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Vurdering af stødvis ventilering og
pneumatisk opsprækning
This report was prepared for the Danish Environmental Protection Agency in 1997 by HOH
Vand & Miljø A/S (previously Kemp & Lauritzen Vand & Miljø A/S). It
describes and evaluates two new in-situ remediation techniques, pulsed venting and
pneumatic fracturing. In addition, the report provides an overview of foreign and Danish
projects which have made use of these techniques, and evaluates the need for testing the
methods under Danish conditions.
The report is limited to a literature review and to a discussion of use of the
techniques in the vadose zone. No laboratory or field testing was carried out.
The two techniques of interest have been developed since approximately 1990 and are
used to remediate landfills or contaminated soil. Remediation occurs through the
introduction of air or oxygen-enriched air into the ground in short bursts. This replaces
existing contaminated soil gas, provides an aerobic environment for the improvement of
biodegradation rates and creates and expands fractures in soil and bedrock, if adequate
pressures are used. The techniques are suitable for remediation of organic contaminants
which are biodegradable or volatile.
The majority of activity regarding the techniques are connected to specific companies
and patented equipment and/or methods. These companies include AGA AB, Sweden (pulsed
venting) and Accutech Remedial Systems, Inc. (pneumatic fracturing).
Pulsed venting
The three major applications of pulsed venting are: 1) odour stabilization/explosion
risk reduction in landfills prior to excavation and waste sorting, 2) acceleration of
biodegradation of landfill wastes and 3) bioventing contaminated soils. Operation times
for these three applications are 10-14 days, 3-8 months and individual from case to case,
respectively.
The remediation process involves the injection of air via specially constructed
pressure lances concurrent with the continuous removal of displaced soil gas via normal
vacuum extraction wells in the proximity. Air injection occurs via individual small
pressurized containers (2-10 bars) placed on top of each pressure lance. The air is
injected all at once in a shock-like manner through a fast-working valve. This pulse is
repeated in each lance at suitable intervals (seconds to minutes).
A description of the theory of air flow during pulsed venting was not found in the
literature.
There is currently only one site in Denmark where pulsed venting is used. The site is a
former service station located in the town of Bredsten. Here, diesel contamination is
currently being remediated by two in-jection lances surrounded by 7 vacuum extraction
wells. Monitoring at the site has not produced data which can support or refute claims of
improved clean-up with pulsed venting as compared to normal bioventing.
Considerable foreign experience with the method has been achieved in Austria (where the
method was developed) and Germany. Here, the main application has been odour
stabilization/explosion risk reduction at landfills prior to excavation and sorting.
Numerous large landfills have been successfully excavated using this method resulting in
waste volume reduction. Application of the technique for accelerated biodegradation of
landfill wastes and bioventing of contaminated soils has been attempted at very few
foreign sites.
The most promising applications for pulsed venting in regard to the Danish law no. 939
of October 1996 on contaminated sites (lovbe- kendtgørelse om affaldsdepoter) is judged
to be accelerated biodegradation of landfill wastes and bioventing contaminated soils.
The price of odour stabilization/explosion risk reduction is reported to be approx.
5-10 US dollars pr. ton waste. Prices for the other applications were not available.
Two recommendations for testing pulsed venting under Danish con- ditions are set forth
at the conclusion of the report: 1) pilot or full-scale accelerated biodegradation of
landfill wastes and 2) field studies to compare air flow patterns of pulsed venting contra
continuous bioventing in inhomogeneous soils.
Pneumatic fracturing
The three major applications of pneumatic fracturing are 1) improving the results of
soil vapor extraction or bioventing projects by increasing the permeability of the soil,
2) accelerating removal of separate phase hydrocarbons, 3) dewatering perched water
tables. In addition, pneumatic fracturing can improve results of pump-and-treat in the
saturated zone. The actual process of pneumatic fracturing requires typically a period of
several days. Potential savings in operation times of the following in-situ clean up is
difficult to quantify.
The fracturing process is based on the injection of short pulses (under one minute) of
air under high pressure in specially constructed wells. The pulse is repeated at various
depths in the same well (each pulse is applied on a screen interval of about 1 meter) as
well as in other wells to ensure the desired soil volume is treated. A high initial
pressure (often about 5 bars) must be used to overcome cohesion and the mass of the
overlaying layers. After initiation, an air flow which exceeds the permeability of the
soil layer is used to extend the fractures away from the injection well. At the conclusion
of the air injection, partial closure of the fractures occurs.
The reported theory of air flow is based on flow in open fractures rather than porous
media. Here, the cubic law - which states that the air flow is proportional to the cube of
the fracture size - is the theoretical basis. It is claimed that this is the explanation
for the greatly increased air flows made possible by pneumatic fracturing.
To date, no in-situ remediation projects in Denmark have used pneumatic fracturing.
Full-scale application of pneumatic fracturing for in-situ remediations has been
extensive in the United States. Improved permeability of the fractured soils or bedrock
has been adequately documented. Following the fracturing a variety of techniques have been
applied, such as soil vapor extraction, bioventing and thermal treatment. In addition,
techniques for the saturated zone such as removal of free-phase hydrocarbon or air
sparging have been investigated.
Various calculations of prices are to be found in the literature. Expenses for renting
fracturing and monitoring equipment for the necessary two weeks have been estimated at
28,000 U.S. dollars and an additional 16,000 for labor. The price pr. ton fractured soil
(in excess of the primary remediation) is estimated at 4 - 10 U.S. dollars.
Due to the fact that many contaminated sites in Denmark are located in areas with
low permeable soils, it is deemed that pneumatic fracturing is a relevant technique. Since
the technique is well-documented in the United States, it is recommended that a full-scale
test be carried out under Danish conditions.
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