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Effluent Collection System

Media

Figure 21.1 Submerged attached growth process. (Example shown is uptlow. Reverse direction of arrows for downflow.)

Media

Influent Distribution System

Figure 21.1 Submerged attached growth process. (Example shown is uptlow. Reverse direction of arrows for downflow.)

uents in the wastewater and the environmental conditions provided in the bioreactor. Flow may be upward, downward, or horizontally across the media, and a wide variety of media types can be used. Table 21.1 summarizes the media characteristics and operating conditions for the SAGBs considered in this chapter. As indicated there, the media may be either packed or fluidized. Effluent is recirculated in some instances to maintain required fluid velocities through the system, as described in Chapter 18. A significant thing to note about the media for all of the SAGBs is the high specific surface area, which is about an order of magnitude higher than the specific surface areas of trickling filter or rotating biological contractor (RBC) media. This suggests that they can carry much more biomass, thereby allowing smaller HRTs to be used, and indeed, this has been their main appeal.

Many SAGBs are operated without a solids separation and recycle system. In such bioreactors, the HRT is less than the minimum SRT required for microbial growth on the substrates provided. By doing this, the growth of suspended microorganisms is minimized and the growth of attached microorganisms is maximized. For these systems, a means for controlled removal of the attached biomass must be provided. The specific means provided varies with system configuration and is discussed in conjunction with each system.

Other SAGBs use both suspended and attached growth biomass. These combined suspended and attached growth (CSAG) systems are illustrated schematically in Figure 21.2 and the characteristics of the media used in them are summarized in Table 21.2. The suspended component of the biomass is maintained by passing the bioreactor effluent through a liquid-solids separation device, such as a clarifier, and recycling the separated biomass to the bioreactor in the same manner as in the activated sludge proccss. Biomass that grows on the fixed media will occasionally slough off and be incorporated into the suspended biomass. The excess biomass produced is removed as waste solids, just like in the activated sludge process.

Heterotrophic microorganisms grow in these systems if biodegradable organic matter is present in the influent wastewater, but the type of metabolism they exhibit depends on the terminal electron acceptor provided. Aerobic metabolism predom-

Submerged Attached Growth Bioreactors

Table 21.1 Packed and Fluidized Bed Reactors
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