Single-step nitrification models erroneously describe batch ammonia oxidation profiles when nitrite oxidation becomes rate limiting

Abstract

Nitrification involves the sequential biological oxidation of reduced nitrogen species such as ammonium‐nitrogen (NH4+‐N) to nitrite‐nitrogen (NO2−‐N) and nitrate‐nitrogen (NO3−‐N). The adequacy of modeling NH4+‐N to NO3−‐N oxidation as one composite biochemical reaction was examined at different relative dynamics of NH4+‐N to NO2−‐N and NO2−‐N to NO3−‐N oxidation. NH4+‐N to NO2−‐N oxidation and NO2−‐N to NO3−‐N oxidation by a mixed nitrifying consortium were uncoupled using selective inhibitors allylthiourea and sodium azide. The kinetic parameters of NH4+‐N to NO2−‐N oxidation (qmax,ns and KS,ns) and NO2−‐N to NO3−‐N oxidation (qmax,nb and KS,nb) were determined by a rapid extant respirometric technique. The stoichiometric coefficients relating nitrogen removal, oxygen uptake and biomass synthesis were derived from an electron balanced equation. NH4+‐N to NO2−‐N oxidation was not affected by NO2−‐N concentrations up to 100 mg NO2−‐N L−1. NO2−‐N to NO3−‐N oxidation was noncompetitively inhibited by NH4+‐N but was not inhibited by NO3−‐N concentrations up to 250 mg NO3−‐N L−1. When NH4+‐N to NO2−‐N oxidation was the sole rate‐limiting step, complete NH4+‐N to NO3−‐N oxidation was adequately modeled as one composite process. However, when NH4+‐N to NO2−‐N oxidation and NO2−‐N to NO3−‐N oxidation were both rate limiting, the estimated lumped kinetic parameter estimates describing NH4+‐N to NO3−‐N oxidation were unrealistically high and correlated. These findings indicate that the use of single‐step models to describe batch NH4+ oxidation yields erroneous kinetic parameters when NH4+‐to‐NO2− oxidation is not the sole rate‐limiting process throughout the assay. Under such circumstances, it is necessary to quantify NH4+‐N to NO2−‐N oxidation and NO2−‐N to NO3−‐N oxidation, independently. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 68: 396–406, 2000.

Applicability of two-step models in estimating nitrification kinetics from batch respirograms under different relative dynamics of ammonia and nitrite oxidation

“…Because the influent was devoid of soluble COD, we express the NH 4 + -N oxidizing biomass and NO 2 − -N oxidizing biomass concentrations as follows (Chandran and Smets, 2000).…”

“…During the first phase, NH 4 + -N oxidation was the sole ratelimiting process (k ns < k nb , ␣ ‫ס‬ 0.05, df ‫ס‬ 36), while in the second phase, both NH 4 + -N and NO 2 − -N oxidation limited overall nitrification (k ns ‫ס‬ k nb , ␣ ‫ס‬ 0.05, ␤ ‫ס‬ 0.25, df ‫ס‬ 7). The temporal shift in the relative kinetics of NH 4 + -N and NO 2 − -N oxidation was attributed to a decrease in the (Chandran and Smets, 2000).…”

Probing the stoichiometry of the nitrification process using the respirometric approach

“…(10) is valid if only the first step nitrification occurs in the system (only ammonia oxidizers were presented or the second step of nitrification was selectively inhibited by sodium azide) (Aleem and Sewell, 1981;Ginestet et al, 1998;Chandran and Smets, 2000). However, it is necessary to determine the correct chemical dosage for a given sludge sample to make sure that it only inhibits nitrite oxidation but not ammonia oxidation.…”

A two-step nitrification model of ammonia and nitrite oxidation under benzene inhibitory and toxic effects in nitrifying batch cultures

“…This is particularly the case considering the heterogeneity of the nitrifying microbial consortium under study. An alternative to the unavailability of X A and X N measurements is to estimate them as fractions of the total biomass concentration according to stoichiometric considerations [1,5] or to consider the nitrification kinetics in terms of volumetric rather than specific oxidation rates. In this work, the volumetric rate of ammonium oxidation r NH 4 = q NH 4 X A , of nitrite oxidation r NO 2 = q NO 2 X N and benzene transformation r BZ,A = q BZ,A X A was introduced.…”

“…However, in some cases, e.g. when nitrite oxidation becomes limiting, single-step models have been shown to erroneously describe batch ammonia oxidation profiles [1]. The use of two-step nitrification models has already been accurately investigated [2][3][4][5][6] and their implementation is especially essential when nitrite accumulation becomes important as a target product or an unwanted limiting intermediate.…”