Title

Biokinetic characterization of ammonium and nitrite oxidation by a mixed nitrifying culture using extant respirometry

Date of Completion

January 1999

Keywords

Engineering, Sanitary and Municipal|Environmental Sciences|Engineering, Environmental

Degree

Ph.D.

Abstract

A comprehensive kinetic characterization of nitrification by a mixed nitrifying culture was conducted using a sensitive respiromettic bioassay. A leading attempt was made to model the initial acceleration in NH4 +-N oxidation, which has thus far been recognized, but not quantified. The possibility of treating complete NH4+-N to NO 3-N oxidation as one composite biochemical process under varying NH4+-N and NO2-N oxidation dynamics was verified. A single-step model that considers nitrification as one composite reaction was adequate for describing the kinetics of NH4+-N to NO3-N oxidation when NH4+-N to NO2-N oxidation was the sole rate-limiting step. However, the independent quantification of each step was required for the accurate description of the overall process when both oxidation steps were rate-limiting. A two-step mathematical model that describes both NH4+-N and NO2-N oxidation explicitly was developed and expressed solely in terms of oxygen uptake. Process kinetics of NH4+-N to NO2-N and NO2-N to NO3-N oxidation were quantified using two approaches. In the first approach, the two steps were metabolically uncoupled using selective nitrification inhibitors, allylthiourea and sodium azide, and the kinetics of each individual step were determined. In the second approach, the respirogram depicting complete NH4 +-N to NO3-N oxidation was correlated to the developed two-step nitrification model. The overall experimental nitrification respirogram did not contain enough information to quantify both steps when NH4+-N oxidation was the only rate-limiting step. However, increasing degrees of dual rate-limitation improved the identifiability of the two-step model parameters. Optimization of the experimental design is recommended for the quantification of both nitrification steps irrespective of whether one or both steps are rate-limiting. ^ An accurate description of biochemical phenomena observed only in batch assays, such as the acceleration phase in NH4+-N oxidation, is essential since biokinetic parameters obtained from batch assays are used in the design and control of continuous biochemical reactors. The advances in computational facilities and an improved understanding of the biochemistry and microbial ecology of activated sludge reactors have facilitated the development of more mechanistic based-descriptors of the activated sludge process, previously treated as a black-box. The development of the two-step respirometry-based model for nitrification represents a significant step towards a better delineation of biological nitrogen removal processes. ^