Ion exchange (IEX) is being increasingly used to remove organics from water or to soften water. The ion exchanger, however, needs to be regularly regenerated in a process which releases a saline waste stream: the used regenerant. Within the joint research programme for the water companies, KWR studied which methods were most suitable for the recovery of salts and humic acids from IEX regenerant streams.
Ideally, the salt and the humic acids in the used regenerant are recovered separately. Vitens applies nanofiltration (NF) and diafiltration (DF) for this, but these methods are not universally applicable because of the wide variation in the composition of used regenerant. The composition is closely associated with the quality of the feed water, the type of IEX (anions, AN or cations, Kat), and the position of the IEX in the process. Over the past few years, the water sector has particularly increased its use of anion exchange (AnIEX), which removes humic acids from water.
Electrodialysis and nanofiltration are most suitable
KWR researched, using a technical-economic analysis, which treatment trains might be of interest for used regenerant for both AnIEX and KatIEX. Electrodialysis (ED) followed by nanofiltration (NF) was the most attractive option for the treatment of used AnIEX regenerant. KWR then carried out lab-scale research into ED at PWM’s Andijk plant. This revealed that ED was a suitable technique for the treatment of used AnIEX regenerant. KWR also tested forward osmosis (FO) on a lab scale. FO appeared to be technically feasible, but, in light of the expected limited savings, is of less interest than ED (+NF). Through the application of ED and NF to used AnIEX regenerant, the separation of salts and humic acids is technically and economically feasible. The first logical follow-up step is to test this approach on a larger scale and for a longer period.