Insight into the role of chitosan in rapid recovery and re-stabilization of disintegrated aerobic granular sludge.

The disintegration and instability of aerobic granular sludge (AGS) systems during long-term operation pose significant challenges to its practical implementation, and rapid recovery strategies for disintegrated AGS are gaining more attention. In this study, the recovery and re-stabilization of disintegrated AGS was investigated by adding chitosan to a sequencing batch reactor and simultaneously adjusting the pH to slightly acidic condition. Within 7 days, chitosan addition under slight acidity led to the re-aggregation of disintegrated granules, increasing the average particle size from 166.4 µm to 485.9 µm. Notably, sludge volume indexes at 5 min (SVI5) and 30 min (SVI30) decreased remarkably from 404.6 mL/g and 215.1 mL/g (SVI30/SVI5 = 0.53) to 49.1 mL/g and 47.6 mL/g (SVI30/SVI5 = 0.97), respectively. Subsequent operation for 43 days successfully re-stabilized previous collapsed AGS system, resulting in an average particle size of 750.2 µm. These mature and re-stabilized granules exhibited characteristics of large particle size, excellent settleability, compact structure, and high biomass retention. Furthermore, chitosan facilitated the recovery of COD and nitrogen removal performances within 17-23 days of operation. It effectively facilitated the rapid aggregation of disintegrated granules by charge neutralization and bridging effects under a slightly acidic environment. Moreover, the precipitated chitosan acted as carriers, promoting the adhesion of microorganisms once pH control was discontinued. The results of batch tests and microbial community analysis confirmed that chitosan addition increased sludge retention time, enriching slow-growing microorganisms and enhancing the stability and pollutant removal efficiency of the AGS system.

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 (SVI30) (90.1 mL/g) than the original sludge (SVI30 = 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.