Page 274 - Materials Chemistry, Second Edition

Groundwater Remediation 257

Many studies have been conducted on oxygen-transfer efficiencies in aera-
tion of water and wastewater treatment, but little information is available
with regard to air sparging of aquifers impacted by organics. The oxygen-
transfer efficiencies should depend on factors such as injection pressure, the
depth of the injection point in the aquifer, and characteristics of the geological
formation, to name a few.

Example 6.17: Determine the Rate of Oxygen Addition by Air Sparging
Three air-sparging wells were installed into the plume of an aquifer
impacted by hydrocarbons. The injection air flow rate into each well is 5 ft /
3
min. Assuming the oxygen-transfer efficiency is 10%, determine the rate of
oxygen addition to the aquifer through each sparging well. What would be
the equivalent injection rate of water that is saturated with air?

Solution:
(a) The oxygen concentration in the ambient air is approximately 21%
by volume, which is equal to 210,000 ppmV. Equations (2.1) or (2.2)
can be used to convert it to a mass concentration:

MW
3
°
1ppmV = × 10 −6 [lb/ft ]at68F
385
32
−6
= ×10 −6 = 0.083 × 10 lb/ft 3 (2.2)
385
Therefore, 210,000 ppmV = (210,000)(0.083 × 10 ) = 0.0175 lb/ft 3
−6
(b) The rate of oxygen injected in each well = (G)(Q)
3
3
= (0.0175 lb/ft )(5 ft /min) = 0.0875 lb/min = 126 lb/day
The rate of oxygen dissolved into the plume through air injection
in each well (using Equation 6.30) = (126 lb/day)(10%) = 12.6
lb/day

(c) The dissolved-oxygen concentration of the air-saturated reinjec-
tion water is approximately 9 mg/L at 20°C. The required water-
reinjection rate to supply 1.26 lb/day of oxygen can be found as:
12.6 lb/day = (12.6 lb/day)(454,000 mg/lb) = (Q)(C) = (Q)(9 mg/L)
Thus, Q = 635,600 L/day = 116.6 gpm

Discussion:
1. The oxygen-transfer efficiency of 10% means that only 10% of the
total oxygen sparged into the aquifer will be dissolved into the
aquifer. But, 90% of the oxygen injected can serve as the oxygen
source for bioremediation in the vadose zone.