Electrohydromodulation for phosphate recovery from wastewater

Abstract

The mining of phosphate rock to produce fertilizer, and subsequent disposal of the municipal wastewater containing a significant fraction of the phosphate applied agriculturally, has led to environmental impacts of phosphate mining as well as surface water eutrophication. However, recovery of phosphate (H2PO4-, HPO42-, PO43-) from wastewater, either chemically or biologically, has not been sufficiently economical at low phosphate concentrations in raw wastewater (~2.4 mg-P l−1) to motivate widespread adoption. In this work, we demonstrate phosphate recovery directly from raw and mineral-spiked septic tank effluent by electrochemical pH shifting, termed electrohydromodulation (EHM), without expensive chemical addition. EHM using a CMI-7000 multivalent cation exchange membrane (MCEM) at 5 V at 1.347 mA/cm2 resulted in the highest efficiency at lowest energy consumption. 94 and 95% phosphate were recovered from mineral-spiked septic tank effluent (simulating advanced oxidation-based direct potable reuse water), and raw septic tank effluent, at an energy demand of 1.046 kWh/m3 (3.960 kWh/1000 Gal) and 1.863 kWh/m3 (7.054 kWh/1000 Gal), respectively, low despite use of non-toxic, inexpensive graphite electrodes. Results from scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray powder diffraction (XRD) indicated that recovered precipitates consisted principally of amorphous calcium phosphate (ACP), with minor amounts of amorphous calcium carbonate (ACC), having an overall Ca:P ratio of 2. CO2 stripping at low pH prior to pH shifting was found to minimize competitive calcite precipitation to improve product purity. Electrodes used intermittently, with polarity reversal, over 1.5 years of experiments were intact from fouling.