Co-immobilization of AOA strains with anammox bacteria in three different synthetic bio-granules maintained under two substrate-level conditions

Hydrogel encapsulation of ammonium oxidizing archaea (AOA) along with anammox bacteria holds potential to enable mainstream partial nitritation (PN)-anammox process attributing to AOA's high affinity to ammonia and oxygen. This study explored the growth of AOA and anammox in hydrogel-based synthetic biogranules by testing two AOA strains, three types of hydrogel beads and two substrate levels, to identify the optimal combination favoring the concomitant growth of AOA and anammox. The AOA Nitrososphaera viennensis (AOA-NV) exhibited higher abundance (10^−2.3±0.6 AOA/16S) than the AOA-DW (10^−4.7±0.8 AOA/16S) during the entire experimental period. Amongst the three types of hydrogel beads, the PVA-SA-BaCl₂ (140 days) and PVA-SA-H₃BO₃ beads (>180 days) exhibited better long-term structural stability than the PEGDMA-SA-CaCl₂ beads. The PVA-SA-H₃BO₃ beads exhibited the best long-term stability and both the PVA/SA BaCl₂ and PVA-SA-H₃BO₃ beads had comparable ability to retain AOA, anammox and the overall microbial community. Substrate conditions rather than the bead type primarily controlled the microbial community structure. Modest substrate concentrations (1 mM NH₄⁺-N in the feed and 0.8 mg/L dissolved oxygen (DO) in the reactor during aeration phase) followed by low substrate conditions (0.1 mM NH₄⁺-N and 0.2 mg DO/L) both supported the growth of AOA and anammox, while the low substrate condition also suppressed the growth of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), with AOA /AOB and anammox/NOB ratio of 0.7 and 0.4 at moderate substrate condition and 16.5 and 2.6 at low substrate condition.