Kinetic and elemental characterization of HAP-based high-rate partial nitritation/anammox system orienting stability...

Anammox-based processes are attractive for biological nitrogen removal, and the combination of anammox and hydroxyapatite (HAP) is promising for the simultaneous removal of nitrogen and phosphorus from wastewater. However, the kinetics of one-stage partial nitritation/anammox (PNA) in which ammonia-oxidizing bacteria (AOB) and anammox bacteria (AnAOB) exist in a reactor are poorly understood. Moreover, inorganic elements are required to promote microbial cell synthesis and growth; therefore, monitoring of elements to prevent the limitation and inhibition of the process is critical. The minimum amounts of inorganic elements required for a one-stage PNA process and the elemental flow remain unknown. Therefore, in this study, kinetics, stoichiometry, and element flow in the long-term, high-rate, continuous, one-stage HAP-PNA process with microaerobic granular sludge at 25 °C were determined using process modeling, parameter estimation, and mass balance. The biomass elemental composition was determined to be CH_2.2O_0.89N_0.18S_0.0091, and the biomass yield (Y_obs) was calculated to be 0.0805 g/g NH₄⁺-N. Therefore, a stoichiometric reaction equation for the one-stage HAP-PNA system was also proposed. The maximum specific growth rate (μ_m) of AnAOB and AOB were 0.0360 and 0.0982 d^-1 with doubling times of 19 and 7.1 d, respectively. Finally, the elemental requirements for stable and high-rate performance were determined using element flow analysis. These findings are essential for developing the anammox-based process in a stable and resource-efficient manner and determining engineering applicability.