We propose a UF-NF cascade design for purification of CNC and acid/sugar separation, as illustrated in Figure . The permeate streams from UF stages become the feed stream for NF stage, and sugars are retained in the NF retentate side. The final NF stage is designed to concentrate the sugars to 5 wt.%. With the NF cascades, the sugars are gradually separated from acid. By solving the model equations (SI Eqs. , we obtain the concentration profiles for sugars and acid (see SI Figure ). Finally, ~ 99% acid can be recycled, while less than 1 % acid is lost in the sugar stream and CNC streams (sulfation and a small amount as impurities). The energy consumption for NF mainly comes from a pump increasing pressure for the feed stream (permeate stream for UF). More details for the NF membrane (the same as the patent ), NF modelling and operating conditions for the two CNC products can be found in SI S1.5.1.