Environmental Engineering Reference
In-Depth Information
case. As two different bacterial strains are involved, the reaction proceeds in two
steps. The intermediate product nitrite (NO
2
−
) is unstable, so that nitrification is
commonly combined to the overall reaction (
19.23
):
NH
4
NO
2
+H
2
O+2H
+
+
1.5O
2
=
(19.21)
NO
2
NO
3
+
0.5O
2
=
(19.22)
NH
4
NO
3
H
2
O+2H
+
+
2O
2
=
+
(19.23)
The kinetics of biodegradation (i.e. biochemical reaction rates of two compo-
nents A and B) are commonly expressed according to a multiplicative-Monod
formulation. Additionally, threshold terms preventing the reaction at very low
concentrations, were included in the model formulation:
C
B
C
B
+
2
C
A
C
A
+
2
C
A
C
A
+
C
B
C
B
+
R
=−
k
max
·
K
1
/
2
·
K
1
/
2
·
·
(19.24)
K
thr
K
thr
where
A
represents ammonium and
B
oxygen.
R
[M L
−
3
T
−
1
] is the reaction rate
at a certain location,
k
max
[M L
−
3
T
−
1
] is the Monod maximum utilization rate
of the reaction,
K
1/2
[M L
−
3
] is the Monod half saturation constant of the reac-
tion for each contaminant. In the given case additional terms are introduced that
specify required minimum concentrations of substrate and electron acceptor using
K
thr
[M L
−
3
] as the threshold concentration of the specific contaminant that is just
sufficient to maintain the reaction.
A frequent prerequisite for acceptance of Natural Attenuation at a site is the
proof that the contaminant plume is not advancing anymore. Therefore, evidence
for steady state conditions is needed, which is typically given by the observation
that the plume is significantly shorter than the travel distance of the groundwater
taking into account the age of the source, flow velocity and possible retardation. In
order to assess if an observed ammonium plume can be assumed to be at steady
state, an estimation of retardation factors may thus be very important. It should be
noted, however, that retardation does not affect the length of a steady state plume,
but only defines how much time is needed until steady state conditions are achieved.
The major process of retardation of ammonium as an inorganic cation is generally
assumed to be ion exchange at negatively charged external and internal surfaces of
clay minerals as well as organic matter in the soil and aquifer material. Ion exchange
is competitive because exchange sites are limited. Ionic species of highest con-
centration and activity within the aqueous solution will predominate the exchanger
composition, which has to be taken into account when estimating retardation factors
of specific species. Potassium (K
+
) may easily replace NH
4
+
due to the similar ion
radius, however, in most environments it is only present in small concentrations. In
the presence of limestone rocks calcium (Ca
2+
) is the major cation in groundwater
aquifers and will therefore dominate in the competition for cation exchange sites.
The reaction equation can be formulated as:
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