Page 267 - Materials Chemistry, Second Edition

250 Practical Design Calculations for Groundwater and Soil Remediation

(d) Assuming the packing material only occupies a small fraction
of the total reactor volume, the hydraulic retention time can be
estimated by:
hydraulic residence time = (V/Q) = (Ah)/Q
= (47.2 ft )(3 ft) ÷ [20 gpm/(1 ft /7.48 gal)] = 53 min
3
2
Discussion:
It is relatively difficult for the effluent of a bioreactor to meet the dis-
charge limit in the ppb level. Activated-carbon adsorbers may be
needed as a polisher to treat the effluent of the bioreactor before
discharge.

6.6 In Situ Groundwater Remediation

6.6.1 Description of the In Situ Bioremediation Process
Biological in situ treatment of organic COCs in aquifers is usually accom-
plished by enhancing activities of indigenous subsurface microorgan-
isms. Most of the in situ bioremediation is practiced in the aerobic mode.
The microbial activities are enhanced by addition of inorganic nutri-
ents and oxygen into the groundwater plume. The typical process con-
sists of withdrawal of groundwater, addition of oxygen and nutrients,
and reinjection of the enriched groundwater through injection wells or
infiltration galleries. In addition to extraction of groundwater, addition
of oxygen-releasing compounds (ORCs) to the plume can also be used
instead.

6.6.2 Addition of Oxygen to Enhance Biodegradation
Groundwater naturally contains low levels of dissolved oxygen (DO).
Even if it is fully saturated with air, the saturated dissolved oxygen
(DO ) concentration in groundwater would only be in the neighborhood
sat
of 9 mg/L at 20°C. Biodegradation of organic COCs in the plume may
need much more oxygen than those dissolved-oxygen molecules present
in groundwater.
Addition of oxygen to the groundwater can be done by air sparging or
pure oxygen sparging. The oxygen in the injected air can raise the DO to its
saturation level of 8 to 10 mg/L. With pure oxygen injection, the DO con-
centrations of up to 40 to 50 mg/L can be achieved. DO level in the water
can also be raised by addition of chemicals such as hydrogen peroxide and