Environmental Engineering Reference
In-Depth Information
addition of surfactants, or physical restructuring, in an attempt to encourage
additional degradation. Treatment times are greatly extended and costs
increase substantially. The treatment options then become: (1) disposal, (2)
short-term treatment, or (3) impoundment. Impounding the treated soils and
allowing natural degradation processes to eventually degrade the high-molec-
ular-weight (HMW) PAHs can be considered. With the impoundment option,
leachable hydrocarbons, mainly the low-molecular-weight PAHs, are not
likely to cause environmental problems because they have been largely
removed during the initial degradation phase. In essence, these soils can be
considered biostabilized (Luthy et al., 1997; Talley et al., 2000); that is, the very
slow leaching of the residual HMW PAHs is counterbalanced by the degra-
dation capabilities of the indigenous microbial communities. However, this
requires that the impounded soil be carefully managed and monitored over
periods of years, a task that can add considerable long-term cost.
Therefore, the problem became one of identifying the causes of the
rapid-to-slow transition of degradation and learning what could be done to
control it.
7.1.3.2 Slurry phase treatment
Slurry phase treatment involves the processing of contaminated soil using
a contained system, or bioreactor, in which the contaminated soil receives
the maximum amount of mixing and aeration. A variety of reactors can be
used: fixed film, plug flow, and slurry reactors. Degradation rates in a biore-
actor are usually considerably faster than solid phase systems (Pinelli et al.,
1997). This is because intimate contact between the PAHs and the micro-
organisms is provided and optimal conditions for microbial growth and
degradation are maintained with considerable uniformity. Because of the
contained nature of the system, inoculation with selected organisms (bio-
augmentation) or the addition of surfactants is reasonable. However, slurry
phase treatment is expensive to set up, operate, and maintain, especially as
only relatively small quantities of soil can be treated at a time. As the size
of the reactor increases, optimal conditions will be compromised due to the
physical nature of the systems. Costs can often can be reduced by using
existing facilities, such as lined lagoons and basins, for the treatment.
7.1.3.3 Performance comparison
From a comparison of the performance of slurry phase and solid phase
treatments, it can be seen that solid phase treatment is slower than slurry
phase treatment (Figure 7.2). Mueller et al. (1991a, 1991b) divided PAHs into
three groups based on the number of rings and added the mixtures to soil
in both slurry and solid treatments. Group 1 consisted of two-ring PAHs
(naphthylene, methyl and dimethyl naphthalenes, and biphenyl). Group 2
consisted of three-ring PAHs (acenaphthylene, acenaphthene, phenanthrene,
anthracene, and methyl-anthracene). Group 3 consisted of various four-,
five-, and six-ring PAHs (fluoranthene, pyrene, benzo(b)fluorene, chrysene,
