Hydrodynamic selection determines whether retained biomass undergoes mechanical maturation or aggregate enlargement during continuous-flow anammox granulation

Abstract

Continuous-flow anammox granulation requires effective biomass retention together with sufficient hydrodynamic selection, yet how their balance governs granule development remains unclear. Two continuous stirred-tank reactors with height-to-diameter ratios of 5.17 and 2.30 were operated under comparable loading conditions, and multiphase CFD was integrated with rheological, EPS, surface-interaction, and microbial analyses. At 150 r/min, the power density and mean turbulent dissipation rate in R1 were 2.05 and 2.77 times those in R2, respectively, although both reactors developed similar particle sizes and mechanical strengths by day 92. Thereafter, their trajectories diverged. Increasing R1 to 180 r/min enhanced axial suspension and hydrodynamic selection, accompanied by moderate particle growth to 412.34 μm and an increase in storage modulus to 23.43 kPa. R1 also exhibited a protein-rich tightly bound EPS matrix, favorable interparticle interactions, and sustained enrichment of Candidatus Kuenenia. In contrast, reducing R2 to 120 r/min improved biomass retention but weakened hydrodynamic selection, resulting in rapid particle enlargement to 537.59 μm, while the storage modulus declined to 10.93 kPa and anammox bacterial dominance decreased. These responses defined two distinct trajectories, namely selection-driven mechanical maturation in R1 and retention-driven aggregate enlargement in R2. Granule maturation therefore depended on coordinated particle growth, mechanical consolidation, and functional enrichment rather than particle enlargement alone. For engineering applications, mixing intensity should be selected according to reactor-specific suspension and biomass-retention requirements rather than prescribed as a fixed stirring rate.

Graphical abstract

Keywords

Anammox granular sludge / Hydrodynamic selection / Mechanical maturation / Computational fluid dynamics / Functional enrichment

Highlight

	● Hydrodynamic selection split anammox granulation into two pathways.
	● Weak selection enlarged aggregates without proportional mechanical maturation.
	● Stronger selection strengthened granules and enriched Candidatus Kuenenia .
	● Similar total EPS concealed greater TB-PN enrichment in mature R1 granules.
	● AHL-rich R2 aggregates enlarged without corresponding mechanical strengthening.

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