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).

A lab-scale ultrasound enhancing Anammox reactor (R1) was established and irradiated once a week by ultrasound with the optimal parameter (frequency of 25 kHz, intensity of 0.2 W cm-2 and exposure time of 3 min) obtained by batch experiments. R1 and the controlled Anammox reactor (R2) without exposure to the ultrasound were operated in parallel. The start-up period of Anammox process (53 days) in R1 was shorter than that (61 days) in R2. The nitrogen loading-enhancing period (day 53-day 135) in R1 was also shorter than that (day 61-day 151) in R2. At the end of the nitrogen loading-enhancing period, NLR (0.76 kg N m-3 d-1) and NRR (0.68 kg N m-3 d-1) of R1 were both higher than NLR (0.66 kg N m-3 d-1) and NRR (0.56 kg N m-3 d-1) of R2. Moreover, The stability of Anammox process in R1 was better than that in R2. The results demonstrated that the periodical irradiation of ultrasound enhanced the start-up and operational performance of Anammox reactor. Microbial community analysis indicated that the ultrasound accelerated the microbial succession from some other bacteria to Anammox bacteria so that shorten the start-up period of Anammox process from the conventional activated sludge. It also indicated that the ultrasound strengthened the competitive advantage of Candidatus Kuenenia stuttgartiensis in Anammox bacteria of the mature sludge so as to enhance the nitrogen removal performance of the Anammox reactor under the operation condition of high nitrogen loading.

Novel honeycomb-like carriers, made of polypropylene, were applied to enhance biomass retention capacity so as to improve Anammox start-up performance in a fixed bed reactor (FBR). The reactor was operated for 3 months. On day 45, Anammox activity appeared. After 61 days operation, the removal efficiencies of ammonium and nitrite were both over 91% based on 70 mg N L⁻¹ of the influent ammonium and influent nitrite, indicating that a remarkable Anammox activity was attained. A final specific Anammox activity of 0.12 g NH₄⁺-N (gVSS*d)⁻¹ was reached. The FBR showed a good capacity of resisting shock loading and preferred resisting shock loading of nitrogen concentration to resisting hydraulic shock loading. Phylogenetic analysis showed that Candidatus Brocadia anammoxidans and Candidatus Kuenenia stuttgartiensis were detected in the mature biofilm, and Candidatus Brocadia anammoxidans was the dominant Anammox strain. Candidatus Kuenenia stuttgartiensis played a positive role in the reactor performance, as it could consumed nitrite quickly and efficiently so as to avoid an adverse effect of temporary nitrite accumulation. The results showed that the honeycomb-like carriers were suitable for start-up of Anammox.