Partial nitrification and simultaneous denitrification in sequential anaerobic and aerobic reactors: performance and microbial community dynamics

The removal of organic matter and nitrogen from domestic sewage was evaluated using a system composed of two sequential reactors: an anaerobic reactor (ANR) with suspended sludge and an aerobic (AER) reactor with suspended and adhered sludge to polyurethane foams. Nitrogen removal consisted of AER operating at low dissolved oxygen (DO) concentrations; this favoured the simultaneous nitrification and denitrification (SND) process. The concentration of COD and N were 440 mgO₂.L⁻¹ and 37 mgTN.L⁻¹, respectively. The operation was divided into three phases (P), lasting 51, 53, and 46 days, respectively. The initial DO concentrations applied in the AER were: 3.0 (PI) and 1.5 mg.L⁻¹ (PII and PIII). In PIII, the AER effluent was recirculated to the ANR at a ratio of 0.25. Kinetic assays were performed to determine the nitrification and denitrification rates of the biomasses (ANR and AER in PIII). Changes in the microbial community were evaluated throughout phases PI to PIII by massive sequencing. In PIII, the best results obtained for chemical oxygen demand (COD) and total nitrogen (TN-N) removal efficiencies, were close to 94% and 65%, respectively. Under these conditions, system effluent concentrations below 30 mg COD.L⁻¹ and 15 mg TN-N.L⁻¹ were verified. The nitritation and nitration rates were 10.5 and 6.5 mg N.g VSS⁻¹.h⁻¹, while the denitrification via nitrite and nitrate were 6.8 and 5.8 mg N.g VSS⁻¹.h⁻¹, respectively. A mixotrophic community was prevalent, with <i>Rhodococcus, Nitrosomonas, Pseudomnas, and Porphyromonas</i> being dominant or co-dominant in most of the samples, confirming the SND process in the AER sludge.