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0 10 20 30 40 50 60 70 Aeration Time or SRT, days

Figure 12.6 Depression of the pH during aerobic digestion. (From D. S. Mavinic and D. A. Koers, Fate of nitrogen in aerobic digesters. Journal, Water Pollution Control Feileration 54: 352-360, 1982. Copyright (Ö Water Environment Federation; reprinted with permission.)

nitrified and then converted to nitrogen gas through denitrification with biomass as the electron donor, the overall molar stoichiometry would be:

4 C,H-0:N + 23 0: -» 20 CO: + 2 N: + 14 H:0 (12.9)

In that case there would be no alkalinity destruction and the oxygen requirement would be reduced to 5.75 moles per mole of biomass destroyed, or 1.63 g 0;/g VSS destroyed, which represents an 18% savings compared to the fully aerobic process. Considerations such as these led to the concept of the A/AD process. Evidence indicates that pH values near neutrality can be maintained with this process if a significant degree of denitrification is obtained.

Figure 12.7 illustrates A/AD process options. The option in Figure 12.7a is a modification of intermittent CAD in which the oxygen transfer system is cycled on and off to create aerobic and anoxic periods during the digester operational cycle. Mixers may be provided to maintain solids in suspension during the anoxic periods, but if a sufficiently high suspended solids concentration is maintained in the bio-reactor, only limited settling will occur, making mixing unnecessary. Other options involve the use of separate anoxic and aerobic zones with recirculation of mixed liquor from the aerobic to the anoxic zone, just as in the modified Ludzak-Ettinger (MLE) process, as shown in Figures 12.7b and 12.7c. SRTs and suspended solids concentrations in A/AD systems are similar to those in CAD systems. Furthermore, the facilities required for A'AD systems are similar to those used with CAD systems,

Feed Sludge Mixer

Aeration (Cycled)

Supernatant

Feed Sludge Mixer

Supernatant

Aeration (Cycled)

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