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Figure 21.4 Downflow packed bed systems for denitrification.

21.1.3 Upflow Packed Bed Bioreactors

The upflow packed bed (UFPB) bioreactor, illustrated in Figure 21.5. is a more recent development. It is similar to the BAF units described above, except that flow is upward rather than downward. Similar media sizes and superficial velocities arc used, as indicated in Table 21.1. Bed depths tend to be between 2 and 4 m. Aeration is provided in the same manner as in downflow BAF bioreactors, thereby creating an aerobic bioreaction zone in the top and a filtration zone in the bottom. Alternatively, aeration can be provided from the bottom, if the entire bed is to be aerobic and used for bioreaction. UFPB bioreactors are used for carbon oxidation, combined carbon oxidation and nitrification, separate-stage nitrification, and combined carbon oxidation, nitrification, and denitrification.2",4 >"1" The last operational mode is achieved by sizing the aerated zone so that nitrification is achieved and recirculating nitrified

Media Retention (Required with Some Media)

Process Air

Influent

Media Retention (Required with Some Media)

Process Air

Influent

Effluent

(Reverse Direction for Backwash)

Media

Spent Backwash (Periodic),

Figure 21.5 Uptlow packed bed (UFPB) bioreactor.

Effluent

(Reverse Direction for Backwash)

Media

Spent Backwash (Periodic),

Figure 21.5 Uptlow packed bed (UFPB) bioreactor.

effluent back to the bioreactor influent to supply nitrate-N to the lower portion of the bioreactor, thereby converting it inlo an anoxic zone.

One commercial version of the UFPB system uses the fired clay media described above.2" This media is heavier than water and rests on an underdrain system. Another commercial system uses 2 to 5 mm plastic beads, which are lighter than water and tend to float.'" As a result, a grid is required at the top of the bioreactor to retain the media, but an underdrain system is not required.

21.1.4 Fluidized Bed Biological Reactors

Chapter 18 provides a detailed description of fluidized bed biological reactors (FBBRs) and a theoretical analysis of their performance. This section summarizes information about FBBRs to allow their comparison with the other SAGBs. Anaerobic FBBRs are discussed in Chapter 13.

Figure 21.6 provides a schematic diagram of the FBBR process. Process influent flow, consisting of a mixture of wastewater and recirculated effluent, is added to the bottom of the bioreactor through a distribution system and flows upward to a collection system.'1 The process influent flow fluidizes and expands the media bed. A variety of media can be used but, as indicated in Table 21.1, silica sand with a diameter of" 0.3 to 0.7 mm and granular activated carbon (GAC) with a diameter of 0.6 to 1.4 mm are used most often. The small carrier particle diameter provides a large specific surface area for biofilm growth. As discussed in Section 18.2.2, the diameter of a bioparticlc increases, but its density decreases, as biofilm growth develops on it. The net result is a decrease in its settling velocity, resulting in the migration of particles with the greatest amount of biofilm to the top of the fluidized bed. Therefore, the mass of biomass in the bioreactor is controlled by periodically removing bioparticlcs from the top of the bed and processing them through a media cleaning system. The media cleaning system typically consists of a pump (where turbulence shears the attached biofilm) and a liquid-solids separation dcvice (such as a cyclone) where the media is separated from the biomass. The removed biomass is directed to solids processing, while the cleaned media is returned to the bioreactor.

Fbbr Media
Figure 21.6 Fluidizcd bed biological reactor (FBBR).

Fluidized bed biological reactors can be applied to aerobic carbon oxidation," combined carbon oxidation and nitrification," separate-stage nitrification,711 denitri-fication,17 and anaerobic treatment.2'11 For aerobic applications, a portion of the process influent flow is processed through a pressurized oxygen transfer vessel where it is saturated with pure oxygen, as illustrated in Figure 21.6. No oxygen is supplied for anoxic applications. However, electron donor must be supplied either by the influent wastewater or by addition of a supplement like methanol. For a wastewater containing both biodegradable organic matter and ammonia-N, combined nitrification and denitrification using the wastewater organic matter will require two FBBRs operating in series in the same manner as described for DFPB bioreactors and illustrated in Figure 21.4b. The first FBBR is anoxic and receives influent wastewater and recirculation from the downstream aerobic FBBR. Anaerobic operation of an FBBR requires the exclusion of both oxygen and nitrate-N.

21.1.5 Combined Suspended and Attached Growth Systems

Combined suspended and attached growth (CSAG) systems, illustrated in Figure 21.2, consist of an activated sludge bioreactor with attached growth media added to it.1K Suspended biomass is removed in the secondary clarifier and recycled to the bioreactor to maintain a desired suspended biomass inventory, just as in the activated sludge process. Biomass also accumulates on the media and provides an additional biomass inventory. Excess attached biomass periodically sloughs from the media and is incorporated into the suspended biomass. Suspended biomass is wasted from the system to maintain either a desired mixed liquor suspended solids (MLSS) concentration or a desired SRT.

The attached biomass contributes both to the removal of biodegradable organic matter and to the production of a suspended biomass with improved settling characteristics, allowing sludge volume indexes (SVIs) on the order of 50 mL/g to be routinely observed.17 Consequently, the addition of media increases the total biomass in the system in two ways: by providing sites for attached growth and by allowing an increased MLSS concentration to be maintained through improved settling properties.

A variety of fixed media have been used in CSAG systems, as listed in Table 21.2. The principal media in use in 1997 include plastic sheet trickling filter media (sec Figure 19.3), Ringlace,® and polyurethane porous foam pads in processes called Captor® and Linpor.®

Ringlace® is a flexible, looped, rope-like material, constructed of polyvinyl chloride woven into strands, that provides a high surface area for biological growth.4 lh1"411 It is hung on racks suspended over the air diffusers in the bioreactor. This placement results in circulation of mixed liquor past the media, thereby transporting substrate and dissolved oxygen to the attached biogrowth. A similar media placement is used for plastic trickling filter media.4'" 4" Figure 21.7 illustrates the placement and use of Ringlace® or trickling filter media in CSAG systems.

In the Captor® and Linpor® processes, polyurethane foam pads are placed in the bioreactor in a free-floating fashion and retained there by effluent screens, as illustrated in Figure 21.8.4'"",ft""' Even when biomass accumulates inside them, the pads may float on the liquid surface due to their low density. The action of the diffused aeration system causes the pads to circulate within the bioreactor, although there may be a tendency for them to accumulate in the effluent end of the bioreactor. Air lift pumps are often used to recirculate the foam pads to counter this tendency and to maintain a uniform distribution throughout the bioreactor. Other mechanisms, such as a stream of air, are used to remove pads from effluent screens.

The oxygen requirements of both the suspended and attached biomass must be met by the oxygen transfer system installed in the bioreactor. As illustrated in Figure

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