Practical Design Calculations for Groundwater and Soil Remediation - Chapter 5

Kuo, Jeff "Vadose zone soil remediation" Practical Design Calculations for Groundwater and Soil Remediation Boca Raton: CRC Press LLC,1999 chapter five Vadose zone soil remediation This chapter illustrates important design calculations for commonly used in situ and above-ground soil remediation techniques. The treatment processes covered include soil vapor extraction, soil bioremediation, soil washing, and low-temperature heating. V.1 Soil vapor extraction V.1.1 Introduction Description of the soil venting process Soil vapor extraction (SVE), also known as soil venting, in situ vacuum extraction, in situ volatilization, or soil vapor stripping, has become a very popular remediation technique for soil contaminated with VOCs. The process strips volatile organic constituents from contaminated soil by inducing an air flow through the contaminated zone. The air flow is created by a vacuum pump (often called a “blower”) through a single well or network of wells. As the soil vapor is swept away from the voids of the vadose zone, fresh air is naturally (through passive venting wells or air infiltration) or mechan-ically (through air injection wells) introduced and refills the voids. This flux of the fresh air will (1) disrupt the existing partition of the contaminants among the void, soil moisture, and soil grain surface by promoting volatil-ization of the adsorbed and dissolved phase of contaminants, (2) provide oxygen to indigenous microorganisms for biodegradation of the contami-nants, 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. Design calculations for the VOC-laden air treatment are covered in Chapter seven. ©1999 CRC Press LLC Major components of an SVE system Major components of a typical soil venting system include vapor extraction well(s), vacuum blower(s), moisture removal device (knock-out drum), off-gas collection piping and ancillary equipment, and the off-gas treatment system. Important design considerations The most important parameters for preliminary design are the extracted VOC concentration, air flow rate, radius of influence of the venting well, number of wells required, and size of the vacuum blower. V.1.2 Expected vapor concentration As mentioned in Section II.3.5, volatile organic contaminants in a vadose zone may be present in four phases: (1) in the soil moisture due to dissolu-tion, (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 = (Pvap )(xA ) [Eq. V.1.1] where P = partial pressure of compound A in the vapor phase, Pvap = vapor pressure of compound A as a pure liquid, and x = mole fraction of com-pound A in the liquid phase. Examples using Raoult’s law can be found in Section II.3. The partial pressure calculated from Eq. V.1.1 represents the upper limit of the contam-inant 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 contaminated 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 venting project. Initially the extracted vapor concentrations will be relatively constant. 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 con-taminants among the three other phases. As the air flows through the pores and sweeps away the contaminants, the contaminants dissolved in the soil moisture will volatilize from the liquid into the void. Simultaneously, the contaminants will also desorb from the soil grain surface and enter into the soil moisture (assuming the soil grains are covered by a moisture layer). Thus, the concentrations in all three phases decrease as the venting process progresses. ©1999 CRC Press LLC Table V.1.A Physical Properties of Gasoline and Weathered Gasoline Molecular weight Compound (g/mole) Pvap @ 20°C (atm) Gest ppmV mg/L Gasoline 95 Weathered gasoline 111 0.34 340,000 1343 0.049 49,000 220 Modified from Johnson, P. C., Stanley, C. C., Kemblowski, M. W., Byers, D. L., and Colthart, J. D., Ground Water Monitor. Rev., Spring, 1990b. With permission. The above phenomenon describes common observations at sites that contain a single type of contaminant. Soil venting has also been widely used for sites contaminated with a mixture of compounds, such as gasoline. For these cases, the vapor concentration decreases continuously from the start of venting; a period of constant vapor concentration in the beginning phase of the project does not exist. This can be explained by the fact that each compound in the mixture has a different vapor pressure. Thus, the more volatile compounds tend to leave the free product, as well as the moisture and the soil surface, earlier and be extracted earlier. Table V.1.A shows the molecular weights of fresh and weathered gasoline and their vapor pressures at 20°C. The table also lists the saturated vapor concentrations that are in equilibrium with the fresh and weathered gasoline. Toestimate the initial concentration of the extracted vapor in equilibrium with the free-product phase, the following procedure can be used: Step 1: Obtain the vapor pressure data of the compound of concern (e.g., from Table II.3.C). Step 2: Determine the mole fraction of the compound in the free prod-uct. For a pure compound, set x = 1. For a mixture, follow the procedure in Section II.1.4. Step 3: Use Eq. V.1.1 to determine the vapor concentration in atm or mmHg unit. Step 4: Convert the concentration by volume into a mass concentration, if needed, by using Eq. II.1.1. Information needed for this calculation • Vapor pressure of the contaminant • Molecular weight of the compounds Example V.1.2A Estimate the saturated gasoline vapor concentration Use the information in Table V.1.A to estimate the maximum gasoline vapor concentration from two soil venting projects. Both sites are contaminated from accidental gasoline spills. The spill at the first site happened recently, while the spill at the other site occurred 3 years ago. ©1999 CRC Press LLC Solution: a. The site with fresh gasoline. Vapor pressure of fresh gasoline is 0.34 atm at 20°C, as shown in Table V.1.A. The partial pressure of this gasoline in the pore space can be found by using Eq. V.1.1. as: PA = (Pvap)(xA) = (0.34 atm)(1.0) = 0.34 atm Thus, the partial pressure of gasoline in the air is 0.34 atm (= 340,000 ´ 10–6 atm), which is equivalent to 340,000 ppmV. Use Eq. II.1.1 to convert the ppmV concentration into a mass concentration unit (at 20°C), as 1 ppmV fresh gasoline = {(MW of fresh gasoline)/24.05} mg/m3 = (95)/24.05 = 3.95 mg/m3 So, 340,000 ppmV = (340,000)(3.95) = 1,343,000 mg/m3 = 1343 mg/L b. The site with weathered gasoline. Vapor pressure (as well as the partial pressure in this case) of weathered gasoline is 0.049 atm, which is equivalent to 49,000 ppmV. Use Eq. II.1.1 to convert the ppmV concentration into a mass concentration unit (at 20°C), as 1 ppmV weathered gasoline = {(MW of weathered gasoline)/24.05} mg/m3 = (111)/24.05 = 4.62 mg/m3 So, 49,000 ppmV = (49,000)(4.62) = 226,000 mg/m3 = 226 mg/L Discussion 1. The saturated vapor concentration of the weathered gasoline can be a few times less than that of the fresh gasoline. (In this case, it is more than five times smaller.) 2. The calculated vapor concentrations are essentially the same as those listed in Table V.1.A. 3. Although gasoline is a mixture of compounds, the mole fraction was set to one since the vapor pressure and molecular weight of gasoline were given as the weighted averages. Example V.1.2B Estimate saturated vapor concentrations of a binary mixture A site is contaminated with an industrial solvent. The solvent consists of 50% toluene and 50% xylenes by weight. Soil venting is considered for site ©1999 CRC Press LLC ... - tailieumienphi.vn