A membrane-based recycling process for minimizing environmental effects inflicted by ion-exchange softening applications

A new process is presented for replacing the typical NaCl-based with a KCl-based solution, for regenerating water-softening ion-exchange resins. The incentive is three-fold: to minimize the release of detrimental brines to the environment, to augment K⁺ in humans’ diet at the expense of harmful Na⁺ and to improve the worth of the treated water with respect to irrigation and soil properties. Since KCl is 3–5 times more expensive than NaCl, the proposed process targets the recycling of the spent regeneration solution through a sequence of membrane processes (nanofiltration, diananofiltration, reverse osmosis) aimed at separating the divalent cations (hardness species) from K⁺, followed by concentration of a fraction of the K⁺ solution using RO to prepare it to serve as the next-cycle regeneration solution. Detailed results of all separation steps (apart from the relatively-simple RO step) are presented, along with a comprehensive cost analysis, that shows that the process becomes cost competitive (vs. the traditional NaCl-based regeneration) after merely five ion exchange/regeneration cycles. The three-step recycling technique resulted in the recovery of 92% of the K⁺ mass, along with 60% of the fresh water used in the process. After five adsorption/regeneration cycles the ion exchange breakthrough curve showed only slight deterioration due to a low divalent cations concentration that accumulated in the recycled regeneration solution ([Ca²⁺] + [Mg²⁺] ≈ 23 meq/L vs. ∼1000 meq/L of K⁺).