Page 169 - Materials Chemistry, Second Edition

152 Practical Design Calculations for Groundwater and Soil Remediation

dissolved and desorption of the adsorbed COCs; (2) provide oxygen to indig-
enous microorganisms for biodegradation of the COCs; and (3) carry away
the toxic metabolic by-products generated from the biodegradation process.
The extracted air is usually laden with VOCs and brought to the ground
surface by the vacuum blower. Treatment of the extracted vapor is normally
required before being released to the ambient air. Design calculations for the
VOC-laden air treatment are covered in Chapter 7.
Major components of a typical soil-venting system include vapor extrac-
tion well(s), vacuum blower(s), moisture-removal device (the knockout
drum), off-gas collection piping and ancillary equipment, and the off-gas
treatment system. The most important parameters for preliminary design
of a soil-venting system are the extracted VOC concentration, air flow rate,
radius of influence of the venting well, number of wells required, locations
of the wells, and the size of the vacuum pump.

5.2.2 Expected Vapor Concentration

As mentioned in Section 2.4, volatile organic COCs in a vadose zone may be
present in four phases: (1) in the soil moisture due to dissolution, (2) on the
soil grain surface due to adsorption, (3) in the pore void due to volatilization,
and (4) as the free product. If the free-product phase is present, the vapor
concentration in the pore void can be estimated from Raoult’s law as:
P A = P ( vap x ) (5.1)
)( A

where
P A = partial pressure of compound A in the vapor phase
P vap = vapor pressure of compound A as a pure liquid
x = mole fraction of compound A in the liquid phase
A
Examples using Raoult’s law can be found in Section 2.4. The partial pres-
sure calculated from Equation (5.1) represents the upper limit of the COC
concentration in the extracted vapor from a soil-venting project. The actual
concentration will be lower than this upper limit because (1) not all the
extracted air passes through the impacted zone, and (2) limitations on mass
transfer exist. Nevertheless, this concentration serves as a starting point for
estimating the initial vapor concentration at the beginning of a soil-venting
project. Initially, the extracted vapor concentrations will be relatively con-
stant if free product is present. As soil venting continues, the free-product
phase will disappear. The extracted vapor concentration will then begin to
drop, and the extracted vapor concentration will become dependent on the
partitioning of the COCs among the three phases. As the air flows through
the pores and sweeps away the COCs, the COCs dissolved in the soil