Aluminum-Cycle Ion Exchange Process for Hardness Removal: A New Approach for Sustainable Softening

Abstract

From a sustainability viewpoint, sodium exchange softening, although used widely, is under scrutiny due to its production of excess Na-laden spent regenerant and subsequent discharge to the environment. Many arid regions are introducing regulations disallowing dumping of concentrated sodium salts, the residuals from popular Na-exchange softening. The sodium content of the softened water is, also, always higher than in the feed, which poses a dietary health concern when used for drinking or cooking. An efficient, easy-to-operate hardness removal process with reduced sodium in both the treated water and in the spent regenerant is an unmet global need. Use of a cation exchange resin in Al-form for hardness removal, that is, exchange of divalent Ca or Mg with trivalent Al, is counterintuitive, and this is particularly so, because the aluminum ion to be exchanged has higher affinity than calcium. Nevertheless, ion exchange accompanied by precipitation of aluminum hydroxide allows progress of the cation exchange reaction leading to hardness removal. Experimental results demonstrated that calcium can be consistently removed for multiple cycles using a stoichiometric amount of AlCl as the regenerant. The process essentially operates at the maximum possible thermodynamic efficiency: removal of one equivalent of Ca corresponds to use of one equivalent of Al as a regenerant. During the Al-cycle process there is no increase in Na concentration and partial reduction in the total dissolved solids (TDS) of the treated water. It is noteworthy that the ion-exchange resin used, components of the fixed-bed column and operational protocol are nearly the same as traditional softening processes on Na-cycle. Thus, existing Na-cycle systems can be retrofitted into Al-cycle operation without major difficulty.

Treated Municipal Wastewater Reuse: A Holistic Approach Using Hybrid Ion Exchange (HIX) with Concurrent Nutrient Recovery and CO2 Sequestration

“…Obviously, an increase in concentration of these electrolytes at the membrane interface at high recovery accompanied by the concentration polarization greatly enhances scaling potential. − Dosing of antiscaling agents, polyacrylic or phosphonic acid, are practiced universally but they promote biological growth and consequent biofouling on the membrane surface . Traditional softening processes for calcium or hardness removal require use of brine or mineral acid as a regenerant, posing disposal problems, and new process routes are underway to overcome such shortcomings. , …”

“…20 Traditional softening processes for calcium or hardness removal require use of brine or mineral acid as a regenerant, posing disposal problems, and new process routes are underway to overcome such shortcomings. 21,22 Since ocean disposal is no longer a feasible option for inland wastewater recovery in arid regions away from the coast, any disposal of the concentrated reject wastewater stream from RO into rivers or lagoons containing high concentration of N and P is unacceptable for their eutrophication potentials and consequent adverse environmental impacts. Conceptually, a pretreatment process that allows phosphate and nitrate recovery while simultaneously eliminating scale-forming constituents like hardness to protect RO membranes without needing externally added chemicals/regenerants will provide a holistic solution toward recovery and reuse of municipal wastewater.…”

Hybrid Ion Exchange Desalination (HIX-Desal) of Impaired Brackish Water Using Pressurized Carbon Dioxide (CO2) as the Source of Energy and Regenerant

“…Salt-free softening and demineralization by CO 2 , although conceived and investigated earlier with traditional ion exchange resins and ion exchange fibers, suffer from major shortcomings. − Also, the inability to remove phosphate or other contaminants selectively and poor regeneration efficiency of commercial strong- and weak-acid cation exchange resins with CO 2 have been the primary impediments for desalting impaired water and wastewater sources. In this study, treated wastewater from Bethlehem, PA, and synthetic brackish water were used for a prolonged laboratory investigation.…”

Thermomorphic Hydrophilicity Base-Induced Precipitation for Effective Descaling of Hypersaline Brines

“…37 The technique controls scaling by chargebased exchange between scale-forming cations (e.g., Mg 2+ and Ca 2+ ) in the feedwater and substituent species (e.g., Na + ) in the ion-saturated resin. 38,39 However, scaling control of hypersaline brines by ion exchange is costly, and more importantly, the effectiveness is severely hampered by the initial high concentration of sodium ions in almost all hypersaline brines. Thermomorphic multiphase systems (TMSs) are switchable solvent mixtures that use temperature as the trigger to toggle between mono-and biphasic states.…”