Rapid transition from complete nitrification to partial nitrification-anammox at low temperatures via thermal inactivation of nitrite oxidoreductase

[Display omitted] •Thermal treatment for activated sludge expands the difference between AUR and NUR.•The proliferation of Nitrotoga in the seeding biofilm may deteriorate the PN/A system.•NUR is mainly contributed by anammox biofilm in the coupling system.•Thermal treatment for anammox biofilm can rapidly restore efficient TNRR.•Thermal treatment mechanism is NOR selective inactivation rather than NOB washout. The stable and efficient biological nitrogen removal at low temperatures has always posed a challenge for the engineering application of the partial nitrification-anammox (PN/A) process. In this study, a complete nitrification system operating at 15 ± 1 °C was rapidly switched to PN/A mode and stably operated through thermal treatment of thickened sludge and anammox biofilm. Batch test results revealed that the ammonia uptake rate (AUR) and nitrite uptake rate (NUR) of activated sludge, after being treated at 35 °C for 2 days, decreased by 22.88 % and 96.10 %, respectively. Similarly, anammox biofilm exhibited no significant change in anammox activity (ram), but a significant decrease in NUR by 52.7 % after being treated at 40 °C for 2 days. Long-term coupling experiments demonstrated that the complete nitrification system at 15 °C was rapidly switched to PN/A mode with a TN removal rate (TNRR) of 87.91 ± 1.53 % through selective inactivation between ammonia monooxygenase (AMO) and nitrite oxidoreductase (NOR) at 35 °C for 2 days for activated sludge. Furthermore, the coexistence of Nitrospira in the seeding anammox biofilm and the proliferation of Nitrotoga in the biofilm led to the deterioration of the PN/A system. However, the collapsed TNRR could be rapidly restored to 96.42 % through selective inactivation between hydrazine synthase (HDH) and NOR at 40 °C for 2 days for anammox biofilm. The results of key enzymatic activity and Illumina MiSeq sequencing indicated a significant decrease in NOR activity but no significant change in community structure after thermal treatment for both thickened sludge and anammox biofilm. Therefore, the rapid transition of the PN/A system in a complete nitrification system at low temperatures was achieved through the selective inactivation of enzymes between NOR and AMO/HDH, rather than NOB washout.