Performance and microbial response during the fast reactivation of Anammox system by hydrodynamic stress control

Anaerobic ammonium oxidation (Anammox) has become a promising method for biological nitrogen removal. However, this biotechnology application is always limited due to the low growth rate and biomass yield of Anammox bacteria. This study investigated the process of fast reactivation of an Anammox consortium idled for 2 years via hydrodynamic stress control. The results showed that the Anammox system was efficiently and quickly reactivated by shortening of the hydraulic retention time (HRT) of the reactor from 12 to 6 hr within 68 days of operation. Moreover, at a 4-hr HRT with an influent total nitrogen loading rate of 1.2 kg N/(m³·day), the reactor maintained high biological performance with an ammonium removal loading rate of 0.52 kg N/(m³·day) and a nitrite removal rate of 0.59 kg N/(m³·day). In the reactivated Anammox reaction, the stoichiometric coefficients of NH₄⁺–N to NO₂^?–N and NH₄⁺–N to NO₃^?–N were 1:1.04 ± 0.08 and 1:0.31 ± 0.03, respectively. The specific Anammox activity and hydrazine oxidoreductase activity, both of which represent the degree of Anammox bacteria present, increased as the hydrodynamic stress increased and were maximally (125.38 ± 3.01 mg N/(g VSS·day) and 339.42 ± 6.83 μmol/(min·g VSS), respectively) at 4-hr HRT. Microbial response analysis showed that the dominant microbial community was obviously shifted and the dominance of Anammox bacteria was enhanced during the hydrodynamic selection.