Kinetic characterization of a low-dissolved‑oxygen oxic-anoxic process treating low COD/N tropical wastewater revealed selection of nitrifiers with high substrate affinity

The design of wastewater treatment plants in the tropics is largely based on default parameters from the studies in temperate climates. This may lead to suboptimal design, such as the intensive aeration required for biological nitrogen removal. To reduce the aeration energy, a low-dissolved‑oxygen oxic-anoxic (low-DO OA) process was developed for treating low chemical oxygen demand-to‑nitrogen (COD/N) tropical wastewater. This study calibrated the growth kinetic parameters of microbes in a conventional anoxic-oxic (AO) and a low-DO OA sequencing batch reactors (SBRs) based on a modified version of Activated Sludge Model No. 1 (ASM1). We selected three parameters to be calibrated, namely the maximum growth rate of heterotrophs (μH), maximum growth rate of nitrifiers (μA) and nitrifiers' affinity towards ammoniacal nitrogen (NH4+−N) (KNH). The low-DO OA SBR selected for microbes with a low μH (2.2 d−1), μA (1.49 d−1) and KNH (0.035 mgNH4+−NL−1), which supported the observed proliferation of K-strategist Nitrospira at low-DO condition (0.4 ± 0.2 mg O2 L−1). The calibrated parameters for the AO SBR (1.7 ± 0.2 mg O2 L−1) were significantly higher (μH=9.3 d−1, μA=4.49 d−1, KNH=6.3 mgNH4+−NL−1) than the low-DO OA SBR. The calibrated ASM1 adequately simulated the low-DO OA SBR performance under different sludge retention times. The findings demonstrated a kinetic insight into the unique K-strategist nitrifiers in a low-DO OA process. Moreover, this study reinforced the importance of using parameters for tropical wastewater rather than relying on default values from studies in temperate climates.