Rapid enrichment of anammox bacteria for low-strength wastewater treatment: Role of influent nitrite and nitrate ratios in sequencing batch reactors (SBRs)

Anaerobic ammonium oxidation (Anammox) bacteria (AnAOB) have a long doubling time, which represents one of the key challenges for starting up anammox-based reactors. This study investigated the effect of varied nitrite/nitrate ratios on the anammox reactor startup process using five lab-scale anammox sequencing batch reactors (SBRs) (R1-R5). Nitrate addition significantly accelerated AnAOB enrichment startup, particularly during nitrite accumulation phase. The total start-up time was shortened by 55–71 days, compared to the reactor without nitrate. In R1 (nitrite/nitrate=10:0), anammox contributed the highest nitrogen removal percentage (91%), while R2 (nitrite/nitrate=7:3), with nitrate added, exhibited the highest anammox activity (53.84 mg NH4+-N/g VSS/day) in the steady-state phase. As nitrate proportions increased (R3-R5), anammox activity gradually declined due to limited available COD required for the conversion of NO3- to NO2-. Furthermore, it was found that nitrate-added improved the sludge settleability, with lower SVI30. 16S rRNA gene revealed that Ca. Brocadia was the predominant AnAOB in R1, R2 and R3, where nitrite was the primary NOx--N species (≥50%) in the influent. While in the reactor with low concentration of nitrite (R4: nitrite/nitrate=3:7), Ca. Kuenenia was the dominant AnAOB. This study successfully established a complex and balanced ecosystem with multi-species coexistence, where AnAOB served as the primary functional group on nitrogen removal, supported by denitrification bacteria and dissimilatory nitrate reduction to ammonium (DNRA) bacteria. This study proposes a rapid start-up strategy for the anammox process, which overcomes the bottleneck of long start-up time and helps to promote the application of anammox in wastewater treatment.