Influence of fermentation conditions and microfiltration processes on membrane fouling during recovery of glucuronane polysaccharides from fermentation broths.

Abstract

We have investigated the recovery of exopolysaccharides produced by Sinorhizobium meliloti M5N1 CS bacteria from fermentation broths using different membrane filtration processes: cross-flow filtration with a 7 mm i.d. tubular ceramic membrane of 0.5-microm pores under fixed transmembrane pressure or fixed permeate flux and dynamic filtration with a 0.2 microm nylon membrane using a 16-cm rotating disc filter. With the tubular membrane, the polysaccharide mass flux was mainly limited by polymer transmission that decayed to 10% after 90 min. The mass flux of polymer produced under standard fermentation conditions (70 h at 30 degrees C) stabilized after 70 min to 15 g/h/m(2). This mass flux rises to 36 g/h/m(2) when the mean stirring speed during fermentation is increased and to 123 g/h/m(2) when fermentation is extended to 120 h. In both cases, the mean molecular weight of polysaccharides drops from 4.0 10(5) g/mol under standard conditions to 2.7 10(5) g/mol. A similar reduction in molecular weight was observed when the fermentation temperature was raised to 36 degrees C without benefit to the mass flux. These changes in fermentation conditions have little effect on stabilized permeate flux, but raise significantly the sieving coefficient, due probably to molecular weight reduction and the filamentous aspect of the polymer as observed from SEM photographs. The polymer-mass flux was also increased by reducing transmembrane pressure (TMP) and raising the shear rate by inserting a rod in the membrane lumen. Operation under fixed permeate flux instead of constant TMP inhibited fouling during the first 4 h, resulting in higher sieving coefficients and polymer mass fluxes. The most interesting results were obtained with dynamic filtration because it allows operation at high-shear rates and low TMP. Sieving coefficients remained between 90 and 100%. With a smooth disc, the polysaccharide mass flux remained close to 180 g/h/m(2) at 1500 rpm and cell concentrations from 1 to 3 g/L. When radial rods were glued to the disc to increase wall shear stress and turbulence, the mass flux rose to 275 g/h/m(2) at the same speed and cell concentration.