Enhanced biological phosphorus removal using granules in continuous-flow reactor

• A novel settling tank called “Tube Settler” was designed and applied.
• Phosphorus removal granules were operated successfully under continuous-flow.
• Granules with large size were effected more seriously in continuous-flow reactor.
• Continuous-flow operation repressed the metabolism of PAOs in the initial period.

To perform the enhanced biological phosphorus removal using granules in continuous-flow reactor (CFR), a novel CFR was designed and applied in this study. During the whole operation process, the variation of efficiency and characteristics of phosphorus removal granular sludge (PRGS) was investigated here. When the PRGS with good efficiency of phosphorus and COD removal was inoculated from a sequencing batch reactor (SBR) into the CFR, the phosphate and COD removal efficiency reduced from 99% to 64% and from 98% to 79%, respectively. Moreover, the structures of the granules turned looser and unstable, and the mean diameter reduced from about 950 μm to around 600 μm. However, after 24 days of adaptation, the efficiency and diameter recovered to the initial level. The phosphorus and COD removal efficiency increased to 95% and 96%, respectively. Meanwhile, the granules adapting to continuous-flow were obtained, and they were characterized with loose structure and irregular shape. In the adaptation period, the PRGS with large size was influenced seriously but that with small size was not when the PRGS was inoculated from the SBR to the CFR. In the initial period of adaptation, the metabolism of PAOs was repressed and then recovered gradually in about 20 days. Finally, the morphological and structural variation mechanism of PRGS in the CFR was discussed, and it would be helpful to guide the operation of continuous-flow system based on PRGS in practice.