An innovative membrane bioreactor (MBR) system for simultaneous nitrogen and phosphorus removal

• Novel integration of post-denitrification with a rough settler and MBR.
• Effective organic degradation and nutrient (N and P) removal by the MBR system.
• Rough settler for sludge–water separation and anaerobic P release.
• Increases of PAO and PHB densities in the sludge for luxury P uptake.

Membrane filtration was integrated with a post-denitrification process to form an innovative membrane bioreactor (MBR) system for effective organic degradation and nutrient (N and P) removal. The system comprised of an aerobic tank, an anoxic tank, an intermediate sedimentation tank, and a membrane filtration tank. The sedimentation tank functioned not only as a rough settler for sludge–water separation before membrane filtration but also as an anaerobic chamber for P release. While half of the influent flowed into the aerobic tank, the other half was fed into the anoxic tank to favor the proliferation of phosphorus accumulating organisms (PAOs). The experiment was conducted continuously for about 430 days. With a short overall treatment time of less than 10 h for municipal wastewater, the MBR-based process could achieve the total organic carbon, total nitrogen, and total phosphorus removals of around 94%, 85%, and 87%, respectively. The growth and activity of PAOs in the MBR system were evidenced by the significant P release in the anaerobic chamber followed by the luxury P uptake in the membrane tank. With the DAPI and PAOmix probe staining, the increases of PAOs and polyhydroxybutyrate (PHB) in sludge during the experiment were well observed under the fluorescent microscope.