Dissolved oxygen control in photosynthetic-oxygen supported algal-bacterial granule system to better coordinate microalgae with bacteria

Algal-bacterial granule (ABG) process is a promising biotechnology for carbon emission reduction, especially for photosynthetic O₂-supported ABG (POS-ABG) systems without mechanical aeration. However, the roles of dissolved oxygen (DO) generated by coexisting microalgae remain unclear in this POS-ABG system. This study aimed to explore the effects of different DO control levels (0.5-1.0, 1.0-1.5, and 2.0-2.5mg/L) on POS-ABG system performance, in which DO levels were automatically controlled via a feedback control unit between DO concentration and light illumination duration. Results show that with the increase in DO level chlorophyll-a content increased together with more assimilation of inorganic carbon, resulting from a prolonged light illumination. However, the low DO levels of 0.5-1.0 and 1.0-1.5mg/L failed to maintain efficient nutrient removal due to O₂ mass transfer limitation. By contrast, when DO was controlled at 2.0-2.5mg/L, the microalgae and functional bacteria associated with nutrient removal were found to well coordinate with each other, achieving a low CO₂ emission of 0.38kg-CO₂/kg of chemical oxygen demand (COD), and highly efficient total nitrogen (95%) and phosphorus (98%) removal. This study provides scientific evidence for the key design parameter DO in POS-ABG systems. © 2025 Elsevier Ltd.