Estimation of nitrogen removal rate in aqueous phase based on delta15N in microorganisms in solid phase.

Microbial nitrification and denitrification are important processes for removing nitrogenous compounds in aqueous systems. Nitrogen removal rate estimation is essential for controlling nitrogen removal processes and modeling the nitrogen cycle in ecosystems. The model described the relationship between ammonium removal rate (aqueous phase) and the nitrogen stable isotope ratio (delta15N) of microorganisms (solid phase) when a coupled nitrification-denitrification process occurs and assimilation and advections are maintained in a steady state. An oxidation ditch in a municipal wastewater treatment plant was evaluated for 3 years using the model. The ammonium removal rate was calculated from the data of delta15N of the activated sludge, it correlated significantly with the observed removal rate. The isotope fractionation factor (epsilon) was determined to be -5.5 per thousand by using a nonlinear method. The model and obtained factor value were applicable for standard activated-sludge processes performed in parallel in the oxidation ditch and a river watershed. The model may help illustrate nitrogen behavior in ecosystems.