Definition of Discipline Activities: Bioremediation
The bioremediation
discipline is a multidisciplinary field led by microbiologists
and supported by a range of other professionals. Woking closely with remediation
engineers, microbiologists have developed bioremediation to a point now demonstrated
to be effective in environmental cleanups throughout the country and overseas.
Many such projects have claimed successes, including bioremediation projects,
but many sucesses are a result of volatilation, not to bioremediation per
se.
The primary need in bioremediation projects is to address hydrogeology
issues, which will characterize the subsurface for use in designing effective
bioremediation programs. Laboratory chemical analyses
are used to characterize subsurface hydrochemical conditions, and a familiarization
with biological processes in hostile, subsurface environments is important
to resolve the numerous, complex issues involved in bioremediation.
Gasoline spills (including MTBE, TBA, etc.) are common and can cause water
contamination issues as hydrocarbons dissolve in ground water and travel offsite
in the aquifer. Hydrocarbons naturally degrade in the subsurface due to microbial-mediated
reactions. However, the reaction rates are slow because electron acceptors,
like oxygen, are quickly depleted in contaminated ground water and are slowly
recharged. Contaminated ground water has significant hydrocarbon concentrations
but depleted electron acceptors, whereas the overlying unsaturated zone contains
oxygen but lower hydrocarbon concentrations. Early response and intervention
is the key to minimizing extent and costs of remedial action for gasoline
and its components and is essential to protecting public health and the environment.
Source Control
When attempting to clean up such spills, timely and comprehensive source control
and associated hydrogeological investigations are needed once a release is
detected. This includes:
· Immediate control and cessation of the release,
· Repair or removal of the release source (tank, pipe, flange, pump, etc.),
· Removal / recovery of free product in both the saturated / unsaturated
zones, and
· Removal of residual free product from the subsurface soils.
Any remedial action initiated before the source is controlled is ineffective
and has the potential of expanding the scope of the remedial action as uncontrolled
sources continue to migrate in the subsurface. Physical action, like excavation,
is the usual approach to source control of small releases. Contaminated soils
removed by excavation can be treated by disposal (asphalt batching, daily
landfill cover), or physical (thermal desorption,) or biological (biopiles)
treatment. For larger releases, however, alternatives include free-phase LNAPL
recovery, barrier installation, and hydraulic control of the ground-water
plume. A variety of single and multi-phase extraction techniques moves both
liquid and gas phases to the surface for treatment. At the surface, dewatering
and subsequent recycling treats the higher concentrations of recovered material.
Direct on-site thermal catalytic processes destroy lower concentrations and
absorbents like Granular Activated Charcoal (GAC) polish the air or water
before discharge.
From Technology to Techniques
Two major objectives of site remediation include destruction of residual or
dissolved gasoline constituents or their removal from the impacted area. Destruction
can range from total mineralization/oxidation or reduction to inorganic components
or transformation to some unlisted form. Chemical reaction, biological means,
or thermal processes accomplish destruction. This is a brief description of
the myriad of innovative techniques developed to refine and enhance the implementation
of four basic technologies that have evolved for the active remediation of
gasoline released to the subsurface:
1. Subsurface ventilation,
2. Pump-and-treat technology,
3. In-situ chemical oxidation and
4. In situ bioremediation.
Specific techniques refining these technologies were developed to protect
human health and the environment, reduce risk, reduce treatment time, reduce
costs, and achieve lower decontamination objectives, among others. Table 1-1
lists the four major technologies and related range of associated techniques
that refine and enhance them. In many cases, the techniques expand the matrix
(i.e. soil, water, and air) capability of the technology. For example, under
subsurface ventilation, the combination of air sparging with soil-vapor extraction
expands the capability of SVE to remove gasoline vapors from both the saturated
and the vadose zones. In contrast to basic technology, specific techniques
exploit unique characteristics of the site as well as those of the contaminant
to expedite remediation.
We will limit this brief discussion of remediation to consideration
of residual and dissolved contaminants, knowing that source control has addressed
and removed the free-phase LNAPL. Several technologies, implemented in a sequence
based on field progress results, schedule, and cost, usually make up the typical
remediation project. The mix of unique surface and subsurface site conditions
and the physical and chemical characteristics of the contaminants will determine
selection of the appropriate remedial technologies and refinement techniques.
For example, the physical characteristics of MTBE are well suited to traditional,
physical remedial approaches that have proven to be effective with the other
components of gasoline.
A comparison of the physical characteristics of the aromatic and ether fractions
of gasoline is presented in Table 1-2. The Henry's law coefficient and water
solubility of MTBE make pump-and-treat technology particularly effective for
removing it from the saturated zone and its vapor-pressure characteristics
facilitate recovery from the vadose zone.
For in situ treatment, recent demonstrations have confirmed the effectiveness
of both chemical and biological oxidation processes for the destruction of
gasoline. Refinements in the formulation of chemical oxidants (e.g. stabilized
hydrogen peroxide, chelated catalysts) for in situ treatment provide
greater control, extended longevity, and more thorough treatment than earlier,
uncontrolled formulations. This together with uniform delivery of the oxidant
using an approach called Deep Remediation Injection Systems (DRIS) has avoided
many of the bounce-back problems observed with well-point injection programs.
For in situ bioremediation, providing higher concentrations of oxygen
than available from simple aeration, accelerates biological processes. Oxygen
concentrators, pure oxygen sources and oxygen release compounds now deliver
abundant electron acceptors to the biologically active zone.
The relationship between destruction, ventilation, and pumping technologies
for remediation of gasoline spills are illustrated in Figure 1-1. Chemically
or biologically mediated processes can destroy gasoline in the subsurface.
While the simplest approach is generally the most efficient and cost effective,
contaminant characteristics and site conditions of geology and hydrogeology
often dictate more complicated place-transfer or phase-transfer systems.
For ex situ destruction, ventilation blowers or pumping
systems transfer gasoline constituents directly from the subsurface to the
surface. Indirect, phase-transfer processes, may require several steps to
move the gasoline constituents to the surface where they can subsequently
be removed from the air or water matrix before discharge. Consider the differences
in the techniques discussed below with respect direct and indirect processes
of place-transfer and phase-transfer to affect ex situ destruction.
In another approach, called Subsurface Ventilation Evacuation or SVE (vacuum
or pressurization), gas exchange is increased in the subsurface and is an
effective method of moving vapor-phase gasoline hydrocarbons to the surface
for destruction. Another form of SVE is performed in situ, in the unsaturated
(or vadose) zone. This type of soil-remediation technology in which a vacuum
is applied to the soil pulls gas-phase volatiles and some semivolatile contaminants
from the soil through extraction wells to the surface. SVE is also known as
in situ soil venting, in situ volatilization, enhanced volatilization,
or soil vacuum extraction. The gas leaving the soil may be treated to recover
or destroy the contaminants, depending on local and state air discharge regulations.
Vertical extraction vents are typically used at depths of 1.5 meters (5 feet)
or greater and have been successfully applied as deep as 91 meters (300 feet).
Horizontal extraction vents (installed in trenches or horizontal borings)
can be used as warranted by contaminant-zone geometry, drill-rig access, or
other site-specific factors.
Bioventing Bioventing (BV) is a process of stimulating the natural, in
situ biodegradation of contaminants in soil by providing air or oxygen
to existing soil microorganisms. Bioventing uses low air-flow rates to provide
only enough oxygen to sustain microbial activity in the vadose zone. Oxygen
is most commonly supplied through direct air injection into residual contamination
in soil.
In addition to degradation of adsorbed fuel residuals, volatile compounds
move slowly through biologically active soil or shallow sediments where the
vapors are biodegraded. The primary mechanism of volatile removal by SVE is
mass transfer. However circulating air in the vadose zone stimulates microbial
activity to degrade volatile and some semi-volatile compounds directly in
the soil or sediment matrix. The major difference between SVE and BV is the
volume of air moved through the subsurface. Low-flow ventilation and gas exchange
favors biological activity while higher flows and extraction favors volatile
removal.
Bioventing can reduce vapor-treatment costs and can be also result in the
remediation of semi-volatiles that cannot be removed by direct volatilization
alone. Bioventing uses the same blowers as SVE systems to provide specific
distribution and flux of air through the contaminated vadose zone to stimulate
the indigenous microorganism to degrade hydrocarbons. SVE thermal enhancement
Direct heating of subsurface soils and sediments by radio frequency (RF) heating
or by resistance heating with electrode pairs or indirect heating through
the injection of steam or hot air through the injection wells enhances SVE
performance dramatically, if the system is designed and operated properly.
Heated air or steam helps to "loosen" some less volatile compounds from the
soil or sediments in a process similar to steam distillation. When sufficient
power is available, direct heating of soils with electrode pairs has been
very effective for the focused remediation of small areas at several sites.
Air Sparging (AS) Injection of compressed air at controlled pressures and
volumes into the water-saturated soils or sediments remediates the sediments
and ground water by three processes:
1. In situ air stripping of dissolved VOCs,
2. Volatilization of trapped- and adsorbed-phase contamination present below
the water table and in the capillary fringe, and
3. Aerobic biodegradation of both dissolved- and adsorbed-phase contaminants.
Stripping and volatilization are the dominant removal mechanisms, with
biodegradation becoming more significant over the long term. Air sparging
is a low-cost technique applicable to a wide range of contaminant concentrations
and is flexible enough to accommodate diverse geological and hydrogeological
limitations. For air sparging to be successful, the soil or sediment in the
saturated zone must have sufficient permeability to allow the injected air
to readily escape up into the unsaturated zone. Air sparging, therefore, will
work fastest at sites with coarse-grained soil or sediment, like sand and
gravel. The Henry's law constant for a given contaminant defines, in part,
the effectiveness of air sparging. Empirical approaches are typically used
to evaluate and optimize the effectiveness of air sparging in the field because
air channeling and residence time, water mixing, and equilibrium considerations
complicate interpretation of the inter-phase mass-transfer efficiency.
Variations in the sparging cycle from continuous sparging to pulse sparging,
to avoid channeling and permeability changes, have met with various degrees
of success depending largely on the geology and hydrogeology. SVE & air sparging
combining SVE with AS expedites removal of dissolved VOCs from the subsurface.
SVE sweeps from the vadose zone the organic compounds that are stripped from
the aqueous phase and volatilized from the saturated zone. Like AS alone,
air stripping and volatilization are the dominant removal factors, however,
overall ventilation of the subsurface stimulates the indigenous microflora
to degrade residual and semi-volatile compounds on the soils.
In-Situ Air Stripping (ISAS)
In-situ air stripping (ISAS) combines three technologies, air sparging,
horizontal wells, and soil-vapor extraction. ISAS uses horizontal wells to
inject (sparge) air into the ground water. The horizontal wells provide more
effective access to a horizontal ground-water plume. As the air comes into
contact with contaminants, they volatize and rise through the soil sediment.
The volatile organic compounds (VOCs) are then extracted from overlying soils
by standard soil-vapor extraction. The air sparging process eliminates the
need for surface ground-water treatment systems such as air strippers.
SVE & air sparging w/ ozone hydrogen peroxide peplacement of air with the
powerful oxidant, ozone, in the SVE/AS combination leads to the in situ
destruction of many gasoline constituents. Ozone, the third strongest direct
oxidant after fluorine and hydroxyl radical, can oxidize organic contaminants
by direct oxidation in addition to the oxidation by free hydroxyl radical.
In situ destruction of VOCs reduces surface treatment costs and often
pre-treats adsorbed semi-volatile compounds to improve their susceptibility
to biodegradation. Subsequent oxygen release also stimulates in situ
biodegradation.
Biosparging like its sister technique Bioventing, uses lower airflow (0.5-3
cfm/injection point) than needed for air stripping and volatilization. The
objective is to provide enough oxygen and gas exchange to drive in situ
aerobic biodegradation in the saturated zone without stripping the hydrocarbons.
This approach is particularly effective and appropriate for biodegradable,
but poorly stripped compounds, i.e. those with a low Henry's Law constant,
like acetone and MTBE. Microflora in the vadose zone generally treat VOCs
stripped during biosparging, so capturing and treating them at the surface
is usually not an issue.
Pump-and-Treat Systems
Pump-and-treat systems operate by pumping ground water containing dissolved
or non-aqueous phase liquid hydrocarbon to the surface. Free-phase hydrocarbons
are separated out, dissolved constituents are removed, and the water is either
reinjected or discharged to a surface water body or municipal sewage plant.
Contaminant recovery is limited by the behavior of the contaminant in the
subsurface (primarily solubility and the adsorption /partition coefficients),
site geology and hydrogeology and the extraction system design. It is further
complicated by residual contamination in the saturated zone, adsorbed contamination
in the capillary fringe as well as insoluble non aqueous phase liquids (NAPLS)
floating on the water table.
Pump-and-treat systems are
also used to contain contaminated ground water, provide hydraulic control
and recover contaminant mass in either gas or liquid phase by creating a capture
zone around the pumping well. The natural hydrogeological property of the
site and the rate at which ground water is extracted limit the capture zone
of the recovery well. As refinements to the pump-and-treat system, Pulse Pumping/
Reverse Flow Pumping address low-permeability formations, channeling, capillary
fringe, cost, and the delivery of nutrients to stimulate in situ bioremediation.
Vacuum Enhanced Recovery (VER)
The application of a vacuum to the extraction point provides a method to further
enhance the capture zone. A high-vacuum or negative pressure applied to a
recovery well and to the formation enhances liquid recovery of the well by
increasing the net effective drawdown. VER increases the mass removal of the
volatile and semivolatile contaminants by maximizing dewatering and facilitating
volatilization from previously saturated sediments via increased air movement.
Physical removal of significant hydrocarbon mass increases subsurface oxygen
levels for aerobic biodegradation of residual contaminant. VER is cost effective
to enhance the overall recovery of contaminants, especially under low-permeability
conditions.
Single-Phase Vacuum Extraction (SPVE)
A single pump removes fluid, via a drop tube, and applies a vacuum to the
well and formation. The well produces both liquid- and vapor-phase material.
Compared to pumping alone, single-phase vacuum extraction increases the capture
zone and therefore, reduces the number of recovery wells needed, and accelerates
the recovery of both liquid and residual contaminants. Although limited to
depths of less than 25 ft, this technique is one of few enhancements for mass
removal from low permeability sites.
Dual (multi-)-Phase Vacuum Extraction (DPVE)
Similar in its advantages to the single-phase system, the multiphase extraction
system uses a submersible pump for liquid recovery at greater depths (>25
ft) and a diaphragm or liquid-ring pump for evacuation of the formation. A
single-vacuum pump can be used for multiple wells under a variety of design
strategies. When coupled with surfactants, a pump-and-treat system can be
used as an in situ soil washing system.
Surfactant Enhanced Aquifer Redemption (SEAR)
SEAR techniques have been particularly effective at removing NAPLs and NAPL
residual from highly permeable subsurface systems. Surfactant selection must
be paired with the target constituents and must exhibit some short-term resistance
to biodegradation by the indigenous microflora.
In Situ Chemical Oxidation
Chemical oxidants for the oxidation of organic compounds in the subsurface
are selected in part, based on their oxidizing power as summarized in Table
1-3.
Hydrogen Peroxide/Fenton's Reagent
Hydrogen peroxide reacts catalytically with naturally occurring or injected
ferrous iron to form the highly reactive hydroxyl radical. Fenton's reagent
is the iron-catalyzed hydrogen peroxide. This strong oxidizer, exceeded only
by fluorine in oxidizing power, oxidizes organic constituents in gasoline
to carbon dioxide and water. Recent advances in chemical stabilization of
the peroxide and controlled release of the iron by chelation has extended
the reaction time and provided more control of this chemistry.
Permanganates
Potassium or sodium permanganate is an important oxidizer for odor control
in sewage treatment plants. The oxidation potential of permanganate increases
with decreasing pH. The temperature sensitive solubility of the dark-purple
crystals of potassium permanganate presents handling challenges circumvented
by the soluble sodium permanganate. Like Fenton's reagent, permanganates are
capable of oxidizing gasoline constituents ranging from alkenes to PAHs, but
are less sensitive to pH than Fenton's reagents, exhibit slower reaction kinetics
and produce manganese oxide as a product.
In Situ-Enhanced Bioremediation
Bioremediation uses indigenous or introduced (augmented) microorganisms (primarily
bacteria) to degrade organic contaminants into harmless substances, biomass
or carbon dioxide and water. The kinetics of biodegradation are limited by
substrate availability, electron donors, nutrients, pH, moisture content,
other carbon and energy sources and other factors beyond the scope of this
summary of techniques employed in remediation, oftem involving some form of
bioremediation.
Techniques to stimulate in situ bioremediation have focused on providing
the factors limiting the biological activity of the system. The oxygen demand
for aerobic biodegradation of gasoline constituents has been satisfied by
a technique called Direct Oxygen Injection, Oxygen Diffusion using Oxygen
Release Compounds (ORC).
Most gasoline hydrocarbons also biodegrade under denitrifying conditions.
A recent report from France indicate that MTBE and ETBE biodegrade under denitrifying
conditions in the laboratory (personal communication with Andre Pauss, 1999).
Depending on the aquifer, supplemental nitrate may be required to achieve
this. After the demand for electron acceptors is satisfied, essential nutrients
like phosphorus and nitrogen become growth and activity limiting. Both nutrients
can be delivered as a liquid or gas for stimulating biological activity in
the saturated or vadose zone respectively.
Monitored Natural Attenuation
Natural attenuation, also known as passive bioremediation, intrinsic bioremediation,
or intrinsic remediation, is a passive remedial approach that depends upon
natural processes to degrade and dissipate petroleum constituents in soil
and ground water. Some of the processes involved in natural attenuation of
petroleum products include aerobic and anaerobic biodegradation, dispersion,
volatilization, and adsorption. In general, for petroleum hydrocarbons, biodegradation
is the most important natural attenuation mechanism; it is the only natural
process that results in an actual reduction of petroleum.
Conclusions and Recommendations
The techniques that refine or enhance the basic technologies briefly discussed
here, reflect an adaptation to unique hydrogeological site conditions, specific
clean-up objectives, physical characteristics of the contaminant and specific
risk concerns. Before any technology or refinement thereof is selected, specific
information must be obtained on site-specific hydrogeologic
conditions, engineering design, data needs, performance data, pilot testing,
duration of operation, sequencing with other technologies, case studies and
anticipated cost. While this is beyond the scope of this presentation, many
of these data are readily available on the Internet at state and federal web
sites and from readily available, published sources, with the exception of
site-specific hydrogeologic conditions, which must be characterized at the
outset of the project.
Refinements and enhancement of remediation technologies continue to produce
a variety of exciting techniques for optimizing in situ destruction
or physical extraction for ex situ destruction. Development and implementation
of these techniques requires flexible designs and flexible, responsive management
styles that can adapt to new information to expedite the remediation process.
For further information on the discipline, the Institute
of Environmental Technology sponsors an Internet Resources Portal, click (here).
The ELA Principal responsible for this discipline's activities is:
Richard E. Woodward, M.A.
Note 1: The environmental field is multi-disciplinary by nature
and, for maximum effectiveness, ELA incorporates input from complimentary
disciplines when appropriate in most projects undertaken.
Note 2: The above presentation is based in part on a recent article in Soil
Sediment & Ground Water (formerly Soil & Ground Water
Cleanup), March, 2000 by R.E. Woodward.
Last Update: May 21, 2000
