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Theoretical Analysis of Microbial Yieldand Pathways Related to N 2 O Productionin Nitrification

Identifying microbial yield and the pathway relating to nitrous oxide (N2O) production in nitrification were essential prerequisites for modeling N2O production and emissions. A thermodynamic method based on standard Gibbs free energy change of bio-chemical reaction was used to evaluate the NH3-NO, NH3-NO2 -, NH4 +-NO, NH4 -NO2 -, NH2OH-NO, and NH2OH-NO2 reactions. Meanwhile, the stoichiometric equations of the relevant reactions were built up to ascertain the microbial yield of the microorganisms responsible for N2O production. The results indicated that: 1) all the above reactions had the possibility of producing N2O in nitrification as the standard Gibbs free energy changes of all the reactions were negative, 2) all the possible reactions can be used as the pathways to describe N2O production in nitrification in mathematical models, and 3) theoretically, the yield coefficient of autotrophs relating to N2O production was in the range of 0.097~0.194 g cell/ g N. Ammonia-oxidizing bacteria were responsible for the production of N2O in nitrification. In order to develop a unified model for better simulation of N2O production and emissions in the biological wastewater treatment process, intensive studies should be carried out to reveal undiscovered pathways relating to N2O production.