Enhancement of Anammox performances in an ABR at normal temperature by the low-intensity ultrasonic irradiation

[Display omitted] •An ultrasound enhancing reactor based on the ABR (ABRU) was established.•ABRU run around 25 °C to start up and operate Anammox process.•Quick start-up of Anammox process was achieved in ABRU.•The max NLR and NRR attained 0.50 kg N m−3 d−1 and 0.43 kg N m−3 d−1 respectively.•Microbial analysis and morphology observation of mature sludge was conducted. A lab-scale ultrasound enhancing Anammox reactor (ABRU) was established and irradiated once a week by ultrasound with the optimal parameter (frequency of 25.0 kHz, intensity of 1.00 W cm−2 and exposure time of 36.0 s) obtained by response surface methodology (RSM). ABRU and the controlled Anammox reactor (ABRC) without ultrasonic treatment were operated in parallel. The start-up time of Anammox process in ABRU (59 d) was shorter than that in ABRC (69 d). At the end of the nitrogen load-enhancing period, NLR (0.500 kg N m−3 d−1) and NRR (0.430 kg N m−3 d−1) in ABRU were both higher than NLR (0.400 kg N m−3 d−1) and NRR (0.333 kg N m−3 d−1) in ABRC. The results of RTQ-PCR demonstrated that the specific low-intensity ultrasound irradiation improved the enrichment levels of AnAOB in mature sludge. SEM images and the observation of the macroscopic morphology of mature sludge showed that the ultrasound irradiation strengthened the formation of Anammox granular sludge, thereby improved the interception capacity and impact load resistance of the reactor, and enhanced the nitrogen removal performance in ABRU. The ultrasonic enhanced Anammox reactor based on an ABR with the optimal parameters can promote the rapid start-up and efficient and stable operation of the Anammox process at normal temperature (around 25.0 °C).