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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> ) to limit the growth of nitrite-oxidizing bacteria ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>NOB</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{NOB} $$</mml:annotation></mml:semantics> </mml:math> ). However, the high sensitivity of Anammox bacteria to operational conditions and the competition with other microorganisms lead to a critical challenge in maintaining sufficient <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> population. In this study, a one-dimensional steady-state model was developed and calibrated to investigate the kinetic constants of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mspace></mml:mspace> <mml:msub><mml:mi>X</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>μ</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {\mu}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> ) was 0.033 to 0.10 d^-1. In addition the other kinetic constants of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> were 0.003 d^-1 for decay rate constant ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>b</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {b}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> ), 0.10 mg-O₂/L for oxygen half-saturation constant ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>K</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>ANA</mml:mi></mml:msubsup> </mml:mrow> <mml:annotation>$$ {K}_{O_2}^{ANA} $$</mml:annotation></mml:semantics> </mml:math> ), 0.07 mg-N/L for ammonia half-saturation constant ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>K</mml:mi> <mml:msub><mml:mi>NH</mml:mi> <mml:mn>4</mml:mn></mml:msub> <mml:mi>ANA</mml:mi></mml:msubsup> </mml:mrow> <mml:annotation>$$ {K}_{NH_4}^{ANA} $$</mml:annotation></mml:semantics> </mml:math> ), and 0.05 mg-N/L for nitrite half-saturation constant ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>K</mml:mi> <mml:msub><mml:mi>NO</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mi>ANA</mml:mi></mml:msubsup> </mml:mrow> <mml:annotation>$$ {K}_{NO_2}^{ANA} $$</mml:annotation></mml:semantics> </mml:math> ). The model simulation results showed that the dissolved oxygen of about 0.10 mg-O₂/L was found to be optimal to maintain high <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> population. In addition, minimal COD concentration is required to control heterotrophs ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>H</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_H $$</mml:annotation></mml:semantics> </mml:math> ) and improve ammonia oxidation by ammonia-oxidizing bacteria ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>AOB</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{AOB} $$</mml:annotation></mml:semantics> </mml:math> ). It was also emphasized that moderate mixing conditions ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>L</mml:mi> <mml:mi>f</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {L}_f $$</mml:annotation></mml:semantics> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>≅</mml:mo></mml:mrow> <mml:annotation>$$ \cong $$</mml:annotation></mml:semantics> </mml:math> 100 μm) are preferable to decrease the diffusion of oxygen to the deep layers of the granules, controlling the competition between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>NOB</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{NOB} $$</mml:annotation></mml:semantics> </mml:math> . A single-factor relative sensitivity analysis (RSA) on microbial kinetics revealed that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>μ</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {\mu}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>ANA</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{ANA} $$</mml:annotation></mml:semantics> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>X</mml:mi> <mml:mi>NOB</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {X}_{NOB} $$</mml:annotation></mml:semantics> </mml:math> . 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>L</mml:mi> <mml:mi>f</mml:mi></mml:msub> </mml:mrow> <mml:annotation>$$ {L}_f $$</mml:annotation></mml:semantics> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>≅</mml:mo></mml:mrow> <mml:annotation>$$ \cong $$</mml:annotation></mml:semantics> </mml:math> 100 μm) are preferable for better performance of Anammox growth in single-stage PN/A processes. Moderate dissolved oxygen (DO <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>≅</mml:mo></mml:mrow> <mml:annotation>$$ \cong $$</mml:annotation></mml:semantics> </mml:math> 0.10 mg-O₂/L) is highly recommended for efficient growth of Anammox bacteria in single-stage PN/A granulation. Minimal COD (COD <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>≅</mml:mo></mml:mrow> <mml:annotation>$$ \cong $$</mml:annotation></mml:semantics> </mml:math> 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.