Contribution of various constituents of activated sludge to membrane bioreactor fouling

Urban effluents were treated in a pilot membrane bioreactor (MBR) fed by ra w wastewater from a sewage plant. The MBR consisted of an aerated tank fed with raw water containing a volume of 600 l of activated sludge which was r ecirculated through an external circuit containing a ceramic membrane of 0. 25 m(2) area with 0.1 mu m pores. Suspended solids (SS) concentration was s tabilized at 10 g/l, The sludge and hydraulic residence times (SRT and HRT) were 60 days and 24 h, respectively. Elimination of chemical oxygen demand was higher than 95%. Rheological tests showed that activated sludges from the MBR and from the aeration tank of a classical plant were non-Newtonian. In order to evaluate the contribution of various sludge fractions to fouli ng, SS were first separated from the sludge by settling, then colloids were separated from the supernatant by a combination of flocculation and settli ng. The relative contributions of SS, colloids, and dissolved molecules (DM ) to the resistance to filtration caused by fouling were found to be 65%, 3 0% and 5%, respectively. The filtration resistance due to each constituent was also measured as a function of its concentration. The permeate flux did not decrease much when cellular concentration rose from 2 to 6 g/l, The me asured total filtration resistance was then compared with the sum of resist ances of each constituents at the same respective concentrations as in the sludge. This calculated sum was found to be 50% higher than the measured to tal resistance, indicating that fouling resistances caused by each constitu ent were not additive. (C) 2000 Elsevier Science Ltd. All rights reserved.