Membrane fouling caused by extracellular polymeric substances during microfiltration processes

The effects of both extractable extracellular polymeric substances (EPS) and the supernatant of sludge flocs on the membrane fouling in submerged membrane bioreactors (SMBRs) were invesetigated. Three laboratory-scale SMBRs were operated at a constant permeate flux (12.5 L/m2 h) with a flat-sheet microfiltration membrane (hydrophilic polyolefin, pore size of 0.25 μm) at different solids retention times (8, 20 and 80 days, respectively). The concentrations of extractable EPS (i.e., extracted by the cation-exchange resin method) and dissolved organic carbon (DOC) in the supernatant at the steady-state period were compared. The results showed that as SRT increased, the organic carbon content in extractable EPS decreased, whereas DOC in the supernatant tended to be independent of SRT. Batch filtration tests were conducted to determine the specific cake resistances of the fouling layer using both the raw sludge and the twice-washed sludge with buffer solution. The supernatant contributed approximately 50% to the total specific cake resistance. The organic carbon and protein content in the extractable EPS decreased with increasing SRT, and protein content in the supernatant also had the same tendency. Therefore, the supernatant played a relatively significant role in membrane fouling.