The current trend towards indirect potable water reuse is driving regulations for extremely low nutrient concentrations in treated wastewater, and the reverse osmosis and ion exchange processes required to meet these environmentally-responsible levels are prohibitively expensive. In addition, phosphorus (P) is a non-renewable resource, production of nitrogen (N) fertilizer is energy intensive, and discharge of these nutrients in treated wastewater causes eutrophication. As a solution, we have developed a new chemical-free electrochemical process to recover nitrogen (N) and phosphorus (P) simultaneously from settled sewage as fertilizer, in a process that may allow affordable compliance with indirect potable reuse regulations. Nutrients are recovered by modulating the pH with a divided electrochemical cell which uses multi-valent cation exchange membrane (MCEM). First, the influent pH is dropped to remove carbonates, then raised to recover nutrients, and finally the effluent is neutralized before discharge. pH modulation performance was evaluated with three different MCEMs and CMI-7000 produced best results. Phosphates were precipitated at high pH, and the recovered precipitates were analyzed using scanning electron microscopy (SEM), energy dispersive X-Ray spectroscopy (EDS) and X-ray powder diffraction (XRD) techniques to characterize the composition. The dry product contained 22% (w/w) amorphous calcium phosphate, having a Ca:P ratio of 2:1, with impurities being 18.8% (w/w) Mg(OH)2, 5.4% (w/w) silt, and the remainder principally organic matter. Nitrogen is recovered by stripping and absorbing into acid to produce a saturated ammonium sulfate solution containing 36.6% (w/w) (NH4)2SO4. The total organic carbon test indicated that the recovered ammonium sulfate product contained only 2.74% impurities based on the NH3 content. Currently, a flow-through reactor is being developed to simultaneously recover both N & P, and achieved average recovery of 91 and 84% respectively. Efficiency is higher in a non-reverse osmosis net-zero water (nearly closed-loop reuse) system.

Simultaneous chemical-free nitrogen and phosphorus recovery from municipal wastewater

2019 Spring National Meeting

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