Environmental Engineering Reference
In-Depth Information
21.2.1.2 Limiting Environmental Factors
Except for insight into the distribution of contaminants over the four phases in a soil
system, the kinetic parameters play the most important role in selection of a proper
remediation technology. The main factors that affect the rate of the soil remedia-
tion processes are temperature, moisture, permeability of the soil, presence of an
electron acceptor like oxygen, nutrients, chemical structure of the contaminant, and
availability.
Temperature
A temperature increase will speed up physical/chemical processes in which soil
vapour pressure and solubility of contaminants play an important role. However,
in North Western Europe the average soil temperature is about 10-15
◦
C. As tem-
perature increase is mainly dependent on the high heat capacity of water, an
antropogenically induced temperature increase of the soil is often too expensive.
Due to adaptation of micro-organisms which are naturally present in soil, relatively
high conversion rates for organic contaminants can be observed at temperatures
from 10 to 15
◦
C. When the temperature of groundwater is increased by more than
10
◦
C, a shift in the microbial population will occur. After adaptation, the maxi-
mum biodegradation rate can be increased by a factor of two for each ten degrees of
temperature increase, according to the Arrhenius equation. However, many microor-
ganisms in soil and wastewater treatments systems have an optimal temperature
range from the psychrophilic to the mesophilic range of about 30-35
◦
C (Schlegel
and Schmidt
1985
).
Wa t er
Biological degradation occurs only via the water phase. This means that sufficient
water needs to be present. However, at a high moisture content, insufficient oxy-
gen transport to the micro-organisms may limit the aerobic biological degradation.
Suspended systems, such as bioreactors, have a very high water content, but by
active aeration optimum degradation can still be ensured. In landfarming systems
in sandy soil, moisture content values of 10-20% are maintained. For anaerobic
biological conversions the high water content has no effect.
Permeability of Soil Water
The permeability of soil for the water (
K
l
) is especially of importance for the biolog-
ical treatment of the saturated zone. The permeability can be calculated with Darcy's
law (Koorevaar et al.
1983
):
Q
l
A
=
K
l
∗
δ
p
(21.13)
δ
s
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