Nutrient recovery from source-separated human urine has been identified by many as a viable avenue towards the circular economy of nutrients. Moreover, untreated (and partially treated) urine is the main anthropogenic route of environmental discharge of nutrients, most concerning for nitrogen, whose release has exceeded the planet’s own self-healing capacity. Urine contains all key macronutrients (N, P, and K) and micronutrients (S, Ca, Mg, and trace metals) needed for plant growth and is, therefore, an excellent fertilizer. However, direct reuse is not recommended in modern society due to the presence of active organic molecules and heavy metals in urine. Many systems have been proposed and tested for nutrient recovery from urine, but none so far has reached technological maturity due to usually high power or chemical requirements or the need for advanced process controls. This work is the proof of concept for the world’s first nutrient recovery system that powers itself and does not require any chemicals or process controls. This is a variation of the previously proposed microbial electrochemical Ugold process, where a novel air cathode catalyst active in urine conditions (pH 9, high ammonia) enables in situ generation of electricity in a microbial fuel cell setup, and the simultaneous harvesting of such electricity for the electrodialytic concentration of ionic nutrients into a product stream, which is free of heavy metals. The system was able to sustain electrical current densities around 3 A m^–2 for over two months while simultaneously upconcentrating N and K by a factor of 1.5–1.7.

Removal and recovery of nutrients from waste streams is essential to avoid depletion of finite resources and further disruption of the nutrient cycles. Bioelectrochemical systems (BESs) are gaining interest because of their ability to recover nutrients through ion migration across membranes at a low energy demand. This work assesses the feasibility of the concept of nutrient bio-electroconcentration from domestic wastewater, which is a widely available source of nutrients in ionic form, collected via sewer networks and easily accessible at centralized wastewater treatment plants. Here, we demonstrate the limits of a three-chamber BES for the recovery of nutrients from domestic wastewater. Because of low ionic conductivity, the measured current densities did not exceed 2 A m^–2, with corresponding limited nutrient ion recoveries. Moreover, in a 3D electrode, forcing higher current densities through potentiostatic control leads to higher Ohmic losses, resulting in anode potential profiles and runaway currents and potentials, with consequent unwanted water oxidation and disintegration of the graphite electrode. At the current density of 1.9 A m^–2, N removal efficiency of 48.1% was obtained at the anode. However, calcium and magnesium salts precipitated on the anion-exchange membrane, putatively lowering its permselectivity and allowing for migration of cations through it. This phenomenon resulted in low N and K recovery efficiencies (12.0 and 11.5%, respectively), whereas P was not recovered because of precipitation of salts in the concentrate chamber.

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