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

As freshwater supplies dwindle globally, large metropolises like Los Angeles and Singapore have made significant progress in municipal wastewater reuse where reverse osmosis (RO) is an integral component of recovery. However, for wastewater plants where inland disposal of RO reject streams is the only choice, nitrate and phosphate-laden RO reject streams warrant near-complete removal prior to disposal to avoid eutrophication effects. In addition, hardness removal prior to RO allows maximum recovery without scaling/fouling of the RO membrane. In this study, we present a hybrid ion exchange pretreatment process that (i) allows near-complete recovery of nitrogen and phosphate, (ii) removes hardness, and (iii) uses CO2 as the only regenerant chemical that is sequestered finally as CaCO3(s). One shallow shell weak-acid cation exchanger (SS-WAC) and a hybrid anion exchanger (HAIX-NanoZr) with specific affinity toward nitrate and phosphate form the heart of HIX-NP recovery process. Studies were conducted using secondary wastewater from the Bethlehem, PA plant and CO2 was the only regenerant used replacing the need for other mineral acids. HIX-NP accomplished near-complete removal of phosphate and nitrate and their recovery in high purity. Thus, feed to subsequent RO is free of N and P posing no major reject disposal for inland treatment