Simultaneous removal of phosphate (PO43-) and ammonium (NH4+) by nanofiltration for phosphorus recovery as struvite

Abstract

In this study, we studied the transport of PO43- and NH4+ in synthetic wastewater (mixture) using three commercial nanofiltration (NF) membranes to elucidate the impact of charge on the membrane surface on the concurrent rejection of PO43- and NH4+ at different water recovery (WR) levels. Initially, we examined the transport of NH3-N and PO43- in each single salt across a range of pH levels at zero WR, encompassing two different concentrations, to understand the influence of pH and feed concentration on ion transport behavior. We found that the rejection of NH3-N in the single salt solutions, governed by Donnan exclusion, increased as pH rose up to pH 7.5, but decreased as the pH became alkaline due to an increased proportion of NH3 species. Conversely, PO43- species were retained over 90% in single salt solutions due to their larger hydrated radius, with a minor increase in rejection caused by increasing electrostatic repulsion against higher valence PO43- species as pH increased. Next, we examined ions transport in synthetic wastewater at zero and high WR. We observed that the magnitude of the negative membrane increased the retention of PO43- by electrostatic repulsion at zero WR. NH4+ was rejected by charge neutrality, predominantly determined by Cl- retention zero WRs in the mixture for all membranes. At high WR, the rejection sequences among ions remained unchanged across all membranes tested. A degree of reduction in rejections on ions were observed due to membrane fouling at this WR in all membranes. This study deepened our understanding of the fundamental processes involved in simultaneous PO43- and NH4+ transport in NF. This work contributed to alleviating environmental issues caused by phosphorus contamination and facilitating the circulation of phosphorus for reuse.