Environmental Engineering Reference
In-Depth Information
degradable organic matter (bulking) and fertilizer. Bulking refers to the
addition of inexpensive, readily available materials (straw, manure, sewage
sludge, wood chips, rice hulls, etc.) that enhance aeration of the soil and
improve soil texture. The bulking agents also dilute the soil contaminant,
reducing the concentration of toxic chemicals. The resulting mineralization
of the added organic matter also aids in the degradation of PAHs. Some
control of aeration and temperature is usually required (Potter et al., 1997).
Kastner et al. (1998) found that the addition of compost enhanced the deg-
radation of PAHs in soil due to the presence of the organic solids in the
compost. Composting was not selected as our treatment option because the
loss of PAHs was due to soil binding, which did not reduce the toxicity of
the soil (Johnson, 1998).
Land-farming remediation of PAH-contaminated soils, based on the deg-
radative activities of natural microbial communities, is a well-used and
generally reliable technology. It has been applied to a variety of soils and
contaminant types (i.e., creosote, coal tar, and petroleum) and is generally
preferred over nonbiological approaches such as stabilization, chemical oxi-
dation, and incineration (Mueller et al., 1995). The focus of land farming is
to stimulate the degradation capabilities of natural microbial communities
by providing oxygen and nitrogen and to use physical mixing of the soil to
distribute the contaminants over a greater surface area and bring them into
contact with the microorganisms. Traditional agricultural procedures are
used to provide mixing (tilling, bulking), moisture (irrigation), and nitrogen
(fertilizer). Fertilizer can be applied as commercial formulations or as organic
waste material (commonly manure). Land farming is often, but not neces-
sarily, limited to the treatment of 6 to 12 in. of soil at a time (normal depth
of tilling), but as one layer (lift) is successfully treated, successive lifts can
be applied on top. The disadvantages of traditional land farming are the
time required, the operation and maintenance costs, and the space required
for the lifts.
The degradation that results from solid phase remediation often follows
a two-stage process involving an initial rapid phase with extensive PAH
degradation (a few months) and a second slower phase of degradation
(months to years) with relatively little further change in PAH concentration
(Brown et al., 1995; Cornelissen et al., 1998; Mueller et al., 1998; Pollard et
al., 1994). Differences in desorption rates from soil particles into the intersti-
tial water are frequently cited as the cause of the two-stage process. Initially,
rapid desorption occurs, and degradation is limited by microbial factors. At
some point, desorption slows and degradation rates decrease accordingly.
The transition from the fast to the slow phase is often critical. If it occurs
before cleanup criteria are met, treatment times are greatly extended and
costs increase substantially. Cleanup criteria are often based on the concen-
tration of benzo(a)pyrene equivalents. If concentrations cannot be reduced
below a previously established cleanup value, then the soil is not considered
clean and must be either disposed of as a hazardous waste or further “engi-
neered” with another treatment technique, such as chemical oxidation, the
