Sludge fractionation as a method to study and predict fouling in MBR systems

Membrane bioreactors (MBRs) are continuously being improved, but membrane fouling still causes severe problems, so effective measures to reduce fouling are imperative to minimize operational cost. Potential foulants can be found in MBR sludge flocs, but it is not known which fraction(s) that mainly influence fouling and how sludge flocs affect fouling. A standard procedure was developed to separate sludge into floc/colloid, colloid/solute, and solute fractions. The fouling properties of MBR sludge and sludge fractions were evaluated in a lab-scale MBR equipped with aerated flat-sheet membranes. The fastest flux decline took place in the solute fraction due to formation of irreversible fouling; this fraction contained foulants smaller than the membrane pore diameter (0.2 μm), causing pore blocking and/or adsorption. Flux declined more slowly in the colloid/solute fraction, where fouling was more reversible. The external fouling layer on the membrane was shown to protect it from pore blocking/adsorption. Flux decline was slowest in unfractionated MBR activated sludge. Thus, presence of sludge flocs reduced the concentration of foulant and also directly reduced the formation of an external fouling layer by shear. This may be due to surface erosion or more turbulence close to the membrane surface by sludge flocs which thereby partly removed the external fouling layer. To reduce membrane fouling, concentrations of solutes (e.g., macromolecular extracellular substances) and colloids (e.g., single cells) should be kept low and concentration of sludge flocs high.