Question

9.20) A groundwater aquifer is contaminated with benzene. Contrast the relative effectiveness of air stripping to activated carbon adsorption by providing a preliminary design of each process.

Answer 1

Answer=

Given Contaminant: Benzene
Water solubility - 1780 mg/L, M.W. - 78.11, Melting point - 5.5°C, Boiling point - 80.1°C,
vapor pressure - 95.2mm @25°C, vapor density - 2.77, specific gravity - 0.8786 @20/4°C,
Chemical formula C6H6, Diffusion coefficient - 0.09234 cm2
/s
Constants for Henry's constant: A = 5.53, B = 3190, Koc = 83 ml/g.
Data from Appendix A, page 1040-41 of text book and from Verschueren, Karel: Handbook of Environmental Data
on Organic Chemicals - Second edition, 2nd edition Van Nostrand Reinhold Co., Inc., New York, NY
Solution:
Assumption: The groundwater is saturated with contaminant at the source.
As no information is given regarding the concentration of the contaminant, One approach would be to consider the
water at the source to be saturated with benzene. A second possible assumption that the level is below saturation,
say 150 mg/L.
From the above properties it is observed that the water solubility of benzene is 1780 mg/L.
In order to obtain a detailed information regarding the extent of the plume, first the site history and geology would
have to be reviewed. Both indirect and direct methods are employed to investigate subsurface conditions.
Indirect methods include different geophysical methods like electromagnetic induction, earth resistivity, ground
penetrating radar, magnetic techniques and seismic refraction and reflection. These geophysical techniques are
employed to decut the parameters of interest. Direct methods are employed to verify the interpretations of
geophysical methods. Boring, sampling and testing are direct field methods of subsurface exploration. These
techniques includes installation of ground water monitoring wells.
Soil gas surveys are performed to investigate the distribution and transportation of contaminants in the subsurface
zone. Soil gas surveys establish a vapor phase plume in the void space of the soil in the vadose zone. This
information can then be interpreted in the context of source detection and plume delineation. They have been used
as a low cost alternative to ground water sampling and testing to assess the nature of the ground water plume.
Sampling is done and the gas is analyzed by using different instruments like organic vapor analyzer, a
photoionization detected and gas chromatography.

Considering that the water is saturated with benzene at source with 1780 mg/L, the concentration is too high to use
air stripping. (Air stripping is not effective for concentrations greater than 200 mg/L.) Therefore carbon adsorption
would be the more suitable method for treatment where the concentrations is greater than 200 mg/L.
Comparison of two treatment methods
Consider a point where the concentration of benzene is 150 mg/L and the concentration has to be reduced to 130
µg/L for discharge to a POTW. (Note: another reasonable value for the effluent requirement would be 5 µg/L, the
maximum contaminant level (MCL) under the safe drinking water act. This lower limit would be difficult to achieve
by either method.
AIR STRIPPING:
The following data is obtaiConsidering that the water is saturated with benzene at source with 1780 mg/L, the concentration is too high to use
air stripping. (Air stripping is not effective for concentrations greater than 200 mg/L.) Therefore carbon adsorption
would be the more suitable method for treatment where the concentrations is greater than 200 mg/L.
Comparison of two treatment methods
Consider a point where the concentration of benzene is 150 mg/L and the concentration has to be reduced to 130
µg/L for discharge to a POTW. (Note: another reasonable value for the effluent requirement would be 5 µg/L, the
maximum contaminant level (MCL) under the safe drinking water act. This lower limit would be difficult to achieve
by either method.