A process for the coupled production of sweet whey and lactic acid from acid whey is suggested, comprising the following steps:(a) providing acid whey having a lactic acid content of about 0.1 to about 1% by weight；(b) nanofiltration of the acid whey, obtaining a first permeate P1 and a first retentate R1； (c) optionally, redilution of the first retentate R1 with water to reconstitute the initial dry matter content, and preparation of the second nanofiltration step；(d) nanofiltration or nano-diafiltration of the retentate R1, obtaining a second permeate P2 and sweet whey as a second retentate R2； (e) combining the two permeates P1 and P2 and subjecting the mixture to reverse osmosis, obtaining a third permeate P3 which, substantially, only contains water, and a concentrate of lactic acid as a third retentate R3.
