The multistage A/O process exhibits strong biological nitrogen removal capability. The anoxic zone is filled with anaerobic bacteria-enriched progressive anaerobic balls, achieving removal of organic matter and total nitrogen through denitrification while simultaneously realizing sludge reduction. The effluent from the anoxic zone enters the multistage aerobic zone, which is filled with biomass-enriched sponge media, enabling the degradation of residual organic matter and nitrification of ammonia nitrogen through nitrification.