A new insight into membrane fouling mechanism in submerged membrane bioreactor: Osmotic pressure during cake layer filtration

Big gap between experimental filtration resistance of cake layer formed on membrane surface and the hydraulic resistance calculated through the Carman–Kozeny equation, suggested the existence of a new membrane fouling mechanism: osmotic pressure during cake layer filtration in SMBR system. An osmotic pressure model based on chemical potential difference was then proposed. Simulation of the model showed that osmotic pressure accounted for the major fraction of total operation pressure, and pH, applied pressure and ionic strength were the key determining factors for osmosis effect. It was found that, variations of osmotic pressure with pH, applied pressure and added ionic strength were well coincident with perditions of model's simulation, providing the first direct evidences of the real occurrence of osmosis mechanism and the feasibility of the proposed model. These findings illustrate the essential role of osmotic pressure in filtration resistance, and improve fundamental understanding on membrane fouling in SMBR systems.

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► A mathematical model of osmotic pressure from cake layer filtration was developed.
► pH, applied pressure and ionic strength were key parameters determining osmotic pressure.
► Osmotic pressure accounted for the largest fraction of the operation pressure.
► Osmotic pressure effect was a major mechanism responsible for fouling in MBRs.