Page 215 - Materials Chemistry, Second Edition

198 Practical Design Calculations for Groundwater and Soil Remediation

Discussion:
1. The amount of available oxygen in the soil moisture, 1.08 g/m soil,
3
is much smaller than that in the air void, 78.1 g/m .
3
2. It would need at least 255 (i.e., 27,720/78.1) void volumes of fresh
air to supply sufficient oxygen for complete biodegradation. The
minimum fresh air requirement = (255)(V air void ) = (255)(280 L/m
3
soil) = 71,400 L/m soil = 71.4 m fresh air/m soil.
3
3
3

5.5 Bioventing

5.5.1 Description of the Bioventing Process
Bioventing is an in situ soil-remediation technique that uses indigenous
microorganisms to biodegrade organic COCs in a subsurface. In biovent-
ing, fresh air is induced into the impacted zone using extraction or injection
wells. Oxygen in the air will promote aerobic biodegradation of organic
COCs. All aerobically biodegradable COCs can be treated by bioventing.
Bioventing is most often used at sites with petroleum products heavier
than gasoline (e.g., diesel and jet fuel), while gasoline tends to volatilize
readily and can be removed more quickly using soil vapor extraction [7].

5.5.2 Design of the Bioventing Process

A bioventing system is very similar to a soil vapor extraction (SVE) system. It
may include vapor extraction well(s), vacuum blower(s), a moisture removal
device (the knockout drum), off-gas collection piping and ancillary equip-
ment, and the off-gas treatment system. The main difference between bio-
venting and SVE is that bioventing promotes biodegradation of COCs and
minimizes volatilization. Generally, bioventing uses lower air-flow rates
than SVE. The main objectives of the induced air flow are to provide oxygen
to promote the biological activities and to carry away metabolic products.
The vapor extraction can also be applied intermittently, instead of on a con-
tinuous basis, just to provide oxygen to the subsurface. If necessary, nutrient
should also be added to the subsurface.
The extent of biological activities can be assessed by determining the car-
bon dioxide concentration in the extracted air. Excluding the background
concentration, carbon dioxide should come from biodegradation of the
organic COCs.

Example 5.26: Determine the Efficiency of Bioventing

Bioventing is used to remediate a site impacted by diesel fuel. The average
concentrations of TPH and CO in the recently extracted air samples are
2