A flat-type membrane bioreactor for aerobic whole cell immobilization was introduced and its performance for citric acid production was investigated using $\underline{Aspergillus}$ $\underline{niger}$.
The reactor consisted of three layers: The top layer contained flowing air for oxygen supply, the next layer had stationary cells, and the bottom layer had flowing aquous nutrients. A hydrophilic membrane separated the nutrient layer from the cells. Hydrophobic membrane was used to supply air and separate the gas layer from the cells. The hydrophobic membrane was made of BIAX (Dongyang nylon co: polyurethane and nylon-6, pore size 0.1-3μm), and the hydrophilic membrane was made of Nylon-6,6(Sartorius, West Germany: pore size 0.45μm).
The fungal cell growth could be controlled by supplying a nitrogen deficient medium at the production stage. With pure oxygen and the nitrogen deficient medium volumetric citric acid productivity reached 0.401 g/L.h, which was about 10-times higher than that of the shake-flask fermentation. The initial pH of the culture medium played an important role in citric acid production and the lower initial pH resulted in a higer citric acid yield. As the feed rate of the culture medium was increased, the productivity was slightly improved. At the feed rates of 1, 2 and, 3 mL/h, the productivities were 0.401, 0.406, and 0.449 g/L.h, respectively. Citric acid production increased when the culture medium was replaced by the nitrogen deficient medium, and citric acid concentration reached 7.48 g/L.
This study shows that the flat-type membrane bioreactor developed in this study can be as successful as dual hollow fiber membrane bioreactor for cell immobilization and citric acid production, furthermore it can be scale up more easily than the dual hollow fiber bioreactor.
