25 - The nitrogen cycle and its application in wastewater treatment

Abstract

The balance of the nitrogen cycle depends on the combined activities of nitrogen fixation, nitrification, denitrification, nitrogen assimilation and dissimilation. The removal of ammonia-nitrogen in wastewater treatment plants is dominated by two mechanisms: a conversion pathway to nitrogen gas via the related processes of nitrification and denitrification, and an assimilatory pathway for the formation of bacterial biomass. For combined nitrification-denitrification, there are at least two routes: anaerobic ammonia oxidation (the Anammox process), and the autotrophic nitrification. Under typical wastewater treatment conditions—10-20°C, with the presence of organic material—the autotrophic nitrification followed by heterotrophic denitrification is the dominant mechanism for the biological nitrogen removal. In the design of a nitrogen-removing plant, the capital cost is usually dominated by the size of the reactor used and its associated solid-liquid separation unit. The dominant factors in the operating cost include aeration, sludge removal, pumping cost (primarily for any recycle stream required), and the chemical dosage requirement.