Page 260 - Materials Chemistry, Second Edition

Groundwater Remediation 243

Step 2: If the stripping factor is known or selected, determine the air-
to-water ratio by using Equation (6.20). Go to Step 4.
Step 3: If the stripping factor is not known or selected, determine the
minimum air-to-water ratio by using Equation (6.19). Obtain the
design air-to-water ratio by multiplying this minimum air-to-
water ratio with a value between 2 to 10. Go to Step 4.
Step 4: Determine the required air flow rate by multiplying the liquid flow
rate with the air-to-water ratio determined from Step 2 or Step 3.

Information needed for this calculation:

• Henry’s constant
• Stripping factor, S
• Design liquid flow rate, Q

Example 6.11: Determine the Air-to-Water Ratio for an Air Stripper
A packed-column air stripper is designed to reduce the chloroform concen-
tration in the extracted groundwater. The concentration is to be reduced
from 50 mg/L to 0.05 mg/L (50 ppb). Determine (1) the minimum air-to-
water ratio, (2) the design air-to-water ratio, and (3) the design air flow rate.
Use the following information in calculations:

• Henry’s constant for chloroform = 128 atm
• Stripping factor = 3
• Temperature of the water = 15°C
• Extracted groundwater flow rate = 120 gpm

Solution:
(a) Use the formula in Table 2.4 to convert the Henry’s constant to its
dimensionless value:

*
*
HRT(1000 γ) H (0.082)(273 + 15)(1000)(1)
H = = = 128
W 18
So, H* = 0.098 (dimensionless)
(b) Use Equation (6.19) to determine the minimum air- to- water-ratio:

H *  Q a   ==1(0.098)  Q a  


 Q w  min  Q w  min
So, (Q /Q ) = 10.25 (dimensionless)
a
w min