Separation of nutrient ions and organic compounds from salts in RO concentrates by standard and monovalent selective ion-exchange membranes used in electrodialysis

Abstract

To allow water recycling and nutrient recovery from reverse osmosis (RO) concentrates from a food industry plant, separation of salts from the organic fraction and from nutrient anions is required. In this study, the use of ion-exchange membranes in electrodialysis was investigated to this purpose. Two types of anion-exchange membranes from PCA, Germany, were investigated: a nonselective membrane (SA) and a membrane selective for monovalent anions (MVA). Different approaches including lowering the initial current density and increasing the initial pH were applied. The transport properties of different anions and different small charged organic compounds through ion-exchange membranes were discussed. The separation efficiency, which represents the selectivity, was compared under different conditions. The results show that separation of non-nutrient anions from nitrate and phosphate was difficult, whereas separation of salts from the organic fraction was feasible provided that the organic solutes are similar to uncharged test solutes used in the experiments; it was shown that a higher molar mass of the organic solute has a positive effect on the separation. Lowering the current density can increase the separation efficiency of monovalent/multivalent anions with either the SA membrane (a nonselective membrane) or the MVA membrane (a monovalent selective membrane). When the initial pH was increased, the separation efficiency of monovalent/multivalent anions can be improved for the MVA membrane, however, no obvious change was found for the SA membrane. Finally, experiments on real RO concentrates proved that the separation of salts from organics by electrodialysis is similar to the separation of salts from uncharged solutes and therefore feasible.