Maximizing the efficiency of single-stage partial nitrification/Anammox granule processes and balancing microbial competition using insights of a numerical model study.
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Granulation is an efficient approach for the rapid growth of anaerobic ammonia oxidation (Anammox) bacteria ( X ANA $$ {X}_{ANA} $$ ) to limit the growth of nitrite-oxidizing bacteria ( X NOB $$ {X}_{NOB} $$ ). 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 ANA $$ {X}_{ANA} $$ population. In this study, a one-dimensional steady-state model was developed and calibrated to investigate the kinetic constants of X ANA $$ {X}_{ANA} $$ 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 ANA $$ {X}_{ANA} $$ ( μ ANA $$ {\mu}_{ANA} $$ ) was 0.033 to 0.10 d-1. In addition the other kinetic constants of X ANA $$ {X}_{ANA} $$ were 0.003 d-1 for decay rate constant ( b ANA $$ {b}_{ANA} $$ ), 0.10 mg-O2/L for oxygen half-saturation constant ( K O 2 ANA $$ {K}_{O_2}^{ANA} $$ ), 0.07 mg-N/L for ammonia half-saturation constant ( K NH 4 ANA $$ {K}_{NH_4}^{ANA} $$ ), and 0.05 mg-N/L for nitrite half-saturation constant ( K NO 2 ANA $$ {K}_{NO_2}^{ANA} $$ ). 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 ANA $$ {X}_{ANA} $$ population. In addition, minimal COD concentration is required to control heterotrophs ( X H $$ {X}_H $$ ) and improve ammonia oxidation by ammonia-oxidizing bacteria ( X AOB $$ {X}_{AOB} $$ ). It was also emphasized that moderate mixing conditions ( L f $$ {L}_f $$ ≅ $$ \cong $$ 100 μm) are preferable to decrease the diffusion of oxygen to the deep layers of the granules, controlling the competition between X ANA $$ {X}_{ANA} $$ and X NOB $$ {X}_{NOB} $$ . A single-factor relative sensitivity analysis (RSA) on microbial kinetics revealed that μ ANA $$ {\mu}_{ANA} $$ 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 ANA $$ {X}_{ANA} $$ and X NOB $$ {X}_{NOB} $$ . 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 $$ {L}_f $$ ≅ $$ \cong $$ 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.
