Mimicking and Inhibiting Urea Hydrolysis in Nonwater Urinals

Concomitant urea stabilization and phosphorus recovery from source-separated fresh urine in magnesium anode-based peroxide-producing electrochemical cells

In this study, we investigated the recovery of nitrogen (N) and phosphorus (P) from fresh source-separated urine with a novel electrochemical cell equipped with a magnesium (Mg) anode and carbon-based gas-diffusion cathode. Recovery of P, which exists primarily as phosphate (PO43-) in urine, was achieved through pH-driven precipitation. Maximizing N recovery requires simultaneous approaches to address urea and ammonia (NH3). NH3 recovery was possible through precipitation in struvite with soluble Mg supplied by the anode. Urea was stabilized with electrochemically synthesized hydrogen peroxide (H2O2) from the cathode. H2O2 concentrations and resulting urine pH were directly proportional to the applied current density. Concomitant NH3 and PO43- precipitation as struvite and urea stabilization via H2O2 electrosynthesis was possible at lower current densities, resulting in urine pH under 9.2. Higher current densities resulted in urine pH over 9.2, yielding higher H2O2 concentrations and more consistent stabilization of urea at the expense of NH3 recovery as struvite; PO43- precipitation still occurred but in the form of calcium phosphate and magnesium phosphate solids.

Benchmarks for urine volume generation and phosphorus mass recovery in commercial and institutional buildings

Phosphorus (P) is a finite resource and necessary nutrient for agriculture. Urine contains a higher concentration of P than domestic wastewater, which can be recovered by source separation and treatment (hereafter urine diversion). Commercial and institutional (CI) buildings are a logical location for urine diversion since restrooms account for a substantial fraction of water use and wastewater generation. This study estimated the potential for P recovery from human urine and water savings from reduced flushing in CI buildings, and proposed an approach to identify building types and community layouts that are amenable to implementing urine diversion. The results showed that urine diversion is most advantageous in CI buildings with either high daily occupancy counts or times, such as hospitals, schools, office buildings, and airports. Per occupant P recovery benchmarks were estimated to be between 0.04-0.68 g/cap·d. Per building P recovery rates were estimated to be between 0.002-5.1 kg/d, and per building water savings were estimated to be between 3 and 23 % by volume. Recovered P in the form of phosphate fertilizer and potable water savings could accrue profits and cost reductions that could offset the capital costs of new urine diversion systems within 5 y of operation. Finally, urine diversion systems can be implemented at different levels of decentralization based on community layout and organizational structure, which will require socioeconomic and policy acceptance for wider adoption.

Transport and deposition of solid phosphorus-based mineral particles in urine diversion systems

The deposition of solid phosphorus-based mineral particles is a common problem in urine diversion systems, which occurs in transport systems, particularly in horizontal pipelines. In this work, particle deposition behaviour in turbulent flow in a 3D horizontal pipe was simulated by using the Euler-Lagrange method. The effects of particle diameter, particle density, particle shape factor and fluid flow velocity on particle deposition behaviour were investigated. The results showed that the deposition rate increased by 9.92%,6.88% and 6.88% with increasing particle diameter (10-90 μ m ), particle density (1400 kg/m3-2300 kg/m3), and particle shape factor (0.2-1), respectively. For particles with larger diameters (>90 μ m ) or larger density (>2300 kg/m3), the deposition rate of these particles was almost reached 100%. It was found that gravitational sedimentation was the dominant deposition mechanism in low fluid flow velocity range (0.1-0.5 m/s). As fluid flow velocity increased (>0.5 m/s), turbulent fluctuation became the dominant factor that affected particle motion behaviour, whereas the effect of gravitational sedimentation on particle deposition behaviour declined significantly, and the increase in fluid flow velocity no longer significantly affects deposition rate. It was found that the deposition rate decreased by 29.13% as the fluid flow velocity was increased from 0.1 m/s to 0.5 m/s, while the corresponding deposition rate only decreased by 14.24% when the fluid flow velocity was increased from 0.5 m/s to 2 m/s. The optimal flow velocity was found to range between 0.75 and 1.25 m/s, which may mitigate the deposition of mineral solids in urine diversion systems.

Factors influencing the recovery of organic nitrogen from fresh human urine dosed with organic/inorganic acids and concentrated by evaporation in ambient conditions

To feed the world without transgressing regional and planetary boundaries for nitrogen and phosphorus, one promising strategy is to return nutrients present in domestic wastewater to farmland. This study tested a novel approach for producing bio-based solid fertilisers by concentrating source-separated human urine through acidification and dehydration. Thermodynamic simulations and laboratory experiments were conducted to evaluate changes in chemistry of real fresh urine dosed and dehydrated using two different organic and inorganic acids. The results showed that an acid dose of 1.36 g H₂SO₄ L^-1, 2.86 g H₃PO₄ L^-1, 2.53 g C₂H₂O₄·2H₂O L^-1 and 5.9 g C₆H₈O₇ L^-1 was sufficient to maintain pH ≤3.0 and prevent enzymatic ureolysis in urine during dehydration. Unlike alkaline dehydration using Ca(OH)₂ where calcite formation limits the nutrient content of fertiliser products (e.g. <15 % nitrogen), there is greater value proposition in acid dehydration of urine, as the products contain 17.9-21.2 % nitrogen, 1.1-3.6 % phosphorus, 4.2-5.6 % potassium and 15.4-19.4 % carbon. While the treatment recovered all phosphorus, recovery of nitrogen in the solid products was 74 % (±4 %). Follow-up experiments revealed that hydrolytic breakdown of urea to ammonia, chemically or enzymatically, was not the reason for the nitrogen losses. Instead, we posit that urea breaks down to ammonium cyanate, which then reacts with amino and sulfhydryl groups of amino acids excreted in urine. Overall, the organic acids evaluated in this study are promising for decentralised urine treatment, as they are naturally present in food and therefore already excreted in human urine.

N, P, K recovery from hydrolysed urine by Na-chabazite adsorption integrated with ammonia stripping and (K-)struvite precipitation

This study investigated the recovery of K⁺ along with NH₄⁺-N and PO₄^3--P from hydrolyzed urine by technical integration. The K adsorption capacities of biochar, clinoptilolite, artificial zeolite and chabazite were firstly compared. Due to the high K recovery efficiency and additional P recovery capacity, Na-chabazite was selected as the adsorbent in this study. Its kinetics and isotherm analysis indicated that the high molarity of NH₄⁺-N seriously hindered the K adsorption onto Na-chabazite in synthetic hydrolyzed urine (SHU). However, this competition between NH₄⁺ and K⁺ got diminished when their molarity is the same, i.e. in the SHU after ammonia stripping (ASSHU). Based on this key finding, Na-chabazite adsorption was integrated with ammonia stripping and struvite precipitation under different configurations. Simultaneous ammonia stripping was inadequate to diminish the competitive effect of NH₄⁺ on K⁺ adsorption. Depending on the demand for fertilizer, two sequential configurations were recommended, respectively.

Simultaneous removal of phosphate and antibiotic from hydrolyzed urine by novel spherical particles

Phosphate recovery from wastewater is regarded as promising strategy to achieve sustainable supply of non-renewable natural resources. In this study, a novel technique for spherical materials preparation was developed to achieve both phosphate recovery and antibiotic removal from urine. Phosphate removal and sulfamethoxazole (SMX) degradation performance of the synthesized spherical materials was studied in synthesized urine and real urine. MgB, made from magnesium oxide (10%) and biochar (10%), was the most effective in phosphate removal and SMX degradation. Struvite formation and radical production were the mechanisms of phosphate removal and SMX degradation, respectively. The phosphate removal capacity of MgB was 0.181 g/g and the removal cost was around 0.245 RMB/g phosphate. Meanwhile, the combination of MgB and persulfate could achieve a 98% degradation efficiency of SMX, which could eliminate the hazardous impurity in final product. Furthermore, this technique has also been validated useful in treating real hydrolyzed urine.

Efficient Hydrogen Generation and Total Nitrogen Removal for Urine Treatment in a Neutral Solution Based on a Self-Driving Nano Photoelectrocatalytic System

Urine is the main source of nitrogen pollution, while urea is a hydrogen-enriched carrier that has been ignored. Decomposition of urea to H2 and N2 is of great significance. Unfortunately, direct urea oxidation suffers from sluggish kinetics, and needs strong alkaline condition. Herein, we developed a self-driving nano photoelectrocatalytic (PEC) system to efficiently produce hydrogen and remove total nitrogen (TN) for urine treatment under neutral pH conditions. TiO2/WO3 nanosheets were used as photoanode to generate chlorine radicals (Cl•) to convert urea-nitrogen to N2, which can promote hydrogen generation, due to the kinetic advantage of Cl-/Cl• cyclic catalysis. Copper nanowire electrodes (Cu NWs/CF) were employed as the cathode to produce hydrogen and simultaneously eliminate the over-oxidized nitrate-nitrogen. The self-driving was achieved based on a self-bias photoanode, consisting of confronted TiO2/WO3 nanosheets and a rear Si photovoltaic cell (Si PVC). The experiment results showed that hydrogen generation with Cl• is 2.03 times higher than in urine treatment without Cl•, generating hydrogen at 66.71 μmol h-1. At the same time, this system achieved a decomposition rate of 98.33% for urea in 2 h, with a reaction rate constant of 0.0359 min-1. The removal rate of total nitrogen and total organic carbon (TOC) reached 75.3% and 48.4% in 2 h, respectively. This study proposes an efficient and potential urine treatment and energy recovery method in neutral solution.

An Appraisal of Urine Derivatives Integrated in the Nitrogen and Phosphorus Inputs of a Lettuce Soilless Cultivation System

Reinforcing and optimizing sustainable food production is an urgent contemporary issue. The depletion of natural mineral resources is a key problem that is addressed by recycling mined potassium and phosphorus, and nitrogen, whose production depends on very high energy input. A closed-loop approach of fertilizer use asserts the necessity for efficient management and practices of organic waste rich in minerals. Human-derived urine is an underutilized yet excellent source for nitrogen fertilizer, and, in this study, processed urine fertilizer was applied to greenhouse soilless cultivation of lettuce (Lactuca sativa L.) cv. Grand Rapids. Biomass increase, biometric parameters, soil plant analysis development (SPAD) index, minerals, and organic acids content of lettuce were analyzed. From eight different urine fertilizer products generated, K-struvite, urine precipitate-CaO, and the liquid electrodialysis (ED) concentrate supported the growth of lettuce similar to that of commercial mineral fertilizer. ED concentrate application led to the accumulation of potassium (+17.2%), calcium (+82.9%), malate (+185.3%), citrate (+114.4%), and isocitrate (+185.7%); K-struvite augmented the accumulation of magnesium (+44.9%); and urine precipitate-CaO induced the highest accumulation of calcium (+100.5%) when compared to the control, which is an added value when supplemented in daily diet. The results underlined the potential of nitrogen- and phosphate-rich human urine as a sustainable source for the fertilization of lettuce in soilless systems.

Ammonium and potassium removal from undiluted and diluted hydrolyzed urine using natural zeolites

There is limited research comparing nutrient removal in concentrated and dilute waste streams. Accordingly, the goal of this research was to study the effect of dilution on ammonium and potassium removal from real hydrolyzed urine using natural zeolites. The performance of two natural zeolites, clinoptilolite and chabazite, was studied and compared using batch equilibrium experiments at four dilution levels defined as urine volume divided by total solution volume (expressed as a percent): 100%, 10%, 1% and 0.1%. The adsorption behavior of other exchangeable ions, namely sodium, calcium, and magnesium, in clinoptilolite and chabazite was studied to improve the understanding of ion exchange stoichiometry. Ammonium and potassium removals were highest in undiluted urine samples treated with clinoptilolite or chabazite. This is a key finding as it illustrates the benefit of collecting undiluted urine via source separation. High removal of ammonium and potassium by clinoptilolite and chabazite was also achieved in 10% urine solutions, which are representative of water-efficient flush systems and show that nutrient recovery is possible for diluted urine as well. Chabazite showed higher ammonium and much higher potassium removal than clinoptilolite. Finally, the results showed that clinoptilolite and chabazite demonstrated stoichiometric exchange between ammonium and potassium in urine solutions with mobile cations in the zeolites.

Life Cycle Assessment of Urine Diversion and Conversion to Fertilizer Products at the City Scale

Urine diversion has been proposed as an approach for producing renewable fertilizers and reducing nutrient loads to wastewater treatment plants. Life cycle assessment was used to compare environmental impacts of the operations phase of urine diversion and fertilizer processing systems [via (1) a urine concentration alternative and (2) a struvite precipitation and ion exchange alternative] at a city scale to conventional systems. Scenarios in Vermont, Michigan, and Virginia were modeled, along with additional sensitivity analyses to understand the importance of key parameters, such as the electricity grid and wastewater treatment method. Both urine diversion technologies had better environmental performance than the conventional system and led to reductions of 29-47% in greenhouse gas emissions, 26-41% in energy consumption, approximately half the freshwater use, and 25-64% in eutrophication potential, while acidification potential ranged between a 24% decrease to a 90% increase. In some situations, wastewater treatment chemical requirements were eliminated. The environmental performance improvement was usually dependent on offsetting the production of synthetic fertilizers. This study suggests that urine diversion could be applied broadly as a strategy for both improving wastewater management and decarbonization.

Impact of acetic acid addition on nitrogen speciation and bacterial communities during urine collection and storage

The rate of urea hydrolysis in nonwater urinals is influenced by the volume of urination events and the frequency of urinal use. Inhibition of urea hydrolysis with acetic acid addition has been demonstrated at the laboratory scale but it was not able to fully represent the conditions of a real restroom with real urine collection. The goal of this study was to understand the effects of acid addition for control of urea hydrolysis on nutrient concentrations and bacterial communities in human urine during collection and storage. Three control logics were used to determine the schedule of acid addition: (i) acid addition after every urination event, (ii) acid addition during periods of high building occupancy, and (iii) acid addition during periods of low building occupancy. Wifi logins were used to approximate building occupancy and to create the control logics used in the study. All three control logics were able to inhibit urea hydrolysis. The bacterial communities were identified to determine the impact of acid addition on the community structure. The collection of urine by nonwater urinals alone did not reduce the presence of enteric bacteria commonly found when collecting urine with urine-diverting toilets. Acid addition reduced the community diversity and created conditions for higher relative abundances of the order Enterobacteriales. Finally, results from stored acidified urine showed that urea hydrolysis inhibition is reversible and is influenced by the amount of acid added at the urinal. The amount of acid added can influence the rate of hydrolysis in the storage tanks and can be used to select for urea- or ammonia-nitrogen for nutrient recovery. This study is the first of its kind to inhibit urea hydrolysis in nonwater urinals in a real restroom with real urine, and is the first to identify the bacterial communities in urine collected solely with nonwater urinals.

Technologies for the recovery of nutrients, water and energy from human urine: A review

The global demand for a constant supply of fertilizer is increasing with the booming of the population. Nowadays more focus is given to the recovery and reuse of the nutrients rather than synthesis of the fertilizer from chemicals. Human urine is the best available resource for the primary macronutrients (Nitrogen, Phosphorus and Potassium) for the fertilizer as it contains 10-12 g/L nitrogen, 0.1-0.5 g/L phosphorous and 1.0-2.0 g/L potassium. For the recovery of these nutrients from human urine, various technologies are available which requires source separation and treatment. . In this review, a wide range of the technologies for the treatment of source-separated human urine are covered and discussed in detail. This review has categorized the technologies based on the recovery of nutrients, energy, and water from human urine. Among the various technologies available, Bio-electrochemical technologies are environmental friendly and recovers energy along with the nutrients. Forward Osmosis is the best available technology for the water recovery and for concentrating the nutrients in urine, without or minimal consumption of energy. However, experimental work in this technology is at its prior stage. A single technology is still not sufficient to recover nutrients, water and energy. Therefore, integration of two or more technologies seems essential.

Alkaline dehydration of source-separated fresh human urine: Preliminary insights into using different dehydration temperature and media

For sanitation systems aiming at recycling nutrients, separately collecting urine at source is desirable as urine contains most of the nutrients in wastewater. However, reducing the volume of the collected urine and recovering majority of its nutrients is necessary, as this improves the transportability and the end-application of urine-based fertilisers. In this study, we present an innovative method, alkaline dehydration, for treating fresh human urine into a nutrient-rich dry solid. Our aim was to investigate whether fresh urine (pH < 7) added to five different alkaline media (pH > 11) could be dehydrated at elevated temperatures (50 and 60 °C) with minimal loss of urea, urine's principal nitrogen compound. We found that it was possible to concentrate urine 48 times, yielding dry end-products with high fertiliser value: approximately, 10% N, 1% P, and 4% K. We monitored the physicochemical properties and the composition of various dehydration media to provide useful insights into their suitability for dehydrating urine. We demonstrated that it is possible to recover >90% nitrogen when treating fresh urine by alkaline dehydration by inhibiting the enzymatic hydrolysis of urea at elevated pH and minimising the chemical hydrolysis of urea with high urine dehydration rates.

Electrochemically Induced Precipitation Enables Fresh Urine Stabilization and Facilitates Source Separation

Source separation of urine can enable nutrient recycling, facilitate wastewater management, and conserve water. Without stabilization of the urine, urea is quickly hydrolyzed into ammonia and (bi)carbonate, causing nutrient loss, clogging of collection systems, ammonia volatilization, and odor nuisance. In this study, electrochemically induced precipitation and stabilization of fresh urine was successfully demonstrated. By recirculating the urine over the cathodic compartment of an electrochemical cell, the pH was increased due to the production of hydroxyl ions at the cathode. The pH increased to 11-12, decreasing calcium and magnesium concentrations by >80%, and minimizing scaling and clogging during downstream processing. At pH 11, urine could be stabilized for one week, while an increase to pH 12 allowed urine storage without urea hydrolysis for >18 months. By a smart selection of membranes [anion exchange membrane (AEM) with a cation exchange membrane (CEM) or a bipolar membrane (BPM)], no chemical input was required in the electrochemical cell and an acidic stream was produced that can be used to periodically rinse the electrochemical cell and toilet. On-site electrochemical treatment, close to the toilet, is a promising new concept to minimize clogging in collection systems by forcing controlled precipitation and to inhibit urea hydrolysis during storage until further treatment in more centralized nutrient recovery plants.

Three-stage treatment for nitrogen and phosphorus recovery from human urine: Hydrolysis, precipitation and vacuum stripping

Source separation of human urine has not been widely adopted because of scaling on urine collecting fixtures and lack of verified technologies for on-site utilization of waterless urine. This study investigated the effects of flushing liquid, temperature and urease amendment on hydrolysis of urea to ammonia, explored ammonia recovery via vacuum stripping in connection with phosphorus recovery via struvite precipitation in different sequences, and performed economic analysis of a proposed nutrient recovery strategy. It was found that acetic acid could be dosed at 0.05-0.07 N to flush urine-diverting toilets and urinals for hygiene and prevention of scaling. However, a high dosage of 0.56 N completely inhibited urea hydrolysis. Source-separated urine could be stored at 25 °C with ample urease for complete urea hydrolysis within approximately 20 h. Fully hydrolyzed waterless urine contained 9.0-11.6 g/L ammonia-N, 0.53-0.95 g/L phosphate-P and only 2.3-9.1 mg/L magnesium. When magnesium was supplemented to attain the optimum Mg²⁺: PO₄^3- molar concentration ratio of 1.0 in hydrolyzed urine, batch operation of a pilot-scale air-lift crystallizer removed 93-95% of phosphate and produced 3.65-4.93 g/L struvite in 1-5 h. Batch operation of a pilot-scale vacuum stripping - acid absorption system for 12 h stripped 72-77% of ammonia and produced 37.6-39.7 g/L (NH₄)₂SO₄. Compared with the ammonia → phosphorus recovery sequence, the struvite precipitation → vacuum stripping sequence produced more struvite and ammonium sulfate. The strategy of urea hydrolysis → struvite precipitation → vacuum stripping of ammonia is a sustainable alternative to the conventional phosphorus fertilizer production and ammonia synthesis processes.

Opportunities for Building-Scale Urine Diversion and Challenges for Implementation

Urine diversion (i.e., urine source separation) has been proposed as a more sustainable solution for water conversation, nutrient removal and recovery, and pharmaceutical sequestration. As wastewater regulations become more stringent, wastewater treatment plants reach capacity, and water resources become more strained, the benefits of urine diversion become more appealing. By using nonwater urinals and urine-diverting toilets, urine diversion systems seek to collect undiluted human urine for nutrient recovery and pharmaceutical sequestration. Urine is a unique, nutrient-rich waste stream that constitutes an overall low volume of waste entering a wastewater treatment plant. If urine is separated at the building-scale, various technologies can be used to recover nutrients and sequester pharmaceuticals at their most concentrated location. However, the implementation of urine diversion requires a paradigm shift from conventional comingling of wastewater and centralized treatment to source separation and decentralized treatment. This Account proposes a vision for building-scale implementation of urine diversion with the goal of clarifying the opportunities and challenges in this context. The main components of urine, i.e., nitrogen, phosphorus, potassium, and pharmaceuticals, are major drivers for technology development and system implementation. Stepping back, the benefits from water conservation and effects on wastewater treatment are an extension of the system boundary that can impact the sustainability of adjacent systems. However, major challenges have been identified in the literature as hurdles for widespread implementation of urine diversion. Challenges include the comparison of recovering nutrients at the wastewater plant versus at the source, the collection and storage of urine, the ability to recover nutrients and sequester pharmaceuticals, and the overall environmental and economic impacts of urine diversion systems. While these challenges exist, studies have been conducted to address some of the underlying research questions. As more research is conducted, the vision of a seamless urine diversion system with building-wide plumbing and storage comes closer to reality. As such, the application of urine diversion systems will benefit from technology development and research to fill gaps that have been identified. It is important to classify urine diversion systems as a process and not a product. This has implications for the way these systems are evaluated, as their impact on peripheral systems can be of benefit to different stakeholders. In the same light, new research areas, such as cyber-physical systems, reverse logistics, and sustainability transitions, can be applied to urine diversion as approaches for ensuring a robust process for widespread implementation. However, established technologies should be constantly reassessed and enhanced by newer techniques. For example, membrane distillation, eutectic freeze concentration, and solar evaporation should be considered for nutrient recovery and volume reduction because they offer benefits over conventional technologies. Finally, the human behavior component of urine diversion cannot be ignored, as negative user acceptance and improper maintenance of these systems can have a detrimental impact on their future implementation.

Real-Time Monitoring and Control of Urea Hydrolysis in Cyber-Enabled Nonwater Urinal System

This research used a cyber-physical system (CPS) to monitor and control the extent of urea hydrolysis in nonwater urinals. Real-time pH and conductivity data were used to control urea hydrolysis inhibition under realistic restroom conditions with acetic acid addition. Variable urination frequencies and urination volumes were used to compare three conditions that affect the progression of urea hydrolysis. Mechanistic and conceptual models were created to evaluate the factors that influence the progression of urea hydrolysis in nonwater urinals. It was found that low urination volumes at low frequencies created ideal conditions for urea hydrolysis to progress. Alternatively, high urination volumes at high frequencies created pseudo-inhibitory conditions because it did not allow for sufficient reaction time or mixing with older urine in the urinal trap. The CPS was used to control urea hydrolysis inhibition by two logics: (1) reactively responding to a pH threshold and (2) predictively responding to past measurements using four lasso regression models. Results from the control logic experiments showed that acid was added once per hour under low use conditions and once in a 4 h experiment for high use conditions. The CPS allowed for full control of urine chemistry in the nonwater urinal, reducing the conditions (i.e., clogging and malodor) that have led to the removal of nonwater urinals in the United States.