Extended filaments of bulking sludge sink in the floc layer with particulate substrate

•Extended filaments sink in the floc layer when particulate substrate was fed.•Limited substrate in bulk was the reason for disappearance of extended filaments.•A model was proposed to explain the evolution of bulking inducing and controlling.

Recent studies have showed that particulate substrate has a positive effect on sludge settleability. This study was driven by the question of how the extended filaments disappeared when soluble substrate was switched to particulate substrate. A lab-scale aerobic completely stirred tank reactor was performed and evaluated for a period of two months. The results showed that extended filaments were quickly induced with soluble substrate. Total extended filament length of activated sludge decreased and sludge settleability was significantly improved after the soluble substrate was switch to particulate substrate. Due to the lack of hydrolysis products in the bulk liquid when particulate substrate was fed, extended filaments were forced to grow towards the inside of the flocs and finally sink in the floc layer. A physical model was proposed to explain the evolution of extended filaments caused by soluble substrate and cured by particulate substrate.