United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/600/R-92/173
September 1992
&EPA
A Technology
Assessment of Soil Vapor
Extraction and Air
Sparging
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EPA/600/R-92/173
September 1992
A TECHNOLOGY ASSESSMENT OF
SOIL VAPOR EXTRACTION AND AIR SPARGING
by
Mary E. Loden, P.E.
Camp Dresser & McKee Inc.
Cambridge, MA 02142
Contract No. 68-03-3409
Project Officer
Chi-Yuan Fan
Superfund Technology Demonstration Division
Risk Reduction Engineering Laboratory
Edison, NJ 08837
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
Printed on Recycled Paper
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NOTICE
This report has been reviewed in accordance with the U.S. Environmental Protection Agency's peer and
administrative review policies and approved for publication. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use.
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FOREWORD
Today's rapidly developing and changing technologies and industrial products and practices frequently
carry with them the increased generation of materials, that, if improperly dealt with, may threaten both
human health and the environment: The U.S. Environmental Protection Agency (EPA) is charged by
Congress with protecting the nation's land, air, and water resources. Under mandate of national
environmental laws, the agency strives to formulate and implement actions leading to a compatible balance
between human activities and the ability of natural resources to support and nurture life. These laws direct
the EPA to perform research to define our environmental problems, measure the impacts and search for
solutions.
The Risk Reduction Engineering Laboratory is responsible for planning, implementing and managing
research, development, and demonstration programs to provide an authoritative, defensible engineering
basis in support of the policies, program and regulations of the EPA with respect to drinking water,
wastewater, pesticides, toxic substances, solid and hazardous wastes, and Superfund-related activities. This
publication presents information on current research efforts and provides a vital communication link between
the researcher and the user community.
The impacts associated with uncontrolled releases of petroleum hydrocarbons from underground
storage tank systems present a major concern to the Risk Reduction Engineering Laboratory. Air sparging,
an innovative technology, is being used at increasing numbers of sites to remediate impacted groundwater
and soil in the saturated zone. This document provides general information on air sparging technology for
remediating soils and groundwater contaminated with petroleum products. It also identifies the research
needed to advance the development and application of this innovative technology.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
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ABSTRACT
Air sparging, also called "In situ air stripping" and 'In situ volatilization* injects air into the saturated zone
to strip away volatile organic compounds (VOCs) dissolved in groundwater and adsorbed to soil. These
volatile contaminants transfer In a vapor phase to the unsaturated zone where soil vapor extraction (SVE)
can then capture and remove them/ In addition to removing VOCs via mass transfer, the oxygen in the
Injected air enhances subsurface biodegradation of contaminants.
The design of an air sparging system requires system component compatibility, optimal selection of
blowers, efficient well configuration, and appropriate air emissions treatment. The technology can treat soil
and water contaminated by gasoline, solvents, and other volatile compounds. Air sparging systems, always
coupled with soil vapor extraction, provide control of the subsurface air flow. Proper hydraulic control
prevents the migration of contaminants.
Air sparging is a relatively new treatment technology. Research efforts have not yet fully elucidated the
scientific basis (or limitations) of the system, nor completely defined the associated engineering aspects.
However, a substantial body of available information describes the effectiveness and characteristics of air
sparging systems. This document summarizes the available literature and addresses case studies of
practical air sparging applications. It also identifies needs for further research.
1992.
This report covers research done between June and August of 1991. The work was completed in April
IV
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CONTENTS
Page
Foreword Hi
Abstract . iv
Figures vi
Tables vii
Abbreviations and Symbols viii
Acknowledgments ix
1. Introduction 1
2. Air Sparging 4
3. Air Sparging Case Studies 14
4. Design Considerations 27
5. Air Sparging System Costs 40
6. Research Needs 58
References .'.... 61
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FIGURES
Number
1
2
3
4
5
6
Mechanisms of mass transport during air sparging
Migration of light non-aqueous phase liquids during air sparging
Air sparging process schematic
Air sparging well configurations .... i
Extracted vapor concentration of air sparging system
Effect of pressure on air sparging system
Eias
6
11
30
3.2
37
38
Vi
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TABLES
Number
1 Conditions Affecting Applicability of Air Sparging
2 Summary of Published Air Sparging Sites
3 Published Air Sparging Construction Details
4 SVE and Air Sparging System Components Capital Costs
5 Vapor Treatment Costs
6 Equipment Specifications
7 Capital Costs
8 Operation and Maintenance Costs
Page
8
16
18
43
47
55
56
57
vii
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ABBREVIATIONS AND SYMBOLS
A Active (recycling facility)
ABN acid/base neutral '
BTEX Benzene, toluene, ethyl benzene, xylene (combined analysis)
COM Camp Dresser & McKee, Inc.
CGI combustible gas indicator
CPVC chlorinated poiyvinyl chloride
DCE dtchloroethylene
DO dissolved oxygen
ORE destruction and removal efficiency
EPA U.S. Environmental Protection Agency
GAC granular activated carbon !
GC gas chromatograph
HSWA Hazardous and Solid Waste Amendments
Ib Pound (weight)
LHL lower exposure limit
LNAPL light non-aqueous phase liquid
MTBE methyl tert-butyi ether
NA not available
NAPL non-aqueous phase liquid
NPDES National Pollution Discharge Elimination System
O&M operation and maintenance
ORD Office of Research and Development
OUST Office of Underground Storage Tanks, U.S. EPA
OVA organic vapor analyzer I
PCE perchioroethyiene
PP polypropylene
PVC poiyvinyl chloride
RCB Releases Control Branch
RCRA Resource Conservation and Recovery Act
RREL Risk Reduction Engineering Laboratory
scfm standard cubic feet per minute
SVE soil vapor extraction
TCA trichlorethane
TCE trichloroethylene
TCLP Toxicity characteristic leaching procedure
THA total hydrocarbon analyzer ,
TPH Total petroleum hydrocarbons
TSDs Transportation/storage/disposal facilities (for hazardous waste)
VOC Volatile organic compounds
UEL upper exposure limit
UST Underground storage tank
$/t Dollars per ton
viii
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ACKNOWLEDGMENTS
This report was prepared for the USEPA Office of Research and Development (ORD) under the
direction of Technical Project Monitor Chi-Yuan Fan, P.E. of the USEPA Superfund Technology
Demonstration Division, Risk Reduction Engineering Laboratory, Releases Control Branch in Edison, New
Jersey.
Tom A. Pedersen, Camp Dresser & McKee Inc. (COM) Project Manager, directed the Work
Assignment No. 3-09 under EPA Contract No. 684)3-3409 to COM Federal Programs Corporation. Mary E.
Loden, P.E. (COM) was the principal author, assisted by Carole Kasltek (COM). Roger Olsen, Ph.D., Charles
Jutras, and David C. Noonan (all of COM) completed a technical review. Marilyn Avery and Francine
Everson, Foster Wheeler Enviresponse, Inc. (FWEI) provided editorial support, and Peter Michaels (FWEI)
provided additional technical support in the development of the document.
The authors would like to express their appreciation for the guidance and assistance provided by
Chi-Yuan Fan, ORD's Technical Project Monitor for this work assignment. They would also like to
acknowledge the assistance of Gale Billings of Billings and Associates, Inc. in Albuquerque, NM; Donald
Jacobs and Michael Marley of VAPEX in Canton, MA; and Andrew MWdleton Of Remediation Technologies
Inc. of Pittsburgh, PA. Thomas C. Schruben of OUST, George Mickelson of the Wisconsin Department of
Natural Resources, and Roy Chaudet of IT Corporation contributed to the technical review of this report.
Special thanks are also extended to Jean O'Brien (COM) for word processing and to Lori Hoffer,
Lisa Cusick and Robert Randazzo for original graphics (all of CDM).
ix
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SECTION 1
INTRODUCTION
The U.S. Environmental Protection Agency (EPA) through the Hazardous and Solid Waste Amendments
of 1984 (HSWA) and its land ban regulations, has encouraged the use of remedial action alternatives to
excavation and land based disposal of contaminated soils resulting from leaking underground storage tanks
(USTs).
EPA, through its Risk Reduction Engineering Laboratory's (RREL) Releases Control Branch (RGB), has
initiated research and development efforts to expedite the remediation of contaminated soil impacted by
leaking USTs. This work includes the investigation of emerging and innovative remedial technologies, such
as air sparging used in combination with soil vapor extraction (SVE), as alternatives to pump-and-treat
technology.
PUMP-AND-TREAT
Pump-and-treat processes have comprised a primary form of groundwater remediation. They employ
groundwater extraction wells, a groundwater treatment system, and a discharge location for treated water.
The treatment system for volatile organic compounds (VOCs) typically consists of air stripping and carbon
adsorption equipment. In designing pump-and-treat systems, the remedial manager may experience
difficulty in obtaining the required state and local permits for discharging the treated water. Several states
restrict recharging treated water back into the aquifer; they make obtaining a National Pollution Discharge
Elimination System (NPDES) permit for surface water discharge a long, difficult process. In addition, further
restrictions limiting aquatic toxicfty apply to surface water discharge.
Several factors control the effectiveness of pump-and-treat systems, such as the following rates:
withdrawal of water from the ground
contaminant diffusion
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desorption and dissolution of contarninants
dissolution of non-aqueous phase liquids (NAPL)
Pump-and-treat Is a slow method of remediating groundwater, with predicted dean-up times ranging
from 10 to 30 years, or even longer due to the presence of NAPL and other physicochemical limitations as
stated above [Mercer et al. 1990]. These long dean-up times increase costs for extraction, water treatment,
and monitoring. Such limitations have promoted great interest in technologies which can achieve
concentration goals in significantly less time than pump-and-treat. Sites treated with aiir sparging have
achieved dean-up levels in time periods less than that expected via pump-and-treat systems.
i .
AIR SPARGING SYSTEMS
I
This innovative technology sends air into a contaminated aquifer in order to force pollutants to leave
subsurface soil and groundwater for soil pore spaces, where SVE can remove them. SVE systems always
accompany air sparging treatments because they can capture the VOCs that air sparging strips from the
saturated zone.
REPORT FORMAT
To accommodate the reader with a specific Interest; the report will cover six different facets of air
sparging as an Innovative treatment for soil and'groundwater contaminated by leaking USTs:
a process description of air sparging and a review of the literature on the subject
the components of the system and the factors that affect their performance
case studies of documented applications
the process design
tha economics of implementing air sparging
the need for future research in this innovative area
Much of the information presented in this report emerged from a review of available literature on air
sparging technology induding case studies and .theoretical papers presenting process mechanisms The
report describes air sparging system components, discusses the subsurface mechanism controlling the
system's effectiveness, and outlines the various factors determining its applicability at a particular site. It
also compares air sparging to conventional pump-and-treat treatment for groundwater remediation.
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A case study section synopsizes over 20 air sparging applications, focusing on five which highlight
various remedial conditions and the results achieved. Next the report describes the process layout and
equipment requirements for an air sparging system. This section addresses contaminant removal and
system performance.
A costs section presents capital, operational, and monitoring costs for soil vapor extraction and air
sparging systems. It also provides costs for vapor emissions treatment and other significant cost factors
associated with air sparging technology.
The final section forecasts the data and research efforts that are needed to further advance this
technology and its application to the remediation of soil and groundwater impacted by the release of
petroleum products from leaking USTs.
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SECTION 2
i
AIR SPARGING
PROCESS DESCRIPTION
Air sparging, also called "in situ air stripping" and "in situ volatilization," is a technology utilized to
remove VOCs from the subsurface saturated zone. It introduces contaminant-free air into an impacted
aquifer system, forcing contaminants to transfer from subsurface soil and groundwater into sparged air
bubbles. The air bubbles are then transported into soil pore spaces in the unsaturated zone where they can
be removed by SVE.
Air sparging systems must operate in tandem with SVE systems that capture volatile contaminants
stripped from the saturated zone. Using air sparging without accompanying SVE could create a net-positive,
subsurface pressure extending contaminant migration to as-yet-unaffected areas. Thus the treatment could
increase the overall zone of contamination. Without SVE, uncontrolled contaminated soil vapor could also
flow into buildings (i.e., basements) or utility conduits (i.e., sewers), creating potential explosion or health
hazards.
REMEDIATION MECHANISMS
The SVE system alone may affect the rate |Of volatilization of VOCs from the saturated zone [Marley,
Walsh and Nangeroni, 1990]. However, transport of immiscible contaminants from the saturated zone to
the vadose zone necessitates channeling them to the air/water interface for removal by an SVE system.
Thus, the rate of contaminant transport from Qroundwater to soil vapor phase has increased with the
addition of air sparging to an SVE system.
The effectiveness of combined SVE/air sparging systems results from two major mechanisms;
contaminant mass transport and biodegradatioii. Depending on the system configuration, the operating
parameters, and contaminant types found on-site, one mechanism usually predominates. In both
remediation mechanisms, oxygen transport in the saturated and unsaturated zones plays a key role.
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Although the exact nature of the saturated zone vapor phase is not completely understood, sparging seems
to create air bubbles, which move through the groundwater to the unsaturated soil, like bubbles in an
aeration basin [Ardito and Billings, 1990; Brown and Fraxedas,1991]. Other theories trace the movement
of air through irregular pathways in the saturated zone and, ultimately, to the surface of the water table
[Middleton and Miller, 1990]. These theories suggest that the air would move as pockets through soil
pathways, rather than forming bubbles, because groundwater travels in a porous medium.
The nature of air transport affects mass transfer to and from the groundwater regime. Bubbles exhibit
higher surface area for transfer of oxygen to the groundwater and for volatile migration to the unsaturated
zone, than the area provided by continuous, irregular air-flow pathways.
Mass Transfer
Mass transfer employs several mechanisms that move contaminants from saturated zone groundwater
to unsaturated soil vapors. Figure 1 illustrates the following major mechanisms: (a) dissolving soil-sorbed
contaminants from the saturated zone to groundwater; (b) displacing water in soil pore spaces by
introducing air; (c) causing soil contaminants to desorb; (d) volatilizing them, and (e) enabling them to enter
the saturated zone vapor phase. Due to the density difference between air and water, the sparged air
migrates upwards in the aquifer. The pressure gradient resulting from the creation of a vacuum in the
unsaturated zone pulls the contaminant vapors toward and Into the SVE wells.
The action of the air passing through the saturated zone in response to sparging leads to turbulence
and mixing of the groundwater. This in turn increases the rate at which contaminants adsorbed to the
saturated zone soils dissolve into the groundwater. Light non-aqueous phase liquids (LNAPLs) floating on
the water table are also subject to increased rate of transfer to the unsaturated zone because they are
volatilized by the air sparging process.
In summary, air sparging increases the speed at which the following occur:
volatilization of contaminants from the groundwater to the vadose zone;
desorption and dissolution of adsorbed contaminants from the soil into the groundwater; and
dissolution of NAPLs due to mechanical mixing.
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The mass transfer of contaminants may be further enhanced by heating the air prior to sparging. The
increase in air temperature will increase the rate of volatilization of contaminants.
Biodearadation Mechanism
Aerobic biodegradation of contaminants by indigenous microorganisms requires the presence of a
carbon source, nutrients, and oxygen. Air sparging increases the oxygen content of the groundwater thus
enhancing aerobic biodegradation of contaminants in the subsurface. Certain organic contaminants, such
as petroleum constituents, serve as a carbon source for microorganisms under naturally occurring
conditions. The rate of biodegradation can be enhanced by optimizing nutrient status of the system.
Remediation of an aquifer via the biodegradation mechanism has distinct advantages since a portion
of the contaminants will be biologically degraded to carbon dioxide, water, and biomass - yielding a lower
level of VOCs in the extracted air. This in turn can substantially reduce vapor treatment costs. The
possibility of off-site contaminant vapor migration is also reduced when sparged vapors entering the vadose
zone contain lower levels of contaminants.
Certain contaminants, such as chlorinated solvents, can undergo biodegradation under anaerobic
conditions. Air sparging, in these instances, could adversely affect this biodegradation process.
TECHNOLOGY APPLICABILITY
Although air sparging is a relatively new technology for contaminated subsurface soil remediation, it
has been applied at hundreds of sites in the United States and Europe since 1985. However, the design
of these systems has been, for the most part, empirically based [Marley, 1991].
The effectiveness of air sparging depends on various site conditions. Table 1 lists these factors, which
are discussed below.
Depth to Groundwater
Air sparging has been effective in an aquifer 150 ft below surface [Looney, Kaback and Corey, 1991].
There appears to be no depth limit at which air sparging would not be effective, but significant cost
implications may accompany the installation of an air sparging system in a very deep aquifer. However, a
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TABLE 1. CONDITIONS AFFECTING APPLICABILITY OF AIR SPARGING
Air sparging
applicability factor
Depth to groundwater
Volatility of contaminants
Solubility of contaminants
Biodegradability
Permeability
Aquifer type
Soil type
Presence of LNAPL
Bedrock aquifer contamination
>5ft
High volati
Low sdub
Favorable
conditions
lity
lity
High biodegradability
>10-3cm/
Unconfine<
Sandy soil
sec
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3
None or thin layer
Highly frac
tured bedrock
Unfavorable
conditions
Oft
Low volatility
High solubility
Low biodegradability
<10"3 cm/sec
Confined
Clays, high organic soils
Thick layer of LMAPL
Unfractured bedrock
water table located at a shallow depth (<5 ft), |may increase the difficulty of recovering vapors with SVE.
It could release VOC emissions to the atmosphere. Capping such a site with pavement or other impervious
material might reduce atmospheric emissions.
Volatility of Contaminants
Enhancing mass transfer of contaminants from the soil and groundwater into the vapor phase, a key
mechanism of the air sparging process, requires highly volatile contaminants. Volatility is directly related
to the Henry's Law Constant of a compound and its vapor pressure - the higher the Henry's Law constant,
j
tha higher the volatility. In general, compounds which are effectively removed from contaminated water by
air stripping are sufficiently volatile for adequate air sparging treatment. Compounds with Henry's Law
Constants of 105 atm-m3/mole or greater can be air stripped or sparged [Brown et al., 19911. Due to their
high volatility, petroleum compounds (e.g., benzene and toluene), and solvents (e.g., trichloroethylene) are
very amenable to air sparging technology.
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Solubility of Contaminants
The solubility of a contaminant in water determines its ability to be stripped by air sparging. In general,
the more soluble a contaminant is in water, the greater the difficulty there is in using air sparging.
Biodearadabilltv of Contaminants
Since biodegradation is enhanced by air sparging, compounds that are readily aerobically degraded
are amenable to remediation by air sparging. Biodegradation of petroleum hydrocarbons, such as those
found in gasoline and diesel leaks from USTs, has been significantly increased with air sparging. Prior to
designing an air sparging system for bioremediation, electrolytic respirometry should be used to analyze
samples of the soils and groundwater. This will make it possible to gauge the effectiveness of the
indigenous microorganisms and their energy sources to metabolize the petroleum hydrocarbons.
Soil Permeability
Soil permeability, which measures the ease of fluid flow through the soil column, is a critical parameter
in the design of air sparging systems. Injected air must flow freely throughout the aquifer to achieve
adequate removal rates. In most aquifers, horizontal permeability is greater, by a factor of ten, than vertical
permeability. Successful sparging systems require air flow in both horizontal and vertical directions [Brown
and Fraxedas, 1991]. Vertical flow is particularly important since the contaminant must migrate to the
vadose zone for removal by SVE.
If the geology restricts the vertical flow, contaminants may migrate laterally into previously
uncontaminated areas. Hydraulic conductivity of 0.001 cm/sec or greater is required to obtain sufficient
subsurface air flow [Middleton, 1990]. Bench-scale experiments have shown coarse sand (d50 = 0.8 mm)
forming the dividing line between soils, which permits injected air to rise by hydraulic uplift alone from soil
that required additional pressure to inject air and through which air escaped at only a few points [Wehrle,
1990].
Due to the heterogeneity of soils at all sites, it may be necessary to concentrate wells in areas with
lower permeability. The spacing of the wells depends on the radius of influence. In general, highly
permeable soils will have larger radii of influence and higher air flow rates than lower permeable soils.
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Screen placement requires a good understanding of the stratigraphy of a site. Well layout should overlap
the radii of influence. This will ensure the treatment of all soil areas.
Clogging of the injection well screen or thje aquifer in the vicinity of the sparging wells could reduce
permeability and, therefore, decrease the effectiveness of the method. Clogging may result from enhanced
bacterial growth under increased oxygen levels. In addition, oxidation at sites with high iron and manganese
levels could cause further clogging. Some applications have injected nitrogen instead of ambient air to
i
minimize problems associated with fouling [MWR, 1990]. However, the use of nitrogen also prevents the
enhancement of aerobic biodegradation.
Confining Layers
Some air sparging proponents point out th|at it can only achieve success at sites with water table (i.e.
unconfined) aquifers. Confined aquifers, where a low permeability layer lies above the water-bearing zone,
would inhibit the flow of air upward from the saturated zone to the vadose zone. The injected air in these
situations would flow radially away from the injection point; the vapor extraction system would not recover
it Such a situation could build up pressure in the aquifer.
i
i
For unconfined aquifers, stratigraphic layers with different permeabilities will also affect air and water
flow patterns as well as influence the air sparging system. In such situations, optimal air flow will occur in
the more permeable zones [Wehrle, 1990]. Air flow may travel horizontally away from the injection point
and create a wider zone of influence than woulcl otherwise be expected [Bohler etal., 1990].
Soil Characteristics
Air sparging systems are most applicable for sites with sandy soil, due to its permeability. Soil
containing a large organic carbon fraction may impede the desorption of volatile organic contaminants, thus
reducing air sparging effectiveness. In extraction wells, the presence of a large amount of monomers in the
soil may cause clogging of well screens possibly due to polymerization.
Presence of LNAPL
Low-density (or light) nonaqueous phase; liquids (LNAPL) floating on the water table presents a
particular problem during air sparging. As Figure! 2 shows, the air sparging action creates a mounding effect
10
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in the proximity of the sparge well. In sites with steep hydraulic gradients, this mounding effect may be
sufficient to move a plume of LNAPL, possibly contaminating dean areas. While it is possible to prevent
the plume movement by modulating the sparged air pressure, it is more important to recover the mobile
portion of the LNAPL to a residual saturation phase.
Contamination In Bedrock Aquifer
The effectiveness of air sparging hinges on the mass transfer of air to the groundwater and movement
of the contaminants' vapor through the saturated zone upward into the unsaturated zone where they can
be extracted. Unless the rock formation is highly fractured, with fractures vertically oriented, this technology
will not provide sufficient mass transfer to effectively remediate a bedrock aquifer.
i
Mefolt In Groundwatef
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In addition to the possibilities of clogged well screens resulting from oxidation of metals in groundwater
and the growth of bacteria previously discussed, precipitation of metals can also be an inhibiting factor.
Since ambient air contains carbon dioxide, calcium carbonate precipitation may occur in some aquifers
during air sparging. This may also reduce the air flow through the system.
Contaminant Location
Air sparging targets contaminants in the saturated zone and the capillary fringe. For compounds with
a density less than water such as many petroleum constituents, much of the contamination may lie in the
capillary fringe and just below the water table, depending on such factors as water table fluctuations, the
amount of product released, contaminant density, and contaminant solubility. Dense non-aqueous phase
liquids (DNAPL), such as trichloroethylene, often migrate through the aquifer to a lower confining unit and
to greater depths. For dissolved contaminants in the aqueous phase, groundwater flow and direction wilt
control the distribution of contaminants throughout the site. Depending on soil characteristics, air sparging
would remediate DNAPL-contaminated soil as well.
I
i
Combination with Other Technologies
Air sparging is always used in conjunction with SVE. The implementation of SVE addresses the vadose
zone contamination, and incorporates air sparging wells to treat saturated zones.
12
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Groundwater extraction at air sparging sites may serve as a hydraulic control. Injected air may mobilize
contaminants adsorbed to soil, either by displacement from the soil matrix or through increased dissolution
of the adsorbed contaminant into the groundwater during mixing caused by air injection [Middleton and
Hiller, 1990]. If this occurs and the rate of volatilization is Insufficient, downgradient groundwater
concentrations could actually increase. Air sparging may have fallen into disfavor in Germany due to
increased downgradient dissolved contamination (Brown and Fraxedas, 1991]. To prevent this situation,
a groundwater pumping system could hydrauiicaily contain the site groundwater flow.
13
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SECTION 3
AIR SPARGING CASE STUDIES
Air sparging technology is a relatively recent remediation method, applied at contaminated sites only
within the past half decade. Early applications of this technique apparently occurred in Germany during the
mtd-1980's [Mlddieton and Miller, 1990]. Due to the technology's short track record, the delay in publishing
the results of field work, and the reluctance of some experts in revealing details about the technology for
proprietary and competitive reasons, a relatively, sparse body of information is available on air sparging.
Wfth Increased application, the quantity and quality of this data should Improve, disseminating helpful
information to the remedial community.
Not surprisingly, documented air sparging experience has not been limited to one chemical group or
soil type. The sites vary In contaminant treated, soil type, geological features, additional techniques used
at the site, and other factors. A study of these sites, however, reveals that some share common
characteristics, from which Important Insights can be drawn.
AIR SPARGING EXPERIENCE
Reviews of case histories for air sparging sites and visits to active sites in New Mexico contributed to
the preparation of this report. A summary of the information gathered during these activities follows below.
Table 2 lists 21 sites remediated by air sparging. It provides data on soil types, contaminant types,
groundwater concentrations (initial and final), and the time needed to achieve those final levels. Table 3
presents construction and operations information for these case studies. Brief treatments of four case
studies from the United States and nine European installations will illustrate how air sparging successfully
remediates the saturated zone.
14
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Contaminants Treated
At the sites studied, air sparging has been used exclusively to treat VOCs, including petroleum
constituents and chlorinated solvents. Gasoline and industrial solvent applications targeted trichloroethyiene
(TCE) and percMoroethylene (PCE). In many instances such contamination originated in releases from USTs
at service stations, tank farms, dry cleaners, manufacturing plants, and other industrial facilities. Among the
case histories reviewed, nine sites were contaminated with gasoline, and twelve were impacted by the
release of solvents. One of the nine gasoline-contaminated sites contained both gasoline and diesei fuel
contamination.
Contaminant Magnitude
Table 2 lists the initial contaminant concentration for each case history site. There appears to be no
upper limit for expectations of air sparging effectiveness. Indeed, as the contaminant levels increase, air
sparging should exceed the results achieved by groundwater pump-and-treat approaches, since the
volatilization mechanism depends on a concentration gradient between the groundwater concentration and
that of the (contaminant-free) introduced air.
Soil Characteristics
Like many in situ remediation technologies, the effectiveness of air sparging is significantly affected by
soil characteristics. Table 2 shows the soil properties found at each site listed. Most of these sites
contained permeable soil types, such as sand, silt, and gravel. The Nordrhein, Westfalen site presented
fractured limestone. Such sites, with highly fractured rock formations, may also provide sufficient
permeability for air sparging application, as noted before:
Depth to Groundwater Table
Air sparging has operated at sites where the depth to groundwater ranges from just two ft [Harress,
1989] to 135 ft [Looney, 1991]. Most of the sites studied, however, measured this depth from 8 to 20 ft
(Table 3).
15
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CASE STUDIES
At many sites, the air sparging application has followed limited success with groundwater pump-and-
treat operations [Marley, 1990; Ardtto and Billings, 1990; MWdleton and Hiller, 1990]. In effect, these sites
were "retrofitted" with air sparging in the hopes of expediting the cleanup and achieving goals in a matter
of months rather than years. In many cases, these goals have been met - with several sites completing site
closure. At most of these sites, SVE addressed vadose zone contamination; air sparging treated saturated
zone contaminants. The following case studies (four in the United States and nine in Europe) describe sites
where air sparging was successful.
Cane Studies In the United States
Gaaoline Service Station, Rhode laland-
A groundwater pump-and-treat and product recovery system, which was initially implemented at this
gasoline spill site in Rhode Island, proved inadequate to meet the closure criteria establishod by the Rhode
Island Department of Environmental Management [Marley, 1991]. In addition to groundwater
extraction/treatment, a soil gas containment system was instituted to control the migration of gasoline
vapors into nearby basements. The vapor containment system was subsequently upgraded to a soil vapor
extraction/air sparging system by increasing vapor extraction flow with air injection into the saturated zone.
A cost/benefit analysis was performed on three respective treatment schemes: two groundwater pump-and-
treat processes and an air sparging process to be used in conjunction with the existing soil vapor extraction
system. A geological study of the site showed fine to coarse sand and some fine to medium gravel; soil
analyses revealed low levels of weathered gasoline constituents.
Based on the results of a pilot study, a full-scale air sparging system was designed. It employed seven
shallow and six deep injection wells, with two vapor extraction wells. Pretreatment concentrations of
benzene, toluene, ethyl benzene, and total xylenes (BTEX) in groundwater measured as high as 21,000 ppb.
Full-scale air sparging treatment over a 60-day period lowered BTEX concentrations to levels well below the
established closure criteria (only hundreds of pp}j).
i . - / .
Dry Cleaning Facility--
i
A vapor extraction/air sparging treatment system was designed to remediate soil and groundwater
contaminated by leaking USTs at a former dry cleaning facility. Groundwater contaminants included
perchloroethylene (PCE), trichloroethylene (TCE), idichloroethylene (DCE), and total petroleum hydrocarbons.
20
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The subsurface environment consisted of miscellaneous occurrences of fill material sporadically overlying
a continuous sheet of naturally occurring Quaternary sediments [Brown, 1991]. A naturally existing barrier
locally minimized the potential for downward migration of dissolved-phase total petroleum hydrocarbons and
chlorinated VOCs from the shallow water-bearing zone to deeper water-bearing units.
A three-phase pilot study employed the following: vapor extraction only, air sparging only, and the
simultaneous operation of both systems. The air sparging tests ran at pressure levels of 10,15, and 20 psi
with corresponding flow rates of 16, 24, and 37 cfm. Vacuum/pressure readings and OVA monitors
measured system performance. The combined system was deemed effective because the OVA readings
showed removals that exceeded those of the single processes.
Based on the results of the pilot study, a full-scale system was designed, consisting of seven nested
vapor extraction/air sparge points, one (vapor) extraction-only well and seven injection-only wells. The
vapor extraction system operated approximately one month prior to start-up of the air sparging system.
Effluent samples indicated that concentrations of PCE and TCE decreased during vapor extraction start-up
and then increased with start-up of the injection system. Initial groundwater concentrations were as high
as 40,000 ppb total VOCs; after 125 days, they dropped by more than 98%.
Horizontal Wellt, Savannah River Site-
Air sparging was demonstrated at a U.S. Department of Energy site as an innovative environmental
technology capable erf remediating unsaturated zone soils and groundwater containing VOCs [Kabek et al.,
1991 ]. A 20-week pilot test evaluated the technology, utilizing two horizontal wells, one each for extraction
and injection. Air Injection flow rates and temperature were also used to evaluate the process. The
horizontal wells were located along a process sewer line that was the apparent source of the contamination.
The horizontal well configuration was chosen for this site because it would provide more surface area for
the injection and extraction needed to treat the linear contamination. Since many water-bearing subsurface
formations extend areally and because the site geology dictates the path of a contaminant plume, horizontal
wells may draw vapors more efficiently from these horizontal formations.
The injection well, installed below the water table at a depth of 150-175 ft, extended 300 ft horizontally;
the extraction well, installed at a depth of 75 ft (approximately 60 feet above the water table), extended 200
ft horizontally. Extensive characterization and monitoring determined that the highest concentrations of PCE
and TCE in groundwater were found at depths greater than 180 ft below the zone of injection.
21
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Helium tracer tests provided a better understanding of the vapor flow paths between the two wells. The
results Indicated connectivity between the two wells, although the recovery rates were slow. After 46 days,
45% of the helium had been recovered.
Mlcrofaiai tests showed an increase in the activity of indigenous microorganisms, as measured by
Increased CO2 levels during air Injection at medium and high flow rates. This activity diminished at the
conclusion of the air Injection test. The injection of heated air had no apparent effect on the amount of
contaminants nor the temperature of the vapors extracted. Comparison of extraction rates achieved in one
vertical well during a vapor extraction test to rates from the air sparging horizontal well showed an increase
of approximately 20% by the air sparging system.
Conservancy Site, Belen, New Mexico-
Contamination at the Conservancy Site consisted of a 6,500 gal gasoline leak from a leaking UST
[Bil'.Ings and Associates, 1991]. A free product layer as thick as 33 inches was found by monitoring wells,
with groundwater benzene concentrations of up to 6 ppm. The soil is silty sand with a clay layer.
Free product recovery and air sparging systems were installed on-site. The air sparging system
consisted of nested sparge and extraction wells, linked in a network. Since the depth to groundwater was
only 6.5 ft, it was possible to manually install the extraction and sparging wells.
The sparging system consisted of 2-in PVC wells and solvent-welded piping. The network was radially
Installed around the source of the contamination to minimize migration of the contaminant plume. Air
Injection and vapor extraction used several blowers, installed in parallel systems with manifolds and piping
networks for operational flexibility.
The system operated intermittently for two months, and then continuously for three months. After the
fifth month, groundwater benzene reductions throughout the site ranged from 37 to 100 percent with an
overall average of 59 percent The following average percent reductions of other parameters were achieved
after the fifth month:
i
benzene-59%
toluene-66%
ethyl benzene - 54%
xyienes - 49% j
22
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Based on these reduction rates, the site might achieve the cleanup criteria established by the State of
New Mexico in about 2.5 years as predicted by the engineer [Billings and Associates, 1991].
Developments and Application* of Air Sparging in Europe
Chief among the firms developing and applying SVE and air sparging technology in Europe are
Hannover Umwetttechnik GmbH and Harress Geotechnik. Hannover Umwelttechnik (HUT) has developed
an inexpensive and relatively effective technique for SVE and groundwater stripping in situ (Nunno and
Hyman, 1988). Compressed air is pulsed into the aquifer through injection wells, stripping the volatile
contaminants from the groundwater. The compressed air is introduced in a pulsed manner in order to
prevent channelling or short circuiting.
Since 1985, in situ groundwater aeration has been used on over thirty sites in Europe (Middleton and
Hiller, 1990). Following are detailed descriptions of two of these remedial installations and their operations.
An additional seven brief case histories of installations in Germany are included.
Example 1-
In the example described here, soil gas measurements inside a building revealed concentrations of
more than 500 ppm for both trichiorethyiene (TCE) and tetrachloroethylene (PCE). Peak concentrations in
soil samples were found to be as high as 2,800 mg/kg for TCE and 64 mg/kg for PCE.
The geology on the site was characterized by quaternary sand and gravel units of more than 110 feet
in thickness, with an interiayer of silty sands at a depth of 44 to 47 feet. The depth to groundwater was
about 27 feet measured from the floor of the building.
Two soil venting units, equipped with radial flow blowers, produced a volume flow of 475 cfm. Within
100 days a total of 5,100 IDS of solvents was removed from the soil. At that point, compressed air was
injected into the groundwater using 5 injection pipes with a length of 37 feet each. The injected volume flow
was about 6 cfm at each pipe.
Exhaust air VOC concentrations decreased by approximately an order-of-magnitude in the first 100 days
due to soil venting. Air injection started at day 100. An increase in the exhaust air VOC concentrations from
a total of 800 mg/m3 to more than 10,000 mg/m3 was observed within 2 hours after the start of the aeration.
From this peak, the VOC concentration again decreased along the typical slope of an air extraction curve.
23
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Soft venting and groundwater aeration removed a total of more than 8,900 Ibs VOC from the unsaturated
and the saturated zone within 240 days. After 3 months of aeration, the concentrations in the groundwater
were reduced from an initial 33,000 fig/L to 270
Example 2-
Groundwater contamination was discovered on the site of a chemical manufacturer. Initial analyses
revealed concentrations of more than 5,000 pg/L of solvents in the groundwater. Following the discovery,
several wells were established up- and downgradient of the contamination sources which had been
previously defined by soil gas investigations. i
The geology of the site was characterized by uniform sandy gravels down to a depth of approximately
36 ft. The sandy gravels were underlain by marly clays, which form the base of the aquifer. The water table
was at a depth of 8 ft. SoU venting was chosen as the process to dean up the vadose zone, starting in June
1986. For the remediation of the contaminated groundwater, eight air injection points were installed at the
base of the aquifer in the immediate vicinity of the soil venting systems. Injection of air into the aquifer
commenced in July 1986.
Groundwater quality was monitored using wells located along the property line downgradient of the
contaminated areas. Within 9 months of operation, the concentration of solvents in the well, which was
located directly downgradient, decreased from 5,417 pg/L to 320 pg/L By May 1990, the concentration
had further decreased to less than 10 pig/L In another downgradient well, the concentrations decreased
from 1,990 pg/L In August 1987 to around 150 pg/L in May 1990. During the same period, the contaminant
concentration In the exhaust air decreased from initial levels of up to 500 ppm to values of 1 ppm and less.
No groundwater was pumped during the period of the remediation.
I
Following are brief case histories of air sparging installations at seven locations in Germany (Harress
Geotechnlk, Inc., 1989). The operations all began with an SVE installation in the vadose zone. After the
VOCs In the vadose zone were reduced to asymptotic levels, the air injection systems were installed in the
saturated zones within the zone of Influence of trie SVE systems.
Case History No. 1
Location: Augsburg, Bavaria
SoU conditions: 36 ft sandy gravel, aquitard - clay
Depth to groundwater: 8ft
Number of air Injection points: 8 at 50 ft spacing
24
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Number of vapor extraction points: 4
VOC contaminant: halogenated hydrocarbons
Initial groundwater concentration:
Effectiveness of VE/GA1 System:
Case History No. 2
(in downgradient monitoring wells B2 and B4)
B2- 1,900 ppb
B4-5,417 ppb
Within 9 months in B2 to 185 ppb, B4 to 320 ppb
Location: Berlin
Soil conditions: 115 ft of sand, with silty lenses from 9 ft to 36 ft below grade, aquitard - day
Depth to groundwater: 15-18 ft
Number of air injection points: 3
Number of vapor extraction points: 1
VOC contaminant: mostly 1,2-DCE-cis, with TCE and PCE
Initial groundwater concentration: 1,2-DCE-cis > 2,000 ppb
Effectiveness of VE/GA"" System: Reduced to 1,000 ppb after 10 months, reduced to 440 ppb after a
total of 2 years
Case History No. 3
Location: Bielefeld, Nordrhein-Westfalen
Soil conditions: 5 ft to 15 ft (thickness varying) of fill and sandy to silty sediments, aquitard - siltstone
Depth to groundwater: approximately 2 ft to 8 ft
Number of air injection points: 5 at 30 to 60 ft spacing
Number of vapor extraction points: 1, plus 1 at 100 ft distance
VOC contaminant: PCE, TCE, TCA
Initial groundwater concentration: PCE - 27,000 ppb, TCE 4,300 ppb, TCA - 700 ppb
Effectiveness of VE/GA"" System: Reduction to total VOC concentration of 1,207 ppb after 11 months of
operation
Case History No. 4
Location: Munich, Bavaria
Soil conditions: 6 ft fill, 14 ft gravel, 6 ft fine grained sand, 9 ft gravelly sand, aquitard - clayey silt
Depth to groundwater approximately 15 ft
Number of air injection points: 7 at 60 - 80 ft spacing
Number of vapor extraction points:
VOC contaminant: PCE, TCE, TCA
Initial groundwater concentration:
Effectiveness of VE/GA1 System:
Case History No. 5
1
PCE - 2,200 ppb, TCE - 400 ppb, TCA -150 ppb
Within 3 months, PCE - 622 ppb, TCE -13 ppb, TCA - 3 ppb. After an
additional month, PCE - 539 ppb, TCE -12 ppb, TCA - 2 ppb
Location: Nordrhein-Westfalen
Soil conditions: 6 ft clayey silt, 30-45 ft sand (fine to medium grained), aquitard - siltstone
Depth to groundwater: 6 - 9 ft
Number of air injection points: 10
Number of vapor extraction points: 2
VOC contaminant: halogenated hydrocarbons
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Initial groundwater concentration:
Effectiveness of VE/GA"" System:
Case History No. 6
Sublocation A: between 1,500 and 4,500 ppb
Sublocation B: (downgradient monitor well) between 10,000 and
12,000 ppb ;
Reduction in Sublocation A: to 25 ppb within 1 month to 10 ppb
within an additional 4 months; B: to 200 ppb within 6 months
Location: Nordrhein-Westfalen (Bergisches Land)
Soil conditions: Limestone, fractured
Depth to groundwater: 90 ft
Number of air Injection points: 8
Number of vapor extraction points: 2
VOC contaminant: haiogenated hydrocarbons
Initial groundwater concentration: 80,000 ppb
Effectiveness of VE/GA*" System: 2,500 ppb to '4,900 ppb after 6 months, 400 ppb after 15 months
Case History No. 7
Location: Pluderhausen, Baden-Wurttemberg j
Soli conditions: 2 ft fill, 7 ft silts, 10 ft gravel, aquitard - clay
Depth to groundwater: approximately 11 ft
Number of air injection points: 5 at 10 -15 ft spacing
Number of vapor extraction points: 1
VOC contaminant: trichloroethene (TCE)
Initial groundwater concentration: 20,000 ppb; reduced to 1,200 ppb after approximately 10 months of
groundwater extraction and treatment
Effectiveness of VE/GA"" System: Starting at 1,200 ppb, a 90% reduction (to 120 ppb) after 5 days of
operation, and a further reduction to 23 ppm after an additional two
months :
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SECTION 4
DESIGN CONSIDERATIONS
The design of air sparging systems depends on various elements, such as well configuration, blower
capacity, compressor size, and vapor treatment systems. The proper placement of process equipment,
gauging, and instrumentation are crucial to monitoring the process. Only then can adjustments ensure
optimal effectiveness. Air sparging systems are diverse in terms of design and operational factors. These
characteristics are discussed below.
INJECTION WELL CHARACTERISTICS
Installation of air injection wells usually employs conventional vertical drilling methods, although
horizontal drilling techniques are gaining increased acceptance. Some contractors drill wells using a truck-
mounted hollow-stem auger [Kresge and Dacey, 1991]; others install wells without, using hand augers
[Billings and Associates, 1991]. At sites where the depth to groundwater is shallow and site conditions
favorable, hand-held, gasoline-powered augers or pneumatic hammers can be used.
Wells typically utilize PVC, galvanized steel, or stainless steel casing and screen/s. Steel pipe is
necessary when injected air will be heated to high temperatures. PVC (Schedule 40 or 80) for ambient air
injection offers the advantage of lower cost. Two-inch diameter pipe can transmit the usual air flow rates.
Screen lengths vary, depending on the zone to be remediated, from 2 ft to 10 ft. A shorter screen
allows greater control over the injection point, whereas a longer screen provides more air dispersion.
Contractors usually backfill screens with sand or gravel packing from 6 in to 2 ft above the screen. A
bentonite seal above the screen is essential to prevent short-circuiting of the injection air. The remainder
of the borehole annulus is then grouted to the surface. The bottom of the casing is plugged.
27
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Spacing
The spacing of Injection points is a key design parameter. Well spacing must be sufficient so that the
sparging system affects the entire zone of contaminated aquifer. Locating the weils too tightly will add
unnecessary cost Too few wells may bypass some areas. In most cases, well spacing is determined by
the results of pilot studies and site-specific conditions. Either the radius of influence for that site or
professional judgment based on soil type, soil layering effects, depth to groundwater table, and
contaminated saturated zone thickness, determine the spacing of the wells.
Radius of Influence
The radius of influence of an air sparging well describes the contaminated areas that the well can
adequately remediate. The radius depends on several factors including the soil type, soil homogeneity,
depth of Injection below the water table, injection air pressure and flowrate, and groundwater flow rate. For
example, the higher the soU permeability, the larger the radius of influence for either a sparging or vacuum
well. The cases studied radii Identified from five ft to 177 ft; typically it Is less than 25 ft. In one sparging
system, the radii of influence of the sparging wells were 72 ft, 76 ft, and 177 ft at injection pressures of 10
psi, 15 psi, and 20 psi, respectively [Brown et a!., 1991]. This shows the effect of additional pressure on the
measured radius of influence.
The literature studied did not describe the radius of influence for a horizontal injection well. However,
it was indirectly measured by a helium tracer at the Savannah River Site. It has also been determined by
monitoring levels of dissolved oxygen (DO) In groundwater. In one case, a three-fold increase In DO
concentrations occurred in wells located in the vicinity of air injection wells; it documented an average radius
of influence of 10 to 15 ft per injection well [Kresge and Darcy, 1991].
Air Injection
Using an injection well, a blower or compressor introduces air into the subsurface. The connection can
be made to the top of the well casing (Middleton, 1991) or directly into the well using packers to seal off
the area of injection. The choice of blower, compressor, or vacuum pump depends on the air flow rate and
Injection pressure desired. Injection at greater depths may require a rotary lobe unit rather than a
regenerative blower. Values for injection pressure were rarely reported but ranged from 3 psi to 20 psi.
28
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Air flow rates correspond to air injection pressures. Not all case studies report pressure values.
Generally they described ranges from 2 to 16 cfm per injection point. Greater air flow rates could cause
greater turbulence and mixing in the saturated zone, leading to increased volatilization.
Several sparging experts noted that the volume of extracted air should exceed the volume of injected
air to maintain a margin of safety and to prevent subsurface pressure buildups. Wisconsin requires at least
a 4:1 ratio of extracted air to injected air when the injection well is in a source area [Mickeison, 1991].
Another system maintained a volume ratio of 5:1 [Marley, 1990].
PROCESS LAYOUT AND EQUIPMENT
The first step in implementing an air sparging system consists of designing the well configuration and
selecting the process equipment. Figure 3 shows the aboveground components of a typical system.
The major components of the air sparging system include the following:
injection wells
oil-free compressor
vacuum blower
air/water separator
air emissions treatment
piping and valves
instrumentation
As Figure 3 illustrates, an air sparging system can operate with a single passage of ambient air, or with
multiple passes of recycled extracted air. Recycling eliminates the need to discharge the extracted air.
The selection of blowers should take into account the site-specific type of operation. Treating
flammable gases such as gasoline vapors may require the installation of non-sparking vacuum pumps. This
requirement is overcome in many installations by locating the vapor treatment system, such as activated
carbon adsorption, upstream of the vacuum pump. The air sparging blowers are not required to be of non-
sparking construction.
29
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30
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Well Configuration
Perhaps the most important design element of an air sparging system is the configuration of the well
system. Both well design and layout play important roles. The placement of air sparging and vapor
extraction wells must take into account factors such as depth to groundwater, hydraulic conductivity,
contaminant/s, and the extent of contamination. Various configurations, as shown in Figure 4, alter the
design of air sparging systems. Each configuration can present its own unique advantages and
disadvantages in conjunction with site-specific soy/aquifer characteristics and project objectives.
Vertical Well Configuration -
Based on their radius of influence, placement of vertical extraction and sparging wells throughout the
site should cover the zone of contamination. Pilot tests, with two to four wells in a portion of the site provide
the best means of determining the radius of influence.
Nested Wells -
Nested wells are extraction and sparging wells that are placed in the same borehole, thus saving drilling
costs. However, proper grouting of the borehole to prevent short circuiting of air is very important The
primarily vertical pressure gradient is another difficulty presented by nested wells. It can lower the radius
of influence per well In comparison with other well configurations.
Horizontal Wells -
Advancement in drilling techniques have made horizontal wells feasible for air sparging systems. This
configuration is particularly effective at sites that present shallow aquifers and long, thin contaminant plumes,
such as those caused by leaking pipelines. In some cases, horizontal wells may increase extraction
efficiency over vertical wells by a factor of five [Looney, Kaback and Corey, 1991]. A horizontal well
provides uniform pressure throughout the length of the well, and more surface area for sparging than a
vertical well. Such wells can reach under buildings and into other hard to reach areas. Also, since less
wells are required, they result in cost savings associated with piping, manifolds, and trenching.
31
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Combined Horizontal/Vertical Wells -
Depending on site conditions, the combination of vertical and horizontal wells may be advantageous.
Conditions such as depth to groundwater, soil permeability, and confining layers will determine whether a
combination of horizontal and vertical wells would be the optimal configuration.
Well Radius of Influence
Soil permeability, among other factors, determines the radius of influence for sparging and extraction
wells. The radius of influence, in turn, determines the well spacing and numbers needed for the site. The
number affects not only the cost, but also the design of an air sparging system.
Air sparging experts have suggested several methods of determining the radius of influence for a
sparging well. These methods study the following:
pressure at various distances from sparging points
dissolved oxygen concentration of the aquifer
groundwater elevations in response to injection
groundwater contaminant concentration isopleths
Pressure measurements provide the most common method for determining the radius of influence of
a sparging well. Some experts state that pressure declines exponentially away from the injection well, and
determining the radius can be accomplished by plotting the natural logarithm of the pressure versus distance
[Brown et al., 1991]. Others measure dissolved oxygen concentrations in monitoring wells or at points
throughout the expected zone of influence. This latter method requires measurements before and during
system operation, but it may be a more relevant measurement of the sparging effect.
Well Installation
Sparging well construction should optimize the injection of air to the contaminated saturated soil and
groundwater zone. The screen level should lie close to the water table in order to effectively capture the
vapors sparged from the saturated zone. However, if the SVE screen is too dose to the water table, the
mechanical action will extract water, which will reduce system efficiency and require the use of an air/water
separator to prevent blower damage.
33
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Infection
Below Water Table
The air Injection point e.g., the base of the aquifer or near the water table, depends on the location of
the contaminants. For example, many chlorinated compounds in the DNAPL phase sink through the aquifer
to a confining unit Petroleum constituents (LNAPLs), on the other hand, may float on or near the water
table. The density of the contaminants determines the location of the dissolved contaminant plume in the
aquifer. l
Ideally, the air should be injected just below the lowest level at which contaminants have been detected.
This will ensure that the sparged air contacts all of the contaminant zone. Because injection pressure is a
function of depth, excessively deep wells will require larger, more expensive blowers and vacuum pumps.
PROCESS MONITORING AND OPERATION
Proper operation and monitoring of the air sparging process are necessary to ensure that sparged
vdatiles are captured and that migration of groundwater contaminants is controlled. The following operating
parameters should be monitored:
sparging pressure ;
vacuum pressure
air flow rates
radius of influence for both vacuum and sparging wells
dissolved oxygen in groundwater
contaminant concentration in extracted air
continuity of blower and compressor operation
[
I ,. .
The air sparging process, coupled with SVE, enhances both mass transfer and biodegradation of
subsurface contaminants. Depending on the mechanism desired and the type of contaminant present, the
operating and monitoring procedures will differ. Regardless of the targeted mechanism, the design must
minimize off-site migration of gases. It is necessary to discuss the steps used to prevent off-site migration,
and specific monitoring requirements in terms of the mechanism they will enhance.
34
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Mas» Transfer Enhancement
Mass transfer systems are characterized by high-vacuum, high-flow wells operations. A high vacuum
provides a large driving force that increases the removal of contaminants. Adequate pressure monitoring
assures net-negative pressure in the subsurface during operations.
Heating the sparging air can enhance mass transfer. The higher air temperature raises the Henry's
Law Constant, thus improving the stripping of contaminants from groundwater and increasing the
volatilization rate of contaminants.
Biodegradatlon Enhancement
The key to enhancing biological activity is adequate oxygenation of the groundwater to maintain an
optimal environment for microorganism growth. However, the addition of nutrients and supplemental carbon
to the subsurface may also be necessary to maintain a healthy microorganism population.
In a successful biodegradation scenario, extracted, sparged gases have relatively low contaminant
concentrations as compared to gases extracted from mass-transfer-enhanced systems. However, it Is still
important to maintain a net-negative subsurface pressure (with vapor extraction wells) to control contaminant
migration. Extracted vapor treatment may still be required.
Monitoring this type of system is similar to that of any in situ biodegradation system. The dissolved
oxygen level in the groundwater determines the effectiveness of oxygen mass transfer. A dissolved oxygen
level of 3 ppm is a good indicator of process performance [Billings and Associates, 1991]. Hydrocarbon
and carbon dioxide levels in the extracted air also monitor the biological process.
Contaminant Migration Minimization
An air sparging system must operate in a manner that will minimize further migration of contaminants.
As previously mentioned, vapors could travel horizontally in the vadose zone and LNAPL plumes could
extend due to mounding effects in the water table during sparging. Increased vapor migration could also
result from the concentration of the contaminant exceeding the equilibrium concentration in the vadose
zone. Untreated soil pores in the unsaturated zone contain air in equilibrium with the contaminated soil.
The contaminant concentration in the untreated soil will register at a relatively high level. SVE replaces the
35
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saturated air with deaner air, as shown in Figure 5; this causes an exponential decline in soil vapor
concentration. If the sparging wells are started too soon, a surge pf contaminated air from the saturated
zone could cause the vapor concentration in the vadose zone to exceed the equilibrium concentration.
Resulting concentration gradients could cause further contaminant migration.
To prevent vapor migration, an SVE system should be in operation prior to start-up of the sparging
wells. Once the vapor concentration has leveled off, the sparging wells should then be activated. The
injection of sir will cause a new concentration peak, which will ultimately level off in an exponential manner,
as shown in Figure 5. The plateau for contaminant concentration in the extracted air of a sparging/
extraction system is regulated by various factors, such as the rates of dissolution and desorptton of
contaminants in the vadose zone, and the rate of dissolution, desorption, and volatilization of contaminants
in the saturated zone. In addition, the rate of vapor migration in the saturated zone vapor phase will
I
influence the concentration of extracted vapor. In order to fully capture the sparged vapors, the extracted
air flow rate should exceed the injected air flow rate.
if properly coordinated, remedial activities at sites containing LNAPLs can minimize migration of the
floating product by implementing a product recovery system prior to sparging, or hydraulically controlling
the depression of the water table. This method, however, adds a need for posttreatmenf of the groundwater
residuals, thus defeating the purpose of an in situ groundwater remediation program.
Adjusting the pressure at which the air sparging wells operate can minimize vapor migration. The
minimum sparge pressure required to overcome water column is 1 psi for every 2.3 ft of hydraulic head
[Brown and Fraxedas, 1991]. To transfer air into the saturated zone, well pressure must remain above this
minimum. However, a pressure too high may move the vapor horizontally, rather than vertically toward the
vadose zone. As shown in Figure 6, this can decrease vapor capture by the extraction system and inhibit
treatment of some saturated zone areas by air sparging.
i
Process Operation
In most cases, the concentration of extracted vapors levels off after the sparging wells have been
operating for a period of time. However, the high costs of treating extracted vapors create a need to extract
less vapor volume at a higher concentration. This can be achieved by pulsing the vacuum and sparging
36
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wells. Shut-down time allows the soil, groundwater, and soil vapor to equilibrate, increasing the vapor
concentration. The system can then restart (vapor extraction wells first, then sparging wells) to pull out the
more highly concentrated soy vapor.
39
-------
SECTION 5
AIR SPARGING SYSTEM COSTS
The published literature on air sparging technology includes little discussion on the costs of designing,
building, and operating a system. However, equipment for air sparging technology is very similar to that
i
used for son vapor extraction, and hence, the posts are comparable. There are 3 major cost elements for
an air sparging system: capital, operating, and monitoring costs.
CAPITAL COSTS
Capital costs for an air sparging system encompass design, engineering, permitting, contingencies,
equipment procurement, installation, and instrumentation. Some components contribute significantly to the
capita] costs of a complete air sparging system:
Wells (extraction, sparging, and monitoring wells) - installation, piping, and trench construction
Mechanical equipment - blowers, compressors, and vacuum pumps
Instrumentation - flow meters, pressure gauges, and analytical equipment for vapor testing
Vapor treatment equipment - includes air/water separator, emissions control (usually activated
carbon devices, or others such as incineration and catalytic oxidation), and water treatment
systems '
In addition to these major components, cost estimates for site remediation must also include funds for
a thorough site Investigation that is required prior to the remedial design.
[
Well Installation
Sparging and extraction wells, which are very similar in design, normally use schedule 40 PVC
(polyvinyl chloride) piping in various diameters (2-in to 12-in). Polypropylene (PP) or chlorinated polyvinyi
chloride (CPVC) pipes are more rigid; they provide an alternative where stronger piping is required. A
40
-------
typical 30-ft well installation will cost from $2,000 to $4,000. Of this cost, materials such as casing (riser),
well screen, plugs, filter pack materials, bentonite,.and cement grout may total from $500 to $2,000 per well,
depending on the method of construction. Table 4 shows the range of costs for various sparging and
extraction well components. PVC piping, for example, costs as little as $2 per linear ft with a 2-in diameter
casing up to $12 with a 6-in casing. Similarly. PVC screens cost from $2 to $15 per linear ft, depending on
diameter. Ball valves (PVC) cost from $60 for a 2-in riser to $300 for a 6-in riser.
Well configuration can achieve savings or add costs to the items described above. For example,
nested wells can cut drilling costs by placing both sparging and extraction wells in the same hole.
Horizontal wells cost several times more than vertical wells, but may increase the VOC extraction efficiency
by a factor of five [Looney, Kaback, and Corey, 1991].
System piping can lie aboveground, or buried in trenches. Aboveground piping can realize savings if
the site is inactive and if barriers to access are acceptable. However, water carried in aboveground piping
may freeze during winter operation, causing operational problems and pipe damage. Pipe freezing problems
may be overcome by applying heat tracing and insulation. This adds a significant cost to the piping
installation. Installation costs will also increase significantly if the piping is buried in trenches.
Mechanical Equipment
Air is sparged into the subsurface saturated zone by mechanical compression equipment. Vacuum
pumps extract the sparged air in addition to the induced air flow that they produce through the vadose zone.
The type of mechanical compression equipment used is a function of the flow rate and pressure required.
An important feature of the equipment employed is that the air injected by the machine be oil-free.
Some of the types of compression equipment that may be employed with the air sparging technology
include:
oil-free rotary screw machines
centrifugal blowers
regenerative and rotary lobe blowers
reciprocating compressors
41
-------
Single-stage oil-free rotary screw compressors are commercially available with flow capacities as low
as 420 scfm, capable of achieving a discharge pressure of 50 psig (Table 4). Rotary lobe machines have
a wide application in soil remediation both as air injection compressors and as vacuum pumps. The rotary
lobe air compressors listed in Table 4 are single-stage units with a discharge pressure of 18 psig. The rotary
lobe vacuum pumps are capable of achieving vacuums of 15" Hg absolute for the flow rates listed.
Regenerative blowers are available and are used as air injection machines for very low pressure applications
(5 psig), as well as in vacuum blower applications.
I
I
Centrifugal blowers and reciprocating compressors are limited in their application. The practical lower
limit of capacity for centrifugal blowers in air injection service is approximately 8000 scfm. Reciprocating
j
compressors would only be employed if pressures higher than 50 psig were required. The reciprocating
machine becomes prohibitively expensive at lower pressures since the cylinders must bo non-lubricated in
order to supply the oil-free air required for injection.
I
Instrumentation and Monitoring
Instruments for monitoring of the process and the extracted vapor stream are vital to air sparging
design and operation. Monitoring equipment should measure the vacuum air flow, vapor characteristics,
and contaminant concentrations.
Vacuum can be measured with a magnehelic gauge. These gauges are typically located at each
extraction well and upstream of the blower. The cost for each magnehelic gauge can range from $50 to
$75. A quick-coupling sampling port may substitute for gauges at each well. Air flow, expressed in standard
cubic feet per minute (scfm) to normalize flow readings taken at different pressures, can be measured in-line
by an annubar flow meter or at flow ports using portable equipment. Air flow should be measured at each
well and upstream of the blower. Annubar flow meters cost about $300. Quick-coupling sampling ports with
two or three connections are available for $25.
Monitoring of the composition and concentrations of the extracted vapors is critical in determining
vapor treatment alternatives and operating procedures. An organic vapor analyzer (OVA), a total
hydrocarbon analyzer (THA) or a combustible gas indicator (CGI) can determine the quantitative vapor
concentration of VOCs. A gas chromatograph (GC) can identify vapor components and concentrations.
42
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TABLE 4.
SVE AND AIR SPARGING SYSTEM COMPONENTS
CAPITAL COSTS
Component
Extraction well
construction
Casing
Screen
Sand pack
Gravel pack
Piping
Valves (ball)
Joints (elbow)
Surface seals
Air compressor
Vacuum pump
Typ.
PVC
PVC
PP
PVC
CPVC
PVC
Single union
PVC
90 degrees - slip
Bentonrte 6 in
Bentonrte 4 in
Polyethylene 10 mil
HOPE 40 mil
asphalt 2 in
Single stage
Rotary screw
Rotary lobe
Rotary lobe
Size
2 in
4 in
6 in
2 in
4 in
6 in
2 in
4 in
6 in
2 In
4 in
6 in
2 in
4 in
6 in
2 in
4 in
6 in
2 in
4 in
6 in
450 scfm (75 HP)
11 20 scfm (200 HP)
2000 scfm (350 HP)
100 scfm (15 HP)
450 sefm (75 HP)
1000 scfm (125 HP)
2000 scfm (250 HP)
100 scfm (5 HP)
450 scfm (25 HP)
1000 scfm (50 HP)
2000 scfm (125 HP)
Capital
costs
($)
12-15/ft
2-3/ft
3-5/ft
7-12/ft
2-4/ft
5-7/ft
10-1 5/ft
15-20/Cuft
0.74/cu ft
2.10/ft
5.60/ft
10.00/ft
0.4/ft
1.10/ft
2/ft
2.50/ft
6.70/ft
12/ft
65
300
700
3
16
51
0.37/sq ft
0.25/sq ft
0.25/sq ft
0.56/sq ft
1.03/sqft
60000
80000
90000
3000
10000
30000
33000
3000
6500
9500
20000
Notes
Matthews Manufacturing
SCH. 40 PVC
Matthews Manufacturing
SCH. 40 PVC
Any slot size
SCH. 40 PVC
SCH. 80 PVC
Vendor - M&T Plastics
SCH. 40 PVC, 2 in & 4 in
threaded socket, 6 in
M&T Plastics, SCH. 40
PVC, threaded, socket
Vendor - Atlas Copco
Vendor - Roots Dresser
Vendor - Roots Dresser
43
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TABLE: 4. (Continued)
Component
Alr/wator separator
Instrumentation
Vacuum gauge
(magnehellc)
Row (annubar)
Sampling port
Concrete pad
Flame arrester
Air relief valve
Soli gas probe
Engines ring/design
Diffuser stack*
Typ*
Knockout pots
Brass T
w/o SS element
w/SS element
Carbon steel
Stainless steel
Size
20 to 800 gal
800 gal
20 gal
35 gal
; 65 gal
105 gal
130 gal
I
;
4 in
i 6 In
4 in
6 In
Capital
coats
($)
1,500-2,400
11,600
1,470
1,560
1,750
2,150
2,350
50-75
300
20-30
450/yd3
665
735-930
225
30-50
8-15% of system
cost
8/ft
10/ft
30/ft
40/ft
Notes
Vendor - Water Resources
Assoc., installation 33% of
capita! costs
Vendor - Stafford Tech.
Vendor - Stafford Tech.
Vendor - K.V. Assoc.
Add 40% for installation
44
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Analysis of the vapor CO2 concentration can track the subsurface biological activity. Monitoring of the vapor
composition usually occurs between the demister (or knockout pot) and the blower. In carbon adsorption
systems, monitoring may also check the exhaust from the carbon bed.
Vapor Treatment
Air/Water Separator -
Air/water separators ('knockout pots') decrease the velocity of the vapor stream and allow the gravity
fallout of water droplets and sediment. They can be very simple (e.g., a 55-gallon drum) or may be
sophisticated in terms of level controls and other instrumentation. The size depends on the flow rate (to
reach a minimum residence time), ranging from 800 to 1,200 gal. Construction materials vary, including cast
iron, stainless steel, or similar material. Demisters are often incorporated into the vapor pretreatment
process. These screens remove particles down to microns in size by coalescing droplets on the demister
material.
Duvall Industries, Inc. manufactures a variable-sized demister ranging in cost from $700 to $1,000 for
flow volumes of 100 to 1,000 scfm. Water Resources Associates, Inc. manufactures knockout pots for use
with their incineratton/SVE systems. The cost for knockout pots may range from $1,500 to $2,500,
according to size and flow rate capabilities.
Liquids that accumulate in the air/water separator must be treated on-sfte, disposed off-site (according
to regulations, possibly to a sewer line), or removed by truck. On-site water treatment can employ liquid
phase granular activated carbon (GAG). Small, easily installed carbon units are appropriate for the small
flows expected from vapor pretreatment units.
Emissions Control -
Vapors removed from the subsurface normally require treatment prior to release to the atmosphere,
depending on local regulations. Several options are available: carbon adsorption, catalytic oxidation,
thermal incineration, combination systems, and internal combustion engines. Where vapor treatment is not
required, drffuser stacks can provide safe emission of the extracted vapors. Vapor phase concentration will
determine which options are appropriate.
45
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Vapor treatment can comprise a significant portion of the total air sparging system cost. Care must
be taken to ensure that the most cost-effective option is used, based on the vapor discharge standards, the
extracted vapor concentration, the expected mass removal over the life of the system, and several other
variables. The operating costs for vapor treatment may dominate the system cost, especially for GAG
systems. For this reason, the forecast of expected removal rate becomes even more important.
Carbon Adsorption
Carbon adsorption is widely used for vapor treatment in industrial and air sparging settings. It applies
to a variety of vapor contaminants and can achieve very high removal rates. Carbon is only economical for
relatively low mass removal rates; high mass [removal rates make the cost of replacing/ regenerating the
carbon prohibitive. In addition, the heat of adsorption may present an explosion hazard in the treatment
of combustible VOCs.
i
Numerous vendors offer cartoon adsorption systems in a large variety of sizes. Table 5 shows a partial
list of these vendors and their respective products. These systems range from very small systems '(55-gallon
drums holding less than 200 Ibs) through larger, skid-mounted systems (up to 5,700 Ibs). For very large
Installations, vendors can customize carbon to the specific requirements of the site. Carbitrol offers G-1 ,G-2,
G-3, and G-5 canisters that are rated for various air flows. These drum systems contain 200,170,140, and
2,000 Ibs of activated carbon, respectively. The G-1 system, rated at 100 scfm, costs $695; the G-2 (300
scfm), $985; and the G-3 (500 scfm), $985. The G-5 system which is rated for 600 scfm is available with a
304 stainless steel (SS) vessel for $11,000 or an epoxy-lined carbon steel vessel for $7,700. TIGG
Corporation offers the Nixtox Series N500 DB, N750 DB, and N1500 DB (deep bed) systems that contain
1,900, 3,200, and 5,700 Ibs of virgin carbon, respectively. Calgon Carbon Corporation also offers a large
variety of carbon adsorbers. The Ventsorb canister can handle average flows up to 100 cfm or high flows
from 400 to 11,000 cfm. The high-flow models available skid-mounted with a fan, flexible connectors, and
a damper. The canisters range in price from $760 to $6,330; the skid-mounted models cost from $5,400
to $10,700. [
The carbon may be virgin or reactivated, j Purchase of reactivated carbon usually saves three to thirteen
percent off the price of virgin carbon. For example, the virgin G-1 (200 Ib) canisters offered by Carbitrol sell
for $660; a reactivated canister sells for $640. Larger containers are usually charged on a weight basis.
Environtrol reactivated carbon sells for $1.15 per Ib plus transportation costs. A one-time RCRA Toxic
Characteristics Leaching Procedure (TCLP) tjest is required ($2,800 to $3,000) for hazardous materials.
46
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48
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A recycling carbon system is an alternative to the replacement of canisters and off-site reactivation.
Such systems regenerate the carbon in place, usually using steam -to desorb the contaminants. The
contaminant/steam mixture is then drawn off and treated or sent for proper disposal. Continental Recovery
System Inc. offers this type of system; it comes in several sizes, using from one to six carbon beds.
Manually-operated systems cost from $20,000 (one bed) to $50,000 (six beds). A fully automated,
\
remotely-monitored, trailer-mounted system sells for $150,000 or leases for $7,400 per month on a 6-mbnth
lease. The cost effectiveness of the system depends on the mass removal rate. The system initially costs
more than non-regenerative systems, but reduced carbon usage may make it a cheaper option on a long-
term basis.
Use of carbon for vapor treatment may develop a need for a heat exchanging unit to cool extracted
vapors heated by compression from the blower. This treatment will ensure maximum contaminant uptake.
Alternatively, GAG can be placed upstream of the blower in a treatment train.
Incineration -
Incineration of contaminant vapors offers an excellent treatment option for high vapor concentrations.
At temperatures of 1,000 to 1,400°F or higher, vapor combustion destroys over 95 percent of the
contaminant concentration.
Fuel supplements may be required to maintain the requisite temperatures for adequate removal. The
amount of supplementary fuel depends on the vapor concentration. Some vendors report that, at gasoline
concentrations above 12,000 ppm, the flame is self-sustaining; at concentrations below this figure, greater
amounts of fuel are needed in proportion to the contaminant. The operating cost of an incineration system
is greatly affected by the need for supplementary fuel. Propane, which costs about $1.00 per gal, is often
used for this purpose.
While higher contaminant concentrations make this method cheaper, safety concerns increase with
higher concentrations. Highly volatile contaminants (such as gasoline) become explosive in certain
concentrations. This range is limited by the lower explosive limit (LEL) and the upper explosive limit (UEL).
Fresh air must be mixed with the extracted vapors at very high concentrations to reduce the concentration
to a safe level.
49
-------
Table 5 shows the cost for various incineration units. These prepackaged units include the burner,
blowers, sampling valves, and other appurtenances. Capital costs depend on the flow rate to be treated;
they range from $23,000 (for 100 scfm) to $4oiooO (570 scfm) from one vendor. A smaller unit (70 scfrn)
costs $12,000. A heat recovery system, which Uses the exhaust to preheat the incoming vapors, can realize
a substantial energy and cost savings.
Catalytic Oxidation -
i
Catalytic oxidation systems employ a catalyst to facilitate the oxidation of the contaminants. Thus, they
operate at much lower temperatures (600 to 800°F) than direct incineration while achieving destruction and
removal efficiencies (DREs) above 85 percent. The catalyst Is a precious metal formulation (typically
platinum or palladium), which can exist either
in the form of beads or a honeycomb bed.
Although most commonly applied to petroleum contamination, special catalysts enable catalytic
oxidation to treat chlorinated contaminant vapors. However, hydrochloric acid, formed during the oxidation,
requires additional treatment processes (scrubbers, neutralization, etc.).
I
Catalytic oxidation requires careful monitpring to prevent overheating and destruction of the catalyst.
If the concentration of vapors in the extracted air exceeds 3,000 ppm, the vapor stream must be diluted with
fresh air to remain below the cutoff level. At lower concentrations, supplemental fuel (propane) may be
needed to maintain the required temperatures. Safety is also a concern for catalytic oxidation. This method
Is best suited for concentrations below ten percent of the LEL
I
f
Available catalytic oxidation units can handle flows from 30 scfm to more than 50,000 scfm. Hasstech
offers a trailer-mounted unit (MCC-2) that can handle 30 to 40 scfm. ORS offers the Catalytic Scavenger
In a 20 kw model (200 scfm) and 35 kw model (500 scfm) that sell for $60,000 and $75,000, respectively.
Installation and training will cost $3.000 for these units. CSM Systems, Inc. produces the Torvex series
Mode! 5A, 5B (500 scfm) and Model 10B (1,000 scfm) that sell for $50,000 and $70,000, respectively. A
I
trailer ($8,500) and ADS dilution system ($20,000) are available for these models. Larger catalytic oxidation
systems are also available from CSM and Dedert Corporation. Dedert sells field- ready units, rated at 5,000
scfm, for $200,000.
50
-------
Diffuser Stacks -
Diffuser stacks, constructed of either carbon steel or stainless steel, merely direct vapors into the
atmosphere. This system is simple and inexpensive, but only an option where treatment of the vapors is
not required. The design of dtffuser stacks should minimize health risks. Costs depend on the height
required and the material of construction.
Other Costs
Implementation of an air sparging system will entail other costs that are neither strictly capital costs or
O&M costs. These include system design, engineering, permit acquisition, contingencies and other
miscellaneous costs. These costs are often treated as capital costs. Engineering and design fees often
comprise 10 to 15 percent of the system cost, as do contingencies. These and other costs are highly site-
specific, however, the figures quoted here are arbitrary.
OPERATION AND MONITORING COSTS
Operation and monitoring costs, depending on the duration of system operation, may comprise a
significant portion of the overall air sparging remediation cost. These costs arise mainly from power for the
blowers; vapor treatment, including fuel costs for incineration methods and GAG regeneration/replacement;
monitoring and analyses for progress and cleanup attainment determination; and other on-going costs such
as labor. Labor costs depend on whether the system is operated manually or by a microprocessor. These
costs are discussed later.
Power Requirements
The cost of electric power depends on the power rating of the fan/s or tdower/s, the hours of
operation, and the local cost of electricity. The following formula determines the cost:
(0.75) x (fan horsepower) x (electricity cost in $/kw-hr) x (hours of operation)
For example, a 10-hp blower operated continuously would use electricity at $0.10/kw-hr. The daily cost
for power would be 10 x 0.75 x $0.10 x 24 = $18.00 per day. Pulsed operation - operating the blowers
51
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Intermittently - would save power costs by decreasing the hours of operation. Power may also be required
for heat exchangers.
Vaoor Treatment
The operating cost for vapor treatment depends on the method used, then concentration of
contaminants, and the flow rate. Generally, GAC adsorption costs increase, while the cost for incineration
and oxidation decreases with higher vapor concentrations. GAC treatment costs will be dominated by
carbon replacement and regeneration; incineration and oxidation treatment will be dominated by fuel costs
to sustain Incineration.
Carbon Adsorption
I
Adsorption of contaminants from the vapor phase concentrates the contaminants onto the carbon.
When the carbon's capacity to hold contaminants has been exceeded, the carbon is considered "spent" and
must be replaced or regenerated. Obviously, higher mass removal rates (flow rate x concentration) will
result In more frequent carbon replacement and higher costs.
i
Carbon costs vary according to the type and quantity ordered. They may range up to $2.00/lb.
Regenerated carbon costs 87 to 97 percent of virgin carbon cost. One vendor quoted $1.15/lb for large
orders. Table 5 shows costs for virgin carbon units. One rule of thumb states that carbon costs about
$20/lb ($130/gal) of gasoline removed [Hinchee et al., 1987].
Where carbon is used and mass removal rates are high, on-site regeneration may become economical.
Continental Recovery Systems offers a unit that uses steam to regenerate carbon in place. Other vendors
offer units that regenerate the carbon and then incinerate the contaminants. These combination units are
Initially more costly, but save on O&M costs. The determination of the most cost-effective option is site-
specific; the pilot system results normally make the determination.
Incineration -
Incineration requires supplementary fuel (typically propane or LPG) for vapor concentrations below
12,000 pprn. This fuel costs about $1.00 per gallon. When the BTU value of the vapor feed cannot sustain
the required temperature (about 1,400 to 1,600°F), fuel supplements must maintain proper temperatures.
52
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Catalytic Oxidation -
This method requires much lower temperatures (600 to 800°F) than incineration; and, it is therefore less
costly to operate. Optimal vapor phase concentration for catalytic oxidation is about 3,000 ppm. Higher
concentrations require dilution (to protect the catalyst from destruction), while lower concentrations may
require supplemental fuel. ORS quotes the cost of a 200 scfm Catalytic Scavenger at about $800/mo to
operate with no incoming hydrocarbons (i.e., just air). As the hydrocarbon concentration increases, the
supplemental fuel requirements decrease.
SYSTEM MONITORING
For air sparging to gain wide acceptance wfth regulatory agency personnel, consultants, and site
owners, methods to confirm the system's success are required. Monitoring ensures that the air sparging
system does not move contaminants away from the treatment zone, especially off-site. Several techniques
have been used for these purposes.
The simplest method to assess effectiveness of an air sparging system, used by virtually all proponents
identified in this project, monitors the extracted vapor stream for VOCs, O^CO* or other contaminants of
concern. Another method analyzes and monitors dissolved oxygen (DO) in groundwater throughout the
treatment zone. Groundwater concentrations in monitoring wells are measured before, during, and after air
sparging to determine the actual effect on in situ contaminant levels, which are usually how the regulated
endpoints are expressed (concentration of BTEX, TPH, or other parameter remaining in groundwater or soil).
Downgradient wells can check whether the system is mobilizing contaminants. In most published case
studies, both monitoring techniques, vapor sampling and groundwater sampling, have been used.
Monitoring and Analyses
Laboratory sampling for soil, groundwater, and vapor contaminant concentrations is relatively costly,
but necessary to assess the effectiveness of the remediation. A comprehensive sampling and analytical plan
using recognized and accepted methodologies is very important. Soil sample analyses will generally cost
$150 for total petroleum hydrocarbons (TPH), $250 for volatile organic contaminants (VOCs), $100 for
benzene, toluene, ethyl benzene, and xylenes (BTEX), $450 for acid/base neutral extractable compounds
(ABNs), and $70 for routine soil parameters, which include organic carbon and partide size distribution.
53
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Analyses for groundwater sampling cost $125 (TPH), $225 (VOCs), $100 (BTEX), $425 (ABNs), and $50
for general groundwater quality parameters; respectively. Soil gas analysis using a GC determines total
hydrocarbons and other specific contaminants; it may cost as much as $250 at a laboratory.
i
Biological assay tests can monitor biological activity in the soil. Dissolved oxygen in groundwater
should be measured on-site with a D.O. probe, which costs about $1,000.
i
CONCEPTUAL ESTIMATE FOR AN SVE AND AIR SPARGING INSTALLATION
i
i
Following is a conceptual estimate for a leaking underground storage tank site remediation using the
air sparging technology. The site is contaminated In both the saturated and unsaturated zones by gasoline.
The equipment that will be included for site remediation will be sufficient to act on a total of up to 10,000
cubic yards of contaminated soil. The depth to the water table is assumed to be 60 feet.
i . ' . -- ' ' '
The capital costs are based on a configuration that includes two (2) vapor extraction wells, one (1) air
Injection well, and four (4) groundwater monitoring wells. The system also consists of a 25 HP rotary lobe
vacuum pump, a 15 HP rotary lobe air injection compressor, two (2) air/water separators, a collection
header and various piping connections. An off-gas emissions control system will be required to capture the
BTEX hydrocarbon compounds, this will consist of canisters filled with granular activated carbon adsorbent.
The size of the site dictates that on-site regeneration of the carbon will not be practical. The cost of carbon
wHI be based on regeneration or reactivation off-site. The canisters containing the carbon will be rented from
the supplier, so that the costs for the emissions control system will appear as an operations and
maintenance cost. ;
i
Table 6 contains the equipment specifications required for the site remediation, Table 7 outlines the
capital costs of the equipment items, and Table 8 contains a summary of the annual operating and
maintenance costs. I
54
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TABLE 6. EQUIPMENT SPECIFICATIONS
Vacuum Blower
Size
Rating
Electrical
Compression ratio
Type
25 HP
500 scfm @ 10" Hg vac
440 V, 3 phase
1.52
Straight lobe rotary (positive displacement), constant volume - variable
discharge pressure
B.
Air Compressor
Size
Rating
Electrical
Type
15 HP
160 scfm, disch. press. 15 psig
440V, 3phase
Rotary lobe, positive displacement V-bett dirve with inlet filter, inlet silencer
and discharge silencer
C. Air/Water Separators
Size
Type
Accessories
800 gallons
Stainless steel
Sight glass
2-4" NPT connections (top)
1-4' NPT connection (bottom sealed to atmosphere)
D.
Piping Network
Type
Length
Elbows
Caps
Valves (2")
Reducers
Type
Length
4-PVC
500ft
20
5
6
10
2" PVC
70ft
Vacuum Well Construction
Type
No. of wells; Screen
3 10'
3 15'
Hole size
Casing
Rotary auger
Depth
20'
60' (to water table)
6"
4"
F. Air Sparging Well Construction
Type
No. of wells
Depth
Hole size
Casing size
Air line
Rotary auger
One
60'
6"
4"
2" PVC. well complete with bottom cap, bentonite seal and inflatable packer
G. Valve Boxes (4)
Type
Size
Additional features
Below grade/cast iron construction
2'x2'x 1'
Gravel packed bottom
H. Trench Construction
Type
Depth
Layout
Length
Cover
Cut and cover
1 foot below grade
4" PVC pipe
50 feet
Concrete
55
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TABLET. CAPITAL COSTS
||*m/ti«*ctlption
1. WELLS
Air sparging wall
Extraction walla
Monitoring walls
VaJv* boxM
SUBTOTAL
2. EQUIPMENT
Air compresaor
Vacuum blower
Separator*
Blower housing
SUBTOTAL
3. MECHANICAL/PIPING
Wellhead pits (4)
W*!l plpo & fittings
Pips
Valvas & fittings
letting
SUBTOTAL
4. ELECTRICAL/INSTRUMENTS
Bee. & Instr. walla
Else. & Instr. - equip.
Hoc. distribution
Main control panel
SUBTOTAL
TOTAL
Install/labor
coat<$)
2,000
4,000
3,000
1.500
$10,500
1,500
2,500
11,600
J.K2
$18,100
2,000
3,000
1 5,500
1,500
_§QQ
$12,300
1,000
2,500
2,000
1.000
$6,500
$47,900
Equlp./nurtL
eo*t<$)
1,000
1,600
1,900
1.000
$5,500
3,000
9.500
23,200
5.000
$41,700
1,200
1,500
4,000
2,100
500
$14,700
1,500
3,000
4,000
2.000
$10,500
$72,400
Total
coat($)
3,000
5,600
4,900
2.500
$16,000
4,500
12,000
34,800
7.500
$59,800
3,200
4,500
9,500
3,600
1.000
$27,500
2,500
5,500
6,000
3.000
$17,000
$120.300
56
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TABLE 8. OPERATION AND MAINTENANCE COSTS
Power
Off-gas emissions control'
Maintenance
Monitoring2
Labor
Contingency
TOTAL
Annual costs
8,000
120,000
5,000
34,000
15,000
10,000
$192,000
1 Assumes an average usage of 2,000 Ib per month of granular activated carbon. The price includes
transportation and off-site regeneration.
2 Assumes twice a month evaluation of extraction well concentrations with a portable GC.
57
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SECTION 6
RESEARCH NEEDS
Air sparging, in combination with soU vapor extraction, promises to be a cost-effective, relatively simple
technology for remediation of volatile organic contaminants in the saturated zone. The recent advent of
this technology suggests the need for additional theoretical evaluation of the design of air sparging systems.
A review of available literature on air sparging technology indicated that the technology, through a topic of
research, employs systems that are designed according to the results of pilot studies or empirical data.
i
An understanding of the process, and of the important design parameters that go into the development
of a predictive mathematical model, are essential prior to field implementation. Several attributes,
mechanisms, and phenomena (such as dissolution, partitioning, etc.) related to air sparging require further
research. For example, although it is dejar that mass transfer plays the most important role in the
remediation of chlorinated VOCs, the role of biodegradation during air sparging of petroleum-contaminated
aquifers has not yet been fully demonstrated.
SATURATED ZONE VAPOR PHASE
The nature of the saturated zone vapor phase requires further definition. Conflicting opinions state that
the air passing through the saturated zone travels in the form of bubbles or in a continuous phase passing
through pathways in the soil, or in some other form.
.
Clearly, the transfer of oxygen to the saturated zone is key to bioremediation during air sparging. The
transfer of contaminants from soil and water to the vapor phase is also important for removal of
contaminants. If these transfer mechanisms can become effective, the rate of contaminant removal would
increase significantly. For example, an increase in surface area between the vapor phase and the soils and
groundwater would increase the rate of mass transfer.
58
-------
Subsurface air injection requires additional study:
What is the optimal well screen size for air injection?
Does the injection of air in the form of microbubbles significantly improve the mass transfer?
What is the correlation between soil permeability, aquifer depth, and optimal injection pressure?
How much of the injected air is recovered in the SVE system, and what is the fate of the
unrecovered air?
SYSTEM DESIGN AND INSTALLATION
Air sparging systems have used various well configurations and designs. Depending on the type of
contaminants, location within the aquifer, and plume shape, some systems are more effective than others.
Additional research should address the following issues:
What is the optimal ratio of sparging to extraction wells?
Should the system be designed differently to enhance biodegradation as opposed to enhancement
of mass transfer of contaminants?
OPERATING CONDITIONS OF SYSTEM
Analyses of soil venting systems indicate the system is most cost effective during intermittent
operations. This allows the soil to equilibrate with the soil vapor so that more contaminants can be removed
with lower energy costs. Certainly, if a site remediation is to operate for several years, pulsing the blower
operation can achieve a significant cost savings. Similarly, pulsed operation of an air sparging system may
save energy. Several questions remain unanswered regarding this mode of operation:
What is the optimal interval for operating the vacuum blowers and air injection equipment?
Can the blowers and air injection equipment be pulsed simultaneously, or should they be pulsed
at different intervals (i.e. operating the vacuum blowers longer than the air injection equipment) to
prevent vapor migration to uncontaminated areas?
What are the optimal injection and vacuum pressures?
59
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RESEARCH METHODS
Many questions remain unanswered regarding air sparging technology. Various phenomena, such as
air transport, can be studied on the bench scale. However, since air sparging is an in situ system, various
operating conditions, such as pulsed operation and system pressures, must be analyzed in an actual field
environment '
60
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63
&U.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-003/60059
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