Continuous urea–nitrogen recycling from human urine: A step towards creating a human excreta based bio–economy

Abstract

The present study argues that human excreta are vital, renewable biological resources (nutrients) which must be considered an important subset of a bio–based economy. A human excreta based bio–economy can be defined as a set of circular and sustainable approaches that promote closed–loop flows of resources and nutrients from sanitation to agriculture. To this effect, from a technological perspective, this study presents a pathway for recovering, concentrating, and reusing fertilizer products from human urine. The development and implementation of urine diversion technologies have facilitated large–scale separation and storage of human urine independent of the faeces; however, processes that help harness the fertilization potential of source–separated urine in an effective, simple, yet efficient manner remain to be found. Hence, this study demonstrates a simple approach to selectively strip urea (a value–added product) from human urine in a continuously operated column packed with activated carbon prepared from agricultural wastes (coconut shells). Investigations were performed to determine the effect of carbon bed height, urine flow rate and concentration on efficiency of urea recovery. In order to model the column breakthrough and understand interactive influence of process parameters, experimental data was modelled against four well–known breakthrough equations. Optimal operating conditions were identified through an objective of simultaneous maximization of column capacity and urea removal efficiency of the activated carbon as well as minimization of sorption time. An initial urea concentration of 60%, urine flow of 6 L h−1 and carbon bed height of 30 cm resulted in 80% urea removal and an equilibrium uptake capacity of 94.03 mg g−1. For quantitative recovery of the adsorbed urea and reuse of the saturated column, regeneration experiments were performed using deionised water as the desorption media pumped in up flow mode at 3 L h−1. Considering the average annual nutrient composition of human urine, this approach could allow production of ∼4.5 kg urea per person per year. The reutilization of human wastes through ecologically–conscious sanitation practices could therefore blur the difference in the cognition of two human constructs, ‘resources’ and ‘wastes’.