F.5 Dissimilatory reduction of nitrate to ammonium

Dissimilatory reduction of nitrate to ammonium (also referred to as DRNA) is the microbial conversion of nitrate to ammonium. Dissolved oxygen is not consumed in this process. The WQ Module assumes the number of moles of nitrate-N consumed is the same as the number of ammonium-N moles produced. The DRNA rate is computed within the WQ Module via Equation (F.11).

\[\begin{equation} R_{DRNA\langle computed\rangle}^{NO_3} = R_{DRNA}^{NO_3} \times \frac{K_{DRNA-O_2}^{NO_3}}{K_{DRNA-O_2}^{NO_3} + \left[DO\right]} \tag{F.11} \end{equation}\]

\(R_{DRNA}^{NO_3}\) is the user specified rate of dissimilatory reduction of nitrate to ammonium without the influence of oxygen, \(K_{DRNA-O_2}^{NO_3}\) is the Michaelis-Menten half saturation dissolved oxygen concentration for DRNA, and \(\left[DO\right]\) is the ambient dissolved oxygen concentration.

The computed DRNA rate from Equation (F.11) is multiplied by ambient nitrate concentration to compute the flux to ammonium at each model timestep in each model cell as per Equation (F.12).

\[\begin{equation} \href{AppDiags.html#WQDiagDRNA}{F_{DRNA\langle computed\rangle}^{NO_3}} = R_{DRNA\langle computed\rangle}^{NO_3} \times \left[NO_3\right] \tag{F.12} \end{equation}\]

Although not recommended, the influence of oxygen in all the processes described in sections F.1 to F.5 can be removed by setting the command oxygen == off in the nitrogen model block. This switch applies to sediment flux, nitrification and denitrification. Anammox and DRNA will be switched off entirely if this command is issued.