Important roles of immobilization in improving low-temperature nitrogen removal of cold-shock Anammox sludge and related mechanism

Efficient nitrogen removal via the Anammox process at low temperatures is challenging, as it relies heavily on the maintenance of Anammox activity and biomass. In this study, a novel combination of biological immobilization and cold shock treatment was successfully applied to enhance specific Anammox activity (SAA) and biomass retention under different low temperatures. The results indicated that the gel particles immobilizing low cold-shock biomass (only 0.53 g/L) achieved superior SAA and exhibited excellent mechanical strength at 25–5°C. Additionally, the SAA and mass transfer performance of the immobilized cold-shock particles were significantly influenced by particle size, with the optimal range determined to be 3.2–4.0 mm. Consequently, immobilized cold-shock particles (M1) were prepared with an optimal biomass content of 0.53 g/L and a particle size of 3.2–4.0 mm. At low temperatures of 10–5°C, the M1 particles demonstrated in-situ nitrogen removal efficiencies of 52 %-72 % and ectopic SAA values of 3.0–4.5 µmol/(g VSS·h), markedly outperforming non-immobilized cold-shock granular sludge (M0). This was attributed to enhanced biomass retention and improved ammonium conversion kinetics facilitated by immobilization, particularly at 10 and 5°C. Moreover, the higher abundances of Candidatus Kuenenia and Candidatus Brocadia within M1 particles were critical for sustaining nitrogen removal performance under extreme low-temperature conditions. Importantly, the upregulated expression of cold proteins (CspA, CspB, and PpiD) and the increased content of C 18 [5]-ladderane lipids in M1 particles significantly contributed to the enhanced low-temperature adaptability of the cold-shock sludge. • Immobilizing low cold-shock biomass achieved effective nitrogen removal at 25–5°C. • Immobilization improved significantly kinetic property of cold-shock sludge at 5–10℃. • Immobilization promoted the low-temperature proliferation of Anammox genus. • Gel particles maintained cold adaptability by upregulating cold proteins expression. • The increased C 18 [5]-ladderane lipid promoted transfer performance of gel particles.