Environmental Engineering Reference
In-Depth Information
and is challenging our concepts
about cleanup standards and risks (Alexander, 1995). This is particularly the
case for biological treatment of recalcitrant compounds, in which one of the
most important site-specific factors is the availability of the compounds held
within solids and how this affects treatment rates and acceptable toxicolog-
ical endpoints.
Biostabilization is a newly developed concept that could significantly
benefit the remediation process for soils and sediments contaminated with
recalcitrant compounds. Biostabilization is the biodegradation of accessible
pollutant fractions in a soil or sediment matrix, leaving a bound residue that
is much more biologically unavailable and immobile (Luthy et al., 1997). The
concept, however, is still in a developmental stage, and endpoints and appro-
priate measures of endpoints have not been defined. Very little research has
been conducted on the applicability of biostabilization principles. It has been
hypothesized that residual hydrocarbons remaining after biotreatment may
represent an acceptable treatment endpoint. This is an important concept,
but for wide acceptance, better understanding of the polycyclic aromatic
hydrocarbon (PAH) release rates and mechanisms that bind or sequester
recalcitrant compounds within contaminated material is required.
remediation (NCR, 1994; Moore et al., 1989)
1.2
Relevance
Recalcitrant compounds present one of the most pressing problems for
biotreatment of contaminated soils or sediments. These compounds consti-
tute a broad class of chemicals that appear as persistent contaminants in
soils and sediments (NRC, 1997, 1994), including petroleum and fuel resi-
dues, tars, and creosotes. Such compounds have low solubility, low volatility,
low intrinsic reactivity, and they typically exhibit very slow release rates
from soil or sediment (Steinberg et al., 1987; Pignatello and Xing, 1996). These
characteristics make biotreatment difficult in any geologic setting, necessi-
tating extensive, site-specific testing. These characteristics also confound risk
assessment, wherein the availability of recalcitrant compounds to organisms
or the environment is unknown.
In theory, soils and sediments contaminated with recalcitrant com-
pounds may be treated utilizing various cleanup strategies. However, many
proposed strategies have significant economic and feasibility problems.
What is needed is an effective technology that supports the economics of
disposal, eliminates adverse contaminant impacts, and supports the reuse
of treated contaminated soils and sediments. In-place (
) biotreatment
combined with biostabilization offers a strong possibility to achieve these
goals. Regardless of whether the biotreatment system is passive (natural
attenuation) or engineered (in-place treatment), a pragmatic solution is to
focus active biotreatment on the available contaminant fraction and confirm
that if residuals are released, the rate is sufficiently slow to allow consump-
tion by the microbial community, i.e., the contaminant is biostabilized. Thus,
an alternative environmental endpoint (cleanup level) may be appropriate,
in situ
