U.S. patent application number 10/801185 was filed with the patent office on 2005-09-22 for in-situ thermochemical solidification of dense non-aqueous phase liquids.
Invention is credited to Hayes, Thomas D., Srivastava, Vipul J..
Application Number | 20050207847 10/801185 |
Document ID | / |
Family ID | 34986454 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050207847 |
Kind Code |
A1 |
Hayes, Thomas D. ; et
al. |
September 22, 2005 |
IN-SITU THERMOCHEMICAL SOLIDIFICATION OF DENSE NON-AQUEOUS PHASE
LIQUIDS
Abstract
A method for in-situ dense non-aqueous phase liquids treatment
in which a subsurface source area comprising at least one rapid
release contaminant is located and free water removed therefrom.
The subsurface source area is then heated to a temperature suitable
for extracting the at least one rapid release contaminant from the
subsurface source area. The at least one rapid release contaminant
is extracted from the subsurface source area, resulting in
solidification of the dense non-aqueous phase liquids remaining in
the subsurface source area.
Inventors: |
Hayes, Thomas D.;
(Schaumburg, IL) ; Srivastava, Vipul J.;
(Woodridge, IL) |
Correspondence
Address: |
MARK E. FEJER
GAS TECHNOLOGY INSTITUTE
1700 SOUTH MOUNT PROSPECT ROAD
DES PLAINES
IL
60018
US
|
Family ID: |
34986454 |
Appl. No.: |
10/801185 |
Filed: |
March 16, 2004 |
Current U.S.
Class: |
405/128.35 ;
405/128.15; 405/128.8 |
Current CPC
Class: |
B09C 1/06 20130101; B09C
1/08 20130101; B09C 1/005 20130101 |
Class at
Publication: |
405/128.35 ;
405/128.15; 405/128.8 |
International
Class: |
B09C 001/00 |
Claims
We claim:
1. A method for in-situ dense non-aqueous phase liquids treatment
comprising the steps of: locating a subsurface source area
comprising at least one rapid release contaminant; removing free
water from said subsurface source area; heating said subsurface
source area to a temperature suitable for extracting said at least
one rapid release contaminant from said subsurface source area; and
extracting said at least one rapid release contaminant from said
subsurface source area, resulting in solidification of said dense
non-aqueous phase liquids remaining in said subsurface source
area.
2. A method in accordance with claim 1, wherein said at least one
rapid release contaminant is extracted using vacuum extraction.
3. A method in accordance with claim 1, wherein a sweep gas is
passed through said subsurface source area.
4. A method in accordance with claim 1, wherein said subsurface
source area is heated to a temperature in a range of about
80.degree. C. to about 200.degree. C.
5. A method in accordance with claim 1, wherein said subsurface
source area is heated by a dry heat source.
6. A method in accordance with claim 3, wherein said sweep gas is
an inert gas.
7. A method in accordance with claim 6, wherein said inert gas is
selected from the group consisting of air, nitrogen, helium and
mixtures thereof.
8. A method in accordance with claim 3, wherein said sweep gas
comprises at least one gaseous oxidant.
9. A method in accordance with claim 1, wherein said subsurface
source area is identified by three dimensional mapping of PID/FID
measurements.
10. A method in accordance with claim 2, wherein said vacuum
extraction is carried out at a vacuum up to about 30 inches of
mercury.
11. A method in accordance with claim 10, wherein said vacuum
extraction is carried out at a vacuum in a range of about 5 to
about 15 inches of mercury.
12. A method in accordance with claim 8, wherein said gaseous
oxidant is ozone.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method for in-situ treatment of
dense non-aqueous phase liquids so as to immobilize them in the
subsurface, thereby reducing the release rates of contaminants of
concern disposed therein, such as benzene and naphthalene, and
providing environmentally acceptable management of a contaminated
site. More particularly, this invention relates to a method for
in-situ solidification of dense non-aqueous phase liquids in
contaminated subsurface sites and for removal of rapidly released
contaminants from such contaminated subsurface sites.
[0003] 2. Description of Related Art
[0004] In the United States alone, there are thousands of sites
that are contaminated with a type of dense non-aqueous phase
liquids (DNAPL) comprising a mixture of components that have
different physical properties and that behave differently from one
another at ambient and elevated temperatures. One example of this
type of contamination is the tar dense non-aqueous phase liquids
generated from coal processing, such as occurred in manufactured
gas plants (MGP) or in coking in the steel industry. This special
type of dense non-aqueous phase liquids is mainly comprised of a
heavy fraction that exists as an immobile solid at ambient
temperatures and a minor fraction of light hydrocarbons (with low
molecular weight and high vapor pressure), e.g. benzene and
naphthalene, that exist as liquids at ambient temperatures. The
compounds of the minor fraction operate to solubilize the heavy
fraction when the two fractions are combined. Although the minor
"solubilizing fraction" of organic compounds may comprise as little
as 3% of the weight of the dense non-aqueous phase liquids, its
presence determines the rheology, physical flow and mobility,
partitioning of contaminants into water and gases, and rates of
contaminant release to groundwater.
[0005] Contaminants of concern from MGP sites can be grouped in the
following three categories:
[0006] 1) Benzene, Toluene, Ethylbenzene, Xylene (BTEX)
[0007] 2) Polyaromatic Hydrocarbons (PAHs)
[0008] 3) Total Petroleum Hydrocarbons (TPHs).
[0009] A certain maximum contaminant level (MCL) has been
established for volatile contaminants in drinking water. For
example, the allowable MCL for benzene is 0.005 mg/l. This value of
benzene for drinking water can be considered as very stringent
regulations. However, for cleanup criteria applied to some areas of
MGP sites, the non-hazardous levels are sufficient for remediation
of sites. These numbers are often an order of magnitude higher than
for drinking water. The benzene level for non-hazardous level
concentration is 0.5 mg/l. While treating MGP sites containing
tar/soil contaminated with BTEX, it is, therefore, important to
accomplish the non-hazardous level of the contaminants while
implementing a treatment technology.
[0010] Thus, soil/tars must be treated at operating conditions
sufficient to accomplish enough removal of benzene so that residual
levels are below 0.5 mg/l levels in the TCLP test. No minimum
levels are defined for lighter PAHs; however, they should be
removed from the soil/tar to render soil/tar immobilized.
[0011] In the U.S., there exist more than 3,000 former MGP sites.
About 400 of the sites have been characterized and about 125-150 of
these have received attention in the form of some type of
remediation. Less than 50 sites have been satisfactorily resolved
or "closed". Currently, it is estimated that over the next two
decades more than 20-40 sites per year will require immediate
solutions for the mitigation of highly concentrated source areas
(defined as source areas with more than 1% contamination in the
soil); each of these sites will contain at least three distinctive
source areas requiring treatment (e.g. gas holders, relief holders,
tar separators, soil hot spots). It is expected that a successfully
developed in situ thermal treatment technology could be applicable
to most of these sites.
[0012] At present, state of the art methods for addressing the
problems of dense non-aqueous phase liquids involve thermal
desorption, which is usually conducted with ex-situ processing,
requiring the contaminated soil to be excavated from the
subsurface. Thermal desorption typically involves heating of the
contaminated soil/DNAPL material in a rotary kiln-type device at
temperatures in the range of about 300.degree. C. to about
700.degree. C., vaporizing more than 98% of all of the organics,
and disposing of the off-gas stream through combustion or
condensation.
[0013] Conventional methods of applying thermal desorption to
achieve in-situ removal of coal-based DNAPL have been less than
highly successful. Attempts to apply classical thermal desorption
temperatures in-situ to achieve efficient removal of the total
DNAPL mass have been largely unsuccessful due to the inability to
reach the required target temperature range of
400.degree.-700.degree. C. using commercial heating hardware.
Barriers to reaching these temperatures in the subsurface include:
1) the heat of vaporization due to the high moisture conditions of
many sites with shallow water tables, and 2) the loss of heat
transfer efficiencies in soils that lose virtually all moisture
(which rapidly occurs at temperatures above 200.degree. C.). A
further disadvantage of conventional thermal desorption methods is
the large outputs of hydrocarbon contaminants resulting from the
high temperature desorption that require expensive handling and
disposal. These limitations point to the need for a new strategy
for the effective use of thermal treatment.
SUMMARY OF THE INVENTION
[0014] It is, thus, one object of this invention to provide a
method for handling dense non-aqueous phase liquids which does not
utilize the higher temperatures required by conventional thermal
desorption methods.
[0015] It is another object of this invention to provide a method
for handling dense non-aqueous phase liquids which can be carried
out in-situ, thereby avoiding the excavation required by
conventional thermal desorption methods.
[0016] It is still a further object of this invention to provide a
method for handling dense non-aqueous phase liquids which reduces
the release rates of subsurface contaminants of concern to achieve
risk based goals for the environmentally acceptable management of a
site.
[0017] It is still a further object of this invention to provide a
method for characterizing a contaminated site based upon the light
fraction of dense non-aqueous phase liquids as a means of locating
the subsurface areas of the site that are the major sources of
rapidly released contaminants of concern from the total dense
non-aqueous phase liquids mass.
[0018] It is yet a further object of this invention to provide a
method for solidification and immobilization of dense non-aqueous
phase liquids on a contaminated site and for removal of rapidly
released contaminants from the subsurface source areas of these
contaminants.
[0019] It is still another object of this invention to provide a
method for handling dense non-aqueous phase liquids which
eliminates the need for expensive post-treatment handling and
disposal.
[0020] These and other objects of this invention are addressed by a
method for in-situ dense non-aqueous phase liquids treatment,
referred to herein as thermochemical solidification (TCS),
comprising the steps of locating a subsurface source area
comprising at least one rapid release contaminant, removing free
water from the subsurface source area, heating the subsurface
source area to a temperature suitable for extracting the at least
one rapid release contaminant from the subsurface source area, and
extracting the at least one rapid release contaminant from the
subsurface source area, resulting in solidification of the dense
non-aqueous phase liquids remaining in the subsurface source area.
The method of this invention exploits the physical differences
between the heavy and light fractions present in the dense
non-aqueous phase liquids to effect an in-situ removal of the lower
molecular weight compounds from the dense non-aqueous phase liquids
mass, thereby leaving behind the higher molecular weight materials
which are solidified when the light fraction is removed. The method
of this invention can effectively treat either neat (concentrated)
dense non-aqueous phase liquids or mixtures of dense non-aqueous
phase liquids with soil, demolition wastes and/or sediment.
[0021] The primary components of this invention include 1) a
protocol for identifying the subsurface location (the true source
area) of the rapid release fraction of contaminants associated with
dense non-aqueous phase liquids; 2) equipment for removing free
water from the source area; 3) equipment for delivering dry heat
into the source area; and 4) equipment for handling and disposal of
the light fraction of organic compounds that are extracted from the
source area.
[0022] Thermochemical solidification technology has considerable
potential to provide substantial benefits in terms of savings for
the industrial and government sectors and in terms of performance
and reliability of the thermal technologies applied to the DNAPL
management. In general, it is expected that the technology will be
applicable to the treatment of concentrated DNAPL in shallow and
deep subsurface locations. In applying the risk-based
thermochemical solidification approach, savings will arise from the
following advantages:
[0023] 1) Volumes of soils requiring aggressive treatment will be
reduced by more than 70 percent.
[0024] 2) Less DNAPL to handle. Of the volumes of soils treated,
DNAPL residues requiring disposal will be reduced by more than 90%
because the thermochemical solidification only removes 3-10% of the
mass of the DNAPL to achieve its risk-based treatment goals.
[0025] 3) Verifiable in effectiveness. New protocols for the
contaminant "availability" to groundwater and other receptors from
IGT make verification of reduced release rates in the source areas
possible.
[0026] Thermochemical solidification technology has considerable
potential to provide substantial benefits for the industrial and
government sectors, including the following: Substantial
billion-dollar level savings nation-wide. Reduces cost of many
commercial thermal treatment systems through:
[0027] 1) Reducing the temperatures for thermal treatment to below
200.degree. C.
[0028] 2) Providing a risk based treatment endpoint
[0029] 3) Substantially decreasing the amount of the DNAPL material
that requires handling
[0030] 4) Environmentally acceptable closure strategy for many
types of sites:
[0031] a) Electric and Gas Utility MGP
[0032] b) Steel/Coking
[0033] c) Creosote
[0034] d) Government (DOE, DOD, etc.)
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] These and other objects and features of this invention will
be better understood from the following detailed description taken
in conjunction with the drawings wherein:
[0036] FIG. 1 is a general schematic diagram of the in-situ
thermochemical solidification method in accordance with one
embodiment of this invention applied to a landfill or a
contaminated site;
[0037] FIG. 2 is a diagram showing the effect of the treatment
conditions of the invention on the total vapor pressure
(representing available contaminants, where available means the
fraction of contaminants that are not tightly bound to the soil
matrix and which are readily leachable upon contact with water) of
a sample of tar-contaminated soil;
[0038] FIG. 3 is a diagram showing the effect of the treatment
conditions of the invention on the benzene vapor pressure
(representing the available fraction of benzene) of a sample of
tar-contaminated soil;
[0039] FIG. 4 is a diagram showing benzene removal from a tar
sample in accordance with the method of this invention;
[0040] FIG. 5 is a diagram showing naphthalene removal from a tar
sample in accordance with the method of this invention;
[0041] FIG. 6 is a diagram showing diesel range organics removal
from a sample in accordance with the method of this invention;
[0042] FIG. 7 is a diagram showing comparative removals from a
sample employing the method of this invention;
[0043] FIG. 8 is a diagram showing TCLP levels of tars treated in
accordance with the method of this invention before and after
treatment;
[0044] FIG. 9 is a diagram showing naphthalene release potential
before and after treatment of a first sample in accordance with the
method of this invention;
[0045] FIG. 10 is a diagram showing naphthalene release potential
before and after treatment of a second sample in accordance with
the method of this invention;
[0046] FIG. 11 is a diagram showing phenanthrene release before and
after treatment of a first sample in accordance with the method of
this invention;
[0047] FIG. 12 is a diagram showing phenanthrene release before and
after treatment of a second sample in accordance with the method of
this invention; and
[0048] FIG. 13 is a diagram showing the stability of residues
remaining from application of the method of this invention.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0049] Although not intended in any way to limit the scope of
applicability of the method of this invention, the following
description of the method of this invention relates to the in-situ
treatment of subsurface non-aqueous phase liquids at former
manufactured gas plant sites or at other industry sites having
similar dense non-aqueous phase liquids characteristics to MGP
operations, such as creosote, steel and coking sites. It will be
understood that the method of this invention may be applied to any
contaminated site containing dense non-aqueous phase liquids.
[0050] In the last several years, vapor pressure measurement tools
have been developed for a rapid field measurement technology that
uses vapor pressures as an indicator of the organic contaminant
release potential from the source areas (i.e. subsurface soils that
are sources of the contaminant transport) to receptors of concern.
New commercial analytical products (e.g. Photo Ionization Detectors
and Field GCs) allow the field measurement of vapor pressures of
individual and/or classes of the contaminants of concern. The
measurement of vapor pressures of chemicals of concern is, in
principle, a good indicator of the availability of chemicals for
transport from the source areas to the receptors of concern; a high
vapor pressure of a contaminant indicates a high mobility and a
high release rate. See, for example, U.S. Pat. No. 6,591,702 to
Hayes et al., which is incorporated by reference herein in its
entirety. These vapor pressures may be measured with commercially
available, low-cost photo ionization detectors or the equivalent
that is capable of providing repeatable and reliable measurements
of the hydrocarbon vapor pressures. These detectors are employed
for the measurement of a variety of volatile organic compounds
(VOCs). The use of low energy lamp (9.8 eV) can exclude a variety
of the hydrocarbons that have an ionization potential higher than
9.8 eV. Additionally, the use of a tube containing absorbing
material filters most of the VOCs such as toluene that can
interfere with the measurement of benzene. These tubes also absorb
humidity to allow for a very accurate reading on the PID meter. The
combination of the filter tube and the PID remove the disadvantages
associated with colorimetric tubes and gas chromatographs.
[0051] This vapor pressure or fugacity-based strategy of estimating
the contaminant release potential from a source area relies on the
general observation that the most mobile contaminants are those
with the highest vapor pressures. Further, the rapid release
compartment contains the contaminant mass that is "available" for
the partitioning from the soil to water, living membranes, or to
pore space gases.
[0052] The first step in the method of this invention is to
identify the location of the dense non-aqueous phase liquids
present in the contaminated site in terms of the "available" or
"rapid release fraction" contaminants of concern, such as benzene
and naphthalene, using protocols that are known to those skilled in
the art, such as the water leaching column method. When applying
this approach to a site, the site is divided into a grid and a soil
core is taken from each grid element. An example of a commercial
system suitable for performing the coring operation is the
GEOPROBE.RTM. soil sampling system available from Geoprobe Systems
in Salina, Kans. using a 2" or 3" diameter Shelby tube sampler.
Soil gas data using a photoionization detector (PID) or flame
ionization detector (FID) is collected from the soil core. The soil
gas data is used as a screening tool to select locations along the
core for soil samples to be collected for laboratory testing for
contaminant mobility and availability to receptors such as
groundwater, ecological organisms and humans. Based upon this body
of data, a three-dimensional mapping of the rapid release fraction
or available fraction of the contaminant(s) is performed, which is
used to effectively place the heating system, vacuum extraction
wells and sweep gas injection wells. The end result of this step is
the identification of the true subsurface source area.
[0053] Having located the subsurface source area, drainable water
is removed from the treatment zone where the thermochemical
solidification will be applied. This may only require physical
pumping if the contamination is located in an area that is
drainable. If it is not readily drainable, the source area to be
treated may have to be hydraulically isolated using commercially
available barriers such as sheet piles, slurry walls, or in-situ
grouted walls. Once the source area is isolated from rapid
infiltration of groundwater, it can be drained using devices such
as mechanical pumps. Alternatively, prefabricated vertical drains
may be used for dewatering if especially tight soils are
encountered in the source areas.
[0054] After removal of the free water, the system components are
installed using the general configuration shown in FIG. 1. The
placement of each of the components in the subsurface is based upon
the data obtained in the initial step regarding the spatial
distribution of the rapid release fraction of the contaminant(s).
If the site is very large and if the distribution of the rapid
release fractions covers an extensive area, treatment may be
conducted in sections. In most cases, the overall strategy of the
placement of the treatment zones will be the targeting of source
areas having a high rapid release fraction mass.
[0055] Following the removal of free water and the installation of
the system components, the dense non-aqueous phase liquids
treatment zone is then heated using thermal heating that does not
add water or steam to the treatment zone. Commercial heating
technologies that can be employed include resistive heating,
6-phase heating, RF heating, and conductive heating using in-ground
heat exchangers, or any other method that delivers dry heat to the
subsurface source area. In accordance with one particularly
preferred embodiment of this invention, the treatment zone is
heated to a temperature in the range of about 80.degree. C. to less
than the boiling point of naphthalene (about 218.degree. C.).
Temperatures in the range of about 120.degree. C. to about
160.degree. C. will typically result in the removal of a sufficient
mass of the solubilizing fraction (volatile aromatic and aliphatic
compounds) to allow the dense non-aqueous phase liquids to be
solidified. As a result, the dense non-aqueous phase liquids
material is transformed from liquids that are characteristically
hazardous to a solid material that can pass the TCLP test as a
non-hazardous waste material. During this transformation, a
significant mass of the lighter hydrocarbons that represent the
solubilizing fraction is volatilized from the dense non-aqueous
phase liquids mass and is removed from the dense non-aqueous phase
liquids through extraction wells located within or proximal to the
heated dense non-aqueous phase liquids treatment zone.
[0056] During heat addition, three phases of system operation may
be contemplated: Phase I--transitional temperature ramp-up--during
which the temperatures in the treatment zone are increased from
ambient temperature to the target temperature and much of the
residual water content of the treatment zone is driven off; Phase
II--rapid mass release of the solubilization fraction--during the
initial time period at which the treatment zone is at the target
temperature (extending for less than a week), the vapor pressures
of the light hydrocarbons in the dense non-aqueous phase liquids
are increased and the solubilizing fraction is released from the
dense non-aqueous phase liquids in accordance with Henry's Law and
Ficks Law predictions. During this phase, a large mass of volatiles
is initially given off from the liquids and solids in the dense
non-aqueous phase liquids treatment zone and into the gas phase of
the interstitial pores of the soil. These gas emissions are
collected through an extraction well system described herein below.
During this period, the extraction wells are operated to generate a
highly concentrated gaseous stream of volatile organic compounds;
and Phase III--enhanced extraction of the residual solubilization
fraction--during which period of time the system is operated to
utilize a combination of methods to enhance the removal of the
solubilizing fraction remaining in the dense non-aqueous phase
liquids zone after the initial release. Enhancements include, high
vacuum, passing of sweep gas into the treatment zone, and the
introduction of gaseous chemical oxidants in the gaseous phase.
[0057] Removal of the solubilizing fraction that is initially
thermally desorbed from the treatment zone dense non-aqueous phase
liquids material is carried out in accordance with one embodiment
of this invention using a system of vacuum extraction wells. Vacuum
extraction wells may be either vertically or horizontally oriented
and may vary in diameter from 1-3 inches or greater. A commercially
available vacuum blower may be used. It may be desirable, but not
necessary, that the blower contain a variable speed drive motor
with a high degree of fluid flow range. Transmission wells and
lines should be compatible for operating vacuum systems up to about
27 inches of mercury. During the contemplated Phase I described
herein above, the vacuum extraction system is operated at a flow
sufficient to remove moisture from the treatment zone. A condenser
may be used during this time to capture the water fractions
released during Phase I heating. During the contemplated Phase II,
the vacuum extraction system removes the solubilizing fraction (VOC
fraction) that is rapidly released once the target temperature is
achieved and residual water is removed from the treatment zone.
[0058] In accordance with one embodiment of this invention, if
needed, the treatment zone is purged with a sweep gas to achieve
more rapid and efficient removal of the solubilizing fraction of
the dense non-aqueous phase liquids. The sweep gas is injected into
or below the dense non-aqueous phase liquids treatment zone. The
extraction wells are simultaneously operated to remove the off-gas
and the sweep gas from the treatment zone. Sweep gases that may be
employed in the process of this invention include air, nitrogen,
helium or any other suitable inert gas.
[0059] In addition to sweep gas, other enhancements may also be
applied to drive the light solubilizing fraction out of the dense
non-aqueous phase liquids and achieve dense non-aqueous phase
liquids solidification. Among these enhancements are: temperature
spiking in the dense non-aqueous phase liquids treatment zone for
short periods of time to achieve very high transient vapor pressure
of the solubilizing fraction and increase the driving gradient for
these compounds to be partitioned into the interstitial gas of the
soil pores; increasing vacuum up to about 30 inches or more of
mercury to drive the volatile solubilizing fraction out of the
solids and liquids into the gas phase according to a more favorable
Henry's Law partitioning; and addition of gaseous oxidants, such as
ozone, into the sweep gas to partially oxidize the gaseous
solubilizing compounds and cause a shift of the partitioning
reaction toward increased partitioning from the dense non-aqueous
phase liquids material to the gas phase.
[0060] Management of the volatile organic compounds (VOCs) that
emerge from the treatment zone is accomplished in accordance with
one embodiment of this invention by the use of appropriate off-gas
treatment equipment. During Phase I described herein above when
heat is applied to ramp the temperature to the target temperature,
an off-gas stream is likely to have a very high humidity, but
variable VOC content. During this time, condenser equipment may be
employed to remove water from the gas stream. The effluent of the
condenser can be passed either through a catalytic oxidation unit
or through an activated carbon adsorption system.
[0061] During Phase II described herein above, a catalytic
oxidation system or commercial flare is used to thermally destroy
the concentrated VOCs in the off-gas stream. The VOCs comprise the
solubilizing fraction that has been driven from the dense
non-aqueous phase liquids treatment zone. An in-line PID
measurement may be taken periodically in the off-gas stream and
stored in a computer. When the cumulative PID measurements level
off to a plateau, the sweep gas injection system is activated,
causing the sweep gas to be injected into or below the dense
non-aqueous phase liquids treatment zone.
[0062] During Phase III described herein above, as soon as the
sweep gas is activated, the off-gas stream is switched from the
catalytic oxidation system to an adsorbent system such as a
granular activated carbon box for VOC control.
[0063] To facilitate operation of the VOC management system,
sensors may be interfaced to a computer for automated control of
the off-gas management system. Humidity sensors in the off-gas
stream and thermistors in the dense non-aqueous phase liquids
treatment zone may be used to determine when the Phase I transition
period has been completed. The PID data taken from the off-gas
stream can be compiled and analyzed to provide a basis for a
decision to channel the off-gas stream from the catalytic oxidation
unit to the activated carbon adsorption system. Computer control of
the off-gas stream solenoid valves, the catalytic oxidation system
and the carbon adsorption process can all be achieved with state of
the art computer programming and controller equipment for ease of
operation.
[0064] Performance of the in-situ thermochemical solidification
method of this invention may be verified through a number of
diagnostic measurements: 1) measurements of PID and/or FID in the
extraction wells and in gas phase piezometers proximal to the
treatment zone during a period when the sweep gas has been
discontinued; and 2) direct follow-up soil sampling and analysis to
determine the mobility of contaminants after application of the
thermochemical solidification method of this invention. Regarding
the first diagnostic measurement, substantial decreases in PID or
FID measurements indicates significant reductions in the content of
the solubilizing fraction of the dense non-aqueous phase liquids.
Regarding the second diagnostic measurement, the same tests used in
the initial step of the disclosed method as discussed herein above
would be used in these determinations.
EXAMPLES
[0065] In this example, thermochemical solidification was applied
to the treatment of dense non-aqueous phase liquids in a gas holder
of an MGP site that was 50 feet in diameter and 22 feet deep. The
holder had already been decommissioned and what remained is the
subsurface holder tank that had been filled in with demolition
materials and soil. Soil borings and PID profile measurements
showed that high PID readings were obtained in a layer that extends
from the bottom of the holder to 6 feet above the bottom of the
holder. Contaminant mobility testing of soil samples showed that 90
percent of the rapid release fraction contaminant mass was located
in the 6 ft dense non-aqueous phase liquids layer at the bottom of
the holder. This layer was thus targeted for thermochemical
solidification. The dense non-aqueous phase liquids layer contained
some free water and the composition of the drained soil/dense
non-aqueous phase liquids material was as follows: 1) DNAPL
content=10%; 2) solubilizing fraction of DNAPL=1%. The 6 ft
treatment zone contained about 61 tons of dense non-aqueous phase
liquids organic compounds, of which the light solubilizing
fractions amounted to about 6 tons.
[0066] Resistance heating elements were installed in the mid
section of the dense non-aqueous phase liquids layer. A commercial
resistance heating system was sized to deliver sufficient thermal
heat input to raise the temperature of the dense non-aqueous phase
liquids layer to the target temperature range within 5 days. Vacuum
extraction wells, air injection wells and an off-gas treatment
system were installed. In operation, the system was first heated to
the targeted temperature over a 5 day period. Operation of the
vacuum extraction well system was initiated during this heating
period. When the treatment zone was brought up to temperature,
about 5 tons of rapid release fraction materials were released
within the first week of operation. During this period, a flare was
operated to treat the off-gas stream. In this initial heating
period, the PID readings in the off-gas decreased in concentration
until a plateau was reached. The off-gas stream was then channeled
to a carbon box and an air sweep gas stream was introduced into the
air injection system at rates of about 30-100 scfm. Periodically,
the air stream was temporarily interrupted and a set of PID
measurements taken from gas phase piezometers to monitor the
progress of treatment. When the piezometer PID leveled out, the
treatment process was discontinued.
[0067] Post treatment coring was performed and it was determined
that the PID readings of the treatment zone had been reduced by
more than 95%. Follow-up sampling showed that more than 90% of the
rapid release fraction of benzene was removed from the dense
non-aqueous phase liquids treatment zone and that the dense
non-aqueous phase liquids material itself passed TCLP testing for
benzene and naphthalene, indicating that the material can be
disposed of or left in place as a non-hazardous waste.
[0068] In this example, large laboratory-based TCS prototype
testings were performed. Elements of this test included:
[0069] [1] An apparatus consisting of an 8-ft long (2" diameter)
vertical glass column that was outfitted with heat tracing,
temperature control, and vacuum extraction (exterted at the top of
the column)
[0070] [2] Depth of concentrated soil/DNAPL in the column of 6
feet
[0071] [3] Dimensions of the large TCS column selected to simulate
the mass transfer of volatile and semi-volatile contaminants in one
direction at a path length that represents a realistic radius of
influence for field-based vacuum extraction
[0072] [4] No sweep gas used in column tests
[0073] [5] Test duration of one week
[0074] [6] Measurements of vapor pressure, total concentrations and
mobility of contaminants of concern before and after TCS
treatment.
[0075] More detailed descriptions of the methods used in large
scale laboratory thermal solidification are given in the following
sections:
Sample Preparation
[0076] Two large samples from two different sites (Sites 1 and 2)
of tar DNAPL/soil mixtures were prepared to pack in the large-scale
column for the thermochemical transformation. The sample from Site
1 was a pumpable DNAPL-saturated free product and did not represent
one typically found on in-situ sites. This material showed a very
high PID (high vapor pressure) in an initial screening test.
Therefore, this material was amended with sand (three parts sand to
one part free product). The sample from Site 2 was more
representative of the in-situ sampling of MGP sites. This sample
was obtained from porous subsurface strata having a high grit
content. The sample measured a moderate initial PID ready and,
therefore, did not require mixing with sand. This sample had some
woody material associated with it. The large particles and rocks
were, however, removed before packing in the column.
Vapor Pressure Measurement
[0077] Vapor pressures in the two test samples were measured using
photoionization detector (PID) equipment. The PID of the samples
was taken by transferring a small aliquot of the samples in a 4-oz
jar, equilibrating the contents of the jar in at least 2 hours and
measuring PID through a special cap for the jar outfitted with a
special fitting. Benzene PID and total PID were measured. Benzene
was measured using UltraRAE PID meter (Ray Instruments). Benzene (5
ppm) was used as standardized gas. This equipment used a resin
absorbing tube to absorb all compounds but benzene. Total PID was
measured using MiniRAE 2000 (Ray Instruments). The instrument was
calibrated using standardized isobutylene gas (100 PPM). These
instruments were calibrated each day before use. The benzene and
total PID were also measured at the bottom of the large laboratory
column by removing the glass wool and the sand at bottom of the
columns and inserting the PID meters through the openings.
Operation
[0078] Large laboratory TCS prototype experiments were performed in
8-foot columns. Two 4-foot columns were combined using a dual
neoprene coupler (Ace glass, Vineland, N.J.). The glass wool was
packed into a 3" space at the bottom followed by 2" of sand. The
material was filled up to 5 to 6.5 ft. in the column. The weight of
the material was in the range of 5-6 Kg. A mechanical
vibrator/vacuum pump was used to pack the columns. A custom-built
profile probe (OSK2K524, Omega Engineering) was used to measure the
temperature at each foot height from the bottom of the column. The
thermisters were housed in a stainless steel tube (1/4").
Temperature data were collected and compiled in a computer using a
data acquisition board purchased from Omega Engineering (Cio-Das-TC
16 Channel T/C). The column was wrapped around with a 20-feet
heating tap and was insulated.
[0079] Vapors escaping from the top of the column were condensed
into a 3-liter condenser held at low temperature of 4.degree. C.
using chiller water heat exchangers. The residual vapors were
trapped in a resin column (40 g Ambersorb-600). The vacuum was
maintained between 5-10 inches Hg throughout the operation. The
target temperature (90.degree. C.) was reached within 4 hours and
maintained using a PID controller. The exit port of the column was
examined routinely to check for any blockage due to escaping vapors
from the column. A gravimetric analysis was performed on the column
by measuring the weight of the tar material before and after
treatment. This was done using a balance (Mettler Model 12001,
Toledo, USA) capable of measuring up to 12.1 kg of the material
with a precision of 0.1 gram. The unfilled portion of the column
was also wrapped and insulated to minimize the condensation within
the column. The following was the mode of operation performed on
the column:
[0080] 1) Heat on day 1 to attain a temperature of 90.degree.
C.
[0081] 2) Apply vacuum gradually to a maximum of 5 inches of
mercury on day 1
[0082] 3) Maintain temperature of 90.degree. C. for 3-days
[0083] 4) Spike and maintain temperature to 120.degree. C. next 24
hrs
[0084] 5) Lower temperature back to 90.degree. C. on day 4
[0085] 6) Maintain at 90.degree. C. for next 4-days
[0086] 7) Periodically monitor PID at top and bottom of column
[0087] 8) Collect 4-7 samples of condensate resin to determine the
nature of material removed
[0088] 9) Measure the weight of material before and after the
treatment
Analysis
[0089] These soil/tar samples were analyzed for different
contaminants such as PAHs (EPA Method No 8270C)), BTEX (EPA Method
No 8260B), & TPHs (Modified EPA Method No. 8015B). The TCLP
BTEX/Naphthalene was determined using EPA Method 1311/8260B.
Vapor Pressure and Individual Compounds Variations
[0090] Large column tests were performed for a period of 7 days on
two different tars DNAPL/soil samples (Site 1 and 2). Total vapor
pressure dropped more than 90% during first 3 days of the treatment
for Site 2 sample and was further reduced to greater than 99%
during one week of treatment (FIG. 2). These findings suggest that
most rapid release light fractions were removed during thermal
treatment.
[0091] Removal of benzene from the soil was almost complete as
noted by the greater than 99% removal of vapor pressure within 3
days (FIG. 3). Thus, both naphthalene and benzene removal was
evident from the large column at bottom of the column which is the
most difficult location for removing light contaminants. These
findings were further confirmed by the mass concentration removal
of benzene and naphthalene (FIGS. 4 and 5). Test results summarized
(FIG. 6) for diesel range organics (DRO) showed relatively less
removal of DROs. This could be due to the presence of heavy oils in
the samples.
[0092] The removal rates of various compounds are summarized in
FIG. 7. From these results, it is evident that benzene showed the
most removal followed by naphthalene, DRO, phenanthrene and the
higher ring compounds. Thus, low temperature treatment removes only
a limited amount of overall mass (<5% of soil weight) but
efficiently removes contaminants of concern and renders soil
harmless.
Effect of TCS on TCLP and Contaminant Mobility
[0093] TCLP tests performed on the two site samples suggest a
significantly reduced level of volatile compounds (FIG. 8). These
treated samples were further tested for mobility of various
compounds as shown in FIGS. 9-12.
[0094] Several tests were performed on the treated and the
untreated samples to determine the contaminant mobility. These
tests included determination of available levels of naphthalene,
phenanthrene and the mobility testing by reconstitution treated
soil/DNAPL samples in various agents (surfactant/solvent). The
available (rapid release) fraction of naphthalene for Site 2 sample
soil was reduced from greater than 70% of total mass for the
untreated sample to less than 0.1% for the treated samples (FIG.
9). Likewise for Site 1 sample, the available fraction was reduced
from 18% to less than 0.1% with TCS treatment as shown in FIG. 10.
The available fraction was also reduced for phenanthrene for the
two site soils (FIGS. 11 and 12). More than 80% reduction was
noticed in the available fraction for each site soil. These
findings suggest that the thermal solidification in the column
successfully reduced the available fraction in site soils for the
compounds of concern.
[0095] Treated samples were further tested for mobility testing by
reconstitution of the TCS treated soils in 1% surfactant solution
and hexane (10%). The results summarized (FIG. 13) suggest that
once a DNAPL mass has been solidified it will not revert to a
flowing DNAPL fluid, nor will it exhibit the treated samples alone
or under surfactant/solvent added conditions. Thus, the thermal
treatment made the contaminant mass immobilized to an extent that,
even under adverse conditions (surfactant/solvent), no leaching of
naphthalene is expected. These findings suggest that thermal
treatment has effectively removed the risk for leaching of the
contaminants of concern into groundwater.
[0096] While in the foregoing specification this invention has been
described in relation to certain preferred embodiments thereof, and
many details have been set forth for purpose of illustration, it
will be apparent to those skilled in the art that the invention is
susceptible to additional embodiments and that certain of the
details described herein can be varied considerably without
departing from the basic principles of the invention.
* * * * *