|
Frequently Asked Questions
Waste Treatment Home
> Frequently Asked Questions
Q: What is solidification/stabilization
treatment?
Q:How is cement-based
solidification/stabilization done?
Q:What is portland
cement?
Q:What kinds of contaminants
can be treated by S/S?
Q:Is S/S an accepted
technology?
Q:How does S/S treatment
protect human health and the environment?
Q:Can treated materials
be reused?
Q:How much cement is
added?
Q:How is the S/S treatment
evaluated?
Q:Why is S/S so
frequently selected for Superfund site remedies?
Q:How
does S/S treat radioactivity?
Q:
What hydraulic conductivity and compressive strengths can be achieved
by soil-cement?
Q:Why
use S/S at former wood preserving sites?
Q: What
low-permeability ranges are possible with cement-bentonite slurry
walls?
Q: Is
roller-compacted concrete as strong as conventional formed concrete
when used in an industrial application like a compost facility pavement?
Q:Why use S/S for MGP sites?
Q:
How can S/S be protective of human health and the environment if
the technology does not lower the concentration of hazardous constituents
in the treated material?
Q:
S/S involves mixing a binding reagent into contaminated environmental
media or waste. What are the advantages of using portland cement?
Q:
How is S/S-treated material different from conventional concrete?
Q: What
is the volume increase with S/S applications?
Q: Why
is unconfined compressive strength used as a S/S performance standard?
Q: How
does EPA view S/S treatment?
What is solidification/stabilization
treatment?
Solidification/stabilization (S/S) is a treatment technology that
is used to prevent or slow the release of harmful chemicals from
waste or contaminated soil, sludge, and sediment.
How is cement-based solidification/stabilization
done?
Cement-based S/S is done by mixing portland cement into waste or
contaminated media such as soil, sludge, or sediment. Cement can
be mixed into waste as it is generated by industry to make the waste
safe for land disposal. During cleanup of contaminated land, cement
can be mixed into contaminated soil, sludge, or sediment while the
contaminated medium remains in place, or in situ. Or the cement
can be mixed into the contaminated media after it has been excavated
(ex-situ mixing).
What is portland cement?
Portland cement is a generic material produced by over 50 companies
at over 125 plants in the U.S. and Canada. The principal use of
cement is in concrete for construction. Concrete is a mixture of
portland cement, aggregates (gravel and sand), and water. The cement
used in S/S is the same as that used in concrete.
What kinds of contaminants
can be treated by S/S?
Cement-based S/S has been used to treat a wide variety of contaminants
including inorganics, such as heavy metals like lead and arsenic,
and organic contaminants, like creosote and petroleum products.
The ability of S/S technology to treat such a wide range of hazardous
constituents is one reason why S/S has been used to treat over 25%
of the U.S. Superfund sites where the sources of contamination have
been addressed.
Is S/S an accepted
technology?
The U.S. Environmental Protection Agency (EPA) considers S/S to
be an established treatment technology. EPA has identified S/S treatment
as Best Demonstrated Available Technology (BDAT) for at least 57
commonly produced industrial wastes (Resource Conservation and Recovery
Act [RCRA]-listed hazardous wastes) and has selected S/S treatment
for 25% of its Superfund (abandoned or uncontrolled) site remediation
projects.
S/S has been used since the 1950s to treat radioactive wastes and
since the 1970s to treat hazardous chemical wastes. Over the years
innovations in mixing methods and additives have extended the application
of cement-based S/S to a greater variety of site conditions and
contaminants.
How does S/S treatment protect
human health and the environment?
Cement-based S/S is an established and proven technology that prevents
contaminants from moving into the environment from treated waste.
S/S immobilizes contaminants within the treated material. This immobilization
occurs through physical and chemical bonding with the contaminants
and, in many cases, chemical changes within the contaminants themselves.
Immobilizing contaminants within treated waste prevents human, animal,
and plant exposure to hazardous constituents.
Can treated materials
be reused?
S/S-treated soils, sediment, and sludge are often reused. Most frequently
this occurs on a property that is being cleaned up. Reusing treated
material on-site, rather than hauling it away and bringing in new
material, protects the surrounding community from the hazards posed
by increased truck traffic, truck noise, air pollution, and damage
to roadways.
How much cement is added?
The amount of cement added varies typically from a range of 5% to
30%. There is no "cookbook" recipe. Quantity of cement
to be added is determined through a treatability study and mix design
conducted on the individual subject waste or medium. Treatability
studies can be simple or more involved depending on the project
requirements. A description of an involved treatability study written
by the U.S. Army Corps of Engineers is found at: http://www.usace.army.mil/publications/eng-tech-ltrs/etl1110-1-158/entire.pdf.
Project managers may wish to review the description against project
requirements, selecting elements of the described treatability study
guide relevant to the project.
How is the S/S treatment
evaluated?
S/S treatability studies and mix designs include tests to determine
the physical and chemical properties of the treated material. Using
leaching and extraction tests scientists can determine the amounts
of hazardous constituents that can leach from treated waste under
worst-case scenarios. Physical tests such as compressive strength
are used to determine the absence of free liquids in treated waste
and also construction properties, if the treated material is destined
for reuse or land disposal.
There are a variety of physical and chemical tests that have been
applied to S/S technology. Perhaps the most comprehensive reference
describing the various tests is EPA’s Stabilization/Solidification
of CERCLA and RCRA Wastes: Physical Tests, Chemical Testing Procedures,
Technology Screening and Field Activities, May 1989. For
a copy of the publication, click
here.
In addition to the chemical tests described in the reference, readers should
be aware of the Synthetic Precipitation Leaching Procedure (SPLP).
Information concerning the SPLP EPA Method 1312 is available from
EPA.
Why is S/S so frequently
selected for Superfund site remedies?
EPA’s goal for a Superfund site is to select a remedy that
is protective of human health and the environment, one that maintains
protection over time, and that minimizes un-treated waste. EPA believes
that treating waste and rendering it non-hazardous (rather than
containing it) is generally the preferred method for achieving long-term
protectiveness. EPA considers S/S to be an established treatment
technology. S/S is considered Best Demonstrated Available Technology
(BDAT) for over 50 RCRA-listed wastes.
Criteria used in Superfund to assess remediation alternatives include
(a) overall protection of human health and the environment, (b)
compliance with applicable or relevant and appropriate requirements
(ARARs), (c) long-term effectiveness and permanence, (d) reduction
of toxicity, mobility and volume, (e) short-term effectiveness,
(f) implementability, (g) cost. Among the different alternatives
considered during the feasibility study phase of a Superfund project,
S/S is often the alternative that best fulfills these criteria.
In brief, S/S treatment (a) has demonstrated protectiveness, (b)
complies with ARARs, including land disposal restrictions, (c) demonstrated
long-term effectiveness through treatability testing and monitoring,
(d) reduces toxicity and mobility of hazardous constituents, (e)
can be carried out safely for site workers and surrounding community,
with in-situ treatment reducing hazards posed by waste transportation,
(f) is an established technology with experienced contractors and
reagents available to do the treatment, and (g) is often the most
cost-effective remedy considered.
How does S/S treat
radioactivity?
Although, S/S does provide additional shielding of radioactivity
immobilized within contaminated material, S/S does not reduce radioactivity
of a material contaminated with radionuclides. The principle action
of S/S on radioactive wastes is to physically immobilize the radionuclides
within the treated material. Immobilization of the radioactive material
prevents release of those materials into the environment. Over time
the level of radioactivity emitted from the immobilized radionuclides
reduces itself through a process of radioactive decay.
Radionuclides decay at a fixed rate, unaffected by factors such
as temperature or pressure. The fixed rate of decay is described
by the “half life,” which is the time required for half
of the atoms of a given radionuclide to decay into a decay product.
The decay product may still be radioactive and will in turn continue
to decay. The decay product may have a shorter or longer half life
than the originating radionuclide.
S/S treatment allows the contaminated material to be safely stored
until the process of radioactive decay reduces the level of radiation
emitted from the treated material to an acceptable level.
What hydraulic conductivity
and compressive strengths can be achieved by soil-cement?
Hydraulic conductivity of as low as 1X10-7
cm/sec has been achieved. Unconfined compressive strengths of more
than 800 psi have also been achieved. Hydraulic conductivity and
compressive strength of soil cement is dependent on a number of
factors including the properties of the soil, mix design, and installation
techniques. A summary of soil-cement properties is found in American
Concrete Institute’s Report titled State-of-the-Art Report
on Soil-Cement ACI 230.1R-90. Click
here for more information or to order.
Why use S/S at former
wood preserving sites?
S/S effectively treats common organic and inorganic hazardous constituents
found in contaminated media at former wood preserving sites. Common
contaminants found at former wood preserving sites include: creosote
(one of the polycyclic aromatic hydrocarbons (PAHs)), pentachlorophenol
(PCP), polychlorinated dibenzo-pdioxin and polychlorinated dibenzofurans
(PCDDs/PCDFs) and metals-containing compounds such as chromated
copper arsenate (CCA).
According to EPA, S/S has demonstrated effectiveness in the treatment
of all these contaminants. S/S treats contaminated media (soil,
sludge, sediment) by reducing contaminant mobility in the treated
material. This happens through physical and chemical changes in
the treated material. The use of S/S to immobilize inorganic hazardous
constituents is well accepted. Immobilization through S/S, is the
EPA presumptive remedy for wood preserving sites contaminated with
inorganic contaminants.
Immobilization of organic contaminants by S/S has been increasingly
demonstrated by full-scale remediation projects involving organic
hazardous constituents at former wood preserving sites and manufactured
gas plant sites. These successful projects have included remediation
of media contaminated with nonaqueous phase liquids (NAPLs).
Why use S/S for MGP sites?
Cement-based solidification/stabilization is
increasingly being used at former manufactured gas plant (MGP) sites
since it can treat contaminated media while the media remains in
place. In-place (or in-situ) treatment can be more protective of
human health and the environment than traditional dig and haul remedies.
In-situ treatment minimizes excavation and the associated releases
of volatiles. Putting excavated material on the road to a landfill
poses additional hazards, disturbance, emissions, and inconvenience
to the surrounding community. In-situ S/S treatment is often lower
in cost than other treatment technologies. Recently at a former
MGP site in Milwaukee in-situ S/S was selected to treat the majority
of contaminated media when it was demonstrated that thermal desorption
treatment would be more than twice the cost per unit treated compared
to S/S.
Is roller-compacted concrete
as strong as conventional formed concrete when used in an industrial
application like a compost facility pavement?
Yes, it is. In a concrete mixture, portland
cement chemically reacts with water to bind the aggregate and sand
into a solid mass. A key determining factor to concrete strength
is water to cement ratio. Low water cement ratio concretes generally
have higher strengths. In the placement of conventional concrete
pavement, formwork is used to create the area and thickness of the
pavement design and a concrete mixture is cast (“poured”)
into the forms and finished. Concrete mix designs used for conventional
formed concrete have excess “water of convenience” to
lubricate the mixture making it easier to place and finish within
the formwork.
Assuming no water reducing admixtures are
used, mix designs have lower water cement ratios than conventional
concrete. They have enough water to react with the cement but very
little extra. Whereas internal vibration is typically used to consolidate
conventional concrete, the dry consistency of RCC requires the use
of external compaction methods in the form of heavy steel drum or
pneumatic tire rollers to achieve full consolidation. The result
is that the RCC mix designs can achieve higher strengths than conventional
concrete mixtures using the same amount of cement in the mix. Alternatively,
lower cement proportions can be used to achieve the same strengths
as conventional concrete mixtures, thus providing additional project
savings.
What low-permeability
ranges are possible with cement-bentonite slurry walls?
Permeability is one of the most important factors
in the design of a slurry wall. Both laboratory and field tests
indicate that the permeability of cement-bentonite slurry walls
range from 1X10-6 cm/sec to 1X10-7 cm/sec.
Compatibility with site conditions is also very important. Permeability
tests should be run with the subject site’s groundwater or
leachate.
How can S/S be protective of
human health and the environment if the technology does not lower
the concentration of hazardous constituents in the treated material?
Solidification/stabilization
(S/S) treatment does not generally result in a reduction
of the total concentration of hazardous constituents (contaminants)
in a treated material. S/S protects human health and the environment
by immobilizing hazardous constituents within the treated material.
Protection is achieved by preventing migration of hazardous constituent
to human and environmental receptors.
Contrast that with "dig and dump" remedies
that merely move the hazardous constituents to another place. S/S
fits into a risk-based remedy decision. Further, an in-situ remedy
lowers risks to surrounding communities since less excavation is
involved. S/S can treat a very broad group of hazardous constituents,
both inorganic and organic. Most other remediation treatment technologies
can not. This lowers cost of the remediation of properties. Protective
remedies at a lower cost conserve resources that can be applied
to other sites. More sites remediated means greater overall protection
of human health and the environment.
S/S involves mixing
a binding reagent into contaminated environmental media or waste.
What are the advantages of using portland cement?
Portland cement has advantages that make it
more economical and easier to use than other S/S binding reagents:
- Cement is manufactured under ASTM standards,
ensuring uniformity of quality and performance
- Cement’s success in S/S is supported by
more than 50 years of use in a variety of projects
- Cement has a long-term performance record
- Cement usage can minimize volume increase compared
with other reagents
- Cement is a non-proprietary product—no
premiums to pay for using a “licensed” or “patented”
product
- Cement is manufactured by many companies in the
U.S. and Canada, readily available across the continent in bag
or bulk quantities
How is S/S-treated
material different from conventional concrete?
 In
the field of solidification/stabilization (S/S), some people view
S/S-treated material as having the properties of concrete building
material. This is partially true. The term solidification in S/S
treatment refers to the physical changes in a treated material.
Just as in concrete, portland cement “cements” the other
unconsolidated material in a contaminated material mixture together
into a concreted mass. However, building material concrete is comprised
of selected ingredients. Concrete mix designs use high quality,
properly proportioned gravel, stone and sand selected for their
durability and compressive strength properties.
In S/S treatment, mix designers are saddled
with the properties of the contaminated media that is being treated.
Selection of aggregate material is generally not an option. So while
concrete used as a building material should have unconfined compressive
strengths (UCS) of 4000 psi (28 MPa) or greater, S/S-treated material
usually have UCS performance standards starting at 50 psi (340 kPa).
Tests used to determine physical properties
of S/S treatment such as UCS, durability, and hydraulic conductivity
are adoptions of tests used for testing other less common cement-based
materials such as soil-cement. Brief descriptions of tests used
in S/S treatment are found in an
EPA publication.
What is the volume increase
with S/S applications?
 Volume
increase is often not as great as one would think in field application
of cement-based S/S. It is dependent on several factors including
void space in the untreated waste, water content of untreated waste,
and compaction after treatment.
In Jesse Conner's book Chemical Fixation and Solidification
of Hazardous Wastes, he writes, "In general, volume increase
for cement-based CFS systems equal about 0.4 times the weight increase;
that is, 10 percent volume increase equals 25 percent weight increase
(mix ratio of 0.25) This applies to water saturated wastes only.
Dry ashes and dusts may actually see a volume decrease due to lowered
void space when the material is wetted."
For a graphic illustration of volume increase, see
PCA's In-Situ
Solidification/ Stabilization of a Former Wood Treatment Site.
In the photographs the treated soil columns appear to be about 2
feet higher than the surrounding untreated soil. Mixing depths were
up to 24 feet with 22% S/S reagent addition rate. This is less than
a 10% volume increase.
A 21% cement addition rate used during
In-Situ
Treatment of Refinery Oil Sludge
Basin resulted in a 3% volume increase.
Why is unconfined compressive strength used as
a S/S performance standard?
 Unconfined
compressive strength (UCS) is a commonly used performance standard
for S/S treatment. Compressive strength of a material may be defined
as the measured maximum resistance of a material to axial loading.
Put very simply how much weight can the material support. It is
generally expressed in pounds per square inch (psi) or in the metric
term—megapascals (MPa).
UCS is used as a performance standard in S/S treatment generally
for a couple of reasons. One reason is to ensure that S/S-treated
material has at least as much bearing strength as surrounding material
at the disposal site. So in the case of an in-situ S/S treatment
project the treated material should be as strong as the surrounding
soil or even stronger depending on the planned use of the site.
A second reason for UCS as a performance specification is rooted
in the use of S/S for treatment of waste containing free liquids.
Free liquids are prohibited from land disposal. S/S is often used
to a treat free liquid-containing wastes. U.S. law prohibits the
use of an absorbent to address free liquids, chemical stabilization
must be used instead. EPA recommends a minimum strength of 50 psi
(0.34 MPa) in S/S-treated material in order to demonstrate chemical
stabilization (chemical binding of free liquids) rather than merely
absorption of free liquids. A discussion of the reasoning for the
use of UCS to demonstrate successful treatment of liquid waste is
found on page 2-2 of EPA’s Stabilization/Solidification
of CERCLA and RCRA Wastes.
How does EPA view S/S treatment?
EPA documentation lists S/S treatment as an
established treatment technology. S/S technology falls into the
category of treatment not merely containment. Further, as a treatment,
use of S/S is part of a permanent remedy. The Superfund Amendments
and Reauthorization Act of 1986 (SARA) directed EPA to have a preference
for permanent remedies (that is treatment) over containment or removal
and disposal in remediation of Superfund sites. EPA’s
Treatment
Technologies for Site Cleanup: Annual Status Report,
12th edition, is an excellent resource for data on the trends
in treatment technologies.
Click here for a summary of the S/S-related comments from the
report.
|