An anoxic period or zone may be used to improve treatment plant performance. The use of an anoxic period or zone of 1-2 hours on an as-needed basis or as part of the standard operating procedures of the treatment process can provide the following benefits for an activated sludge process:
• Control of undesired filamentous organism growth
• Improvement in floc formation
• Decrease in sludge production
Some filamentous organisms such as Haliscomenobacter hydrossis, Nocardio-forms, Sphaerotilus natans, and type 1701 are strict aerobes. By exposing strict aerobic filamentous organisms to anoxic periods of 1-2 hours, their growth is controlled, because these organisms can only use free molecular oxygen for cellular activity and degradation of soluble cBOD.
Floc particles contain strict aerobic bacteria and facultative anaerobic bacteria. Several of each bacteria are floc-forming organisms. By exposing these organisms to anoxic periods, the number of strict aerobes is gradually reduced, while the number of facultative anaerobes is gradually increased. Because floc particles become denser and firmer with an increasing percentage of facultative anaerobic bacteria, the use of anoxic periods produces floc particles that settle better and are more tolerant of shearing action.
Because the degradation of soluble cBOD with nitrate produces less sludge as compared to the degradation of soluble cBOD with free molecular oxygen, the use of anoxic periods results in decreased sludge production. Decreased sludge production results in the use of less polymers and coagulants for thickening and dewa-tering purposes providing an overall decrease in sludge disposal costs.
Controlled anoxic periods can be established by the following operational measures:
• Terminating the addition of air to an aeration tank and permitting the use of nitrate in the aeration tank for 1-2 hours provided the settled solids do not adversely affect the aeration equipment during restart
• Establishing an anoxic tank in the first aeration tank in plug-flow mode of operation (Figure 11.5)
• Establishing an anoxic zone in an aeration tank with a surface aerator (Figure 11.6) or an oxidation ditch (Figure 11.7)
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