Environmental Engineering Reference
In-Depth Information
is the most rapid process to occur, whereas for the biodegradation of TPH, PAH and
the aromatics benzene, toluene, ethylbenzene and xylene maximal biodegradation
rates occur under aerobic conditions. For biodegradation under aerobic conditions,
the supply of sufficient oxygen is important. For In Situ bioremediation in the water
saturated zone, the maximal oxygen concentration is approximately 8 mg O
2
/L
water
(20
◦
C) (
0.25
∗
10
−
3
(mol/L), whereas the vadose zone contains a maximum of
300 mg O
2
/L
air
(9.375
∗
10
−
3
(mol/L). Therefore one liter pore volume in the unsat-
urated zone may contain 37.5 times more oxygen than the saturated zone. In case
of low permeable soils it is reported that the use of air as a carrier for oxygen is a
thousand times more effective than the use of water (Wilson and Ward
1986
).
=
Bioaugmentation
In general, biomass will be formed during the biological soil remediation process. In
principle all types of micro-organisms are everywhere, as was stated by early micro-
biologists (Beijerinck et al.
1940
). However, for relatively new and hard to degrade
xenobiotics like MTBE, the inoculation with enriched microbial populations may
speed up the time needed for the total bioremediation of a specific site (Salanitro
et al.
2000
). Many studies on bioaugmentation with specific micro-organisms have
been published (Hinchee et al.
1995
), but for regular observed contaminants like
TPH, PAH and chlorinated solvents as such bioaugmentation is not essential.
Nutrients
To stimulate the growth of biomass, sufficient nutrients (such as nitrogen) should
be supplied. Nitrogen may be supplied to the system as NH
3
. Other elements which
are needed for the growth of microorganisms are sulphur (S) and phosphorus (P). In
the past, the growth of biomass in chemostats was studied based on mass balancing
principles from chemical engineering. At that time, general element formulas were
developed for biomass which include all relevant elements essential for growth of
micro-organisms. A common formula for biomass is (Roels and Kossen
1978
):
CH
1.8
O
0.5
N
0.2
S
0.0046
P
0.0054.
In the balance for the biodegradation of organic contaminants, S is often included
as sulphuric acid (H
2
SO
4
), and P as phosphoric acid (H
3
PO
4
). In practice, N, S
and P are added in order to create an optimum environment for biodegradation.
In laboratory experiments in which the degradation of, for example, TPH is investi-
gated in the absence of soil in order to determine the maximum conversion rate, also
trace elements are added. In practice, such elements are usually present in sufficient
amounts in the soil, therefore not leading to limiting conditions for biodegradation.
Nutrients seem to be present in soil in sufficient quantities, as several biodegradation
studies showed no improvement of biodegradation rates after addition of nutrients
(Miller et al.
1991
; Norris et al.
1994
).
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