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

Nitrification involves the sequential biological oxidation of reduced nitro gen 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 o xidation as one composite biochemical reaction was examined at different re lative 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 co nsortium were uncoupled using selective inhibitors allylthiourea and sodium azide, The kinetic parameters of NH4+-N to NO2--N oxidation (q(max,ns) and K-s,K-ns) and NO2--N to NO3--N oxidation (q(max,nb) and K-s,K-nb) were det ermined by a rapid extant respirometric technique. The stoichiometric coeff icients 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 N O3--N oxidation was noncompetitively inhibited by NH4+-N but was not inhibi ted 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 oxida tion 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 indic ate that the use of single-step models to describe batch NH4+ oxidation yie lds erroneous kinetic parameters when NH4+-to-NO2- oxidation is not the sol e 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 ox idation, independently. (C) 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 68: 396-406, 2000.