'A
SO/UST-88-001
vvEPA
United States
Environmental Protection
Agency
Office of
Underground Storage Tanks
Washington D.C. 20460
EPA/530/UST-88/001
April 88
Cleanup of Releases
from Petroleum USTs:
Selected Technologies
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EpA/530/USt-*8/001
Cleanup of Releases
From Petroleum USTs:
Selected Technologies
OFFICE OF UNDERGROUND STORAGE TANKS
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, DC 20460
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Preface
Cleaning up a gasoline release from an underground storage tank (LIST) normally requires short-term
emergency measures as well as long-term corrective actions. Short-term emergency measures are the
immediate steps taken to abate imminent safety and health hazards. This handbook focuses on long-term
remediation and site restorations. It has been assumed that emergency measures have already been
taken to remove any immediate explosion or health threat and that the task at hand is to clean up the
gasoline that has leaked from the tank and moved into the environment.
The objective of this handbook is to provide engineering-related information on costs, efficiencies, and lim-
itations of corrective action technologies aimed at removing petroleum-related contaminants (principally
associated with gasoline) from the subsurface. While many technologies exist for the cleaning of soil, air,
and water, only a limited number possess demonstrated performance records and have progressed to
full-scale applications. This handbook concentrates on those technologies that have been widely applied.
This is not a design manual and should not be used as such. Sufficient detail on corrective action
technologies is provided so that state and local government personnel can adequately evaluate corrective
action methods and plans proposed by responsible parties and their consultants. Although this report will
help everyone in understanding the basics of corrective action technologies, some of the information may
be more helpful if the reader has training or experience in chemistry or engineering.
To compare and contrast the various corrective action techniques, each proven technology is evaluated
on the basis of several important criteria:
Effectiveness. How effective is the technology in removing contaminants?
Cost. What are the capital and operation and maintenance (O&M) costs of the technologies? What
are the projected service lives? How does cost vary with time and removal efficiency? (Costs are
reported in this document in 1986 dollars.)
Reliability. How consistently can the technologies remove the contaminants of concern and over how
long a period of time?
Ease of Operation. How complex is the technology? Are specially trained personnel required for
O&M activities?
Limitations. What factors might reduce the effectiveness or reliability of a technology or limit its
applicability in a given situation?
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Contents
Preface
Figures
Tables
Acknowledgments
Summary of Findings
Section 1
Section 2
Section 3
Introduction
Objective of This Handbook
Limitations of This Handbook
Other Studies
Organization of This Handbook
Fate and Transport of Gasoline in the Subsurface
Factors Affecting Transport
Multiphase Movement of Gasoline
Vapor Transport in the Unsaturated Zone
Gasoline Degradation in the Subsurface
Summary
References
Recovering Free Product
Methods of Gasoline Plume Containment
Gasoline Recovery Equipment
Disposal of Recovered Gasoline and Contaminated Water
Conclusions
Case Studies
v
vi
vii
viii
1
1
2
2
3
6
8
12
12
13
13
15
15
18
28
28
30
Section 4
Section 5
Section 6
Section 7
Gasoline Removal From Soils Above the Water Table 35
Excavation and Disposal 35
Incineration 42
Venting 42
Soil Washing/Extraction 45
Microbial Degradation 47
Summary 51
References 53
Removing Gasoline Dissolved in Groundwater 57
Air Stripping 57
Activated Carbon Adsorption 74
Using Air Stripping and Granular Activated Carbon in Combination 86
Case Studies 88
Biorestoration 87
References 94
Point-of-Entry Treatment and Alternative Water Supplies 99
Point-of-Entry Treatment 99
Extension of Existing Water Distribution System 104
References 108
Index 109
IV
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Figures
Number Page
1 Schematic of the subsurface environment and four phases of contamination 5
2 Contaminant transport through unsaturated homogeneous and heterogeneous soils 8
3 Schematic of contaminant plumes showing methods by which groundwater can
be contaminated 11
4 Contaminating effect on soil caused by fluctuating water table 11
5 The trench method of recovering free product 15
6 Using overlapping cones of influence to contain gasoline plume 17
7 Single pump and dual pump gasoline recovery systems 17
8 Low temperature thermal stripping pilot system 38
9 Costs of low temperature thermal stripping pilot plant unit 41
10 Costs of low temperature thermal stripping unit 41
11 Vacuum extraction system 42
12 Soil flushing system 46
13 Countercurrent extractor process flow diagram 46
14 Schematic of a typical diffused aerator 57
15 Schematic diagram of redwood slatted tray aerator 58
16 Schematic diagram of packed tower aerator 59
17 Ranges for feasible aeration alternatives for the removal of volatile compounds 59
18 Differential element for an air stripping tower 60
19 A comparison of stripping rates for TCE and gasoline compounds 61
20 Temperature dependence of Henry's law constant 61
21 Generalized pressure drop curve for packings (English units) 64
22 Generalized pressure drop curve for packings (metric units) 64
23 Total cleanup costs as a function of residual aquifer concentration 68
24 Capital costs for packed tower (based on size) 69
25 Capital costs for clearwell 69
26 Capital costs for water pump 69
27 Capital costs for air blower (based on pressure drop) 69
28 Operating costs for pump (based on packing depth) 70
29 Operating costs for blower (based on pressure drop) 70
30 Representative volatile organic compound discharge rates 73
31 Mass transfer of solute from liquid to carbon particle 75
32 Idealized diagram of internal pore structure of GAC 75
33 Freundlich isotherm for benzene 76
34 Idealized diagram of zones within GAC reactor 78
35 Breakthrough and exhaustion in an operating GAC reactor 78
36 Idealized single-solute breakthrough curve 78
37 Schematic diagram of multistage GAC contactors 79
38 Displacement from GAC of dimethylphenol (DMP) by more strongly adsorbable dichlorophenol (DCP) 81
39 Mean adsorption capacities of various compounds in gasoline 82
40 Capital costs of low capacity package GAC contactor 83
41 Capital costs of pressure GAC contactor 83
42 Capital costs of gravity steel GAC contactor 83
43 Capital costs of gravity concrete GAC contactor 83
44 Effect of air stripping as a pretreatment to GAC 86
45 Flow diagram of biocraft biorestoration 91
46 Schematic of Oxitron® process 92
47 Schematic of Mars™ process 92
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Tables
Number Page
1 Fate and transport characteristics of toxic gasoline components 7
2 Organic carbon partition coefficients (Koc) for gasoline compounds 9
3 Well drilling costs 21
4 Shallow well (water table =£ 20 feet) product recovery equipment 22
5 Shallow well (water table =s 20 feet) water table depression equipment 22
6 Deep well (water table > 20 feet) water table depression equipment 23
7 Deep well (water table > 20 feet) product recovery equipment 24
8 Trench product recovery equipment 25
9 Advantages and disadvantages of dual pump systems and oil/water separator systems 30
10 Cost summary for case studies 33
11 Gasoline retention at residual saturation 36
12 Optimal operation conditions for McKin pilot study configuration 40
13 Estimated volumes to renovate hydrocarbon residually saturated soils 50
14 Soil corrective action summary evaluation 52
15 Physical characteristics of common packing materials 72
16 Relative cost factors for treatment of groundwater 74
17 Carbon adsorption capacities for selected compounds 77
18 Microbial degradation screening test results 89
19 Three cases of carbon usage 100
20 Carbon adsorption point-of-entry treatment system costs 103
21 Cost of proposed Camp Dresser & McKee project in Rhode Island, 1984 103
22 Cost of Culligan Inc. project in Rhode Island, 1986 103
23 Cost of Hall and Mumford Project in Wisconsin, 1987 103
24 Water treatment equipment commonly used in carbon adsorption systems 104
25 Cost breakdown per linear foot for water distribution and transmission mains 105
26 Cost breakdown for booster pump stations 106
27 Capital costs of water distribution extension for a community of 10 homes 106
28 Capital costs of water distribution extension for a community of 40 homes 107
29 Capital costs of water distribution extension for a community of 250 homes 107
VI
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Acknowledgments
This report was prepared under the direction of Michael R. Kalinoski and Richard A. Valentinetti of the
EPA Office of Underground Storage Tanks on EPA Contract No. 68-01-7053 with Camp Dresser & McKee
Inc. (COM).
The COM project director was Guillermo J. Vicens, and the project manager and principal author was
David C. Noonan. Portions of the report were written by James T. Curtis, John L. Durant, Tom A. Peder-
sen, Joanne S. Barker, William K. Glynn, Mary Tabak, and Andrea E. Sewall. Technical review was con-
ducted by Steven J. Medlar and Stewart L. Abrams. Elizabeth G. Schultz and Linda M. O'Brien prepared
the original manuscript, and A. Russell Briggs and Lori Hoffer prepared the original graphics.
Additional technical review and comments were provided by Gerald W. Phillips of EPA Region V and the
following State officials: Thomas S. Suozzo (New York), Mary Jean Yon (Florida), Gary Blackburn (Kan-
sas), Anne P Couture (Michigan), and Tom Crosby (Delaware). Comments were also provided by S.
Robert Cochrane of PEI Associates Inc., Richard L. Stanford of PRC Engineering Inc., and H. Kendall Wil-
cox of Midwest Research Institute.
The final document was prepared under EPA Contract No. 68-01-7383 with Midwest Research Institute.
Doris Nagel and Erika Drinkwine provided editorial review, with assistance from Harold Orel. Erika
Drinkwine prepared the final manuscript.
VII
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Cleanup of Releases From Petroleum USTs:
Selected Technologies
Summary of Findings
Only a limited number of technologies to clean soil,
air, and water of the contaminants principally as-
sociated with gasoline are available that (a) have
demonstrated performance records and (b) have
progressed to full-scale application. This Summary
reviews these technologies in terms of their re-
moval efficiencies, limitations, and costs.
Recovery of Free Product From
Water Table
The two technologies most commonly used to limit
the migration of floating gasoline across the water
table are the trench method and the pumping well
method. A variety of equipment can be used to re-
cover the free product. Typically, skimmers, filter
separators, and oil/water separators are used in
trench recovery, and single- and dual-pump sys-
tems are used with the pumping wells.
Containment of Free Product
The trench method is most effective when the
water table is no deeper than 10 to 15 ft below the
ground surface. Excavation of the trench is easy to
undertake, and with this method the entire leading
edge of the gasoline plume can be captured. How-
ever, the trench method does not reverse ground-
water flow, so it may not be appropriate when a
potable well supply is immediately threatened.
Cost: about $100/yd5 of soil excavated.
A pumping well system is normally used for deep
spills, when water table depth exceeds 20 ft below
the ground surface. The direction of groundwater
flow can be reversed with this system. Cost: $100
to $200/ft for 4-in to 10-in gravel-packed galvanized
steel wells. This cost includes engineering and
labor.
Recovery Equipment
Skimmers, filter separators, surface-mounted prod-
uct recovery pumps, aboveground oil/water
separators, and dual pump systems can all be
used to separate gasoline from groundwaters. Dual
pump systems and oil/water separators are typi-
cally used for deeper releases. Skimmers can
achieve up to 99 percent recovery of all hydrocar-
bons floating on the water surface. Cost: $6,000 to
$7,000; the addition of a water table depression
pump to expedite gasoline flow can increase capi-
tal cost of skimming system to $12,000 to $13,000
(approximately doubles the cost). Filter separators
can reduce spill thickness to a sheen. For addi-
tional recovery, the top layer of the gasoline-water
mixture must be removed from the well and treated
aboveground. Filter separators can only be used to
recover spills 20 ft or less below ground surface,
and only with surface-mounted pumps. (Submersi-
ble pumps would cause the floating separator to
sink.) Cost: $6,000 to $7,000; the addition of a
water table depression pump to increase gasoline
flow can increase capital costs of separation sys-
tem to $12,000 to $13,000 (about double).
Aboveground oil/water separators are large tanks
into which the recovered gasoline-water mixture is
pumped and allowed to separate. Tanks range in
size from 1,000-gal units to 10,000-gal units. To
achieve the necessary retention time, separators
must be sized at least 10 times larger than the
groundwater extraction rate. Cost: $6,000 (1,000
gal) to $16,000 (10,000 gal).
Dual pump systems can remove up to 99 percent
of free floating product. The most commonly used
gasoline/water separation units, these systems
consist of a water table depression pump and a
product recovery pump. The depression pump
creates cones of influence that allow gasoline to
accumulate; the product recovery pump, which is
equipped with gasoline sensors, brings only the
gasoline to the ground surface. Cost: $12,000 to
$14,000 for the two types of pumps. Because at
least two pumps are required, operation and main-
tenance (O&M) costs are higher than with other
methods.
Case Studies
Case studies of groundwater contamination have
led to three conclusions:
• Cost of recovering free product at a site depends
more on the recovery method and equipment re-
quired for the cleanup than on the size of the spill.
• More than one gasoline-recovery option may be
feasible at a given spill.
• Costs of free product recovery are small com-
VIII
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pared to the cost of restoring hydrocarbon-
contaminated groundwater to drinking water
standards.
The case studies reviewed involved spills from 2,
000 gal to 100, 000 gal. Costs of recovery ranged
from $43, 000 to $225, 000 (including equipment,
labor, engineering, and hydrogeologic services).
On average, only 29 percent of the spilled prod-
uct was recovered, at a cost range of $2 to $93 per
gallon.
Removal of Gasoline From
Unsaturated Soils
Soil treatment is an essential component of a cor-
rective action plan. After a spill, hydrocarbons in the
unsaturated zone can eventually enter the ground-
water, if the soil is not treated. A number of
techniques are used, but they vary in cost and ef-
fectiveness. Excavation and disposal is the most
widely used corrective action for contaminated soil.
Other methods include enhanced volatilization, in-
cineration, venting, soil washing/extraction, and
microbial degradation.
Excavation and Disposal
The most widely used corrective action, excavation
and disposal, can be 100 percent effective.
Moreover, soil excavation as an adjunct to removal
of underground storage tanks (USTs) may help to
eliminate the major source of continuing gasoline
migration to the subsurface. The following limita-
tions must be kept in mind:
• Standard backhoes (0.5 yd3 capacity) can reach
only a maximum depth of 16 ft. Larger backhoes
(3.5 yd3 capacity) are available that can remove
soils at depths of up to 45 ft.
• Excavation is difficult in heavily congested areas
or in areas close to or under buildings.
• The more soil brought to the surface, the greater
the risk of exposure.
• Although tipping fees at some landfills are a
reasonable $12/yd3, disposal can cost up to $1607
yd3 if the soil is considered hazardous.
• The lack of uniform guidelines among the states
for the disposal of contaminated soils means that
transport risks may run high, as soil is sent from
states with strict guidelines to the more permis-
sive states.
Cost: $200 to $300/yd3, which is relatively expen-
sive. The result is that only small quantities of con-
taminated soil, say, 500 yd3, are normally exca-
vated and disposed. The trend is toward applying
alternatives to land disposal, such as incineration
or biodegradation, by which contaminants are de-
stroyed.
Disposal of contaminated soils in batch asphalt
plants is a practice not yet reported in the literature,
but may be more common than most people
realize. Some plants charge $55/yd3 for accepting
gasoline-contaminated soils; other plants refuse
such soils because they must then observe the
state laws governing hazardous waste treatment
facilities.
Enhanced Volatilization
Theoretically, up to 99.99 percent of volatile organic
compounds (VOCs) can be removed by enhanced
volatilization, but this soil treatment method has not
been widely applied in the field. Different methods
of enhancing volatilization include rototilling, me-
chanical aeration, pneumatic conveyor systems,
and low-temperature thermal stripping. Only ther-
mal stripping has been documented to successfully
remove contaminants with vapor pressures com-
parable to those of gasoline constituents. Limita-
tions to enhanced volatilization include soil charac-
teristics that constrain the movement of gasoline
vapors from the soil to the air; contaminant concen-
trations that may create an explosion hazard; and
the need to control dust and organic vapor emis-
sions. Cost: $245 to 320/yd3 soil treated; most ef-
fective with 15,000 to 18,000 tons of soil.
Incineration
By complete oxidation, incineration can eliminate
99.99 percent (or more) of gasoline constituents in
soil. This technology is widely practiced and highly
reliable. The associated limitations are that the soil
must be brought to the surface, which increases
the risk of exposure; incineration is usually appro-
priate only when toxics other than volatiles are pre-
sent; and the permitting requirements may cause
time delays. Cost: $200 to $640/yd3 of soil. Soil vol-
umes of less than 20,000 yd3 will increase costs
considerably.
Venting
Venting allows for the removal of gasoline vapors
from unsaturated soils without excavation. It has
been demonstrated to be effective in recovering as
much as 99 percent of gasoline components in un-
saturated soil. The technology has not been widely
applied in the field, however, partly because critical
design parameters remain undefined. Moreover, its
effectiveness is uncertain because soil characteris-
tics may impede free movement of vapors, create
an explosion hazard, or cause high levels of or-
ganic emissions. Venting is relatively easy to imple-
ment and causes minimal disturbances to struc-
tures or pavement. Cost: $15 to $20/yd3, which is
inexpensive. It would become even more cost-
effective when soil volumes exceed 500 yd3.
IX
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Soil Washing/Extraction
With this approach, contaminants are leached from
the soil and into a leaching medium, after which the
extracted contaminants are removed by conven-
tional methods. Removal of 99 percent of volatile
organic compounds is possible under ideal condi-
tions, but typical removal rates are less. High per-
centages of silt and clay in the soil may impede the
separation of the solid and liquid after the washing
phase. Since the process requires physical separa-
tion techniques (e.g., distillation, centrifugation, and
evaporation), pilot studies are recommended be-
fore final design and implementation. Cost: $150 to
$200/yd3 of contaminated soil.
Microbial Degradation
Theoretically, gasoline removal efficiencies of 99
percent or more can be achieved with microbial de-
gradation of contaminants. The technique has not
been widely applied in the field, and additional re-
search is required to confirm cost and effective-
ness. The advantages of this technique are that the
soil is usually treated in situ and the volatiles are
completely destroyed. Gasolines composed princi-
pally of alkenes in the C5 to C10 range would be the
quickest to degrade. For its effectiveness, the
technique is dependent on oxygen levels, nutrient
levels, temperature, and moisture content of the
soil. Cost: $66 to $123/yd3. The combination of soil
venting and microbial degradation is often one of
the least costly and most effective corrective ac-
tions for treating gasoline-contaminated soils.
Current Soil Treatment Procedures
Much confusion exists about the hazard posed by
gasoline-contaminated soil and how the soil should
be treated. An informal survey of several states re-
vealed that none require soil testing during UST ex-
cavations. Many states do require a fire marshal to
be present to determine explosion hazards at sites
where visual inspection shows soil to be contami-
nated. Landfill is the principal mode of disposal of
contaminated soils, and time delays are common.
None of the states surveyed have regulations pre-
venting open aeration of contaminated soils to re-
duce the volatile organic compounds. Many ex-
cavators admit placing gasoline-contaminated soils
on plastic sheets until the volatiles disperse so that
the soil can be trucked to the local landfill.
Removing Gasoline
Dissolved in Groundwater
The two most widely used technologies (employed
in more than 95 percent of all cases) for removing
that portion of the gasoline plume dissolved in
groundwater are air stripping and filtration through
granular activated carbon (GAG). Biorestoration is
a cost-effective and promising alternative, but it has
not been widely applied in the field.
Air Stripping Towers
For most volatile organics found in gasoline,
packed towers have maximum removal efficiencies
of 99 to 99.5 percent. Through air stripping, effluent
concentrations of 5-|x/L volatile organics can be
achieved. Concentrations less than 5 p,/L are not
usually achievable because the technology is con-
strained by the size of the tower that would be re-
quired to achieve such a high removal efficiency.
Critical design parameters include the type of pack-
ing material used, the air-to-water ratio, the strip-
ping factor, and the tower height. Plastic packings
are the most widely used; they are inexpensive and
lightweight. Air-to-water ratios from 20:1 to 100:1
are common for aromatics removal in general and
for those in gasoline in particular. Stripping factors
between 3:1 and 5:1 are best suited for gasoline-
related constituents. In designing a packed air
tower, the following considerations are important:
• Zoning laws may restrict the maximum height of a
tower.
• The tower, blower, and pumps may have to be en-
closed, not only for noise reduction but also for
aesthetic reasons.
• Influent air must be free of VOCs, so air intake
must be situated to avoid "short circuiting" be-
tween the influent air and the tower effluent air.
• Gaseous demisters may be needed to prevent
water from leaving the top of the tower.
• Vapor-phase treatment, if required, will double the
cost of the packed air tower.
Because more contaminant can be adsorbed in an
air-to-carbon loading than in a water-to-carbon
loading, vapor-phase treatment with GAG may be
advantageous. Cost: $50,000 to $100,000 (includ-
ing labor, engineering, and contingencies), which is
50 to 80 percent less than comparable costs to
treat with GAC. On a volume-treated basis, typical
costs at a leaking UST site are $5 to $25 per thou-
sand gallons.
Granular Activated
Carbon Adsorption
GAC adsorption can remove as much as 99.99 per-
cent of the organic compounds found in gasoline.
To achieve effluent concentrations of 5 |x/L or less
for gasoline constituents, GAC is almost always re-
quired. Designing a GAC system is complex, as the
following points illustrate:
• Each contaminant competes for carbon pore
space.
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• The EBCT (empty bed contact time) is directly re-
lated to the size of contactor needed; 15 minutes
is the usual minimum contact time for gasoline
spills.
• Fixed bed columns and pressure filters are nor-
mally used in cleaning leaking UST sites; use of
pressure filters saves repumping costs because
they allow higher surface loading rates and pres-
sure discharge to the distribution system.
• The ability of a compound to be removed with
GAC is a function of its solubility. Low-solubility
compounds adsorb better than high-solubility
compounds. The order in which gasoline com-
ponents break through (from earliest to latest)
is: benzene, ethylbenzene, toluene, xylene,
naphthalene, and phenol.
• Some compounds found only in certain gasolines
might break through earlier than benzene be-
cause of their low adsorption capacities: methyl-
tertiary butyl ether (MTBE), ethylene dibromide
(EDB), and ethylene dichloride (EDC). Less than
40 percent of today's gasolines contain EDB or
EDC. Only 10 percent contain MTBE. It is more
expensive to design for the removal of these com-
pounds than it is to design for benzene removal.
• Effectiveness of system may be reduced by ex-
cessive iron or manganese, and hardness of the
water. If iron concentration exceeds 5 mg/L, re-
moval prior to carbon filtration is recommended.
• Spent carbon from leaking UST sites is usually
landfilled. Caution must be exercised in handling
gasoline-saturated carbon tanks because they
can self-ignite.
• GAC is most effective when used with air strip-
ping. Carbon life can be extended by treating
gasoline-contaminated groundwater with packed
air towers. A two-phase approach is best. The
first phase is to install a packed air tower. Its per-
formance would then be monitored to determine
effluent concentrations, and the need for a sec-
ond-phase treatment with GAC.
Cost: $300,000 to $400,000 for a typical GAC unit.
Costs include labor, engineering, and contingen-
cies. O&M costs range from $25,000 to $30,000/yr.
Biorestoration
Under proper conditions, trace concentrations of
aromatic hydrocarbons can be reduced by 99 per-
cent with this technique. Its distinct advantage is
that the gasoline contaminants are completely de-
stroyed, not merely transferred to another environ-
mental medium. Its applicability depends on dissol-
ved oxygen concentrations, available nutrients,
temperature, pH, salinity, concentrations of contam-
inants, presence of predators, and water con-
tent.Through biorestoration, effluent concentrations
in the ppm-range (mg/L) can probably be attained;
treatment to ppb-levels (|xg/L) requires manipula-
tion of the system (encouragement of co-
metabolism or degradation by an added substrate).
Currently, the technology appears to work best as a
"polishing" step. Cost: few data exist, but costs
range from $30 to $40/yd3 treated to $10,000/acre
treated, and from $4 to $6/lb of contaminant re-
moved.
Point-of-Entry Treatment
and Extension of
Water Distribution Systems
Restoration of the polluted aquifer can often take
months or years, during which time users of the
water must find alternative water sources. Two al-
ternatives are point-of-entry treatment systems and
extension of the water distribution system.
Point-of-Entry Treatment
Systems which treat water at the point of entry into
a home are preferable to point-of-use systems that
can be placed on individual taps. Research indi-
cates that showering in water that contains volatile
gasoline compounds may pose a serious health
threat; therefore, only point-of-entry systems are
considered appropriate at homes with gasoline-
contaminated well water. There are several types of
devices: reverse osmosis, ion exchange, distilla-
tion, aeration, and carbon adsorption. Carbon ad-
sorption is the most effective in eliminating dissol-
ved gasoline compounds. Carbon adsorption is
capable of removing more than 99 percent of dis-
solved gasoline compounds, including benzene, to-
luene, and xylene.
Activated carbon can adsorb dissolved compounds
for water, but only up to a point. To eliminate the risk
of contaminant breakthrough, two carbon tanks in
series are installed, and the effluent water is tested
periodically for the presence of VOCs. The most
serious limitation associated with carbon adsorp-
tion point-of-entry treatment systems is that signifi-
cant changes in contaminant concentrations may
go undetected. If the influent concentrations fluc-
tuate and exceed the design capacity of the sys-
tem, contaminant breakthrough could occur without
the resident knowing it. For this reason, it is recom-
mended only as an interim remedial measure in a
home. Cost: Carbon tanks, from $700 to $900; car-
bon replacement and disposal, from $100 to $200
per replacement; testing for VOCs, $250. Additional
water quality improvement equipment, such as
chemical feed units, softeners, filter, retention
tanks, and polishers, are in the $500 to $950 range
for each piece of equipment. Case studies reported
in this manual indicate that annual capital and O&M
costs are from $4,000 to $5,000 per household.
XI
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Extending Water Distribution System for 6,n mains to $40/|f for 12,n mainS) and to $84/|f
Cost is usually the primary consideration in extend- for 24-in mains. Distribution mains (for short dis-
ing water mains to homes affected by a contami- tances between the transmission main and the indi-
nated well field. This measure is often the appropri- vidual home or building) range from $44/lf for 6-in
ate long-term solution. Cost: transmission mains mains, to $56/lf for 12-in mains, to $100/lf for 24-in
(for long distances) range from $27/linear foot (If) mains.
XII
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Section 1
Introduction
Gasoline is a complex mixture of hydrocarbons
comprised principally of alkane, alkene, and
aromatic hydrocarbons. Gasoline spilled or leaked
into soil volatilizes because of its high vapor pres-
sure, filling pore spaces with vapors. Gasoline
vapors, as well as gasoline in the liquid phase, are
subject to further dispersal and migration as pre-
cipitation moves into and through the subsurface.
Gasoline in either state can dissolve in water and
eventually move into groundwater supplies. It is
important, therefore, that gasoline in unsaturated
soils, as well as free product and that dissolved in
groundwater, be removed to minimize further dis-
persal of the contaminants.
Cleaning up a release from an underground stor-
age tank (UST) requires both short-term emer-
gency measures and long-term corrective actions.
Short-term emergency measures involve taking
immediate steps to abate imminent safety and
health hazards, including potential explosions.
These emergency steps include notifying appropri-
ate government officials, stopping the release, and
removing hazardous substances as necessary to
prevent further releases and to allow inspection
and repair of the tank system.
The focus of this handbook is on long-term remedi-
ation and site restoration that occurs after emer-
gency measures have already been taken to
remove any immediate explosion or health threat.
The task at hand is to clean up the released
gasoline that has moved into the environment and
is adsorbed to soil particles, floating as free product
on the water table, or dissolved in groundwater. In
cleaning up a site, it is essential that corrective
actions be designed to address the released con-
taminants in each and every medium in which they
are found. Cleaning up contaminated groundwater
without cleaning up contaminated soil could result
in continued contamination as rainfall percolates
through the soil. Cross-media contamination issues
also must be addressed in developing a corrective
action plan for a particular site, for example, vapor
emissions from air stripping towers and soil venting
systems.
A variety of corrective actions can be used at a
leaking UST site to treat contaminated soil, recover
free product, treat vapors generated by evaporating
aromatics, and remove contaminants dissolved in
groundwater. They differ in their cost, removal
efficiencies, reliability, and applicability.
Objective of This Handbook
The objective of this handbook is to provide
engineering-related information regarding the re-
moval efficiencies, limitations, and costs of alterna-
tive corrective action technologies for removing
contaminants (principally associated with gasoline)
from the subsurface. While a large number of
technologies exist to clean soil, air, and water, only
a limited number possess demonstrated perfor-
mance records and have progressed to full-scale
applications. This handbook focuses on those cor-
rective action technologies that have been widely
proven to be effective and reliable, or that are prom-
ising but lack full-scale application and review.
Ultimately this handbook can serve as a reference
document for local and state personnel who must
evaluate and make decisions about the most
appropriate corrective actions to use at a particular
site. The cost curves, design equations, and related
implementation issues will assist them in making
informed and effective decisions. The corrective
action field is changing, but the information pre-
sented here will still help direct attention to the main
elements and factors in conducting cleanups.
The overall intent of this handbook is to provide
personnel involved with corrective actions at UST
sites with a summary of the principal components,
design considerations, and costs behind the
technologies, and to identify conditions and situa-
tions where one corrective action might be pre-
ferred over another.
This is not a design manual and should not be
used as such. Each situation is unique. A profes-
sional engineer or similarly qualified individual
should be sought to design and install any equip-
ment described in this report.
To compare and contrast the various corrective
action techniques, each proven technology is
-------
evaluated on the basis of several important criteria:
• Effectiveness. How effective is the technology in
removing contaminants?
• Cost. What are the capital and operation and
maintenance (O&M) costs of the technologies?
What are the projected service lives? How does
cost vary with time and removal efficiency?
(Costs are reported in this document in 1986
dollars.)
• Reliability. How consistently can the technologies
remove the contaminants of concern and over
how long a period of time?
• Ease of Operation. How complex is the technol-
ogy? Are specially trained personnel required for
O&M activities?
• Limitations. What factors might reduce the effec-
tiveness or reliability of a technology or limit its
applicability in a given situation?
Limitations of This Handbook
This handbook necessarily focuses on widely
applied and proven technologies, ones that could
be recommended at a site to secure desired
results. The technologies are described with spe-
cific attention focused on removing gasoline from
the subsurface, especially the major constituents of
concern in gasoline: benzene, toluene, and xylene
(BTX).
There are a number of regulatory issues associ-
ated with the implementation of corrective action
technologies, particularly with soil treatment
technologies. The regulatory issues include secur-
ing the appropriate permits, establishing standards
for cleanup, determining when contaminated soil is
"hazardous," and ultimate disposal of recovered
free product, contaminated soil, and groundwater.
This handbook does not cover these issues, except
for those touched upon under the "Limitations" sec-
tion for each technology. Rather, the focus of this
study is on engineering-related considerations for
each technology.
Other points to keep in mind when reading this
handbook are:
• Composition of Gasoline
This handbook focuses on corrective actions for
cleaning up the principal constituents of concern
in gasoline for which reliable data exist. Reliable
data on the toxicity, chemical characteristics, and
weight for most of the approximately 240 com-
pounds which have been identified in gasoline do
not exist for every constituent. Consideration
must be given to the "minor" constituents as well.
In addition, there are a number of proprietary
additives for which little or no data exist.
• Site-Specific Conditions
The optimal design and performance of all tech-
nologies is highly dependent upon site-specific
conditions. To optimize system performance, field
and pilot testing at individual sites should be
undertaken prior to full implementation. The
generalized analyses in this report are based on
"typical contamination incidents" and the typical
concentrations one is likely to encounter in a leak
situation.
• Soil Treatment Technologies
Soil treatment has not received widespread appli-
cation. Much research has been gathered on
various soil treatment techniques, but a great deal
of uncertainty remains about how the techniques
work, and what the controlling factors are to
achieve maximum effectiveness. Although the
theories and equations are not as thoroughly de-
veloped as they are for other technologies, such
as air stripping and carbon adsorption, soil treat-
ment techologies are not less useful nor less
important. Soil contamination must always be
addressed, and some kind of soil cleanup is
usually necessary.
Other Studies
Several reports would serve as useful companion
documents to this one:
• Camp Dresser & McKee Inc. 1986. Interim Report -
Fate and Transport of Substances Leaking From
Underground Storage Tanks.
Describes in detail the various compounds that
make up gasoline and how they move in and
through the environment.
• PEL 1987 Underground Storage Tank Corrective
Action Technologies. Prepared by US EPA,
Hazardous Waste Engineering Laboratory, Cin-
cinnati, Ohio.
Provides detailed information on the "universe" of
technologies that are available to remove
gasoline from the subsurface. The report is a
comprehensive examination of what is available
(compared to the few technologies that are typi-
cally used).
• Radian Corporation. 1987 Air Strippers' Air Emis-
sions and Control (Draft).
Provides information on vapor-phase discharges
from air stripping towers, including cost data.
-------
Organization of This Handbook
The principal corrective actions for UST releases
are discussed in detail separately in the sections
that follow. Section 2 introduces the chemicals of
concern and describes how they move through the
environment. Section 3 describes various methods
aimed at recovering free product from the water
table. Section 4 describes soil treatment
techniques, and Section 5 addresses the removal
of contaminants dissolved in groundwater.
Because Section 5 addresses the treatment of con-
stituents dissolved in groundwater, an area about
which much information exists, it is printed on col-
ored paper to set it apart as the section of this
report that contains the most extensive engineering
information. As such, it will likely be the most useful
section to a reader reviewing corrective action
plans. Section 6 provides information on alternative
water supplies such as point-of-entry systems and
water distribution system extension, possible shor-
ter term solutions while the longer term corrective
actions are being implemented. Section 7 is a sum-
mary of findings, and Section 8 is an index of key
words.
-------
Section 2
Fate and Transport of
Gasoline in the Subsurface
To implement corrective actions effectively, it is
essential to understand how gasoline behaves in
the subsurface. Fate and transport mechanisms
are complex. The behavior of chemicals in the sub-
surface is governed not only by their physical and
chemical properties but also by the characteristics
of the soil and rock formations through which the
chemicals move.
Chemicals can exist in the subsurface in four gen-
eral states: as free product (pure compound);
adsorbed to soil; as vapor; or as solutes in water
(see Figure 1). In conducting corrective action, the
person responsible needs to address the removal
of contaminants in each state. The following sec-
tions of this handbook pertain to the cleanup of one
or more of these four chemical states. Recovery of
free product is discussed in Section 3. Removal of
chemicals adsorbed to soil is discussed in Section
4. Removal of contaminants dissolved in ground-
water, as well as treatment of the vapor phase of
the contaminant, is discussed in Section 5.
UNDERGROUND
STORAGE TANK
GROUND SURFACE .
UNSATURATED
ZONE
V
(7) GASOLINE
VAPORS
(2) ADSORBED
— GASOLINE
/' -'-,' '. , .''{ CAPILLARY ZONE
-..i..L.*..
-------
The extent to which each phase of a chemical parti-
tions and migrates in the subsurface is a function of
several variables, including moisture content, bulk
density and permeability of the soil and rock forma-
tions, air and gas vapor pressure within pore
spaces, temperature, pH, and the presence of bac-
teria that decompose the contaminants. Analyzing
the fate and transport of gasoline in the subsurface
is a particularly complicated process because
gasoline is a mixture of chemicals. Each compound
used in commercial gasolines has a unique set of
physical and chemical properties that determines
its multiphase flow characteristics. Consequently, it
is often difficult to predict how each gasoline com-
ponent will behave and the extent to which it will
migrate in soil and rock formations. A brief discus-
sion follows that covers the current understanding
of the fate and transport of gasoline and its compo-
nents in the subsurface.
Factors Affecting Transport
Properties of the Gasoline
To understand how gasoline behaves in the sub-
surface, its relevant characteristics must be con-
sidered.
• Gasoline ranges in density from 0.72 to 0.78 g/
cm3 and is less viscous than water. (Viscosity is a
measure of a liquid's resistance to flow; since
gasoline is less viscous than water, it moves more
easily through soil).
• Gasoline is immiscible in water (i.e., the fluids dis-
place one another without mixing); however,
there are many components of gasoline which
readily dissolve upon contact with water.
• Some gasoline constituents are highly volatile.
• Some compounds are readily biodegraded in the
presence of soil bacteria and oxygen.
Gasoline is a mixture of different compounds. A
typical blend contains nearly 200 different hydro-
carbons in addition to additives which serve as
anti-knock agents, anti-oxidants and sweetening
inhibitors, metal deactivators, corrosion inhibitors,
deicing and anti-stall agents, preignition preven-
tors, dyes, and upper cylinder lubricants. Each
compound exhibits different physical and chemical
properties which control its fate and transport in a
soil system, and therefore, it is difficult to study the
behavior of a specific gasoline as a whole.
Thirteen chemicals commonly found in gasoline
(nine hydrocarbons and four additives) are regu-
lated as hazardous substances under the Com-
prehensive Emergency Response, Compensation
and Liability Act (CERCLA). This group includes
benzene, toluene, and xylenes (BTX), and addi-
tives such as ethylene dibromide (EDB) and tet-
raethyl lead. These chemicals are listed in Table 1,
along with values for their toxicity, water solubility,
vapor pressure, and degree of biodegradability.
Although the physical and chemical properties
shown in Table 1 are usually adequate for charac-
terizing the behavior of gasoline under laboratory
conditions, they may not be adequate for describ-
ing behavior in a subsurface environment.
Structure of the Subsurface
Soil particles and rock fragments are separated by
voids called pore spaces. The pores are often inter-
connected, forming a network of fine channels
through which water and air can circulate. Subsur-
face formations are characterized based on their
pore structure and water-bearing capacity. Figure 1
depicts three distinct zones: the unsaturated zone,
the capillary zone, and the saturated zone.
The Unsaturated Zone
The unsaturated zone (also referred to as the aera-
tion zone or vadose zone) is the region between
the ground surface and the top of the capillary
zone. Water is retained there by adsorption on the
surface of particles and by capillary forces (suc-
tion). Capillary forces are adhesive and cohesive
forces which bind water molecules to solid sur-
faces.
The maximum volume of water that can be held in
the pores by adsorption and capillary forces is
referred to as the residual saturation of water. As
defined by CONCAWE (1979), residual saturation is
the minimum content a fluid must attain in order to
move in a porous medium; or alternatively, the
threshold content below which the fluid is no longer
able to move. Unsaturated zone pore spaces that
are not filled with water contain air that can circulate
freely.
The Capillary Zone
The capillary zone is a transition region between
the unsaturated zone and the saturated zone.
Moisture content in the capillary zone ranges from
residual saturation near the unsaturated zone to
complete saturation at the water table. The capil-
lary zone varies in thickness depending on the size
of the soil particles and the diameter of the pore
spaces. The finer the pores, the higher the capillary
rise. The homogeneity of the subsurface formation
also influences the thickness of the capillary zone.
In homogeneous porous media, the zone thickness
will tend to be constant, whereas in nonhomo-
geneous formations, the thickness can vary
considerably.
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The Saturated Zone
The saturated zone is the region below the unsatu-
rated zone and the capillary zone where the pore
spaces are completely saturated with water. In the
saturated zone, the water pressure increases with
depth. The boundary where the pressure in the
water phase equals the atmospheric pressure is
called the piezometric surface or water table. The
area below the water table may be thought of as a
reservoir, the capacity of which equals the total vol-
ume of the pore spaces filled with water.
Subsurface reservoirs, or aquifers, perform two
important functions: they act as storage reservoirs
and water-carrying bodies. Water-carrying capacity
is a function of effective porosity (i.e., total volume
of interconnected pore spaces) and permeability.
Permeability is the measure of a soil's resistance to
the flow of liquids and is dependent on the diameter
of the pores and the shape and orientation of the
soil and rock particles. Groundwater flow through
an aquifer is influenced by gravitational forces, but
the rate at which the groundwater moves can vary
significantly. Depending on the permeability of the
aquifer and the flow gradient, groundwater can
move at a velocity of only a few meters per year to
several meters per day.
Multiphase Movement of
Gasoline
Because gasoline is comprised of some highly vol-
atile and soluble hydrocarbon fractions, its move-
ment in the subsurface occurs in more than one
phase. Gasoline components can move as solutes
in the water phase, free product in the admissible
phase, and vapor in the air phase. Multiphase flow
is further complicated because each subsurface
formation has different characteristic properties
which govern the transport of substances through
it. In the following discussion, multiphase transport
of gasoline components is described for the
unsaturated zone, the capillary zone, and the satu-
rated zone.
Transport in the Unsaturated Zone
The depth to which gasoline penetrates the subsur-
face is most dependent on the volume discharged.
As gasoline moves into the soil, it begins to migrate
both vertically and horizontally. The vertical compo-
nent is due to gravity, while the horizontal compo-
nent is due to capillarity (the adhesive forces
between the gasoline and the soil and rock parti-
cles). Migration occurs by successive permeation
of larger areas. In a highly permeable homogene-
ous formation, the migration is mainly downward. In
a less permeable homogeneous formation, capil-
lary forces will have greater influence and migration
has a more significant horizontal component. As
shown in Figure 2, migration through heterogene-
ous formations results in a widely varying shape of
the infiltrating gasoline body.
Vertical penetration in the unsaturated zone can be
impeded in three ways: (1) when the threshold of
residual saturation is reached (occurs when the
gasoline body is adsorbed to soil and rock particles
and trapped in capillary spaces); (2) when an
impermeable layer exists in the path of the gasoline
(migration is lateral until residual saturation is
reached or until it reaches a discharge point); or (3)
when the gasoline reaches the water table.
LAND SURFACE
"J J ;•'"•'/ ~^
HIGHLY PERMEABLE
HOMOGENEOUS SOIL
'V"1** •' » * ** » " '^*«"
LESS PERMEABLE
HOMOGENEOUS SOIL
STRATIFIED SOIL WITH
VARYING PERMEABILITY
Figure 2. Contaminant transport through unsaturated homogenous and heterogenous soils.
-------
The gasoline that remains in the unsaturated zone
is partitioned among four phases: free product
retained in pore spaces by capillary forces; solutes
of gasoline components adsorbed onto particles;
vapor in the soil air; and dissolved product in the
soil water.
Retention by Capillary Farces
Capillary forces are attractive forces between
gasoline and particles in pore spaces through
which the gasoline moves. The dominant capillary
force, adhesion (the attraction of liquid molecules
for solid surfaces), holds gasoline molecules rigidly
to soil and rock particles. These molecules in turn
hold by cohesion (attraction of molecules to each
other) other gasoline molecules which are further
removed from the soil and rock particle surfaces.
Together, adhesion and cohesion enable soil and
rock particles to retain gasoline against the force of
gravity. Capillary forces result in vertical and hori-
zontal movement of liquids. The extent of capillary
movement depends on pore size. In general, the
finer the pore size, the greater the movement.
Adsorption Onto Particles
Sorption (the bonding of a solute to sorption sites
on a solid surface) occurs through the following
mechanisms: van der Waals forces, hydrophobic
bonding, hydrogen bonding, charge transfer, ligand
exchange, ion exchange, ion/dipole interactions,
magnetic interactions, and chemisorption. The
extent to which gasoline compounds adsorb to a
specific soil or rock particle depends on the sorp-
tion potential of the chemicals; the organic carbon
content of the particles; the texture, structure, and
bulk density of the particles; clay and moisture con-
tent; cation exchange capacity; and pH.
Table 2 lists adsorption coefficients for specific
gasoline compounds. The values presented are a
measure of each compound's mobility potential
based on its affinity for organic carbon. Of the toxic
gasoline compounds listed, tetraethyl lead and
naphthalene have relatively low mobility values and
are likely to be adsorbed to the soil. Toluene, the
xylenes, benzene, and phenol have high mobility
values and, therefore, are more likely to appear in
either the dissolved or gaseous phases.
VDlatilization to Soil Air
Volatilization of gasoline compounds to the soil air
depends on the potential volatility of the com-
pounds and on soil and environmental conditions
which modify the vapor pressure of the chemicals.
Vapor pressure (the pressure exerted by a gas at
equilibrium with respect to its liquid or solid phase)
is directly proportional to volatility. Factors which
influence vapor pressure and, consequently, volatil-
ity include soil parameters such as water content,
clay content, and surface area, in addition to
environmental factors such as temperature, wind
speed, evaporation rate, and precipitation
(Fleischer, 1987). Vapor pressure values for toxic
gasoline components are listed in Table 1. The
values range from 0.2 torr (760 torr = 1 atm) for
tetraethyl lead to 75 torr for benzene.
For more information, see Vapor Transport in the
Unsaturated Zone, p. 12.
Dissolution in Soil Water
Dissolution occurs as soluble gasoline compounds
come in contact with water. The potential for dissol-
ution of gasoline compounds to soil water is a func-
tion of each compound's solubility. Solubility is the
Table 2
Adsorption Coefficients for Gasoline Compounds
Chemical
Value
Tetraethyl Lead1
(n) Heptane
(n) Hexane
Naphthalene2
(n) Pentane
Ethyl benzene2
Toluene2
1-Pentane
(o) Xylene2
Benzene2
Phenol2
Ethylene Dibromide
4,900 mL/g
2,361
1,097
976
568
565
339
280
255
50
50
44
Source Lyman et al (1982).
1 Koc is a measure of the tendency for organic compounds to be adsorbed by soil. The
higher the Koc value for each compound, the lower the mobility and the higher the
adsorption potential
2 Also listed in Table 21 as a toxic compound.
-------
partitioning of a chemical between the nonaqueous
(gasoline) and dissolved phases. Not only does sol-
ubility determine the extent to which a contaminant
will dissolve, it also affects other fate mechanisms.
For example, a highly soluble substance often has
a relatively low adsorption coefficient and also
tends to be more readily degradable by microor-
ganisms.
As shown in Table 1, several compounds are more
soluble than benzene: phenol, EDC, EDB, and
dimethylamine. Another constituent of gasoline
known to be more soluble than benzene is methyl
tertiary butyl ether referred to as MTBE, which is
not shown on Table 1 because it is not considered
toxic.
MTBE is being more widely used as an octane-
enhancing additive to gasoline, especially because
tetraethyl lead is being phased out. Although
MTBE has been produced commercially only since
1979, it is now among the top 50 chemicals pro-
duced in the United States. Still, only about 10 per-
cent of U.S. gasoline contains MTBE. The health
effects of MTBE are generally poorly understood,
especially at low levels. It has been classified vari-
ously as an irritant, as a possible central nervous
system depressant, and formerly as having medici-
nal value (used to treat gallstones).
MTBE is extremely soluble in water: about 24 times
more soluble than benzene (43,000 mg/L vs. 1,780
mg/L). Because of its high solubility, MTBE is easily
transported by groundwater away from a spill site
and will often have a larger plume than gasoline
components such as benzene; the MTBE plume is
thought to occur as a "halo" around the benzene
plume. It has been detected at concentrations as
high as 47 ppm, but is typically found at concentra-
tions of 1 ppm or less (1 ppm = 1 mg/L). MTBE
actually has a cosolvent effect, causing some of the
other compounds in gasoline to solubilize at higher
concentrations than they would normally in "clean"
water.
Transport in the Capillary Zone
When free gasoline first reaches the capillary zone,
its vertical migration is stopped. As more gasoline
descends, a layer of increasing thickness forms
and hydrostatic pressure is exerted, depressing the
water table. As buoyant forces act to restore the
original water level, lateral movement begins and a
lens of gasoline forms and spreads out. Lateral
spreading occurs in all directions, but the predomi-
nant movement is with the slope of the water table.
Heterogeneities and permeability differences often
influence the direction and extent of free gasoline
migration in the capillary fringe. In heterogeneous
soils, gasoline migration is along the path of least
resistance. Soil permeability affects the rate and
thickness of lateral spreading. In low permeability
soils, resistance to flow is high and a thicker lens
will form; lenses formed in higher permeability soils
are thin and fast-moving.
As in the unsaturated zone, transport in the capil-
lary zone is governed by multiphase flow
phenomena. However, the increased water content
in the capillary zone affects the rates of volatiliza-
tion and dissolution. As soil water content
increases, volatilization and vapor transport gener-
ally decrease, and dissolution and solute transport
generally increase. Free product migration occurs
on top of the water table, but as the gasoline con-
tinues to spread, it is held by capillary forces in the
soil matrix. When the free gasoline is exhausted,
migration stops and residual saturation is reached.
Several technologies that are available to recover
free product are discussed in detail in Section 3.
Transport in the Saturated Zone
Dissolved gasoline compounds reach the saturated
zone in several ways:
• Infiltrating water passes through the gasoline
bound in the unsaturated zone and leaches some
compounds and carries them into the aquifer.
These compounds then move with the groundwa-
ter gradient as a single phase.
• Free gasoline reaches the water table where
some of the compounds dissolve and move with
the groundwater gradient as a single phase (see
Figures).
• Free product held in residual saturation in the
unsaturated zone is submerged following a rise in
the water table. Capillary forces binding the sub-
merged free gasoline to the soil and rock particles
resist buoyant forces pushing the gasoline up
toward the elevation of the new water table. As a
result, the gasoline remains in the saturated
zone, and dissolution occurs freely (see Figure 4).
The movement of dissolved gasoline compounds in
the saturated zone is governed by advection and
dispersion. Advection is the movement of dissolved
contaminants with the mean groundwater gradient.
Dispersion describes how dissolved contaminants
spread out and become diluted as they move. The
effects of dispersion explain the observation that
contaminants occupy more of the saturated zone
than can be due to advection only.
Once gasoline components have dissolved in
groundwater, removal becomes very costly. Typi-
cally, packed air towers and/or carbon adsorption
are required for the removal of dissolved com-
ponents of gasoline in groundwater. These ground-
water treatment technologies are discussed in
Section 5.
10
-------
SATURATED ZONE
(A) CONTAMINANT IN DIRECT CONTACT WITH THE WATER TABLE
GROUNDSURFACE
WATER TABLE
SATURATED ZONE
(B) GROUNDWATER CONTAMINATION RESULTING FROM SOLUTION
OF CONTAMINANT IN PERCOLATING RECHARGE WATER
Figure 3. Schematic contaminant plumes showing methods by which groundwater can be contaminated.
ORIGINAL
WATER TABLE
OIL AT RESIDUAL
SATURATION
FREE GASOLINE
NEW WATER TABLE
Figure 4. Contaminating effect on soil caused by fluctuating water table.
11
-------
Vapor Transport in the
Unsaturated Zone
Vapor phase transport of gasoline components in
the unsaturated zone can pose a significant health
and safety threat because of inhalation and explo-
sion potential.
For vapors to move in the unsaturated zone, the
soil and rock formations must be sufficiently dry to
permit interconnection of air passages among the
soil pores. Two parameters then govern movement:
vapor concentration and vapor flow. Leaked
gasoline will have its greatest vapor concentration
at the leak site, where the free gasoline is evaporat-
ing at the liquid-vapor interface. The rate of vapori-
zation depends on the vapor pressure of the
gasoline constituents, the pore pressure and mois-
ture content of the soil, and the ambient tempera-
ture. The natural vapor flow (or flux) is away from
areas of high concentration to areas of lower con-
centration and ultimately to the atmosphere. In
warm weather, vapors of benzene, toluene, or
xylene (BTX) readily escape upward by diffusion. If
there is an impermeable layer above the rising va-
pors, however, such as a paved road, building, or
parking lot, or if the ground surface is frozen, the
vapors are able to move only by lateral under-
ground travel; thus, migration can occur over rela-
tively long distances.
The principal modes of gasoline vapor transport in
soils are diffusion and advection. Diffusion is the
mass transport that results from the random motion
of vapor molecules and is generally away from
areas of high concentration towards areas of low
concentration. Advection results from changes in
the total pressure gradient and is the net downgra-
dient migration of gases. Pressure changes that
cause advection vapor movement result from
barometric pumping, imposed pressure gradient,
and density differences.
Barometric Pumping
Pore air pressure deep in the unsaturated zone typ-
ically reflects the mean atmospheric pressure at
the ground surface. A rise or fall in atmospheric
pressure with respect to the pore pressure will
result in vapor flow into or out of the soil. This
mechanism is most important where the depth to
the free gasoline is small compared to the depth of
the unsaturated zone. It can increase the rate at
which the free gasoline volatilizes.
Imposed Pressure Gradient
In cold weather, a heated basement may cause the
density of the column of air in the building to be less
than that found outside in the ground. In addition,
the action of a furnace draws air into the basement
from the surrounding subsurface. Vapors will seep
into a basement from the soil pores through path-
ways such as cracks in basement walls, unfinished
floors, or crawl spaces.
Density Differences
If a vapor has a density sufficiently different from
that of other gases in the soil pores (such as air),
there will be a gravity-driven density current of the
vapor. In particular, a relatively heavy vapor will
tend to "pour" down to the bottom of the unsatu-
rated zone and pool as a lens on top of the water
table (or on top of another lens of even denser
vapor).
A number of soil treatment techniques can be used
to collect vapor emissions or enhance volatilization.
These and other technologies are discussed in
Section 4.
Gasoline Degradation in
the Subsurface
Gasoline compounds that reach the subsurface are
subjected not only to the physical processes of dis-
solution, adsorption, and volatilization, but also to
chemical processes. The most important of these
are biotic and abiotic chemical transformation.
There are two biotic processes: biodegradation and
biotransformation. These processes are oxidation-
reduction reactions performed by microorganisms.
Biodegradation is the decomposition of a contamin-
ant by microorganisms such as bacteria, fungi, and
yeasts. The end products of biodegradation are
water, carbon dioxide, and energy for cellular
growth and reproduction. Biotransformation is the
partial biodegradation of compounds. In biotrans-
formation, contaminants are partially degraded to
simpler compounds which may be more or less sol-
uble or toxic than the original compounds.
Abiotic chemical transformations are reactions—
not performed by bacteria—that decrease conta-
minant concentrations by degrading the chemicals
into other products. The most important chemical
transformations are hydrolysis and oxidation/reduc-
tion reactions.
In the subsurface, both aerobic (meaning in the
presence of oxygen) and anaerobic (in the absence
of oxygen) conditions exist. Biodegradation occurs
under both conditions; however, for most gasoline
components, the rate of decay is greater under
aerobic conditions. Other parameters that can influ-
ence the rate of biodegradation include: soil mois-
ture content, compound availability, oxidation/
reduction potential of the compounds, ambient
temperature, pH of soil, inorganic nutrients, and
concentration of microorganisms (Fleischer, 1987).
12
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Degradation is often the result of the combined
effects of chemical transformations and biodegra-
dation. For example, the oxidation/reduction of
complex hydrocarbons can produce simple com-
pounds such as peroxides, primary alcohols, and
monocarboxylic acids. These compounds can then
be further degraded by bacteria, leading to the for-
mation of carbon dioxide, water, and new bacterial
cell materials (CONCAWE, 1979).
Summary
• The migration of gasoline through the subsurface
depends on the quantity released, the multiphase
flow characteristics of the individual gasoline
compounds, and on the structure of the soil and
rock formations through which the gasoline
moves.
• Gasoline compounds in the subsurface may be
partitioned by phases: as free product retained in
pore spaces and floating on the water table,
adsorbed to soil particles, as vapor in soil and air,
and as dissolved compounds in soil water. These
multiphase characteristics are determined by the
physical and chemical properties of the com-
pounds. For example, benzene, toluene, and
xylene are highly volatile and, therefore, are com-
monly found in the vapor phase; naphthalene and
ethylbenzene exhibit relatively low solubilities and
vapor pressure and, therefore, are most common
in the free product phase; and phenol is highly
soluble in water, has relatively low vapor pres-
sure, and is therefore common in the dissolved
phase.
• As gasoline moves through the unsaturated zone,
it migrates both vertically (downward) and hori-
zontally. The vertical component is due to gravity,
while the horizontal component is due to capillar-
ity. Gasoline transport in the unsaturated zone is
a multiphase phenomenon: free product migrates
through the zone or is held in pore spaces; gase-
ous product or vapor moves in the soil air; and
dissolved product moves in soil water and
adsorbs to soil.
• Gasoline movement in the capillary zone is lim-
ited to lateral spreading. As gasoline accumulates
on the water table, it forms a lens that moves hori-
zontally with the groundwater gradient. The free
product phase continues to migrate until it is com-
pletely adsorbed to soil and rock particles and
residual saturation is reached. The dissolved
phase flow occurs as compounds move into solu-
tion from the free product phase.
• The transport of gasoline components in the satu-
rated zone is limited to the dissolved phase. Solutes
enter the groundwater and move in the general
direction of the groundwater gradient according to
the mass transport laws of advection and disper-
sion.
• A gasoline compound in the free product phase
will enter the vapor phase according to its specific
vapor pressure (the higher the vapor pressure of
the compound, the more likely it is to volatilize).
Once in the vapor phase, the contaminants will
move by advection and diffusion. Vapor phase
transport poses a significant health and safety
threat because of inhalation and explosion poten-
tial.
• Gasoline components are degraded in the sub-
surface by biotic and abiotic chemical transforma-
tion processes. Biotic processes include biodeg-
radation and biotransformation by microorganisms.
Abiotic chemical transformations include hydro-
lysis and oxidation reduction reactions. Degrada-
tion by microorganisms occurs in both aerobic
and anaerobic environments; however, for most
gasoline compounds the rate of biodegradation is
higher under aerobic conditions (i.e., in the
unsaturated zone).
Numerous treatment technologies are available to
remove gasoline from air, soil, and water. Detailed
discussions of some of them are presented in the
following sections. Section 3 covers treatment
technologies for removing free product from the
water table; Section 4 covers gasoline adsorbed in
the unsaturated zone; and Section 5 covers
gasoline dissolved in groundwater.
References
Bear, J. 1979. Hydraulics of Groundwater. New
York: McGraw Hill.
CONCAWE, 1979. Protection of Groundwater From
Oil Pollution. The Hague, Netherlands: CON-
CAWE.
Davis, J.B., Farmer, V.E., Kreider, R.E., Straub,
A.E., and Reese, P.M. 1972. The Migration of
Petroleum Products in Soil and Groundwater, Prin-
ciples and Countermeasures. American Petroleum
Institute. Washington, D.C.: American Petroleum
Institute Publication No. 4149.
Fleischer, E.J. 1987 An Evaluation of the Subsur-
face Fate of Some Organic Chemicals of Concern.
M.S. Thesis Presented to the University of Mas-
sachusetts Department of Civil Engineering.
Johnson, R.C., and Dendron, S.R 1984. Groundwa-
ter Transport Modeling as a Regulatory Technique
for Protection from Hydrocarbon Contamination.
Lyman, W.J., Reehl, W.F, and Rosenblatt, D.H.
1982. Handbook of Chemical Property Estimation
Methods. New York: McGraw Hill.
Maynard, J.B., and Sanders, W.N. 1969. Determi-
nation of the Detailed Hydrocarbon Composition
and Potential Atmospheric Reactivity of Full-Range
Motor Gasolines. Journal of the Air Pollution Con-
trol Association. Vol. 19.
13
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Section 3
Recovering Free Product
The principal means of recovering floating conta-
minants from the groundwater surface is the use of
the natural water gradient to control the movement
of the contaminants. This is accomplished either by
inducing a water gradient or by influencing an exist-
ing one artificially. Pumping wells and trenches are
the devices most commonly used to influence the
flow of groundwater.
Pumping wells sunk several meters below the
water table surface remove water from the aquifer,
creating depressions in the water table into which
floating oil and gasoline accumulate. Trenches dug
perpendicularly to a groundwater gradient intercept
the flow of floating contaminants. Once enough
floating free product has accumulated in a water
table depression or a trench, it can be recovered
with skimmers, filter separators, or special pumps.
The use of pumping wells and trenches to influence
the flow of an aquifer minimizes the threat to adja-
cent groundwater bodies and soil by containing the
spread of floating free product. Also, by accumulat-
ing floating product in water table depressions and
trenches, both free product and dissolved con-
stituents can be removed. It should be noted that a
basic understanding of the hydrogeology and soil
characteristics at the site and the extent of the spill
is essential to the effective recovery of floating free
product.
In this section, the different methods of free product
recovery and available types of oil/water separation
equipment are discussed and analyzed for cost,
efficiency, and limitations. The information pre-
sented is based on contacts with groundwater con-
sultants, with free product recovery equipment
designers and technicians, and on Camp Dresser
& McKee Inc.'s experience.
Methods of Gasoline Plume
Containment
Trench Method
One of the simplest free product recovery
strategies is to dig a trench with a mechanical
excavator down to the water table and intercept the
flow of the floating gasoline. This method is applica-
ble only when the water table is relatively shallow
and the gasoline plume is less than 10 to 15 ft
below the ground surface. Once the groundwater
flow direction and plume size have been estab-
lished, a trench is dug in the path of the migrating
plume. The trench is dug deep enough so that the
groundwater "ponds" and the floating gasoline is
exposed (Figure 5). To increase the flow of gasoline
to the trench, water in the trench below the surface
may be pumped out. In doing so, a hydraulic gra-
dient is created, more groundwater is pulled toward
the trench, and the aquifer is induced to redirect the
movement of the floating gasoline. To ensure that
the intercepted gasoline does not escape back into
the soil, an impermeable membrane can be placed
on the downgradient side of the trench. The mem-
brane can serve as a baffle preventing the flow of
gasoline but allowing water to pass under it.
CROSS SECTION
FREE GASOLINE
GBOUNDWATIR ROW
Figure 5. The trench method of recovering free product.
15
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Gasoline ponding in the trench can be removed
with a variety of portable, free-floating contaminant
recovery devices. Some equipment, such as filter
separators, work automatically only when gasoline
is present in the trench, separating and removing
the gasoline from the water. Other devices include
hand-held skimmers, which are no more than
sophisticated floating vacuum cleaners with hydro-
carbon sensors. In cases where both gasoline and
water are pumped out of the trench, standard
gasoline recovery equipment can be used. Large,
nonportable oil/water separation tanks, like those
used for industrial applications and at gasoline and
oil refineries, are commonly used. (For further dis-
cussion, see Gasoline Recovery Equipment, p. 18).
For the trench method to be implemented success-
fully, the groundwater and soil conditions must be
favorable. The water table should be high (i.e., less
than 10 to 15 ft below the ground surface), and the
soil above the water table must be firm and well
aggregated so that the trench is self-supporting.
Temporary trenches may not need support to pre-
vent the trench from caving in, but for long-term
recovery the trench may be partially backfilled with
crushed stone and coarse gravel on the sides, or
supported with plywood siding or concrete slurry
walls. As a rule, a wide trench has no particular
advantage over a narrow one. But in general, the
longer the trench, the faster it will collect gasoline,
provided that the water table is kept depressed.
Pumping Well Method
For sites with wells or with a water table that is too
deep for trenches to be effective, well pumping
strategies are used to influence the aquifer and
recover gasoline spilled on the groundwater sur-
face. Once the characteristics of the aquifer have
been established and the direction of groundwater
flow and size of plume are known, water pumping
rates can be calculated which will contain the mi-
gration of the contaminant plume. Groundwater
models and other analytical techniques are availa-
ble to assist in the proper siting and sizing of con-
tainment wells. If a single well and the "cone of
influence" or depression it produces are not suffi-
cient to contain the spread of the plume, multiple
wells may be drilled. The wells should be
positioned with respect to the plume and in proxim-
ity to one another in such a way that the cones of
influence overlap and thereby prevent the migration
of the plume beyond the influence of the wells
(Figure 6).
Single Pump Systems
In a single pump system, both gasoline and water
are recovered through a single pipeline to above-
ground storage tanks or oil/water separators (Fig-
ure 7). Two problems are associated with single
pump systems: (1) large volumes of contaminated
water must be stored, treated, and disposed; and
(2) during pumping, gasoline and water are mixed,
thereby complicating aboveground separation. For
these reasons, single pump systems are most
commonly employed for smaller spills when the
gasoline-water recovery rates are relatively low
(e.g., less than 500 gal/h).
Dual Pump Systems
In cases where large amounts of gasoline must be
recovered, two pump systems are frequently
employed. Dual pump systems using separate
gasoline and water pumps facilitate separation of
gasoline and water in the well, thus significantly
reducing the amount of water that must be treated.
As in the trench method, water pumps are lowered
into the wells up to depths of 10 to 15 ft below the
water table surface. The pumps draw in water from
all directions and establish a cone of influence or
depression in the water table. Floating gasoline is
drawn into the depression where it can be reco-
vered as free product with a product recovery
pump (Figure 7).
The "water table depression pump," as it is called,
should maintain a constant, or nearly constant,
cone of influence to prevent the migration of the
gasoline plume. If a constant depression is not
maintained and the water table and the gasoline
plume are allowed to rise, gasoline droplets may
adhere to soil particles. As the water table con-
tinues to rise, the density differential between the
gasoline and water would not be great enough to
overcome the adhesive forces of the soil particles,
and the gasoline droplets would remain in the soil.
If the cone or depression is allowed to recover com-
pletely, the gasoline plume will once again be free
to migrate along the natural groundwater gradient.
Dual pump systems operate in the following way.
Initially, the water table depression pump probe is
set at an arbitrary depth in the well to which the
water table will be depressed. The water table
depression pump is then lowered approximately 10
ft beyond the probe and pumping is begun. As
water is pumped out of the well, the water table and
floating product are drawn down until the water
pump probe detects the presence of hydrocarbons.
When this occurs, the water pump will cease
pumping and the depressed water table will rise
slightly. As soon as the water pump probe detects
water again, however, it will resume pumping and
the depression will be maintained. Once a constant
depression has been established, the product
pump is deployed.
16
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DIRECTION OF
GROUNDWATER FLOW
EXTENT OF
GASOLINE PLUME
,' EXTENT OF CONE
OF INFLUENCE
Figure 6. Using overlapping cones of influence to contain gasoline plume.
SINGLE PUMP SYSTEM
CONCENTRATED
FREE PRODUCT
CX-WAfEff SEPARATOR
DUAL PUMP SYSTEM
PBCOUCT RECOVEW PUMP
Figure 7. Single pump and dual pump gasoline recovery systems.
17
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The inlet and probe of the product pump are set at
the same depth, a few inches above the probe of
the water table depression pump. As the water
pump draws in groundwater, gasoline will accumu-
late in the depression until the hydrocarbons are
detected by the product pump probe.
The probe of the product pump has the same func-
tion as the probe of the water table depression
pump: it activates the pump when gasoline is pre-
sent and turns the pump off when the gasoline
plume reaches an arbitrary minimum thickness or
when the water table fluctuates and water is
detected.
In addition to limiting the amount of water that must
be treated, the dual pump system has another
advantage over the single pump system: the dual
pumps function automatically. Barring equipment
failures, water table depression and product
removal are constant, and the system can operate
for weeks or months with only periodic inspections.
Once the gasoline plume has been drawn down to
within a fraction of an inch, the product pump probe
will no longer be able to detect the remaining
gasoline. At this point, the product pump turns off,
and the water table depression pump is elevated to
the depression and allowed to pump the mixture of
water and the remaining gasoline out of the well.
When the levels of contaminants in the recovered
gasoline-water mix have dropped to acceptable
water quality limits, pumping is terminated and the
well is considered clean.
It should be noted that in order to achieve drinking
water quality standards, considerable amounts of
money and technological resources must be
invested in the cleanup. Some sites could take
years to restore depending upon the complexity of
subsurface conditions and the volume of gasoline
spilled.
Limitations
Although trenches are commonly employed as an
effective means of containing the spread of subsur-
face spills, there are limitations associated with
their construction and use. The most serious limita-
tion is that trenches are feasible only when the
water table is relatively shallow and the floating free
product is less than 10 to 15 ft below the ground sur-
face. The cost of the trench excavation and mate-
rials (for example, concrete slurry walls and riprap
to support the walls of the trench, and gasoline-
impermeable liners to prevent the flow of gasoline
through the downgradient side of the trench),
increases significantly with depth. Below 10 to 15 ft
the cost of the trench method becomes more than
the cost of using other containment methods, for
example, the pumping well method.
Another limitation is the problem of extracting the
free product once it enters the trench. Pumping and
skimming must be continuous to maintain a flow
gradient to the trench. Otherwise, the floating free
product will tend to move to the ends of the trench
and pass around the impermeable liner. When
using only pumps to extract the gasoline-water mix,
rather than including skimmers and filter separators
to perform in situ free product recovery, above-
ground storage and separation of the trench liquids
may pose problems. For example, storage and
transportation of the gasoline-water mix requires
special handling precautions; likewise, purchasing
or renting an oil/water separator, or finding a suita-
ble disposal alternative for the gasoline-water mix
will considerably increase the costs of free product
recovery.
Gasoline Recovery Equipment
In the United States, more than 25 companies
design and sell equipment and provide technical
advice on gasoline recovery from subsurface spills.
Many of the companies deal strictly in aboveground
oil/water separators such as those typically used at
petroleum refineries and wastewater treatment
plants. Others have created their own lines of in situ
oil/water separation devices specifically designed
to separate oil and water underground and recover
free product. Site-specific, state-of-the-art equip-
ment is available which can recover free product
from a variety of adverse subsurface conditions.
There are narrow pumps for small wells, filter
separators which operate passively, and special
dual pump systems for deep wells. Yet, even as
designers produce new and improved equip-
ment, none claim that their oil/water separation
equipment can recover 100 percent of the spilled
product.
It is an accepted fact that a certain percentage of
the spill will always be trapped in the unsaturated
soil as the plume migrates from the spill site to the
water table. Only the portion of the original spill that
ends up as free product on the water table is readily
recoverable.
The following subsection is a discussion of the dif-
ferent types of oil/water separation equipment
available for recovering gasoline that has reached
the groundwater table. The equipment is evaluated
for ease of operation, removal efficiency, limita-
tions, and cost. For more detail on the pumps
and recovery equipment available on the market,
see Tables 4 through 8 at the conclusion of this
discussion.
18
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Skimmers and Filter Separators
for Trenches
Skimmers
Skimmers are designed to float and automatically
pump gasoline off the water surface. The most
effective skimmers are equipped with conductivity
sensors to detect gasoline. When gasoline-free
water is present, an electric signal is passed
between the sensors, and the gasoline pump does
not operate. But when gasoline, which is noncon-
ductive, is present, the electric signal is interrupted
and the pump is automatically turned on. The
pump extracts gasoline from the trench until clean
water is detected by the sensors. Skimmers are
easily deployed and may be set up temporarily or
permanently, or they may be attached to a handle
and operated manually.
For manually operated skimmer equipment, the
gasoline recovery pump is not attached to the float-
ing sensor. Rather, it is set on the ground above the
trench and connected to the skimmer with a syn-
thetic hose to remove the gasoline. A 1/4-hp pump
can recover product at a rate of 2.8 gal/min or
4,000 gal/d.
One advantage of skimmers is that they can pass
grit and debris up to a quarter of an inch thus allow-
ing unfiltered gasoline to be recovered. Once the
skimmed product has been removed from the
trench, it is stored in recovery drums for further
treatment or disposal. Skimmers can recover
water-free gasoline to the limit of the sensor's ability
to distinguish gasoline from water (usually at a
depth of a fraction of an inch). Then, with the
gasoline sensor turned off, skimmers suck up the
remaining gasoline mixed with small amounts of
water from the water surface of the trench. The
average capital cost of a skimmer is $6,000 to
$7,000, but combined with a water table depression
pump to increase the flow of gasoline to the trench,
a skimming system could cost as much as $12,000
to $13,000.
Filter Separators
Like some of the skimmers, filter separators float
on the trench water surface and pump gasoline
automatically and continuously. Yet, unlike skim-
mers, which operate with the aid of conductivity
sensors, filter separators have special filters that
allow gasoline and other petroleum products to
pass but repel water. The filter separator floats so
that the oleophilic-hydrophobic ("oil-loving"—
"water-hating") membrane is positioned at the
gasoline-water interface. Both gasoline and water
contact the filter, but only the gasoline moves
through. Once a small amount of gasoline (approxi-
mately one liter) has accumulated within the
separator's compartment, a floating arm is raised
which sends an electric signal to activate the
gasoline recovery pump and the compartment is
automatically drained.
The gasoline recovery pump is located above the
trench and is connected to the filter separator with
a gasoline-resistant hose. A 1/4-hp pump can
remove gasoline from the separator at a rate of 5
gal/min. Filter separators of this kind are portable
and easily installed, and can reduce a gasoline
plume in a trench down to a sheen. They generally
cost about the same as skimmers ($6,000 to
$7,000), but if a water table depression pump is
required, the filter separator system could cost as
much as $12,000 to $13,000.
Filter Separators and Dual Pump
Systems for Shallow Wells
Filter Separators
The same type of filter separators that are used for
trench equipment may be used in shallow wells.
The design and operation of the unit are the same,
but there are more variables to consider when
using filter separators in shallow wells.
It should be noted that pumps are generally clas-
sified according to their pumping position with
respect to the well structure, regardless of the
depth of the well. A pump installed above a well is
called a shallow well pump, and a pump below the
ground surface inside a well is called a deep well
pump. This means that it is possible to have shal-
low well pumps pumping from greater depths than
deep well pumps. To avoid confusion in this handbook,
shallow wells are defined as wells in which the
depth from the top of the well to the liquid surface is
less than 20 ft, and deep wells are wells in which
the depth from the top of the well to the liquid sur-
face is greater than 20 ft.
The first consideration when using filter separators
is that they be deployed only to a maximum depth
of 20 ft. Although the separation unit floats on the
water table surface, its surface-mounted pump is
physically unable to provide more than 20 ft of lift
(head). To achieve greater pumping heads, sub-
mersible pumps would be needed, but submersible
pumps cannot be attached to filter separators
because the heavy pump would cause the floating
separator to sink. Therefore, filter separators can
be used only with surface-mounted pumps in shal-
low wells. (For more detail, see Dual Pump and
Single Unit System for Deep Wells, p. 20.)
A second consideration when using a filter
separator is maintaining a steady flow of gasoline
to the separator. This is done by deploying a water
table depression pump below the groundwater sur-
19
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face. The pump removes water from the well, creat-
ing a depression in the water table. The floating
gasoline flows into the recovery well and accumu-
lates on top of the depression where it can be eas-
ily separated by the filter.
Filter separators are more difficult to deploy in shal-
low wells than in trenches because a water table
depression pump is required. A cone of influence
must be maintained to trap the floating gasoline
and, as a result, the system is more expensive and
requires more time and supervision to achieve and
maintain conditions amenable to the filter
separator. Gasoline removal efficiencies of filter
separators in shallow wells are comparable to filter
separators in trenches. In both cases, the filter
separator can reduce the gasoline plume to a
sheen on the water table. To achieve additional
recovery, the top layer of gasoline and water must
be removed from the well and treated above-
ground.
Dual Pump Systems
Of all the oil/water separation equipment available
on the market, dual pump systems composed of
water table depression pumps and product recov-
ery pumps are the most common. In all cases, the
water table depression pumps are designed to
pump water out of the well and thereby create
depressions in the water table into which floating
free product accumulates. The product recovery
pumps are designed to pump water-free gasoline
out of these depressions. Dual pump systems
come in a range of sizes and pumping capacities to
meet a variety of well diameter, depth, and pumping
conditions. Water pumps come in sizes as small as
3 1/2-in diameter for4-in wells and as large as 10-in
diameter for 12-in and 24-in wells. Water pumps
range in pumping capacity from 1/3-hp units, which
have a maximum pumping rate of 15 gal/min and a
maximum total dynamic head (TDH) of 130 ft, to
7 1/2-hp units with a maximum pumping rate of 230
to 500 gal/min and a maximum TDH of 300 ft.
Product recovery pumps come in similar sizes and
pumping capacities, though, as a rule, they are not
required to do as much pumping as water table
depression pumps.
Water table depression pumps and product recov-
ery pumps are equipped with sensors which allow
them to pump only pure product. As the free prod-
uct is removed from the water table depression and
the lens of gasoline becomes too thin for the prod-
uct recovery pump sensor to detect, water-free
gasoline recovery will cease. To remove the
remaining portion of the gasoline lens, as well as
the gasoline constituents dissolved in the ground-
water, the top layer of water in the well is pumped
out to bring a mixture of gasoline emulsions, dissol-
ved materials, and water to the ground surface.
Once the level of constituents in the groundwater is
within acceptable limits, the well is considered
clean and pumping is terminated.
Dual pump systems are capable of removing up to
99 percent of the free product and dissolved
gasoline constituents from groundwater. One
hundred percent removal is impossible because
dissolved constituents migrate both vertically and
horizontally away from the plume and are therefore
dispersed through a much greater volume of water
than is the undissolved free product. Furthermore,
as aquifer recharging and water table fluctuations
occur, gasoline emulsions and dissolved con-
stituents trapped in the soil may make their way
down to the aquifer, thus further contaminating it.
Dual Pump and Single Unit
Systems for Deep Wells
The technology of deep well pumps is different
from that of shallow well pumps. When the pump-
ing level in a well exceeds 20 ft, physical restric-
tions limit the type of pumps that can be used. Sur-
face-mounted pumps, which rely on atmospheric
pressure to provide suction lift, have a theoretical
maximum lifting capacity of 34 ft. Beyond 34 ft, the
pull of gravity exerted on the rising liquid column
exceeds the capacity of the pump to provide lift. In
practice, the 34-ft theoretical maximum is never
achieved—20 ft is the highest lift that can be
expected with surface-mounted pumps. As a
result, submersible pumps must be used in wells
where pumping water depths exceed 20 ft. Sub-
mersible pumps do not rely on suction lift; rather,
they are submerged in the well, below the liquid
surface and, with the aid of pistons, rotors, vertical
turbines, jets, or compressed air, they push the liq-
uid out of the well.
Surface-mounted pumps have three distinct advan-
tages over submersible pumps: (1) they are easier
to operate and maintain because they are above-
ground; (2) they are generally less expensive (sub-
mersible pumps must be made explosion-proof
due to the presence of volatile hydrocarbons and
also must be able to pump in corrosive environ-
ments); and (3) they generally have a longer life-
span (on average, two to three years longer than
submersible pumps, which are exposed to
gasoline, oil, and other corrosive chemicals).
Dual Pump Systems
Dual pump systems for deep wells operate gener-
ally in the same manner as dual pump systems for
shallow wells. Two pumps are employed. A water
table depression pump contains the migration of
the gasoline plume, and a product recovery pump
20
-------
draws off the gasoline that has accumulated in the
water table depression. The two pumps are usually
set some 10 to 15 ft apart to ensure adequate draw-
down and to ensure that the water table depression
pump does not come in contact with the gasoline.
Each pump is equipped with its own sensor to pre-
vent the pumping of gasoline-water mixtures, and
each is operated independently so that a constant
depression in the water table is maintained while
gasoline is being recovered.
The main difference between dual pump systems
for deep wells and shallow wells is that in deep
wells greater pumping distances and more extreme
pumping conditions are found, requiring more pow-
erful, durable pumps. Water table depression
pumps and product recovery pumps are available
that can pump from depths as great as 500 ft and
can withstand the corrosive effects of saltwater and
water laden with sediments. The 2- and 3-hp water
table depression pumps commonly used in deep
well recovery operations are rated to pump a maxi-
mum of 60 gal/min and have a maximum head of
150 ft. Product recovery pumps used in deep wells
are seldom required to pump as much as water
table depression pumps and therefore have lower
ratings.
Another important distinction between dual pump
systems for deep and shallow wells is that surface-
mounted, suction lift pumps cannot be used, since
deep wells (as defined here) are wells in which
pumping depths exceed 20 ft. Only submersible
pumps can be used in deep wells. Moreover, due to
their greater pumping capabilities and other fea-
tures that allow operation under adverse conditions
(for example, explosion-proof drive units, water-
tight seals, electric cables), deep well submersible
pumps are more expensive than surface-mounted
pumps. On the average, submersible pumps are 10
to 15 percent more costly than surface-mounted
pumps. As a result, dual pump systems for deep
wells are more expensive than dual pump systems
for shallow wells.
Another economic distinction between shallow well
and deep well recovery systems is seen in well dril-
ling costs. As Table 3 shows, drilling costs increase
linearly with depth.
Costs include engineering and labor, and it is
assumed that the wells are auger-drilled and
gravel-packed, and that they have galvanized steel,
gasoline-resistant screens. It is also assumed that
the wells are drilled in sandy-gravel soil and that the
wells yield pumping rates of 20-100 gal/min. These
cost figures were taken from a survey of practicing
drillers.
Diameter
4-m.
6-in.
8-in
10-in
24-in
Table 3
Well Drilling Costs
Cost/ft
$ 90-120
$100-130
$120-160
$150-200
$300-350
A 4-in well drilled to 20 ft costs $2,100, while the
same size well drilled to 40 ft costs $4,200. The
diameter of the well also affects cost. As Table 3
indicates, the larger the diameter of the well, the
greater will be the per-foot drilling costs. For exam-
ple, an 8-in well costs, on the average, $35 more
per foot to drill than a 4-in well, while a 10-in well
costs about $70 more per foot than a 4-in well.
Well drilling costs are important because they influ-
ence the treatment options available at the site.
Unlike recovery equipment costs, which increase
only moderately with incremental changes in
design capacity, well drilling costs show steep
increases with slight changes in diameter and
depth and may, in some cases, exceed the costs of
the recovery equipment. For this reason, when
recovery options are being considered, close atten-
tion should be paid to optimizing drilling and recov-
ery costs.
Single Unit Systems
In wells with limited access, such as small diameter
wells, single unit dual pump systems can be
deployed. Single unit systems equipped with both
water table depression and product recovery
pumps are available to fit wells as narrow as 4 in. in
diameter. The product recovery pump is attached
above the water table depression pump, and both
pumps are equipped with sensors that control
pumping in the same manner as described above
for dual pump systems. Single unit dual pumps for
narrow wells have low pumping rates (i.e., 0.6 gal/
min at a maximum depth of 160 ft), but they sell for
as little as $12,000.
21
-------
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25
-------
Aboveground Oil/Water
Separators
Aboveground oil/water separators can be used as
an alternative to in situ gasoline and water separa-
tion with water table depression pumps and prod-
uct recovery equipment. Oil/water separators are
typically used at oil refineries and wastewater treat-
ment plants, but they can also be used to treat
groundwater that has been contaminated with
hydrocarbons. The separators are little more than
large tanks into which the hydrocarbon and water
mixture is pumped. Their main function is to slow
the flow of the incoming water and to allow gravity
separation of the less dense hydrocarbon emul-
sions. Separators have been successfully used at
many sites but seem to be most effective when the
hydrocarbon spill is relatively small and the rate of
water flow through the separator is slow enough to
allow for complete separation.
Oil/water separators are composed of two or more
chambers. The first (the inlet or preseparation
chamber) is for the deposition of settleable solids,
and the second (the separation chamber) is for the
separation of liquids of dissimilar specific gravities
and the removal of the lighter liquid from the
heavier liquid. Hydrocarbon emulsions and water
recovered from a well are pumped into the
separator through the inlet nozzle. The high veloc-
ity flow is directed against a baffle that is sloped at
a 45° angle to the inlet. The baffle slows and dis-
perses the incoming flow into a diffuse cascade
that tapers outward and spreads across the entire
width of the separator. Once the flow moves
beyond the baffle, its turbulence is significantly
reduced and gravity separation and settling can
begin.
Primary coalescence of hydrocarbon emulsions
occurs in the preseparation chamber. The less
dense hydrocarbon droplets rising with the density
gradient collide and fuse with adjacent droplets.
According to Stokes1 law, the larger the diameter of
the particle, the faster the rate of rise. Thus, as
small droplets coalesce to form larger droplets,
their upward vertical velocity increases. Separation
will continue as long as turbulence is minimized
because turbulence interferes with coalescence
and separation by breaking large globules of hydro-
carbons into smaller globules that are more easily
dispersed into water.
In some separators the preseparation and separa-
tion chambers are partitioned by coalescing tubes
or coalescing plates.Coalescing tubes stand verti-
cally across the width of the tank and are coated
with an oil-attracting, petroleum-based chemical.
As droplets coalesce on the tube surface, larger
droplets form which rise to the water surface.
Coalescing plates are also designed to enhance
the separation of hydrocarbon emulsions, but their
mode of operation is somewhat different from that
of the tubes. Coalescing plates are composed of a
stack of corrugated metal plates which rise at an
angle up to the water surface and extend across
the width of the tank. Water containing hydrocarbon
droplets flows between the plates, which are about
an inch apart. Droplets rising with the density gra-
dient accumulate and coalesce on the underside of
the plates, forming larger droplets with faster rising
rates. At the same time, solid particles suspended
in the water settle onto the top sides of the plates
and move by gravity to the bottom of the separator.
As the separated hydrocarbons begin to accumu-
late on the water surface, emulsion-free water is
directed away from the corrugated plates or tubes
and enters the separation sections. This quiescent
zone allows for further gravitational separation of
the remaining hydrocarbon emulsions. Once a dis-
tinct product layer has developed, it can be reco-
vered with filter separators, product recovery
pumps, or rotary pipe skimmers. A rotary pipe
skimmer is essentially a pipe with the top quarter
removed. The pipe is bolted to the side of the sep-
aration chamber and runs across its width. The
pipe is rotated manually into the flow causing the
layer of hydrocarbons to enter the pipe opening.
The skimmed hydrocarbons are poured into 30-gal
drums for disposal or re-refining. When skimming is
complete, the pipe opening is returned to the verti-
cal position.
Some oil/water separators are built with an outlet
zone for the discharge of clarified water. This third
chamber is separated from the separation chamber
by a partition that extends across the width of the
tank and down a few inches below the water sur-
face. The partition is designed to block the flow of
the hydrocarbon layer while allowing emulsion-free
water to move underneath to the discharge pipe.
Oil/water separators range from 100-gal units to
50,000-gal units, but they are sized to treat specific
volumes of water. Typically, separators are built to
hold 10 times the extraction rate of the well. For
example, a well being pumped at a rate of 100 gal/
min would require a 1,000-gal separator to ensure
adequate separation of hydrocarbons from water.
Water is retained in the separator for at least 10 to
12 min, which is the minimum time in which com-
plete gravity separation can be achieved. By under-
sizing the separator or increasing the extraction
rate from the well, the water flow rate through the
separator increases, thus reducing the retention
time. But this reduction in retention time decreases
the efficiency of the separator and allows hydrocar-
bon emulsions to remain in the separator effluent.
26
-------
Therefore, it is critical in the design of the system
that the volume of the separator tank be at least 10
times the extraction rate from the well. Under
optimum conditions, an oil/water separator can
reduce the amount of hydrocarbon emulsions in
water to 15 ppm (1 ppm = 1 mg/ L).
The cost of oil/water separators is a function of the
design capacity of the tank. For instance, a 1,000-
gal separator (designed to handle a well extraction
rate of 100 gal/min) costs between $5,500 and
$6,000; a 5,000-gal separator costs between
$10,500 and $12,000; and a 10,000-gal separator
costs between $15,500 and $17,500. The costs will
vary depending on what additional features are
purchased. Exterior corrosion protection, for exam-
ple, will increase separator costs by 10 percent,
additional coalescence units will increase costs by
20 to 30 percent, and sensors and automatic prod-
uct recovery equipment will cost an extra $5,000 to
$7,000.
Subsurface Installation of
Oil/Water Separators
A recent innovation in using oil/water separators to
recover free floating hydrocarbons from subsurface
spills has been to install the separator unit below
ground, flush with the water table.
The main advantage of this technique is that the
gasoline plume, which moves with the groundwater
gradient, can be intercepted and recovered with
minimum energy input. The plume is trapped and
directed to the separator influent nozzle with either
a subsurface drainage network—similar to an
aboveground municipal storm drain system—or
with a dike and an impermeable membrane to
retard the flow of the plume. Both water and the
intercepted hydrocarbons move by gravity flow
through the separator inlet and into the separator
chamber. Once separation of emulsions from water
has occurred and the gasoline plume has rede-
veloped at the top of the separator, it is recovered
with a product recovery pump, and the emulsion-
free water is allowed to flow through the discharge
back to the groundwater.
Because underground installation of oil/water
separators is a relatively new remedial technique,
little cost information is available. Despite this lack
of information, several noneconomic considera-
tions may make underground installation advan-
tageous. For example, underground installation
reduces the likelihood that water will freeze in the
separator, eliminates the evaporation of potentially
dangerous volatile hydrocarbons, and saves
aboveground space for other uses. The disadvan-
tages include the problem of excavating a hole
large enough and deep enough to install the
separator at the water table, as well as the quality
of the separator effluent, which normally has a
residual dissolved concentration of 15 ppm.
Limitations
The main limitation of using pumping wells is that
they are time-consuming to install and cannot
always be deployed soon enough to contain the
migration of rapidly moving spills. When imple-
menting the pumping well method, a careful hydro-
geologic investigation of the groundwater flow
characteristics and spill size is needed to deter-
mine the optimum location and number of wells to
contain the spill. Hydrogeologic investigations are
lengthy procedures, however, and often require
weeks and even months to complete. Likewise,
once a well has been drilled and a water table
depression pump has been installed, there is a lag
period (dependent on the conductivity of the
aquifer) between the start of pumping, the creation
of the depression, and the containment of the spill.
Therefore, in cases where rapid deployment of
containment measures are required, other less
time-consuming methods such as the trench
method may be more suitable.
A second limitation associated with using the
pumping well method is that the water table
depression must be kept constant. If a constant
depression is not maintained and the water table is
allowed to fluctuate, gasoline droplets may adhere
to soil particles and be trapped below the water
table. If the depression is lost completely, the float-
ing free product will once again be free to flow with
the groundwater gradient. Likewise, if the depth of
the depression is lowered, a greater volume of soil
will be exposed to the gasoline plume and further
contamination of unsaturated soil may result.
When using single pump systems to recover free
product from the water table, two problems arise.
First, since only one pump is used, large volumes
of gasoline-laden water are recovered. Once the
gasoline-water mix is pumped out of the well, it
must be stored, treated, and properly disposed.
The second problem is that gasoline and water
become well mixed during pumping, further com-
plicating the separation process. In northern cli-
mates, it is usually necessary to winterize free
product recovery equipment. Ice can form inside
oil/water separators and other types of equipment,
thereby reducing their effectiveness.
Finally, caution must be exercised when digging
trenches or drilling wells so as not to rupture under-
ground utilities. Locations of water and sewer
pipes, gas mains, electrical wiring, telephone
cables and other types of underground conduits
should be determined before excavation.
27
-------
Disposal of Recovered
Gasoline and Contaminated
Water
Gasoline recovered from subsurface spills can be
disposed of by incineration, or, in many cases, it
can be reused with little or no treatment. If the dis-
posal option is chosen, the gasoline must be stored
and transported with care to ensure that no further
spills occur. If the gasoline is to be reused, it must
be re-refined or mixed with other gasoline because
it degrades ("weathers") while in the soil.
The cumulative effects of three processes leads to
the degradation of gasoline. In the presence of oxy-
gen, aromatic hydrocarbons such as benzene,
toluene, and xylene are oxidized; gasoline con-
stituents are metabolized by soil microbes; and
hydrocarbons are dissolved in water stored in soil
pores and at the water table surface. If recovered
and used immediately, weathered gasoline can
cause pinging and knocking in automobile engines
thereby necessitating blending or re-refining.
Recovered water that contains small amounts of
floating free product and dissolved constituents
must first be passed through oleophilic-hydro-
phobic absorbent filters to remove the remaining
free product. Once the emulsions are removed,
four options are available to dispose of the water
and dissolved hydrocarbons. The first option is to
recharge the aquifer with the recovered water in
order to flush out the remaining pockets of free
gasoline. The main drawback to this technique,
however, is that the recharging water still contains
dissolved constituents and by flushing the unsatu-
rated zone, the constituents are merely recycled
back into the groundwater.
The second option is to discharge the water to a
natural water course where dilution and exposure
to oxygen will reduce the threat posed by dissolved
gasoline constituents. To do this, a National Pollut-
ant Discharge Elimination System (NPDES) permit
is required. At this writing, there are no established
Federal water quality standards for discharging
contaminated groundwater to natural water
courses, although some states and local authorities
have established their own standards for the quality
of discharged groundwater. It is advisable that "best
engineering judgment" be exercised and consider-
ation be given to maintaining the quality of receiv-
ing waters when discharging.
The third option is to send the water through a
wastewater treatment plant where adsorption sys-
tems can remove the remaining dissolved con-
stituents. An obvious problem with this technique is
whether sewer lines are available near the recov-
ery well and whether the wastewater treatment
plant can handle the increased flow.
The fourth option, to treat the emulsion-free water
with on-site air strippers and carbon adsorption filt-
ration systems, is the most expensive. Air strippers
facilitate the volatilization of dissolved components
by increasing the water surface area exposed to
oxygen, and carbon adsorption systems adsorb
dissolved constituents out of the water. Together,
the two systems can reduce the level of dissolved
constituents to within the range of most drinking
water quality standards and produce highly potable
water. For a further discussion of air stripping and
carbon adsorption, see Section 5.
Conclusions
Before deciding which treatment options and
recovery equipment would be most effective in
remediating a subsurface gasoline spill, the
characteristics of the site must be known. A hydro-
geological investigation, complete with monitoring
wells and chemical testing, is needed to determine
the geology and soil characteristics of the site, the
depth to the water table, the groundwater gradient,
the size and migration patterns of the gasoline spill,
and the thickness of the plume. Without this basic
information, recovery operations could be severely
hindered. The location of underground utilities must
also be considered when drilling or digging.
Once the site analysis is complete, the most impor-
tant consideration is how to contain the migration of
the gasoline plume. In a shallow spill, it is usually
possible to respond more rapidly and effectively
with a trench than with a well system. The reason
for this is that equipment and contractors for this
type of installation are readily available in most
areas, and recovery from trenches is less compli-
cated.
Once the recovery method has been selected, the
equipment must be considered. Skimmers, filter
separators, surface-mounted product recovery
pumps, and aboveground oil/water separators can
all be used in recovering gasoline from trenches.
Another advantage of using a trench is that it can
be dug long enough to intercept the entire plume,
thus allowing complete aboveground recovery of
free product. Aboveground recovery is desirable for
several reasons. First, all the product recovery
equipment is surface-mounted and is therefore
easier to operate and maintain than submersible
equipment. Second, more recovery equipment
options are available (for example, filter separators,
portable skimmers, surface-mounted pumps, and
oil/water separators). Finally, aboveground recov-
ery is less costly and time-consuming than subsur-
face recovery.
Although the trench method is a time-saving,
economical, and effective alternative to the pump-
28
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ing well method in shallow water table situations,
the advantages of trenches diminish as the depth
to the water table increases. At a given depth, the
total cost of excavating, installing a slurry wall to
hold up the sides of a deep trench, adding riprap to
cover the bottom, and lining the downgradient wall
with an impermeable, gasoline-resistant mem-
brane will exceed the costs and advantages of
implementing a pumping well system. The depth at
which the advantage shifts from one method to the
other is, of course, site specific. It depends on
many factors, including the size and thickness of
the plume, whether the plume is migrating in more
than one direction, soil moisture content, and
whether the trench can be maintained without col-
lapsing, not to mention other considerations such
as local zoning laws and whether the land available
is suitable for trench excavation. Therefore, at sites
with shallow water tables, there are no strict rules
governing when to implement the different
methods. Situations may arise in which the advan-
tages of using one method may offset the advan-
tages of using the other.
Beyond depths of 10 to 15 ft the feasibility of using
the trench method is significantly reduced, and at
these depths the pumping well method is almost
always implemented. Like trenches, pumping wells
can be used to contain the migration of the gasoline
plume, but unlike trenches, which must be dug
across an area large enough to intercept the flow of
the plume, a single well installed below the water
table can draw in water from all directions and
reverse the spread of the gasoline plume. Wells are
particularly useful in recovering large spills that
have spread over a wide area. By drilling several
wells and coordinating their pumping rates, the
cones of influence created by the wells can be
overlapped to contain the migration of a diffuse
plume.
All four trench recovery equipment options (skim-
mers, filter separators, surface-mounted pumps,
and oil/water separators) are equally capable of
recovering free product. Similarly, they can each be
purchased for an initial capital cost of under
$10,000 (with the exception of large oil/water
separators).
When considering the overall ease of operation,
however, filter separators have several advantages.
First of all, they are lightweight, portable, and can
float in trenches; second, gasoline is passively (i.e.,
no energy inputs are required) separated from the
trench water with an oleophilic filter; and finally,
water does not have to be removed from the trench
to facilitate recovery.
The main disadvantage associated with filter
separators, though, is that they are only able to
reduce a plume's thickness to a sheen on the water
surface. The sheen, defined as an iridescent oily
film on the water surface, is still considered to be
free product. To remove the sheen and the remain-
ing dissolved gasoline constituents, trench water
must be pumped out and treated with secondary
treatment equipment such as air strippers and acti-
vated carbon filters. If the gasoline plume poses an
immediate threat to adjacent groundwater sources
and rapid recovery is required, single pump and oil/
water separating systems offer a distinct advan-
tage: by pumping a steady flow of gasoline and
water from the trench, significant recovery of free
product and dissolved constituents can be
achieved quickly.
The two types of recovery systems most commonly
used in pumping wells are the dual pump systems
and the single pump and oil/water separator sys-
tems. The two are comparable in many respects.
Both have high gasoline recovery efficiencies, they
are similarly priced in many instances, and both
have been successfully employed in numerous
recovery operations.
In deciding which system offers the most advan-
tages, the characteristics of the plume and the
recovery site must be considered. If, for example,
the gasoline spill is at or near a critical groundwater
source and rapid recovery is paramount, oil/water
separation systems may be advantageous. The
two reasons for this are: (1) gasoline and water
pumped together in a single pump can be removed
faster than if two free-product-only pumps are
used; and (2) by generating only shallow cones of
depression with single pumps, less soil will be
exposed to the gasoline plume. On the other hand,
if large volumes of water must be extracted, it may
be more economical to use a dual pump system.
By recovering free product from within the well
itself, the amount of water that must be treated is
significantly reduced.
The differences between the two systems are out-
lined in Table 9 on the following page.
29
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Table 9
Advantages and Disadvantages of Dual Pump Systems vs. Single Pump and Oil/Water Separator Systems
Dual Pump Systems
Single Pump and Oil/Water
Separator Systems
Advantages
Two pumps recover gasoline and water separately
Pumps only when product is present
Instantaneous separation of gasoline
Pumps are portable
Advantages
Requires one pump with no sensor
Shallow depressions in water tables will minimize additional
soil contamination by plume
Allows more rapid recovery of product
Lower operation and maintenance costs
Multiple well costs are lower because a single separator can
be used for many wells
Disadvantages
Requires two pumps with sensors
Deep depression in water table may cause further soil
contamination by plume
High operation and maintenance costs
Multiple well costs are high because two pumps are needed
for each well
Disadvantages
Single pump mixes gasoline and water thus complicating
separation
Requires 10- to 12-min detention time for separation
Pumps continuously (even if product is not present)
Separator is stationary
The higher the pumping rate, the larger the separator and the
greater the equipment costs
30
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Case Studies
When considering alternative remedial techniques and the costs and advantages of different equipment,
it is useful to study solutions that have already been applied to leaks. A review of case histories provides
insights into approaching problems and anticipating complications.
Case Study No. 1: 83,000 Gallons
In October 1975, a leak developed in an above-
ground storage tank at a defense fuel supply sta-
tion in Virginia. An estimated 83,000 gal of JP-4 jet
fuel was lost. The spill did not pose an immediate
threat to drinking or irrigation water supplies, but it
was determined that the fuel could migrate off-site
and contaminate a spring-fed pond. Different con-
tainment options were considered.
The first option involved digging a trench down to
the groundwater surface to intercept the free prod-
uct plume. This idea was abandoned, though,
when it was decided that the sandy soil on the site
and the depth to the water table (17 ft) would inhibit
containment and recovery of the fuel. The soil
above the water table was too wet to support a
trench.
The second option was to drill a production-type
well and use a single pump system to recover both
fuel and water for aboveground separation. It was
thought that by using one or more wells, the
groundwater gradient could be reversed and the
free product plume could be contained. This plan
was not implemented, however, because a qual-
ified contractor and the necessary equipment could
not be located in time.
The third alternative was a "well point system"
using two lines of 4-in PVC piping to intercept the
flow of the plume. It was thought that by setting the
two lines of 25-hole pipes flush with and perpen-
dicular to the flow of the water table, a mixture of
plume and water could be skimmed off the surface
and the free product would be thus contained. This
option was finally selected.
In implementing the well point system, the PVC
pipes were connected to a surface-mounted 6-in
centrifugal pump. The recovered mixture of fuel
and water was pumped into a preexisting dike
drainage collection system that led to an oil/water
separator. During the first two weeks of pumping,
the extraction rate decreased from 650 gal/min to
200 gal/min and the amount of fuel recovered
decreased from 1,200 gal/d to 600 gal/d. After an
additional two weeks of continuous pumping, the
system yielded only 30 gal/min and fuel recovery
fell to an average of 380 gal/d. At the end of the
fifth week a total of 20,800 gal of fuel had been
recovered and the well point system was shut off.
Later, a 40-point well system was installed a few
feet below the first, but no additional fuel was
recovered.
Installation and equipment rental costs for the five
weeks was $21,500, and an equal amount was
spent on operational and overtime costs. Cost per
gallon of recovered fuel was $2.07 overall, with a
marginal recovery cost of $3.14/gal at the end of the
recovery operation. Of the original 83,000 gal of jet
fuel spilled, only 25 percent was recovered. The
remaining fuel was assumed to be contained within
the interstices of the soil where it would be subject
to physical, chemical, and biological degradation.
The recovered fuel was mixed and burned with
boiler fuel.
Case Study No. 2: 3,000 Gallons
A gasoline storage tank leak in Provincetown, Mas-
sachusetts, endangered the town's drinking water
supply. The gasoline spill occurred at a service sta-
tion located 600 ft from the South Hollow well field,
which supplied more than 60 percent of Provin-
cetown's drinking water. The 3,000-gal spill con-
taminated a half-acre area above an aquifer which
contributed directly to the well field. Concentrations
in excess of 1,000 ppm of gasoline-related hydro-
carbons (including benzene) were found in the
groundwater. When it was determined that the con-
taminants were migrating toward the well field, the
well field was shut down.
To control the flow of the free product plume, an
innovative containment system was devised. The
system was composed of two recirculation cells: a
smaller cell within a larger one. In the inner cell,
four recovery wells were drilled downgradient of the
service station to intercept the plume. Single pump
recovery systems installed in the wells extracted
both water and gasoline from the surface of the
aquifer. The recovered gasoline-water mix was
then pumped through an oil/water separator to
separate the undissolved gasoline emulsions.
Next, the separated water was passed through an
air stripping tower and activated carbon filters to
remove the dissolved hydrocarbon constituents.
After being treated, the water was placed in a
recharge bed upgradient of the contaminant plume
in order to flush trapped gasoline emulsions from
the soil and provide a constant flow of water
through the cell. The inner cell was designed to cir-
culate 36,000 gal/d. The outer cell, comprising a
single production well and recharge chamber, was
capable of circulating over 100,000 gal/d and was
31
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installed to provide additional containment in case
the plume moved beyond the influence of the inner
cell.
In all, 700 gal, or 23 percent of the original gasoline
spill, was recovered. The total cost of recovering
the free gasoline, including well drilling costs, the
oil/water separator, the pumps, and construction
and engineering services, was approximately
$49,220, or $70.31/gal. These figures were extrapo-
lated from costs for the entire water treatment sys-
tem and are therefore somewhat higher than the
cost of a free product recovery system purchased
separately; also, due to the small size of the spill,
the economies of scale were poor.
Case Study No. 3:
2,000 to
4,000 Gallons
In 1984, a retail gasoline station in eastern Mas-
sachusetts reported a leak of regular leaded
gasoline from an underground tank. At the time, the
operator drained the tank and discontinued its use,
but the other tanks were maintained and the station
remained open until 1985. Because the leak oc-
curred at a low rate over a long period of time, it
was difficult to estimate how much gasoline was
lost. Best estimates are that between 2,000 and
4,000 gal leaked from the tank.
In order to contain the spread of the plume and
recover the leaked gasoline, a 6-in diameter recov-
ery well was drilled. A stainless steel, submersible
water table depression pump installed in the well
removed water at a rate of 75 gal/min or 108,000
gal/d, and a submersible petroleum pump was
deployed to recover the gasoline from the water
table depression. Water extracted from the well
was run through a 3-ft diameter packed air strip-
ping tower to remove dissolved, volatile hydrocar-
bons. After passing through the air stripping tower,
the treated water was discharged into an aquifer
recharge trench located upgradient of the recovery
well.
The system was installed and activated in March of
1985 and has been in operation since. To date,
approximately 1,200 gal of gasoline have been
recovered. Since May of 1986, no free floating
hydrocarbons have been detected in the wells. At
the most recent monitoring (November 11, 1986),
the maximum dissolved hydrocarbon level
detected was 9.1 ppm.
The total cost to recover the spilled gasoline,
including well installation, water table depression
and product recovery pump systems, trench exca-
vation, O&M costs, and gasoline disposal costs,
was $112,000, or $93/gal of recovered gasoline. It
should be noted that in Massachusetts, contamin-
ants recovered from subsurface spills are classified
as hazardous wastes and must be disposed of
accordingly.
Case Study No. 4: 100,000 Gallons
In May of 1983, a gasoline leak was reported in an
underground tank at a service station in North
Babylon, New York. An estimated 100,000 gal had
been lost. Observation wells installed in the service
station property revealed that floating free product
was present 11 to 12 ft below the ground surface.
In some places the gasolineplume was found to be
as much as 18 in thick. Forty-four observation wells
were installed to develop groundwater contours
and to determine the direction of groundwater flow.
Once the plume was located, surface-mounted
product recovery pumps were sent down some of
the 4-in observation wells to initiate the cleanup
process while an automated system was being
devised. In the first week, 750 gal of gasoline was
recovered.
To contain the migration of the plume and increase
product recovery rates, three 26-in wells were
installed. The wells were placed 100 to 200 ft apart
in a line parallel to the water gradient. In the upgra-
dient well, which was nearest to the center of the
gasoline plume, a 15-hp submersible water table
depression pump was deployed. The pump
extracted water from below the water table surface
at a rate of 300 gal/min. As gasoline accumulated
in the well, it was removed with surface-mounted
product recovery pumps. The system began
operating on July 3, 1983, and within five months
27,000 gal of gasoline had been recovered.
Gasoline that had migrated beyond the influence of
the upgradient well was recovered in the first down-
gradient well. The second downgradient well has
yet to recover any gasoline. To date (November
1986), 28,500 gal of gasoline has been recovered.
The water being extracted from the wells contains
dissolved hydrocarbons, and it therefore requires
further treatment with air stripping towers. Air strip-
pers are able to remove from 90 to 95 percent of
the dissolved constituents. Once air stripping is
complete, the treated water is being discharged to
a natural water course, and the recovered gasoline
is being re-refined and sold. The total cost of recov-
ery, including the three wells, three dual pump sys-
tems, labor, engineering, and O&M costs to date,
has been nearly $225,000 ($337,500 if air and
water testing and indoor vapor monitoring costs are
included). This translates to $789 ($11.84) per gal-
lon of gasoline recovered.
32
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Discussion of Case Studies
By comparing the four case studies, an important
conclusion can be drawn: the cost of recovering
free product at a site depends more on the recov-
ery method and equipment required to perform the
cleanup than on the size of the spill. Case No. 3, for
example, was a gasoline spill of between 2,000
and 4,000 gal in which $112,000 was invested in
recovery operations, while in Case No. 1, 83,000
gal of jet fuel was lost, but only $43,000 was
invested in recovery.
The differences in costs result from the different
recovery methods used. In Case No. 3, a 48-ft by
6-in well was drilled, and a dual pump system was
used. In Case No. 1, a 4-in PVC pipe-interceptor,
which acted in much the same way as a trench,
was embedded at 17 ft, and an oil/water separator
was used for free product recovery. In comparing
costs, well drilling is (as a rule) considerably more
expensive than PVC pipe installation. Moreover, in
Case No. 3, recharge trenches were dug to fuel the
recirculation cell, and the recovery equipment was
purchased outright by the service station owner in
anticipation of a long-term cleanup operation. In
Case No. 1, on the other hand, recirculation was not
needed and the recovery equipment was rented,
not purchased, by the polluter because of the short
duration of the cleanup.
It should be noted that, although the renting of
recovery equipment is an option in cleanup opera-
tions, it may become uneconomical if long-term
recovery is required. Also, had recovery equipment
been purchased, not rented, the increased costs
would still have left the small costs of Case No. 1
far short of those of Case No. 3.
Other examples of cost disparities among the case
studies can be seen in Table 10
Cases 2 and 3 also show that free product recovery
costs are more dependent on the cleanup method
and equipment selected than on the size of the
spill. Both cases involved gasoline spills of roughly
3,000 gal, both occurred in sandy-gravel aquifers,
and both had relatively deep water tables. Yet,
despite these similarities, different recovery sys-
tems were implemented in the two cases, and the
costs of recovery in Case No. 3 were twice what
they were for Case No. 2. The main difference
between the two spills was the threat of migration
that each posed to adjacent, uncontaminated
groundwater sources.
In Case No. 2, the spill was migrating rapidly
toward a drinking water well field. As a result, single
pumps were sent down four recovery wells, a mix-
ture of gasoline and water was pumped off to an oil/
water separator, and in a relatively short period of
time, the migration of the gasoline plume was con-
tained.
In Case No. 3, the spill did not endanger other
groundwater sources, and the more time-consum-
ing process of establishing and maintaining a cone
of depression with a dual pump system was under-
taken.
Another important conclusion that can be drawn
from the four case studies is that in any given
gasoline spill, more than one recovery option may
be available. For example, in Case No. 1, two alter-
natives were considered: (1) to drill a well and use
a single pump and an oil/water separator; and (2) to
install a PVC pipe interceptor network, a surface-
mounted pump, and an oil/water separator. Both
choices were viable, but because a qualified con-
tractor and the necessary equipment could not be
located in time, the well system was abandoned
and the interceptor pipe had to be installed.
As a general rule, the shallower the water table at
the site, the greater the number of effective reme-
dial techniques available. This is especially true of
sites where the water table is less than 15 ft from
the ground surface, in which case the trench
method, the dual pump method, or the single pump
and oil/water separator method may be used.
One final point that should be made regarding the
case studies is that although recovery costs may
vary from site to site, these costs are relatively
small when compared with the total cost of restor-
ing gasoline-contaminated groundwater to drinking
water standards. Removing dissolved gasoline
constituents with air strippers and activated carbon
filters is often so costly that any savings achieved
by economizing on free product recovery strategies
are insignificant.
Case
No
Spill Size
gal
Table 10
Cost Summary for Case Studies
Amount
Duration of Recovered
Recovery gal
Total
Cost
Cost/gal
Recovered
83,000
3,000
2,000-4,000
100,000
5wk
15 mo
7 mo
20,800
700
1,200
28,500
$ 43,000
49,220
112,000
225,000
$ 2.07
70.31
93.00
7.89
33
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Section 4
Gasoline Removal From
Soils Above the Water Table
This section provides an overview of corrective
action technologies for removing gasoline from
unsaturated soils. The discussions cover the basic
mechanisms of removal, the effectiveness of the
corrective action under different situations, and
potential limitations of the technology. Also included
are estimated ranges of costs for implementation of
the alternatives.
A number of regulatory issues are associated with
the cleanup of contaminated soil. These regulatory
issues include determinations of what is consid-
ered a contaminated soil, how those determina-
tions are made, and how and where contaminated
soils shall be disposed of. It is not the intent of this
section to focus on and resolve these issues; rather,
this section provides useful information on the cost
and effectiveness of soil treatment alternatives.
The corrective actions for gasoline-contaminated
soils discussed in this section include:
• Excavation and disposal. Contaminated soil is
dug up and sent to a landfill.
• Enhanced volatilization. Rototillers and other
mechanical devices are used to increase the
evaporation of volatiles.
• Incineration. Contaminated soils are burned at
high temperature.
• Venting. Gasoline vapors are removed from the
soil without excavation.
• Soil washing. Gasoline constituents are leached
from the soil matrix.
• Biodegradation. Bacteria degrade gasoline either
in situ or aboveground in reactors.
Much information has been gathered on various
soil treatment techniques, but a great deal of uncer-
tainty remains about how the techniques work, and
what the controlling factors are to achieve maxi-
mum effectiveness. Soil treatment has not been
used as widely as groundwater treatment such as
air stripping and carbon adsorption. A large body of
engineering information exists for groundwater
treatment technologies; they are widely under-
stood, the theories and related equations have
been thoroughly developed, and the principal
design parameters are well known. This is not the
case for soil treatment, however, where the tech-
nologies are not well understood or well developed.
The information contained in this section for many
of the soil treatment technologies is based on
research results and preliminary pilot studies.
Excavation and Disposal
Gasoline-contaminated soil may be excavated and
transported to an approved disposal facility with
conventional construction equipment. It is probably
the most widely used corrective action undertaken
for gasoline-contaminated soils at this time; how-
ever, the increasing costs and ultimate disposal
problem of this alternative will make it less attrac-
tive in the future. The EPA (1984) provides detailed
descriptions of soil excavation and transport sys-
tems for remedial actions at uncontrolled hazard-
ous waste sites. The equipment types generally
used for excavation of soil include backhoes,
cranes, dozers, and loaders, and they would be
expected to be used for removing leaking under-
ground storage tanks (USTs) and piping.
Effectiveness
Excavating contaminated soils as an adjunct to
tank removal may be an appropriate way to elimi-
nate the major source of continued gasoline migra-
tion to the subsurface environment. Product will
drain from a soil saturated with gasoline under the
force of gravity until residual saturation is reached.
At residual saturation no additional fluid migration
from the soil should occur unless precipitation
washes gasoline from the soil profile.
The characteristics of a soil largely determine its
capacity to retain gasoline in liquid and gaseous
35
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phases under unsaturated conditions. Excavation
of soils at residual saturation can effectively remove
product from the environment. As shown in Table
11, excavating dry, fine-textured sands would be
more effective than coarser textured sands or
those that are at field capacity, because coarser
sands retain less gasoline.
Hoag and Marley (1986) evaluated the residual sat-
uration of gasoline in soil columns filled with coarse
sand, medium sand, fine sand, or a mixture of
coarse, medium and fine sand. The residual satura-
tion of the sands was evaluated under dry condi-
tions at field capacity as well as three different col-
umn-packing densities. Table 11 lists the degree of
saturation by gasoline of the soils determined by
Hoag and Marley (1986), expressed as the ratio of
the volume of gasoline to the volume of pore space.
They determined that gasoline residual saturation
decreases with increasing particle diameter, and
that a soil's capacity to retain gasoline decreases
when soils are at field capacity as compared to dry
soils. They also determined that at increased
densities the soil is able to retain more gasoline
because of the increase in total available surface
area per unit volume and the attendant decrease in
the average soil pore diameter.
Corrective actions that remove soils saturated with
gasoline would be expected to minimize effectively
the further migration of gasoline from the soils to
the water table. Soils at residual saturation would
not be expected to release substantial quantities of
product provided percolating precipitation or a fluc-
tuating water table is not a factor.
Limitations
Although gasoline-contaminated soils can be exca-
vated with conventional construction equipment,
the depth at which these implements can remove
soils is limited. Backhoes with 0.5 yd3 of capacity
have a maximum reach of 26 ft and a maximum
excavation depth of 16 ft. Larger backhoes (e.g., 3.5
yd3 capacity) have the ability to remove soils at
depths of up to 45 ft at maximum digging angles of
45° (EPA, 1984). Because of the shallow angle of
repose expected to be encountered in most situa-
tions, a significant amount of surface area will be
disturbed relative to the depth excavated.
Leaking USTS are found in various settings. Those
under paved areas, under buildings, or where sub-
stantial underground or overhead utilities exist may
not be as amenable to excavation as those where
little pavement or few structures exist. Congested
or heavily traveled areas may also limit excavation
techniques as a means of corrective action. Exca-
vation operations which interfere with the business
may be in some instances an unacceptable alter-
native.
Excavation requires the contaminated soils to be
removed to a considerable depth and then placed
on the soil surface or into transport vehicles. The
nature of excavation increases the potential of
exposing workers and the public in general to the
contaminants. For example, the removed soil is
susceptible to the effects of rainfall, which could
lead to runoff of contaminated materials from the
site. Therefore excavation facilities should be
Sand
Type
Table 11
Gasoline Retention at Residual Saturation
Moisture
Status
Residual Saturation
Percent
9/kg
Fine
Dry
Field capacity
54-60
20-26
92-122
34- 44
Medium
Dry
Field capacity
15-27
13-18
35- 47
24- 37
Coarse
Dry
Field capacity
15-19
34- 44
Mixed
Dry
Field capacity
46-60
55- 68
Source: Adapted from Hoag and Marley (1986)
36
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designed and operated to adequately protect the
health and safety of workers and the public, as well
as the environment.
The void created as a result of excavation must
generally be filled with clean soil as part of the cor-
rective action. The clean soil, however, can be con-
taminated by the fluctuation of water table eleva-
tions containing gasoline components. Therefore,
excavation may be most appropriate in situations
where soils are contaminated with gasoline compo-
nents above the residual saturation level.
In general, excavation may be an effective means
for removing contaminants from the soil environ-
ment; however, for the overall corrective action to
be effective, there must be a suitable means of dis-
posal.
Pursuant to RCRA and the Hazardous and Solid
Waste Amendments of 1984 (HSWA), EPA promul-
gated Land Disposal Restrictions (40 CFR 268) on
November 7, 1986 (FR 51(216):40572-40654).
Effective November 8, 1986, certain solvent- and
dioxin-contaminated soils are prohibited from land
disposal unless they result from CERCLA or RCRA
response actions. Soils resulting from CERCLA
and RCRA response action are excluded from pro-
hibition through November 8,1988.
Soils prohibited from land disposal include those
containing dioxin and those contaminated with
F001-F005 solvent constituents at concentrations
greater than 1 percent. The F001-F005 solvents
include components typically encountered in
gasoline, such as ethylbenzene, toluene, and
xylene. Gasoline products typically contain approxi-
mately 2 percent ethylbenzene, 12 percent toluene,
and 8 percent xylene by weight. Therefore, it is
likely that soils saturated with gasoline could
exceed the 1 percent limit for solvents and thus be
subject to land disposal restrictions.
Land disposal appears to be the most common
method for disposal of gasoline-contaminated soils
even though the potential liability associated with it
poses severe limitations. Since gasoline is amena-
ble to thermal destruction, volatilization, and
biodegradation, it may be best to opt for such treat-
ment rather than land disposal.
Costs
Costs for excavation and disposal can be segre-
gated into the following components: site prepara-
tion, excavation, material handling/staging, backfill
material, final grading, hauling, and disposal.
Site preparation costs may be minimal where only
minor excavation is required but may be significant
when large areas must be cleared. Site clearing
costs can range from $1,500 to $2,300/acre when
grubbing and stump removal is required. Where
paved areas are to be excavated, site preparation
costs may not be incurred.
Excavation costs will vary depending on the type of
equipment used. Backhoes/front end loaders with
capacities of 0.5 to 0.75 yd3 range from $3.55 to
$5.00/yd3 (Means, 1987); backhoes with 1 to 3.5
yd3 capacity range from $1.75 to $3.00/yd3 (EPA,
1984).
Material handling/staging costs range from $1.20 to
$4.50/yd3 depending on unit costs for dozers and
loaders moving soils on site.
Backfill material will cost from $10 to $20/yd3
depending on the distance the material is hauled.
Grading at the backfill will add an additional $2.50
to $3.50/yd3 to the costs of the backfill placement.
Hauling costs for removal of the soil to the disposal
site are largely dependent on the distance traveled
but range from $0.50 yd3/mi to $1.00 yd3/mi.
Disposal costs are highly affected by the type of
waste. Landfill tipping fees for ignitable wastes are
estimated to be $120/ton and for toxic wastes,
$240/ton (EPA, 1974). Landfill disposal, including
transport, of gasoline-contaminated soils typically
ranges from $125 to $200/yd3 according to a num-
ber of corrective action contractors. Tipping fees as
low as $5/yd3 were reported for "clean soils" and as
high as $120/yd3 at licensed hazardous waste
facilities. The majority of landfills reportedly require
flash point analysis at a minimum, with soils having
flash points below HOT being rejected. One tank
installer indicated that flash point is the characteris-
tic that most often causes soils to be rejected for
disposal by a landfill. This contractor has found that
spreading contaminated soil out on plastic sheets
and allowing volatilization to occur renders the soil
"clean" enough for disposal in a municipal landfill at
a tipping fee of $5/yd3. In this type of operation, soil
disposal costs may be as low as $40/yd3 including
excavation and hauling.
A significant amount of gasoline-contaminated soil
is apparently also being disposed at batch asphalt
plants. These facilities utilize the contaminated
soils in their production process. One plant charges
$55/yd3 for disposal provided the soil has passed
the EP toxicity test and does not contain chlori-
nated solvents. (EP refers to an extraction proce-
dure, one test of several to determine whether or
not a solid waste is a hazardous waste under
RCRA.) States differ with regard to control of
asphalt plant operations. In Massachusetts, for
instance, asphalt plants do not accept gasoline-
contaminated soils because they would then be
subject to the hazardous waste regulations applica-
ble to transport, storage, processing, and disposal
37
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facilities. Rhode Island, on the other hand, appar-
ently has no such requirement; asphalt plants will
accept gasoline-contaminated soils provided they
do not contain chlorinated solvents.
Enhanced Volatilization
Enhanced volatilization is any technique that
removes volatile organics from unsaturated soil by
putting contaminated soils in contact with clean air
in order to transfer the contaminants from the soil
into the air stream. The air stream is further treated
through the use of carbon canisters and/or water
scrubbers or afterburners to reduce air emission
impacts. A number of different methods are availa-
ble that can achieve this effect: mechanical rototill-
ing, enclosed mechanical aeration systems, low
temperature thermal stripping systems, and pneu-
matic conveyer systems.
• Mechanical Rototilling
This method involves turning over soils to a depth
of about 1 ft below the surface to increase the rate
of volatilization. Several passes of the rototilling
equipment over the soil may be required to effect
sufficient volatilization. Following treatment, the
topsoil is moved to a nearby pile, and rototilling is
performed on the next 1 ft of soil. The effectiveness
of this method is highly dependent on weather con-
ditions. High speed rototillers and soil shredders
can enhance the rate of volatilization.
• Enclosed Mechanical Aeration Systems
To effect volatilization, contaminated soils are
mixed in a pug mill or rotary drum. The gasoline
components are released from the soil matrix by
the churning action of air/soil contact. The induced
airflow within the chamber captures the gasoline
emissions and passes them through an air pollution
control device (e.g., water scrubber or vapor-phase
carbon adsorption system) before they are dis-
charged through a properly sized stack.
• Low Temperature Thermal Stripping Systems
This configuration is similar to the enclosed
mechanical aeration system except that additional
heat transfer surfaces allow the soil to heat by com-
ing into contact with a screw-auger device or rotary
drum system. The induced airflow conveys the
desorbed volatile organics/air mixture through a
combination afterburner where organic contami-
nants are destroyed. The air stream is then dis-
charged through a properly sized stack.
• Pneumatic Conveyer Systems
These systems consist of a long tube or duct to
carry air at high velocities, an induced draft fan to
propel the air, a suitable feeder for addition and dis-
persion of particulate solids into the air stream, and
a cyclone collector or other separation equipment
for final recovery of the solids from the gas stream.
Several such units heat the inlet air to SOOT to
induce volatilization of organic contaminants.
Pneumatic conveyers are primarily used in the
manufacturing industry for drying solids with up to
90 percent initial moisture content.
Of the four methods described above, documenta-
tion exists to support the contention that low tem-
perature thermal stripping systems have the
greatest ability to successfully remove contami-
nants that are similar to gasoline constituents (i.e.,
compounds with high vapor pressures) from soil.
Roy F Weston Inc. (1986) conducted studies using
a pilot system comprised of several conveyer belts,
a heated screw auger conveyer, and storage hop-
pers along with primary process equipment. The
heart of the treatment system is the thermal pro-
cessor which heats the soils sufficiently to volatilize
the organics. Once volatilized, the organics are
destroyed in an afterburner. An indirect heat trans-
fer fluid, in this case oil, is used to heat the thermal
processor, and the soil is conveyed from the feed
end of the thermal processor to the discharge end
by twin screws. Hot oil (ranging from 100-300°C)
travels the full length of each screw, then returns
through the center of each shaft continuously
throughout system operation (Figure 8). The con-
tinuous movement of the screws conveys and
thoroughly mixes the contaminated soils. The soils
have a residence time of 30 to 60 min in the screw
auger-type dryer. The exhausted air stream passes
through an afterburner at a temperature of 1000°C
to destroy organics.
Figure 8. Low temperature thermal stripping pilot system.
38
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A low temperature thermal stripping system was
used to remove trichloroethylene (TCE) from soil
during a full-scale pilot study conducted by
Canonie Environmental Services Corporation at
the McKin Superfund Site in Gray, Maine (Webster,
1986). Although the principal volatile of concern
was TCE, the system also effectively reduced ben-
zene levels to less than 1 ppm, or 1 mg/L (see
Effectiveness, this page). The system involved con-
veyer belts, a large rotating drum-type materials
dryer and storage hoppers, in addition to primary
process equipment. Excavated soil was fed into the
drum and mixed at 300°F Exhaust air from the
enclosed aeration process was treated in a
baghouse, a scrubber, and a vapor-phase carbon
bed prior to release.
The materials dryer used for soil aeration in this
pilot study was an asphalt batch plant to remove
moisture from fine and coarse aggregates. The
dryer was a large, rotating cylindrical drum approxi-
mately 9 ft in diameter and 28 ft in length. Pre-
aerated, contaminated soils were introduced to the
dryer by conveyer belt and fed by a front-end
loader and hopper. Forced hot air was generated
by an oil burner and introduced to the drum to
enhance vaporization of volatile organic com-
pounds (VOCs). During aeration runs, the drying
temperature varied from 150 to 330°E The exhaust
air from the materials dryer was treated in a three-
stage process to remove particulates and organic
vapors. The first stage of air pollution control
utilized a baghouse normally used with the asphalt
batch plant. Exhausted air from the baghouse was
conveyed via ducts to the packed tower air scrub-
ber, the second phase of air pollution control. The
dryer retention time was 6 to 8 min, with a typical
soil volume of approximately 3 yd3 passing through
the dryer a minimum of three times. The scrubber
was used to condition the air prior to vapor-phase
carbon adsorption and to remove water-soluble
chemical constituents and remaining particulates.
In the final stage of air pollution control, a vapor-
phase carbon adsorption bed was used to remove
VOCs.
The system used during a full-scale pilot study con-
ducted at a Superfund site in Region IV was some-
what similar to the system described for the McKin
pilot study with the exception of having less strin-
gent air quality control equipment. It included an
asphalt drying unit, an asphalt preheater, conveyer
belts, and a particulate collection system. In this
system, soil was gravity fed through a rotating pre-
heater, and fuel was burned at the opposite end of
the system. Organics were vaporized from the soil
through agitation of the rotating preheater and
exposure of soil to gas at 375°F at a feedrate of 10
to 15 tons/h. Particulate emissions from the treat-
ment unit were collected in a cyclone and a cloth
baghouse. Some volatile material emissions were
discharged into the surrounding atmosphere.
Effectiveness
The three full-scale pilot studies described above
demonstrated greater than 99.99 percent removal
of VOCs based on a review of post-aeration soil
sampling data. Based on a review of air monitoring
organic emissions data, it can be concluded that
none of the treatment systems jeopardized public
health or the environment.
The McKin pilot study (Webster, 1986) was under-
taken to determine optimum operating conditions
to produce a consistent, post-aeration soil TCE
concentration less than the 0.1 ppm target level
established by EPA. The sandy soils treated were
contaminated with up to 3,310 ppm TCE. To allow
for evaluation of such operating parameters as
dryer temperature, dryer airflow, soil volume per
run, number of passes through dryer, total dryer
retention time, dust control, and handling of col-
lected baghouse particles, the study was con-
ducted in four phases. The optimal operating condi-
tions determined in the four phases are presented
in Table 12.
The results suggest that for highly contaminated
soils, dryer temperature is a significant factor in
meeting treatment objectives, with higher tempera-
tures yielding lower post-aeration TCE concentra-
tions. Control of the dryer airflow is also an impor-
tant operating parameter because of its effect on
air temperature. Maintaining consistent treatment
efficiencies and baghouse temperatures to protect
the synthetic filters was difficult with low airflows.
Although the primary compound of concern in
the McKin soil aeration pilot study was TCE, the
results suggest that the aeration process was also
effective in removing other volatile organic com-
pounds from soils. Tetrachloroethylene and 1,1,1-
trichloroethane detected in 1- to 100-ppm ranges in
pre-operation samples were routinely not detected
above 1 ppm in post-aeration samples. The effi-
ciency of removing aromatic volatiles such as ben-
zene was also examined. Among the excavated
soil samples, two had 680 ppm and 2,600 ppm of
benzene, the highest concentrations by several
orders of magnitude.
In post-aeration analyses of these soil batches,
benzene was not detected at a 1.0 ppm detection
limit. Similar significant decreases were found for
other aromatic volatiles such as ethylbenzene,
toluene, and xylenes. In addition, the pilot study
showed that controlled soil-handling techniques
and treatment of the process air contributed only
negligibly to air quality impacts due to organic
vapors. To reduce dust emissions from the opera-
39
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Table 12
Optimal Operation Conditions for McKin Pilot Study Configuration
Parameter
Dryer temperature
Dryer air flow
Dust control
Handling of collected
baghouse particulates
Soil volume per run
Number of passes through dryer
Total dryer retention time
Optimal Condition
SOOT
15,000 cfm
Enclosed bucket conveyer system,
wetting soil only after final pass
through dryer
Treated separately in enclosed,
heated conveyor
3yd3
Minimum of three
6 to 8 mm
Source: Webster, 1986.
tion, enclosed handling of treated soils proved
necessary.
The pilot study conducted at a Superfund site in
Region IV (1986) demonstrated that the treatment
unit was able to effectively treat 1,670 tons of con-
taminated soil by reducing the concentration of
volatile organics present. Concentrations of 1,1,1-
trichloroethene, TCE, toluene, and xylene were
reduced by at least 99 percent. For example, sandy
soil TCE concentrations of 20 ppm were reduced to
0.055 ppm following thermal treatment, and soil
ethylbenzene concentrations of 10 ppm were
reduced to 0.018 ppm following treatment.
Limitations
The limitations of low temperature thermal stripping
systems as an enhanced volatilization technique
are associated with soil characteristics that inhibit
the mobility of gasoline vapors from the soil to the
air, contaminant concentrations that may cause an
explosion or fire, and the need to control air quality
impacts due to dust and organic vapor emissions.
Costs
Rototilling or other mechanical means would not be
considered a potentially appropriate corrective
action unless the contaminated soil could be
spread over a large area and treated for extended
periods of time. This type of corrective action may
be appropriate at sites that are close to existing
land treatment facilities or that have substantial
acreage on which land farming could be under-
taken. In most cases, however, it is expected that
corrective actions at UST facilities may be more
appropriately addressed by low temperature ther-
mal stripping.
Roy F Weston, Inc. (1986), under contract to
USATHAMA, performed an economic evaluation of
low-temperature thermal stripping of volatile
organics from contaminated soils in the following
categories: 1,000 tons; 10,000 tons; and 100,000
tons. The results of the economic analysis for the
four Holo-Flute systems evaluated are presented in
Figure 9. Based upon evaluation, it was concluded
that System B was the most cost-effective
approach for sites with 15,000 to 80,000 tons of
soils to be treated. The unit costs for this system
ranged from $74 to $160/ton ($99 to $213/yd3) with-
out flue gas scrubbing and from $87 to $184/ton
($116 to $245/yd3) with flue gas scrubbing. Operat-
ing costs for stripping 1,000 tons of soil ranged from
$42 to $89/ton ($56 to $119/yd3) for the four sys-
tems shown in Figure 9. However, the capital costs
for the systems are a significant portion of the total
costs for processing, as Figure 10 illustrates. Using
this type of system, actual costs for processing less
than 10,000 tons of soil would be expected to be in
excess of $200/ton ($270/yd3). Estimates for ther-
mal stripping of soils using asphalt batch plants
may also range upwards of $300/yd3.
40
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o
o S
"o o
"o f.
a
09 -
08-
07 -
06 -
05 -
04 -
03
02 -
01 -
System B
20
SYSTEM A -ONE THERMAL PROCESSOR WITH
TWO 24 INCH DIAMETER AND TWO
24 FOOT LONG HOLO-FLUTE® SCREWS
SYSTEM B -ONE THERMAL PROCESSOR WITH FOUR
24 INCH DIAMETER AND FOUR 24 FOOT
LONG HOLO-FLUTE® SCREWS
SYSTEM C -TWO SYSTEM B UNITS ARRANGED
IN SERIES
SYSTEM D -FOUR PROCESSORS CONSISTING OF
TWO PARALLEL SYSTEM C UNITS
ALL WITH FLUE GAS SCRUBBING
40
60
—T~
80
Quantity o) Soil Processed (Tons)
(Thousands)
100
Figure 9. Costs of low temperature thermal stripping pilot plant unit.
400
350 -
300 -
250 -
SYSTEM B WITHOUT FLUE GAS SCRUBBING
100
Quantity of Soil Processed (Tons)
(Thousands)
Figure 10. Costs of low temperature thermal stripping unit.
41
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Incineration
Incineration can effectively eliminate gasoline from
soils by complete oxidation. Rotary kiln and
fluidized beds as well as other systems may
achieve destruction and removal efficiencies (ORE)
of 99.99 percent or greater as required by RCRA
for hazardous wastes. Incinerators may be either
fixed facility types or mobile units.
Although incinerators may effectively remove gaso-
line from soils, the same limitations as are
associated with soil excavation would be encoun-
tered. Use of mobile units is further limited by the
permitting process, which may take considerable
time and is expensive.
Costs for incineration vary significantly depending
on the particular characteristics of the soil and
waste material. COM (1986) conducted a survey of
mobile treatment technology vendors and obtained
price estimates ranging from $150 to $480/ton
($200 to $640/yd3) for incineration of 20,000 yd3 of
hypothetical hazardous waste. Unit costs for less
than 20,000 yd3 of material would be anticipated to
be significantly greater than those reported.
Venting
Soil venting refers to any technique that removes
gasoline vapors from unsaturated soil without exca-
vation. It is accomplished in situ by using vents of
various designs (Figure 11) consisting of gravel
packs extending to the soil surface, slotted or
unslotted well casings installed with or without a
gravel pack, or any other configuration that allows
gases to move from the soil. Passive systems con-
sist of vents that are open to the atmosphere and
do not require energy for extraction of gases. Use
of a wind-driven turbine on a vent stack is consid-
ered a passive system. Active systems make use
of pressure or vacuum pumps to accelerate the
removal of gasoline vapors from the soil. In pres-
surized venting, air is forced into the soil by an infil-
trating vent. In vacuum venting, a vacuum created
on the extraction well removes vapors. Pressure
and vacuum systems could be used in tandem to
increase the rate at which gasoline is removed from
soils.
With venting, the vapors are either discharged to
the atmosphere or treated before discharge
depending on vapor concentrations and regulatory
requirements.
INJECTION
MANIFOLD
ELECTRIC
AIR FLOW FORCED
HEATER DRAFT INJECTION
FAN
EXTRACTION
MANIFOLD
VAPOR
CARBON
PACKAGE
TREATMENT
UNIT
INDUCED
DRAFT EXTRACTION
FAN
VERTICAL EXTRACTION
VENT PIPE (TYP)
SOIL CONTAMINATION
SLOTTED
VERTICAL INJECTION
VENT PIPE (TYP)
Figure 11. Vacuum extraction system.
42
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Effectiveness
There is much uncertainty regarding the effective-
ness of soil venting systems because the technol-
ogy has not been widely applied. In order to predict
the effectiveness of soil venting, several resear-
chers have attempted to develop theoretical mod-
els of vapor movement in soil. Hoag et al. (1986)
undertook studies to examine the mechanisms and
kinetics that control venting of soil residually satu-
rated with gasoline. By using an experimental ves-
sel, they developed a saturated vapor-phase
equilibrium model to predict theoretical hydrocar-
bon mass loss. The experimental data showed
good agreement with the predicted values.
However, it is questionable whether this model
could be applied to field situations because of the
difficulties involved in determining initial values for
key variables in the equations. For example, in
order to use the equations, the vapor-phase con-
centration in the soil pores must be determined.
Because of the constant flux of soil air, it may be dif-
ficult to estimate an initial vapor-phase concentra-
tion for gasoline in soil under field conditions.
In addition, the model presupposed that soil parti-
cle size, density, and moisture had no effect on the
mechanisms involved in the venting process.
Although this may have been the situation in the
experiment, it is questionable whether the model
could be applied to field conditions, where soil
characteristics would be expected to significantly
affect the rate at which gasoline vapors could be
vented from the soil.
The experimental results of Hoag et al. (1986) do
indicate, however, that more than 99 percent of
gasoline initially present at residual saturation in
sands could, theoretically, be removed by soil vent-
ing under ideal conditions.
Baehr and Hoag (1986) developed a mathematical
model to derive an equilibrium approximation for
gasoline present as a solute in the water phase, as
a vapor in the air phase, or as a constituent in the
immiscible phase. The model was adapted to
describe conditions encountered in experimental
column tests with one-dimensional flow. The soils
in the column had an average particle diameter of
0.89 mm and a porosity of 0.429. The residual
gasoline content of the column at the start of the
experiment was 0.077 and the experiment was run
at an air-phase specific discharge of 1.88 cm/s. The
model predictions generally compared very well
with the experimental data and provided informa-
tion leading to these conclusions:
• Removal rates decrease with time during venting
as the immiscible phase composition shifts
towards a mixture comprised of less volatile com-
pounds corresponding to a lower total vapor pres-
sure of the gasoline.
• Rates at which vapors escape from residual sat-
urations are faster than the maximum rate at
which they can be swept from soil above the
water table.
• Depression of the water table would result in an
increased rate of removal of vapor-phase compo-
nents due to the removal of the rate-limiting diffu-
sion barrier that results from water in the soil
pores within the capillary fringe.
Although the model shows promise for use in
designing soil venting installations and for predict-
ing performance of alternative designs, Baehr and
Hoag state that additional research and field testing
are required before a computer code of practical
value can be developed.
The Texas Research Institute, Inc. (1984) under-
took a series of four experiments to examine forced
venting of gasoline-contaminated soil. These
experiments examined the efficiency of removing
gasoline from the underground environment and
lowering gasoline vapor concentrations in the
unsaturated soil under various venting system
geometries and flow rates. Results indicated that
venting is an applicable technique for removing
gasoline vapors in soils. The reduction in gasoline
vapor concentrations in the unsaturated zones
were on the order of at least 100-fold at flow rates
of 4 L/min or above. Concentrations of gasoline
vapor were reduced to 1,000 ppm or less when flow
rates of 4 L/min or greater were used. These find-
ings lead to the following general recommenda-
tions regarding soil venting techniques for gasoline
removal from unsaturated soils:
• Using short slotted sections at the bottom of the
import vents may provide more efficient vapor
removal than continuous slot vents.
• Sealing the soil surface of the venting area may
optimize venting by helping to ensure that
exhaust air is drawn laterally, not downward from
the soil surface.
• Initiating venting at high flow rates (16 L/min or
higher) would remove the majority of vapors, sub-
sequently reducing the flow rate to conserve
energy.
Malot and Wood (1985) advocate soil venting
before removing free product. The advantage of
this approach is that it minimizes the amount of
groundwater extracted and treated. Employing
product recovery systems which depress the water
table results in additional soil volume becoming
contaminated as the floating product moves down-
ward with the water table in response to pumping.
Although it can be argued that free product removal
43
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activities could in certain instances disperse the
contaminants beyond that which would occur if
venting systems alone were employed, the benefits
of implementing a free product recovery system as
the initial corrective action are substantial enough
to justify such an approach in most instances.
Malot and Wood (1985) applied a soil venting sys-
tem at a site where 15,000 gal of carbon tet-
rachloride was spilled in an area where the top of
the unconfined Karst aquifer was 300 ft below
ground surface. It was estimated that 4.4 million
cubic yards of unsaturated soil materials consisting
of clayey silts and silty clays were contaminated.
Soil venting systems consisting of slotted screens
installed at depths of 75 to 180 ft were used at a
vacuum of 29.9 in. Hg and a flow rate of 240 ft3/min.
After 90 days the vacuum stabilized within a radius
of influence in the soil of 10 ft.
Hoag and Cliff (1985) reported on an actual appli-
cation of soil venting for remediation of a site where
approximately 400 to 500 gal of gasoline had been
spilled. Of tiiis total, approximately 80 gal was recov-
ered as free product before soil venting was
initiated. The groundwater table was approximately
18 ft beneath this site, and the total aerial extent of
the contaminant plume (to 1 mg/L) was 12,000 ft2.
Within the contaminated area, three soil vents
made of 6-in diameter PVC pipe were installed and
connected to 21 ftVmin vacuum pumps. Over the
90-day period of operation, 364 gal of gasoline was
recovered; 90 percent of this volume was removed
in the first 40 days. After the initial 40 days of opera-
tion, only a skim of gasoline remained on the sur-
face of the groundwater table; after 90 days no
detectable layer of gasoline was observed in the
monitoring wells. The gasoline level in the soil was
reduced to levels ranging between 0.5 to 0.1 mg/L
over much of the area originally contaminated. The
soil venting system employed was effective in
removing large volumes of gasoline from the
residually saturated soils and from the top of the
capillary zone.
Using test cells Crow et al. (1985) investigated
hydrocarbon venting at a petroleum fuels market-
ing terminal. Their studies demonstrated the effec-
tiveness of soil venting at removing gasoline vapors
from unsaturated soil and also the potential for
using soil venting to remove spilled hydrocarbons
from a shallow aquifer. The tests were run using 2-
in diameter slotted well casing installed to 20-ft
depths in 4-in diameter bore holes backfilled with
pea gravel over the 14- to 20-ft slotted depth. Liquid
ring vacuum pumps were used to extract vapors at
rates ranging from 18.5 to 39.8 ft3/min, and the
effective radius of influence extended from each
extraction well approximately 50 to 110 ft in the low-
and high-rate experiments, respectively.
Payne et al. (1986) reported on the use of a closed-
loop forced air circulation system to remove tet-
rachloroethylene from unsaturated soils at a site in
Michigan. Tetrachloroethylene, or PERC as it is
commonly called, is a degreaser solvent with a
relatively high vapor pressure. Approximately 1,000
to 2,000 yd3 of fine sandy soil was contaminated
with PERC at concentrations ranging from 8.3 to
5,600 mg/kg. Soil venting was accomplished with a
2-in diameter galvanized casing withdrawal well
installed to a depth of 17 ft in the center of the con-
taminated area. Five air injection wells were con-
structed at a radius 50 ft from the withdrawal well,
and a sixth was installed 70 ft away. The injection
wells consisted of 1.25-in diameter PVC casing
extending to a depth of 19 ft. The 5-in diameter
borehole for the injection wells was gravel-packed
from 19 to 25 ft and in the withdrawal well from 17 to
25ft.
Air extracted from the withdrawal well was passed
through a 96 ft3 filtration bed charged with 1,200 Ib
of granulated active carbon (GAG). Vacuum levels
of 4.5 in. Hg were reached using a 1-hp oilless rot-
ary vane vacuum pump initially. Gaseous levels of
PERC reached 92,000 mg/m3 at 48 h declining to
6,000 mg/m3 at 72 h. The PERC levels remained at
5,000 mg/m3 through day 12 and declined to 1,000
mg/m3 on day 19 and to 10 mg/m3 on day 35. After
45 days of pumping, split spoon samples collected
contained 0.84 and 0.64 mg/kg. PERC has charac-
teristics similar to BTX components, including high
Henry's law constants and similar vapor pressure
characteristics. Therefore, it is anticipated that
gasoline would respond to venting in a manner
comparable to that experienced with PERC.
Anastos et al. (1986) reported on a pilot demonstra-
tion of soil venting at a site contaminated with TCE.
The pilot tests were run in one area where soil TCE
levels ranged from 5 to 50 mg/kg and in another
area with levels of 50 to 5,000 mg/kg of TCE. The
pilot tests were undertaken to demonstrate the
feasibility of soil venting and to allow for the
development of design data for a full-scale soil ven-
ting system. The pilot system consisted of a series
of perforated PVC pipe vents 3-in. in diameter
installed vertically into the contaminated soil,
through which volatiles were removed, and another
series of PVC pipes installed in the soil, into which
air was pumped under pressure. The pilot system
also included space heaters to heat air used in the
system to a constant temperature and a vapor-
phase activated carbon unit to treat the exhaust air.
The extraction pipes were spaced 20 ft apart in the
area where soil TCE concentrations were 5 to 50
mg/kg, and 50 ft apart in the area of TCE concen-
trations of 50 to 5,000 mq/kg. Airflow rates of 50 ft3/
min and 50 to 225 fr/min, respectively, were
applied.
44
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The exhaust gases from the system were meas-
ured with a gas chromatograph/photoionization
device (GC/PID) and found to decrease over the 3-
month period of the study from an initial value of 5
to 12 ppm in the soil area contaminated with 5 to 50
mg/kg TCE to 500 to 800 ppb. In the highly con-
taminated soil area, the TCE content in the exhaust
gases remained at 250 to 350 ppm over the 3-
month project duration. The conclusion was that
TCE removal to exhaust-gas levels below 100 ppb
might have been achieved through continued sys-
tem operation beyond the 3-month test period.
Because of the chemical similarities between TCE
and BTX, comparable removal efficiencies for BTX
could likely be achieved.
Limitations
The limitations of venting are associated with soil
characteristics that impede free movement of vap-
ors to the extraction well, emissions of volatiles,
and explosion hazards.
Soils with limited pore space due to compacted
conditions or fine-grained texture could restrict the
rate at which air moves through the soil and also
the ability of the air to pass effectively over all con-
taminated soil particles. These types of conditions
would require the use of more closely spaced vent-
ing wells and possibly higher capacity pumps.
Where air quality restrictions apply, volatiles gener-
ated during the venting process can be readily cap-
tured with GAG. Also, soil bed filters could be used
to scrub vented vapors.
Soil bed filters or biofilters have been used exten-
sively for the treatment of malodorous gases
associated with wastewater treatment plants
(Terasawa et al., 1986). These systems use a soil's
chemical, physical, and microbial characteristics to
filter odors from gases.
Prokop and Bonn (1985) reported on the use of a
soil bed filter for the removal of VOCs including pro-
pane and isobutane. This technology could poten-
tially be used with soil venting to scrub contami-
nants from exhaust vents in lieu of carbon or other
high cost systems.
Explosion hazards associated with gasoline vapors
can be overcome by using intrinsically safe equip-
ment and by ensuring that adequate volumes of air
are moved through the system to keep vapor con-
centrations below the lower explosion limit.
Costs
The major capital costs for soil venting systems are
associated with the venting well installation, pump
purchase, and the costs associated with air emis-
sion control. Venting wells are installed with con-
ventional drilling equipment and materials. Costs
for a vent well (20 ft) constructed of 2-in diameter
slotted Schedule 40 PVC would be expected to be
in the range of $40 per linear foot (If) installed, and
attendant piping would cost approximately $3 to $57
If. Vacuum pump sizing would be based upon the
area and volume of soil to be vented. Vacuum
pumps capable of moving 40 to 60 ft3/min at 1 Vz in.
H2O, similar to that used by Roy E Weston (1985) in
their studies, range in price from $500 to $2,000,
and those capable of moving 1,000 standard ft3/
min at 25 in. Hg vacuum, such as that used by
Crow et al. (1985), cost approximately $4,000.
Operating costs vary depending on utility costs and
time of operation. Payne et al. (1986) reported that
soil venting was effective in the cleanup of soil
contaminated with PERC, and that it was more
cost-effective than excavation and removal if soil
volumes exceeded 500 yd3. Anastos et al. (1986)
estimated that full-scale remediation of the TCE-
contaminated site using soil venting techniques
would range in cost from $15 to $20/yd3 exclusive
of air emission treatment costs.
Soil Washing/Extraction
Soil washing is any technique that removes
gasoline constituents from the soil matrix by
actively leaching the contaminants from the soil into
a leaching medium. The extracted constituents can
then be removed from the washing fluid by con-
ventional treatment methods. Soil washing is
accomplished either in situ as a water flushing sys-
tem, as shown in Figure 12, or processed through a
countercurrent extractor system as shown in Figure
13. Water is the fluid most often used for soil flush-
ing, and it may contain additives such as acids,
alkalis, and detergents. However, washing fluids
can also consist of pure organic solvents such as
methanol, hexane, or triethylamine (TEA).
The slurry of soil and washing fluid can be de-
watered by conventional techniques such as
sedimentation, filtration, evaporation, dissolved air
flotation, or drying beds. The treated soils can then
be put back into the original excavation or sent to a
sanitary landfill. This technology has been
developed extensively in the mining and oil recov-
ery industries to both remove and concentrate
gasoline contaminants. The leachate collected
from the extraction process can be treated conven-
tionally and recycled in a closed system. Contami-
nated solvents are separated by physical separa-
tion techniques such as distillation, evaporation, or
centrifugation.
Treated effluent can be reinjected into the ground;
however, this method presumes the need for site
controls. Suitable site controls may consist of
above- or below-ground barriers.
45
-------
ATMOSPHERE
SPRAY
RECHARGE
SYSTEM
WITHDRAWAL WELLS
Figure 12. Soil flushing system.
Cl-lElANT
THICKENER! ACIDIFICATION
FILTER RINSE TANKS
Figure 13. Countercurrent extractor process flow diagram.
46
-------
Effectiveness
The effectiveness of a soil washing system
depends in large part on the residual gasoline
capacity of the soil. Creosote-coal tars adhere
more tightly to the soil matrix because of their low
solubility in groundwater. Diesel, kerosene, and
gasoline are not so tightly bound to the soil matrix,
and thus the soil washing system is very effective
on these constituents.
Richard and Trost (1986) have evaluated the effec-
tiveness of soil washing in several organic contami-
nated soils. Their system utilizes alkaline agents
(such as NaOH and Na4SiO4) and biodegradable
surfactants to liberate the organic contaminants
from the clays and sands. The slurried soil is sepa-
rated using froth flotation equipment, and the
cleaned soils are returned to the site. Test results
have shown this soil-washing process will remove
up to 99.4 percent toluene, 99.5 percent gasoline,
96.7 percent diesel, 96.1 percent kerosene, 974 per-
cent TCE, 99.9 percent tetrachloroethylene, and
99.0 percent creosote-coal tars. However, Richard
and Trost caution that the actual percentage of con-
taminant removal is dependent on the relative
amounts of clay and sand, the nature of contami-
nants, and the concentration and type of reagents.
The Basic Extraction Sludge Treatment (BEST)
method developed by Resource Conservation
Company (Bellevue, Washington) is a solvent
extraction process. The system separates viscous
oily wastes into three fractions: clean oil that will be
sent to a blending company for use as a fuel; water
that can be treated in a biological treatment sys-
tem; and oil-free, dry solids that can be returned to
the site excavation. The key to the process is the
use of TEA as the leaching fluid. TEA is completely
soluble in water below 66°F but insoluble at higher
temperatures. Thus the oils and water from the
soils dissolve in the TEA below 66°F The resulting
solids are dewatered conventionally by vacuum fil-
ters, filter presses, or centrifuge and then are dried.
The solvent-oil-water mix is heated and collected in
a decanter where water and TEA separate. Both
sludges and soils are currently being treated by this
process on a large oil sludge lagoon in Savannah,
Georgia.
The process can handle a wide variety of organic
wastes and organic-contaminated soils and has
resulted in removal rates of up to 99.5 percent of
organic constituents such as asphalts, diesel fuels,
creosote-coal tars, gasoline, and kerosene.
Limitations
Limitations with the use of soil washing or flushing
are associated with soil characteristics that impede
the solid-liquid separation after the washing phase.
This may result from a high percentage of silt or
clay in the soil material. In situ soil flushing can
result in decreased permeability with the use of
surfactants or other additives.
Hoag (1985) has found that water used to flush soil
residually saturated with petroleum products is not
effective in mobilizing the immobile phase, and sur-
factant treatment may be necessary to remove
these materials effectively. Engineering-Science
Inc. (1986) in evaluating aquifer restoration
techniques found that if the equivalent of 120 years
of precipitation was applied to a soil saturated with
gasoline, the leachable hydrocarbon fraction
decreased from 1,500 ppb to 400 ppb. Gasoline
was still detected in the column after flushing with
844 pore volumes. As noted in Table 13, signifi-
cantly greater volumes of water than air are
required to renovate residually contaminated soils.
Whether in situ or excavation systems are utilized,
laboratory and pilot testing will be necessary to
determine feasibility. Contaminant removal rates
may not be adequate to reduce soil contamination
below required action levels.
Costs
MTA Remedial Resources, Inc., which has
developed a commercial soil washing process,
reports processing costs of about $100/ton for both
capital amortization and operating costs. This cost
does not include excavation or disposal expense.
Resource Conservation Co. with its BEST treat-
ment system has estimated a processing cost of
about $120 to $150/wet ton. This cost would not
include excavation or disposal expense.
Several systems have been employed at hazard-
ous waste sites. A soil washing system that is being
tested at Lee's Farm, Wisconsin, has an estimated
cost of about $150 to $200/yd3 excluding develop-
ment and excavation costs. The major costs are
usually associated with the washing fluid treatment
system.
Microbial Degradation
Soils harbor a plethora of microorganisms that can
degrade hydrocarbons and other environmental
contaminants. Soil bacteria, actinomycetes and
other microbes, have been shown to acclimate
readily to hydrocarbons and to use these com-
pounds in their metabolic processes. Hydrocarbon
components acted upon by soil microbial popula-
tions under ideal conditions will be converted to
microbial biomass and carbon dioxide.
The ability of microbial populations to degrade
hydrocarbons has been exploited by the petroleum
industry through land farming techniques. In land
farming, petroleum refining wastes and by-products
47
-------
are placed on soils, taking advantage of the ability
of indigenous microorganisms to degrade these
materials. Whether indigenous or introduced,
microbial populations can potentially be used to
degrade gasoline-contaminated soil in situ or in a
reactor or otherwise modified environment. This
section discusses the principles involved in bio-
degradation and issues relevant to the use of such
systems.
The scientific literature is replete with laboratory
research reports on the biodegradation of gasoline
in soils and aqueous systems. The American
Petroleum Institute (API) has compiled much of this
information in two publications (Brookman et al.,
1985a and 1985b).
Although the laboratory research summarized in
these documents provides useful data for develop-
ing corrective action strategies, field application
data are somewhat more limited.
Effectiveness and Limitations
Bossert and Bartha (1984) report that n-alkanes, n-
alkylaromatics and aromatic petroleum compo-
nents of the C1I? to C22 range are the least toxic and
the most readily biodegradable of the petroleum
components, whereas those in the C5 to C9 range
have relatively high solvent type membrane toxicity.
Those petroleum components above C22 have low
toxicity but are not readily degraded because of
their physical characteristics. Cycloalkanes and
branched alkanes in the C10 to C22 range are more
resistant to biodegradation than aromatics and n-
alkanes due to their branched nature.
Gasoline composed principally of alkanes in the C5
to C10 range and cycloalkanes would therefore be
expected to be subject to microbial degradation in
the soil environment provided conditions were not
limiting. According to Brookman et al. (1985b), fac-
tors that affect the rate at which degradation will
occur include:
• Indigenous soil microbial population
• Hydrocarbon type and concentration
• Soil extraction, expressed as pH
• Nutrient availability
• Temperature
• Moisture content
• Oxygen content
These factors are discussed in the following sub-
sections.
Indigenous Soil Microbial Population
Soil microbes capable of degrading petroleum
products include Pseudomonas, Flavobacterium,
Achromobacter, Arthrobacter, Micrococcus and
Acinetobacter, among others. In fact, more than
200 soil microbial species have been identified
which can assimilate hydrocarbon substrates (Sav-
age et al., 1985). Although numerous methods are
available to enumerate the microbial population in
soil, it is questionable if such determinations are of
value in implementing corrective actions. Total
microbial counts of fertile soils range from 107 to
109 per gram of dry soil, and hydrocarbon degrad-
ers counts range from 105 to 10G per gram in soils
with no history of pollution (Bossert and Bartha,
1984). Soils which have been exposed to petro-
leum have counts on the order of 106 to 108 per
gram. Indigenous soil microbial populations would
therefore be expected to respond to releases of
gasoline provided that environmental conditions
support growth.
Microbial populations can also be augmented by
the introduction of acclimated microbes. Accli-
mated microorganisms are receiving increased
attention for use in degrading hydrocarbons in
soils. Acclimated microorganisms are developed
through genetic manipulation or enrichment cultur-
ing techniques. Some firms that sell microbial cul-
tures or systems that use indigenous organisms or
enrichment processes to degrade environmental
contaminants are:
PolyBac Corporation
Allentown, Pennsylvania
Groundwater Decontamination Systems, Inc.
Paramus, New Jersey
Solmar Corporation
Orange, California
Although Wilson et al. (1986) acknowledge that
acclimated microorganisms have been used suc-
cessfully to degrade hydrocarbons in soils, they
identify the following obstacles to aquifer restora-
tion:
• Acclimated microbes must be able to survive in
the environment and compete successfully with
indigenous microbes for nutrients.
• Acclimated microbes must be able to move from
point of injection to location of contaminant.
• Acclimated microbes must retain selectivity for
degrading compounds for which they were ini-
tially adapted.
These same obstacles to restoration would be
expected in soils to some extent. However, should
the soil be amenable to mechanical mixing, some
of these limitations could be overcome.
Hydrocarbon Type and Concentration
Although indigenous soil microbes and introduced
microorganisms can theoretically degrade gasoline
48
-------
in soil, the concentration of the products in the soil
may limit the rate of degradation. Alexander (1985)
has reported that microbes may not assimilate car-
bon from chemicals in trace amounts in natural
environments. Co-metabolism, the process by
which a microbe oxidizes a substance without
being able to use the energy derived from the oxi-
dation to support its growth, may be the primary
mode of degradation of hydrocarbons at parts-per-
billion levels (1 ppb = 1
White et al. (1985) used soil samples obtained from
gasoline spill sites in Virginia, Pennsylvania and
New York to demonstrate reduction of methanol
from 105 mg/L to 0 mg/L in 30 days and of TBA, a
gasoline additive, from 115 mg/L to 0 mg/L in 55
days.
Bossert and Bartha (1984) report that when petro-
leum is added to soils at rates of 0.5 to 10 percent
by weight, rates of degradation are limited in the
first 30 to 90 days by factors other than substrate
availability, whereas at later stages of degradation
at extremely low petroleum addition rates, sub-
strate availability limits the rate of degradation.
Overash and Pal (1979) indicate that a soil's capac-
ity to assimilate oil in land treatment systems
ranged from 0.2 to 0.4 percent per month by weight.
So/7 Reaction
Soil reaction, expressed as pH, can influence the
rate at which gasoline is degraded by microor-
ganisms. Optimal oil sludge degradation has been
demonstrated to occur between pH 75 and 78
(Atlas, 1981 ). Under acidic soil conditions, fungi will
be more prevalent than bacteria. Although fungi
can degrade petroleum products, the rate of degra-
dation will be less than that attained by a mixed
fungi-bacterial community such as would occur in
neutral to slightly alkaline soils (Bossert and
Bartha, 1984).
Nutrient Availability
The availability of the soil micronutrients nitrogen
and phosphorus are often cited as being rate-limit-
ing to hydrocarbon degradation. The availability of
macronutrients present in soil to microbes is
optimum at near neutral pH values, although in
some soils the nutrient status may be low. The opti-
mal quantity of nitrogen and phosphorus required
for microbial degradation is related to the organic
carbon content of the soil-waste mixture. According
to Bartha and Bossert (1981 ) the optimum organic
carbon/nitrogen/phosphorus ratio for oil sludge
degradation is 60/1/0.075.
Temperature
The majority of microorganisms responsible for
degradation of petroleum hydrocarbons are
mesophiles (30°C) and thermophiles (40°C),
although degradation by psycrophiles (4°C) has
been reported (Bossert and Bartha, 1984). Report-
edly, optimum temperatures for microbial degrada-
tion are above 20°C (Atlas, 1981), although signifi-
cant increases in degradation rates above 40°C
have not been widely reported. Soil temperatures in
the continental United States during all but winter
months in northern latitudes, therefore, should not
limit microbial degradation.
Moisture Content
Microbes require water to carry out metabolic pro-
cesses. The amount of water a soil contains varies
with time in response to precipitation, drainage, and
evapotranspiration. The quantity of moisture
retained by soil after free drainage is termed field
capacity, whereas the total amount of water a soil
can hold at saturation is termed moisture-holding
capacity. Optimal microbial activity occurs between
50 and 80 percent water-holding capacity, and at 10
percent or less, metabolic activity becomes margi-
nal (Bossert and Bartha, 1984).
Oxygen Content
Lack of oxygen is normally the rate limiting factor
for aerobic hydrocarbon degradation in most soil
situations (Nyer, 1987). Product leakage into soils
may effectively fill pore spaces with liquid and
gaseous components, thus excluding oxygen. A
high water table or wet soil conditions can further
exacerbate the oxygen supply.
As soils become saturated, anoxic conditions result
and anaerobic microbial activities predominate.
Although some studies indicate that anaerobic
degradation of petroleum products does occur, the
rate at which these reactions occur is significantly
less than that encountered under aerobic condi-
tions. Therefore microbial degradation of gasoline
under negative redox potentials would not be
expected to be significant. The redox potential is an
electric potential established by the ratio of oxidized
materials to reduced materials in a soil system.
Aerobic environments are generally characterized
by redox potentials in the positive range, whereas
anaerobic environments are characterized by
negative values.
Lack of oxygen in aquifer systems has been
reported as a major limiting factor for in situ aquifer
microbial degradation of petroleum products. Wil-
son et al. (1986) note that microorganisms in a well-
oxygenated groundwater containing 4 mg/L of oxy-
gen can degrade only 2 mg/L of benzene and that
the solubility of benzene in water (1,780 mg/L) is
much greater than its capacity for degradation.
Yaniga and Smith (1985) reportedly used mechani-
cal systems to add air to the aquifer but were able
49
-------
only to induce 10 ppm of oxygen (roughly the sat-
uration concentration of oxygen in water at 20°C)
into the groundwater. To overcome the limited effi-
ciency of the system due to insufficient oxygen,
injection of hydrogen peroxide to the groundwater
was investigated. Hydrogen peroxide injected at
concentrations of 100 ppm stimulated microbial
degradation. According to Raymond (1987), hydro-
gen peroxide can increase dissolved oxygen in
groundwater to between 250 and 400 ppm. Liquid
oxygen may also be injected and can result in dis-
solved oxygen concentrations of about 40 ppm.
Chan and Ford (1986) reported on the use of in situ
soil techniques and a bioreactor to degrade No. 2
fuel oil that had leaked from storage tanks. The field
application of these techniques convinced them
that the bioreactor was at least four times more effi-
cient than in situ methods due to higher oxygen
content (9 mg/L vs 2.5 mg/L in the water).
Borden and Bedient (1986) developed theoretical
equations for simulating the simultaneous growth,
decay, and transport of microorganisms as well as
transport and removal of hydrocarbons and oxygen
in aquifer systems. Based on their studies, they
concluded:
• A zone of reduced hydrocarbon and oxygen con-
centration will develop between the oxygenated
formation water and the plume in which microbial
degradation rates are reduced.
• A large microbial population will develop in the
region contiguous to the hydrocarbon source in
which an instantaneous reaction of hydrocarbons
and oxygen takes place.
• Adsorption to the aquifer material may signifi-
cantly enhance the biodegradation of hydrocar-
bon spills.
• Exchange of oxygen and hydrocarbon vertically
with the unsaturated zone may significantly
enhance the rate of biodegradation.
This study and the accompanying study on field
application of the model (Borden et al., 1986) point
out the importance of oxygen exchange to micro-
bial degradation in aquifer materials. In soil mate-
rials above the water table, the rate of oxygen
exchange will be greater than that associated with
aquifer materials. The rate at which oxygen can be
brought into contact with the microbial population
and gasoline-contaminated soils will be related to
the depth of contamination, the texture of the soil,
and its water and gasoline content. Table 13 pro-
vides a comparison of the amounts of air or water
required to be exchanged with soil materials in
order to renovate residually saturated soils (Wilson
and Ward, 1986). These values were developed
based upon the assumption that the oxygen con-
tent of air was 200 mg/L and that of water was 10
mg/L and that the hydrocarbons were completely
metabolized to carbon dioxide. Wilson and Ward
caution that actual values at specific sites would
vary from these typical estimates. Nonetheless, this
information further points out the significance of
oxygen in the degradation of hydrocarbons.
Table 13
Estimated Volumes to Renovate Hydrocarbon Residually Saturated Soils
Proportion Occupied by
Texture
Stone to coarse gravel
Gravel to coarse sand
Coarse to medium sand
Medium to fine sand
Fine sand to silt
Hydrocarbons1
0.005
0.008
0.015
0.025
0.040
Air1
0.4
0.3
0.2
0.2
0.2
Water2
0.4
0.4
0.4
0.4
0.5
Pore/Volumes
Air
250
530
1500
2500
4000
Water
5000
8000
15000
25000
32000
Source: Wilson and Ward, 1986.
1 Drained
"Saturated
50
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Costs
Costs for microbial degradation of gasoline-con-
taminated soil are not widely reported because
these techniques are most often applied to
remediate groundwater systems or recirculation
systems. FMC provided cost estimates ranging
from $400,000 to $600,000 to clean up a hypotheti-
cal spill of 10,000 gal of jet fuel in a fine gravel for-
mation using their hydrogen peroxide enhanced
microbial degradation system. (FMC's Ground-
water Remediation Operations have recently been
acquired by International Technology.) Olsen et al.
(1986), however, report that bioreclamation costs
are in the range of $50 to $100/ton ($66 to $1237
yd3).
Summary
Soils saturated with gasoline will drain under the
forces of gravity until they reach a point called
residual saturation. At that point the gasoline
retained in a soil normally does not migrate to
groundwater supplies as free product. However,
infiltrating rainfall or fluctuating water tables can
flush gasoline from the soil matrix or transport com-
ponents in a dissolved phase. Gasoline present in
unsatu rated soils, however, may migrate to ground-
water supplies and pose health risks, or it may mig-
rate as vapors to enclosed structures and pose
explosion hazards. Thus corrective actions that
effectively mitigate these risks need to be consid-
ered under corrective action programs for leaking
USTs.
The corrective actions that are potentially applica-
ble to soils contaminated with gasoline include
excavation and disposal, enhanced volatilization,
incineration, venting, flushing (washing), and
biodegradation. In certain instances, other correc-
tive actions such as in situ stabilization may also be
appropriate. However, because of the relatively
high volatility of gasoline and the ability of microbes
to degrade its components, corrective actions
which render the contaminated materials less
hazardous should be favored. Table 14 summarizes
the major issues associated with the corrective
actions for gasoline-contaminated soils.
Excavation and disposal is probably the most
widely used corrective action to recover and treat
contaminated soil at LIST sites at the present time.
Various contractors contacted indicated that soils
were typically disposed at landfills with reported tip-
ping fees in some cases as low as $12/yd3. Exca-
vation and disposal costs may approach $200 to
$300/yd3, however. Therefore, this alternative is
cost inefficient for all but small quantities of soil. In
situ venting is more cost-effective for treating soil
contaminated with volatiles when volumes exceed
500 yd3. Increasing costs and disposal restrictions
will make this alternative less attractive in the
future.
States exercise varying degrees of control of the
disposal of gasoline-contaminated soils. This situa-
tion leads to the export of soil from states with more
stringent controls to those that are less restrictive.
Uniform guidelines for disposing contaminated
soils would minimize these types of practices and
thus serve to minimize risks associated with trans-
port over long distances.
Disposing contaminated soils in batch asphalt
plants is a practice that is not reported in the litera-
ture but apparently is another widespread method
of disposal. A number of contractors reported that
asphalt plants accepted gasoline-contaminated
soils and, in certain states, at fees of about $55/yd3.
In other states with more stringent regulations,
asphalt plants will not accept gasoline-contami-
nated soils because they would then be classified
as hazardous waste treatment facilities and subject
to all pertinent regulations.
One asphalt plant reported that tests required
before the soil was accepted included EP toxicity,
flash point, and priority pollutant analyses. Those
materials considered toxic or which contained
chlorinated solvents were not accepted.
Excavation and disposal may be an appropriate
corrective action when undertaken simultaneously
with removal of leaking USTs. However, because
this action results in significant disturbance of sur-
face and subsurface infrastructure, it may not be
applicable in certain settings. Disposal of the con-
taminated soils also carries with it potential
liabilities attendant to its ultimate disposition. Treat-
ment technologies which provide for destruction or
detoxification of the soil materials such as
enhanced volatilization, incineration, or biodegra-
dation would therefore be favored means of dis-
posal provided they are cost-effective.
Enhanced volatilization can be accomplished by
simply turning the contaminated soils and thereby
increasing exposure of the gasoline to the atmos-
phere; or by using mechanical systems with a heat
source to drive the volatile compounds from the
soil. The latter technique is referred to as low tem-
perature thermal stripping and has the greatest
flexibility and control in system operation. In this
system, gases volatilized from the soil can be cap-
tured and destroyed rather than discharged to the
atmosphere as would occur with a rototilling opera-
tion. The low temperature thermal stripping system
is capable of removing 99.99 percent of volatile
organic compounds including BTX. Cost estimates
for processing soil using low temperature for this
51
-------
5
Table 14
I Correction Action Summary Evaluatu
'S
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Non-UST Sites
Presently Using Met
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te
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Is
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3?
O •—
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< tr
Limitations
Effectiveness
of
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Si
o.-y
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•a
|
'S
2
CO
0
'co
^
1?)
CO
.0
s
l!
Brings contaminant to surface, thereby
possibly increasing exposure
Not efficient if large quantities are to be
removed.
Significant amounts of surface area
disturbed relative to depth excavated.
Difficult to undertake in heavily
urbanized areas near buildings, utilities.
Requires suitable means of disposal.
Reduction in mobility.
Reduction in volume.
100% removal of
contaminants in soil
excavated.
o
o
CO
8
(A
c.
o
1 8
IT, &
X C CO
1 LU CO "O
^
Z co
CO" O
E"^ "^
_. ^ m
Mckin, ME
Metaltee Aerosyste
Caldwell Trucking, 1
Triangle Chem, TX
Hollingsworth Soldi
$
1 1
1 1
2 3
Requires vapor phase treatment and
dust control.
Reduction in toxicity.
99.99% removal of VOC's.
Most effective if 15,000 to
80,000 tons of soil require
treatment.
8
C\J
«0
^e
CD "fS
8-M
5S
,5 o
LU >
At least 28 sites
0
CO
3
T3
d
!l
e
Permitting requirements can be signifiCc
Brings contaminants to surface thereby
Reduction in volume.
Reduction in mobility.
+
o
§
(f>
c
g
'S
m
.£
C
—
I i
possibly increasing exposure.
Typically the most expensive soil treatm
technology.
Appropriate usually when toxics other th
Total destruction of all
contaminants.
just volatlles are present.
<
-S
Tyson's Dump, PA
VernaWellField.lV
Ponder's Corner's,
CO
o
CD
» 1
X C/>
Effectiveness depends on soil
characteristics.
May require vapor phase treatment of
emissions.
Reduction in mobility.
99.99% removal of
VOC's.
S
LT>
W
D)
C
•^=
C
CD
>
o
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'•^
u
Care must be taken to avoid explosions
At least 5 sites
CO
(U
en
CO
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o
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X CO
May require vapor phase treatment.
Not as effective as active venting.
Reduction of mobility.
in
o
r-
Q>
>
CO
CO
0.
<
§
3 g>
Leeds, AL
Goose Farm, NJ
Lee's Farm, Wl
Bog Creek Farm, Is
Western Processir
Volk Air Field, Wl
0)
CO
3
I
!!
CO
Requires separation techniques such a
distillation, evaporation, centrifugation.
Less effective for textured soils.
Reduction of mobility.
99.99% removal of
contaminants.
Accelerated removal
of contaminants.
o
a
3
fft
0)
c
r-
co
co
'o
CO
At least 7 sites.
0)
CO
3
CD
(— ^
§ 1
x co
Biologic systems subject to upset.
Reduction in toxicity.
Variable effectiveness.
m
CM
6
&
c
-S
CO to
!Q T3
o co
O O)
'^ CO
2 -D
52
-------
system range from $245 to $320/yd3 including air
emissions control.
Incineration techniques can provide destruction
removal efficiencies (ORE) in excess of 99.99 per-
cent; however, the added costs associated with
incineration and the permitting process which may
be attendant to the use of mobile units would
suggest that this type of corrective action is not
appropriate unless the soil is also contaminated by
constituents that render it toxic or otherwise
untreatable. Incineration costs range from $200 to
$640/yd3 exclusive of costs of transportation and
final ash disposal.
Soil venting systems have demonstrated 99 per-
cent effectiveness in certain applications. These
techniques are relatively easy to implement and
cause minimal disturbance to structures or pave-
ment. Soil venting is also easily incorporated into
systems for cleanup of contaminated aquifers.
There is much uncertainty regarding the overall
effectiveness of soil venting systems, however,
because the technology has not been widely
applied. Unit costs for soil venting have been
reported to be as low as $15 to $20/yd3 of soil
treated exclusive of air emission control costs.
Soil flushing (washing) systems may provide for
effective removal of contaminants in certain hydro-
geologic settings at competitive rates. However,
because this operation further disperses contami-
nants, its usefulness should be carefully evaluated
in light of the potential for exacerbating the prob-
lem. Costs for soil washing range from $150 to
$200/yd3 exclusive of excavation costs.
Biodegradation of gasoline by indigenous or intro-
duced soil microorganisms is effective in unsatu-
rated soils provided environmental conditions can
sustain microbial metabolism. Lack of oxygen may
be the factor that most limits degradation in subsur-
face soils. Costs for microbial degradation of petro-
leum products in soil range from $66 to $123/yd3
making this one of the least costly of the corrective
actions evaluated. However, because in situ degra-
dation techniques are akin to "black boxes," they
require extensive monitoring networks to demon-
strate effectiveness. The biological systems
associated with these techniques must be carefully
nurtured to ensure optimal performance.
Although a number of soil treatment techniques are
available for removing gasoline from soil, only a few
have been applied in the field. This is largely
because of uncertainties regarding the effective-
ness of soil treatment technologies like venting and
biodegradation. Unlike groundwater treatment
technologies, where the science is well understood
and the principal design parameters are well
known, soil treatment techniques are considered
"black boxes." Other soil treatment technologies
such as incineration and soil washing are expected
to have only limited applicability to cleanup of leak-
ing USTs, either because they are prohibitively
costly or because they create other undesirable
environmental problems.
Even though excavation and landfilling are the
most widely used corrective action for soils, this
approach may not be the best solution for soil
cleanup. Because of time delays at local landfills,
excavators have had to store large piles of contami-
nated dirt, and it is not known to what extent vapors
emanating from these dirt piles present a health
hazard. This concern exists for any corrective
action that necessitates bringing contaminated soil
to the ground surface.
Although the two technologies have not been fully
developed, it appears that a combination of soil
venting and microbial degradation may provide the
most efficient corrective action because they limit
the public's exposure to the contaminated sub-
stances. Further research is needed, however, to
confirm the effectiveness of these two techniques.
References
Alexander, M. 1961. Introduction to Soil Microbiol-
ogy. John Wiley and Sons, Inc., N.Y
Alexander, M. 1981. Biodegradation of Chemicals of
Environmental Concern. Science, 211:132-138.
Alexander, M. 1985. Biodegradation of Organic
Chemicals. Environmental Science & Technology,
Anastos, G., Corbin, M.H. and Coia, M.F 1986. In-
situ Air Stripping: A New Technique for Removing
Volatile Organic Contaminants from Soils. In:
Superfund 86.
Atlas, R.M. 1981. Microbial Degradation of Petro-
leum Hydrocarbons: An Environmental Perspec-
tive. Microbiological Reviews, 45(1):180-209.
Baehr, A.L. and Corapcioglu, M.Y 1987 A Composi-
tional Multiphase Model for Groundwater Contami-
nation by Petroleum Products 2. Numerical Solu-
tion. Water Resources Research, 23(1):201-213.
Baehr, A.L. and Hoag, G.E. 1986. A Modeling and
Experimental Investigation of Venting Gasoline
from Contaminated Soils. University of Mas-
sachusetts.
Barker, J.F, Patrick, G.C. and Major, D. 1987 Natu-
ral Attenuation of Aromatic Hydrocarbons in a Shal-
low Sand Aquifer. Ground Water Monitoring
Review, 7(1 ):64-71.
Bartha, R. and Bossert, 1.1981. The Treatment and
53
-------
Disposal of Petroleum Wastes. In: Petroleum
Microbiology. MacMillan Publishing Co., N.Y
Baver, L.D., Gardner, W.H. and Gardner, W.R. 1972.
Soil Physics, Fourth Edition. John Wiley and Sons,
Inc., N.YBIackmer, A.M. and Bremmer, J.M. 1977
Gas Chromatographic Analysis of Soil Atmos-
pheres. Soil Science Society of America Journal,
41(5):908-912.
Bonn, H.L. 1977. Soil Treatment of Organic Waste
Gases. In: Soils for Management of Organic
Wastes and Waste Waters, pp. 607-618. American
Society of Agronomy, Madison, Wise.
Borden, R.C., and Bedient, RB. 1986. Transport of
Dissolved Hydrocarbons Influenced by Oxygen-
Limited Biodegradation 1. Theoretical Develop-
ment. Water Resources Research, 22(13):1973-
1982.
Borden, R.C., Bedient, RB., Lee, M.D., Ward, C.H.
and Wilson, J.T 1986. Transport of Dissolved
Hydrocarbons Influenced by Oxygen-Limited
Biodegradation 2. Field Application. Water
Resources Research, 22(13):1983-1990.
Bossert, I. and Bartha, R. 1984. The Fate of Petro-
leum in the Soil Ecosystem. In: Petroleum Micro-
biology, R.M. Atlas, Editor. MacMillan Publishing
Co., N.Y
Bremmer, J.M. and Blackmer, A.M. 1982. Composi-
tion of Soil Atmospheres. In: Methods of Soil Analy-
sis, Part 2. pp. 873-901. Chemical and Microbiologi-
cal Properties. Agronomy Monograph No. 9.
Brookman, G.T., Flanagan, M. and Kebe, J.O.
1985(a). Literature Survey: Hydrocarbon Sol-
ubilities and Attenuation Mechanisms. API Pub. No.
4414.
Brookman, G.T, Flanagan, M. and Kebe, J.O.
1985(b). Literature Survey: Unassisted Natural
Mechanisms to Reduce Concentrations of Soluble
Gasoline Components. API Pub. No. 4415.
Castle, C., Bruck, J., Sappington, D. and Erbaugh,
M. 1985. Research and Development of a Soil
Washing System for Use at Superfund Sites. In:
Management of Uncontrolled Hazardous Waste
Sites.
COM. 1986(a). Mobile Treatment Technologies for
Superfund Wastes. EPA 540/2-86-003(f).
COM. 1986(b). Superfund Treatment Technologies:
A Vendor Inventory. EPA 540/2-86-004.
Crow, W.L, Anderson, E.R and Minugh, E.M. 1987.
Subsurface Venting of Vapors Emanating from
Hydrocarbon Production Ground Water. Ground
Water Monitoring Review, 7(1):51-55.
Crow, W.L, Anderson, E.R and Minugh, E. 1985.
Subsurface Venting of Hydrocarbon Vapors from
an Underground Aquifer. API Pub. No. 4410.
Davidson, D., Wetzel, R., Pennington, D., Ellis, W.
and Moore, T. 1985. In-situ Treatment Methods for
Contaminated Soils and Groundwater. HMCRI
Seminar, November 4-5,1985. Washington, D.C.
Eklund, B. 1985. Detection of Hydrocarbons in
Groundwater by Analysis of Shallow Soil Gas/
Vapor. API Pub. No. 4394.
Engineering-Science, Inc. 1986. Cost Model for
Selected Technologies for Removal of Gasoline
Components from Groundwater. API Pub. No.
4422.
EPA. 1984. Review of In-place Treatment
Techniques for Contaminated Surface Soils, Vol-
ume I: Technical Evaluation. EPA-540/2-84-003a.
EPA. 1985. Remedial Action at Waste Disposal
Sites/Handbook. Revised. EPA/625/6-85/006.
Federal Register. 1986.51(216):40572-40654.
Fuller, W.H. and Warrick, A.W. 1985a. Soils in
Waste Treatment and Utilization: Volume I - Land
Treatment. CRC Press Inc., Boca Raton, FL.
Fuller, W.H. and Warrick, A.W. 1985b. Soils in
Waste Treatment and Utilization: Volume II - Pollut-
ant Containment Monitoring and Closure. CRC
Press Inc., Boca Raton, FL.
Hazaga, D, Fields, S. and Clemens, G.R 1984.
Thermal Treatment of Solvent Contaminated Soils.
In: Fifth National Conference on Management of
Uncontrolled Hazardous Waste Sites. Washington,
D.C.
Hoag, G.E. and Cliff, B. 1985. The Use of the
Soil Venting Technique for the Remediation of
Petroleum Contaminated Soils. University of
Connecticut.
Hoag, G.E. and Marley, M.C. 1986. Gasoline
Residual Saturation in Unsaturated Uniform
Aquifer Materials. ASCE-EE Draft.
Hoag, G.E., Marley, M.C. and Bruell, C.J. 1986. Soil
Venting of Gasoline Contaminated Soils. ASCE-EE
Draft.
Jaynes, D.B. and Rogowski, A.S. 1983. Applicability
of Pick's Law to Gas Diffusion. Soil Science Society
of America Journal, 47(3): 425-430.
Lai, S.H., Tiedje, J.M. and Erickson, A.E. 1976. In-
situ Measurement of Gas Diffusion Coefficients in
Soils. Soil Science Society of America Journal,
40(1):3-6.
54
-------
Malot, J., and Wood, PR. Low Cost, Site Specific,
Total Approach to Decontamination. No Date.
Marley, M.C. and Hoag, G.E. 1984. Induced Soil
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Hydrocarbons in the Vadose Zone. Proceedings of
the NWWA/API Conference on Petroleum Hydro-
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Nov. 5-7 Houston, TX. pp. 473-503.
Means. 1987 Site Work Cost Data.
Noland, J.W., McDevitt, N.R and Koltuniak, D.L.
1986. Low Temperature Thermal Stripping of Vol-
atile Compounds - A Field Demonstration Project.
Presented at the National Conference and Exhibi-
tion on Hazardous Wastes and Hazardous Mate-
rials, Atlanta, GA.
Nyer, E.K. 1985. Groundwater Treatment Technol-
ogy. Van Nostrand Reinhold Co., N.Y
Nyer, E.K. 1987 Lecture in Philadelphia, Pennsyl-
vania. April 29.
Olsen, R.L., Fuller, PR, Hinzel, E.J. and Smith, P
1986. Demonstration of Land Treatment of Hazard-
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Overcash, M.R. and Pal, D. 1979. Design of Land
Treatment Systems for Industrial Wastes - Theory
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Arbor, Ml.
Payne, F.C., Cubbage, C.R, Kilmer, G.L. and Fish,
L.H. 1986. In-situ Removal of Purgeable Organic
Compounds from Vadose Zone Soils. Presented at
Purdue Industrial Waste Conference, West
Lafayette, Indiana. May 14.
Prokop, W.H. and Bohn, H.L. 1985. Soil Bed Sys-
tem for Control of Rendering Plant Odors. Journal
Air Pollution Control Association, (12):1332-1339.
Radian Corp. 1985. Detection of Hydrocarbons in
Groundwater by Analysis of Shallow Soil Gas/
Vapor. API Pub. No. 4394.
Rao, RS.C., Hornsby, A.G, Kilcrease, D.R and
Kedikizza, RN. 1985. Sorption and Transport of
Hydrophobic Organic Chemicals in Aqueous Mixed
Solvent Systems: Model Development and Prelimi-
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14(3):376-383.
ient-state Method With a Time Dependent Surface
Condition. Soil Science Society of America Jour-
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Roy F Weston, Inc. 1985. In-situ Air Stripping of
Soils Pilot Study. Prepared for U.S. Army Toxic and
Hazardous Materials Agency. Contract DAAK 11-
82-C-0017 Task 11.
Roy F Weston, Inc. 1986. Economic Evaluation of
Low Temperature Thermal Stripping of Volatile
Organic Compounds from Soil. U.S. Army Toxic
and Hazardous Materials Agency Report No.
AMXTH-TE-CR 86085.
Savage, G.M, L.F Diaz and C.G. Golueke. 1985.
Biological Treatment of Organic Toxic Wastes.
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Phenol By Soil. Journal of Environmental Quality,
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Raymond, R. 1987 Lecture
Pennsylvania. April 29.
Rolston, D.E. and Brown, B.D. 1977 Measurement
of Soil Gaseous Diffusion Coefficients by a Trans-
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Terasawa, M, M. Hurai, and H. Kubota. 1986. Soil
Deodorization Systems. Biocycle, (27):28-32.
Texas Research Institute, Inc. 1984. Forced Venting
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Ulbrich, FR. 1986. The Hydrocarbon Contaminated
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Webster, D. 1986. Enclosed Thermal Soil Aeration
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Journal Air Pollution Control Association,
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Wilson, J.T., L.E. Leach, M. Henson, and J.N.
Jones. 1986. In-situ Biorestoration as a Ground
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Yaniga, RM. and W. Smith. 1985. Aquifer Restora-
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Yaniga, RM. and W. Smith. 1986. Aquifer Restora-
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Organic Contaminants. Superfund 86.
55
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Section 5
Removing Gasoline Dissolved in Groundwater
For removal of gasoline constituents dissolved in
groundwater several methods are available: air
stripping, activated carbon adsorption, biorestora-
tion, resin adsorption, reverse osmosis, ozonation,
oxidation with hydrogen peroxide, ultraviolet irradi-
ation, and land treatment. Under the right cir-
cumstances any of these methods can remove,
destroy, or detoxify all or some of the gasoline con-
taminants. Air stripping and activated carbon
adsorption are the most cost-effective and widely
applied in practice, however, and have been used
in over 95 percent of groundwater cleanups.
Together air stripping and activated carbon adsorp-
tion are applicable to most cases where gasoline
has contaminated local groundwater. They offer the
best combination of effectiveness in removing con-
taminants to low levels over a wide range of situa-
tions, as well as being fairly cost-effective.
Biorestoration is a technology that has only recently
begun to receive attention. Its potential, although
promising, has yet to be proven as a viable, wide-
spread method for controlling groundwater contam-
inants because of an inability to predict and model
the timing, kinetics, or reduction that is due to
biorestoration. The other methods mentioned may
be effective in certain situations but are expected to
be used rarely at leaking UST sites because of their
limitations and/or their high cost.
The following subsections address air stripping,
carbon adsorption, and biorestoration in terms of
AIR SUPPLY
their operation, removal efficiencies, cost-effective-
ness, and limitations.
Air Stripping
Background
Air stripping is a proven, effective means to remove
volatile organic compounds (VOCs) from ground-
water. It works by providing contact between air
and water to allow the volatile substances to diffuse
from the liquid to the gaseous phase. In many
cases it is the most cost-effective option for
gasoline-contaminated groundwater. It has been
used at many sites, either alone or with other
methods (usually activated carbon); and with effec-
tiveness. There are several methods of air strip-
ping, including diffused aeration, tray aerators,
spray basins, and packed towers.
Diffused Aeration
In a diffused aeration system, air (usually com-
pressed air) is injected into the water through a dif-
fuser or sparging device that produces fine bub-
bles. Mass transfer occurs across the air-water
interface of the bubbles until they leave the water or
become saturated with contaminant. This type of
aeration is usually conducted in a contact chamber,
although it can take place in holding ponds. A
schematic diagram of a diffused aerator appears in
Figure 14.
INFLUENT
DIFFUSER GRID-
/*.
I
i
I
"?
3
*
<
.
-
-
- 1
_
•\
b,'k'
T7
1 - : _: '—
:— • — — ; •• — •—
1 * -, ' ' i *
•*-
J
•
_* -
•._
2s
s
',
'1
%
j
u»
EFFLUENT
Figure 14. Schematic of a typical diffused aerator.
57
-------
Mass transfer rates can be improved by producing
smaller bubbles, increasing the air-water ratio,
improving basin geometry, or using a turbine to
increase turbulence. Increasing the depth of the
tank will also improve the mass transfer rate if the
bubbles do not reach saturation before exiting to
the atmosphere (Kavanaugh and Trussell, 1981).
In practice, diffused aerators have removal efficien-
cies in the range of 70 to 90 percent for organics
such as trichloroethylene, carbon tetrachloride, tet-
rachloroethylene, and vinyl chloride (Kavanaugh
and Trussell, 1981; Dyksen et al., 1985). In some
cases this may be an acceptable level of treatment;
where higher removal rates are required, though,
diffused aerators are not practical.
Tray Aeration
Tray aeration is a simple, low maintenance method
of aeration that does not use forced air. Water is
allowed to cascade through several layers of slat
trays to increase the surface area available to the
atmosphere (Figure 15). Full-scale tray aerators
used for the removal of trichloroethylene, tet-
rachloroethylene, trans-1,2-dichloroethylene, 1,2-
dichloroethane, and other chemicals have shown
removal efficiencies of 10 to 90 percent; usual
values are between 40 and 60 percent (Hess et al.,
1983). In certain situations, tray aeration could be a
cost-effective method of reducing somewhat the
VOC concentrations (for example, prior to activated
carbon treatment). Like diffused aeration, this
method cannot be used where low effluent concen-
trations are required; it has not been widely applied
at leaking UST sites.
DISTRIBUTOR
NIPPLES
STAGGERED
SLAT TRAYS
BAFFLES
AIR STACKS
Spray Aeration
Spray aeration involves setting up a grid network of
piping and nozzles over a pond or basin. Contami-
nated water is simply sprayed through the nozzles
and into the air to form droplets. Mass transfer of
the contaminant takes place across the air-water
surface of the droplets. Mass transfer efficiency
can be increased by passing the water through the
nozzles multiple times; removal of 1,1,1-
trichloroethane is reported to have increased from
40 percent to 85 percent by passing water through
nozzles two-and-a-half times (Hess et al., 1983).
Spray aeration has also been used as a means of
aquifer recharge; water treated by granulated acti-
vated carbon (GAC) and air stripping was sprayed
over an 8-acre area to recharge the aquifer being
cleaned (Mclntyre et al., 1986).
Spray aeration could result in higher removal of
VOCs, as well as an increased rate of aquifer resto-
ration due to the recharge. Two disadvantages of
spray aeration are, however, the large land area
necessary for the spray pond and the formation of
large amounts of mist that could be carried into
nearby residential areas. Also, the possibility of ice
formation (both of the mist and on the nozzles)
would lower the usefulness of this technique in
colder climates.
Packed Towers
The packed tower method involves passing water
down through a column of packing material while
pumping air countercurrently up through the pack-
ing (Figure 16). The packing material breaks the
water into small droplets, causing a large surface
area across which mass transfer can take place.
This high air-water ratio and the large void volume
can result in very high removal efficiencies, greater
than those attainable by any other aeration
technique.
These countercurrent packed towers are the most
common of the air stripping methods; in fact, the
term "air stripping" often refers to packed tower aer-
ation. The towers are very effective in removing
VOCs; reported removal efficiencies can reach as
high as 100 percent (i.e., to not-detectable levels),
but are typically in the range of 90 to 99 percent for
the compounds normally found at gasoline-con-
taminated sites. Packed towers are also the most
cost-effective of the air stripping methods for most
situations. For these reasons, this section of the
manual focuses on countercurrent packed towers.
Figure 17 shows graphically the VOC removal
ranges for feasible aeration alternatives.
Figure 15. Schematic diagram of redwood slatted tray
aerator.
58
-------
OFF-GASES
INFLUENT
BLOWER
\ \ N1^ \ \ \ V,
DEMISTOR
-DISTRIBUTOR
-PACKING MATERIAL
• SUPPORT PLATE
WET WELL
EFFLUENT
"05I1
BOOSTER
PUMP
TO
STORAGE TANK,
DISTRIBUTION
SYSTEM
OR ADDITIONAL
TREATMENT
Figure 16. Schematic diagram of packed tower aerator.
w
99
99.9
99.99
99.999
99.9999
SPRAY TOWERS
DIFFUSED
AERATION
CROSS FLOW
TOWER
PACKED
TOWER
NOT FEASIBLE
EDB
EDC
TEL
BENZENE
TOLUENE
XYLENES 11
ETHYLBENZENE DICHLOROETHYLENE
PCE
0.! I.O 10 100 1000 10,000
HENRY'S CONSTANT (ATM)
Figure 17. Ranges for feasible aeration alternatives for the removal of volatile compounds.
59
-------
Theory of Air Stripping
The basic principles of air stripping are straightfor-
ward (Treybal, 1980). The kinetic theory of gases
holds that molecules of dissolved gases can pass
freely between the gaseous and liquid phases. At
equilibrium, the same number of molecules move
in both directions through a unit of area in a unit of
time. The departure from equilibrium provides the
driving force for the mass transfer. The rate of mass
transfer is proportional to the difference between
the liquid-phase concentration of a contaminant in
the influent (the operating concentration) and that
substance's equilibrium liquid-phase concentration.
The equilibrium concentration of a contaminant
depends on its Henry's law constant. Henry's law
describes the relative tendency for a substance to
separate between the liquid and gaseous phases
at equilibrium. Thus, Henry's constant can be
thought of as a partitioning coefficient. As will be
discussed later, the magnitude of Henry's constant
is integral to the feasibility of air stripping for a par-
ticular compound. Henry's law can be expressed
mathematically as:
e,Yout
Pa = HXa
(1)
where
pa - Partial vapor pressure of contaminant a
(atm)
H = Henry's law constant (atm)
Xa = Mole fraction of contaminant a in water
(mole/mole)
The phenomenon of air stripping can best be
described as "controlled disequilibrium." Introduc-
ing fresh, contaminant-free air into the system
results in a net mass transfer from the liquid phase
to the gaseous phase. By continually replenishing
the air with contaminant-free air, the contaminants
are eventually reduced to very low levels.
Design Parameters
The design of an air stripping tower can also be
described mathematically; the equations are well-
developed in the literature (Treybal, 1980;
Kavanaugh and Trussell, 1981; Hand et al., 1986).
The equations are derived by setting up a mass
balance in the air stripper (Figure 18). Four basic
assumptions are incorporated in these equations.
First, that the influent air is free of VOCs. Second,
that plug flow conditions (i.e., where there is no dif-
ferential flow) hold for the air and water flow. The
use of an inlet water distribution system (weir tray
or nozzles) helps to preserve this condition. Third,
that the changes that occur in the liquid and air vol-
MASS BALANCE:
L-Xh =G-Yout
dZ
L,Xr
G.Y,r
L = volume liquid
G = volume gas
X = concentration in
liquid
Y = concentration in
gas
Z = depth of packing
Figure 18. Differential element for an air stripping tower.
umes during mass transfer are negligible. Fourth,
that Henry's law holds true for these conditions.
To solve for the master design equation, two vari-
ables first need to be determined: the flow rate to
be treated and the percent removal desired. The
flow rate depends on many factors, such as the
extent of the contamination, the rate at which the
contaminant plume is migrating, the future use of
the water, and the physical characteristics of the
aquifer (its permeability or transmissivity). The
desired removal efficiency is strongly dependent on
the future use of the water as well as the immediate
health threat posed by the contamination.
The remaining design parameters can be deter-
mined once the flow rate and desired removal effi-
ciency are known. When designing a tower for a
specific removal efficiency, a number of parameters
(e.g., size and type of packing, height and/or
diameter of the column, water temperature, air-
water ratio, gas pressure drop) can be adjusted to
achieve similar results. Some of these, such as air-
water ratio and tower height, are inversely related.
The objective of the design of an air stripping tower
is to maximize the rate of contaminant removal
from the water at the lowest reasonable cost. This
is usually done by iterating various parameters to
find the best combination.
The required design parameters are Henry's con-
stant (which is both contaminant- and temperature-
dependent), the mass transfer coefficient (which is
60
-------
dependent primarily on the packing material), and
the stripping factor and the air pressure drop (both
of which are selected to minimize total cost while
satisfying the removal efficiency goals).
Henrys Law Constant
Theoretical Henry's constants are available in the
literature for most compounds of interest (ICF, 1985;
Perry and Chilton, 1973). Figure 19 shows some
Henry's constants, including several gasoline con-
stituents as well as trichloroethylene (TCE) and 1,1-
dichloroethylene. These values are computed from
data on a compound's gram-molecular weight,
water solubility data, temperature, and the equilib-
rium vapor pressure of pure liquid. The concern
has been expressed that Henry's constants derived
from these values may not extrapolate correctly to
field design work. The low-solute concentration typ-
ical of groundwater, the temperature dependence
of Henry's constant, and the fact that the inside of
an air stripper does not represent true equilibrium
are all reasons given to cast doubt on laboratory
data. Recent work by Munz and Roberts (1987)
has shown that solute concentrations do not affect
Henry's constant, however, at least to concentra-
tions as low as 0.001 molar. Temperature was again
shown to have a major effect on Henry's constant
and thus on stripper performance (Figure 20).
Munz and Roberts state that each rise of 10°C in
temperature corresponds to an increase in the
Henry's constant by a factor of 1.6. Thus, tempera-
ture is very important when designing a stripping
tower.
u_ CONCENTRATION IN AIR mcg/L
CONCENTRATION IN WATER mcg/L
001
EASE OF STRIPPING
01 ]
100
1
1
Illllll
, 1
1
\ T
BE
•JZEN
mill i
ITCE
limn i mm
i
TOLUENE
XY
-ENES
ETHYLBENZENE
NAPHTHALENE
MTBE
nirm nunFTHYl ENF
EDB
EDC
TEL
EDB - ETHYLENE DIBROMIDE (1 2 - DIBROMOETHANE)
EDC . ETHYLENE DICHLORIDE (1 2 - D1CHLOROETHANE)
TEL - TETRAETHYL LEAD
MTBE = METHYL TERTIARY BL/TYL ETHER
TCE - TRICHLOROETHYLENE
400
300--
200--
100-
(1 987) K
10 20
TEMPERATURE (°C)
30
40
Figure 20. Temperature dependence of Henry's law
constant.
Henry's law constants are typically expressed as
either "dimensionless" or in atmospheres. Dimen-
sionless units are valid only for systems that oper-
ate at standard pressure because the actual units
are:
(atmospheres of pressure)
cubic meters of water
cubic meters of contaminant
Typical ranges of "dimensionless" Henry's con-
stants for gasoline components are 0.02 to 0.30
(see Figure 19).
The more common unit, atmospheres, is expressed
by:
moles of water
(atmospheres of pressure) mo|es of contaminant
Typical values for gasoline components (expressed
in atmospheres) range from 20 to 500 (Figure 17). It
is very important when designing air strippers to use
correct units for Henry's constants.
Moss Transfer Coefficient
The rate of mass transfer per unit time per unit vol-
ume is first-order, proportional to the difference
between the liquid-phase concentration of the con-
taminant in the influent and the equilibrium concen-
tration:
Figure 19. A comparison of stripping rates for TCE and
gasoline compounds.
JA = -K,a(C*-C,)
(2)
61
-------
where
where
K,a
C,* =
JA = the rate of mass transfer of contaminant A
(kg/hr/m3)
the mass transfer coefficient (K,) and the spe-
cific interfacial surface area (a) (K,a is also
known as the proportionality constant.)
equilibrium liquid-phase concentration (kg/m3)
Cj = operating liquid-phase concentration (kg/m3)
The proportionality constant, K,a, is composed of the
overall liquid mass transfer coefficient and the spe-
cific interfacial area. The mass transfer coefficient, K,,
represents the rate at which the system moves
towards equilibrium. The specific interfacial area, a, is
a measure of the available total surface area of water
that is exposed to the air. This value is dependent on
the packing material. The best packing material will
optimize the surface area per volume (m2/m3).
K| is a function of the geometry and physical charac-
teristics of the system, the compound being stripped,
and the temperature and flow rate of the liquid.
To describe the kinetics of air stripping, the two-phase
resistance model of mass transfer is generally used
(Perry and Chilton, 1973). This model incorporates
the resistance to mass transfer in both the liquid and
gas phases. K, is related to these by:
D
1 1
(3)
where
K,
k,
H =
overall liquid mass transfer coefficient
liquid-phase diffusional resistance
gas-phase diffusional resistance
molar density of water (55.6 kmole/m3)
Henry's constant (atm)
When a compound has a large Henry's constant
(above 50 atm), the term including k? is negligible.
In this case, K, ~ k,. In these applications, it is valid
to assume that the liquid-phase resistance domi-
nates.
Values for K,a are sometimes supplied by manufac-
turers or may be found in the literature. However,
because of the importance of this parameter in
packed tower design, it is recommended that K,a
values be determined from pilot studies.
In the absence of field data, there are two general
methods by which these values can be determined.
The first is the Sherwood-Hollaway empirical corre-
lation:
(4)
x
= molecular diffusion coefficient in water (ft2/
hr)
x,n = empirical constants
L' = liquid mass flux rate (Ib/ft2/hr)
U| = viscosity of water
= density of water
K,a = units of sec"1
A second, more common method is the Onda
equations (Onda et al., 1968). These equations
estimate the wetted surface area of the packing
material and the liquid-phase and gas-phase mass
transfer coefficients. These values are then used to
obtain K,a:
(5)
=0.0051
(6)
(7)
where:
NRe = Reynolds number (dimensionless)
computed as L
atuL
NFr = Froude number (dimensionless) computed
as ,2,,
La,
= Weber number (dimensionless) computed
as
aw
at
PiTwat
wetted area of packing per unit volume
(m2/m3)
total surface area of packing material per
unit volume (obtained from manufacturer
or literature) (m2/m3)k| = liquid-phase mass
transfer coefficient (m/sec)
62
-------
U|
g
PI
L
D
Da
G
UG
PG
= air-phase transfer coefficient (m/sec)
= viscosity of water (kg-m/sec)
= acceleration of gravity (9.8 m/sec2)
= density of water (kg/m3)
= liquid flow rate (kg/sec/m2)
= diffusivity in water (m2/sec)
= equivalent diameter of sphere with same
surface area as a piece of packing
material (m)
= critical surface tension of packing material
(obtained from manufacturer or from the
literature) (kg/sec2)
= surface tension of water (kg/sec2)
= diffusivity in air (m2/sec)
= gas flow rate (kg/sec/m2)
= viscosity of air (kg-m/sec)
= density of air (kg/m3)
Several researchers (Hand et al., 1986; Wallman
and Cummins, 1986) have reported good agree-
ment between K,a values derived from the Onda
equations and pilot plant data. In general, the
Onda-derived coefficients were somewhat lower
than pilot plant data and would result in a conserva-
tive design. An important conclusion by Wallman
and Cummins (1986) was that K,a values increase
with tower diameter. This trend was attributed to
sidewall effects, which were less important as
tower diameter increased. Because of this finding,
it was predicted that pilot plant determinations of
K,a are also conservative.
Stripping Factor
The stripping factor, R, is a ratio of the actual
operating air-water ratio to the theoretical minimum
ratio. The theoretical minimum air-water ratio for
100 percent removal is determined by a mass bal-
ance in the stripper. It is based on the concept of
Henry's law, which states that a certain amount of
air must be brought into contact with the water to
remove the contaminants. That minimum air-water
ratio is described by:
min
(8)
where
(G/L)min = minimum air-water ratio
H = Henry's constant (dimensionless)
C,,Ce = concentrations of influent and effluent
As described above, R is the ratio of the actual air-
water ratio to this minimum ratio:
R. ^actual
(9)
Combining these two equations by substitution,
and assuming a given removal efficiency, R can be
expressed as:
R = (G/L)(H/Pt)
(10)
where
(G/L)
P, = operating pressure (= 1 atm)
H = Henry's constant (atm)
As can be seen, the stripping factor is directly
related to the air-water ratio. In turn, these are
related to the gas pressure drop through the
packed column. There is more than one combina-
tion of air-water ratio and air-pressure drop that will
achieve a certain removal level. Therefore, these
values are iterated to obtain the most cost-effective
design (considering both capital and O&M costs).
Studies have shown that the most cost-effective
stripping factor (on a present-worth basis) usually
falls between R = 3 and R = 5 for most gasoline
constituents (Hand et al., 1986).
Gas Pressure Drop
The gas pressure drop through the stripping unit is
usually determined from a gas pressure drop
curve. Many packing vendors will supply a brand-
specific pressure drop curve; otherwise, a gen-
eralized curve may be used (Figures 21, 22). Using
this graph, it is possible to calculate the allowable
gas and liquid flow rates for a variety of gas pres-
sure drops. To use the pressure drop curve, find the
appropriate value on the x-axis based on the
selected air-water ratio. Read up to the chosen gas
pressure drop (generally 0.25 to 0.50 in. H2O per
foot is used). It is usually better to use lower pres-
sure drops for lower air-water ratios (COM, 1986).
By reflecting off the curve and reading the corre-
sponding value on the y-axis, it is possible to calcu-
late the allowable gas flow rate from the dimen-
sionless group. Dividing the gas flow rate by the
air-water ratio gives the liquid flow rate.
The pressure drop is a function of the gas and liq-
uid flow rates and the size and type of the packing.
It is important because it relates to the overall cost
of the air stripper and the flexibility of stripper per-
formance. A stripper operating at a high pressure
drop will require a smaller volume than a similar
stripper at a lower pressure drop. This reduces
capital costs for the tower but increases the blower
cost, and because the fan will be larger, more
power will be required; thus, O&M costs will
increase. The various combinations of pressure
drops and air-water ratios should be iterated to find
the most cost-effective choice. The pressure drop
min
63
-------
PARAMETER OF CURVES IS PRESSURE DROP IN
INCHES OF WATER/FOOT OF PACKED HEIGHT
Figure 21. Generalized pressure
drop curve for packings
(English units).
0.01 0.02 0.040.06 0.1 0.2 0.4 0.6 1.0 2.0 4.0 6.0 10.0
-
G \f.
Figure 22. Generalized pressure
drop curve for packings (metric
units).
A£MS!.6J7«10-'^
Z ft m
•""2° =1.224x10-' ^
0.001
>\
5\
x
\
, \
\
SS
0.01 o.o: 0.04
0.1
0.2
0.4
1.0
10
£. ( "c
C' (PL-PC
64
-------
is also important as it relates to tower flexibility.
Towers designed and built to operate at a low pres-
sure drop have the flexibility to increase the gas
flow rate and hence the air-water ratio, should the
future influent concentrations increase or the
effluent limitations decrease. This capability will
allow higher removal efficiencies and, thus, pre-
serve the current effluent concentrations or allow
attainment of stricter limits. Towers designed for
high pressure drops do not have this flexibility and
would have to decrease the liquid loading to
increase the air-water ratio.
The flooding line in Figures 21 and 22 refers to a
point at which the stripper no longer functions due
to inappropriate air-liquid flow rates. As the gas flow
rate is increased (at a constant liquid flow rate), one
of a number of changes may occur. These include
inversion, by which the liquid is not dispersed;
development of a slug of foam; or formation of a
layer of liquid at the top of the tower (Treybal, 1980).
Above the flooding line, stripping towers do not
operate efficiently; operation in this region should
be avoided.
Design Equations
After Henry's constant, the mass transfer rate coef-
ficient, the stripping factor, and the gas pressure
drop have been determined, all the variables of the
master design equation are satisfied. The following
equation results from the solution of the mass bal-
ance equation:
(11)
where
NTU = number of theoretical transfer units
where
Z, = depth of packing (m)
L = liquid loading rate (m3/m2/sec)
K.,a = overall liquid mass transfer coefficient
(sec1)
R = stripping factor (dimensionless)
C, = influent concentration (mg/L)
Ce = effluent concentration desired (mg/L)
This equation gives the total depth of packing nec-
essary to reach the desired flow rate under the
stated conditions. This can be thought of conceptu-
ally as:
Z = (NTU) • (HTU)
(12)
(13)
HTU = height of theoretical transfer unit
= L/K,a
(14)
NTU is a mathematical expression that charac-
terizes the difficulty of removing a compound from
solution. It bears a general relationship to the
height of the stripping column. The value of NTU is
predominantly influenced by the desired removal
efficiency and, to a lesser extent, the stripping fac-
tor (air-water ratio).
HTU characterizes the rate of mass transfer from
the liquid-phase to the gas-phase. The value is
primarily influenced by the mass transfer coefficient
and, to a lesser extent, the liquid loading rate. The
value bears a general relationship to the tower
diameter.
Design Procedure
There is no single procedure that must be followed
when designing an air stripping tower. General pro-
cedures are suggested in the literature (Kavanaugh
and Trussell, 1981; Ball et al., 1984). Regardless of
the procedure followed, values are first required for
the flow rate, influent and effluent concentration,
operating temperature, and the Henry's constant
for the limiting contaminant. After these initial
values are determined, a suggested general design
procedure is:
1. Select the packing material. There are many
commercial packings available, each with differ-
ent mass transfer and pressure drop charac-
teristics. The two broad categories of packing
are dumped and stacked (see Removal Efficien-
cies, p. 70, for a discussion of packing material).
A packing should be selected that exhibits a
high mass transfer rate with a low gas pressure
drop. For water treatment applications, plastic
packings are most common because they offer
low price, corrosion resistance, and lightweight
(2-10 Ib/ft3) material that is easily dumped into a
tower. Table 15 lists physical characteristics of
common packing materials.
2. Select a reasonable stripping factor (between 2
and 10, with 3 to 5 being the best). Calculate the
air-water ratio from Equation 10.
65
-------
3. Refer to Figures 21 and 22. Select a reasona-
ble gas pressure drop. Generally, it is better to
choose lower pressure drops (defined as being
less than or equal to 100 N/m/m2) for low air-
water ratios. Read graph to find a value for the
dimensionless group. Calculate the gas flow
rate.
4. Based on the chosen air-water ratio, calculate
the required liquid loading rate.
5. Find the tower diameter from D =|
where Q = flow rate (cfs).
6. Find the height of transfer unit from Equation 14.
7 Find the number of transfer units from Equation
13.
8. Find depth of packing (Equation 12). Use an
appropriate safety factor (1.2 is common).
9. Repeat for various values of the stripping factor
and gas pressure drop. Determine the most
cost-effective combination of parameters based
on present worth calculations.
Design Considerations
Several factors should be considered when design-
ing an air stripping tower. One consideration is the
character of the area surrounding the air stripper. If
the area is residential, the tower, blower, and
pumps may need to be enclosed for aesthetic rea-
sons and/or to control noise levels. Depending on
various factors (especially the gas flow rate), air
strippers can be loud. Zoning laws may also affect
stripper design. Many communities have maximum
height limitations.
A second consideration would be the prevailing
wind patterns of the area. One of the assumptions
of air stripping is that the influent air is free of VOCs.
In order to ensure this condition, the air intake
should be situated in such a manner as to prevent
"short-circuiting" between the tower effluent air and
influent air. Such a condition would result in lower
removal efficiencies.
A third consideration is proper distribution of the
influent water throughout the packing. A common
problem is channeling along the wall of the tower.
Known as "sidewall effect," channeling is caused
by the lower flow resistance along the wall, due to a
greater void volume. To correct this condition, water
is redistributed by side wipers, normally every 20 ft
of packing. In general, this problem is more severe
with smaller diameter columns.
A fourth consideration is the need for a mist
eliminator. This is a device which captures any
water entrained in the air before it exits to the
atmosphere. These screens are fairly cheap ($200-
$300) and can prevent potentially significant quan-
tities of water from leaving through the top of the
column.
A fifth consideration is the effect of influent water
quality on the material used for stripper construc-
tion. Aluminum is often used for construction
because it is not susceptible to rusting. Fiberglass-
reinforced plastic (FRP) or stainless steel could be
used where water is especially aggressive. Resins
used for FRP towers should be potable water/food-
grade and have EPA and FDA approval. Carbon
steel is generally unacceptable because it tends to
rust. If used, the steel should have potable water-
grade coating. Concrete is sometimes used.
Other considerations, which include efficiency
problems associated with high iron/manganese
content of the water and air pollution impacts, are
addressed more fully under Limitations, p. 71.
Cost of Air Stripping
One of the main benefits of air stripping as a treat-
ment technology for contaminated groundwater is
its general cost-effectiveness compared to other
cleanup methods, such as activated carbon. How-
ever, the cost of air stripping can vary widely
because it depends on many factors and is highly
site-specific.
The total cost of any treatment method is a combi-
nation of the initial capital costs and the ongoing
O&M costs. Capital costs are associated with the
startup of the air stripping facility. Included are costs
for the process equipment, such as the tower and
packing material, air blowers, pumps, piping valves,
and electrical equipment; a clearwell and holding
tank (if needed); any site-related costs, such as
land purchase, bulldozing, and access; vapor-
phase control, if required; materials and construc-
tion costs for housing, (if required); and miscellane-
ous costs such as painting, plumbing, and cleanup.
Also included in the capital costs are fees for
engineering and contingencies, such as legal fees.
O&M costs are basically comprised of power for
the pumps and blowers and maintenance costs (in-
cluding labor and materials).
It is sometimes useful to determine the cost of
treatment on a volume-treated basis. This is often
done to compare the costs at different sites or to
compare the costs of different types of treatment. A
common expression used is the cost per 1,000 gal
treated ($/1,000 gal). This cost represents the mar-
ginal cost of treatment. Typical treatment costs on a
volume-treated basis are $0.05 to $0.25/1,000 gal.
As described above, the total cost includes both
capital and O&M costs. Determining the marginal
66
-------
O&M costs is fairly easy: divide the costs of power
and maintenance for a certain time period by the
volume of flow treated in that period. Finding the
marginal capital cost is more difficult: estimates
must be made for the design life of the facility, the
interest rate over that period, and the flow to be
treated over the project life. The initial capital costs
can then be annualized over the life of the project.
Dividing by the estimated yearly flow will yield the
marginal capital cost.
Cleanup costs at a particular site are a function of
the length of the cleanup, the flow rate to be
treated, the desired removal efficiency and/or the
final concentration goal, the selected air-water ratio,
the physical properties of the limiting contaminant,
the residual concentration remaining in the aquifer,
conventional water quality parameters, and the
need for vapor-phase treatment, among other
items. Each of these factors has a particular effect
on the overall cost and the marginal cost of treat-
ment. The following paragraphs summarize the
various factors and their effects.
Length of Cleanup Time
The length of cleanup time can be one of the most
important determinants of both the total and margi-
nal costs. A longer cleanup will usually mean
higher initial capital costs but lower marginal costs,
because the capital costs can be annualized over a
greater number of years. The total operating costs
will increase with time, but marginal operating costs
are unaffected by the duration of the cleanup.
Flow Rate
The flow rate treated has a direct effect on the
costs of treatment. A high flow rate will require a
larger tower, clearwell, pumps, and blowers. It will
also require more electrical power than a low flow
rate. Thus the total capital and O&M costs will
increase with the flow treated. The marginal costs,
however, will generally decrease as flow rate
increases because of economies of scale.
Desired Removal Efficiency
The desired removal efficiency and/or the final
effluent goal has a primary influence on the total
costs. In general, the higher the desired removal
percentage (or the lower the effluent concentration
limitation), the higher the capital and O&M costs.
More complete contaminant removal (that is, lower
effluent concentrations) requires a higher air-water
ratio, increased packing depth, or both (all other
things being equal). Either factor increases capital
costs, and a higher air-water ratio also increases
operating costs.
Air-Water Ratio
The air-water ratio is a design parameter chosen
on the basis of cost-effectiveness and the Henry's
law constant of the limiting contaminant. A higher
ratio will increase power requirements but
decrease tower volume. The engineer should
determine the long-term costs of higher operational
costs versus higher initial costs and choose this
parameter based on the lowest present-value cost.
For aromatic compounds, typical air-water ratios
are 20-100:1.
Residual Concentration in the Aquifer
The contaminant concentration allowed to remain
in the aquifer is an important cost consideration. As
shown in Figure 23, costs are fairly constant for
residual aquifer concentrations of 200 to 1,000 ppb
(1 ppb = 1 (juL) of hydrocarbon. However, as the
desired residual concentration approaches the low
ppb range, costs increase exponentially. These
total costs reflect the need for prolonged pumping
life, reinjection of water to flush out the contami-
nants, and perhaps the use of detergents to loosen
contaminants adsorbed to the soil particles. The
residual concentration goal should depend on the
present danger of the contamination and the future
use of the site.
Conventional Parameters
The quality of the water in terms of traditional water
quality parameters such as pH, hardness, and iron
and manganese may affect the cost of any VOC
treatment scheme. Abnormal pH, very hard water,
and/or high levels of iron/manganese may require
pretreatment of the influent. This could add consid-
erably to the total cost.
Vapor-phase Treatment
If treatment of the stripper off-gas is desired or
required, the total cost of stripping can be expected
to double (as a rule-of-thumb) (Medlar, 1987). This
assumption is based on the use of GAC for treat-
ment and allows for the cost of the initial carbon
charge, the contractor, and other site-related and
construction costs. Vapor-phase treatment is dis-
cussed more fully in Off-Gas Air Pollution Control
Systems. As can be seen, many factors influence
the cost of air stripping. Because an air stripping
tower can reach a certain removal efficiency
through a variety of design parameters, an
engineer should decide on the most cost-effective
combination. To help with this complicated process,
several computer cost models have been
developed (Nirmalakhandan et al., 1987; Cummins
and Westrick, 1982; Clerk et al., 1984). Through the
67
-------
•£> 1.200.000
3
O
o
in
o
o
o
o
o
I
o
in
ui
cc
o
-J
1,000,000
800.000
eoo.ooo -
400,000 -
200,000 -
BENZENE (CARBON ADSORPTION PLUS MR STRIPPING AND
FREE HYDROCARBON RECOVERY)
BENZENE (AIR STRIPPING PLUS FREE HYDROCARBON RECOVERY)
0 100 200 300 400 500 800 700 800
RESIDUAL AQUIFER HYDROCARBON CONCENTRATION (PPB)
Figure 23. Total cleanup costs as a function of residual aquifer concentration.
900
1000
use of these models, it is possible to isolate one
parameter and optimize costs. For example, two
studies optimize cost by iterating the gas pressure
drop against the stripping factor, another uses the
stripping factor and the liquid loading rate, and a
fourth presents cost curves based on liquid flow
rates for a variety of alternatives. From these
studies, it appears that the most economical strip-
ping factor is between 3 and 5.
To get actual cost figures for this manual, three
sources were used: a survey of manufacturers and
suppliers of packed tower equipment; case studies
from published data which reported cost estimates
and after-the-fact costs of cleanups; and cost
curves developed by Camp, Dresser & McKee
(1987). A survey of tower suppliers resulted in a
range of costs from a low of $5,000 (rated to treat
22 gal/min) to $40,000 (rated to treat 450 gal/min).
These cost quotes generally include the tower,
packing material, mist eliminator, blower fan and
motor, and flow meter. The costs depend primarily
on the rated flow rate but are also influenced by
"extras" such as sampling valves. Because many
suppliers custom-design towers for each particular
case, their costs varied more widely. Large strip-
pers (rated over 500 gal/min) were generally
always custom-built, and thus there are no quoted
prices for towers this size. It is assumed that these
cost proportionally more than the tower costs
quoted above.
The survey of costs from cases reported in the liter-
ature yielded a range of capital costs from $27,000
to $1,100,000, and O&M costs from $7,000 to
$50,000 annually. According to these reports, the
cost of the process equipment (tower, packing,
pumps, and fans) accounted for between 20 and
75 percent of the overall capital cost, with higher
numbers if air pollution control was required. The
fees for engineering and contingencies normally
ranged between 20 and 30 percent of the total cap-
ital costs. Where necessary, buildings and sitework
contributed a significant part of the total cost of the
facility (up to 50 percent). The wide range of costs
exhibited can be attributed to the factors listed pre-
viously, especially the flow rate and whether off-gas
pollution control is included. For example, the
$27,000 case treated 70 gal/min; the site which
cost $1,100,000 included five towers, each 12 ft in
diameter and 50 ft high, which combined treated
3,500 gal/min to drinking water levels.
Figures from Camp Dresser & McKee (Figures 24
through 29) give general capital and operating cost
estimates for air strippers over a wide range of con-
ditions.
Typical costs for air stripping towers at UST sites
are about $130,000 to $150,000 (capital) and
$6,000 to $8,000 annual O&M costs.
68
-------
1,000,000
10,000
Water Flow Rate (MGD)
Figure 24. Capital costs for packed tower (based on size). Figure 25. Capital costs for clearwell.
ipoo,ooo -
100,000 -
3 10,000
Plant Flow (MGD)
10,000
Air Flow Rote (SCFM)
Figure 26. Capital costs for water pump.
Figure 27. Capital costs for air blower (based on pressure
drop).
69
-------
0.030
10 20 30
Packing Depth (feet)
Power Cost = 96(t / kw-hr
Figure 28. Operating costs for pumping (based on
packing depth).
10 20 30 40 50
Air- to- Water Ratio
Power Cost = 96 t/kw-hr
Figure 29. Operating costs for blower (based on pressure
drop).
Removal Efficiencies
The ability of an air stripping tower to reduce VOCs
to low levels has been demonstrated in hundreds of
pilot-scale and full-scale operations. Like the cost
of air stripping, the removal efficiency varies for dif-
fering sites and is influenced by a number of fac-
tors. Some of these are summarized below.
Water Temperature
The temperature of the influent water significantly
affects removal efficiency, as shown in Figure 20,
because of the temperature dependence of
Henry's constant (see Design Parameters, p. 60).
Henry's constant increases with temperature (by
about a factor of 1.6 per 10°C increase in tempera-
ture), resulting in higher rates of stripping for
warmer groundwater. The temperature of ground-
water is fairly constant throughout the year at a
given location, although it varies in different areas
of the country by as much as 15°C. This can have a
strong bearing on the success of an air stripping
facility.
Influent VOC Concentration
The influent contaminant concentration also affects
the percentage removal. For similar conditions, a
higher influent concentration will have a higher
removal efficiency. This can be explained by recal-
ling that the driving force for mass transfer is pro-
portional to the difference between the operating
concentration and the equilibrium concentration of
the contaminant. As the operating concentration
approaches the equilibrium concentration, the driv-
ing force decreases, and relatively less contami-
nant is removed. For this reason, the final effluent
concentration as well as the percentage removal
should be considered when designing to achieve a
particular effluent goal.
Physical Properties of the Contaminants
Because of their particular Henry's constant, the
contaminants to be removed will influence removal
efficiency. Compounds with higher Henry's con-
stants can be removed to a higher percentage than
those with lower Henry's constants. In cases where
multiple VOCs are present, the compound with the
lowest Henry's constant will generally be the limit-
ing compound. A compound with a higher Henry's
constant at a much higher concentration, however,
could be limiting.
70
-------
Pocking Material
The type of packing material can also affect the
removal efficiency. The two broad categories of
packing are randomly dumped packing and
stacked packing. Dumped packing utilizes ran-
domly placed small plastic, metal, or ceramic pack-
ings to provide a high surface area and a high void
volume. Stacked packing can be described as per-
forming like a bundle of tubes. Dumped packing
has been much more common, but stacked pack-
ings may offer some advantages. According to
manufacturers, stacked packings are less suscepti-
ble to biological and mineral fouling due to their
higher (in some cases) void space and the fact that
stacked packings do not have horizontal surfaces.
Table 15 lists physical characteristics of several
common dumped and stacked packings.
Air-Water Ratio
Increasing the air-water ratio will usually result in
increased removal efficiency. However, this effect
may have diminishing marginal returns (Hand et
al., 1986). For most gasoline compounds, very
high-level removal (99 + %) requires a very high
air-water ratio.
Data from full-scale operations have shown that of
95 to 99 percent of the influent concentration of
VOCs can normally be removed. In some cases in
fact, the product water is used for drinking water.
Air stripping is most effective for removing low-
molecular-weight, nonpolar compounds with low
solubilities; benzene, toluene, xylene, and other
aromatics are normally removed to very low levels.
It is important to realize that the removal efficiency
of an air stripping tower is fixed by the design and
will not change over the life of the cleanup (assum-
ing initial conditions do not change). This differs
from the use of activated carbon, whose removal
effectiveness depends on the life of the carbon and
generally decreases over time for each carbon
change.
Ease of Operation
One of the main advantages of air stripping is its
relative ease of operation. Once the tower, blower,
pumps, valves, electrical instrumentation, and
appurtenances are in place and operating, the facil-
ity is practically self-operating. There is no recurring
maintenance (such as carbon replacement) that
requires the services of an engineer beyond nor-
mal maintenance. Iron and manganese or biologi-
cal interferences could cause operational prob-
lems, however, which would require the services of
an engineer.
Reliability
The ability of air stripping to consistently produce
high-level removal efficiencies for volatile ground-
water contaminants is well documented. In the
past, removal to low ppb levels or to below-mini-
mum-detection levels of benzene, toluene, and
xylene has normally been achieved. Each site has
its own characteristics and problems, however, and
complicating factors may prevent the achievement
of such low levels at every site. The more conser-
vative designs may add a safety margin for low-
level removals.
Limitations
The use of air stripping for the removal of dissolved
gasoline from groundwater may be limited by sev-
eral factors. These include the types of chemicals
which can be removed effectively by air stripping;
possible air pollution impacts of the stripping tower
effluent; high iron and manganese and/or sus-
pended solids concentrations in the influent water;
and possible high noise levels from the stripper.
Perhaps the most important limitation of air strip-
ping is that many types of groundwater contami-
nants cannot be removed by this method. It is
applicable only to the removal of volatile com-
pounds. The major constituents of interest in
gasoline, such as benzene, toluene, xylene, and
ethylbenzene, are all fairly volatile and thus easily
removed. Compounds with low volatility, such as
1,2-dichloroethane (EDC), are not readily removed
by this technique. In general, very soluble com-
pounds, high-polarity compounds, and high-
molecular-weight compounds are not easily
removed by stripping.
The possibility of air pollution from the gaseous
effluent from air stripping towers has caused con-
cern. The operation of a stripping tower does not
destroy the contaminant; it simply transfers the
contaminant from the liquid to the gaseous phase.
It is assumed that through the dilution occurring in
the tower and the mixing in the atmosphere, the
ambient concentration of the contaminant entering
the atmosphere will be below safe levels. New Jer-
sey, California, and Michigan have regulations that
limit the discharge of volatiles to the atmosphere. In
New Jersey, no source is permitted to discharge
more than 0.1 Ib/h of any particular VOC, including
benzene. For strippers exceeding this limit, off-gas
air pollution control is required. Typically, carbon
71
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Dumped Packings
Type
Glitsch
Mini-Rings
(Plastic)
Tellerettes
(Plastic)
Intalox
Saddles
(Plastic)
Pall Rings
(Plastic)
Raschig Rings
(Ceramic)
Jaegar
Tri-Packs
(Plastic)
Stacked Packing
Delta
(PVC)
Flexipac
(Plastic)
Table 15
Physical Characteristics of Common Packing Materials
Surface Area Void Space
Size (in.) (st/cf) (%)
OA
1A
1
2A
2
3A
1"(#1)
2" (2-R)
3" (3-R)
3" (2-K)
1"
2"
3"
5/a"
1"
11/2"
2"
31/2"
1/2"
3/4"
1"
r/2"
2"
3"
1"
2"
31/2"
—
Typel
Type 2
TypeS
Type 4
106
60.3
44
41
29.5
24
55
38
30
28
63
33
27
104
63
39
31
26
111
80
58
38
28
19
85
48
38
90
170
75
41
21
89
92
94
94
95
95.5
87
93
92
95
91
93
94
87
90
91
92
92
63
63
73
71
74
78
90
93
95
98
91
93
96
98
Packing Factor2
(1/ft)
60
30
28
28
15
12
40
18
16
12
33
21
16
97
52
40
25
16
580
255
155
95
65
37
28
16
12
-
33
22
16
9
Taken from manufacturers' data and Treybal (1980)
Represents "typical" value; actually a variable.
72
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adsorption is used to treat the vapor-phase con-
taminant. Figure 30 shows the amount of
a particular volatile or total volatiles which would be
released to the air at the stated flow rates and
removal efficiencies. (It is interesting to note that at
gas stations, VOC discharges of 10 Ib/h have been
measured.)
Another limitation of air stripping may be high noise
levels resulting from tower operation. If the facility is
in a residential neighborhood, the noise could be
very disturbing, especially if the tower is being
operated at a high gas loading rate. One solution is
to surround the tower with walls extending above
the tower.
High concentrations of iron and manganese and/or
suspended solids in the influent water can limit the
effectiveness of air stripping. Iron and manganese
facilitate the growth of bacteria on the packing,
causing decreased mass transfer rates and higher
gas pressure drops. The presence of toluene in the
influent is thought to contribute to this effect
(Abrams, 1987). Suspended solids can cause simi-
lar problems if they are trapped by the packing.
Many methods have been used to remediate pack-
ings clogged with iron hydroxides or biological foul-
ing. Some facilities remove the packing and physi-
cally remove attached growth. Normally, however, a
rinse of some type is used. According to Jarnis et
al. (1987), a strong chlorine or hydrogen peroxide
rinse can be used for biological fouling, while a
dilute acid rinse may be used for iron hydroxide
clogging. Stacked packings appear to have less of
a tendency to clog because they do not have any
horizontal surfaces on which bacteria/hydroxides
can gather.
Off-Gas Air Pollution Control Systems
Possible air pollution from the operation of stripping
towers is a major concern in some areas and is a
potential limiting factor for the use of this treatment
technique. In cases where treatment of the stripper
off-gas is desired or required, vapor-phase GAC is
the most common treatment. This method transfers
the contaminant onto the GAC after it has vol-
atilized from the liquid. Other treatment methods
include incineration and catalytic oxidation.
The advantage of using vapor-phase GAC after a
stripper (as compared to using liquid-phase GAC
and foregoing the stripper) is in the greatly
FLOW RATE (gpm) x PERCENT REMOVAL x CONTAMINANT CONCENTRATION (mg/L) x 0005 =
Ibs /hour TO ATMOSPHERE
o
x
NEW JERSEY LIMIT = 010 LB/HOUR
GPM
Figure 30. Representative volatile organic compound discharge rates.
73
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increased adsorption capacity of the GAC in the
vapor-phase. By transferring the contamination to
the vapor-phase (via air stripping) prior to removal
by GAC, the carbon can adsorb much more con-
taminant and therefore will last much longer; thus
O&M costs are significantly reduced. For example,
Zanitsch (1979) reported a vapor-phase adsorption
capacity for toluene of 26 percent by weight (260
mg/g). This compares favorably with a liquid-phase
capacity of 2.6 percent (26 mg/g) (Dobbs and
Cohen, 1980). Depending on the chemical in ques-
tion, the vapor-phase adsorption capacity can be
from 3 to 20 times higher than the liquid-phase
capacity (Medlar, 1987).
In order for vapor-phase GAC to be properly
utilized, the off-gas relative humidity must be
reduced to below 50 percent. This can be done by
using desiccants or heating the air. If the relative
humidity is not reduced, the capacity of the carbon
is significantly reduced because the water
molecules occupy adsorption sites preferentially.
Another consideration in the design of a vapor-
phase GAC system is the approach velocity: it must
be kept below 100 ft/min for effective adsorption.
The cost for vapor-phase GAC systems is typically
$100,000 for single tank (bed) units and $120,000
for dual tank units (COM, 1987). These costs do not
include the cost of the carbon or the operational
cost. These costs are fairly constant over a range
of treatment sites. Table 16 gives approximate rela-
tive cost ranges for several treatment alternatives.
Table 16
Relative Cost Factors for Treatment of Groundwater
Relative Cost Factors'
Technique
Air stripping
Air stripping &
vapor-phase GAC
Air stripping &
liquid-phase GAC
Air stripping &
liquid-phase &
vapor-phase GAC
Liquid GAC only
Capital
1*
2.0
3.0
4.0
1.5
O&M
1*
3.0
30
5.0
4.0
O&M (RCRA)2
1
4.0
4.5
7.5
8.0
Source: COM, 1987.
'Assigned
'Cost factors indicated are relative to air stripping.
indicates cost if spent carbon must be treated as a
hazardous waste under RCRA.
Activated Carbon
Adsorption
Background
Carbon has been used as an adsorbent for cen-
turies; the ancient Hindus reportedly filtered their
water with charcoal (Cheremisinoff and Ellerbusch,
1978). The beverage industry has used GAC for
water treatment since the 1930s. In the mid-1960s
increasingly large numbers of municipal water
treatment facilities began choosing GAC to control
taste and odor problems (Bright and Stenzel,
1985). Because GAC has the ability to remove a
large variety of compounds (including organics)
from water, its use has increased greatly over the
past 20 years as a treatment for organic contami-
nation of surface waters and groundwaters. Today,
along with air stripping, it is one of the most com-
mon methods for treating groundwater contami-
nated by VOCs, including gasoline.
Activated carbon can be either powdered (PAC) or
granular (GAC). Powdered carbon refers to par-
ticles that are smaller than U.S. Sieve Series No.
50; granular carbon is anything larger than this
(Cheremisinoff and Ellerbusch, 1978). PAC is gen-
erally not recoverable in usable form. It is normally
used as part of a treatment train, where it is added
to the water and later removed by sedimentation or
coagulation. Thus, PAC use is limited to complete
treatment systems, in which the product water is to
be used for drinking water. Since most leaking LIST
sites will not require extended treatment trains,
GAC is the usual choice when activated carbon is
to be used. The GAC is normally recovered for
reuse.
Adsorption Processes
Adsorption is a natural process by which molecules
of a dissolved compound collect on and adhere to
the surface of an adsorbent solid. Either chemical
or physical forces cause the molecules to collect on
the solid. Whether chemical or physical, adsorption
occurs when the attractive forces at the carbon sur-
face overcome the attractive forces of the liquid.
Chemical adsorption is said to have occurred when
the attraction is so strong at the carbon surface that
a chemical compound is formed. Physical adsorp-
tion is due to van der Waals' forces, which in com-
parison to chemical adsorption, are extremely
weak bonds. In environmental engineering applica-
tions, adsorption usually refers to physical adsorp-
tion.
Van der Waals' forces are common to all matter
and are thought to be the result of the motion of
74
-------
electrons. Molecules held by van der Waals' forces
are weakly adsorbed and can be removed by
changing the solute concentration or by adding
enough energy to overcome the bonds. This ability
to remove certain molecules adsorbed on carbon
and to reuse the carbon several times is what
allows GAC adsorption to be a cost-effective
technology.
The mass transfer of a solute from the bulk liquid to
the carbon surface has three basic phases (Figure
31). First, bulk transport carries the solute (contam-
inant) among the carbon particles themselves. This
type of transport is affected by the type of carbon
and the liquid velocity. Second, film transport
occurs as the solute diffuses from the bulk liquid
across the theoretical hydrodynamic layer sur-
rounding the carbon particle. The rate of mass
transfer across this layer is assumed to depend on
the mass transfer coefficient k (Perry and Chilton,
1973). Third, the particle undergoes intraparticle
transport throughout the carbon pores. This step
can be divided further into pore diffusion, surface
diffusion, and micropore diffusion. The internal
pores of activated carbon are classified, based on
their size as micropores (10-1000 A) or macropores
(over 1000 A) (Cheremisinoff and Ellerbusch,
1978). Pore diffusion describes the process
whereby the solute is transported into and through
the macropores. The only reaction that occurs is
adsorption on the macropore walls. Surface diffu-
sion occurs when particles already adsorbed on
the pore walls move further into the carbon particle.
Micropore diffusion is the transport mechanism by
which the adsorbate is carried into the micropores
where it reacts with the carbon walls.
LIQUID
Activated Carbon as
an Adsorbent
Activated carbon is used as an adsorbent because
of its large surface area, a critical factor in the
adsorption process. The typical range for surface
areas of commercially available activated carbon is
1,000 to 1,400 m2/g. This very large surface area
results from the unique internal pore structure of
activated carbon (Figure 32). Most of the available
surface area is internal.
Figure 31. Mass transfer of solute from liquid to carbon particle.
Figure 32. Idealized diagram of internal pore structure
of GAC.
Activated carbon is a general term that refers to a
group of substances. It originates from several dif-
ferent sources, including bituminous coal, coconut
shells, lignite, wood, tire scrap, and pulp residues,
with coal being the most common. To form GAC,
the particular base is subjected to three steps:
dehydration, carbonization, and activation. The
dehydration step removes water by heating the
material to 170°C. Further increasing the tempera-
ture drives off other vapors (CO2, CO, CH3COOH)
and decomposition begins, resulting in carboniza-
tion. Activation occurs when superheated steam is
released into the system, enlarging the pores by
removing the ashes produced during the carboni-
zation step.
GAC Evaluation: The Isotherm
The basic instrument for the evaluation of activated
carbon treatment is the adsorption isotherm. The
isotherm is a function that relates the amount of
solute adsorbed per weight of adsorbent to the sol-
ute concentration remaining in the liquid at equilib-
75
-------
rium. As the term implies, isotherms are tempera-
ture-dependent, so values are given in terms of
temperature. The isotherm function (shown in Fig-
ure 33) can be thought of as a means of describing
the capacity of carbon for a particular compound,
or the efficiency of carbon to remove that com-
pound.
100
10
1
'.1 ' ' ' "
t
f
"'i.o '
1
/
4 '
"~\
. A-
'10 '
00
4 *
1000
RESIDUAL CONC (C,), mg/l
Figure 33. Freundlich isotherm for benzene.
The carbon capacity is influenced by a variety of
factors: the solute to be adsorbed, the adsorbent
(carbon) itself, the water temperature, the pH of the
liquid, and other things. Isotherms are usually
determined for a single-solute solution. If more than
one compound is present in the water, as is usually
the case at gasoline contamination sites, the
isotherms are useful only for comparative purpose,
not for design purposes.
The equations most commonly used to describe
experimental isotherm data are those by
Freundlich and Langmuir (see Figure 33). The
Langmuir isotherm is of the form:
_X_ QbC
M~ 1+bC
(15)
where
X/M = amount of adsorbate (X) per weight of
adsorbent (M)
Q = amount of adsorption per unit weight form-
ing a complete monolayer
C = concentration of solute in water at equilib-
rium
b = b0exp (-E/RT)
where
b0 = a constant that includes the entropy term
E = energy of adsorption
R = universal gas constant
T = absolute temperature (°K)
The Langmuir isotherm equation was developed
theoretically to closely model the adsorption proc-
ess, as evidenced by the term Q, which assumes
that a monolayer forms on the carbon. The more
commonly used Freundlich isotherm, on the other
hand, represents an empirical equation. It has the
general form:
X/M = KG
1/n
(16)
where
X/M
C
K,n
= amount of adsorbate per weight of adsor-
bent
= concentration of solute in water at equilib-
rium
= empirical constants specific to the com-
pound
The empirical constants, K and n, are determined
by plotting experimental results, with the amount of
solute adsorbed on the y-axis and the equilibrium
solution concentration on the x-axis. The isotherm
is typically linear. The slope of the line is equal 1/n,
while the y-intercept is equal to K. Although the
constants have no physical significance, they are
useful for comparing the adsorption capacities of
different compounds or for the same compound on
different carbons. Isotherms are specific to the type
of carbon used.) Values for these parameters are
commonly found in the literature. Table 17 sum-
marizes reported K values, representing carbon
capacities for some gasoline components.
Activated Carbon Life and
Breakthrough
Within an operating carbon tank, three distinct
zones are present (Figure 34). The equilibrium
zone, located at the influent end of the tank, is the
area where the carbon is saturated with contami-
nant. At the downstream end of the carbon tank is
an area where the carbon retains its complete
adsorptive capacity. Between these two zones is
76
-------
Table 17
Carbon Adsorption Capacities for Selected Compounds
Compound
Vinyl Chloride
Methylene Chloride
1,2-Dichloroethane (EDC)1
Benzene1
Ethylene Dibromide (EDB)1
Toluene1
Ethylbenzene1
p-Xylene
Naphthalene1
Phenol1
bis (2-Ethylhexyl) phthalate
Adsorption Capacity (mg/gr)
Trace
Avg: 1.2
1.3
1.6
0.8
1.3
Avg: 2.5
3.6
2.0
3.6
03
Avg- 16
1.0
27.4
80
4.1
1.73
Avg 17.0
17.0
Avg- 22.5
26
50
2
12
Avg: 24
53
18
2.2
Avg -46
85
55
50
28
13
Avg. 68
132
5.6
Avg: 91
161
22
11,300
Reference
Nyer, 1987
1CDM, 1987
2CDM, 1987
Nyer, 1985
Dobbs & Cohen, 1980
Dobbs & Cohen, 1980
Nyer, 1985
Hall & Mumford, 1987
Hall & Mumford, 1987
Dobbs & Cohen, 1980
COM, 1987
Verschueren, 1977
Hall & Mumford, 1987
Hall & Mumford, 1987
Neulight, 1987
Dobbs & Cohen, 1980
Verschueren, 1977
Hall & Mumford, 1987
Hall & Mumford, 1987
Dobbs & Cohen, 1980
Verschueren, 1977
Hall & Mumford, 1987
Dobbs & Cohen, 1980
Hall & Mumford, 1987
Hall & Mumford, 1987
Bright &Stenzel, 1985
Bright & Stenzel, 1985
Dobbs & Cohen, 1980
Nyer, 1985
Verschueren, 1977
Dobbs & Cohen, 1980
Dobbs & Cohen, 1980
1Gasoline Components
77
-------
INFLUENT
BED
DEPTH
EQUILIBRIUM ZONE
" (ZONE OF EXHAUSTION)
MASS TRANSFER ZONE
(MTZ)
UNUSED CARBON
EFFLUENT
Figure 34. Idealized diagram of zones within GAC reactor.
the mass transfer zone (MTZ), where adsorption is
taking place. Within the MTZ, a concentration gra-
dient develops, with a high concentration at the
influent end of the MTZ decreasing to near-zero
concentrations for most contaminants at the down-
stream end of the MTZ. The length of this MTZ
depends on the loading rate and the characteristics
of the adsorbent and adsorbate. The total length of
the MTZ represents the resistance to adsorption.
The MTZ moves downward through the column as
the total volume of water treated increases. Eventu-
ally, the leading edge of the MTZ reaches the end
of the column (Figure 35), and the effluent contains
increasingly higher concentrations of contamina-
tion as time passes. When the effluent concentra-
tion reaches a given concentration (determined
arbitrarily or based on effluent standards), break-
through is said to have occurred, and the carbon is
normally replaced. Figure 36 shows an idealized
breakthrough curve. The breakthrough characteris-
tics are an important determinant in deciding
whether GAC is appropriate for a particular site.
Breakthrough is discussed in more detail on p. 81.
Design of Carbon Systems
The design of an activated carbon system is not as
straightforward as the design of an air stripping
tower. Rather than the basic equations that are
used to determine the size and operating parame-
ters of a stripping tower, design of a GAC system
requires more complete pilot testing and engineer-
ing judgment. The adsorption characteristics of any
particular combination of contaminants are not
generally predictable, except in a few situations
where certain common chemicals are found and
the engineer has vast experience. Even under
these conditions, a pilot test using the water of
interest is often required to forecast accurately the
optimal empty bed contact time (EBCT) and carbon
usage rate at a specific site.
When designing a GAC system, the EBCT is cho-
sen first. The EBCT is defined as the volume of car-
bon divided by the flow rate. The EBCT relates
directly to the size of the contactor needed; a high
EBCT requires more carbon. The EBCT is
inversely related to the carbon usage rate; the
higher the EBCT, the lower the usage rate. The
goal of the GAC system design is to find the optimal
point in the tradeoff between a lower carbon usage
rate and a smaller contactor size. A typically used
minimum EBCT for gasoline spills is 15 min. For a
standard 20,000-lb supply of carbon in a 10-ft
diameter column, this EBCT results in a liquid load-
ing rate of 2 gal/min per ft2. Experience has shown
that this configuration results in a system with a
good removal rate and high flexibility, should future
conditions change (Neulight, 1987).
VIRGIN
CARBON
BREAKTHROUGH
EXHAUSTION
a
a
BOTTOM
TIME
Figure 35. Breakthrough and exhaustion in an operating
GAC reactor.
INFLUENT
CONCENTRATION
EFFLUENT
CONCENTRATION
1
IDEAL FRONT >•
BREAKTHROUGH /
^ u^
^
f EXHAUSTION
TIME IN OPERATION
Figure 36. Idealized single-solute breakthrough curve.
78
-------
The second design variable is the decision to use a
single-stage or multistage operation (discussed
below in Operation of Carbon Systems). This deci-
sion is based on the breakthrough characteristics
of the influent stream, as well as financial consider-
ations. Influents that exhibit a long MTZ are better
operated in a multistage fashion (Figure 37)
because this mode allows more efficient use of the
carbon, although at a higher overall cost.
INFLUENT
EFFLUENT
Figure 37. Schematic diagram of multistage GAC
contactors.
Operation of Carbon Systems
Facilities using GAC at leaking UST sites are nor-
mally operated as fixed bed facilities. The contac-
tors may be either gravity or pressure filters and
may be single-stage or multistage. Each choice
offers benefits for specific conditions.
Fixed-bed columns may employ upflow or down-
flow of the liquid. If downflow is used, the carbon
bed acts as a filter for suspended solids in addition
to removing organics. Filtering may be undesirable
in some cases (for example, where the suspended
solids concentration is high) because of the high
head losses which result and extra backwashing
that is necessary. In these cases, upflow of the
water would be preferred.
Gravity GAC filters are usually made of concrete
and are operated similarly to sand filters. They are
generally used for very high flows, such as are
common at municipal water treatment plants (1 to 5
million gallons per day). Gravity filters are not used
at most leaking UST sites. Rather, pressure filters
are used because they allow higher surface load-
ing rates (5 to 7 gal/min per ft2) than do gravity fil-
ters at 2 to 4 gal/min per ft2; and they also pres-
sure-discharge, which saves repumping costs.
They are limited to diameters of 12 ft or less, sizes
in which the cylinders are normally available. They
are typically 10 ft in diameter. A vessel 10 ft in
diameter and 10 ft high holds approximately 20,000
Ib of carbon. When wet, this amount weighs about
40,000 Ib, which is the maximum allowable weight
that can be shipped on U.S. highways and thus
determines the typical size for carbon filters.
GAC contactors may be operated either as single-
stage or multistage. In multistage use (Figure 37),
the leading contactor removes the majority of the
contamination, while the second contactor acts as
a "polishing" step, removing any residual organics
from the water. In series operation, the entire
adsorptive capacity of the carbon is used. The lead
contactor can be used past breakthrough (i.e., to
exhaustion) because the second contactor con-
tinues to remove the constituents. After the spent
carbon is replaced, the piping is reversed so that
the new carbon becomes the polishing bed. Multi-
stage operation is the optimal use of carbon. The
cost for this method, however, is higher than single-
stage and may not always be justified, especially
where discharge limitations are not stringent.
Removal Efficiency
Many case studies have demonstrated the ability of
activated carbon to remove a variety of compounds
in gasoline to nondetectable levels (99.99 + %
removal). The effectiveness of GAC at a particular
location depends on several factors, but primarily
on the compounds to be removed. The appro-
priateness of GAC for a site depends primarily on
cost and how it is influenced by factors such as
influent concentrations, effluent use (concentration
limits), composition of the groundwater, and availa-
ble alternatives to GAC treatment. For example,
GAC can almost always reduce gasoline-contami-
nated groundwater to less than 1 ppb of benzene.
However, in cases where the influent concentration
is very high, and/or the discharge requirements are
not strict, air stripping (either alone or prior to GAC)
may be a more cost-effective and appropriate
means of removing the benzene. The following
paragraphs summarize the factors that influence
the choice of GAC for groundwater remediation.
fffecf/veness
Although activated carbon has been used success-
fully to remove many gasoline compounds from
water, not every compound can be removed. GAC
works best for low-solubility, high-molecular weight,
nonpolar, branched compounds (Bourdeau, 1987).
According to Brunotts et al. (1983), a compound's
solubility in water is the key parameter in determin-
ing how well it will adsorb. Low-solubility com-
pounds are adsorbed better than high-solubility
compounds, all other things being equal. For this
reason, alcohols, ketones, and ethers are poor
adsorbers, whereas most solvents and pesticides
are excellent adsorbers. High molecular weight
compounds adsorb better than low molecular
79
-------
weight compounds, perhaps because of their
higher van der Waals' forces. Extremely high
molecular weight compounds, such as sugars,
however, do not adsorb at all, but these com-
pounds are not usually found in groundwater.
GAC has a higher affinity for nonpolar compounds
than for polar compounds due to the surface
chemistry of the carbon. The polarity of a com-
pound depends on the chemical and physical
structure of its molecules. Polar compounds
behave more like ionic compounds, while nonpolar
compounds are more neutral electrically. Most
components of gasoline, particularly benzene,
toluene, and xylene, are nonpolar. The molecular
structure of a compound will also influence its abil-
ity to adsorb on GAC. Molecules which are
branched or have attached functional groups, such
as chlorine, fluorine, or nitrogen, adsorb well. Pes-
ticides generally exhibit extremely high adsorbabil-
ity, due in part to their complex molecular structure.
Other factors also influence the effectiveness of
GAC treatment: properties of the carbon product
itself, temperature of the water, iron and man-
ganese concentration of the water, the EBCT,
desorption, and bacteria activity.
As discussed earlier, GAC originates from several
different materials and can be prepared by a variety
of methods. For these reasons, different GAC prod-
ucts have different adsorptive capacities. The sur-
face area of the carbon is the most important factor
in determining its efficiency, because the amount of
adsorption is directly proportional to this value. The
surface chemistry of various carbons differs also,
but this effect is minor compared to that of surface
area. Regenerated carbon also differs from virgin
(unused) carbon. According to Bourdeau (1987),
virgin carbon is normally used in cases where the
effluent is to be used for drinking purposes. Reacti-
vated carbon, which costs significantly less, is nor-
mally acceptable for sites where the effluent is dis-
charged to surface or groundwater.
The temperature of the water also affects adsorp-
tion (Snoeyink, 1983). As the temperature
increases, adsorptive capacity decreases. The
effect of temperature in groundwater cases is mini-
mal, as the groundwater temperature in a given
locale is fairly constant throughout the year.
Groundwaters containing significant (above 5 mg/
L) levels of iron and manganese must be treated to
remove these compounds before GAC treatment. If
the iron and manganese are not removed prior to
GAC treatment, they will precipitate onto the car-
bon, clog the carbon pores, cause rapid head loss,
and eventually prevent flow through the carbon.
As stated above, the volume of GAC divided by the
flow rate to the column is defined as the empty bed
contact time (EBCT). It represents the theoretical
time that the GAC is in contact with the water; how-
ever, the actual time of contact is about half of the
EBCT because the interparticle porosity of GAC is
roughly 50 percent. The optimum EBCT is unique
to each facility. It depends on the type of carbon
used, contaminants being removed, bed depth,
flow rate, and influent and effluent concentrations.
Values for EBCT reported in the literature vary
widely, from 3 min to 2,000 min, certainly a reflec-
tion of the variety of situations to which GAC has
been applied. A typically used minimum EBCT for
gasoline compounds is 15 min, which corresponds
to a surface loading rate of 2 gal/min per ft2. Where
the compounds present are more difficult to
adsorb, a 30-min minimum EBCT is used. To deter-
mine the optimum EBCT for a particular site, pilot
studies can be used.
Desorption is the reverse of adsorption. Desorption
may occur with a sudden decrease in the influent
concentration. If this occurs, previously adsorbed
contaminant molecules may desorb so that equilib-
rium in the solution is maintained. This can result in
an effluent concentration that is higher than the
influent concentration. The phenomenon of dis-
placement may also occur if more strongly adsorb-
able contaminants appear in the influent and dis-
place the previously adsorbed compounds (Figure
38). This also results in higher concentrations of
those compounds in the effluent than in the
influent.
GAC beds are excellent media to support biological
growth. Once there, the bacteria are able to
degrade certain compounds from the bulk liquid
and the surface of the GAC (Speital and DiGiano,
1987). The occurrence of biodegradation has sev-
eral benefits. Perhaps the most important benefit is
the increased service life of the carbon. Com-
pounds that are degraded do not occupy sorption
sites, so those sites are available for other
molecules. Speital and DiGiano found that a reser-
voir of empty sorption sites may serve to dampen
variations in the effluent, preventing higher effluent
concentrations resulting from increased influent
concentrations. Van der Kooij (1983) discussed
possible negative aspects of biological growth on
GAC, including the formation of endotoxins, high
colony counts, and possible anaerobic conditions.
Appropriateness of Using GAC
After determining whether GAC could effectively
remove the contaminants of concern at a leaking
LIST site, the most cost-effective technique must be
determined. The decision whether this is GAC will
be based on the primary factors affecting cost:
influent concentrations of the contaminant(s) and
total organic carbon (TOC); desired effluent con-
80
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1000 2000 3000 4000
DCP introduced
into influent
5000
1000 2000 3000
Bed Volumes
4000
5000
Figure 38. Displacement from GAC of dimethylphenol (DMP) by more strongly absordable dichlorophenol (DCP).
centration; and breakthrough characteristics of the
contaminants in the influent.
GAC is best suited for reducing low influent
gasoline concentrations to nondetectable levels. In
situations where the influent concentration of TOC
is high, the carbon usage rates increase dramat-
ically. O'Brien and Fisher (1983) report the results
of 31 contamination case studies (not all gasoline-
related, however) in which GAC was used. In 17
cases where the influent TOC was above 1 mg/L
(1,000 ppb), the median carbon usage rate was
1.54 lb/1,000 gal treated. For the 14 cases where
influent TOC was below 1,000 ppb, the median car-
bon usage rate was 0.35 lb/1,000 gal. It can be
seen that treating a high influent concentration
uses much more carbon and is therefore signifi-
cantly more expensive than treating a low influent
concentration.
Carbon is well suited to remove most gasoline con-
taminants to nondetectable levels. It is therefore an
excellent choice where effluent standards are strin-
gent, such as drinking water standards. Unlike air
stripping, which has a specific percentage removal
of less than 100 percent, carbon can remove com-
pounds to nondetectable limits prior to break-
through.
Waters with many contaminants, such as gasoline-
contaminated waters will increase the carbon
usage rate significantly. This is due to competitive
adsorption. Conceptually, carbon has a limited
number of adsorption sites. Each site can accom-
modate one molecule; once the site is filled, no
other molecules are able to adsorb there. An
influent with many compounds will have a carbon
usage rate between that predicted by the com-
pound of earliest breakthrough and that predicted
by the sum of the usage rates of the individual com-
pounds (Hall and Mumford, 1987). In some cases
of competitive adsorption, displacement may occur
if a more strongly adsorbable compound is intro-
duced into the contaminant stream.
The breakthrough characteristics for each influent
stream are also important in determining the
appropriateness of GAC as a treatment technique.
The following section discusses breakthrough in
detail.
Breakthrough
Breakthrough occurs when the adsorptive capacity
of the carbon for a particular compound is
exhausted and that compound begins to appear in
the effluent. Because each compound has a
unique adsorptive capacity and because influent
concentrations vary, compounds will break through
at different rates.
The relative order of breakthrough of a group of
compounds can usually be predicted based on the
mean capacity of those compounds (X/M from
isotherm studies). This is true in cases where the
compounds have similar concentrations in the
influent. Compounds with low capacities will be the
first to appear in the effluent, whereas compounds
with high capacities would likely appear later. Of the
major components of gasoline, the order of break-
through (from earliest to latest) is generally ben-
zene, ethylbenzene, toluene, xylene, naphthalene,
and phenol (Figure 39). Other compounds some-
times found as additives to leaded gasoline—such
as methyl-tertiary butyl ether (MTBE), 1,2-
dichloroethane (EDC), and ethylene dibromide
81
-------
MEAN ADSORPTION CAPACITY, mg/gm
@ EQUILIBRIUM CONCENTRATION = 600 mcg/L'
1
<
10
10
EDC
t t
100 ,
1
| XVLENE
TOLUENE
ETHVLBENZENE
<
PHENOL
NAPHTHALENE
•SOME EXPERIMENTS CONDUCTED AT 1000 mcg/L
Figure 39. Mean absorption capacities of various
compounds in gasoline.
(EDB)—might appear in the effluent even before
benzene due to their very low adsorption
capacities. Table 17 gives the adsorptive capacities
for several compounds including gasoline con-
stituents.
In theory, breakthrough occurs when the leading
edge of the MTZ just reaches the end of the carbon
bed and the effluent begins to contain a detectable
amount of contamination (Figure 35). In practice,
however, breakthrough usually refers to a point at
which the effluent reaches a certain level, or
threshold, of contamination. This level is some-
times arbitrarily set, such as the commonly used 5
percent of the influent concentration (Reynolds,
1982); or the level may be based upon environmen-
tal regulations, such as discharge limits or drinking
water standards. In either case, the level of con-
tamination may refer to the total of all volatiles in the
water (TOC is the usual indicator) or to a specific
compound or compounds upon which the dis-
charge limits are based.
For example, for an influent stream contaminated
by a variety of compounds that is to be used as a
drinking water source, an effluent limit may be set
for total VOCs. This was the case reported by Mac-
Leod and Allan (1983). GAG was used to treat
municipal well water contaminated by several
organics to levels of 150 ppb. In that municipality,
an effluent standard of 5 ppb TOC was established
for water for household use. Sometimes an effluent
standard is based on a single contaminant. In these
cases, the single contaminant may be the com-
pound in the influent stream that is the first to break
through, or it may be the compound considered
most hazardous.
Usually, the compound with the earliest break-
through is used as an indicator for carbon change,
especially where the effluent is to be used for drink-
ing water. In the case of gasoline contamination,
benzene is normally the first compound to break
through. For this reason and the fact that benzene
is usually considered one of the most toxic compo-
nents in gasoline, the effluent is typically monitored
for benzene, and the carbon is changed when the
benzene reaches the threshold concentration. It
should be noted that other compounds which may
not be found in all gasoline (such as MTBE and
EDC) may breakthrough earlier than benzene.
Cost of GAC Treatment
The cost of GAC treatment is dependent on site-
specific conditions, and thus varies widely. In gen-
eral, though, GAC is more expensive than air strip-
ping for similar situations because the capital cost
of equipment and the O&M costs for GAC are
higher than those for air stripping.
Capital Costs
The capital costs of GAC treatment include the ini-
tial carbon charge, carbon vessel, the pumps and
piping, electrical equipment, a clearwell (if neces-
sary), housing (if necessary), and engineering
design and contingencies. The need for pretreat-
ment may also significantly increase the cost of
GAC treatment. The flow rate and the discharge
requirements are the criteria used for design and
thus have a controlling effect on capital costs. For
waters which require removal to nondetectable
levels, two carbon contactors normally are oper-
ated in series, which will increase the capital cost.
Very high flow rates may be treated by using sev-
eral pressure contactors in parallel or by using a
gravity carbon contactor. Gravity contractors are
often made of cement, and operate similarly to
sand filtration tanks. They can accommodate sur-
face loadings of only 2 to 4 gal/min per ft2 (Neulight,
1987) and thus must be larger than a correspond-
ing pressure tank, which can treat 5 to 7 gal/min per
ft2. Housing for the contactor(s) is often unneces-
sary. Engineering and contingencies average about
30 percent of the total capital cost. Figures from
Camp Dresser & McKeel (Figures 40 to 43) give
approximate construction costs for four types of
GAC contactors.
82
-------
100,000
lo.ooo
1,000
10,000,000
5 1,000,000
g 100,000
10,000 100,000
gallons per day
1,000,000
10,000
1,000 10,000
Individual Contactor Volutne-ft3
100,000
May 1986 ENR CCI= 4229
May 1966 ENR CCI =4229
Figure 40. Capital costs of low capacity package GAC Figure 41. Capital costs of pressure GAC contactor
contactor.
10,000,000
1,000,000
| 100,000
10,000
1,000
10,000,000 F
1,000,000
0 100,000
10,000 100,000 1,000,000
Individual Contactor Volume—ft3
10,000
100 1,000 10,000
Individual Contactor Volume -ft
100,000
Figure 42. Capital costs of gravity steel GAC contactor. Figure 43. Capital costs of gravity concrete GAC
contactor.
83
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Operation and Maintenance
The O&M costs include labor, building upkeep,
energy costs for pumps and instrumentation, car-
bon replacement/regeneration, and miscellaneous
expenses. Labor, building upkeep, and miscellane-
ous costs would be similar to those associated with
an air stripping facility. Where pressure filters are
used, significant savings may occur because the
water does not have to be repumped to the system.
The cost of the carbon replacement or regenera-
tion can be considerable, in most cases dominating
the O&M costs.
Carbon costs depend on the type of carbon used
and the carbon usage rate. Carbon prices supplied
by a manufacturer (Calgon) ranged from roughly
$0.75/lb for their highest quality virgin carbon to
$0.60/lb for service carbon (regenerated). The car-
bon usage rate is often expressed as pounds per
thousand gallons of water treated (lb/1,000 gal).
The usage rate is influenced by several factors,
including the breakthrough characteristics of the
contaminants to be removed, the concentration of
contaminants, and the required effluent concentra-
tion.
Each compound has a unique adsorption capacity
that can be described by its Freundlich isotherm
(discussed in Section 5.2.8). The greater the X/M
value (the weight of contaminants removed per
weight of carbon), the lower the carbon usage rate.
Studies have shown that the actual carbon usage
rate for a typical influent water with several con-
taminants lies between the rate predicted by the
compound with earliest breakthrough and the rate
predicted by adding the usage rates of all the com-
pounds (Hall and Mumford, 1987). When designing
a carbon system from theoretical isotherms, a large
safety factor is normally used. Table 17 and Figure
39 give tabular and graphical representations for
adsorption capacities of gasoline components. If
the spill contains MTBE, EDC, or EDB, which are
sometimes found as additives to leaded gasoline,
the carbon usage rate may be even higher.The
concentration of the contaminants in the influent
stream has a direct effect on the carbon usage rate,
as does the background level of organic carbon.
Naturally occurring organic carbon includes com-
pounds such as humic substances. The carbon
usage rate increases dramatically with increasing
levels of contamination in the influent. O'Brien and
Fisher (1983) discuss 31 cases of contamination by
various compounds where the carbon usage rate
ranged from 0.1 to 13.3 lb/1,000 gal. The high fig-
ures are associated with very high influent concen-
trations. A typical leaking UST contamination site
has influent concentrations in the range of 100 to
20,000 ppb of TOC.
This wide range accounts for both the size of the
spill and the amount of dilution the gasoline has
undergone in the aquifer. Amy et al. (1987) showed
that high TOC levels may significantly increase the
carbon usage rate. After cleanup has progressed
for 6 to 12 months, the influent concentration often
drops by an order of magnitude (Bourdeau, 1987)
because the source has been removed and the
treatment has removed much of the contaminant.
The discharge requirements for the effluent water
also influence the cost. Water for potable use will
have more stringent treatment requirements than
water for groundwater recharge or surface dis-
charge. These requirements may necessitate mul-
tistage treatment or a longer contact time, both of
which tend to increase overall costs. Multistage
operation actually decreases the carbon usage
rate, though, because the complete adsorption
capacity of the carbon is utilized.
Because so many factors influence the cost of
GAG, generalizations are difficult. As mentioned
previously, costs differ widely depending on influent
concentration, and reported figures are often
segregated based on this variable. O'Brien and
Fisher (1983) report treatment costs on a per-vol-
ume basis. For influent concentrations above 1 mg/
L, costs ranged from $0.45/1,000 gal to $2.52/1,000
gal. Costs for lower concentrations (< 1 mg/L)
were between $0.22 and $0.54/1,000 gal.
Total capital costs may vary from $100,000 to
$800,000 but normally fall in about the $350,000
range. O&M costs range from $25,000 to $250,000
annually; figures of $25,000 to $40,000 are typical.
Figures 40 to 43 may be used to find capital costs
based on contactor size. Table 16 gives GAG costs
relative to air stripping costs.
It should be noted that the relative cost factors in
Table 16 are general rules of thumb, and may not
be accurate in specific instances. For example, it
may be less expensive to use air stripping and
vapor-phase GAG than liquid-phase GAG where
volatile concentrations exceed 100 ppb, because
carbon usage is less in the vapor-phase than it is in
the liquid-phase. If volatile concentrations are less
than 100 ppb, it may be less costly to use the liquid-
phase GAC than the air stripper in combination with
the vapor-phase GAC. Capital costs for one air
stripper and vapor-phase GAC contactors will be
greater than for liquid-phase GAC contactors only.
O&M costs for the liquid phase GAC, however, will
be greater. Initial contaminant concentrations and
length of the cleanup time ultimately determine
which alternative is most cost-effective.
Reliability
Activated carbon has been a proven means for
removing dissolved organic compounds for over 15
84
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years. During this time, it has been used to treat
industrial wastewater, public water supplies, and as
one of the main corrective action technologies for
contaminated groundwater, including gasoline
spills. Although GAG is an excellent technique for
most organic chemicals found in gasoline, espe-
cially those with low solubilities, it is generally not
suitable for highly soluble, highly polar, low-molecu-
lar-weight compounds. These compounds either
do not adsorb significantly, or they break through
very early. Methanol, methylene chloride, and
acetone are examples of compounds that are not
readily removed.
Desorption is a phenomenon that could render
GAC unreliable for certain treatment situations.
This phenomenon was discussed in Removal Effi-
ciency, p. 70. Regarding desorption, pilot plant
studies should be made on a case-by-case basis.
Ease of Operation
The use of GAC requires a different type of
monitoring than air stripping for effective operation.
Because effluent quality decreases as time passes,
the product water must be monitored regularly to
ascertain when breakthrough occurs (unlike air
stripping, which does not require constant monitor-
ing). At breakthrough, carbon replacement is nec-
essary. As breakthrough becomes imminent, the
system will require higher levels of attention. The
replacement of carbon in the system requires an
engineer and/or company technician to supervise
the operation. Depending on the facility, replacing
of the carbon can take from 1 to 12 hours; pressure
tanks require significantly shorter periods of time
than gravity filters. It is advisable to have an
engineer make regular inspections to make sure
the facility is operating properly.
Limitations
The potential use of GAC to remove all dissolved
gasoline constituents from groundwater may be
limited by several factors. These factors include the
adsorbability of the various components of
gasoline, high iron and manganese content of the
water, and disposal of the exhausted carbon.
Not every compound found in gasoline is amenable
to adsorption by GAC. The compounds MTBE and
disopropyl ether (DIPE) are sometimes found as
additives to gasoline. Although they can be
removed by GAC, both have very high carbon
usage rates (Garrett et al., 1986; McKinnon and
Dyksen, 1984). Thus, the cost of removing these
compounds by GAC is prohibitive, especially if the
influent concentrations are substantial. One com-
munity is reported to have found MTBE and DIPE
in the drinking water at levels of 23 ppb and 14 ppb,
respectively. GAC was used to remove the com-
pounds and 40,000 Ib of carbon had to be replaced
every 4 weeks at a cost of $32,000 per replace-
ment (McKinnon and Dyksen, 1984). Other com-
pounds normally found in gasoline, such as ben-
zene, toluene, xylene, ethylbenzene, EDB, and
EDC, are all removable by GAC, with varying car-
bon usage rates (all less than MTBE).
Therefore, the presence or absence of highly solu-
ble compounds such as MTBE and DIPE, or other
additives, may determine the appropriateness of
GAC for a particular gasoline spill. None of these
compounds are believed to pose as significant a
health concern as BTX in general, or benzene in
particular. In addition, these additives are not found
in all gasolines, unlike the BTX compounds, which
are contained in over 99 percent of all gasolines.
MTBE, for example, is found in only 10 percent of
the gasoline being manufactured today (Garrett et
al., 1986).
Iron and manganese levels in the influent water
may also limit the use of GAC at a particular site. If
these elements are present at levels above 5 mg/L,
they must be removed prior to GAC treatment. If
the iron and manganese are not removed, they will
precipitate onto the carbon during treatment. If this
happens, head losses will increase rapidly, the
removal of organics will be hindered, and the car-
bon filter may eventually clog, making it ineffective.
At sites where iron and manganese are present at
high levels, treatment to remove these elements to
acceptable levels must precede use of the GAC
unit. This could increase costs substantially or
could be impractical due to space constraints.
A major potential limitation of GAC use is the dis-
posal of the spent carbon. Usually, it is either land-
filled or regenerated. Regeneration is generally
accomplished by heating the carbon to very high
temperatures in a kiln to desorb the attached
organics and then incinerating the contaminants to
destroy them. Regeneration can take place on-site
or off-site, but on-site regeneration is economically
feasible only for the very largest projects. In gen-
eral, UST sites would not use this option.
Off-site regeneration facilities have many limita-
tions. After GAC is used to remove contaminants
either from the water or in the vapor-phase, it is
laden with compounds and could be hazardous.
For example, some spent carbon vessels may self-
ignite; any carbon with a flash point below 200°F is
considered hazardous and may not be shipped
over U.S. highways nor accepted by a regeneration
facility. Likewise, most facilities will not accept car-
bon that has been used to remove dioxin or
polychlorinated biphenyls (PCBs) because of pos-
sible harmful air emissions. In addition, regenera-
tion facilities have air effluent limitations and may
not accept all carbon for regeneration. Under
85
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RCRA rules, many contaminant-laden carbons are
considered hazardous materials necessitating dis-
posal in a permitted landfill. For example, this is the
case for carbon used to remove tetraethyl lead
(TEL), an additive in leaded gasoline. Because TEL
precipitates onto the carbon, carbon contactor and
piping, the carbon and all equipment must be land-
filled as a hazardous waste. Another limitation of
regeneration facilities is that quantities of carbon of
less than 20,000 Ib are normally not accepted.
Summary
GAC is an excellent technique to remove organic
compounds dissolved in water. Normally, gasoline
constituents, particularly benzene, toluene, and
xylene, can be removed by GAC to nondetectable
limits. But this method is often more costly than air
stripping and may not always be the most cost-
effective and appropriate method to clean up a
gasoline spill. GAC use is limited by site-specific
conditions (such as high iron and manganese
levels) or by the disposal of the spent carbon.
Using Air Stripping and
Granular Activated Carbon
in Combination
Background
Discussion in the previous sections has estab-
lished that air stripping and activated carbon
absorption are cost-effective techniques for remov-
ing organic chemicals. In most situations involving
gasoline-contaminated groundwater, either air
stripping or GAC is the technique of choice. How-
ever, in some situations, the combined use of air
stripping and GAC is the best alternative.
The decision to use the methods in combination
would normally be based on effluent quality and
financial considerations. In all cases, using both
should produce an effluent of as good or better
quality than either method alone. A phased
approach is typically best suited for leaking USTs.
The first phase consists of installing a packed air
tower. Its performance can then be monitored to
determine effluent concentrations and the need for
additional treatment with GAC.
Removal Efficiencies
Where effluent quality is required to be very high,
such as potable water situations, the combination
of air stripping and GAC is perhaps the best
technique to reduce effluent to nondetectable con-
taminant levels. In these cases, air stripping is used
first to remove a large percentage of the VOCs, fol-
lowed by GAC to remove residual organic contami-
nants and any nonvolatile compounds to non-
detectable levels. As seen in Figure 44, the use of
air stripping as a pretreatment effectively puts an
upper bound on the effluent concentrations of
VOCs (even at breakthrough) that is considerably
lower than if air stripping were not used. Also, as
the figure illustrates, GAC life is greatly extended. A
properly designed air stripping tower can remove
more than 95 percent of the volatile compounds
from the influent. More importantly, many of the
compounds that are easily removed by stripping,
such as benzene, methylene chloride, and
dimethylamine, are those with the lowest carbon
adsorption capacities. By removing these com-
pounds, the GAC will perform better and last
longer, and effluent quality will be improved.
INFLUENT CONCENTRATION
ACTIVATED CARBON
- BREAKTHROUGH -
NO AIR STRIPPER
AIR STRIPPER EFFLUENT
ACTIVATED CARBON
BREAKTHROUGH -
WITH AIR STRIPPER
TIME
VOLUME TREATED
Figure 44. Effect of air stripping as a pretreatment of GAC.
Mclntyre et al. (1986), MacLeod and Allan (1983),
and Camp Dresser & McKee (1986) have all
reported the use of groundwater treatment systems
which used air stripping as a pretreatment to GAC
treatment. The influent to all the systems studied
contained numerous compounds at varying levels,
and effluent concentrations were below the detec-
tion limits in all cases for all the contaminants.
Cost-Effectiveness
Often, the cost of combining GAC and air stripping
is the variable that controls the decision. Because
this combination will nearly always yield higher
quality effluent than either treatment method alone,
it is safe to presume that the combination would be
used for any situation where a more cost-effective
cleanup would result. Capital and O&M costs on a
present-worth basis should be determined for situ-
ations in which contaminant removal by a combina-
tion of air stripping and GAC might be suitable.
86
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Admittedly, capital costs will increase if both air
stripping and GAC are used. The cost advantage of
using both methods results from the decreased
O&M costs, specifically the lower costs for carbon
usage. Adding a stripping tower for use prior to a
GAC contactor is justified economically only if the
savings on carbon replacement or regeneration
equal or exceed the additional capital and O&M
costs of the stripper. Stated simply, if the total over-
all (capital and O&M) cost of treatment is lowered
by adding a stripping tower, then that step should
always be taken.
Conceptually, this is obvious. For an actual situa-
tion, determinations must be made of the decrease
in carbon usage due to the air stripper and the
associated cost savings. The decrease in carbon
usage can be estimated based on the percent
removal capability of the air stripping tower. A pilot
test, either laboratory or field scale, using water
from the site, should normally be performed.
An example of how a combination of the two
technologies saves money is given by O'Brien and
Stenzel (1984), using TCE. Air stripping was
assumed to have removed 80 percent of the
influent concentration from (1,000 ppb to 200 ppb).
This lowered the adsorption capacity of the carbon
from 57 mg/g to 27 mg/g, so the carbon usage rate
fell from 0.146 lb/1,000 gal to 0.062 lb/1,000 gal.
Thus, an 80 percent decrease in TCE concentra-
tion via air stripping resulted in a 575 percent
reduction in carbon use. Because of chemical
similarities between TCE and BTX, comparable
reductions could be expected for BTX.
Summary
Using air stripping in combination with GAC is an
excellent way to remove high levels of many com-
pounds. The use of air stripping and GAC may be
justified in some cases as the least costly alterna-
tive. Such cases are likely to be those in which the
carbon replacement costs are a significant portion
of the overall cost of operation. Usually, these
cases will have influent concentrations that are very
high and/or increasing, or a contaminant in the
influent that breaks through very early.
Biorestoration
Indigenous microorganisms that have been selec-
tively adapted or genetically altered can be used to
degrade gasoline components dissolved in ground-
water. This use of microbes to renovate contami-
nated aquifers is termed biorestoration. Although
not yet as well known or as widely used as air strip-
ping or carbon adsorption, biorestoration shows
promise. Unlike air stripping and GAC, which are
separation techniques, biorestoration is a destruc-
tion technique. The end products of aerobic micro-
bial degradation are carbon dioxide and water.
Also, where applicable, biorestoration is often the
cheapest alternative available. Disadvantages of
biorestoration include that it cannot be used where
a quick startup is needed (biorestoration typically
takes 4 to 6 weeks for acclimation), and that it is not
successful in a start/stop mode; that is, it must be
continued 24 hours per day, 7 days a week.
Biorestoration can be accomplished in situ by
either natural or induced methods. Natural in situ
biorestoration occurs in aquifers as the microbial
populations become acclimated to the pollutant
and degrade the contaminants into simpler com-
pounds and ultimately carbon dioxide and water.
Induced biorestoration makes use of systems to
modify the groundwater regime to optimize degra-
dation rates. Modification of the groundwater en-
vironment may be accomplished by various
withdrawal, injection, and recirculation pumping
systems that mix the contaminant with the ground-
water and its microbial population; introduction of
elements required for microbial growth including
oxygen, nitrogen, and phosphorus, as well as
growth substrates; or modification of the chemical
characteristics of the groundwater to maximize
rates of microbial degradation. Biorestoration can
also be accomplished by bioreactors constructed
specifically to promote microbial growth in a vessel
through which groundwater is pumped.
Regardless of which type of mechanical system is
used for biorestoration, the fundamental processes
are essentially identical. This section provides a
brief overview of the dynamics associated with
microbial degradation of gasoline components and
a review of the effectiveness, limitations, and costs
associated with available biorestoration systems.
Microbial Processes
Microbial degradation of gasoline components can
occur by aerobic respiration, anaerobic respiration,
or fermentation. Aerobic microorganisms use oxy-
gen in the process of decomposing hydrocarbons;
anaerobes use inorganic compounds such as sul-
fate, nitrate, or carbon dioxide as terminal electron
acceptors; and under fermenting conditions, mi-
crobes use organic compounds for both the elec-
tron donor and acceptor.
Major gasoline components such as the aromatics
and alkanes as well as some minor constituents
such as EDB and EDC, have been shown to be
more readily degradable under aerobic than either
anaerobic or fermenting conditions. Also, the by-
products of anaerobic decomposition, such as
methane and sulfide, and of fermentation reactions,
such as organic acids and alcohols, may pose
greater system management problems than those
associated with the aerobic decomposition prod-
ucts carbon dioxide and water.
Although complete degradation of hydrocarbons
will yield carbon dioxide and water, under certain
environmental conditions complete degradation
87
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Case Studies
Two examples of actual cases where air stripping and GAC were used in combination are given below.
They illustrate typical situations, where air stripping was added after carbon regeneration costs became
prohibitive.
Rockaway Township, New Jersey
In 1979, trichloroethylene (TCE) was detected in the
municipal wells of Rockaway Township, a small
town in north-central New Jersey (McKinnon and
Dyksen, 1984). Subsequently, disopropyl ether
(DIPE) and methyl-tertiary butyl ether (MTBE) were
also found in the water. The source of all three
compounds was thought to be a leaking under-
ground storage tank containing gasoline. Concen-
trations of TCE, DIPE, and MTBE in the water were
200 to 300 |xg/L, 70 to 100 jj-g/L, and 25 to 40 |xg/L,
respectively.A decision was made to use GAC for
water treatment. Based on initial estimates, the car-
bon supply (two 20,000-lb contactors) was
expected to last 6 to 8 months before replacement.
However, after just 3 months of operation at a flow
rate of 2 million gallons per day, DIPE and MTBE
had broken through and were measured in the
effluent at levels of 14 (xg/L and 23 (xg/L, respec-
tively.
The carbon was replaced at a cost of $32,000.
Thereafter, carbon was replaced every 2 months.
By the end of 1981, the carbon was being replaced
every 4 to 6 weeks (the annual O&M cost had risen
to about $200,000). Therefore, the decision was
made to add an air stripper prior to the GAC, at a
capital cost of $375,000.
The stripper was sized to remove 99.9 percent of
the influent DIPE concentrations, the least volatile
compound (COM, 1986). After the stripper became
operational, the effluent was below detectable
limits for all three chemicals. The GAC contactors
were subsequently taken off-line, because they
were thought to be unnecessary. However, resi-
dents began to complain of a scaling problem in
their hot water heaters (the water in Rockaway
Township is very hard). It was thought that this
problem was caused by the change in the water
chemistry during stripping. The GAC contactors
were put back on-line, and the scaling problem was
solved. The carbon has not had to be replaced
since installation of the airstripper.
Acton, Massachusetts
In December of 1978, two of the municipal wells in
Acton, Massachusetts, were taken out of service
because of the presence of several organic chemi-
cals, including trichloroethylene, benzene, and
methylene chloride (MacLeod and Allen, 1983).
GAC was chosen to reduce the contamination from
an average influent concentration of 42 (jig/L to less
than 5 jxg/L total, and less than 1 ^.g/L for any
single compound (Nyer, 1984). The high cost of car-
bon replacement soon became prohibitive. Every 5
months, a complete replacement of 40,000 Ib of
GAC was required, at a cost of $37,000. In addition,
the influent concentrations were expected to rise,
causing even more frequent carbon changes.
Therefore, the decision was made to use air strip-
ping as a pretreatment.
The column was sized to handle 1 million gallons
per day at 95 percent removal of the VOCs. In prac-
tice, the removal ranged between 96 and 99 per-
cent (to less than 1 p,g/L each), due to the safety
factor designed into the system. The stripper cost
$31,000; and the building, electrical equipment, and
miscellaneous equipment cost $109,000. Over the
life of the project, this cost will be more than recov-
ered by the decrease in carbon usage.
88
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may not result. In such instances, intermediary
degradation products may accumulate that could
either be resistant to further degradation or inhibit
further growth (Horvath, 1972). Growth factors
which affect the rate of microbial degradation
include oxygen requirements, temperature, nutrient
availability, and characteristics of the contaminant.
Oxygen Requirements
Aerobic degradation is the most attractive of the
microbial processes for breaking down gasoline
components in groundwaters because it proceeds
at a more rapid rate and does not produce the nox-
ious by-products associated with anaerobic
decomposition. In order for aerobic degradation to
occur, however, significant quantities of oxygen
must be available to the microbes. Barker et al.
(1986) calculated that 23.2 mg/L of oxygen are
required for degradation of 1 mg/L of benzene,
toluene, and xylene in groundwater, and Wilson et
al. (1986) noted that in a well oxygenated ground-
water containing 4 mg/L of molecular oxygen,
microbes can degrade only 2 mg/L of benzene. The
solubility of benzene in water (1,780 mg/L) is there-
fore much greater than the capacity of microbes to
degrade the compound under natural conditions.
Because microbes will consume oxygen as the
hydrocarbon is degraded, an aerobic groundwater
can quickly became anaerobic. This onset of
anaerobic conditions is the most significant factor
limiting the rate of biodegradation in the groundwa-
ter environment, according to Raymond (1987).
Because of the importance of available oxygen in
microbial degradation, this factor would be most
closely controlled when operating an in situ biode-
gradation cleanup. Three means of increasing the
dissolved oxygen content of the groundwater are
the injection of air, liquid oxygen, and hydrogen
peroxide. According to Raymond (1987), the sat-
uration concentration of oxygen in water from air
injection is about 10 mg/L. Hydrogen peroxide
injection can provide between 250 to 400 mg/L of
dissolved oxygen. The very high amount of oxygen
supplied by hydrogen peroxide makes it an excel-
lent choice to maintain the aerobic condition of a
groundwater system.
Temperature
Optimal growth of microbial populations respon-
sible for biodegradation of petroleum products
occurs between 20°C and 35°C. Microbial degra-
dation rates would be expected to moderate with
changes in the groundwater temperatures.
Decreasing rates would be expected during winter
months in northern portions of the country. Experi-
ence has shown, however, that biodegradation can
occur at any groundwater temperature once the
microbes become acclimated.
Nutrient Availability
Macronutrients, such as nitrogen and phosphorus,
must be available for microorganisms in order for
biological processes to take place. The quantities
of nutrients required for degradation are generally
expressed as a ratio of the nutrients to the carbon
source. For petroleum products the ideal carbon-
nitrogen-phosphorus ratio is 160:1:0.08 (Bartha and
Bossert, 1984).
Micronutrients, such as magnesium and sulfur, are
also required for optimal growth, although in very
small quantities. The micronutrients would not be
expected to limit growth of microbes in aquifer sys-
tems as often as oxygen deficiency. The specific
nutrient requirements needed to optimize microbial
degradation of gasoline components is a site-spe-
cific factor that must be determined experimentally
for each groundwater contaminant problem, con-
sidering the relationship of the particular substrate
characteristics to the microbial populations.
Characteristics of the Contaminant
The behavior of a mixed microbial population in
reaction to the introduction of hydrocarbons will
vary depending on the constituents and concentra-
tions of the contaminant.
It has been demonstrated that bacteria, yeasts,
fungi, or algae have the capacity to grow on
straight-chain and branched alkanes (Sunger and
Finnerty, 1984), cyclic alkanes (Perry, 1984), and
aromatic hydrocarbons (Ceringha, 1984). Tabak et
al. (1981) undertook investigations to determine the
biodegradability of various organic contaminants
utilizing static culture enrichment techniques and
wastewater microbiota. Table 18 summarizes the
results of these 7-day screening tests at contami-
nant concentration levels of 5 and 10 mg/L for
selected compounds.
Table 18
Microbial Degradation
Screening Test Results'
Performance2
Compound
5mg/1
10 mg/1
Benzene
Ethylbenzene
Toluene
Phenol
Naphthalene
1, 2-dichloroethane
D
D
D
D
D
B
D
A
D
D
D
B
1 Source. Tabek et al., 1981.
2 Performance at noted concentrations.
A = Significant degradation with gradual adaption
B = Slow to moderate biodegradation concomitant with significant
rate of volatilization.
D = Significant degradation/rapid adaption
89
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EPA is currently undertaking a comprehensive
research program aimed at determining the land
treatability of hazardous wastes (Matthews, 1987).
The biodegradation of benzene, toluene, xylene,
phenol, tetraethyl lead, EDB, and EDC is being
investigated. Interim reports on this work (available
in early 1988) should provide significant data for
evaluating the fate of gasoline components in soil
and ground water.
Component Concentration
Alexander (1985) reports that the rates of minerali-
zation of some organic compounds are directly
proportional to their concentration, and that there is
a threshold level below which certain compounds
which usually are subject to biodegradation are not
converted to carbon dioxide and water. At the
higher concentrations of hydrocarbons in ground-
water, such as that directly beneath a floating oil
slick, microbial toxicity may occur (Cooney, 1984).
As the concentration of contaminants decreases
and microbial populations become adapted to the
compounds, the microbes may be able to over-
come the effects of toxicity and degrade the com-
pounds. The resilience of microbial populations to
service and repopulate areas in which toxic levels
of contaminants were initially present has been
demonstrated repeatedly. Therefore, it is antici-
pated that biorestoration techniques could be
adapted to deal with initially high contaminant
levels. The more difficult question to address is the
level to which a contaminated aquifer can be
cleaned up with these techniques.
Litchfield and Clark (1973) found that microbial
populations encountered where hydrocarbon con-
tamination exceeded 10 ppm were greater than
1Q6/ml_ groundwater, whereas populations of less
than 103/ml_ were encountered where contaminant
levels were less than 10 ppm. As Cooney (1984)
reports, the rate of microbial degradation may be
slow or nil below a certain threshold level. At low
substrate concentrations, other mechanisms may
be more significant in reducing contaminant levels.
Schmidt and Alexander (1985) report on the effects
of low levels of organic carbon on the degradation
of low concentrations of substrate. They
demonstrated that the rate and extent of biodegra-
dation of low concentrations of synthetic organic
compounds may be controlled by the presence of
other organic molecules in the system. Pure cul-
tures of bacteria were demonstrated to utilize low
levels of aromatics simultaneously in the presence
of other organic compounds. Theoretically, the
addition of organic substrates in biorestoration sys-
tems could further enhance biodegradation of
hydrocarbons in groundwater at low levels.
Degradation of compounds may also occur in situa-
tions wherein the microbe effecting degradation
does not derive any nutrients or energy from the
process. This process, co-metabolism, is defined
by Horvath (1972) as any oxidation of substances
that takes place without the energy derived to sup-
port microbial growth. Co-metabolism may be a sig-
nificant process for degradation of low levels of
hydrocarbon contaminants in groundwater. This
process should be evaluated on a site-specific
basis when biorestoration techniques are being
considered since gasoline components such as
xylene and ethylbenzene have been demonstrated
to be subject to co-metabolism (Horvath, 1972).
Bouwer and McCarty (1984) showed that trace
levels of the aromatic hydrocarbons ethylbenzene,
styrene, and naphthalene could be reduced sig-
nificantly in the presence of a primary substrate.
Specifically, with acetate as a primary substrate at
concentrations of 1.0 mg/L, ethylbenzene levels
were reduced from 9.1 (jug/L to 0.1 (xg/L. Ethylben-
zene present alone at these low concentrations
could not trigger biodegradation, whereas the pres-
ence of acetate as a primary substrate was effec-
tive in stimulating it.
Effectiveness
The effectiveness of biorestoration in laboratory
studies has been demonstrated to be significant.
However, actual field applications of biorestoration
techniques underscore their site specificity and
variable nature.
Amdurer et al. (1986) summarize case histories of
in situ treatment techniques applied to subsurface
contaminants. Among the case histories sum-
marized is the Biocraft site in Waldich, New Jersey.
At this facility an estimated 30,000 gal of methylene
chloride, acetone, n-butyl alcohol, and dimethyl-
aniline leaked from USTs to subsurface soils and
groundwater. The biorestoration system con-
structed for use at this site was comprised of a
downgradient dewatering trench and well, two
mobile biological activating tanks, two mobile set-
tling tanks, and two upgradient reinjection trenches
(Figure 45). The system was used to treat 14,000 to
20,000 gal of groundwater per day. The median
reduction of contaminant mass ranged from 88 to
98 percent, except for dimethylaniline for which
only 64 percent of the mass was reduced. Each
pass through the system had a retention time of 12
hours. Operation began in 1981, and as of 1985, 95
percent of the contaminants in the groundwater
had been removed.
Yaniga (1982) reports on the biorestoration of a
groundwater aquifer in Montgomery County,
90
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MANHOLE
EXCAVATED
SOIL
COVER
COLLECTION
TRENCH
9 EQUALLY
SPACED AERATION
WELLS
RECHARGE
TRENCH
Figure 45. Flow diagram of biocraft biorestoration.
Pennsylvania, where gasoline had leaked from an
UST at a service station. The contaminant plume
initially contained up to 15 ppm of dissolved hydro-
carbons. The biorestoration system for this site
consisted of a central pumping well and a reinjec-
tion gallery situated over the pit where the tank had
been. The extracted groundwater was passed
through an air stripping tower to remove VOCs then
oxygenated and enriched with nutrients before
being reinjected. After 20 months of system opera-
tion the hydrocarbon concentration in the ground-
water aquifer was reduced to 2.5 ppm.
Yaniga and Smith (1985 and 1986) report additional
information on the above site following 3 years of
the abatement program. They indicate that from 30
to 3,500 gal of oxygenated and nutrient-rich water
per day was injected into the infiltration gallery, and
after the first 11 months of operation, the organic
content of the groundwater was reduced by 50 to
85 percent. The authors report that use of hydro-
gen peroxide is superior to use of air in oxygenating
the groundwaters. Although others have reported
that hydrogen peroxide can cause microbial toxic-
ity, Yaniga and Smith (1986) indicate that at a 100-
ppm concentration it did not kill, but rather
enhanced, microbial growth and yielded a concen-
tration of 50 ppm of oxygen in the groundwater.
Barker et al. (1987) undertook investigations to
evaluate the fate of benzene, toluene and xylene
(BTX) in a shallow sand aquifer by intensively
monitoring the aquifer following the injection of
1,800 L of water containing about 76 mg/L of BTX.
The attenuation of these components was
evaluated. The field studies were compared to
microcosm studies in which conditions approximat-
ing that encountered in the field were maintained.
The study conclusions support the general findings
that oxygen is rate-limiting with regard to microbial
degradation of petroleum products in the ground-
water regime. BTX components were found to per-
sist in aquifer layers where dissolved oxygen levels
were near zero. In a little over 1 year of injection of
the BTX, complete natural removal was observed
in the field experiment, with benzene exhibiting the
greatest resistance to breakdown.Chan and Ford
(1986) report on the use of both in situ and bioreac-
tor systems to remediate a 22,000-ft2 site contami-
nated with No. 2 diesel fuel. The estimated removal
efficiency of the combined system following 100
days of operation was 80 percent. The bioreactor
was 16 times more efficient than the in situ process
in removing contaminants from the groundwater.
The increased efficiency was attributed to the
higher oxygen concentrations which could be main-
tained in the bioreactor (9 mg/L) as compared to
the in situ levels (2.5 mg/L).
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Dorr Oliver Incorporated has developed two
bioreactor systems, OXITRONR and MARS™, that
potentially could be utilized for the biorestoration of
groundwater contaminated with gasoline. The
OXITRONR system is an aerobic or anoxic fluidized
bed system that uses either sand, activated car-
bon, or other media on which biomass buildup
occurs (Figure 46). Contaminated groundwater is
passed through the system from the bottom of the
reactor vessel at sufficient velocity to fluidize the
bed. Because of the increased space between
fluidized particles, as compared to suspended
growth media systems, 5 to 10 times greater
biomass concentration is reportedly achieved. The
increased biomass concentration reduces the
hydraulic retention time required for treatment.
MARS™ (stands for membrane aerobic or
anaerobic reactor system) uses a well-mixed sus-
pended growth reactor and ultrafiltration system
(Figure 47) for biorestoration of groundwater (Sut-
ton, 1986).
EFFLUENT
WASTEWATER
MEDIA
Figure 46. Schematic of Oxitron® process.
NUTRIENT AND/
OR CHEMICAL ADDITION
WASTEWATER
-o
SCREENING
AIR OR O0
EXCESS BIOMASS
SUSPENDED GROWTH
REACTOR
Figure 47. Schematic of Mars™ process.
92
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Site-specific evaluations of the degradability of the
groundwater constituents in either system are
needed, as with all bioreactor or in situ techniques,
but Dorr Oliver has used these systems success-
fully for treating various wastes similar to those
found in gasoline-contaminated groundwaters,
including benzene and toluene.
Although indigenous microbial populations have
the capability of degrading hydrocarbons, interest
in using mutant, genetically engineered or labo-
ratory-adapted organisms has increased in recent
years.
Polybac Corporation, located in Allentown,
Pennsylvania, markets Polybac and Hydrobac, for-
mulations described as mutant, adapted microbes
and biochemical accelerators. Polybac is made
specifically for municipal and food processing
wastewater treatment systems, and Hydrobac is for
petroleum refinery/petrochemical plant wastewat-
ers. Other products available from Polybac Corpo-
ration, include Petrobac for degrading hydrocar-
bons in salt water and Phenobac for degrading
hydrocarbons in fresh water. Polybac Corporation
also markets bioreactor systems and services
(CTX-BIOX System) with reported removal efficien-
cies "in excess of 99 + % for most organic com-
pounds" (Polybac, 1980; 1981).
Solmar Corporation of Orange, California, markets
microbial cultures for use principally in wastewater
collection and treatment systems. One product is
Advanced BioCulture Formulation L-104, which is
used for "heavy, tarry types of oils, coal tars, and
organic sludges." It is reportedly well suited for
aromatic and phenolic wastes (Solmar, 1984).
Other firms providing groundwater biorestoration
services and systems include:
• Groundwater Technology
Chadds Ford, Pennsylvania
• Groundwater Decontamination Systems, Inc.
Paramus, New Jersey
• NEPCCO
Foxboro, Massachusetts
• Detox
Dayton, Ohio
• Emtek
Bedford, New Hampshire
• TerraVac, Inc.
San Juan, Puerto Rico
Limitations
Limitations on the use of biorestoration techniques
relate to sociopolitical issues as well as technical
factors. Because in situ treatment is accomplished
largely underground, little if any evidence of activity
may be discernible by the general public. This lack
of observable activity could lead to a public percep-
tion of no-action. In general, the public may be
more prone to respond positively to corrective
actions that manifest significant levels of activity,
such as air stripping. Therefore, efforts to educate
the public about the mode and effectiveness of in
situ degradation may be required where high levels
of public awareness exist.
The technical limitations of biorestoration tech-
niques may be related more closely to site-specific
characteristics than to the overall theoretical ability
of microorganisms to degrade gasoline compo-
nents in groundwater. As discussed, microbial
growth factors appear to be readily modified either
in situ or by bioreactors to allow for degradation of
gasoline in groundwater systems. Degradation of
hydrocarbons to ppm levels should be relatively
attainable; however, reaching ppb levels may
require that the system be manipulated to encour-
age co-metabolism or degradation in the presence
of an added primary substrate. These limitations
can be assessed only by performance of site-
specific laboratory and pilot-scale evaluations. It is
possible to meet the proposed maximum contami-
nant level of 5 jjig/L for benzene and 0.44 mg/L for
xylene with a system that is appropriately designed
and optimally operated.
The fact that biorestoration can work in saline as
well as fresh systems makes it adaptable to situa-
tions where other treatment systems may not be
suitable.
Biorestoration may pose major limitations in terms
of the ability of the recovery system to capture con-
taminated groundwaters and the restrictions on
reinjection of treated or nutrient enriched waters
into the aquifer system. Capturing hydrocarbon-
contaminated groundwater is an engineering issue
that should be readily addressable based upon
site-specific hydrologic considerations, whereas
reinjection involves institutional issues that may be
more difficult to overcome.
Reinjection of treated waters, which in some cases
may be nutrient enriched, potentially could be seen
as an environmental threat in and of itself. In some
situations, there could be a problem obtaining per-
mits necessary for reinjection. Manipulating
groundwater characteristics to optimize microbial
degradation may require the introduction of various
growth substrates and nutrients such as nitrogen.
Although these materials should be applied at the
rates required for microbial metabolism, excess
loadings might be needed to ensure adequate dis-
tribution in the aquifer.
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Because complete hydraulic isolation may not be
possible in most situations where biorestoration is
employed, the added materials could move beyond
the zone of contamination. These potential impacts
must be assessed in relation to the potential bene-
fits derived from biorestoration. In undertaking any
biorestoration system, therefore, consideration
must be given to the tradeoffs between injecting
materials into the groundwater and remediating the
gasoline components to acceptable levels.
Costs
Costs for biorestoration are dependent on specific
factors such as hydrogeology and groundwater
chemistry, groundwater quantity and quality, the
quantity of contaminants, and the required level of
cleanup. For the first case history discussed in this
section, the total capital cost for remediation was
$926,000 (Amdurer et al., 1986). Approximately
$446,000 was expended during the feasibility
evaluation, which was completed within 2 1/2 years.
The O&M for treating groundwater once the system
was operational were $0.0165/gal (about $225.50/d
for 13,680 gal/d).
Other representative cost estimates for biorestora-
tion projects are listed below:
• Ehrenfeld and Bass (1984) estimate the 6-mo
cost for in situ biodegradation of wastes on a 1-
acre site to be $1,200.
• A. M. Kirby of Suntech, Inc. estimates costs of
approximately $50,000 for a 5-acre site with a 6-
mo clean-up period.
• Richard L. Raymond, Jr., of Biosystems estimates
a cost of $4 to $6/lb of contaminant removed
(compared to $15 to $20/lb for air stripping and
$40/lb for carbon adsorption).
• Dr. Ralph Portier of Louisiana State University
estimates a cost of $30 to $50/yd3 of contami-
nated soil (compared to $125 to $130/yd3 for haul-
ing and disposal of the soil).
Summary
Although biorestoration techniques provide
methods whereby groundwaters contaminated with
gasoline components can be effectively
remediated, the technology has not been widely
applied. Packed air towers and activated carbon
adsorption systems are the preferred technologies
for groundwater treatment. Biorestoration does
show promise, however, as a cost-effective alterna-
tive. Biorestoration accomplished in situ and in
bioreactors has been demonstrated to be effective
in degrading hydrocarbons, although the degree of
cleanup is highly dependent on specific environ-
mental conditions affecting microbial growth. The
time required for biorestoration techniques to effec-
tively mitigate gasoline-contaminated aquifers is
expected to be on the order of years rather than the
shorter times needed for physiochemical
techniques such as air stripping or carbon adsorp-
tion.
Biorestoration techniques should be considered for
gasoline-contaminated aquifers where control of
contaminant migration can be achieved hydrauli-
cally. Because these corrective action techniques
require minimal construction impacts, they are
highly suited for implementation at active facilities.
One of the main advantages of biodegradation over
other techniques is that the contaminants are com-
pletely destroyed, with the end products of aerobic
degradation being carbon dioxide and water. Air
stripping and activated carbon, on the other hand,
are both separation techniques whereby the con-
taminant is simply transferred to a different
medium. Thus, biorestoration avoids problems
such as the vapor-phase treatment (associated
with air stripping) and the disposal or regeneration
of hazardous spent carbon.
The most beneficial aspect of biorestoration
techniques may be that they can be used with other
physiochemical corrective actions. For example, in
situ biorestoration techniques can be coupled with
soil gas venting and groundwater extraction and
treatment techniques to accelerate restoration of
the aquifer. Bioreactors can be used with air strip-
pers or carbon adsorption systems to yield high
levels of treatment.
Because of the relatively low costs associated with
biorestoration techniques, application of these
methods to gasoline contaminated aquifers should
be given serious consideration during project
development and scoping.
References
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Dresser & McKee Inc., Edison, NJ. March 18.
Alexander, M. 1985. Biodegradation of Organic
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Amdurer, M., Fellman, R.T., Roetzer, J. and Russ,
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Section 6
Point-of-Entry Treatment and
Alternate Water Supplies
Gasoline spills and underground storage tank
(UST) leaks can contaminate groundwater that
supplies private wells. High levels of dissolved
gasoline constituents such as benzene, toluene,
xylene, ethylbenzene and other volatile organic
compounds (VOCs) can render well water unfit for
human consumption. Because aquifer restoration
frequently takes months or even years to complete,
users of well water must find alternative drinking
water sources. Two common alternatives include
point-of-entry treatment in the homes where con-
taminated water is found and extension of the exist-
ing water distribution system to the well user. In the
following section, these two options are analyzed
for their cost, feasibility, and limitations.
Point-of-Entry Treatment
Background
The constituents of gasoline most commonly found
at spills and at leaking UST sites include the
aromatics benzene, toluene, and xylene (BTX).
Aromatics can comprise up to 20 percent of the
weight of gasoline and even higher percentages of
the soluble fraction. Of these three constituents,
benzene is of greatest concern; it has been shown
to cause cancer in humans. Also, benzene is highly
soluble, and it can move easily into groundwater.
As a result, a principal design criterion of ground-
water treatment systems is the removal of ben-
zene.
The benefit of point-of-entry treatment devices is
that they can remove dissolved chemicals from
gasoline-contaminated well water at a single point.
There are two types of devices used. The term
point-of-use applies to devices for individual taps or
for drinking water only; point-of-entry devices are
used to treat all water as it enters the home or
building. Small-scale equipment is available that
can treat a single faucet, while larger units installed
adjacent to the water pump can treat enough water
for an entire household. A significant problem
associated with single-tap and drinking-water-only
treatment is noted in research done by NUS corpo-
ration (Symms, 1986). Symms contends that taking
showers in water containing groundwater contami-
nants probably leads to "far greater exposure to the
chemicals than drinking the same water," because
gasoline is a mixture of highly volatile hydrocarbons
that vaporize readily in the presence of oxygen. By
drawing tap water contaminated with gasoline
components, the hydrocarbons are allowed to vap-
orize. Daily exposure to gaseous hydrocarbons for
weeks or months can pose a significant health
threat. For this reason, point-of-use treatment
devices cannot be said to eliminate totally the
health risks associated with gasoline-contaminated
well water. Hence, the following discussion covers
only point-of-entry devices.
Point-of-Entry Devices
Several different processes have been successfully
adapted to point-of-entry devices for treating con-
taminated well water as it enters the home. The
most common treatment processes are reverse
osmosis, ion exchange, distillation, aeration, and
carbon adsorption. Although these processes have
been widely applied, they are not all equally appro-
priate for removing dissolved gasoline constituents.
Reverse Osmosis
Osmosis is the process by which a fluid diffuses
through a semipermeable membrane until there is
an equal concentration of fluid on either side of the
membrane. In reverse osmosis the natural osmotic
gradient is reversed, so that the fluid is forced
through the membrane, producing unequal con-
centrations on either side of the membrane. With
contaminated well water, high pressure pumps (as
high as 200 psi) move the water through the mem-
brane and separate the contaminants. Reverse
osmosis is most effective in removing chemicals
with a molecular weight greater than 250. Ben-
zene, toluene, and xylene have molecular weights
99
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of 78, 92, and 106, respectively, and are therefore
too light to be retarded by the membrane. Another
drawback to using reverse osmosis point-of-entry
devices is that filtering enough water to meet
household needs would require a costly high pres-
sure, commercial system and a large (60-100 gal)
storage tank.
Ion Exchange
Ion exchange is generally associated with water
softening. The process involves replacing cations
such as calcium and magnesium, which cause
hardness, with other cations, usually sodium.
Because organic compounds are not ionic, ion
exchange would not be an effective point-of-entry
treatment device for recovering dissolved gasoline
constituents from well water.
Distillation
Distillation is the process by which water is vap-
orized and then recondensed into liquid. During
distillation, volatile organics also vaporize, but
because they are not easily recondensed, the
organics are effectively separated from the water.
The main drawbacks to using distillation point-of-
entry devices are that vaporization requires much
energy and costs are high. Also, in most cases, not
enough water can be produced to meet household
needs. Since the vaporized organics are generally
vented to the atmosphere immediately outside the
home, they could continue to pose a health hazard.
Aeration
Aeration is the process by which the surface area
of water exposed to oxygen is increased allowing a
large fraction of the organic compounds to vol-
atilize. Aeration is capable of 90 to 95 percent
removal efficiencies and is most effective in reduc-
ing high (> 100 ppb) levels of volatile contaminants
(1 ppb = 1 (xg/L. Since aeration is less effective at
removing low (< 10 ppb) contaminant levels, it is
often used as pretreatment for other removal pro-
cesses. As a point-of-entry treatment, aeration has
not been widely used because it must be done in a
closed vessel vented to the outside. Also, aeration
is a complicated and expensive process requiring
an additional water pump to move water through
the aerator.
Carbon Adsorption Systems
Usefulness of Carbon
Several factors make carbon adsorption the most
widely used point-of-entry treatment. The prime
benefit of using carbon is its ability to adsorb
and concentrate organic compounds several
thousandfold from dilute solutions. For a discussion
of carbon porosity, see Activated Carbon Adsorp-
tion in Section 5. Carbon is also generally less
expensive and easier to work with than any of the
other treatment types. Because of the prevalent
use of carbon adsorption in point-of-entry applica-
tions, there is a well established network of carbon
adsorption equipment manufacturers and service
companies.
Designing Carbon Adsorption Point-of-
Entry Treatment Systems
Isotherms are critical components in the design of
carbon adsorption systems. They are used to
describe the capacity of carbon to adsorb dis-
solved compounds from solution. The calculations
for determining carbon's capacity and Freundlich
isotherm relationships are discussed in detail in
Section 5. Table 19 illustrates a case in which
benzene, toluene, and xytene were found in
groundwater.
Using the concentrations shown and the K and 1/n
values determined experimentally, X/M can be cal-
culated for each chemical. Next, to determine the
carbon requirement (the amount of carbon neces-
sary to adsorb the chemical from a liter of solution),
the initial concentration of the chemical is divided
by X/M. The resulting value, which has the units
g/L, is converted to lb/1,000 gal. The sum of the
three single-solute carbon requirements is the total
carbon requirement. This number can be used to
calculate the lifetime of the carbon in the treatment
unit.
Table 19
Three Cases of Carbon Usage
Chemical
Benzene
Toluene
Xylene
Concentration
(mg/L)
10
6
8
K
4.1
26.1
85
1/n
0.545
0.44
0.19
X/M
(mg/g)
14.38
57.42
126.18
g/i
0.695
0.104
0.063
Carbon
Requirement
(1b/1,000gal)
5.860
.878
.532
TOTAL
727
100
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In the carbon tank life calculations shown below, a
standard 1.5 ft3 carbon column (a size typically
employed in home treatment systems) is used:
Bed Volume: 1.5 ft3
Bed Mass: 34 Ib 23 Ib/ft3
Tank Capacity: (34 Ib/tank) (727 lb/1,000 gal) =
4,677 gal/tank
Assumptions: 2 people 70 gal/day
Service Time: (4,677 gal/tank) / (140 gal/day) =
33 days/tank
Estimated Tank Life: 1 month/tank
The estimated tank life is the amount of time that
the carbon in the tank can remove contaminants
before "breakthrough" occurs (tank life calculations
assume that concentrations and adsorption rates
are static). Once contaminants are found in the
tank effluent, breakthrough is said to have occurred
and it is necessary to replace the carbon in the
tank.1
In most point-of-entry applications, two carbon col-
umns of equal volume are used in series to ensure
that if breakthrough occurs in the first column, con-
taminants will be adsorbed by the second.
Because the estimation method used above is less
accurate when predicting low-level breakthrough,
tank life estimates should be conservative.
Also, as the number of compounds in solution
increases, the design of carbon adsorption sys-
tems becomes more complicated. For these rea-
sons, total carbon requirements are based on the
sum of the single-solute carbon requirements,
rather than on the one solute that has the highest
carbon requirement and can be expected to break
through first. Hall and Mumford (1987), however,
concluded that in most cases the observed equilib-
rium capacity of the carbon normally lies between
the amount predicted using the single-solute with
the greatest carbon requirement and the amount
predicted using a simple summation of single-
solute requirements.
Carbon Adsorption Efficiency
Case studies and experiments have demonstrated
that carbon has the ability to remove a variety of
organic solutes in water to very low levels. In 11
chemical spills and 18 groundwater contamination
case studies cited by Brunotts et al. (1983), acti-
vated carbon was shown to remove most contami-
nants to within 1 ppb (.001 |xg/l_). Carbon has also
been shown to be effective in removing gasoline
constituents from contaminated groundwater. Hall
and Mumford (1987) discussed the use of carbon
adsorption as a point-of-entry treatment in both a
gas station and a private home where gasoline
constituents were found in the well water. In each
case two carbon tanks in series were used, and
more than 99 percent of the dissolved gasoline
constituents was removed.
Limitations
The use of carbon adsorption point-of-entry water
treatment devices is limited by several factors. As
noted earlier, water pH, temperature and hardness,
as well as solute type and concentration, all influ-
ence the adsorption capacity of carbon. Also, cer-
tain types of pathogenic bacteria that frequently
colonize on the carbon in treatment units can be
released to the treated water. To mitigate these
problems and enhance the adsorptive capacity of
the activated carbon, it is necessary to analyze the
quality of the influent water and then either set up a
pilot system to ensure adequate water treatment or
closely monitor the full-scale system for contami-
nant breakthrough or any other undesirable effects.
Most water quality complications (e.g., pH, hard-
ness, iron, bacteria, viruses) can be reduced or
treated so that the carbon adsorption unit can oper-
ate effectively, but the importance of effluent
monitoring cannot be overemphasized. Research-
ers note that effluent monitoring is one of the most
critical criteria in designing and implementing point-
of-entry carbon adsorption systems (Hall, 1987).
Perhaps the most serious limitation associated with
carbon adsorption point-of-entry treatment sys-
tems installed in the home is that significant
changes in the concentrations of contaminants in
the influent water may go undetected by the home-
1 The general order of breakthrough—from earliest to latest—of toxic gasoline compounds is benzene,
toluene, ethylbenzene, xylene, naphthalene, and phenol. It should be noted that gasoline additives ethylene
dibromide (EDB), ethylene dichloride (EDC), MTBE, and DIPE may breakthrough even before benzene, due
to their low carbon adsorption capacities.
101
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owner until periodic (monthly or bimonthly) water
quality analysis is performed. The design of the
carbon adsorption system is based on a set of
known or predicted maximum contaminant concen-
trations. Should these concentrations fluctuate and
exceed the design capacity of the system, or if
water quality complications reduce the effective-
ness of the carbon, earlier breakthrough of contam-
inants could occur. In such a situation, residents
would be unaware that contaminants had moved
through the treatment system. As a precaution,
water quality analyses could be performed more
frequently. However, because each test could cost
as much as $250, the additional cost to the home-
owner (not to mention the added inconvenience
and anxiety) may make other water treatment alter-
natives more appealing. These sentiments will be
especially prevalent in larger communities where
many homes are affected.
Costs of Carbon Adsorption Point-
of-Entry System
Factors that influence the costs of carbon adsorp-
tion point-of-entry treatment systems are:
• Types of contaminants present
• Concentrations of contaminants in the influent
• Water quality complications in the influent
• Flow rates
• Amount of carbon used
• Chemical testing
Tables 20, 21, 22, and 23 show costs typically
associated with point-of-entry treatment systems
installed in the home. These figures are based on
data generated for a 1984 Camp Dresser & McKee
project in Rhode Island; a 1986 project by Culligan,
Inc., also in Rhode Island; and from a 1987 Hall and
Mumford project in Wisconsin.
The total equipment and annual operating costs for
carbon adsorption point-of-entry treatment sys-
tems are most dependent on the types and con-
centrations of organic compounds in the influent
water. Each organic compound has a characteristic
affinity for carbon, which ultimately determines how
long the carbon will last before its adsorption
capacity is exhausted. Some highly soluble organic
compounds found in gasoline, such as MTBE and
DIPE, have very high carbon usage rates. If these
compounds are found in high concentrations in the
influent water, frequent carbon replacement will be
required, thus making carbon adsorption consider-
ably more expensive. Likewise, as the concentra-
tion and number of organic compounds in solution
increases, the life of the carbon decreases, and it
must be replaced more often.
Water quality problems such as low pH; high iron,
calcium, and magnesium content; the presence of
high molecular weight (> 200) organics; and the
presence of bacteria and viruses may all reduce
the adsorptive capacity of carbon. Water treatment
equipment is available which can remove these
complications; however, as shown in Table 24, they
can add considerable costs to the system.
Changes in the flow rate through the system also
have a direct impact on costs. Assuming concen-
trations are static, higher flow rates caused by
increased water use in the home will lead to more
rapid carbon exhaustion and more frequent
replacement. Thus, as flow rates increase, so do
costs.
The cost of high quality, activated, virgin carbon is
approximately $1.00 per pound. Enough carbon to
fill a 1.5 ft3 tank (34 Ib) would cost roughly $34. The
costs shown on Table 20 for carbon replacement
and disposal are higher than this because they
reflect the associated labor, equipment, and carbon
disposal costs. As mentioned previously in Section
5, regenerated carbon is not used in drinking water
treatment systems. The rate at which carbon is
exhausted—and thus the interval between replace-
ment—is a function of the concentrations and types
of organic compounds in the influent water. Chemi-
cal testing, on the other hand, can be considered a
variable cost. Once the system is in operation and
the estimated carbon tank life has been verified,
the frequency of effluent testing can be reduced;
however, it still must be done periodically as a
safeguard against early breakthrough. Hall and
Mumford (1987) conclude that it is more cost-effec-
tive to replace the carbon at the end of its esti-
mated tank life than it is to continue monthly testing
of the effluent for VOCs. In point-of-entry treatment
systems where only 30 to 40 Ib of carbon is needed
per column at any one time, carbon costs may
seem insignificant in comparison to costs for larger
scale wastewater treatment plant carbon beds,
which can require up to 40,000 Ib of carbon each.
However, when organic compound concentrations
in the influent are high and frequent carbon
replacement is necessary, the annual carbon
budget may become considerable, especially for a
homeowner assuming the cost burden.
Feasibility of Carbon Adsorption
Point-of-Entry Treatment
Few studies have been done to determine the max-
imum influent concentrations for which carbon
adsorption point-of-entry treatment is feasible. The
higher the influent concentrations of organic com-
pounds, the more rapidly the carbon is exhausted
and the sooner it will have to be replaced. Likewise,
102
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Table 20
Carbon Adsorption Point-of-Entry Treatment System Costs
Component Cost1
1 -11/2 ft3 Carbon Column $700-800
Installation & Plumbing $100-150
Carbon Replacement & Disposal $ 100-200/replacement
Chemical Testing (9 VOCs)2 $250/test
1 All costs are based on average annual water usage estimates for 2-3 person household at 80 gpcd (160-240 gallons per
household per day).
2 It is assumed that EPA Method 624 is used to test for the presence of benzene, toluene, xylene and six other VOCs (volatile
organic compounds).
Table 21
Cost of Proposed Camp Dresser & McKee Project
Component Cost1
2 Carbon Columns (1 ft3) $900-1000
Installation & Plumbing $100-150
Carbon Replacement & Disposal $200 x 6 per year = $1800
Chemical Testing $250 x 12 per year - $3000
Total Per Household Per Year = $5800-5950
' Note: These are average costs for 10 homes.
Table 22
Cost of Culligan Project, Inc. in Rhode Island, 1986
Component Cost1
1 Carbon Column (11/3 ft3) $725
Installation & Plumbing $130
Carbon Replacement & Disposal $190 x 3 per year = $570
Chemical Testing $250 x 6 per year = $1500
Total Per Household Per Year = $2925
These are average costs for 13 homes which received point-of-entry carbon adsorption systems. The systems were installed for
21/2 years while a centralized water distribution network was being extended to the homes.
Table 23
Cost of Hall and Mumford Project in Wisconsin, 1987
Component Cost1
2 Carbon Columns $29502
Installation & Plumbing $100
Carbon Replacement & Disposal $100 x 4 per year = $400
Chemical Testing $150 x 12 per year = $1800
Total Per Household Per Year = $5250
1 Note' These are average costs for a service station and private home which received point-of-entry carbon adsorption systems.
2 Cost includes a chemical feed unit, retention tank, and ion exchange softening unit.
103
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Equipment
Table 24
Water Treatment Equipment Commonly Used in Carbon Adsorption Systems
Purpose
Cost1
Chemical feeder
Ion exchange softener
Manganese greensand filter
Calcium carbonate/magnesium oxide bed
120 Gallon retention tank
Reverse osmosis
Raise pH, precipitate iron $500 00
Remove Ca and Mg hardness 950.00
Filter out Fe +2 and Fe(OH)3 950.00
Remove iron 825.00
Mixing, flocculation, precipitation 525.00
Reduice mineral content, remove high molecular weight 730.00
organics, bacteria, viruses
'Costs are from Culligan, Inc. Costs are for the unit only and do not include installation or maintenance.
the higher the influent concentrations, the more
concern there is that early breakthrough will occur;
and, therefore, more effluent testing for VOCs is
performed. To further complicate the economic
considerations, homeowners may insist that addi-
tional safeguards such as point-of-use treatment or
bottled water be provided. Also, for well water with
water quality parameters that are adverse to car-
bon adsorption (such as low pH or excessive hard-
ness), additional water treatment units may be nec-
essary, which would add considerable costs to the
system. Therefore, the decision to apply point-of-
entry treatment should be based on site-specific
conditions, including the overall quality of the
influent water and the needs of the homeowner.
Finally, because the total cost of remediating a
gasoline spill depends on the number of homes
affected, at some point the marginal cost to the pol-
luter; Federal, state, or municipal financiers; or the
homeowners of installing and maintaining point-of-
entry treatment systems will exceed the cost of
other treatment alternatives.
In considering carbon adsorption and other point-
of-entry treatment systems, it should also be recog-
nized that their use in treating gasoline-contami-
nated well water is a newly applied remedial strat-
egy. Although equipment and service companies
are available in most towns and cities, the retailers
and service personnel may not fully understand the
capabilities and limitations of carbon adsorption
units in removing organic compounds.
Furthermore, most research projects and home
installations have focused on carbon adsorption
(and other point-of-entry treatment systems) as an
interim remedial action rather than a long-term
solution to the contamination problem. Commonly,
most point-of-entry treatment projects of this kind
have lasted 1 to 3 years. The interim is used to
implement other remedial action such as aquifer
restoration, to locate suitable alternative water
supplies such as new well fields, or to extend water
distribution mains from local systems.
Extension of Existing Water
Distribution System
Alternative Water Supplies
In situations where gasoline from surface spills or
leaking USTs threaten domestic well fields,
extending nearby water distribution systems is
often an appropriate long-term solution to the prob-
lem of supplying the affected homes with clean
water. Supplying homes with bottled water or instal-
ling point-of-use and point-of-entry treatment sys-
tems are considered interim remedies only,
whereas extending the local water distribution sys-
tem is a permanent solution and is often more
acceptable to homeowners. Other long-term solu-
tions such as drilling new wells in as yet uncontami-
nated parts of the well field may be impractical,
especially if the new wells are located downgra-
dient of the spill site where they could eventually be
contaminated by the migrating gasoline plume.
Costs of Extending Distribution
Systems
Cost is one of the primary considerations in extend-
ing existing water distribution systems to homes
with contaminated well water. Table 25 lists costs
typically associated with installing water transmis-
sion and distribution mains. A "transmission" main
transports water over long distances from cen-
tralized pump stations to "distribution" mains; distri-
104
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Table 25
Cost Breakdown Per Linear Foot for Water
Distribution and Transmission Mains
Cost
Category
Excavation
Pipe
Fittings
Hydrants
Line Valves
Service Conn.
Paving
Dewatering
Miscellaneous
Total Cost
Per Linear Foot
6-inch
Dist
$ 8.70
6.70
2.30
3.80
0.50
11.00
510
1.20
4.40
$43.70
Trans.
$ 8.70
6.70
2.30
—
0.50
—
5.10
1.20
2.90
$27.40
8-inch
Dist.
$ 870
9.20
2.70
380
070
11.00
5.10
1.20
4.90
$47.30
Trans.
$ 8.70
9.20
2.70
—
0.70
—
5.10
1.20
3.40
$31.00
12-inch
Dist.
$ 9.00
15.30
3.70
3.80
1.20
11.00
5.10
1.20
5.90
$56.20
Trans.
$ 9.00
15.30
3.70
—
1.20
—
5.10
1.20
4.70
$40.20
Cost
Category
Excavation
Pipe
Fittings
Hydrants
Line Valves
Service Conn.
Paving
Dewatering
Miscellaneous
Total Cost
Per Linear Foot
16-inch
Dist
$ 9.70
23.20
5.00
3.80
2.60
11.00
5.75
1.25
8.00
$70.30
Trans.
$ 9.70
23.20
5.00
—
2.60
—
5.75
1.25
6.50
$5400
24-inch
Dist.
$ 11 50
38.00
7.40
11.00
5.50
11.00
8.80
1.25
12.75
$10000
Trans.
$ 11.50
38.00
740
—
5.50
—
8.80
1 25
11.25
$83.70
30-inch
Dist.
$ 12.30
55.40
6.80
11.00
8.10
11.00
9.40
1.40
1510
$123.30
Trans.
$ 12.30
55.40
6.80
—
8.20
—
9.40
1.40
13.60
$107.10
NOTE: Costs were generated by Camp Dresser & McKee Inc. Costs include labor, design, and engineering services.
Excavation costs will vary significantly depending on the depth of excavation and on the material that must be excavated.
Distribution pipe costs are based on two service connections per 100 feet pipe laid. Costs will be lower in less densely
settled areas.
bution mains have service connections and dis-
charge outlets for fire hydrants, and they carry
water relatively short distances from transmission
mains to homes.) Table 26 lists costs for booster
pump stations. To enable comparison of costs for
different-sized water distribution extension projects,
cost scenarios have been generated for com-
munities of 10, 50 and 250 homes. These are
shown in Tables 27 through 29. These distribution
costs are lower than the costs listed in Table 25
because fewer connections are required to service
homes. The first item listed on each of these tables,
the 8-in transmission main, is usually the smallest
of its type used.
Certain assumptions have been made in generat-
ing these cost scenarios. The distance between the
existing distribution system and the communities
was set at 10,000 ft; and amounts of transmission
pipe needed were estimated from this distance. It
was assumed that bridge crossings and highway
piping work were not needed. Total costs would be
different, of course, with different assumptions. If,
for instance, the existing distribution system were
closer to the communities in any of the three
scenarios, say a half mile away, the cost of trans-
mission piping would be reduced by nearly
$232,000. Likewise, if a more severe terrain were
assumed and bridge crossings and additional
pump stations were required, the costs for these
scenarios would be considerably higher. These fac-
tors and others such as head requirements and
obtaining access to property through which the
extension must pass complicate the decision of
whether to extend existing water distribution sys-
tems and need to be carefully analyzed before
making the final decision.
105
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Table 26
Cost Breakdown for Booster Pump Stations
Brick
Total Cost $125,000 $121,000
1 Costs are based on construction and installation of 1 MGD stations.
Pre-Fabricated
Cost
Category
General Cleanup
Excav & Bkfill
Site Work
Concrete
Masonry
Carpentry
Process-Pump
Pump Controls
Instrumentation
Misc. Contract.
Misc. Hardware
Suction Lift
160 ft. 100 ft.
7,400
1,900
1 1 ,600
4,100
6,800
30,700
6,300
30,000
16,300
9,900
6,100
1,900
1 1 ,600
4,100
6,800
-
28,200
6,300
30,000
16,100
9,900
Suction Lift
160 ft. 100 ft.
7,400
1,900
1 1 ,600
4,100
3,400
-
30,700
6,300
30,000
8,150
9,900
6,100
1,900
11,600
4,100
3,400
-
28,200
6,300
30,000
8,150
9,900
$113,450
$109,650
Table 27
Capital Costs of Water Distribution Extension for a Community of 10 Homes
Cost
Category
8-inch Transmission Mam1
6-inch Distribution Main
Stream Crossings
Booster Pump Station (Unnecessary)
Storage Facility (Unnecessary)
Subtotal
Engineering Services & Contingency (30% of Subtotal)
Quantity
10,000ft.
3,000 ft.
40 ft.
Unit
Cost
$31 .00/ft.
$35.00/ft.2
$60.00/ft.
Total Cost
For Item
$310,000
$105,000
$ 2,400
$417,400
$125,220
Total Cost
$542,620
Description: This cost scenario was generated for a 10 home community located about 2 miles (10,000 feet) from a suitable
connection point to an existing water distribution system. It is assumed that 2-3 people live in each home and that each person uses
110 gallons of water per day—peak summer demand. The total water requirement for the 10 homes is approximately 0.0033 MGD. It
is assumed that there is no significant increase in elevation, and therefore, a booster pump station is not needed to ensure
adequate water supply. Because of the small size of the community, it is further assumed that water storage facilities (i.e., elevated
tanks, earthen reservoirs) are unnecessary. Finally, it was assumed that there was one 40-foot stream crossing, but no bridge
crossings (which at $200 + per foot can be prohibitively expensive).
Engineering services account for the design of the system; contingency covers unexpected costs during construction.
1 The 8-inch pipe is generally the smallest transmission main used.
2 Distribution pipe costs are lower than the costs listed in Table 25 because fewer connections are required to service homes.
106
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Table 28
Capital Costs of Water Distribution Extension for a Community of 50 Homes
Cost
Category
8-inch Transmission Main1
6-inch Distribution Main
Stream Crossings
Booster Pump Station
Storage Facility (Unnecessary)
Subtotal
Engineering Services & Contingency (30% of Subtotal)
Quantity
10,000ft.
10,000ft.
40ft.
Unit
Cost
$ 31.00/ft.
$ 36.00/ft.2
$ 60.00/ft.
$110,000
Total Cost
For Item
$310,000
$360,000
$ 2,400
$110,000
$782,400
$234,720
Total Cost
$1,017,120
Description: This cost scenario was generated for a 50 home community located about 2 miles (10,000 feet) from a suitable
connection point to an existing water distribution system. It is assumed that 2-3 people live in each home and that each person uses
110 gallons of water per day—peak summer demand. The total water requirement for the 50 homes is approximately 0.02 MGD. It is
assumed that there is a significant increase in elevation, requiring that a 1.0 MGD booster pump station be installed to provide
adequate water supply. Because of the small size of the community, it is further assumed that water storage facilities are
unnecessary. Finally, it was assumed that there was one 40-foot stream crossing.
1 The 8-inch pipe is generally the smallest transmission mam used.
2 Distribution pipe costs are lower than the costs listed in Table 25 because there are fewer connections to homes.
Table 29
Capital Costs of Water Distribution Extension for a Community of 250 Homes
Cost
Category
8-inch Transmission Main1
6-inch Distribution Main
Stream Crossings
Booster Pump Station
Storage Tank
Subtotal
Engineering Services & Contingency (30% of Subtotal)
Quantity
10,000ft.
18,750ft.
40ft.
100,000 gal.
Unit
Cost
$ 31,00/ft.
$ 43.00/ft.8
$ 60.00/ft.
$110,000
$ 1.20/gal.
Total Cost
For Item
$310,000
$806,250
$ 2,400
$110,000
$120,000
$1 ,348,650
$404,595
Total Cost
$1,753,745
Description: This cost scenario was generated for a 250 home community located about 2 miles (10,000 feet) from a suitable
connection point to an existing water distribution system. It is assumed that 2-3 people live in each home and that each person uses
110 gallons of water per day—peak summer demand. The total water requirement for the 250 homes is approximately 0.08 MGD. It
is assumed that the community is large enough to require both a 1.0 MGD booster pump station and an elevated storage tank.
Finally, it was assumed that there was one 40-foot stream crossing.
1 The 8-inch pipe is generally the smallest transmission main used.
2 Distribution pipe costs are lower than the costs listed in Table 25 because there are fewer connections to homes.
107
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Limitations
Other than cost, there are few limitations to extend-
ing water distribution systems, because centralized
systems have been used in cities and towns in the
United States for over a hundred years. A well-
established network of materials, manufacturers,
engineers, and construction companies is capable
of water distribution system installation. The most
significant limitations are the obtaining of access to
privately and publicly held land needed for trans-
mission main installation and the problem of sup-
plying homes with clean water while the distribution
system is being designed and constructed. Most
transmission mains are buried beneath road sur-
faces, allowing excavation equipment easy access
for installation and maintenance. Instances may
arise, however, when it is more practical or
economical to install transmission mains in fields,
lawns, and vacant lots. If these open areas are pri-
vately owned, a "right of way" (a legally binding
agreement between the landowner and the water
distribution authority) must be obtained, or when an
agreement cannot be reached, a court order for
"eminent domain" must be invoked. The process of
obtaining rights of way and court orders can take
months or even years to complete. To circumvent
the process and avoid costly delays, the decision is
often made to install the transmission main around
the property or properties in question, even though
this increases the costs of the system.
References
Brunotts, V.A. et al. 1983. Cost Effective Treatment
of Priority Pollutant Compounds With Granular
Activated Carbon. National Conference on Man-
agement of Uncontrolled Hazardous Waste Sites,
Washington, D.C.
Hall, D.W and R.L. Mumford 1987 Interim Private
Water Well Remediation Using Carbon Adsorption.
Ground Water Monitoring Review, Winter 1987
Hall, D.W. 1987 Warzyn Engineering Inc., Personal
Communication with J. L. Durant, Camp Dresser &
McKee Inc., Boston, MA.
Rozelle L.T 1986. Point-of-Use Treatment of
Organics. Culligan International Company.
Symms, K. 1986. Unpublished Research. Clean
Water Report, November 18,1986.
108
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Section 7
Index
Activated carbon, 57-60,76-79, 86-87,100
adsorption capacity, 74-75, 79-82
biological growth, 80
breakthrough, 76-78, 80-82, 85-87
competitive adsorption, 80
cost, 82-84
design, 78-79
desorption, 78-80, 85
effect of molecular weight, 79-80
effect of polarity, 80
empty bed contact time (EBCT), 78,80
iron (effect), 67,71-74, 85
isotherms, 75-77,100
limitations, 85
mass transfer, 61-63,75, 76-78
multistage, 79
regeneration, 75
removal efficiency, 79
sources of, 75
surface area, 75
van der Waals forces, 74-7,80
with air stripping, 86-87
with point-of-entry systems, 99-104
Air stripping, 57-74, 86-89
air-water ratio, 63-65,71
cost, 66-70
design equations, 65
design parameters, 60-65
design procedure, 65
gas pressure drop, 63-65
Henry's law, 61-63, 70
iron (effect), 67, 71-74
limitations, 71, 73
noise, 66,73
Onda equations, 62-63
pollution control devices, 73
removal efficiency, 67, 70-71
stripping factor, 63
vapor-phase treatment, 68-70
water temperature, 61, 70
with GAC, 86-87
Biorestoration (saturated zone), 88-94
cost of treatment, 94
effectiveness, 90-93
effect of temperature, 89
limitations, 93-94
microbial processes, 87-90
nutrient requirement, 89
oxygen requirements, 89
Case studies
air stripping and GAC, 88
free product recovery, 31-33
Costs
activated carbon, 82-84
air stripping, 66-70
biorestoration, 51-53,94
case studies, 33
enhanced volatilization, 40,41,51,52
excavation and disposal, 37,51,52
extension of distribution system, 104-107
free product recovery equipment, 18-25
incineration, 42, 52-53
microbial degradation (unsaturated), 52-53
oil/water separators, 27
point-of-entry systems, 102-103
soil venting, 45,52-53
soil washing, 47, 52-53
well-drilling, 21
Designs activated carbon, 78-79
air stripping, 60-65
free product recovery methods, 15-18
point-of-entry GAC units, 100-101
Distillation, 100
Excavation and disposal, 35-37,51-53
Extension of distribution systems, 104-108
costs, 104-107
limitations, 108
Fate and transport of gasoline, 5 to 13
capillary zone, 6,9-10
degradation, 12, 13
gasoline properties, 6,7
gasoline transport, 5-12
Free product recovery, 15-33
case studies, 31-33
cost, 18-25, 27, 30-33
deep wells, 20-21, 24-25
disposal of recovered gasoline, 28
filter separators, 19-20
limitations, 18,27
oil/water separators, 26-27
pumping well method, 16-18
shallow wells, 19-20
109
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skimmers, 19
trench method, 15,16
GAG (granular activated carbon), 75,77-87
Gasoline properties, 6,7
Gasoline removal from soils, 35-55
cost, 37,40-42, 45, 47, 51-53
effectiveness, 35, 36,39-40,43-45,47-53
enhanced volatilization, 38-41
excavation and disposal, 35-38
incineration, 42
soil venting, 42-45
soil washing/extraction, 45-47
Gasoline removal from groundwater, 57-98
mass transfer coefficient, 61-63
methods, 57
Henry's law, 61-63,70
Hydrogen peroxide
control of fouling, 73
in biorestoration (saturated zone), 89
microbial degradation (unsaturated zone), 50
oxidation, 57
Incineration, 35,42, 52-53
Ion exchange, 1-6
Iron
activated carbon, 67, 71, 74,85
air stripping, 67,71, 74
Microbial degradation (unsaturated zone), 35,
47-53
Onda equations, 62-63
Partition coefficients (gasoline), 9,13
Point-of-entry treatment, 99-194
costs, 102-103
devices, 99
limitations, 114
systems, 100-101
use of carbon, 100-104
Pumping wells, 16-18
dual pump, 16-21,27
single pump, 16-18,27
Resin adsorption, 57
Reverse osmosis, 57,99-100
Soil venting, 35,42-45,52-53
Soil washing, 35,45-47,52-53
Stripping factor, 63
Structure of subsurface
capillary zone, 6,9-10
saturated zone, 8,10-11
unsaturated zone, 5-10,12,13
Trench method, 15-18
Ultraviolet irradiation, 57
Volatilization, 9, 38-40
110
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