Nitrification Inhibition by Ethylenediamine-Based Chelating Agents

Abstract

Nitrification inhibition by the ethylenediamine-based chelating agents ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), and ethylenediamine (EDA) was evaluated in batch extant respirometric assays employing biomass from a continuously operated nitrifying enrichment reactor. All three chelating agents inhibited ammonium oxidation but did not inhibit nitrite oxidation within the concentration range tested (0-3 mM). The order of inhibition (molar-based) was: EDA >> EDTA > DTPA. The concentration causing 50% inhibition was estimated at 0.6, 2.4, and 3.1 mM for EDA, EDTA, and DTPA, respectively. Inhibition by EDTA reached its ultimate value within 6 h, whereas inhibition by EDA increased up to 6 h and then decreased. Addition of multivalent cations (i.e., Fe³⁺, Ca²⁺, and Mg²⁺) relieved inhibition by EDTA but not EDA. Inhibition by both EDTA and DTPA, but not EDA, correlated with the depletion of cellular Ca²⁺. Further, inhibition by EDA correlated with substantial leakage of cellular K⁺ and disruption of cellular membrane integrity inferred from LIVE/DEAD^® Baclight^™ viability assays. EDA appears to inhibit nitrifying activity by a different operative mechanism than EDTA and DTPA. Finally, the extant batch assays accurately predicted inhibition by EDTA in the continuous flow parent reactor, suggesting the potential application of the batch assay to predict inhibition of nitrifying activity in full-scale biological treatment systems.