Operational characteristics in varied gradients of low carbon-to-nitrogen ratios utilizing a novel integrated bacteria-algae synergistic biofilm reactor for wastewater treatment

Evaluating a novel algal-bacterial biofilm integrated reactor (A-BBR) designed to reduce energy consumption and greenhouse gas emissions, wastewater treatment characteristics were simulated at low carbon-to‑nitrogen ratios. Pollutant removals were investigated under low organic loads at different carbon-to‑nitrogen ratios (ranging from 7.24 to 4.59). COD and NH4+-N removal rates surpassed 90 %, with TN removal exceeding 80 %, demonstrating a notable performance improvement over the fixed-bed biofilm reactor (FBBR). The efficient utilization of organic substrate during denitrification by the bacteria-algae symbiosis system resulted in an increase in nitrogen removal per unit concentration of organic matter. Tracking studies of photosynthetic oxygen production rate versus specific aerobic rate and chlorophyll in the reactor demonstrated that additional aeration was unnecessary during the photoreaction phase, reducing energy losses. The decrease in the carbon-to‑nitrogen ratio was positively correlated with the increase in facultative diatom species within the biofilm. As the carbon-to‑nitrogen ratio had decreased from 7.24 to 4.59, the content of diatoms had increased by over tenfold compared to the initial stage. Furthermore, as the diversity of algae species increased, the difference between the photosynthetic oxygen production rate and the specific aerobic rate had risen from 7.6 to 13.8. This indicated that the algal-bacterial biofilm system with a richer diversity of algae exhibited a higher photosynthetic oxygen production rate compared to the monoalgal system. These results furnish valuable insights for future inquiries into the utilization and underlying mechanisms of bacterial and algal biofilms in the context of water treatment.