Environmental Engineering Reference
In-Depth Information
done due to the increase of organic material in the soil, which increases the
ability of contaminants such as PAHs to adsorb to the soil particles (Tugun
et al., 2003). Furthermore, this organic material contains many molecules
that stimulate microbial growth, and those microbes can consume the immo-
bilized contaminant (Kamath et al., 2004). The plants improve microbial
productivity by aerating the soil subsurface with their roots and by removing
much of the moisture in the soil through evapotranspiration. Both of these
actions create an aerobic environment in which the microbes can be very
effective (Tugun et al., 2003). The main disadvantage is that this form of
remediation can only be done close to the surface and does not affect any
contamination deeper in the soil.
3.4.8 Dechlorination
Aerobic bacteria can break down only chlorinated substances with fewer
than five chlorine substituents. In dechlorination, anaerobic bacteria remove
one or more of the chlorine atoms and replace it with a hydrogen ion,
resulting in a compound that is less toxic, less carcinogenic, and easier to
break down. The effectiveness of this process increases with the increase in
contaminant concentration (Magar, 2003). Dechlorination can also be done
through chemical and mechanical means. When chlorinate substances are
mixed with metallic calcium in a ethanol solvent and mixed for 24 hours
under atmospheric pressure and at room temperature, the polychlorinated
compounds lose chlorine ions and become more easily degradable. The
advantage of this process is that it does not require high temperatures and
pressures, which could potentially make it cost effective. This method is still
in the experimental stages and not ready for use on a large scale (Mitoma
et al., 2004). The major drawbrack to this method is that dechlorination used
by itself often does not meet regulatory standards for contaminant removal.
These standards are often based on mass removal instead of toxicity reduc-
tion, even though toxicity reduction makes contaminated soils as safe as
other methods do (Magar, 2003). Dechlorination, therefore, must be done in
conjunction with another remediation technique.
3.4.9 Compost
Compost has long been used to degrade sewage, food wastes, and yard
wastes, but it is now also being used to degrade certain contaminants, such
as explosives and PAHs. Contaminated soil is removed from the ground and
mixed with natural materials such as cow manure, activated sludge, and
corn cobs. These additives provide the biomass necessary for microbial
growth and worm growth. The worms aerate the soil, and the microbes
destroy the contaminant. The compost naturally goes through stages of
microbial growth and decline, which self-regulates the system. Once the
compost is mixed with the contaminated soil, the only human action required
is an occasional turning of the pile. The disadvantage is that this method
