Production of microbial alginate in a membrane bioreactor

The extraction of bacterial alginate from fermentation broths of Azotobacter vinelandii NCIMB 9068 was performed using crossflow filtration through ceramic membranes of varying nominal pore sizes (0.2–1.4 μm). The 1.4 μm membrane did not retain bacteria and was found inadequate for this application. For the other membranes, the permeation percentage and the (viscometric) average molecular weight of filtered alginate increased with the membrane pore size. Membrane fouling was observed with all membranes but decreased with the pore size. Therefore, the 0.8 μm membrane was selected for continuous alginate production in a membrane bioreactor. Alginate production was efficient for about 70 h, membrane fouling subsequently occurred. A total of 7.55 g of polysaccharide was recovered from the permeate, the production rate (0.09 g h−1), yield (0.21 g filtered alginate g−1 sucrose), the volumetric productivity (0.014 g alginate dm−3 h−1) and the specific productivity (0.022 g alginate g−1 cell h−1) being significantly higher than in the batch process. The average molecular weight of alginate varied over the production period to stabilize at a very low value (6 kDa) in the later phase of incubation.