Ultra-fast green hydrogen production from municipal wastewater by an integrated forward osmosis-alkaline water electrolysis system

Producing clean water via FO using KOH as a draw solution

To integrate FO with AWE, KOH and two other electrolytes including sodium bicarbonate (NaHCO₃), and potassium pyrophosphate (K₄P₂O₇) were considered as potential draw solutions for FO instead of the conventional NaCl, as NaCl induces a CER, which negatively affects H₂ generation and damages both the FO membrane and electrodes^30,31,32,33. Among these electrolytes at a concentration of 1 M, KOH shows current densities 18 and 12 times higher than those of NaHCO₃ and K₄P₂O₇ at 2 V, respectively (Supplementary Fig. 1). The higher current density of KOH represents a higher H₂ production rate in AWE compared to the other two electrolytes. To not compromise the AWE performance, we tested if KOH could be used as the draw solution in FO for extracting water from simulated wastewater effluent containing sodium acetate (NaAc), ammonia/ammonium ions (NH₃/NH₄⁺), and chloride. Besides, although the current density of KOH increases with increasing concentration³⁴, KOH at concentrations higher than 1 M causes the pH of the draw solution to exceed 14, which carries the risk of damaging commercial TFC-FO membranes. Therefore, we tested 1 M KOH to evaluate its impact on the TFC-FO membranes (details given in the Methods section).

Figure 2a shows the water flux (i.e. the permeated water volume per unit of time and per unit of membrane areas, L h^–1 m^–2 (LHM)) and reverse salt flux (RSF) (i.e. draw solute permeated to the feed solution side per unit of time and per unit of membrane areas, g h^–1 m^–2 (GHM)) of the FO process using 1 M KOH as the draw solution for five cycles of five hours each. The water flux decreased from 17.2 ± 0.3 to 14.9 ± 0.1 LHM after 5 h in each cycle, while the RSF remained constant at ~121 ± 3 GHM. The water flux is comparable to a system using NaCl as a draw solution (Supplementary Fig. 2), indicating that the KOH is an effective draw solution to extract water through FO processes. The primary concern associated with the use of KOH as a draw solution is its potential to damage the FO membrane, as the reverse flux of KOH may cause hydrolysis of the polymer chains that constitute the membrane selective layer (on the feed side)³⁵. The integrity of the commercial TFC-FO membrane after operating in 1 M KOH solution was examined by comparing the water flux and RSF of the used membrane after the tests shown in Fig. 2a and a new pristine membrane, both using deionised water as the feed solution and 1 M NaCl as the draw solution (Fig. 2b). Figure 2b shows that the water flux of the used membrane was reduced by less than 5% compared to the pristine membrane, while the RSF of the two membranes was comparable. The membrane integrity in terms of water flux and RSF was not compromised. In addition, results from scanning electron microscopy (SEM) revealed no change in the “leaf-like” surface of the selective layer on the unused membrane and the used membrane after five cycles (Fig. 2c, d and Supplementary Fig. 3a, b) and in the peaks of the functional groups in the Fourier transform infra-red spectroscopy (FTIR) (Supplementary Fig. 4). The comparable water flux and RSF between the pristine and the used membrane suggest that the FO membrane was not damaged by KOH. The high RSF may be attributed to the smaller hydrated ionic size and higher ionic mobility of KOH³⁶, or the ionisation of the membrane via deprotonation of the functional groups on its surfaces facilitating the transport of cations across the membrane^37,38. Nonetheless, the RSF accounted for less than 1.5% of the initial KOH concentration after a cycle, suggesting the impact due to this issue was minor. Moreover, since KOH is inexpensive, any losses can be easily replenished in the system.

Since impurities in the wastewater effluent may permeate through the FO membrane and thus affect electrolysis, we examined the rejection of common impurities in the wastewater effluent by the FO membrane, including NaAc, NH₃/NH₄⁺, and chloride (Fig. 2e). The rejection rates of NaAc, NH₃/NH₄⁺, and chloride remained stable throughout the five cycles at about 85%, 63%, and 98%, respectively. The lower rejection of NH₃/NH₄⁺ occurred as the TFC-FO membrane surface is negatively charged, which allows anions to be intercepted more readily than cations or unionised NH₃ which is the dominant form of NH₃/NH₄⁺ at high pH. The rejection of additional common impurities found in wastewater and varied pH levels was also examined, with rejection rates exceeding 90% (Supplementary Figs. 5 and 6). The potential impacts to the electrolysis process from the permeated impurities were then investigated using a commercial 5-cell alkaline stack with nickel alloy-based electrodes by comparing the current density–voltage (J–V) curves generated from: (i) 1 M KOH, (ii‒iv) 1 M KOH mixed separately with NaAc, NH₃/NH₄⁺, and chloride at a concentration of 10 mM for studying their accumulation effect, respectively, or (v) a 1 M KOH mixture consisting of 1.1 mM of NaAc, 0.7 mM NH₃/NH₄⁺ as N, and 0.08 mM chloride observed in the permeate from the rejection test, as shown in Fig. 2f. All tested solutions exhibited a consistent behaviour in their J–V curve performances, suggesting that the presence of NaAc, NH₃/NH₄⁺, and chloride in KOH minimally impacted the current densities. We infer that the high concentration of KOH, sustaining a high pH (>8), mitigated the negative effects of NaAc and NH₃/NH₄⁺ and suppressed side reactions such as chlorine evolution^31,39. Therefore, the above results of the high and constant water flux, the intact FO membrane, and the minor impact of impurities permeated from the FO membranes on electrolysis suggested using KOH as the draw solution in the FO process to extract water from wastewater effluent is feasible.

Integrating FO with AWE

A water-hydrogen balance model was established to quantify and balance the FO and AWE units in the FOWS_AWE system, as shown in Fig. 3a, with the objective of continuous operation to effectively produce H₂ from wastewater effluent. In the integrated system, clean water is first extracted from the wastewater effluent by the FO process using KOH as the draw solution and then split into H₂ and oxygen (O₂) by AWE using the 5-cell alkaline stack containing nickel alloy powder combined with nickel foam electrodes separated by polymer diaphragms (detailed setup is shown in the ‘Methods’ section). A sustainable operation of this system thus requires a dynamic equilibrium that enables the water production rate crossing the FO membrane (Q_FO) to match the water rate consumed by AWE (Q_AWE) as shown in Eq. 1, while maintaining a consistent concentration of KOH.

Q_FO is a function of controllable parameters in the FO process including the membrane area (S, cm²) and the concentration gradient between draw solution and feed solution (ΔC, M)⁴⁰. The membrane permeability coefficient (K_w, L atm^–1 s^–1 cm^–1), when multiplied by the van’t Hoff factor of the feed solution (I), represents how easily water can be extracted through the membrane, remaining constant throughout the test. Other parameters, including the membrane thickness (d = 0.1 mm), the ideal gas constant (R = 0.0821 L atm K^–1 mol^–1), and temperature (T = 296 K), also remained constant during the test. Q_AWE is only controlled by the applied current of AWE (i_cell, A)⁴¹, as other parameters, including the Faradaic efficiency (FE), the number of cells constituting the electrolyser stack (N_cell = 5 for our setup), V_m (as the molar volume of H₂O at 0.018 L mol^–1 at 296 K of T and 1 atm), and Faraday’s constant (F = 96,485 C mol^–1) are constant throughout the test. Therefore, through balancing Q_FO and Q_AWE, a linear correlation was established to describe the specific normalised current of the FOWS_AWE system (J_i, A cm⁻²), which is defined as the required current of this integrated system for handling the permeated water per area of FO membranes (i_cell  S^–1), changed with ΔC as shown in Eq. 2.

where Φ is the H₂ production potential (L A mol^–1 cm^–2), determined by the configurations of the FO membranes and AWE, types of feed and draw solutions, and operational conditions. H₂ production potential (Φ) was calculated by substituting the values of K_w × I, d, R, T, FE, N_cell, and V_m into Eq. 2, and K_w × I was obtained experimentally by measuring Q_FO under given conditions of S and ΔC in an independent FO module using Eq. 1, whereas FE was calculated by measuring the H₂ production rate and purity at a given i_cell of the AWE module (detailed in the ‘Methods’ section). In this study, Φ values for feeding deionised water and the wastewater effluent collected from a wastewater treatment plant in Hong Kong (HK-WWE1) to the FOWS_AWE system were calculated as 1.55 and 0.95 A mol^–1 cm^–2, respectively. The lower Φ for wastewater effluent was attributed to impurities in the wastewater effluent reducing K_w × I. The obtained Φ values were then used to generate the relationship between J_i and ΔC when reaching a dynamic equilibrium for the FO and AWE units in the FOWS_AWE system, as shown in Fig. 3b. Therefore, to maintain ΔC at 1 M, J_i was calculated as 1.55 and 0.95 A cm^–2 for deionised water and HK-WWE1, respectively²¹.

The two setups of FO and AWE were then experimentally integrated using a 1 cm² FO membrane and applying an i_cell of AWE of 1.55 and 0.95 A for deionised water and HK-WWE1 according to their J_i values, respectively (integrated process shown in Supplementary Fig. 17). As shown in Fig. 3c, d, the continuous operation of this integrated system over two cycles of 5 h each, demonstrated a stable water flux crossing the FO membrane, constant ΔC, and a stable H₂ production rate when fed with either deionised water or secondary wastewater treated effluent. In addition, the molar ratio of the H₂ generation rate to Q_FO was close to one after the two test cycles (Supplementary Fig. 7), indicating that nearly all the water produced from FO was consumed by AWE. These observations indicate a successful integration of FO and AWE processes and validate our established dynamic equilibrium for the FOWS_AWE system. Using wastewater effluent as the feed resulted in a 38% decrease in both Q_FO and H₂ production rate compared to feeding deionised water, which aligns with the decrease observed in the value of K_w × I from wastewater effluent. This indicates that these impurities mainly affect the capacity of the water produced from FO, and the ability of the FOWS_AWE system to produce H₂ relies on this water production capacity. Nevertheless, using wastewater effluent as the feed did not compromise the H₂ purity, as evidenced by the comparable H₂ percentage observed in samples produced using both deionised water and wastewater effluent, as shown in Fig. 3d. Additionally, the stability of H₂ production was not affected either, as the FOWS_AWE system operated at a near-constant voltage of 1.79 ± 0.01 V and maintained a Faradaic efficiency of over 99% throughout two cycles of 5 h each (Fig. 3e), indicating that there were no side reactions caused by impurities crossing the FO membranes or accumulation of undesired impurities.

A continuous 168-h operation of the FOWS_AWE system was conducted to assess its long-term stability under consistent conditions using a wastewater effluent sample collected from a second wastewater treatment plant in Hong Kong (HK-WWE2), and the result is shown in Fig. 3f. The water flux decreased by ~12% from 15.6 to 13.7 LHM, whereas the voltage increased by 4% during the initial 24 h, starting from 1.75 V and maintaining at 1.82 V. In addition, the system sustained a ΔC consistently at around 1 M during the entire 168-h testing period. The 12% reduced water flux after the long-term operation was attributed to fouling from organic matter in the wastewater⁴², because the ratio of C to O elements on the membrane surface was reduced by ~13%, and no degradation of integrity for both the selective and support layers of the FO membrane was observed in SEM coupled with Energy Dispersive X-ray (Supplementary Table 3 and Fig. 8, respectively). FO membrane fouling was shown to be largely reversible, unlike the often irreversible fouling challenges faced in RO⁴³, which means that the reduced water flux in our system can be easily recovered by backwashing or chemical washing. Notably, the H₂ gas purity remained stably high at over 99% throughout the entire long-term operation (Supplementary Fig. 9). These results demonstrated the operational stability of the FOWS_AWE system throughout the 168-h testing period. In addition, as the FOWS_AWE system relies on the water flux to generate H₂, we tested more wastewater samples from the southern and northeastern regions of China, with their composition and concentrations detailed in Supplementary Table 4. The results show that our system exhibited a constant water flux, voltage, and ΔC for various impurities and pH values from different wastewater samples (Supplementary Figs. 10–12). Despite the varied K_w × I caused by different impurities, the resulting H₂ production rates were consistent and remained within a range of 5% (Supplementary Fig. 11d). These results validate the stability and adaptability of our system for producing H₂ under varying wastewater conditions.

H₂ production and energy efficiency of the FOWS_AWE system

The H₂ production and energy efficiency of the FOWS_AWE system showed superior performance compared with two other membrane-based P2H systems for seawater purification integrated with electrolytic water splitting: (i) the seawater electrolysis system (SES) using hydrophobic PTFE membranes with 30% KOH as a self-dampening electrolyte (SDE), and (ii) the FOWS employing cellulose triacetate (CTA) membranes with 0.8 M sodium phosphate as the electrolyte^21,22. The H₂ produced in the three systems was compared by normalising the H₂ production rate over the area of water-producing membranes, as shown in Fig. 4a. The FOWS_AWE system produced the highest H₂ rate at 448 Nm³ day^–1 m⁻² of TFC-FO membrane, which is 102 and 14 times higher than those of SES and FOWS, respectively. This significant improvement in H₂ production compared with SES is primarily due to the higher Φ provided by FO membranes in producing more water. Unlike SES using hydrophobic PTFE membranes, the TFC-FO membrane is hydrophilic and thus offers higher K_w to allow water to permeate. This is verified by the 23‒32 times higher J_i of the TFC-FO membrane to reach equilibrium than the PTFE membrane at the same values of S, ΔC, and using identical AWE setups (Supplementary Fig. 13). Besides, PTFE membranes face wetting issues during operation, which reduces the ability to reject impurities as water penetrates the pores^44,45. The enhanced H₂ production compared to FOWS is mainly attributed to the 5× higher ΔC between wastewater effluent and 1 M KOH compared to the 0.2 M ΔC between seawater and 0.8 M sodium phosphate, enhancing the driving force for producing water. The H₂ flux comparison along with the water-hydrogen model suggests the water production rate dominates in determining the H₂ flux in a membrane-electrolysis system, highlighting the importance of optimising both the membrane type and the feed water source. Moreover, high water production rates offer additional benefits of requiring fewer membranes and associated equipment, thereby reducing space and capital costs which are particularly important considerations for scaling up the system.

The FOWS_AWE system exhibits low energy consumption, as shown in Fig. 4b, where the SEC of the three systems is compared in addition to direct wastewater electrolysis. The FOWS_AWE system requires the lowest SEC at 4.43 kWh Nm^–3 at an i_cell of 0.95 A using wastewater effluent as feed. This SEC is comparable to commercial alkaline electrolysis systems that use deionised water and is 10%, 34%, and 84% lower than SES, FOWS, and direct electrolysis of wastewater, respectively⁴⁶. The lower SEC highlights the importance of preventing undesired impurities from electrolysis and using a pH-tolerant membrane to enable the use of KOH as an electrolyte as it has higher ionic conductivity for more efficient electron transfer. Such excellent energy efficiency is also applicable to other commercial AWE configurations, if the applied current matches the water-hydrogen equilibrium, making the FOWS_AWE system easily adaptable and scalable to meet varying market demands. Therefore, the hydrophilic property of TFC-FO membranes, the higher ΔC resulting from wastewater effluent usage instead of seawater, and the use of an optimal electrolyte (i.e. KOH) all contribute to a high H₂ flux and energy-efficient P2H conversion.

In addition, electrolysis is an exothermic process that generates heat, so capturing and utilising waste heat during AWE operations is critical in achieving the energy-efficient operation of the system. By simulating the capture and utilisation of the waste heat during AWE operations to heat up the KOH draw solute temperature to 40 °C, the FOWS_AWE system shows an increase of H₂ flux by 11% and a decrease of SEC from 4.43 to 3.96 kWh Nm^–3 (Fig. 4c and detailed calculations provided in Supplementary Note 1). This brings us closer to the target of 3.75 kWh Nm^–3 proposed by the International Renewable Energy Agency⁴⁷. The enhanced H₂ flux is mainly due to the increased water production at higher temperatures, indicated by Eq.1⁴⁸, while the reduced SEC is attributed to the increased KOH ionic conductivity reducing the overall resistance of the cell and enabling more efficient electron transfer (Supplementary Fig. 14). These results also suggest that the FOWS_AWE system can perform better in hot regions where the wastewater has temperatures of up to 35 °C and can potentially heat the draw solutions during membrane operational processes²⁸, or in areas with abundant solar energy that can be used for heating.

Implications for H₂ production using wastewater effluent

This study demonstrated the use of municipal wastewater effluent by a FOWS_AWE system for fast and energy-efficient green H₂ generation to address the large-scale H₂ production need associated with potential water constraints. The FOWS_AWE system exhibits stable and continuous H₂ production at the highest yields of H₂ to date (448 Nm³ day^–1 m^–2) using alternative water resources achieving a high H₂ purity of >99% and low SEC of 4.43 kWh m^–3 during the entire testing period of over 10 h. In addition, the FOWS_AWE system demonstrated long-term stability by being operated continuously for 168 h using real wastewater effluent and proved its capability and adaptability in producing H₂ across diverse wastewater conditions found in different regions.

Using wastewater effluent is crucial for sustainable H₂ production in regions where seawater is unavailable or freshwater resources are scarce, especially as H₂ projects are rapidly expanding amid the foreseeable intensified worldwide water stress. The modular FO and AWE system along with the established water-hydrogen balance model allow the FOWS_AWE system to operate with different influent water quality at different scales, ensuring an easy yet versatile approach for sustainable P2H conversion from the household to the city scale. The integration of TFC-FO with electrolysis can reduce the capital costs of water treatment by up to 46% compared to conventional RO and FO–RO processes⁴⁹, providing a strong economic incentive for FOWS_AWE adoption. Addressing pressing global challenges such as water scarcity, climate change, and energy insecurity requires urgent action. Existing technologies, such as the FO and AWE, can substantially impact on solving these challenges if applied and integrated more effectively. The FOWS_AWE system exemplifies leveraging synergistic technologies to rapidly develop economical, efficient, and sustainable solutions for interconnected environmental issues.

In addition to the H₂ production market for industrial use and energy storage, the FOWS_AWE system provides advantages to wastewater treatment plants and many industries, as it serves as a tool for water reuse. The system has the potential to reduce the treatment load and wastewater discharge while simultaneously generating onsite energy, and mitigating environmental impacts⁵⁰. It is estimated that an onsite FOWS_AWE station with a capacity of 5‒6 MW can produce O₂ at ~20,000 kg day^–1, which can supply the aerobic processes of a wastewater treatment plant in treating 50,000 m³ day^–1 of wastewater (calculation shown in Supplementary Note 2)^51,52 or support high-density in-land and offshore aquaculture^53,54. Moreover, fuel cells that use H₂ to generate electricity also produce high-purity water as a byproduct, which can be supplied to industries that require it, e.g. semiconductor and pharmaceutical industries.