Effect of mean cell residence time on the performance and microbial diversity of pre-denitrification submerged membrane bioreactors

The effect of mean cell residence time (MCRT) (5, 8.3, 16.7, and 33.3d) on domestic wastewater treatment performance had been investigated using four bench-scale pre-denitrification submerged membrane bioreactors (MBR) operated in parallel. The 33.3-d MCRT MBR had the lowest microbial activities in terms of specific oxygen uptake rate, specific denitrification rate and observed sludge yield. Excellent COD removal efficiency (more than 95%) and nitrification (more than 97%) were observed in all the four MBRs investigated. Even though high nitrification can be achieved in all the MBRs, total nitrogen (TN) removal efficiency was found to be affected by MCRT with a maximum of 77% at 33.3-d MCRT. Better TN removal efficiency achieved in the 33.3-d MCRT MBR was due to the combined effect of high mixed liquor concentration and lower dissolved oxygen concentration in the recycled mixed liquor. A comparison of terminal-restriction fragment length polymorphisms (T-RFLP) fingerprints based on 16S rRNA and nirS gene revealed that the microbial communities of 5- and 8.3-d MCRT are grouped under the same branch while 16.7- and 33.3-d MCRT are grouped in another branch. T-RFLP based on amoA gene shows that members from the Nitrosomonas genus were more dominant under shorter MCRT operating environment. Clustering analysis did not show any correlation with the organic and nitrogen removal performance obtained in this study.