A hybrid nanofiltration and reverse osmosis process for urine treatment: Effect on urea recovery and purity

Supply, demand and the economic effectiveness of urine-diverting technologies and products: A systematic literature review

The broader adoption of urine-diverting technologies (UDTs) and related products has been proposed as a strategy for moving towards a more circular economy. While some studies have explored the performance of UDTs, the interconnected factors involving supply, demand, and economic feasibility of UDTs remain under-researched. Our systematic review addresses this gap. Our search identified only 64 relevant, peer-reviewed studies, 71 % of which addressed the supply side (primarily the technical aspect of UDTs) and 58 % of which addressed the demand side (focusing on consumers' perceptions). Approximately one-third (18) of these studies delved into the economic feasibility of UDTs, with only 9 employing a cost benefit analysis (CBA) framework. However, none of these studies have analysed the economic performance of UDTs that have been fully deployed, indicating a significant knowledge gap. Our review suggests that overcoming challenges in scaling up UDTs can be achieved by engaging those stakeholders driving the uptake, developing business cases that offer an overall understanding of both market and non-market benefits of UDTs, addressing technological constraints by optimising urine treatment options for efficiency and economic viability, and enhancing stakeholders' acceptance of UDTs.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Superfast adsorption of small and uncharged urea from water using post-sonicated iron-based metal-organic framework

Urea is widely used in fertilizer production for agricultural purposes which risks runoff into soil and water sources. An excess of urea can result in algal or toxic blooms which can poison wildlife or even humans by accumulation in food sources. The removal of urea from water is challenging due to the small size (0.254 nm) and uncharged surface of urea. Intensive research has been conducted on a variety of methods to remove environmental concentrations of urea using adsorbents, but most of them lack effective removal, require long (>2 h) process time, and lack re-generability. Metal-organic frameworks (MOFs) are the new generation of adsorbents with excellent structural and functional group tunability. In this study, we synthesized MIL-100 (Fe), an iron-based MOF, as an efficient adsorbent for the removal of uncharged urea from water. The urea adsorption capacity of MIL-100 (Fe) was tested under varying experimental conditions such as pH (2-10), temperature (25-65 °C), MOF concentration (25-400 ppm), and urea concentration (25-1000 ppm). The results showed the superfast adsorption (more than 85% removal within 2 min) of neutrally charged urea molecules on MIL-100 (Fe). The MOF was able to reach a maximum adsorption efficiency of around 85% with a maximum uptake capacity of 3321 mg/g. The MIL-100 (Fe) showed acceptable re-generability by retaining up to 90% removal efficiency after four regeneration cycles. The urea adsorption followed pseudo 2nd-order adsorption kinetics and dipole-dipole interactions and π-NH bonding were the primary adsorption mechanism.

Concentrating stabilized human urine using eutectic freeze crystallization for liquid fertilizer production

Resource recovery from source-separated urine can be used to produce fertilizers and provide a more sustainable alternative to mineral fertilizers. Reverse osmosis can be used to remove up to 70% of the water in urine that has been stabilized with Ca(OH)₂ and pre-treated with air bubbling. However, further water removal is limited because of membrane scaling and equipment operating pressure limitations. A novel hybrid eutectic freeze crystallization (EFC) and RO system was investigated as a method to concentrate human urine, whilst simultaneously crystallizing salt and ice under EFC conditions. A thermodynamic model was used to predict the type of salts that would crystallize, their associated eutectic temperatures, and how much additional water removal was required (using freeze crystallization) to reach eutectic conditions. This innovative work showed that at eutectic conditions, Na₂SO₄∙10H₂O crystallizes simultaneously with ice in both real and synthetic urine, thus providing a new method to concentrate human urine for liquid fertilizer production. A theoretical mass balance of a hybrid RO-EFC process, including ice washing and recycle streams, showed that 77% of the urea and 96% of the potassium could be recovered with a 95% water removal. The final liquid fertilizer would have a composition of 11.5% N and 3.5% K, and 3.5 kg of Na₂SO₄∙10H₂O could be recovered from 1000 kg of urine. Over 98% of the phosphorus would be recovered as calcium phosphate during the urine stabilization step. A hybrid RO-EFC process would require 60 kWh m^-3 of energy, which is substantially less than other concentration methods.

Facile, Controllable, and Ultrathin NiFe-LDH In Situ Grown on a Ni Foam by Ultrasonic Self-Etching for Highly Efficient Urine Conversion

As the primary source of nitrogen pollutants in domestic sewage, urine is also an alternative for H₂ production via electrochemical processes. However, it suffers from sluggish kinetics and noble-metal catalyst requirement. Here, we report a non-precious ultrathin NiFe-layered double hydroxide catalyst for the remarkable conversion of urea into N₂ and H₂, which is in situ grown on a Ni foam via ultrasonic self-etching in Fe³⁺/ethylene glycol (EG). EG regulates the etching rate of Fe³⁺, resulting in an ultrathin nanosheet structure with the aid of ultrasonication. This structure dramatically promotes the dehydrogenation process via decreasing the nanolayer thickness from 120 to 3.4 nm and leads to a 4.8-fold increase in the generation of active sites. It exhibits record urea oxidation kinetics (390.8 mA·cm^-2 at 1.5 V vs RHE) with excellent stability (120 h), which is 11.8 times better than that of commercial Pt/C catalyst (33.1 mA·cm^-2). Tests with real urine at 20 mA cm^-2 achieve 74% total nitrogen removal and 2853 μmol·h^-1 of H₂ production. This study provides an attractive landscape for producing H₂ by consuming urine biowastes.

A Comprehensive Review on Wastewater Nitrogen Removal and Its Recovery Processes

Discharging large amounts of domestic and industrial wastewater drastically increases the reactive nitrogen content in aquatic ecosystems, which causes severe ecological stress and biodiversity loss. This paper reviews three common types of denitrification processes, including physical, chemical, and biological processes, and mainly focuses on the membrane technology for nitrogen recovery. The applicable conditions and effects of various treatment methods, as well as the advantages, disadvantages, and influencing factors of membrane technologies, are summarized. Finally, it is proposed that developing effective combinations of different treatment methods and researching new processes with high efficiency, economy, and energy savings, such as microbial fuel cells and anaerobic osmotic membrane bioreactors, are the research and development directions of wastewater treatment processes.