Donnan dialysis for phosphate recovery from diverted urine

There is a critical need to shift from existing linear phosphorous management practices to a more sustainable circular P economy. Closing the nutrient loop can reduce our reliance on phosphate mining, which has well-documented environmental impacts, while simultaneously alleviating P pollution of aquatic environments from wastewater discharges that are not completely treated. The high orthophosphate, HxPO4(3-x)-, content in source-separated urine offers propitious opportunities for P recovery. This study examines the use of Donnan dialysis (DD), an ion-exchange membrane-based process, for the recovery of orthophosphates from fresh and hydrolyzed urine matrixes. H2PO4- transport against an orthophosphate concentration gradient was demonstrated and orthophosphate recovery yields up to 93% were achieved. By adopting higher feed to receiver volume ratios, DD enriched orthophosphate in the product stream as high as ≈2.5 × the initial urine feed concentration. However, flux, selectivity, and yield of orthophosphate recovery were detrimentally impacted by the presence of SO42- and Cl- in fresh urine, and the large amount of HCO3- rendered hydrolyzed urine practically unsuitable for P recovery using DD. The detrimental effects of sulfate ions can be mitigated by utilizing a monovalent ion permselective membrane, improving selectivity for H2PO4- transport over SO42- by 3.1 × relative to DD with a conventional membrane; but the enhancement was at the expense of reduced orthophosphate flux. Critically, widely available and low-cost/waste resources with sufficiently high Cl- content, such as seawater and waste water softening regenerant rinse, can be employed to improve the economic viability of orthophosphate recovery. This study shows the promising potential of DD for P recovery and enrichment from source-separated urine.