The metabolic versatility of nitrogen-converting bacteria offers new concepts for nitrogen removal processes to treat wastewater. A partial nitrification of ammonia to nitrite (nitritation) would have the advantage of saving a significant amount of oxygen for the nitrification process, as compared to nitratation of ammonia. However, nitrite is toxic and must be kept below toxic concentrations. In a subsequent denit-rification process, fewer reducing equivalents from the degradation of different carbon sources, which often are a limiting factor in wastewater treatment plants, are then required. In the SHARON process, which is a single reactor system for rapid ammonia oxidation to nitrite and subsequent denitrification of the nitrite with methanol, nitrite oxidizers that would generate nitrate are outcompeted by the slowly metabilizing ammonia oxidizers after a temperature shift toward higher temperatures, due to the higher temperature tolerance of the ammonia oxidizers, and by no sludge retention or sludge return in the reactor (Hellinga et al., 1998). Aeration was switched off periodically and denitrification of nitrite was initiated by adding methanol as a carbon source. The SHARON process in full scale was used to treat sludge liquor at the Rotterdam wastewater treatment plant (Mulder et al., 2001). The partial nitrification of ammonia to nitrite led to an ammonia/nitrite ratio that was suitable for a subsequent Anammox process in which nitrite was denitrified to N2 with ammonia instead of methanol as the electron donor. This process was finally established in full-scale at the Rotterdam wastewater treatment plant (Schmidt, 2003).
Partial ammonia oxidation to nitrite and anaerobic oxidation of ammonia in one aerated reactor is the concept behind the CANON process (completely autotrophic nitrogen removal over nitrite) (Van Lossdrecht and Jetten, 1997). Aerobic and anaerobic ammonia-oxidizing bacteria cooperate as long as the nitrifying bacteria consume oxygen and create anoxic conditions for the anaerobic ammonia-oxidizing bacteria.
Another possibility for removing ammonia in one single step without using an organic carbon source is the OLAND process (oxygen-limited nitrification and denitrification) (Kuai and Verstraete, 1998). Conversion of ammonia to N2 is catalyzed by aerobic nitrifiers and anaerobic granular sludge as a source of Planctomycetes including anaerobic ammonia-oxidizing bacteria (Pynaert et al., 2004).
Other nitrogen-removal processes also exist, such as the NOx process (simultaneous nitrification and denitrification of Nitrosomonas-like microorganisms under fully oxic conditions) and aerobic deammonification (conversion of ammonia to N2 and nitrate), which allow ammonia removal without COD. An overview of these new concepts was presented by Schmidt et al., 2003.
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