Rapid start-up of an aerobic granular sludge system for nitrogen and phosphorus removal through seeding chitosan-based sludge aggregates

This study presents a novel strategy to accelerate the start-up of aerobic granular sludge (AGS) system and ensure the nutrient removal during cultivation. This new method consists of preparing the chitosan-based sludge aggregates outside the reactor and then seeding the reactor with such sludge aggregates. To prepare chitosan-based sludge aggregates, chitosan was dissolved in acetic acid solution acting as a cationic flocculant to bind negatively charged sludge together, and then the dissolved chitosan was in situ precipitated by readjusting pH to form stable sludge aggregates. The chitosan-induced charge neutralization and water-insolubility of chitosan were the two main reasons for the super-rapid formation of chitosan-based sludge aggregates. The as-prepared chitosan-based sludge aggregates had a much lower sludge volume index at 30 min (SVI 30 ) (90.1 mL/g) than the original sludge (SVI 30  = 328.0 mL/g). They also had some AGS-like characteristics such as large particle size (1300 μm) and fast settling velocity (23.8 m/h). Consequently, short settling time can be achieved and excessive biomass wash-out can be avoided in the rapid start-up of AGS system with chitosan-based sludge aggregates as inoculant, which was beneficial to accelerating sludge granulation while maintaining nutrient removal. Additionally, the abundances of filamentous bacteria and Candidatus Accumulibacter and the content of extracellular polymeric substances increased during cultivation, which could also contribute to the AGS formation. By seeding chitosan-based sludge aggregates in the anaerobic/oxic sequencing batch reactor , complete granulation was rapidly achieved in 10 days, and good removals of nitrogen and phosphorus was obtained after 14–18 days of cultivation.