Effect of temperature decrease on anammox granular sludge: Shock and adaptation

Cryopreservation is one of the effective methods for the preservation of anammox granular sludge (AnGS). However, the effects of cooling pretreatment on AnGS are still unclear. In this study, the effects of temperature decrease on AnGS property were investigated by designing different cooling modes: constant at room temperature 20–25 °C (CK), sharp cooling to 4 °C (S4), −20 °C (S20) and stepwise cooling to 4 °C (A4), −20 °C (A20). The results showed that compared with CK, the cooling modes in S4, S20, A4 and A20 improved the physical preservability of AnGS, slowing down the changes of color, shape and structure; and elevated the preservation rate of functional bacteria Planctomycetes (phylum level) and Candidatus Brocadia (genus level). The preservation rate of live cells in different experimental groups was 48.4 ± 1.8%(CK), 61.1 ± 3.3%(S4), 37.8 ± 0.8%(S20), 81.7 ± 4.8%(A4), 61.9 ± 3.1%(A20), respectively. The Anaerobic Ammonium Oxidation Bacteria (AnAOB) in the stepwise cooling mode (A4 and A20) were found to enter the dormant state and form “dormant zoogloea”, while the AnAOB in the sharp cooling mode (S4 and S20) were observed to enter the shock state with a little change. The findings in this work (especially the dormant state of AnAOB) are helpful to understand the effect of temperature decrease on AnGS and to promote the development of AnGS preservation technology. [Display omitted] •Sharp cooling could cause AnAOB to enter the shock state in AnGS.•Stepwise cooling could cause AnAOB to form dormant zoogloea and enter dormant state in AnGS.•Stepwise cooling was more suitable for the pretreatment of AnGS before preservation.