Page 284 - Materials Chemistry, Second Edition

Groundwater Remediation 267

where C is the COC concentration in groundwater, V is the reactor volume,
Q is the groundwater flow rate, k is the rate constant, and τ is the hydraulic
retention time.
Another design approach is to use electrical energy per order of destruction
(EE/O) to scale up an AOP reactor. An EE/O of 5 kWh/1,000 gal/order of COC
destruction means that it will take 5 kWh of energy to reduce the COC concen-
tration from 1 ppm to 0.1 ppm in 1,000 gal of groundwater. It will take another 5
kWh of energy to reduce the concentration from 0.1 ppm to 0.01 ppm. It should
be noted that the value of EE/O is specific to the groundwater and COCs treated.

Example 6.22: Sizing the Reactor for an Advanced Oxidation Process
UV/ozone treatment is selected to remove trichloroethylene (TCE) from an
extracted groundwater stream (TCE concentration = 400 ppb). A pilot study
was conducted and found that, with a hydraulic retention of 2 min, the sys-
tem could reduce TCE concentration from 400 ppb to 16 ppb. However, the
discharge limit for TCE is 3.2 ppb. Assuming the reactors are of ideal plug-
flow type and the reaction is first-order, how many reactors would you rec-
ommend to use?

Solution:
(a) Use Equation (6.39) to determine the reaction rate constant:
16
C out
= exp[ − τ=k()] = exp[ − k2]
C in 400
So, k = 1.61/min

(b) Use Equation (6.39) to determine the required retention time to
reduce the TCE concentration below the discharge limit:
3.2
τ
C out
= exp[ − τ= = exp[ −1.61()]
k()]
C in 400
τ = 3.0 min. Thus, it requires two reactors.
(c) Use Equation (6.39) to determine the final effluent TCE concen-
tration (τ = 4 min because two reactors were used):

C out C out
k
= exp[ − τ=()] = exp[ −1.61(4)]
C in 400
C = 0.64 ppb
out
Discussion:
1. For PFRs, the final concentration from two identical reactors in
series is the same as that from two identical reactors in parallel.