A novel mixing mechanism for effective stabilisation of urea in urinals for subsequent nutrient recovery

Abstract

Wastewater recycling plays a crucial role in meeting the environmental challenges currently facing society. Specifically, nutrient recycling from human urine can be used for fertiliser production, thus contributing to sustainable agriculture and food production. However, if not stabilised, the urea in urine undergoes enzymatic hydrolysis, and results in significant loss of nitrogen, which is vital for fertiliser production. Furthermore, the collection of human urine is challenging and hinders the widespread adoption of urine separating technologies, especially in existing buildings with urinals.In this regard, the main aim of this work was to develop a novel mixing mechanism in a urinal to effectively mix calcium hydroxide and fresh urine such that enzymatic urea hydrolysis is limited. Four urinal designs were tested. The pH of the solution in the urinal containers was used as a proxy for enzymatic urea hydrolysis and showed how effective the design/mixing was. A novel mechanical manual mixing design that utilised a pedestal mechanism connected to spring operated discs as well as an electrical mixing mechanism were identified as effective designs to inhibit enzymatic urea hydrolysis, achieving pH values of 12.0 and 12.1 respectively over a four-day period. These results were above the pH threshold of 11, below which enzymatic urea hydrolysis could occur. The fertiliser produced from the urinals can be recovered in two forms: solid (rich in calcium phosphate) and liquid (rich in urea and potassium).Our novel urinal system is characterised by its simplicity, easy installation, and low maintenance requirements, thus offering a new method for collecting, mixing and treating human urine on-site for subsequent nutrient recovery.