Membrane fouling in a submerged membrane bioreactor: Impacts of floc size

• Small floc had high specific contact interaction energy, and easily adheres to membrane.
• Presence of SMP and colloids in cake layer highly increased cake resistance.
• Floc size highly affected cake resistance due to the osmotic pressure mechanism.

A submerged membrane bioreactor (MBR) treating synthetic wastewater was continuously operated to investigate the impacts of floc size on membrane fouling. Particle size distribution (PSD) analysis showed that almost all the sludge flocs in sludge suspension had size larger than 1 μm, which corresponded to low pore clogging fouling. Thermodynamic analyses showed that adhesion of sludge flocs on membrane surface needed to overcome a repulsive energy barrier. Decrease in floc size slightly increased the specific energy barrier, but highly increased the attractive specific interaction energy in contact, and as whole, facilitated adhesion of small flocs to the membrane surface. Presence of biopolymer matters in sludge suspension remarkably gave rise to the cake resistance. This result could be explained by the osmotic pressure mechanism. Decrease in floc size would greatly increase both of hydraulic cake resistance and osmotic pressure-induced resistance. The findings shed significant light on membrane fouling control.

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