F.4 Anaerobic oxidation of ammonium

Anaerobic oxidation of ammonium (also referred to as anammox) is the microbial conversion of ammonium and nitrite to free nitrogen gas in low dissolved oxygen environments. Oxygen for this process is stripped from nitrite. The equation representing this process is as follows.

\[\begin{equation} NH_4^{+} + NO_2^{-} \rightarrow N_2 + 2H_2O \tag{F.8} \end{equation}\] \(NH_4^{+}\) and \(NO_2^{-}\) are ammonium and nitrite, respectively, and N\(_2\) is free nitrogen gas. In the absence of simulating nitrite, the WQ Module assumes that nitrite and nitrate are related (only at dissolved oxygen concentrations below 0.1 mg/L) as follows.

\[\begin{equation} \left[NO_2\right] = \left[NO_3\right] \times \left( 1.0 - \frac{\left[ DO \right]}{0.1 + \left[ DO \right]}\right) \tag{F.9} \end{equation}\] \(\left[NO_2\right]\), \(\left[NO_3\right]\) and \(\left[DO\right]\) are the ambient nitrite, ammonium and dissolved oxygen concentrations, respectively. The anammox flux to free nitrogen gas N at each model timestep in each model cell that has an ambient dissolved oxygen concentration less than 0.1 mg/L is computed as per Equation (F.10).

\[\begin{equation} \href{AppDiags.html#WQDiagAnammox}{F_{anmx\langle computed\rangle}^{N_2}} = k_{anmx}^{N_2} \times \frac{\left[NH_4\right]}{K_{anmx-NH_4}^{N_2} + \left[NH_4\right]} \times \frac{\left[NO_2\right]}{K_{anmx-NO_2}^{N_2} + \left[NO_2\right]} \tag{F.10} \end{equation}\] \(k_{anmx}^{N_2}\) is the user specified anaerobic volumetric mass oxidation rate without the influence of ammonium or nitrite, \(K_{anmx-NH_4}^{N_2}\) and \(K_{anmx-NO_2}^{N_2}\) are the Michaelis-Menten half saturation ammonium and nitrite concentrations for anammox respectively, and \(\left[NH_4\right]\) and \(\left[NO_2\right]\) are the ambient ammonium and nitrite concentrations, respectively. For ambient conditions that have dissolved oxygen concentrations greater than 0.1 mg/L, the anammox rate is set to zero.