Reversibility of fouling formed in activated sludge filtration

This paper investigates the reversibility of membrane fouling by activated sludge in a membrane bioreactor equipped with a 0.1 mu m pore ceramic membrane. The membrane was submitted to a series of tests in which the permeate flux, the transmembrane pressure (TMP) or the circulation velocity were successively varied in cycles by step increments or decreases. When the permeate flux is set below the critical flux, the TMP remains stable and fouling is reversible. On the contrary, when the critical flux is exceeded, the TMP increases and does not stabilize, as in dead-end filtration. The fouling formed is partly irreversible when the dux is lowered again. When the TMP is first increased up to 400 kPa and then decreased back at constant velocity, no hysteresis is found on the flux-TMP graph, showing that fouling is reversible in this case. Velocity cycles were performed by first lowering the velocity from 5 to 1 m/s and raising it again to 5 m/s. In this case again, the fouling induced by reducing the velocity was found to be reversible. However, when the same pressure and velocity cycle tests were performed with activated sludge collected in the aeration tank of a classical wastewater treatment plant, fouling was found to be partly irreversible, showing that the cake formed in the absence of shearing is much more cohesive. In the final part of the paper, we tested a hydrodynamic method of fouling control consisting in alternating short periods of filtration (1-4 s) and shea periods of washing (1 or 2 s) at low TMP and high velocity. This method yielded to a 20% permeate flux increase with a 10% reduction in hydraulic energy consumption for classical plant activated sludge. (C) 1999 Elsevier Science B.V. All rights reserved.