Phosphorus removal in a membrane-assisted BNR process with focus on evolutions of PAOs and DPAOs

A bench-scale UCT (University of Cape Town)-type membrane bioreactor (UCT–MBR) fed with low-strength synthetic wastewater was operated to investigate phosphorus removal with reference to poly-phosphate accumulating organisms (PAOs) and denitrifying poly-phosphate accumulating organisms (DPAOs). A series of kinetic assays of PAOs and DPAOs were also conducted to analyze the metabolic activities of PAOs and DPAOs. Results showed that 93% of chemical oxygen demand (COD) and 77% of total nitrogen could be removed at 0.08 kgCOD kg⁻¹ MLSS d⁻¹ and 0.015 kgN kg⁻¹ MLSS d⁻¹ loading (MLSS: mixed liquor suspended solids). Removal efficiencies of total phosphorus increased during the experimental phase, with an ultimate removal efficiency of 96.1%. Kano and Kaer increased from 1.95 and 6.29 mgPO₄³⁻-P g⁻¹ MLSS h⁻¹ to 5.47 and 11.13 mgPO₄³⁻-P g⁻¹ MLSS h⁻¹ for DPAOs and PAOs respectively, with the increased ratio of DPAOs to PAOs from 31 to 49% implying DPAO metabolic activity increased faster than that of PAOs during the DPAO accumulation phase. Pano-uptake increased by 6.6 mg L⁻¹ and the ratio of PTano-uptake to PTupt increased from 58.97 to 91.62%. The ratio of DPAOs to PAOs tended to stabilize at around 50% over time.