Immobilization of Azospira sp. strain I13 by gel entrapment for mitigation of N2O from biological wastewater treatment plants: Biokinetic characterization and modeling

Abstract

Development of a strategy to mitigate nitrous oxide (N₂O) emitted from biological sources is important in the nexus of wastewater treatment and greenhouse gas emission. To this end, immobilization of N₂O-reducing bacteria as a biofilm has the potential to ameliorate oxygen (O₂) inhibition of the metabolic activity of the bacteria. We demonstrated the effectiveness of calcium alginate gel entrapment of the nosZ clade II type N₂O-reducing bacterium, Azospira sp. strain I13, in reducing levels of N₂O, irrespective of the presence of O₂. Azospira sp. strain I13 cells in the gel exhibited N₂O reduction up to a maximum dissolved oxygen concentration of 100 μM in the bulk liquid. The maximum apparent N₂O uptake rate, [Formula: see text] , by gel immobilization did not appreciably decrease, retaining 72% of the N₂O reduction rate of the cell suspension of Azospira sp. strain I13. Whereas gel immobilization increased the apparent half-saturation constant for N₂O, [Formula: see text] , and the apparent O₂ inhibition constant, [Formula: see text] , representing the degree of O₂ resistance, correspondingly increased. A mechanistic model introducing diffusion and the reactions of N₂O consumption was used to describe the experimental observations. Incorporating Thieles modulus into the model determined an appropriate gel size to achieve N₂O reduction even under aerobic conditions.

Keywords: Azospira sp.; Biokinetics; Gel immobilization; Mathematical modeling; N(2)O-reducing bacteria; Nitrous oxide; Oxygen inhibition; Respiration system; Wastewater treatment.