DISCUSSION
The present study demonstrates that Acinetobacter sp. Y1 can remove ammonium effectively. The maximum removal rate by the strain is twice higher than that by Alcaligenes faecalis No. 4 under
the same NH4 þ-N load (about 100 mg/L) (6). Furthermore, for Bacillus sp. LY (16) and Pseudomonas alcaligenes AS-1 (17), the ammonium removal rates were just 0.43 mg-N/L/h and 1.15 mg-N/ L/h, respectively, which were significantly lower than that of the present study. TN was removed by 87.8% from the medium in 22 h, indicating the heterotrophic nitrogen removal capability by Acinetobacter sp. Y1. A few intermediates were observed in the nitrifi- cation process, possibly due to the fast conversion to the downstream products such as nitrogen. However, accumulation of nitrite has been reported during ammonium oxidation by Bacillus methylotrophicus L7 (18), Alcaligenes faecalis C16 (19) and Marinobacter sp. F6 (20), while production of both nitrite and nitrate were found for Rhodococcus sp. CPZ24 (21). The concentration of TN was slightly higher than that of NH4 þ-N after 12 h. The nitrogen released from bacteria decay might be responsible for this phenomenon (7,22). Similar results have been found for Pseudomonas stutzeri YZN-001 (23) and Acinetobacter sp. Y16 (22).
Many researches indicated that heterotrophic nitrifier utilizes and converts NH4 þ-N with certain carbon sources during the growing phase (6). The carbon sources favoring heterotrophic nitrification are not unique for different organisms. Citrate is commonly used as an organic substrate for heterotrophic nitrifi- cation by some heterotrophic microorganisms such as Alcaligenes faecalis No. 4. Suitable carbon source for Providencia rettgeri strain YL and Pseudomonas sp. AS-1 was acetate compared to citrate (24). However, there is no convincing explanation about the dependency of heterotrophic nitrification on carbon source. Potentially, the most important factor in determining heterotrophic nitrification might be carbon source (25). Organisms may perform heterotrophic nitrificationeaerobic denitrification via different mechanisms on different carbon sources (18). Among the tested carbon sources, organic acids with simple structure appear to be more efficient than saccharides. A possible reason is that the organic acids could be used directly while the saccharides must be changed to simpler compounds firstly (26). Denitrification could regenerate alkalinity and increase pH. Utilization of pyruvate, citrate and acetate made the media more alkaline, suggesting the possibility of denitrification. The carbon sources that made the media alkaline supported nitrification whereas the others that made the media acidic did not. It has been reported that alkaline environment was favorable to heterotrophic nitrification because free ammonia (NH3), not NH4 þ, was required by AMO and more available in alkaline condition (27). However, Brierley and Wood (25) think this was not solely affected by pH.