LACTIC-ACID SEPARATION FROM FERMENTATION BROTHS BY REVERSE-OSMOSIS AND NANOFILTRATION

Laboratory scale and pilot plant nanofiltration (NF) and reverse osmos is (RO) experiments with fermentation broths were performed with the f ollowing aims: (i) to quantify lactic acid rejection and to determine whether a theoretical model developed in a previous paper could be use d to predict lactic acid rejection; and (ii ) to quantify fouling of N F membranes and to determine the major fouling mechanism. It was found that the rejection model developed, based on the extended Nernst-Plan ck equation, can be used to quantify lactic acid rejection of RO and N F membranes. Especially at high fluxes the prediction of lactic acid r ejection using parameters determined with lactic acid/water mixtures w as quite good. At low fluxes the predicted rejection of lactic acid wa s usually lower. Fouling of the membrane could be quantified in terms of three resistances: a membrane resistance, an initial fouling resist ance, and a time-dependent fouling resistance. Empirical equations for the initial fouling resistances were developed and time-dependent fou ling could be described either by a colloidal fouling model (ultrafilt ered fermentation broth) or a gel layer model (fermentation broth). Ev aluation of the three resistances by simulation of continuous and batc h concentration experiments showed that during NF of an ultrafiltered fermentation broth the initial fouling resistance, resulting from conc entration polarization effects. was the predominant resistance. For a fermentation broth the time-dependent fouling becomes more important t han the initial fouling resistance. Protein fouling is the main cause of the time-dependent fouling. Therefore, it is recommended to remove proteins by ultrafiltration before NF.