 has a half-life of 2.1 days only, most of it decays into
Pa 234 , which is even more short-lived with a half-life of only 12 min. The next
daughter product U 234 has a long half-life of 2.5
10 5 a.
There is a basic mathematical formulation that is commonly used to describe
decay and degradation. More complex approaches will be presented in the follow-
ing Chaps. 7 and 9. One general approach recognizes losses q being proportional to
a power of the concentration c :
q ΒΌ l c n
(5.1)
Search WWH ::




Custom Search