<p>Ammonia-oxidizing bacteria (AOB) are slow-growing autotrophs prone to washout. Biofilm carriers improve retention; however, conventional types suffer from low roughness, poor hydrophilicity, and unfavorable surface charge, limiting biofilm formation. In this study, a composite carrier was fabricated by loading chitosan (CS) and layered double hydroxide (LDH) onto a polyurethane (PU) sponge, which introduced hydroxyl (–OH) and amino (–NH₂) functional groups as well as Mg²⁺ and Al³⁺ ions onto the surface, increased surface roughness, and enhanced the carrier hydrophilicity by 29.2%. During a 55-d nitritation biofilm cultivation experiment, the carrier modified with 0.8 wt% LDH and 0.8 wt% chitosan exhibited significantly enhanced performance. Compared to the unmodified carrier, the sludge adsorption capacity increased by 80.3%, the biofilm biomass increased by 46.8%, and the biofilm growth rate increased by 198.3%, reaching 333.4 ± 9.5 mg/carrier, 2023.2 ± 31.5 mg/carrier, and 103.2 mg/(carrier·d) respectively. In addition, the biofilm stability on the carrier was significantly enhanced, with a 54.1% reduction in sludge detachment under ultrasound treatment compared with the unmodified carrier. The nitritation reactor with the CS/LDH-PU carrier maintained stable nitritation performance under high ammonia loading (1.0 g/(L·d)) and a higher sludge concentration (5.5 g/L), while the reactor without the carrier collapsed at a lower sludge concentration (4.6 g/L). These findings suggest that the CS/LDH-PU carrier provides an effective strategy for optimizing conventional nitritation carriers and enhancing the resilience of nitritation systems under high ammonia load conditions.</p>