Intensifying Mainstream Wastewater Treatment via a Dual-Hydrogel Process Coupling Acidic Partial Nitritation and Anammox

Abstract Partial nitritation and anammox (PN/A) process has been proposed as a sustainable biotechnology for mainstream nitrogen removal. However, it remains challenging to sustain high activities of ammonia-oxidizing bacteria (AOB) and anammox bacteria and suppress the growth of nitrite-oxidizing bacteria (NOB). In this study, a novel dual-hydrogel system comprising two reactors with distinct immobilized microbial communities was developed to integrate acidic PN and anammox for low-strength mainstream wastewater treatment. The system achieved a total inorganic nitrogen removal efficiency of 81.7% at a short overall hydraulic retention time of 5.5 h. In the acidic PN reactor, hydrogel beads effectively immobilized and enriched acid-tolerant AOB (“Candidatus Nitrosoglobus”), while creating a localized low-pH niche with sufficient free nitrous acid microenvironment to selectively inhibit NOB growth. This enabled high-rate PN without strict dissolved oxygen limitation. A pH-based feeding strategy allowed the bulk pH of the acidic PN reactor to be self-maintained at ∼6.1, achieving an appropriate effluent ratio of nitrite to ammonium and minimizing acid inhibition of the downstream anammox process. In the anammox reactor, the beads maintained a relatively higher internal pH, forming a favorable niche for retained anammox bacteria (“Candidatus Brocadia”) and facilitating high-rate nitrogen removal. The hydrogel beads also exhibited excellent structural stability during long-term operation (over 300 days). These results demonstrate an effective approach for simultaneously sustaining AOB and anammox activities while suppressing NOB, providing a promising approach for stable PN/A to treat mainstream wastewater.