Kinetics and simulation of nitrification at various pH values of a polluted river in the tropics

Nitrification is simulated in existing water quality models by multiplying the rate with the attenuation factors for water temperature and dissolved oxygen. The effect of pH, substrates and bacteria is mostly neglected. Moreover, the two-step nitrification is simplified to a one-step process. The aim of this study is to investigate the kinetics of two-step nitrification and to simulate this process. Consequently, the importance of considering the effect of pH, substrate and bacteria in nitrification modelling of river basins is emphasized. For this, an experiment was performed on water taken from the Tay Ninh River in Vietnam. The results show a low actual ammonium oxidation rate in the water column of the river. The ammonium oxidation is completely inhibited at pH 5 while the nitrite oxidation is strongly inhibited at pH 8.5. In comparison to the ammonium oxidizers (AOB), the nitrite oxidizers (NOB) are rarely inhibited at low pH. Numerical simulation of the experiment delivered ammonium oxidation rates kA and nitrite oxidation rates kN for different pH values. The kA for pH values lower than 6 is very low. It increases rapidly as the pH increases from 6 to 6.5. We also found out that pH values of 7.5 and 7 are optimal for the activity of AOB and NOB, respectively. Our results emphasize the importance of considering the effect of pH, substrate and bacteria on the nitrification research in river basins. In doing so, nitrification can be simulated more realistically and sustainable water management can be carried out more successfully.