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secondary clarifier.

Spent Backwash (Periodic)

Spent Backwash (Periodic)

Air Backwash

(Optional)

Figure 21.3 Downllow packed bed (DFPB) bioreactor.

Air Backwash

(Optional)

Figure 21.3 Downllow packed bed (DFPB) bioreactor.

Downflow packed bed bioreactors have been used for carbon oxidation, combined carbon oxidation and nitrification, and separate stage nitrification."~h'w'J" Oxygen must be supplied to accomplish nitrification, and is often supplied in carbon oxidation systems. This is done by adding air through a distribution system located about two-thirds of the way from the top of the bioreactor. Oxygen transfer can be quite efficient due to the tortuous paths that air bubbles must follow as they pass upward through the packed bed. The addition of air to the bioreactor results in its division into two zones. The upper two-thirds is the aerobic bioreactor section, while the lower one-third functions as a filter. One proprietary version of this bioreactor type uses media with an effective size of 2 to 6 mm that is manufactured from a fired clay.-" Experience indicates that the porous media formed in the firing process provides internal surface area to retain bacteria in the packed bed during the back-washing process. Activated carbon has also been used as media. Aerated DFPB bioreactors arc sometimes referred to as biological aerated filters (BAFs).

Downflow packed bed bioreactors can also be used for denitrification. One proprietary devicc containing 2 mm diameter rounded sand has proven effective for combined denitrification and suspended solids removal.11- Because both biodegradable organic matter and nitrate-N must be present in the influent wastewater for denitrification to occur, external organic matter must be added when the wastewater has been nitrified in an upstream treatment system, as illustrated in Figure 21.4a. Materials such as methanol,1 4" fermented wastewater, and fermented solids1'14 can be used for this purpose. Alternately, effluent from a downstream nitrification process can be recirculated, mixed with influent wastewater, and applied to a DFPB denitrification bioreactor,s'" " ""' much in the manner of the modified Ludzack-Ettinger (MLE) suspended growth system, as illustrated in Figure 21.4b. In this case, however, a distinct microbial community will develop in each bioreactor. Nitrogen gas will accumulate in a DFPB bioreactor when it is used for denitrification. This gas must be purged periodically from the bioreactor or it will displace fluid from the interstitial spaces and restrict liquid flow. Purging is often accomplished by temporarily stopping the influent flow and using an air wash cycle.

Supplemental Carbon

(i.e Methanol, Fermentation Products)

Nitrified Secondary Effluent

Air & Water Backwash (Periodic!

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