Integrating Thermodynamics and Mathematical Modelling to Investigate the Stoichiometry and Kinetics of Sulphide Oxidation-Nitrate Reduction with a Special Focus on Partial Autotrophic Denitrification

Abstract

The high complexity of Sulphur Oxidising-Nitrate Reducing (SO-NR) processes challenges a straightforward identification of the mechanisms and the metabolic stoichiometry and kinetics underlying successful accumulation of nitrite in Partial Autotrophic Denitrification (PAD) -nitrite accumulation based on the oxidation of reduced sulphur compounds-  implementations. In this study, the stoichiometry of the SO-NR process, with a focus on the PAD case, has been evaluated through a thermodynamic-based study whereas a model-based approach has been adopted to assess process kinetics. Experimental data on process performance and biomass yields were available from a previous work achieving efficient PAD. First, the free Gibbs energy dissipation method has been implemented, in order to provide a theoretical framework exploring the boundaries for sulphur oxidizing biomass yields. Second, a screening of available mathematical models describing SO-NR process was conducted and five published models were selected, in order to assess the most suitable model structure for describing the observed PAD kinetics. To the best of our knowledge, none of reported biomass yields are estimated in systems operating PAD as the main process and none of the proposed models have been applied to case studies aiming at partial denitrification only. The work showed that the very low biomass yield of 0.117±0.007 gVSS/gS, observed in a PAD system in our previous work, suggests that the conditions applied to achieve partial denitrification resulted in a high energy-dissipating metabolism compared to complete denitrification applications. Sensitivity analysis of the selected models showed significant relevance of maximum specific growth rate parameters (μmax) as well as nitrite inhibition constants, especially under S/N ratios close to 1 gS/gN. Models’ analysis revealed that nitrite accumulation can be described by a classical Monod kinetics if different μmax are adopted for each intermediate reaction, but adopting a Haldane-type kinetics for nitrite uptake infers higher parameter identifiability to the model structure.

Keywords: Partial Autotrophic Denitrification, Nitrite accumulation, SOB, thermodynamics, modelling