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 m(3) m(-2) h(-1) with sparge lengths of 10 to 60 seconds, a low dissolved oxygen residual of 0.05 to 0.90 mg O(2) 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(O(2)), of 0.35 mg O(2) l(-1) min(-1). Once oxygen was depleted, denitrification proceeded. When intermittently sparging at a SAD(m)<0.39 m(3) 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).