Insights into the assistance of bacteria in digested wastewater pollutants removal by microalgae from the aspect of flow and mixing characteristics.

Photosynthetic symbiotic microorganisms (PSM, photosynthetic microalgae and heterotrophic bacteria) show great potential for the treatment of digested piggery wastewater (DPW). However, how bacterial metabolic activities affect substance exchange ability between microalgae and bacteria, and their subsequent impact on pollutant removal remains unclear. Thus, the flow characteristics, gas exchange, and mass mixing of PSM suspension were investigated using a hydrodynamic method. The findings demonstrated that the viscosity of the PSM increased to 1.09 mPa s due to the extracellular polymeric substance secreted by bacteria, which was higher than the value of 0.83 mPa s observed in photosynthetic microorganisms (PM, photosynthetic microalgae). The increased viscosity of the suspension extended residence time of the CO2 bubble (845 ms) in liquid phase, which facilitated sufficient contact between CO2 and microalgae and enhanced capture probability. In addition, flow simulations indicated that the increased CO2 bubble residence time resulted in a 2.7% higher gas holdup in PSM compared to PM, thereby enhancing the total dissolved inorganic carbon (DIC) concentration to 329.7 mg L-1. Simultaneously, compared with PM (2.79E-8 m2 s-2), the higher turbulent kinetic energy in PSM (4.77E-8 m2 s-2) facilitated material mixing and exchange. Therefore, the maximum COD removal rate in PSM was 649 mg L-1 d-1, significantly higher than the 565.5 mg L-1 d-1 observed in PM. The maximum NH4+-N removal rate in PSM increased by 20.1%. In summary, this study provided new insights into the collaborative treatment of DPW using PSM from the perspective of flow characteristics.