Enrichment and adaptation yield high anammox conversion rates under low temperatures

This study compared two anammox sequencing batch reactors (SBR) for one year. SBR_constantT was kept at 30 °C while temperature in SBR_loweringT was decreased step-wise from 30 °C to 20 °C and 15 °C followed by over 140 days at 12.5 °C and 10 °C. High retention of anammox bacteria (AnAOB) and minimization of competition with AnAOB were key. 5-L anoxic reactors with the same inoculum were fed synthetic influent containing 25.9 mg NH₄⁺-N/L and 34.1 mg NO₂^--N/L (no COD). Specific ammonium removal rates continuously increased in SBR_constantT, reaching 785 mg NH₄⁺-N/gVSS/d, and were maintained in SBR_loweringT, reaching 82.2 and 91.8 mg NH₄⁺-N/gVSS/d at 12.5 and 10 °C respectively. AnAOB enrichment (increasing hzsA and 16S rDNA gene concentrations) and adaptation (shift from Ca. Brocadia to Ca. Kuenenia in SBR_loweringT) contributed to these high rates. Rapidly settling granules developed, with average diameters of 1.2 (SBR_constantT) and 1.6 mm (SBR_loweringT). Results reinforce the potential of anammox for mainstream applications.