In the microfiltration of colloids it has become apparent that there i
s a critical Flux below which there map be no fouling of the membrane
by the colloids. The literature supporting this conclusion is reviewed
and experimental results presented which show that there is no foulin
g of the membrane in laboratory and full scale systems operated below
the critical flux in the region which is termed sub-critical. Operatio
n above the critical flux however causes fouling which reduces flux to
the critical value over time. Increasing transmembrane pressure only
increases flux for a transient period with the stable flux eventually
falling to the critical flux. Reduction of transmembrane pressure caus
es the flux to fall below the critical region if fouling of the membra
ne has already occurred. It may be possible once flux is in the sub-cr
itical region for spontaneous cleaning to occur. The value of the crit
ical flux is known to be a function of particle size, hydrodynamics an
d membrane-colloid interactions.