Selective recovery of phosphorus and urea from fresh human urine using a liquid membrane chamber integrated flow-electrode electrochemical system

Advanced electrochemical membrane technologies for near-complete resource recovery and zero-discharge of urine: Performance optimization and evaluation

The depletion of nutrient sources in fertilizers demands a paradigm shift in the treatment of nutrient-rich wastewater, such as urine, to enable efficient resource recovery and high-value conversion. This study presented an integrated bipolar membrane electrodialysis (BMED) and hollow fiber membrane (HFM) system for near-complete resource recovery and zero-discharge from urine treatment. Computational simulations and experimental validations demonstrated that a higher voltage (20 V) significantly enhanced energy utilization, while an optimal flow rate of 0.4 L/min effectively mitigated the negative effects of concentration polarization and electro-osmosis on system performance. Within 40 min, the process separated 90.13% of the salts in urine, with an energy consumption of only 8.45 kWh/kgbase. Utilizing a multi-chamber structure for selective separation, the system achieved recovery efficiencies of 89% for nitrogen, 96% for phosphorus, and 95% for potassium from fresh urine, converting them into high-value products such as 85 mM acid, 69.5 mM base, and liquid fertilizer. According to techno-economic analysis, the cost of treating urine using this system at the lab-scale was $6.29/kg of products (including acid, base, and (NH4)2SO4), which was significantly lower than the $20.44/kg cost for the precipitation method to produce struvite. Excluding fixed costs, a net profit of $18.24/m3 was achieved through the recovery of valuable products from urine using this system. The pilot-scale assessment showed that the net benefit amounts to $19.90/m3 of urine, demonstrating significant economic feasibility. This study presents an effective approach for the near-complete resource recovery and zero-discharge treatment of urine, offering a practical solution for sustainable nutrient recycling and wastewater management.

Metal element-based adsorbents for phosphorus capture: Chaperone effect, performance and mechanism

Excessive phosphorus (P) enters the water bodies via wastewater discharges or agricultural runoff, triggering serious environmental problems such as eutrophication. In contrast, P as an irreplaceable key resource, presents notable supply-demand contradictions due to ineffective recovery mechanisms. Hence, constructing a system that simultaneously reduce P contaminants and effective recycling has profound theoretical and practical implications. Metal element-based adsorbents, including metal (hydro) oxides, layered double hydroxides (LDHs) and metal-organic frameworks (MOFs), exhibit a significant chaperone effect stemming from strong orbital hybridization between their intrinsic Lewis acid sites and P (Lewis base). This review aims to parse the structure-effect relationship between metal element-based adsorbents and P, and explores how to optimize the P removal properties. Special emphasis is given to the formation of the metal-P chemical bond, which not only depends on the type of metal in the adsorbent but also closely relates to its surface activity and pore structure. Then, we delve into the intrinsic mechanisms behind these adsorbents' remarkable adsorption capacity and precise targeting. Finally, we offer an insightful discussion of the prospects and challenges of metal element-based adsorbents in terms of precise material control, large-scale production, P-directed adsorption and effective utilization.

Sustained Phosphorus Removal and Enrichment through Off-Flow Desorption in a Reservoir of Membrane Capacitive Deionization

In this study, we used a membrane capacitive deionization device with a reservoir (R-MCDI) to enrich phosphorus (P) from synthetic wastewater. This R-MCDI had two small-volume electrode chambers, and most of the electrolyte was contained in the reservoir, which was circulated along the electrode chambers. Compared with conventional MCDI, R-MCDI exhibited a phosphate removal rate of 0.052 μmol/(cm2·min), approximately double that of MCDI. This was attributed to R-MCDI's utilization of OH- alternative adsorption to remove phosphate from the influent. Noticing that around 73.9% of the removed phosphate was stored in the electrolyte in R-MCDI, we proposed a novel off-flow desorption operation to enrich the removed phosphate in the reservoir. Exciting results from the multicycle experiment (∼8 h) of R-MCDI showed that the PO43--P concentration in the reservoir increased all the way from the initial 152 mg/L to the final 361 mg/L, with the increase in the P charge efficiency from 5.5 to 22.9% and the decrease in the energy consumption from 28.2 to 6.8 kW h/kg P. The P recovery performance of R-MCDI was evaluated by viewing other similar studies, which revealed that R-MCDI in this study achieved superior P enrichment with low energy consumption and that the off-flow desorption proposed here considerably simplified the operation and enabled continuous P enrichment.

Enhancing efficient reclaim of phosphorus from simulated urine by magnesium-functionalized biochar: Adsorption behaviors, molecular-level mechanistic explanations and its potential application

Magnesium-functionalized Magnolia grandiflora Linn leaf-derived biochar (MBC) capable of efficiently reclaiming phosphorus from urine was synthesized by slow co-pyrolysis. Four adsorption kinetic and seven adsorption isotherm models were fitted to the batch adsorption and desorption experimental data, and it was found that pseudo-first-order kinetic model and multilayer model with saturation best described the phosphate-phosphorus (PO43--P) adsorption process by MBC. MBC and phosphorus-saturated MBC (P-MBC) were found to offer outstanding phosphorus adsorption and slow release properties, respectively. Based on material characterization, statistical physics, adsorption energy distribution and statistical thermodynamics, a multi-ionic, inclined orientation, entropy-driven spontaneous endothermic process of MBC on PO43--P was proposed, involving physicochemical interactions (porous filling, electrostatic attraction, ligand exchange and surface precipitation). Further, seed germination and early seedling growth experiments proved that P-MBC can be used as a slow-release fertilizer. Overall, MBC offers prospective applications as an efficient phosphorus adsorbent and then as a slow-release fertilizer.

Adsorption-electrochemical mediated precipitation for phosphorus recovery from sludge filter wastewater with a lanthanum-modified cellulose sponge filter

In this study, the sludge filter wastewater is confirmed to investigate the effects of adsorption-electrochemical mediated precipitation (EMP) driven phosphorus recovery on the basis of lanthanum-modified cellulose sponge filter (LCLM) material. The adsorption-EMP method relies on in situ recovery phosphate (P) from the used desorption agent (NaOH-NaCl binary solution) via the formation of Ca5(PO4)3OH all while preserving the alkalinity of the desorption agents which benefited long-term application. The lanthanum content of LCLM was 9.0 mg/g, and the adsorption capacity reached 226.1 ± 15.2 mg P/g La at an equilibrium concentration of 3.9 mg P/L. After adsorption, 55.7 % of P was recovered, and the corresponding alkalinity increased from 1.9 mmol/L to 2.2 mmol/L. Adsorption mechanism analysis revealed that the high lanthanum usage of LCLM was attributed to the synergistic effect of the lattice oxygen of LaO and LaPO4·0.5H2O crystallite formation. Additionally, the Ca5(PO4)3OH was found precipitated in the precipitation in the cathode chamber (P-CC) rather than on the surface/section of cation exchange membrane (CEM) and cathode indicating that the P recovery process was controlled by the saturation of CaP species in the EMP system and the electromigration effect. These findings present a new strategy to promote the effective utilization of rare earth elements for P adsorption and demonstrate the potential application of adsorption-EMP systems in dephosphorization for wastewater treatment.

Emerging electro-driven technologies for phosphorus enrichment and recovery from wastewater: A review

The recovery of phosphorus from wastewater is a critical step in addressing the scarcity of phosphorus resources. Electro-driven technologies for phosphorus enrichment have gathered significant attention due to their inherent advantages, such as mild operating conditions, absence of secondary pollution, and potential integration with other technologies. This study presents a comprehensive review of recent advancements in the field of phosphorus enrichment, with a specific focus on capacitive deionization and electrodialysis technologies. It highlights the underlying principles and effectiveness of electro-driven techniques for phosphorus enrichment while systematically comparing energy consumption, enrichment rate, and concentration factor among different technologies. Furthermore, the study provides a thorough analysis of the capacity of various technologies to selectively enrich phosphorus and proposes several methods and strategies to enhance selectivity. These insights offer valuable guidance for advancing the future development of electrochemical techniques with enhanced efficiency and effectiveness in phosphorus enrichment from wastewater.

Simultaneous desalination and molecular resource recovery from wastewater using an electrical separation system integrated with a supporting liquid membrane

Separating molecular substances from wastewater has always been a challenge in wastewater treatment. In this study, we propose a new strategy for simultaneous desalination and selective recovery of molecular resources, by introducing a supported liquid membrane (SLM) with molecular selectivity into an asymmetric flow-electrode capacitive deionization. Salts and molecular substances in wastewater are removed after passing through the ion separation chamber and the molecular separation chamber, respectively. Faradaic reactions, i.e., the electrolysis of water with OH-, occurred in the electrochemical cathode electrode provides a sufficient and continuous chemical potential gradient for the cross-SLM transport of phenol (a model molecule substance). By optimizing the formulation of the liquid membrane and the pore size of the support membrane, we obtained the SLM with the best performance for separating phenol. In continuous experiment tests, the electrochemical membrane system showed stable separation performance and long-term stability for simultaneous salts removal and phenol (sodium phenol) recovery from wastewater. Finally, we demonstrate the potential application of this technology for the recovery of different carbon resources. Overall, the electrochemical system based on SLM is suitable for various wastewater treatment needs and provides a new approach for the recovery of molecular resources in wastewater.

Modular Functionalization of Metal-Organic Frameworks for Nitrogen Recovery from Fresh Urine

Nitrogen recovery from wastewater represents a sustainable route to recycle reactive nitrogen (Nr). It can reduce the demand of producing Nr from the energy-extensive Haber-Bosch process and lower the risk of causing eutrophication simultaneously. In this aspect, source-separated fresh urine is an ideal source for nitrogen recovery given its ubiquity and high nitrogen contents. However, current techniques for nitrogen recovery from fresh urine require high energy input and are of low efficiencies because the recovery target, urea, is a challenge to separate. In this work, we developed a novel fresh urine nitrogen recovery treatment process based on modular functionalized metal-organic frameworks (MOFs). Specifically, we employed three distinct modification methods to MOF-808 and developed robust functional materials for urea hydrolysis, ammonium adsorption, and ammonia monitoring. By integrating these functional materials into our newly developed nitrogen recovery treatment process, we achieved an average of 75 % total nitrogen reduction and 45 % nitrogen recovery with a 30-minute treatment of synthetic fresh urine. The nitrogen recovery process developed in this work can serve as a sustainable and efficient nutrient management that is suitable for decentralized wastewater treatment. This work also provides a new perspective of implementing versatile advanced materials for water and wastewater treatment.

Simultaneous Urea and Phosphate Recovery from Synthetic Urine by Electrochemical Stabilization

Urine is a widely available renewable source of nitrogen and phosphorous. The nitrogen in urine is present in the form of urea, which is rapidly hydrolyzed to ammonia and carbonic acid by the urease enzymes occurring in nature. In order to efficiently recover urea, the inhibition of urease must be done, usually by increasing the pH value above 11. This method, however, usually is based on external chemical dosing, limiting the sustainability of the process. In this work, the simultaneous recovery of urea and phosphorous from synthetic urine was aimed at by means of electrochemical pH modulation. Electrochemical cells were constructed and used for urea stabilization from synthetic urine by the in situ formation of OH- ions at the cathode. In addition, phosphorous precipitation with divalent cations (Ca2+, Mg2+) in the course of pH elevation was studied. Electrochemical cells equipped with commercial (Fumasep FKE) and developmental (PSEBS SU) cation exchange membranes (CEM) were used in this study to carry out urea stabilization and simultaneous P-recovery at an applied current density of 60 A m-2. The urea was successfully stabilized for a long time (more than 1 month at room temperature and nearly two months at 4 °C) at a pH of 11.5. In addition, >82% P-recovery could be achieved in the form of precipitate, which was identified as amorphous calcium magnesium phosphate (CMP) by using transmission electron microscopy (TEM).

Basket anode filled with CaCO3 particles: A membrane-free electrochemical system for boosting phosphate recovery and product purity

Phosphorus (P) is often regarded as the primary stimulant for eutrophication, while its importance as a crucial life element is also well acknowledged. Given its future scarcity, P recycling from waste streams is suggested and practiced. Electrochemically mediated precipitation (EMP) is a robust and chemical-free process for P removal and recovery, yet it requires further developments. The first generation of the CaCO₃-packed electrochemical precipitation column successfully solved the problem of H⁺-OH^- recombination, achieving enhanced P removal efficiency with less energy consumption but suffering from low Ca-phosphate purity in recovered products. Herein, a new concept of a basket-anode electrochemical system is proposed and validated to prevent direct H⁺-OH^- recombination and enhance product purity. The CaCO₃ pellets packed basket anode alleviates the OH^- depletion by CaCO₃-H⁺ interaction and provides extra Ca²⁺ for enhanced P removal. The novel structure of the basket anode, by its derived acidic anode region and alkaline cathode region, completely avoids the precipitation of Ca-phosphate on the packed CaCO₃ and greatly facilitates the collection of high-quality Ca-phosphate product. Our results suggest that almost 100% of the removed P was in high-purity, highly crystalline Ca-phosphate on the cathode. The recovered products contained significantly more P (13.5 wt%) than in the previous study (0.1 wt%) at similar energy consumptions (29.8 kWh/kg P). The applied current density, pellets size, and influent P concentration were critical for P removal performance, product purity, and power consumption. We further demonstrated the long-term stability of this novel system and its technical and economic feasibility in treating real stored urine. Our study provides new cell architectural designs to enhance the performance of EMP systems and may inspire innovations and developments in other electrochemical water treatment processes.

Precise manipulation of the charge percolation networks of flow-electrode capacitive deionization using a pulsed magnetic field

Magnetic field is a simple and powerful means that enables controlled the transport of electrode particles in flow electrode capacitive deionization (FCDI). However, the magnetic particles are easily stripped from hybrid suspension electrodes and the precise manipulation of the charge percolation network remains challenging. In this study, a programmable magnetic field was introduced into the FCDI system to enhance the desalination performance and operational stability of magnetic FCDI, with core-shell magnetic carbon (MC) used as an alternative electrode additive. The results showed that the pulsed magnetic field (PMF) was more effective in enhancing the average salt removal rate (ASRR) compared to the constant magnetic field (CMF), with 51.6% and 67.7% enhancement, respectively, compared to the magnetic field-free condition. The outstanding advantage of the PMF lies in the enhancement in the trapping and mediating effects in the switching magnetic field, which keeps the concentration of the electrode particles near the current collector at a high level and greatly facilitates electron transport. In long-term operation (20,000 cycles), the pulsed magnetic FCDI achieved a stable desalinating rate of 0.4-0.68 μmol min^-1 cm^-2 and a charge efficiency of >96%. In brief, our study introduces a new approach for the precise manipulation of charge percolation networks of the suspension electrodes and provides insight into the charging mechanism of the magnetic FCDI.

Nitrogen and phosphorous recycling from human urine by household electrochemical fixed bed in sparsely populated regions

Decentralized treatment of human urine in sparsely populated regions could avoid the problem of sewage collection in traditionally centralized treatment schemes and simultaneously utilize the recovered N/P fertilizer in-situ to nurture gardens. Herein, an integrated electrochemical fixed bed packed with divided magnesite and carbon zones was constructed for the pretreatment of human urine, followed by the recovery of 95.0% NH₄⁺ and 85.8% PO₄^3- via struvite precipitation and NH₃ volatilization as well as the on-site employment of the produced struvite as fertilizer. In the process, the acid/base zones created by electrochemical water splitting dissolved the magnesite filler as the Mg²⁺ source of struvite, further creating an ideal pH environment for struvite precipitation and NH₃ volatilization in the effluent. Without the need to control solution pH by chemical addition, the system can resist impacts from changes in water quality by adjustment of the current density and flow rate, indicating its great potential for automatic operation. Life cycle assessment indicated that the on-site employment of produced struvite avoids the long-distance fertilizer transportation required for fertilization, thus reducing carbon emission by a hundred million tons per year if the household facility is driven by clean electricity.

Enhancing Brackish Water Desalination using Magnetic Flow-electrode Capacitive Deionization

Flow-electrode capacitive deionization (FCDI) is viewed as a potential alternative to the current state-of-the-art electrodriven technology for the desalination of brackish water. However, the key shortcoming of the FCDI is still the discontinuous nature of the electrode conductive network, resulting in low electron transport efficiency and ion adsorption capacity. Here, a novel magnetic field-assisted FCDI system (termed magnetic FCDI) is proposed to enhance brackish water desalination, simply by using magnetic activated carbon (MAC) as flow electrodes. The results show that the assistance from the magnetic field enables a 78.9% - 205% enhancement in the average salt removal rate (ASRR) compared with that in the absence of a magnetic field, which benefits from the artificial manipulation of the flow electrode transport behavior. In long-term tests, the stable desalination performance of magnetic FCDI was also demonstrated with a stable ASRR of 0.70 μmol cm^-2 min^-1 and energy-normalized removed salt (ENRS) of 8.77 μmol J^-1. In addition, magnetic field also enables the regeneration of the electrode particles from the concentrated electrolyte. In summary, the findings indicate that the magnetic FCDI is an energy-efficient and operation convenient technology for brackish water desalination.

Reduced cathodic scale and enhanced electrochemical precipitation of Ca2+ and Mg2+ by a novel fenced cathode structure: Formation of strong alkaline microenvironment and favorable crystallization

Electrochemical precipitation is a promising technique for hardness abatement without the addition of external ions. However, the scale layer on cathode deteriorated the removal efficiency and limited the practical application. Herein, a fenced cathode structure was designed to prevent cathodic precipitation. The cathode was fenced by a crystallization-inducing material for separating the OH- production and crystallization processes. Precipitation on the cathode was confirmed to shift to the crystallization-inducing material, and the clean fenced cathode provided efficient long-term OH- production. At a current density of 40 A/m2, the Ca2+ or Mg2+ removal efficiency increased by 12.8% or 46.1%, respectively, compared to those of a traditional cathode. Thermodynamic equilibrium in synthetic water and mine water, mass transfer and the location of precipitation were analyzed to elucidate the electrochemical precipitation process. The enhanced mechanism was ascribed to the crystallization-inducing material, which remarkably promoted the crystallization process, and hindered OH- migration, thereby increased the pH of alkaline microenvironment. Notably, a recovery design was proposed to recover pure calcite and brucite from alkalinity-free wastewater. The design reveals a promising strategy for enhancing the crystallization process and reducing cathodic scale, also initiating a new research direction toward hardness removal.

Membrane-Current Collector-Based Flow-Electrode Capacitive Deionization System: A Novel Stack Configuration for Scale-Up Desalination

The stack configuration in flow-electrode capacitive deionization (FCDI) has been verified to be an attractive and feasible strategy for scaling up the desalination process. However, challenges still exist when attempting to simultaneously improve the desalination scale and the cell configuration. Here, we describe a novel stack FCDI configuration (termed a gradient FCDI system) based on a membrane-current collector assembly, in which the charge neutralization enables the in situ regeneration of the flow electrodes in the single cycle operation, thereby realizing a considerable increase in the desalinating performance. By evaluating standardized metrics such as the salt rejection, productivity (P), average salt removal rate (ASRR), energy-normalized removed salt (ENRS), and TEE, the results indicated that the gradient FCDI system could be a performance-stable and energy-efficient alternative for scale-up desalination. Under optimal operating conditions (carbon content = 10 wt %, feed salinity = 3000 mg L^-1, cell voltage = 1.2 V, and productivity = 56.7 L m^-2 h^-1), the robust desalination performance (ASRR = 1.07 μmol cm^-2 min^-1) and energy consumption (ENRS = 7.8 μmol J^-1) of the FCDI system with a desalination unit number of four were verified at long-term operation. In summary, the stacked gradient FCDI system and its operation mode described here may be an innovative and promising strategy capable of enlarging the scale of desalination while realizing performance improvement and device simplification.