Rapid aerobic sludge granulation by short-term in-situ chitosan dosing: Performance, mechanism and microbial response

This study investigated the feasibility of accelerating aerobic granular sludge (AGS) formation by short-term in-situ dosing of chitosan (CTS) in sequencing batch reactors. Compared with the control reactor, short-term CTS dosing during start-up markedly promoted biomass aggregation, shortened the complete granulation time from 27 to 9 days, and produced larger and denser granules with superior settleability. In the CTS-amended reactor, the mean granule size reached 752 µm, while sludge volume index at 30 min (SVI₃₀) rapidly decreased to 59.4 mL/g within 24 h and stabilized at 38.2-44.8 mL/g during steady operation. Mechanistic analyses showed that CTS reduced sludge surface electronegativity, stimulated extracellular polymeric substance (EPS) secretion, and functioned as both a cationic bridging agent and a physical nucleation core. These roles were further supported by direct visualization using CTS-coated Fe₃ O₄ particles as a tracer. Temporary pH control applied to activate the cationic properties of CTS initially suppressed nitrification, but the inhibition was reversible. After recovery, the CTS reactor achieved superior total nitrogen (TN) removal, with an average effluent TN of 8.6 mg/L compared with 15.3 mg/L in the control. The enhanced TN removal may be associated with the combined effects of larger and more compact granules and the possible contribution of residual CTS as supplementary electron donor. Microbial analysis further revealed the enrichment of EPS-producing and structural bacteria, especially Thiothrix, in the CTS reactor. Overall, short-term in-situ CTS dosing provides a simple and biocompatible strategy for rapid AGS start-up and improved nitrogen removal.