Analysis of Free Ammonia Inhibition of Nitrite Oxidizing Bacteria Using a Dissolved Oxygen Respirometer

Abstract

Free ammonia (NH3-N) inhibition of nitrite-oxidizing bacteria (NOB) has been widely studied for partial nitrification (or nitrite accumulation) and denitrification via nitrite (NO2-N) as a low-cost treatment of ammonium containing wastewater. The literature on NH3-N inhibition of NOB, however, shows disagreement about the threshold NH3-N concentration and its degree of inhibition. In order to clarify the confusion, a simple and cheap respirometric method was devised to investigate the effect of free ammonia inhibition of NOB. Sludge samples from an autotrophic nitrifying reactor were exposed to various NH3-N concentrations to measure the maximum specific nitrite oxidation rate (  ) using a respirometer. NOB biomass was estimated from the yield values in the literature. Free ammonia inhibition of nitrite oxidizing bacteria was reversible and the specific nitrite oxidation rate (KNO) decreased from 0.141 to 0.116, 0.100, 0.097 and 0.081 mg NO2-N/mg NOB․h, respectively, as the NH3-N concentration increased from 0.0 to 1.0, 4.1, 9.7 and 22.9 mg/L. A nonlinear regression based on the noncompetitive inhibition mode gave an estimate of the Inhibition concentration (KI) of free ammonia to be 21.3 mg NH3-N/L. Previous studies gave    of Nitrobacter and Nitrospira as 0.120 and 0.032 mg/mg VSS․h. The free ammonia concentration which inhibits Nitrobacter was 30 ~ 50 mg NH3-N/L and Nitrospira was inhibited at 0.04 ~ 0.08 mg NH3-N/L. The results support the fact that Nitrobacter is the dominant NOB in the reactor. The variations in the reported values of free ammonia inhibition may be due to the different species of nitrite oxidizers present in the reactors. The respirometric method provides rapid and reliable analysis of the behavior and community of the nitrite oxidizing bacteria.