EVALUATION OF NITRIFICATION INHIBITION BY HEAVY METALS NICKEL AND ZINC

Heavy metals are postulated as one group of chemicals causing significant nitrification inhibition in wastewater treatment facilities. Using biomass from a well controlled continuously operated lab-scale nitrifying bioreactor, the effect of nickel and zinc on ammonia and nitrite oxidation was measured. The degree of inhibition was calculated from the kinetics of ammonium oxidation and nitrite oxidation, inferred from maximum specific oxygen uptake rates (SOUR) measured in batch respirometric assays. Nickel and zinc inhibited ammonia oxidation, but not nitrite oxidation up to total analytical concentrations of approximately 1. 5 mM. The speciation of nickel was determined using experimental and computational tools (i.e., total, soluble, truly soluble, free cationic, internalized) and the various nickel fractions were correlated to inhibition of ammonia oxidation in the absence or presence of intentionally added metal complexing agents (e.g., EDTA, NTA, citrate, SO42−). The observed inhibition was not a function of total analytical nickel concentration, rather the free cation concentration, [Ni2+], and this relationship was well described by an empirical non-competitive inhibition model in terms of [Ni2+]. Further, the free Ni2+ concentration could be manipulated by the addition of metal complexing agent (e.g., EDTA) at appropriate concentration, resulting in a reduced biological effect. The observed inhibitory effect of zinc and nickel on ammonia oxidation measured in short-term batch assays underpredicted the observed effect in the shock-dosed continuous reactor where exposure was prolonged. Experiments on metal adsorption and internalization revealed that this discrepancy might be due to the surprisingly slow kinetics of zinc and nickel internalization by the nitrifying biomass.