Iranian Journal of

Background and Objectives: In order to optimize wastewater nitrogen removal and to reduce the problems of entering nutrients in final receptors, for example, a lake, partial nitrification, as a novel nitrogen removal method, was studied.

Materials and Methods: The efficiency of simultaneous nitrification and denitrification (SND) in partial nitrification through nitrification/denitrification in fixed-film reactor was surveyed. In this process, ammonium was converted to nitrite by ammonium oxidizing bacteria (AOB) but the activity of nitrite oxidizing bacteria (NOB) was limited at low dissolved oxygen (DO) level. The inflection points of oxidation-reduction potential (ORP) profile were used as the indicators of process optimization.

Results: This research showed that in period 2 at fixed DO level of 0.5 mg/L, nitrite accumulation rate (NAR) was higher than period 1 in which DO was declined from 1 to 0.5 mg/L. In contrast to period 1, SND efficiency was reduced in period 2. In period 3, by increment of the carbon to nitrogen ratio (C/N) to 12.5, NAR increased to 71.4 % and SND efficiency increased to 96.7%. In the long term analysis of proposed method, SND efficiency was, at least, 90%.   

Conclusion: Proper C/N ratio and minimum DO level resulted in higher nitrogen removal efficiencies than the operation in which DO was decreased during aerobic phase. By using a fixed-film reactor and without considering an anoxic step, at DO level of 0.5 mg/L, maximum SND efficiency and maximum NAR would be achieved.