Sustainable use of reverse osmosis concentrates as alternative draw solutions for phosphorus recovery by Donnan dialysis

To reduce operational costs and improve process sustainability, we propose the use of high salinity wastes as alternative draw solutions for orthophosphate (P(V) as H _2 PO _4 ^– ) recovery by Donnan dialysis. Brackish groundwater, seawater, and reverse osmosis (RO) concentrates have high chloride contents, which can be exploited to induce an electrochemical potential gradient and recover P(V) via Donnan dialysis. In this study, a novel framework was developed and employed to determine P(V) recovery by Donnan dialysis with multicomponent draw solutions. Brackish groundwater, brackish water RO concentrate, seawater, and seawater RO concentrate were predicted to achieve 67.7%, 93.8%, 98.2%, and 98.8% P(V) recovery, respectively, from 10 mM P(V) wastewater, with chloride contributing 18.4%, 86.9%, 97.8%, and 98.4%, respectively, to the overall P(V) recovery. The remaining contributions were mostly from bicarbonate and sulfate. Brackish water RO concentrate, a widely available inland resource, was experimentally evaluated as an alternative draw solution for P(V) recovery by Donnan dialysis. Results confirmed 99.8% and 74.5% P(V) were recovered from wastewater containing 1 and 10 mM P(V), respectively. Magnesium and calcium in the brackish water RO concentrate favorably interacted with recovered P(V) to form struvite and hydroxyapatite solids. The precipitation reactions were successfully integrated into the Donnan transport model for systems operated with draw solutions containing synthetic and real RO concentrate. The outcomes highlight new opportunities for sustainable resource recovery via Donnan dialysis with alternative draw solutions composed of saline wastes.