Among the options for high productivity in fermentation processes, bioreactor system development, as well as development of strains with high activity and optimization of operating conditions, are very important. In order to obtain high cell densities it is essential to retain cells inside the reactor as long as possible. For this purpose, cell recycle with internal membrane was adopted.
All the ceramic filters used in this study for the internal filter system were tubular type and made by a sintering process. The average pore size of symmetric and asymmetric ceramic membrane was 0.3 and 0.005㎛ respectively. The pore size of stainless steel used in this study was 2㎛.
For the optimum design of the internal membrane module, a relationship between the radius of the membrane tube and the ratio of the membrane surface per effective fermentation volume was worked out. It was found that there is an optimum dimensionless radius (r/R) in order to achieve the maximum ratio of the membrane surface area : fermentation volume.
Factors affecting the permeation flux during the filtration of fermentation medium and broth by using a symmetric and an asymmetric ceramic membrane were investigated in an internal filter bioreactor. After sterilization of the fermentation medium the flux of the asymmetric membrane was 1.8 folds higher than that before sterilization. At 100g/l glucose, flux index (Fi) at steady state was only 21.8% for the asymmetric membrane and 42.2% for the symmetric membrane. When cell free broth was filtered, Fi at the steady state was 48% for the symmetric membrane and 21% for the asymmetric membrane. For the asymmetric membrane, a decline in the flux by nearly 80% was observed when 100g/l glucose and 8.5g/l yeast extract coexisted in the medium.
The dual function of the cake layer during the filtration of the fermentation broth was investigated in depth and a mathematical model was also developed in order to describe the dual function of the cake layer quantitatively.
Periodic backflushing of tubular ceramic membrane filters with filtrate was employed to alleviate membrane fouling. There existed an optimum backflushing interval and time to give a maximum flux recovery. At 16g/L of yeast cell concentration, the mean flux increased about 4 times by employing repeated operation cycles consisting of 4.53 minutes of filtration, 4.5 seconds of intermission, and 40 seconds of backflushing. Effects of aeration, agitation speed, and yeast cell concentration were also investigated.
The use of ultrasonic waves (20 kHz and 60~130 W) facilitated in situ regeneration of membranes after fouling by yeast cells or particles in the cake layer. A flux 20 ~80% higher than normal was achieved at 16g/l yeast cell suspensions. Optimum ultrasonic incidence conditions were 25sec/5min, 60W to achieve a maximum 84% of initial Fr after 10 minutes. No apparent influence on yeast cell growth, glucose consumption, and ethanol productions were observed at 60W ultrasonic incidence. However, at 110 W and longer time, incidence retarded glucose consumption and induced lower cell and ethanol concentrations during batch fermentation.
When the stainless steel membrane had a negative charge, the permeation flux decreased very slowly compared with one with no charge. On the other hand, the adhesion of macromolecules to the membrane increased when the stainless steel membrane had a positive charge and the flux was even lower than the one without a charge in the end.
Long-term continuous yeast culture with glucose was carried out successfully for more than 2 months employing the internal filtration system incorporated with backflushing unit as a model. A dilution rate of 0.2~$0.3hr^{-1}$ and a bleed ratio of 0.1~0.15 were used. Productivities of about 10 and 18g/l-hr were obtained at dilution rates of 0.2 and $0.3hr^{-1}$ respectively.
In the continuous culture experiment using internal filtration system with wood hydrolysate and dilution rate $0.22hr^{-1}$, the maximum cell and ethanol concentrations were 1.5×$10^9$ cells/ml and 76g/l respectively and a productivity of 17g/l-hr was obtained.