The bioremediation discipline is a multidisciplinary field led by microbiologists and supported by a range of other professionals. Working 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 successes are a result of volatilization, 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.
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 onsite 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:
- Subsurface ventilation,
- Pump-and-treat technology,
- In-situ chemical oxidation, and
- 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 treatments 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 (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:
- In-situ air stripping of dissolved VOCs,
- Volatilization of trapped- and adsorbed-phase contamination present below the water table and in the capillary fringe, and
- 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 volatilize 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 with ozone hydrogen peroxide replacement 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 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.
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.
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, often 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.
To review the technical literature on these technologies and techniques, search “bioremediation” and related key words in the I2M Web Portal (here).
Feel free to contact us to discuss your project needs or to arrange a speaking engagement by one of our Associates for a professional training session, a technical conference, society meeting, or for a graduation ceremony or other function where the knowledge and experience of our Associates may be of interest to your group.
The I2M Associate responsible for this discipline’s activities is:
Groundwater Systems Design, Analysis, and Monitoring: Michael D. Campbell, P.G., P.H.
Soil Vapor Extraction and Air Sparging Systems Design and Operation: Richard C. Bost, P.E., P.G.