Maximizing the efficiency of single‐stage partial nitrification/Anammox granule processes and balancing microbial competition using insights of a numerical model study

Granulation is an efficient approach for the rapid growth of anaerobic ammonia oxidation (Anammox) bacteria ( $X_{\mathrm{ANA}}$ ) to limit the growth of nitrite-oxidizing bacteria ( $X_{\mathrm{NOB}}$ ). However, the high sensitivity of Anammox bacteria to operational conditions and the competition with other microorganisms lead to a critical challenge in maintaining sufficient $X_{\mathrm{ANA}}$ population. In this study, a one-dimensional steady-state model was developed and calibrated to investigate the kinetic constants of $X_{\mathrm{ANA}}$ growth and mass transport in individual granules, including the liquid film. According to the model calibration results, the range of the maximum specific growth rate constant of $X_{\mathrm{ANA}}$ ( ${\mu }_{\mathrm{ANA}}$ ) was 0.033 to 0.10 d^-1. In addition the other kinetic constants of $X_{\mathrm{ANA}}$ were 0.003 d^-1 for decay rate constant ( $b_{\mathrm{ANA}}$ ), 0.10 mg-O₂/L for oxygen half-saturation constant ( $K_{O_2}^{\mathrm{ANA}}$ ), 0.07 mg-N/L for ammonia half-saturation constant ( $K_{{\mathrm{NH}}_4}^{\mathrm{ANA}}$ ), and 0.05 mg-N/L for nitrite half-saturation constant ( $K_{{\mathrm{NO}}_2}^{\mathrm{ANA}}$ ). The model simulation results showed that the dissolved oxygen of about 0.10 mg-O₂/L was found to be optimal to maintain high $X_{\mathrm{ANA}}$ population. In addition, minimal COD concentration is required to control heterotrophs ( $X_H$ ) and improve ammonia oxidation by ammonia-oxidizing bacteria ( $X_{\mathrm{AOB}}$ ). It was also emphasized that moderate mixing conditions ( $L_f$ $\cong$ 100 μm) are preferable to decrease the diffusion of oxygen to the deep layers of the granules, controlling the competition between $X_{\mathrm{ANA}}$ and $X_{\mathrm{NOB}}$ . A single-factor relative sensitivity analysis (RSA) on microbial kinetics revealed that ${\mu }_{\mathrm{ANA}}$ is the governing factor in the efficient operation of the single-stage PN/A processes. In addition, it was found that nitrite concentration is a rate-limiting parameter on the success of the process due to the competition between $X_{\mathrm{ANA}}$ and $X_{\mathrm{NOB}}$ . These findings can be used to enhance our understanding on the importance of microbial competition and mass transport in the single-stage PN/A process. PRACTITIONER POINTS: A one-dimensional steady-state model was developed and calibrated for simulating the single-stage partial nitrification/Anammox (PN/A) granule process. Moderate liquid films ( $L_f$ $\cong$ 100 μm) are preferable for better performance of Anammox growth in single-stage PN/A processes. Moderate dissolved oxygen (DO $\cong$ 0.10 mg-O₂/L) is highly recommended for efficient growth of Anammox bacteria in single-stage PN/A granulation. Minimal COD (COD $\cong$ 0) is preferable for successful operation of the single-stage PN/A granule process. Nitrite concentration is a rate-limiting parameter on the competition between Anammox and nitrite-oxidizing bacteria in the single-stage PN/A processes.