Nanofiltration efficiency in the purification of lactose from ultrafiltered acid whey

The recovery of lactose from acid whey has not yet been implemented at an industrial scale, primarily due to high concentration of lactic acid and minerals. This study demonstrates the feasibility of purifying and concentrating lactose derived from the permeate of ultrafiltered acid whey. A simple batch nanofiltration process was developed to achieve simultaneous deacidification and demineralization by integrating a concentration step with a subsequent constant-volume diafiltration step. The process enables efficient lactose recovery and provides an aqueous by-product containing primarily lactic acid and monovalent salts. Experiments were conducted with real solutions consisting mainly of 30 g/L lactose, 4.3 g/L lactic acid and 2.5 g/L electrolytes. Commercial spiral wound polyamide membranes were tested at pH 4.0 and 50 °C, and membrane performance was assessed based on rejection measures and a preliminary quantification of fouling. The diafiltration step was essential to ensure a final product with a lactose concentration close to 100 g/L and a lactic acid to lactose ratio not exceeding 0.030 g/g. Process efficiency was determined by the optimal combination of the transmembrane fluxes imposed in both steps: high lactose purity (95%), yield (99.6%) and demineralization efficiency (86%) were obtained when operating at fluxes at 30 L/(m2h) with a minimum pumping requirement of 3.1 kWh/m3 of feed. However, while increasing the transmembrane flux improved performance and energy efficiency, a significant increase in water demand was observed in the diafiltration step. Nevertheless, water recovery via an additional reverse osmosis step accounts for no more than 20 % of the total energy requirement.