Effects of various factors on critical flux in submerged membrane bioreactors for municipal wastewater treatment

Critical flux is an important concept in membrane bioreactor (MBR) process for wastewater treatment, and the study on it could enhance the understanding of MBR process and facilitate its application. In this study, the effects of various factors, such as sludge concentration, aeration intensity, soluble chemical oxygen demand (SCOD) of the mixed liquors, and measurement parameters including initial flux, step length and step height, and membrane properties including membrane materials and membrane history, on critical flux were critically investigated. Test results showed that the critical flux decreased with the increase of sludge concentration and it could be enhanced by improving aeration intensity. SCOD, which was used to indicate sludge characteristics, was found to have a negative effect on critical flux. It was also observed that the increase of step length or step height would lower the critical flux; however, the variations of initial fluxes did not clearly affect critical flux. In addition, membrane materials also had significant influences on critical flux, and membrane M3 had a higher critical flux value, which was more suitable for the application to the submerged MBR compared to M1 and M2 membranes. Membrane history was found to have negative impacts on membrane critical flux, and the critical flux decreased as the membrane operation prolonged.