The current linear economy model of nutrients, specifically P and N, is unsustainable: they are fixed or mined from the environment, utilized as fertilizers for food production, then are excreted by humans into wastewater infrastructure creating environmental and health hazards as well as depletion of essential nutrients. To combat both harmful nutrient emissions in wastewater and the current detrimental fertilizer production practice: P and N present in wastewater can be (1) separated in order to prevent harmful nutrient emissions to aquatic environments, and (2) synthesized as fertilizer products to provide sustainable supply for food production. The high concentrations of phosphate and ammonia in source-separated urine offer propitious opportunities to utilize green pathways to remove P and N as contaminants and capture these nutrients as fertilizer products. First, through Donnan dialysis, HPO₄^- in fresh urine exchanges with Cl^- in the receiver chamber across an anion-exchange membrane via an electrochemical potential gradient. The Cl^-in the receiver chamber is provided by a waste stream, such as reverse osmosis brine. Using this technique phosphate recovery of ~95% and enrichment of ~ 2.5X was achieved. Then, phosphate minerals can be precipitated from the enriched receiver solution and applied as fertilizer; alternatively, receiver solutions containing beneficial nutrients and micronutrients (KCl, MgCl₂, and CaCl₂), such as water softening recharge waste, can be applied to capture phosphate as a liquid fertilizer source. The remaining fresh urine is then stored for a number of days to enable the naturally occurring hydrolysis of urea, CH4N2O, which forms ammonia, NH₃, and bicarbonate, HCO₃^-. Then, a novel technique titled isothermal membrane distillation with acidic collector, or IMD-AC, is applied to drive the permeation of volatile NH₃ across a hydrophobic microporous membrane, while retaining the other constituents in hydrolyzed urine. NH₃ is captured in a low-cost non-toxic acetic acid collector solution, which can then be diluted as needed for application as fertilizer. The described pathway applies waste resources, such as reverse osmosis desalination brine, water softening recharge waste, and waste-heat, and other non-toxic chemical inputs, such as acetic acid to yield a green and affordable pathway for nutrient recovery from urine.

Hanqing Fan

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