Effects of relaxation and backwashing conditions on fouling in membrane bioreactor

Relaxation and/or backwashing have been incorporated in most membrane bioreactor (MBR) designs as standard operating strategies to limit fouling. However, to maintain a certain net permeate production, higher instantaneous fluxes have to be applied to compensate for the loss of permeate caused by those modes of operation. To assess the effects of the different operational parameters on fouling for the same water productivity, and to study the fouling mechanisms, three relaxation and five backwashing conditions were applied in a lab-scale aerobic MBR and compared to the continuous filtration mode. Moreover, the fouling behaviour was analyzed in terms of the consequential increase in trans-membrane pressure, resistance fractions and the biopolymeric composition of the fouling layer. The Pearson correlation analysis was applied to identify the major contributor to fouling. From this study, it was found that fouling could be retarded effectively when the appropriate filtration mode was applied. The instantaneous flux applied to compensate for the loss of productivity during backwashing or relaxation periods was the major factor affecting fouling rate for either relaxation or backwashing, which was significantly correlated with the amount of solids and soluble fractions deposited on the membrane surface. The relaxation and backwashing conditions (i.e. duration, interval, strength) also considerably affected the fouling rate. In addition, it was found that pore blocking was more likely to be due to protein rather than carbohydrates and that the cake layer was the major contributor to fouling.