Donnan dialysis is an ion-exchange process that separates ionic chemicals between waste and draw solutions. To advance resource-efficient wastewater treatment and nutrient recovery efforts, we investigated Donnan dialysis with synthetic wastewater (waste solution) and real reverse osmosis (RO) concentrate (draw solution) for phosphorus recovery. The process exploited chemical potential differences for orthophosphate (P(V), waste ion) and chloride (draw ion) across an anion-exchange membrane. We hypothesized that P(V) would be transported from the waste solution to the draw solution, with chloride being transported in the opposite direction to maintain electroneutrality. The following objectives were addressed: (1) to investigate P(V) removal efficiency with RO concentrate-based draw solutions; (2) to examine P(V) recovery efficiency and the composition of recovered solids; and, (3) to test the reusability of RO concentrate over multiple Donnan dialysis cycles. The P(V) recovery efficiencies were 92.3% and 79.3% for solutions with 1 and 10 mM P(V), respectively, after 76 h; moreover, up to 88.6% of the recovered P(V) precipitated in the draw solution. Scanning electron microscopy with energy dispersive spectroscopy revealed the presence of calcium phosphate particles. By systematic dilution of the RO concentrate, we reduced the extent of calcium phosphate precipitation without loss in P(V) removal efficiency. The integration of dilute RO concentrate draw solutions and addition of exogenous magnesium can facilitate formation of more desirable magnesium phosphate precipitates. Due to the high chloride concentration, RO concentrate was successfully reused without chemical modification, and the P(V) removal efficiency for the 10 mM wastewater after 74 h was 83.9% for stage 1 and 78.5% for stage 2. Overall, this work showcases high P(V) removal and recovery efficiencies for Donnan dialysis with RO concentrate-based draw solutions. The successful use of these two waste streams to produce valuable products addresses grand challenges in wastewater treatment, nutrient cycles, and RO concentrate disposal.<br/>

Donnan dialysis with reverse osmosis concentrate for phosphorus recovery from wastewater

ACS Fall 2020 Virtual Meeting

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