Energy recovery from urea in decentralised wastewater streams

This work presents the design of a new process that enables energy recovery from wastewater, based on the selective adsorption of urea from urine and its subsequent decomposition into hydrogen. Removal of nitrogen compounds is at the core of wastewater treatment, as their uncontrolled discharge would lead to eutrophication and low quality of the water bodies. Currently, nitrogen-based contaminants are removed using energy-intensive biological processes that can represent more than 50% of the energy consumption of wastewater treatment plants [1]. Alternatively, decentralised treatment is a new approach where waste effluents are treated close to the generation point, where nutrients is at its highest concentration and the presence of other pollutants is minimized. In this regard, urine diversion toilets and urinals can be used to collect undiluted urine, which represents 80% of the nitrogen produced in households, in the form of urea ((NH2)2CO) [2]. Rather than just a contaminant, urea is a hydrogen-rich compound and a potential source of green energy. Cost-effective adsorbents have been developed by modifying activated carbon with different oxidising agents to introduce oxygen functional groups that increased urea adsorption capacity by 25%. A correlation between the concentration of acid functional groups and the adsorption capacity was found, providing mechanistic insights into the adsorption of urea. After the adsorption step, the urea was desorbed through thermal treatment at 200°C with 90% efficiency, leading to urea decomposition into ammonia and carbon dioxide. Coupling the process with the in-situ catalytic ammonia decomposition demonstrates the production of hydrogen to be used as an energy fuel. This process opens the door to new treatment systems for nitrogen-based compounds that enable their valorisation as energy sources, promoting circularity of wastewater treatment. Preliminary energy analysis shows that, if a decentralised approach with this energy recovery system was deployed in a city of 150,000 people, 250 kg/day of hydrogen would be produced and the municipal wastewater treatment plant would save 2,500 kWh/day, which is equivalent to 20% of their energy consumption.