Evaluation of intermittent air sparging in an anoxic denitrification membrane bioreactor

The impact of intermittent air sparging on the operation of an anoxic (dissolved oxygen <0.1 mg l⁻¹) immersed membrane bioreactor (iMBR) applied to potable water denitrification is discussed. Air sparge length and specific aeration demand per unit membrane area (SAD_m) were varied to determine impact on oxygen transfer and membrane fouling. For SAD_m > 0.39 m³ m⁻² h⁻¹ with sparge lengths of 10 to 60 seconds, a low dissolved oxygen residual of 0.05 to 0.90 mg O₂ l⁻¹ was formed which typically inhibited denitrification; oxygen transfer efficiency increased with increasing sparge time. Residual oxygen was rapidly consumed at a rate, r_O2, of 0.35 mg O₂ l⁻¹ min⁻¹. Once oxygen was depleted, denitrification proceeded. When intermittently sparging at a SAD_m < 0.39 m³ m⁻² h⁻¹ for 30 seconds (following 10 minute dead-end filtration cycles in the iMBR), no dissolved oxygen residual was observed and a flux of 21 l m⁻² h⁻¹ was sustained with fouling rates <0.001 m bar min⁻¹ recorded. This method provides for effective integration of air sparging into anoxic/anaerobic iMBR environments to simplify process design and delivers a tangible reduction in specific energy demand from 0.19 kWh m⁻³ (for constant sparging) to 0.007 kWh m⁻³.