This paper investigates the reversibility of membrane fouling by activated
sludge in a membrane bioreactor equipped with a 0.1 mu m pore ceramic membr
ane. The membrane was submitted to a series of tests in which the permeate
flux, the transmembrane pressure (TMP) or the circulation velocity were suc
cessively varied in cycles by step increments or decreases. When the permea
te 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 f
ormed 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 rev
ersible 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. H
owever, 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 fi
nal part of the paper, we tested a hydrodynamic method of fouling control c
onsisting in alternating short periods of filtration (1-4 s) and shea perio
ds of washing (1 or 2 s) at low TMP and high velocity. This method yielded
to a 208 permeate flux increase with a 10% reduction in hydraulic energy co
nsumption for classical plant activated sludge. (C) 1999 Elsevier Science B
.V. All rights reserved.