Performance of an aerobic/anaerobic hybrid bioreactor under the nitrogen deficient and low F/M conditions

A bioreactor system without a biomass-liquid separation unit is evaluated for its chemical oxygen demand (COD) removal and biomass retention capabilities under the nitrogen deficient and low F/M conditions that are known to produce bulking biomass. A fully oxygenated stream recycled from an external oxygenator delivers the oxygen to an upflow bioreactor in which a biomass zone is formed and maintained in the absence of gas effervescence. COD is removed with up to 90% efficiency by means of aerobic and anaerobic bacterial activities occurring in the biomass zone. The biomass is bulking which is brought about by the extensive filamentous growth caused by the nitrogen deficient and low F/M conditions adopted. However, the biomass zone is undisturbed at superficial upflow velocities as high as 0.66 cm/min, because it has a porous, mat-like matrix that is augmented by the entanglement of filamentous bacteria with the cell clusters. A low-VSS effluent (i.e.,<10 mg/L) is produced directly from the bioreactor.