Feasibility and performance of frozen algal-bacterial granular sludge for rapid startup of brackish aquaculture wastewater treatment system

This study assessed the feasibility of using freezing (−20 °C) to preserve the viability and functionality of salt-tolerant functional species in algal-bacteria granular sludge (ABGS) for the rapid startup of processes treating brackish aquaculture wastewater (BAW). After 24 days of adaptation during the cryo-reactivation, both R1 (fresh aquaculture wastewater, FAW) and R2 (BAW) achieved stable performance and nearly 100 % recovery of their pre-freezing permanganate index (COD_Mn) removal efficiencies. R1 restored 94.5 % of the pre-freezing total inorganic nitrogen (TIN) removal level and 70.0 % of the PO₄³⁻-P removal level, while R2 regained 87.8 % and 75.8 % of these levels, respectively. The rapid growth of filamentous microorganisms (e.g., norank_f__Saprospiraceae, Leptothermofonsia, and Jaaginema) led to worsening of granule settleability and subsequent biomass loss in R1. In contrast, R2 exhibited good granule stability and settleability, despite its slower biomass accumulation rate. The good settleability of ABGS in R2 can be attributed to the enrichment of EPS-secreting bacteria (Proteobacteria) and greater algal richness. Key salt-tolerant bacteria involved in organic carbon metabolism, phosphorus removal, and nitrification-denitrification were well preserved in R2. Cryo-reactivation shows potential as a strategy for the rapid startup of ABGS processes for BAW treatment, although further optimization is needed to ensure complete functional restoration.