Combined magnetic ion exchange and cation exchange for removal of DOC and hardness

• Combined ion-exchange of groundwater achieved >75% DOC and 95% hardness removal.
• Combined ion-exchange of NF concentrated achieved ∼85% DOC and hardness removal.
• DOC removal similar at 2% and 20% NaCl regeneration.
• Hardness removal greater at 20% NaCl regeneration than 2% NaCl.
• Combined ion-exchange can replace coagulation/softening or intermediate NF.

The overall goal of this research was to evaluate an innovative combined ion-exchange process for multi-contaminant treatment. Magnetically enhanced anion exchange resin and conventional cation exchange resin were used in a single completely mixed vessel for simultaneous removal of dissolved organic carbon (DOC) and hardness. Following treatment, exhausted anion exchange resin and cation exchange resin were regenerated together in a single completely mixed vessel using sodium chloride (NaCl). The performance of the combined ion-exchange process was evaluated using three groundwaters and two nanofiltration (NF) membrane concentrates. The test waters provided a wide range of water quality in terms of DOC (2.9–56 mg/L), total hardness (300–1000 mg/L as CaCO3), and sulfate (10–830 mg/L). Combined ion-exchange treatment of groundwater achieved 76% DOC removal and 97% total hardness removal using a 20% (m/v) NaCl regeneration solution. Combined ion-exchange also achieved high removals of DOC (e.g., 83%) and total hardness (e.g., 86%) from NF concentrate using a 30% (m/v) NaCl regeneration solution. These results provide water treatment engineers with a new application of ion-exchange that can be used in place coagulation and precipitative softening or as an intermediate treatment step between primary and secondary NF or reverse osmosis membranes.

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