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Hydraulic loading rate dictated by media size and density. Specific surface area excludes inlernal micropores.

Media

Mixed Liquor (ML^

Mixed Liquor (ML^

Influent

Media

Figure 21.2 Combined suspended and attached growth (CSAG) process.

inates if oxygen is supplied to the system, either dissolved in the influent wastewater or transferred to the bioreactor contents by the addition of air or pure oxygen. Denitrification will occur if dissolved oxygen is not supplied but nitrate-N is provided. Anaerobic processes will occur if both oxygen and nitrate-N are excluded and suitable organic matter is available. Anaerobic systems are discussed in Chapter 13.

Nitrogenous compounds in the wastewater can also be transformed if appropriate environmental conditions are established in the SAGB. Influent ammonia-N can be nitrified if sufficient oxygen is supplied and the total organic loading (TOL) is low enough for nitrifying bacteria to grow. Alternatively, influent nitrate-N can be denitrified if oxygen is excluded and organic matter is available. The development of a microbial community capable of achieving biological phosphorus removal has also been demonstrated for at least one bioreactor configuration.10 Thus, a wide range of biological conversions can be accomplished in these reactors.

21.1.2 Downflow Packed Bed Bioreactors

Figure 21.3 provides a schematic diagram of a downflow packed bed (DFPB) bioreactor. Wastewater is applied to the top and passes downward through the media to an effluent collection system. Relatively small diameter media and modest superficial velocities are used, as indicated in Table 21.1, which prevents biomass from being detached and transported through the media bed. These characteristics allow DFPB bioreactors to function as filters and to remove particulate matter contained in the wastewater. As a result, these bioreactors provide both biological treatment and filtration. Because suspended solids contained in the wastewater will accumulate in the bioreactor, they must periodically be removed by backwashing. Backwashing also removes microorganisms that grow within the media, thereby controlling the biomass inventory. As a result of the need for backwashing, the effluent collection system is designed much like the underdrain system of conventional granular media filters. This means that it must both collect effluent and distribute backwash water uniformly over the bottom of the media bed. Backwash collection facilities are also required.

Submerged Attached Growth Bioreactors 953

Table 21.2 Example Media for Combined Suspended and Attached Growth (CSAG) Processes

Media

Media dimensions

Volume % of aeration basin

Biomass/activity

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