Comparison of nitrification inhibition by metals in batch and continuous flow reactors

Stringent effluent discharge limits for nitrogen require development of assays to accurately and rapidly determine whether a wastewater stream will inhibit nitrification. The suitability of a short-term extant batch respirometric assay to predict nitrification inhibition by metals (Cu, Zn, Ni and Cd) in continuous flow reactors was evaluated. Linear metal partition coefficients were determined to be 78.42+/-2.46, 0.54+/-0.04, 0.05+/-0.01 and 0.60+/-0.01 L/g biomass COD in batch experiments (pH=7.5) for Cu, Zn, Ni and Cd, respectively. A mass-balance model, incorporating metal partitioning, adequately fit the metal concentration profiles in the continuous flow reactor during and after a 24-hour shock load. However, the short-term batch assays significantly underestimated observed inhibition in the continuous flow reactor. The underestimation was, most likely, due to slow kinetics of Zn, Ni and Cd internalization and an exacerbation effect due to continued metal exposure in the continuous flow reactor. The discrepancy in inhibition between batch and continuous systems was the largest for Cu, presumably due to its unique mode of action, which involves loss of cell membrane integrity. Hence, using results from short-term batch respirometric assays to infer responses in continuous flow reactors with longer exposure should be done with caution and must accommodate partitioning behavior and internalization kinetics.