xvEPA
Air/Superfund
Coordination Program
United States
Environmental
Protection Agency
Office of
Air Quality Planning
and Standards
September 1989
EPA-450/1-89-017
SOIL VAPOR EXTRACTION VOC CONTROL
TECHNOLOGY ASSESSMENT
The Soil Vapor Extraction (SVE) Process
Typically, a SVE system consists of extrac-
tion .wells, inlet wells (optional), piping head-
ers, vacuum pumps, flow meters, vacuum
gauges, sampling ports, an air/water separa-
tor (optional), a volatile organic compound
(VOC) control system (optional), and a cap
(optional). Extraction wells are usually de-
signed to fully penetrate the contaminated
soil. These wells are normally constructed of
slotted, plastic pipe placed in permeable pack-
ing to allow vapor flow into the pipe. The VOC
in the contaminated area migrate through the
soil into the pipe where it is either released to
the atmosphere or vented to an air pollution
control device. The decision to employ an air
pollution control device is usually dependent
upon the expected VOC concentration and
applicable regulations. The VOC compounds
migrating to the extraction wells will typically
be the lighter and middle fraction compounds
with molecular weights up to approximately
200 g/mol or organic compounds containing
up to 12 to 15 carbon atoms. Heavier com-
pounds have more of a tendency to remain in
the soil unless the vacuum is increased.
SVE Operation
The operation of an SVE system is relatively simple. The blower (vacuum pump) and other nec-
essary equipment are turned on and the flow comes to equilibrium. The steady state flow rate reached
for a given system is usually a function of the equipment, flow control devices, system geometry, soil
permeability, and site characteristics.
The blower provides reduced pressure in the extraction wells and induces airflow into any inlet wells
present. If injection wells are employed, the discharge pressure from the vacuum pump (or after the
VOC treatment device if present) is used to inject air into the wells. The reduced pressure in the
extraction wells (combined with net airflow from inlet or injection wells if present) is sufficient to
volatilize a large number of organic compounds and induce VOC migration to the extraction wells. At
the extraction wells, compounds pass through the permeable membrane within the pipe and into the
well itself where the VOCs are drawn out of the soil, and towards the vacuum pump.
In some cases, an air/water separator is employed prior to the vacuum pump, to prolong the system
life and increase the efficiency of any VOC treatment system present. In addition, a nonpermeable
cap is often placed over the contaminated area to prevent fugitive VOC migration out of the soil, and
promote movement towards the extraction wells.
-------
The exhaust air from the vacuum pump is sampled on a routine basis and used in conjunction with
flow rate measurements to determine the VOC extraction rate and total amount of VOC extracted from
the site. Typically, the extraction rate is initially high and gradually decreases overtime. In the latter
stages of an extraction operation, the blower is often cycled to conserve energy. This is typically done
as follows: the blower is turned on, flows come to equilibrium and the extraction rate is measured. After
a period of time has elapsed, the extraction rate is again mea-
sured. If the rate has decreased appreciably, the blower is turned off and the site is allowed to settle.
After settling, the process is repeated. This procedure is employed because in the latter stages of
operation (i.e., after the initial extraction rate has decreased appreciably), the VOC extraction rate be-
comes diffusion limited by soil moisture, and is not a function of the vacuum applied to the extraction
well.
EMISSION CONTROLS
• carbon adsorbers
• thermal incinerators
• catalytic incinerators
• condensers
Project Officer
Mr. Norm Huey
U.S. EPA Region VIII
999 18th Street
Suite 500
Denver, Colorado 80202
(303)293-1760
WHERE CAN I OBTAIN COPIES OF THIS VOLUME?
Office of Air Quality Planning and Standards
Joe Padgett
919-541-5589 (FTS 629-5589) .
or
National Technical Information Service
5285 Port. Royal Road
Springfield, Virginia 22161
(703) 487-4650
------- |