Environmental Engineering Reference
In-Depth Information
sources, an electron acceptor, and nutrients are required for a successful process. Naturally
occurring microorganisms are often used as these are adaptable to changing conditions.
Biostimulation is the seeding of known organisms if the site is deicient in required organ-
isms. Nutrients can be pumped in or percolated.
For aerobic processes, dissolved oxygen is added either by aerating water used for
saturating and adding hydrogen peroxide. For shallow soils, nutrients and oxygenated
water can be added via iniltration galleries or spray irrigation. For deeper soils, injec-
tion wells are required. High temperatures enhance bioremediation rates. However, at
lower temperatures bioremediation still occurs but more slowly. Heat blankets to cover the
soil surface can increase the soil temperature and subsequently the biodegradation rate.
Bioremediation can take several years to clean the site.
Under anaerobic conditions, methane, carbon dioxide, and trace amounts of hydrogen
gas will be produced. Various electron acceptors are necessary in place of oxygen. These
include sulfate, Fe(III), or nitrate. Under sulfate-reduction conditions, sulide or elemen-
tal sulfur is produced, and under nitrate-reduction conditions, nitrogen gas is the inal
product. A disadvantage of anaerobic procedures is that contaminants can be degraded to
products that are as or more hazardous than the original contaminant. A good example of
this is the biodegradation of TCE to the more toxic vinyl chloride. To avoid this problem,
aerobic conditions can be created to biodegrade the vinyl chloride.
Bioremediation techniques have been successfully used to remediate soils, sludges, and
groundwater contaminated with petroleum hydrocarbons, solvents, pesticides, wood pre-
servatives, and other organic chemicals. Bench- and pilot-scale studies have demonstrated
the effectiveness of anaerobic microbial degradation of nitrotoluene in soils contaminated
with munitions wastes. Bioremediation is especially effective for remediating low-level
residual contamination in conjunction with source removal.
The most common contaminants treated by bioremediation include PAHs, nonhaloge-
nated SVOCs, and BTEX. Superfund sites are commonly bioremediated if they contain
wastes associated with wood preserving (creosote), and petroleum reining and reuse
(BTEX). Excavation of contaminated soil is not required. Bioremediation is often less costly
compared with other technologies including thermally enhanced recovery with heating,
chemical treatment with expensive chemical reagents, and in situ lushing (which may
require further treatment of the lushing water) and thermal desorption and incineration,
which require excavation and heating. Typical costs for enhanced bioremediation range
from $30-100/m 3 .
Although bioremediation cannot degrade inorganic contaminants, bioremediation can
be used to change the valence state of inorganics and cause adsorption, immobilization
onto soil particulates, precipitation, uptake, accumulation, and concentration of inorgan-
ics. These techniques are promising for immobilizing or removing inorganics from soil
and other wastes. Bacteria (Bader et al., 1996) and biosurfactants (Massara et al., 2007) can
reduce Cr(VI) to Cr(III), which is less toxic and mobile. Sulfate-reducing bacteria can form
insoluble metal sulides. Heap leaching and in situ leaching have been used by the metal
industry for copper recovery (Rawlings, 1997). Thiobacillus bacteria produces sulfuric acid
that can be used to solubilize metal sulides to metal sulfates. Fungus such as Aspergillus
niger can produce citric and gluconic acids (Mulligan et al., 1999).
11.5.8 Bioventing
Bioventing (Figure 11.13) is an in situ process that involves forced aeration in the vadose
zone to enhance the biological degradation of SVOCs and nonvolatile contaminants with
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