Air stripping volatiles from aquifers

Removing a floating volatile organic component (VOC) from the surface of an aquifer by vaporization into a flowing air stream was the subject of investigation. The research objectives were to (1) demonstrate the workability of an idealized aquifer, (2) determine the extent that VOCs could be removed from such a system, (3) identify factors that influenced the removal of the VOCs, and (4) develop a mathematical model to describe the volatilization process. The factors that were investigated included volatility of the VOCs, flow rate, equilibration period of the VOC before commencement of removal operations, use of vented pipe versus solid pipe for gas extraction, and reproducibility of the runs.

The idealized aquifer was simulated using a 50-gal tank containing a layer of sand that rested on a 4.45-cm layer of 3-mm beads. The beads were used to simulate the aquifer region. Isopentane or hexane was placed on the bottom of the tank to ideally represent a floating layer of VOC. Air flow through the bead area was introduced by using a vacuum that pulled centrally on the system. Discrete samples were taken periodically of the eluting vapors and examined by gas chromatograph.

The tank system performed well as an ideal aquifer, producing highly reproducible results. The VOCs removed increased with increasing gas velocity through the simulated aquifer region. The most efficient stripping of VOC from the total system of sand and beads occurred when a perforated vacuum pipe rather than a solid pipe was used.

A mathematical model based on the evaporation of VOCs into the flowing airstream was developed. The predictive equation for the mass-transfer coefficient was developed by modifying an analogous expression from heat transfer. The mathematical model developed in this activity agreed well with the experimental data.