Environmental Engineering Reference
In-Depth Information
Autotrophic microorganisms are those that obtain essential “biologically useful energy” (BUE)
from light (phototrophs) or inorganic chemical (chemotrophs) reactions. BUE is used by autotrophs
to synthesize the biomass from inorganic nutrients (e.g., CO 2 , N, P, K, and Si). The process can be
represented by the following equation (Gordon and Higgins 2007):
CO H UE NH
+
X
+
+
CHONcells
(
)
+
X
(
unbalanced
)
(14.5)
2
2
3
5 72
When X indicates oxygen, photosynthesis results in DO production. When X indicates sulfur, non-
oxygenic growth results in sulide production.
Heterotrophic bacterial growth uses organics for both BUE and cell synthesis. Since organics
are oxidized for BUE, there must also be a reduction in materials so that the oxidation and reduc-
tion are in equilibrium. These types of reactions in which electrons are exchanged are referred to as
oxidation-reduction reactions or redox reactions.
In redox reactions, one substance gives up electrons and another receives electrons. These two
materials are referred to as a redox pair, where the material that loses electrons is oxidized while
material that gains electrons (the TEA) is simultaneously reduced. Since oxygen has one of the
highest afinities for electrons, the term oxidized is used, but oxygen is not necessary for a redox
reaction to occur.
The oxidation-reduction potential (ORP) is a measure of the ability of a substrate to gain or
lose electrons, and is typically measured in millivolts (mV). Energy sources that are more highly
oxidized have greater ORP values, therefore high and positive values are associated with oxic con-
ditions and negative values are associated with anoxic conditions. The ORP is a commonly mea-
sured limnological parameter (where there are suficient TEAs), particularly where data are being
collected to support the computation of metal speciation. The reader is referred to Nordstrom and
Wilde (2005) for a discussion of measurement techniques.
TEAs must be available for heterotrophic organisms to utilize organic matter and facilitate
growth. The equation representing the process is (Gordon and Higgins 2007)
(14.6)
Organics TEA
+
CO EP (end products)
2
+
However, the value of the BUE obtained varies depending on the quality of the food (organics)
and the TEA used. Table 14.3 and Figure 14.4 show the common TEAs in an aquatic environment
and their end products and the BUE yields as reported by Bouwer (1992) and Fenchel et al. (1998),
as cited in Gordon and Higgins (2007).
TABLE 14.3
Terminal Electron Acceptors and Their End Products and BUE
Terminal
e Acceptor
Desirability
End Products
BUE Yield
Most favorable
O 2
H 2 O
29.9 kcal eq -1
Less favorable
NO 3
N 2
28.4
Undesirable
MnO 2
Mn 2+
23.3
Undesirable
FeO(OH)
Fe 2+
10.1
Undesirable
SO 4
H 2 S
5.9
Unfavorable
Organics
Reduced organics
Na
Least favorable
CO 2
CH 4
5.6
Source: Gordon, J.A. and Higgins, J.M., Energy Production and Reservoir Water Quality ,
American Society of Civil Engineers, Reston, VA, 2007.
 
Search WWH ::




Custom Search