Page 207 - Materials Chemistry, Second Edition

190 Practical Design Calculations for Groundwater and Soil Remediation

Example 5.21: Determine the Efficiency of Soil Washing
(Two Reactors in Series)
The single reactor described in Example 5.20 could not reduce the 1,2-
DCA concentration to below 10 mg/L. An engineer proposed using two
smaller washers in series. The washer still accommodates 1,000 kg of soil,
but only 500 gal of fresh water is added to each washer. Can this system meet
the cleanup requirements?

Solution:
Use Equation (5.31) to find the final concentration for two reactors in
series as (V = V = 500 gal = 1,893 L):
l,2
l,1
1 1
X final = × × X initial
1 ( M s,dryp) 1 ( M s,dryp)
+
+
V l,1
V l,2
K
K
1 1
= 1 ( (889)(0.11)) × 1 ( (889)(0.11)) × 500 = 1.2 mg/L
1,893
1,893
+ +
Discussion:
1. In both cases, the same amount of water, 1,000 gal, is used for
1,000 kg of soil. However, use of two small reactors in series
yields a lower final concentration.
2. The calculated values are based on an assumption that the liquid
and the soil are in equilibrium. For a practical reactor design, an
equilibrium condition is seldom reached. Consequently, the actual
final concentration would be higher.

5.4 Soil Bioremediation
5.4.1 Description of the Soil-Bioremediation Process
Soil bioremediation utilizes microorganisms or their metabolic products to
degrade organic COCs in soil. Soil bioremediation can be conducted under
aerobic or anaerobic conditions, but aerobic bioremediation is more popular.
The final products of complete aerobic biodegradation of hydrocarbons are
carbon dioxide and water.
Bioremediation may be conducted either in situ or ex situ. Ex situ soil-bioreme-
diation processes are more developed and widely used than in situ processes.
Ex situ bioremediation is typically performed using one of three systems: (1)
static soil pile, (2) in-vessel, and (3) slurry bioreactor. The static soil pile is the