Lactic acid recovery from whey ultrafiltrate fermentation broths and artificial solutions by nanofiltration

In this work, lactic acid recovery from clarified fermentation broths by nanofiltration was studied. The effect of feed concentration, flow rate, transmembrane pressure and pH on flux and rejection were analysed. The separation efficiency of nanofiltration membranes in the treatment of ionic solutions may be explained by the combination of size and charge effects. In the case of lactic acid solutions, the ratio of lactate ions to non-ionized lactic acid affects the rate at which both pass through nanofiltration membranes. At pH values around 5.0, which are typical values in industrial fermentations, lactic acid is mainly found in the dissociated form. Two polyamide nanofiltration membranes were used: DK2540C® (Filtration Engineering) and AFC80® (PCI Membrane Systems). A strong influence of pH on lactic acid transport through these membranes was observed. The pH value affected both rejection and permeate flux. Rejection increased with pH while flux decreased with this variable. However, at high feed concentrations, solute retention was much lower as the Donnan exclusion effect was attenuated. At the acidic pH required to convert lactate into undissociated lactic acid (pH 3.0), the recovery of lactic acid must be improved in order to reduce lactic acid losses and increase purity.