A Comparative Study of Nitrification Inhibition by Heavy Metals: the Influence of Metal Exposure Time on Biological Effect

Stringent effluent discharge limits for nitrogen from wastewater treatment facilities require development of assays to accurately and rapidly determine whether a wastewater stream will inhibit nitrification. Using biomass from a well-controlled continuously-operated lab-scale nitrifying bioreactor, the suitability of batch extant respirometric assays to predict nitrification inhibition in a continuous flow reactor was evaluated. The effect of Cu, Zn, Ni and Cd on ammonium and nitrite oxidation was quantified. Metal partitioning coefficients (Kp) were first determined in separate batch experiments at 78.42 2.46, 0.54 0.04, 0.05 0.01 and 0.60 0.01 L/g XCOD for Cu, Zn, Ni and Cd, respectively. A reactor mass-balance model incorporating partitioning adequately fit the dynamics of all metals in the continuous flow reactor during and after a 24-hour shock load. However, short-term batch assays significantly underestimated inhibition in the continuous flow reactor if no corrections were made. The underestimation was, most likely, due to the slow kinetics of Zn, Ni and Cd internalization and the effect of continued metal exposure in the continuous flow reactor. However, the discrepancy in inhibition by Cu was the largest among the four metals tested, presumably due to its unique mode of action. Hence, using results from short-term batch inhibition assays to infer responses in the continuous flow reactors with longer exposure should be done with caution.