Effect of on-line ultrasound on the properties of activated sludge mixed liquor and the controlling of membrane fouling in SMBR

Abstract

Membrane fouling is one of the major obstacles for further application of submerged membrane bioreactors (SMBRs). In this study, a bi-frequency on-line ultrasound of 25 kHz and 50 kHz was applied to a laboratory-scale SMBR (SMBR-US) to investigate the alleviating of membrane fouling by ultrasound. Experiments were also carried out in another laboratory-scale SMBR without ultrasound (SMBR-Control). The properties of the activated sludge including mixed liquor suspended solids (MLSS), particle size distribution, viscosity, extracellular polymeric substances (EPS) contents and the total organic carbon (TOC) in supernatant were analyzed. The membrane filtration resistance was then calculated to identify the membrane fouling type in two reactors. During the experiment period, the transmembrane pressure of SMBR-Control system increased very fast compared with the SMBR-US system, indicating a significant mitigating of membrane fouling with on-line ultrasound. The MLSS concentration and mean particle size in SMBR-US system was apparently lower than that of SMBR-Control system, which deduced that the ultrasound can reduce extra sludge production and disintegrate activated sludge flocs in reactors. The mixed liquor viscosity in SMBR-US was consistently lower than that of SMBR-Control system. The MLSS and supernatant TOC played a significant role in membrane filtration of SMBR-Control system, while viscosity had an apparent relationship with the filtration resistance in SMBR-US system. The total membrane filtration resistance in SMBR-US was of 51.85% lower than that of SMBR-Control after 35 days operation, which confirms that the ultrasound has a positive effect on mitigating membrane fouling. The membrane filtration resistance caused by blocking cake layers accounted for 86.63% of the total resistance in SMBR-US system, indicating that the blocking cake layer was the main reason for membrane fouling in SMBR-US system. The resistance caused by blocking cake layers and membrane pore blocks were all relatively high in SMBR-Control system, accounting for 59.26 and 38.18% of the total resistance, respectively, suggesting a higher irreversible membrane pollution in SMBR-Control system.