| Membrane bioreactor (MBR) is a new technology on sewage treatment which combines with membrane separation unit and activated sludge unit, the technology has a series of advantages, and has very promising application in wastewater treatment and reuse. However, membrane fouling is one of obstacle restricting this technology’s development. Membrane fouling will lead to flux decline, increasing the frequency of membrane cleaning and membrane module replacement, affecting the economy and practicability of MBR technology seriously. So it’s very necessary to research the membrane fouling. This study will choose submerged membrane bioreactor (SMBR) system during long-term operation as investigated subject, and will research the interfacial interaction mechanisms of membrane fouling from thermodynamic point of view based on XDLVO theory.In the operation process of SMBRs, the concentrations of COD%total nitrogen and total phosphorus in influent and effluent were measured, respectively. It was found that SMBR had a highest removal rate for COD, and the average removal efficiency achieved95%. However, being very low average removal rate for total nitrogen and total phosphorus,50%and35%respectively.Sludge layer formation on membrane surface is a major fouling form in MBR. Based on the understanding that sludge layer formation via sludge flocs adhering to membrane surface is a thermodynamic adsorption process, three types of interaction force:Lewis acid-base pair force (AB)、van der Waals interaction force (LW) and electrostatic force (EL) variation profiles with separation distance between sludge flocs and membrane surface were calculated. The result showed that AB force was far outweigh than LW and EL force at close separation distance, and existed an energy barrier during entire adsorption process. Theoretical calculation illustrated that the attraction force between membrane and bovine serum albumin (BSA) was stronger than dextran (DEX with40000Da molecular weight), which showed that protein had more contribution on membrane fouling than polysaccharide. Besides, small granule sludge had higher per unit mass adsorption energy, adhering to membrane surface easier. The interaction energy barrier between sludge particles-sludge layer was lower than sludge particles-membrane surface obviously after initial sludge layer adsorption on membrane surface, illustrating that situation was propitious to subsequent sludge layer formation. The study showed that choosing membrane with high electron donor component surface tension would be helpful to control sludge layer formation in MBR.In the operation process of SMBRs, the physiochemical properties of cake layer attached on membrane surface were characterized, FTIR showed that polysaccharide and protein were major components of cake sludge. It was found that specific filtration resistance (SFR) of cake layer was far outweigh than that of activated sludge, which illustrated that cake sludge could cause worse membrane fouling. The experiment also demonstrated that cake sludge was liable to aggravate membrane fouling from the viewpoint of energy. It was further found that, the main reason for high SFR of cake sludge was attributed to its surface elements components.Compared with activated sludge, gel layer had higher SFR value, membrane fouling had direct correlation with gel layer formation. Total interaction energy barrier between activated sludge flocs and PVDF membrane was higher than gel layer, showing that gel layer could attach to membrane surface easier. The primary elements of gel layer and activated sludge were C、H、N and O, protein was component of both sludge, however, gel layer still existed nucleic acids. |
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