Cake formation and consolidation: Main factors governing the applicable flux in anaerobic submerged membrane bioreactors (AnSMBR) treating acidified wastewaters

A laboratory scale anaerobic submerged membrane bioreactor (AnSMBR) was operated for over 200 days, with a volatile fatty acid mixture as substrate. Gas sparging was used for mixing and to provide membrane surface shear. Results show that cake formation was entirely governing the applicable flux. Cake formation showed to be mainly reversible on a short-term basis, however cake consolidation was observed when a long-term operation was performed at a flux close to the critical flux. Consolidated cake could not be removed by the back-flush cycles, and required an external physical cleaning procedure. Surprisingly, low levels of internal pore fouling were observed, significantly decreasing the need for chemical cleaning. The critical flux towards the end of the operation period reached 20 L/m2 h, at a solids concentration of 40 g/L and a gas superficial velocity of 57 m/h. Further increases in gas superficial velocity were ineffective in achieving a substantially higher critical flux.