Wastewater Reclamation and Ion Fractionation by Electrodialysis

With the development of modern society, water shortage and mineral depletion have become the sword of Damocles to civilization. On the other hand, wastewaters contain salts, minerals and nutrients (e.g., phosphate and organic ions). These cannot be efficiently reclaimed, so they pollute receiving water bodies. These facts mean that both the water circle and the mineral circle are open due to human activities. In view of environmental impact and sustainable development, actions have to be taken to close these two circles. Although wastewater treatment processes have been developed, many problems still remain. This PhD dissertation investigates wastewater reclamation by means of salt removal and phosphate recovery by means of ion fractionation, using electrodialysis (ED) to close these circles. The target waters in this thesis include reverse osmosis (RO) concentrates and an anaerobic digestion of potato processing wastewater which typically contain inorganic and organic salts as well as phosphate. In this thesis, a theoretical and practical understanding of the ED process is developed for RO concentrate recovery and ion fractionation in view of wastewater reclamation. On the one hand, inorganic and organic ion transport mechanisms in the ED stack were studied under various current density and pH. Ion characteristics, including ionic size, charge, functional groups and hydrophilicity were investigated to illustrate their effect on the membrane selectivity and ion competition during electrodialysis. Results show that larger ions are retained more efficiently than smaller ones, which points in a direction of a size-exclusion effect. On the other hand, zwitterions are retained almost completely in the diluate side. The results also imply that binary organic anions with a larger molar mass (>100) can be adsorbed into the membrane surface and hence form a charged double layer, which affects membrane selectivity. On the other hand, experiments were performed on both lab scale installations and a pilot scale ED installation. Lab scale ED was evaluated by feeding synthetic water and a real RO concentrate to obtain an optimized parameter set for the pilot. Afterwards, the pilot ED was run to treat the real RO concentrate from a WWTP to produce 75% desalinated water. The pilot system for RO concentrate recovery was evaluated technically and economically to examine the feasibility. Results indicate that the critical scaling concentration (CSC) of Ca2+ can be used to predict scaling problems. Ozonation can successfully improve the biodegradability of ED effluent and hence can reduce the accumulation of organic compounds in the circulation system. A high overall water recovery (95%) of the WWTP system can be achieved from the integrated RO-ED system. An economical analysis shows that the operational cost can be lowered to 0.19 EUR m-3 if the ED concentrate is subjected to decarbonation (DEDC method) to reduce the scaling potential. Moreover, based on the research on the membrane selectivity and ion transport mechanisms, a specifically designed ED stack for ion separation and fractionation, named “selectrodialysis” was developed. This novel stack can be used for desalination and for ion fractionation. The stack was investigated to fractionate multivalent ions from monovalent ions, i.e., fractionate sulphate from chloride. The results show that the sulphate purity can reach over 85% from a sulphate free stream. The investigation proves that the novel electrodialysis stack is feasible and effective to fractionate divalent ion (SO42-) from monovalent ion (Cl-) in the mixture. Furthermore, fractionation of phosphate from chloride and carbonate by selectrodialysis has been studied on synthetic water and a potato digester wastewater. In the investigation, a lower current (0.2 A) and a higher pH (10) was chosen to obtain a better selectivity towards phosphate. Results show that the concentration of phosphate in the product stream reached more than 8 mmol.L-1. In conclusion, this thesis showed that ED is feasible to desalinate and reclaim RO concentrate. It also proved that ED can be used for advanced separations, such as the separation of inorganic ions and small organic acids. A specifically designed ED stack, named “selectrodialysis” (SED), was used to desalinate wastewater and fractionate ions with the same charge sign to recover phosphate.nrpages: 199status: publishe