Influence mechanism of sludge bed position on microalgal-bacterial granular sludge process

Sludge bed position in the reactor is one of the key parameters for microalgal-bacterial granular sludge (MBGS) process, which lacks of study. To fill this gap, this study investigated the influence of sludge bed position on MBGS. The sludge bed located closer to the bottom of the bioreactor demonstrated the optimal pollutant removal performance due to a close synergistic effect between microalgae and bacteria, resulting in the high growth rate as well as agglomeration rate of MBGS. Specifically, organics and ammonia removals were closely related to the sludge bed position. For the bottom bed position, the removals of organic matter, ammonia, and phosphate were 75.1 %, 73.1 %, and 82.5 %, whereas for the top bed position, they were only 13.2 %, 9.6 %, and 68.9 %, respectively. Additionally, a significant correlation between the position of the sludge bed and the relative abundance of Rotifera (R² = 0.931) and Chlorophyta (R² = 0.733) was observed, while the microbial communities at the lower sludge bed positions underwent rapider succession. It can be inferred that that a sludge bed located closer to the bottom of the bioreactor ensures that the light source and substrate matrix are transmitted in the same direction, thereby resulting in a close synergistic effect between microalgae and bacteria for achieving the excellent performance of MBGS. These results can provide basis knowledge for engineering application of MBGS process.