BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 152

than is generally achievable in either the passive or semi-passive approach. Active recirculation
systems use conventional injection wells for electron donor delivery and usually also use
injection wells for culture injection (Table 5.2 ).
Active recirculation systems generally pump groundwater continuously. Electron donor is
usually blended with the extracted water prior to its reinjection and can be added continuously
to the extracted water (generally at low concentrations), or it can be pulsed into the extracted
water periodically (generally at higher concentrations). Soluble electron donor frequently is
used in recirculation systems, because it is easiest to mix and pump and allows for the
distribution over the largest distances.
The type and amount of equipment can vary significantly depending on the size of the site,
the desired extraction/injection flow rate, and the amount of automation required. A simple
recirculation system may involve extraction wells, injection wells and temporary conveyance
(i.e., hoses) to transport water between them. More complex systems may have permanent
piping, surge tanks, flow meters, valves, transfer pumps and process instrumentation/controls
such as level switches, alarms and a programmable logic controller (PLC). Amendment dosing
can be accomplished by manually mixing batches at the desired concentration, using propor-
tional flow mixers, or by using metering pumps. An example of a recirculation system is shown
in Figure 5.4 .

5.4.1.1 Advantages/Disadvantages for Biostimulation
In general, recirculation is most appropriate for biostimulation at sites that have moderate
to high hydraulic conductivity. It has been used for biostimulation (with bioaugmentation in
some cases) at many sites (e.g., Lee et al., 2008 ; Ellis et al., 2000 ; Lendvay et al., 2003 ; Major
et al., 2002 ; Hood et al., 2008 ; Wymore et al., 2009 ; Brown et al., 2009 ). The recirculation
approach provides the greatest engineering control for biostimulation because of the ability to
manipulate hydraulic gradients using the injection/extraction system. Compared to passive and
semi-passive approaches, other advantages include:

Rapid onset of reducing conditions because of the use of soluble donors;

Largest electron donor distribution from an individual injection point (i.e., largest
radius of influence during injection); and

Ability to add large amounts of amendments over a relatively short timeframe.
The most significant disadvantage for active recirculation is that it generally has the highest
capital costs and O&M requirements of any approach. Continual system monitoring, either by
automated instrumentation or by onsite staff, is needed to ensure upset conditions are not
encountered and that all above ground equipment is operating as designed. Besides requiring
more O&M, other disadvantages of active recirculation approaches include:

Logistical constraints at active facilities may impact placement of above ground
infrastructure;

Active systems are more prone to biofouling; and

While good donor distribution can be achieved from individual injection points,
multiple recirculation loops may be required to cover larger treatment areas.

5.4.1.2 Implications for Bioaugmentation
Active recirculation systems are costly and rely on frequent pumping. As a result, bioaug-
mentation may be relatively desirable for several reasons: (1) the relative cost of the