Ion exchange (IEX) is being increasingly used for the removal of organics, for example, for purposes of removing water colour and water hardness. KatIEX is used for removing hardness, and AnIEX for the removal of colour in the form of humic acids. The spent regenerant contains numerous salts and, in the case of AnIEX, humic acids. Discharging this complex stream is not always an option. But, in any event, by treating the spent regenerant, one can possibly recover valuable components, salts and/or humic acids. By separating the salts and the humic acids, both salt (NaCl) and the humic acids can be usefully reused. Vitens uses nanofiltration (NF) and diafiltration (DF) to treat its spent regenerant but, because of the wide variation in the composition of spent regenerant streams, Vitens’ approach is not universally applicable. This is why, in this project, we studied which treatment trains might be of interest on the basis of a technical-economic analysis of spent regenerant from both AnIEX and KatIEX.
Technical-economic analysis and lab-scale research
After surveying the technical possibilities, we conducted a scenario study to determine which options for the treatment of spent regenerant from both AnIEX and KatIEX are of most interest from an economic perspective. We then carried out lab-scale tests with forward osmosis (FO) and electrodialysis (ED) on the spent AnIEX regenerant from the Andijk plant (PWN) and the spent KatIEX regenerant from the DPW Botlek plant (Evides).
Application of FO technically is feasible, of ED (+NF) on spent AnIEX regenerant is promising
The technical-economic analysis shows that, for the treatment of spent AnIEX regenerant, NF+DF and ED+NF offer the biggest cost-reductions and (even) added value; one of the reasons being that the humic acid can be recovered and sold, resulting in high expected net savings (99%). Lab-scale tests with FO on both spent AnIEX and KatIEX regenerant suggest that the approach is technically possible, but the expected savings are limited (15%). Application of ED with mono-selective membranes on spent AnIEX regenerant from Andijk led to a good separation of singly-charged salts and multiply-charged salts plus NOM. In combination with a second ED step and NF, this indicates that the application of ED on spent AnIEX regenerant is technically and economically feasible.