Substrate concentrations and pH effects, which are two environmental factors likely to affect the usefulness of this procedure, were studied in separate experiments. An acceptable compromise which resulted in the optimal response for the whole system (i.e., maximal activity and no toxicity phenomena at high nitrogenous substrate concentrations) was found to be 10 to 20 mg of N (nitrite and ammonia) per liter and pH 7.6 (data not shown).
This method is very simple and could be extended to characterization and control of other nitrifier-containing environmental samples, such as activated sludges (Fig. 3) and biofilms, provided that the sensitivity to inhibitors is similar. This method could also be used to study the toxicity of a given compound for the different fractions of a mixed culture.