Recovery of lactose from acid whey by nanofiltration: An experimental study

The flowsheet of an overall process to recover valuable products from raw acid whey is proposed. It considers typical pretreatments, an ultrafiltration step to remove proteins, a decalcification step to remove calcium and magnesium by precipitation, and a nanofiltration/diafiltration step for demineralization and deacidification of lactose. The performance of each step is evaluated and the characteristics of the main streams are estimated. The feasibility of lactose recovery after the decalcification step by nanofiltration is demonstrated by using a spiral wound module. Membrane characterization was performed at pH 4 and 50 °C, with an artificial solution containing lactose, lactic acid and sodium chloride, prepared to mimic the clarified supernatant from the decalcification unit. The feasibility of the simultaneous concentration and deacidification of lactose is finally validated by processing a real solution. With the real solution, the maximum removal of lactic acid, close to 87%, is obtained by operating with a nanofiltration (NF) step (at a concentration factor of 3.5) followed by a NF step operated in diafiltration mode (DF) at constant volume (at a dilution factor of 1.8). A lactic acid/lactose ratio of 0.018 g/g is achieved, with an overall lactose purity and yield of 93.6% and 98.2%, respectively, when operating at low pressure values (up to 12-14 bar). A preliminary process simulation is finally performed to identify the premises for process optimization of the NF+DF configuration. The success of the integrated NF+DF process is contingent upon the correct balance between the choice of the membrane and the operating conditions, which ensure lactose rejections exceeding 98.5 % and lactic acid rejections falling below 20-30 %, while maintaining transmembrane fluxes between 8 and 15 dm3/(hm2).