[Nitrogen Removal in Low-C/N Rural Sewage Treatment by Anoxic/Oxic Biofilter Packed with New Types of Fillers]

When low-concentration rural sewage is treated biologically, the effluent total nitrogen (TN) concentration often does not meet the discharge limit because of its low carbon-to-nitrogen ratio (C/N). To solve this problem, a laboratory-scale anoxic/oxic (A/O) biofilter packed with Arundo donax and activated carbon as the anoxic and aerobic column fillers (No. 2) was operated for treatment of simulated rural sewage and advanced nitrogen removal, while an ordinary gravel-packing A/O biofilter (No. 1) was set up as the control group. The results were as follows. When the influent chemical oxygen demand (COD), ammonia nitrogen (NH₄⁺-N), and TN concentrations were (79.47±14.21), (34.49±2.08), and (34.73±3.87) mg·L^-1, respectively, the No. 1 and No. 2 reactors achieved removal efficiencies of (88.00±7.00)% and (89.00±10.00)%, (90.00±2.00)% and (97.00±7.00)%, and (37±15)% and (68±7)%, respectively. The results revealed that using Arundo donax and activated carbon new fillers could significantly enhance NH₄⁺-N and TN removal. High-throughput sequencing results indicated that the microorganisms involved in the nitrification process in the No. 1 reactor mainly belong to Proteobacteria, whereas those in the No. 2 reactor belong to Proteobacteria and Nitrospirae. In addition, the main denitrification bacterial phyla in the anoxic column of the No. 1 reactor were Chloroflexi, Proteobacteria, Bacteroidetes, and Planctomycetes, whereas those in the anoxic column of the No. 2 reactor were primarily Bacteroidetes, Proteobacteria, Firmicutes, and Patescibacteria. Quantitative real time polymerase chain reaction (qPCR) results showed that the microbial nitrification (amoA and Nitrospira 16S rDNA), denitrification (narG, nosZ, nirS, and nirK), and anaerobic ammonium oxidation functional genes (ANAMMOX) in the No. 2 reactor were significantly higher than those in the No. 1 reactor. All the genes, except for the narG and nosZ genes, had one to two orders of magnitude of improvement in the No. 2 reactor compared to those in the No. 1 reactor.