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 measu red by means of pH-stat titration, which exploits the stoichiometric relati onship between ammonium oxidation and acidity production. First, the maximu m 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 in tervals into the sludge sample while its nitrifying activity is continuousl y monitored. Finally, nitrification is fully inhibited by allylthiourea to detect the presence of interference. Experiments on three nitrification inh ibitors (allylthiourea, aniline, and cyanoguanidine) demonstrated a good re peatability (coefficient of variation on EC50 determinations of 11, 38, and 14%, respectively) and good correlation with literature values.