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amec�9 <br /> bioaugmentation versus that caused by biostimulation at many field sites (Alvarez and Illman, <br /> 2006). <br /> 2.4 DESIGN CONSIDERATIONS <br /> Bioaugmentation can be implemented by adding microbes to the treatment zone via injection <br /> wells oriented in a grid like pattern for area-wide treatment, or distributed along an alignment <br /> to create a permeable treatment "bio-curtain". Bio-curtains are intended to function as a <br /> continuous zone of biodegradation that spans the plume width to treat groundwater flowing <br /> across the bio-curtain alignment. Key considerations for bioaugmentation are: <br /> • Target in situ microbial concentration , <br /> • Delivery method, <br /> • Injection and extraction well spacing, and <br /> • Geochemistry of ambient groundwater conditions. <br /> These are discussed below. <br /> 2.4.1 Target In Situ Microbial Concentration <br /> The rate of VOC destruction increases with the number of microbes, and therefore microbial <br /> concentration in the subsurface is an important design parameter. Although there is little <br /> guidance on this topic in the literature, recent practice suggests that in situ concentrations of <br /> 104 cells per milliliter (cells/mL) are required to sustain treatment (Steffan and Vainberg, 2010). <br /> This concentration has been adopted as the target concentration for implementing <br /> bioaugmentation in the WWTP area and forms the basis for the conceptual bioaugmentation <br /> system design. During bioaugmentation, relatively small volumes (usually tens of liters) of <br /> concentrated aqueous suspensions containing microbes at concentrations of 109— 1012 <br /> cells/mL are mixed with a larger volume of an aqueous nutrient solution prior to injection into <br /> the target treatment zone (Alvarez and Illman, 2006). The addition of nutrient solution results <br /> in a dilution of the concentrated microbial suspension and is intended to promote cell growth <br /> after injection. <br /> 2.4.2 Delivery Method <br /> Two different methods are often used for delivery of substrate and microbes to subsurface <br /> treatment zones during bioaugmentation: <br /> 1. Direct injection: substrates (and/or microbes) is injected through temporary casing (e.g. <br /> drill rods) or injection wells at low pressure (gravity feed) or higher pressure (hydraulic <br /> and pneumatic fracturing), and <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTFS Assessment_Apx B_01 2711\Attachment B.2\Attach B-2.doc 132-5 <br />