Domestic wastewater treatment in a 40 L photobioreactor without sludge inoculum: Process performance and insights into nitrous oxide dynamics

• Successful ABGS formed without inoculum in real wastewater, showing stable granulation. • High COD and NH₄⁺ removal achieved; phosphorus removal remained limited. • N₂O emission factors ranged from 0.8 % to 3.4 % under varying DO and light. • High aeration increased k L a and enhanced N₂O stripping from the liquid phase. • DO availability had greater effect on N₂O than the presence or absence of light. This study investigated the performance and nitrous oxide (N₂O) emission dynamics of algal-bacterial granular sludge (ABGS) cultivated in a 40 L sequencing batch photobioreactor (SBPBR) treating real domestic wastewater, without initial external inoculum. ABGS formation was successfully achieved and remained stable over 180 days, with stable granule structure (> 1000 µm), good settling properties (SVI₃₀ of 42 mL· g VSS ⁻¹), and chlorophyll-a content of 1.2 ± 0.1 mg· g VSS ⁻¹. The system remained resilient to disturbances, including Tubifex tubifex proliferation, confirming the structural viability of ABGS under non-sterile conditions. Regarding treatment performance, the reactor achieved consistent COD removal (> 80 %) and efficient ammonium removal (> 97 %) after microbial community adaptation. However, phosphorus removal was moderate (52 %), limited by the lack of anaerobic cycling and absence of excess sludge removal. N 2 O dynamics were monitored under four operational scenarios: low/high dissolved oxygen (DO) (2-3 and 6-7 mg· L⁻¹) and with/without light. N₂O production on liquid phase was mainly influenced by DO concentration, as lower DO levels resulted in higher N₂O emissions, while light had only a minor effect on its dynamics. However, under high DO conditions (10 L· min⁻¹, k L a = 283 h⁻¹), N₂O in the gas phase (emission factor, EF) reached 3.4%, which was considerably higher than under low DO (3 L· min⁻¹, k L a = 100 h⁻¹), where EF remained below 1%. This outcome indicates that oxygen availability is the dominant driver of N₂O formation, with light exerting only a secondary influence. These results emphasize the dual control of N₂O by microbial pathways and physical mass transfer, underscoring the need to optimize aeration strategies in ABGS reactors to balance nitrogen removal and greenhouse gas mitigation.