The nitrogen removal and sludge reduction performance of a multi-stage anoxic/oxic (A/O) biofilm reactor

To overcome the problems of high excess sludge yield and poor nitrogen removal efficiency in traditional biological treatment processes, a multi-stage A/O biofilm reactor was developed by combining the multi-stage A/O process with novel floating spherical carriers, resulting in repeated coupling of anoxic and aerobic environments. Results showed that the system achieved COD, $$ , and TN removal efficiencies of 93.8%, 84.5%, and 75.7%, respectively, with average effluent concentrations lower than: 29.8 COD mg/L, 4.3 $$  mg/L, and 13.2 TN mg/L. The observed sludge yield was 0.139 g MLSS/g COD, which was lower than that of the conventional activated sludge process. Microbial analysis showed that the community structure and cell morphology of microorganisms changed greatly with alternating aerobic-anoxic condition; high-throughput sequencing results proved that functional microorganisms can be enriched on the surface of the carries and therefore improved the nitrogen removal efficiency and meanwhile minimize the sludge yield within the system. PRACTITIONER POINTS: The research innovatively developed a novel floating spherical carrier and coupled it with multi-stage A/O process. The complex redox environments inside the floating spherical carriers improves the nitrogen removal efficiency and the sludge reduction effect. Nitrospirae, Hydrogenophaga promoted the nitrogen removal, Firmicutes, Bacteroidetes and Dechloromonas promoted the in-situ sludge reduction of the system.