Separation of lactic acid-producing bacteria from fermentation broth usinga ceramic microfiltration membrane with constant permeate flow

The influence of several operating parameters on the critical flux in the s eparation of lactic acid-producing bacteria from fermentation broth was stu died using a ceramic microfiltration membrane equipped with a permeate pump . The operating parameters studied were crossflow velocity over the membran e, bacterial cell concentration, protein concentration, and pH. The influen ce of the isoelectric point (IEP) of the membrane was also investigated. In the interval studied (5.3-10.8 m/s), the crossflow velocity had a marked effect on the critical flux. When the crossflow velocity was increased the critical flux also increased. The bacterial cells were retained by the mem brane and the concentration of bacterial cells did not affect the critical flux in the interval studied (1.1-3.1 g/L). The critical flux decreased whe n the protein concentration was increased. It was found that the protein wa s adsorbed on the membrane surface and protein retention occurred even thou gh the conditions were such that no filter cake was present on the membrane surface. When the pH of the medium was lowered from 6 to 5 land then furth er to 4) the critical flux decreased from 76 L/m(2)h to zero at both pH 5 a nd pH 4. This was found to be due to the fact that the lowering in pH had a ffected the physiology of the bacterial cells so that the bacteria tended t o adhere to the membrane and to each other. The critical flux, for wheat fl our hydrolysate without particles, was much lower (28 L/m(2)/h) when using a membrane with an IEP of 5.5 than the critical flux of a membrane with an IEP at pH 7 (96 L/m(2)h). This was found to be due to an increased affinity of the bacteria for the membrane with the lower IEP. (C) 2001 John Wiley & Sons, Inc.