Closed-loop fertility cycle: Realizing sustainability in sanitation and agricultural production through the design and implementation of nutrient recovery systems for human urine

•Microwave activated carbon with BET surface area >1000 m22/g prepared from coconut shells.•Activated carbon was able to adsorb >90% of the urea from urine.•Sorption data followed the Langmuir model with a pseudo-first-order kinetic fit.•Soil analysis indicated increase in soil total nitrogen and CEC due to urea adsorbed.•Study demonstrates working of closed-loop fertility cycle to promote sustainable sanitation.

As a solution to the drawbacks of modern sanitation systems and to shift towards a recycling society, source separation of human wastes coupled with resource recovery could be seen as a potential solution. In this research, microwave activated coconut shells were utilized to recover urea from human urine. Batch adsorption studies were carried out to determine the effect of initial concentration, adsorption temperature, microwave output power and irradiation time on the urea uptake capacity of the tailored activated carbon. The shells pretreated with microwave irradiation of 360 W, for 15 min (MACCS-360W-15) shown to be promising adsorbents with BET surface area >1000 m2  g−1. The sorption data were tested against different isotherm models and found to closely follow Langmuir isotherm with a maximum monolayer sorption capacity of 312 mg g−1. Kinetic data over temperature range of 30–60 °C was found to closely follow pseudo-first-order at all adsorbate concentrations. Gibbs free energy (ΔΔG°), enthalpy (ΔΔH°) and entropy (ΔΔS°) indicated the spontaneity and physical nature of the sorption; sorption experiments indicated a urea recovery of ∼95% from urine. Finally, the application of urea adsorbed carbon as a soil conditioner in field trials resulted in significant improvement in the number of seed germination and plant biomass (132%) with a substantial increase in the soil nitrogen and cation exchange capacity. Results also indicated that nearly all the urea was desorbed within the soil during irrigation becoming readily available to the plants. This study demonstrates a closed-loop sanitation cycle that channels nutrients from human beings back to agricultural fields.

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