Hydraulic resistance and osmotic pressure effects in fouling layers during MBR operations

The fouling mechanism in membrane bioreactors (MBRs) is complex, and some studies argue that, in the fouling layer, the counterion osmotic pressure is more important than the hydraulic resistance. In laboratory-scale MBR experiments, activated sludge was centrifuged and the supernatant replaced with different salt solutions having ionic strengths of 0.05–500 mM and monovalent-over-polyvalent cation (M/P) ratios of 0.2–20. High ionic strength increases the average fouling rate, which cannot be explained by counterions, but may be due to sludge floc erosion or disruption and the subsequent production of smaller particles, forming a more compact fouling layer. High ionic strength lowers the Debye length, which may further compact the fouling layer. Simulations document that the hydraulic resistance is high for polymers that form a gel layer or are entrapped in the fouling layer structure and is comparable to the measured resistance. Although counterion osmotic pressure may affect membrane performance, the resistance from soluble microbial products and small particles seems to be dominant in MBR fouling. In addition, Donnan equilibrium simulations show that the M/P ratio in the fouling layer is lower than in the bulk, which may further induce gel formation and thereby high fouling resistance.