Sludge reflux promotes biofilm formation in upflow membrane aerated biofilm reactor for partial nitritation: insight from computational fluid dynamics stimulation and experiments.

Achieving partial nitritation (PN) process using membrane aerated biofilm reactor (MABR) is a novel approach to provide nitrite for mainstream anammox, gained increasing interests. However, insufficient microbial-membrane attachment raises a challenge for biofilm formation in MABR-PN. This study employed computational fluid dynamics modeling to evaluate sludge volume fraction in biofilm formation zones, using liquid reflux and sludge reflux as comparative strategies for enhancing biomass attachment. The optimal performance was obtained in sludge reflux (5 min/4 h), with 40 % sludge volume fraction and the uniformity of sludge distribution in the biofilm formation zone was 80 %. Two bench-scale MABR were developed with liquid reflux (R1:400 % reflux) and sludge reflux (R2:5 min/4 h) under influent NH₄⁺-N concentration of 50 ± 2 mg/L. The biofilm formation time in R2 was significantly shortened 30 % compared to R1, and the nitrite accumulation rate of R2 was significantly higher than that of R1 by 18.5 %. 16S rRNA gene sequencing coupled with observation revealed that the extracellular polymeric substance (EPS)-producing microorganism was enriched in biofilm of R2, meanwhile the abundance of the Nitrosomonas in R2_biofilm was significantly higher than flocs. Moreover, in R2, the abundance of key synthesis genes for EPS in biofilm was 2.4-fold higher than in flocs. The EPS concentration determination showed that R2_biofilm > R1_flocs > R1_biofilm > R2_flocs, confirmed that the high EPS-producing capacity in R2 biofilms could promote biofilm formation. Sludge reflux could promote the formation of MABR biofilm and enable rapid start-up of the PN process. Additionally, the energy consumption of R2 was only 2.5 % of R1.