Recovery of l-tryptophan from crystallization wastewater by combined membrane process

The combined membrane process of electrodialysis (ED) and reverse osmosis (RO) was carried out to recover the l-tryptophan from its crystallization wastewater which had 0.5–1.5% residual l-tryptophan and around 15% impurity of NaCl salt. In the process of ED desalination, the operating current should be controlled below the limiting current density to avoid the polarization phenomenon. The pH of dilute solution significantly affected the recovery ratio of l-tryptophan, and the salt removal ratio varied with current density and voltage. The operating current density also had a great impact on the performance of the electrodialytic desalination process. After ED desalination, two methods including raising the temperature and adjusting the pH of the dilute solution coming from ED process were used to increase the solubility of l-tryptophan in aqueous solution during the RO concentration process. It was observed that the recovery ratio of l-tryptophan was closely correlated with not only the pH of RO feed but also the salt removal degree of ED process. After the treatment of the combined ED and RO process, 60.4% of the optimal recovery ratio of l-tryptophan was achieved, and the purity of final product of l-tryptophan reached 98%.