pH-stat titration to assess nitrification inhibition

This paper describes a standardized method to assess the inhibitory effect of a toxicant on nitrifying microorganisms. Nitrification activity is measured by means of pH-stat titration, which exploits the stoichiometric relationship between ammonium oxidation and acidity production. First, the maximum ammonium oxidizing capacity of a sample of nitrifying activated sludge is assessed in the presence of ammonium as substrate (activity of the blank). Subsequently, five increasing amounts of toxicant are spiked at regular intervals into the sludge sample while its nitrifying activity is continuously monitored. Finally, nitrification is fully inhibited by allylthiourea to detect the presence of interference. Experiments on three nitrification inhibitors (allylthiourea, aniline, and cyanoguanidine) demonstrated a good repeatability (coefficient of variation on EC50 determinations of 11, 38, and 14%, respectively) and good correlation with literature values.