F.2 Nitrification

Nitrification is the pelagic biological oxidation of ammonium to nitrate. The equations representing this process (including intermediates) are as follows.

\[\begin{equation} \left.\begin{aligned} 2NH_4^{+} + 3O_2 \rightarrow & 2NO_2^{-} + 4H^+ + 2H_2O \\ \\ 2NO_2^{-} + O_2 \rightarrow & 2NO_3^- \end{aligned}\right\} \tag{F.2} \end{equation}\]

Dissolved oxygen is consumed by nitrification of ammonium, in a molar ratio of 1:2 N:O\(_2\) (i.e. 2 moles of ammonium consume 4 moles of diatomic oxygen). Nitrite is an intermediate product of nitrification, and simulation of this quantity is possible within the WQ Module although not currently activated. Contact if this capability is required.

The pelagic nitrification rate is computed within the WQ Module via Equation (F.3).

\[\begin{equation} R_{nitrif\langle computed\rangle}^{NH_4} = R_{nitrif}^{NH_4} \times \underbrace{\frac{\left[ DO \right]}{K_{nitrif-O_2}^{NH_4} + \left[DO\right]}}_{\text{Influence of oxygen}} \times \underbrace{ \vphantom{\frac{\left[ DO \right]}{K_{nitrif-O_2}^{NH_4} + \left[DO\right]}} \left[\theta_{nitrif}^{NH_4}\right]^{\left(T-20\right)}}_{\text{Influence of temperature}} \tag{F.3} \end{equation}\]

\(R_{nitrif}^{NH_4}\) is the user specified ammonium nitrifcation rate at 20\(^o\)C without the influence of dissolved oxygen, \(\left[DO\right]\) is the ambient dissolved oxygen concentration, \(K_{nitrif-O_2}^{NH_4}\) and \(\theta_{nitrif}^{NH_4}\) are the half saturation concentration of dissolved oxygen and temperature coefficient for nitrification of ammonium respectively (both are user specified), and \(T\) is ambient water temperature.

The nitrification rate from Equation (F.3) is multiplied by ambient ammonium concentration to compute the flux of ammonium to nitrate at each model timestep in each model cell via Equation (F.4).

\[\begin{equation} \href{AppDiags.html#WQDiagNitrif}{F_{nitrif\langle computed\rangle}^{NH_4}} = R_{nitrif\langle computed\rangle}^{NH_4} \times \left[ NH_4 \right] \tag{F.4} \end{equation}\]

Equation (F.3) reveals a linear relationship between \(R_{nitrif\langle computed\rangle}^{NH_4}\) and both (constant) dissolved oxygen concentration and half saturation oxygen concentration. In dissolved oxygen conditions away from oxygen sinks other than nitrification itself (such as sediments), this leaves ambient temperature as the key external determinant of the computed nitrification rates.

A suite of demonstration model simulations were executed to illustrate this, with each simulation having its own temperature, and a constant nitrification rate \(R_{nitrif}^{NH_4}\) of 0.1/day. Ammonium and nitrate initial conditions were always set to 4 and 0 mg/L respectively, \(K_{nitrif-O_2}^{NH_4}\) to 4 mg/L and \(\theta_{nitrif}^{NH_4}\) was set to 1.05. In order to illustrate only the workings of nitrification, all sediment fluxes were set to zero. The predicted temporal evolution of water column ammonium and nitrate concentrations in these simulations are provided in Figure F.3. Corresponding dissolved oxygen concentrations are also included to demonstrate the impact of nitrification on water column oxygen. Use the “play” button or drag the slider to see how different ambient temperatures change the ambient ammonium, nitrate and dissolved oxygen concentrations (ordinate) in time (abscissa).

Figure F.3: Move the slider to see the effect of changing ambient water temperature on the concentrations of ammonia, nitrate and oxygen

The figure demonstrates the expected behaviour, where ammonium is consumed and nitrate produced. The drawdown in oxygen presented in the figure is due only to nitrification.

The figure also reveals the mass conserving behaviour of the WQ Module: the decrease in ammonium (N) concentration is the same as the corresponding increase in nitrate (N) concentration for each of the different temperature simulations. This confirms the 1:1 ratio of nitrogen conversion from ammonium to nitrate in moles (Equation (F.2)), and hence concentration in an otherwise closed system.

The drawdown in oxygen also demonstrates the WQ Module mass balance. In the 12\(^o\)C case for example:

  • The change in dissolved oxygen concentration is 4.37 mg/L
  • Converting this to moles of O\(_2\) gives 0.1366 moles O\(_2\) (= 4.37/32)
  • The change in ammonium (or nitrate) concentration is 0.955 mg/L
  • Converting this to moles of N gives 0.0682 moles of N (= 0.955/14)

The number of moles of dissolved oxygen consumed (0.137) is twice that the number of moles of N transformed from ammonium to nitrate (0.0682), to two decimal places. This is consistent with Equation (F.2).

A full WQ Module mass balance is presented in Appendix S.