Interconnected macroporous foamed hydrogels enable stable and efficient nitrogen removal in low temperature Anammox...

Anammox process suffers from low nitrogen removal efficiency at low temperature due to the slow proliferation of functional bacteria and biomass loss. Conventional entrapment immobilization retains biomass, but its transport performance is jointly influenced by carrier size and pore architecture, while highly porous structures may compromise mechanical stability. To address this challenge, a foaming-freezing strategy was applied to fabricate a PVA-SA foamed hydrogel with three-dimensional interconnected thick-walled macropores for AnAOB entrapment, and its nitrogen removal performance was evaluated over a 130-day stepwise cooling experiment (25-15 °C). The foamed hydrogel achieved 82.25% porosity, with macropores (>10 μm) comprising 16.68%. Low field nuclear magnetic resonance (LF-NMR) and three-dimensional pore characterization revealed enhanced water mobility and an interconnected internal pore architecture, with porosity-based estimated NH₄⁺-N effective diffusivity was higher than those of gel beads and nonfoamed hydrogels. The carrier withstood 1.65 MPa at 80% compressive strain and maintained fatigue resistance after 100 compression cycles. At 15°C, it delivered a nitrogen removal rate of 72.5 ± 1.0 g N m^-3 d^-1, exceeding those of free sludge, gel beads, and nonfoamed hydrogels. Microbial analyses showed that foamed hydrogel was associated with enrichment of Candidatus Kuenenia (24.44% ± 2.27%) and a broader internal distribution of AnAOB within the hydrogel carrier. Quantitative PCR and metagenomics further revealed higher abundances of Anammox-related genes and lower functional potential associated with denitrification and DNRA, indicating a functional profile more strongly oriented toward Anammox. Overall, integrating foaming with freezing-based pore formation provided favorable diffusion characteristics while maintaining mechanical stability, offering a promising strategy for low-temperature Anammox immobilization.