Changes in activity and microbial community composition of Anammox-HAP granules during long-term preservation under different conditions.

Preservation of anammox granular sludge is important for anammox technology applications. Although previous studies have explored preservation methods, their long-term effects on microbial communities and functional genes remain underexplored. This study investigated the long-term preservation of anammox-hydroxyapatite (HAP) granules with storage durations of up to six years and examined the effects of different preservation methods. Results show that 4°C with 5 mM molybdate not only prevented the blackening of granules but also maintained a lower decay rate of specific anammox activity, preserving >50% after 6 months and 10% after 1 year. Functional gene analysis revealed an increase in sulfur-reducing genes such as dcyD and NADPH, particularly in samples without molybdate. These changes may result in hydrogen sulfide production, which contributes to sludge blackening. This study provides critical insights into optimizing the preservation conditions for anammox-HAP granules, facilitating rapid reactor start-up, and offering a viable solution for long-term storage.