Ion Exchange Potable Reuse: Organics and Salinity Removal Without RO

An innovative treatment train, coined ion exchange-based advanced treatment (XBAT), was developed and validated for potable reuse. XBAT combines suspended ion exchange (SIX) and lime softening, which are integrated by regenerating the spent resin using bicarbonate. XBAT simultaneously removes TOC and salinity without the use of RO and the resulting concentrate disposal challenges. As such, XBAT provides the cost efficiency of non-RO based purification and the salt reduction of RO based purification. Salinity is the result of all dissolved anions and cations in water, which is often characterized by total dissolved solids (TDS). For potable reuse, salinity represents a specific challenge because wastewater effluents are typically more saline than the conventional surface water or groundwater that supplies a given community, and often exceeding drinking water standards. RO-based advanced treatment (RBAT) and carbon-based advanced treatment (CBAT) are the two prevailing advanced treatment approaches for potable reuse. Although RO rejects most dissolved salts, the resulting RO concentrate constitutes a large portion of total feed flow and creates a challenging residual disposal problem. CBAT integrates multiple treatment barriers for chemical and pathogenic contaminants, but none of the unit treatment processes removes measurable amounts of ionic salts. Therefore, there is a pressing need for an alternative treatment approach that overcomes the limitations of RBAT and CBAT by removing salinity without the residual challenge. To provide the efficiency of CBAT while providing the salt removal of RBAT, through years of research, our team developed an innovative treatment concept, namely ion eXchange-Based Advanced Treatment (XBAT), that simultaneously removes TOC and salinity without RO. XBAT consists of (1) suspended ion exchange (SIX) for the removal of negatively charged constituents (TOC, nitrate, sulfate, chloride, bromide, phosphate, etc.), (2) resin regeneration using bicarbonate, and (3) lime softening for cation (i.e., Ca, Mg) and excess bicarbonate removal (figure 1). SIX is a steady-state ion exchange process that keeps a strong base anion (SBA) resin fluidized in the reactor. A significant feature of SIX is its high resin regeneration frequency, which keeps the resin's ion exchange capacity only slightly utilized, thus allowing spent resin to be regenerated by a weaker and less concentrated regenerant (4% NaHCO3 in lieu of 10% NaCl). Bicarbonate regeneration adds alkalinity into SIX effluent, making the treated water more suitable for hardness removal and further TDS reduction through calcium carbonate precipitation. Figure 1 - Conceptual diagram of XBAT (IX with SBA resin and bicarbonate regeneration + lime softening) We have successfully demonstrated XBAT at the bench scale and evaluated its performance for TOC and salinity removal from secondary and tertiary wastewater effluents. Four resin types, two resin doses, three contact times, and their various combinations were tested over up to 50 resin exhaustion and regeneration cycles to assess long-term XBAT performance. Results indicated in figure 2 show an average TOC removal rate of 50% regardless of the effluent type and initial TOC concentrations. Average percent removal rates for sulfate, nitrate, bromide, and chloride were 99%, 90%, 94%, and 74%, respectively. Subsequent lime softening resulted in 92% calcium removal and 96% magnesium removal. Taken together, TDS decreased from an average of 680 mg/L down to 370 mg/L, achieving about 50% salinity reduction. This level of TDS removal would be sufficient to return effluent salinity levels to that of conventional drinking water sources, thus enabling the implementation of potable reuse without concerns about up-cycling conservative ions. A pilot study demonstrating XBAT has recently concluded (January 2024); results will be shared as part of this presentation. Figure 3 shows the XBAT pilot. Figure 2 Bench-scale results of bicarbonate-form IX performance on a tertiary effluent with four different resins Table 1 Bench-scale results of XBAT (SIX + softening) on tertiary effluent Figure 3 - XBAT Pilot - SIX (L) and Pellet Softener (R)