Probing the stoichiometry of the nitrification process using the respirometric approach

Quantifying oxygen demand and nitrifier yield are important in the design and operation of advanced wastewater treatment systems. However, the accurate stoichiometry of the autotrophic nitrification process has not been fully developed. In this research, stoichiometric links between nitrifier yield, ammonia and nitrite oxidization, ammonia assimilation, and oxygen uptake for each step of the nitrification process were determined. A pulse-flow respirometer was used to measure the oxygen uptake for complete nitrification and nitrite oxidation reactions. Results indicated that the specific oxygen uptake was 4.23 mg-O(2)/mg-N oxidized for complete nitrification, with 3.17 mg-O(2)/mg-N oxidized for ammonia oxidation (first step nitrification) and 1.06 mg-O(2)/mg-N oxidized for nitrite oxidation (second step nitrification). For the complete nitrification, fractions of ammonia used for electron donation, synthesis of ammonia oxidizers, and synthesis of nitrite oxidizers were 97.1%, 2.2%, and 0.7%, respectively. The fractions of electrons transferred into cell synthesis were approximately 7.5% for ammonia oxidation and 7.3% for nitrite oxidation. Biomass yield coefficients for ammonia oxidizers and nitrite oxidizers were 0.18 and 0.06 g-VSS/g-N oxidized, respectively. These parameters are critical when calculating oxygen needs and nitrifier biomass concentrations during the design of advanced wastewater treatment processes.