High Spatiotemporal Availability of Hydrogen by Electrolysis of Municipal Reclaimed Water in China

High Spatiotemporal Availability of Hydrogen by Electrolysis of Municipal Reclaimed Water in China | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Analysis High Spatiotemporal Availability of Hydrogen by Electrolysis of Municipal Reclaimed Water in China Lu Lu, Weixiang Chao, Xiuling Yan This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5380448/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The large-scale electrolytic H 2 production requires a steady supply of both electricity and water as well as nearby H 2 demands, which is usually challenged in China due to a spatiotemporal mismatch of these resources. This necessitates reconfiguring them or finding alternatives. Reclaimed water produced from municipal wastewater treatment plants shows widespread distribution with human activities and huge reserves. Here, we conducted a life-cycle assessment of H 2 production from reclaimed water in terms of potential capacity, cost and carbon emissions based on available water and electricity and H 2 market in each province of China, which was compared to that using pure water and seawater. Reclaimed water with easy access to nearby electricity and H 2 demand shows a great H 2 production potential of 95.7–213.1 million tons per year between 2021 and 2060, which can theoretically meet the national H 2 demand (33.4–130.9 million tons) alone and allows H 2 self-sufficiency in most provinces. In contrast, other electrolytic H 2 routes only have a potential of 19.8–79.1 million tons. The current cost ($8.8 ± 0.6/kg H 2 ) and carbon emissions (40.4 ± 0.7 kg CO 2 -eq/kg H 2 ) of H 2 produced from reclaimed water are comparable to other electrolytic approaches, but expect to reach $4.0 ± 0.5 kg H 2 and 4.99 ± 0.05 kg CO 2 -eq/kg H 2 by 2050 due to advances in electrolysis technology and electricity decarbonization, indicating a highly competitive performance with those of fossil H 2 (~$4.3/kg H₂) and defined green H 2 (~4.9 kg CO 2 -eq/kg H 2 ). This study provides a new insight into sustainable electrolytic H₂ production by using unconventional water resources. Earth and environmental sciences/Environmental sciences/Environmental impact Physical sciences/Energy science and technology/Renewable energy/Hydrogen energy Scientific community and society/Water resources Scientific community and society/Energy and society/Energy supply and demand Physical sciences/Energy science and technology/Energy infrastructure/Energy grids and networks Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction China is aggressively advancing hydrogen (H 2 ) energy development to meet its pledge of achieving carbon neutrality by 2060 1 , and the nation’s H 2 demand is expected to rise from the current ~ 33 million tons 2 to > 130 million tons by 2060 3 . Electrochemical splitting of pure water is one of the important approaches to produce H 2 , as it is able to produce green H 2 by using renewable electricity. Although several analyses have demonstrated the sustainability of electrolytic H 2 production 2 , steady access to both low-cost or renewable electricity and pure water remains challenged for large-scale electrolysis. For instance, China’s water electrolysis facilities are concentrated in the eastern provinces and rely on relatively abundant freshwater or desalinated seawater 4 , but the extensive energy stress caused by dense population and industry in this region may restrict further scale-up 5 . In contrast, northwest provinces with abundant renewable solar and wind electricity, such as Xinjiang, Gansu, and Ningxia, are usually suffer from water deficiency 6 (Fig. 1 a). Additionally, the challenges of transporting hydrogen 7 will generally require local H 2 production in the future to avoid current mismatches between H 2 demand and production capability among regions. To meet these challenges, co-location of electricity, electrolysis-grade water and H 2 markets is crucial to minimize the cost of H 2 production and its environmental footprint 8 . This necessitates a reconfiguration of current electrolysis industries to better utilize spatially uneven electricity and water resources. In China, substantial infrastructure investment in the energy sector, especially renewable energies, has significantly improved access to electricity 9 . However, the widespread water scarcity per capita severely restricts exploitation of current freshwater resources for other purposes 4 . The surplus freshwater is only available in areas with moderate populations and abundant water resources, such as Southwest China 10 . The utilization of unconventional water resources, such as seawater and brine, to generate pure water is also constrained by the capacity of installed desalination facilities 11 . Emerging H 2 production via direct electrolysis of seawater is restricted to coastal areas and remains technologically immature, although it allows easy access to nearby offshore wind power and unlimited seawater 12 . Reclaimed water, treated municipal or industrial wastewater that is produced from wastewater treatment plants (WWTPs) and can be reused 13 , might be a promising feedstock of water splitting, as it is intrinsically linked to domestic and industrial activities with widespread spatial distribution 14 . For example, > 4800 municipal WWTPs have been constructed in China 15 (Fig. 1 b) with a total wastewater treatment capacity of 2.3×10 8 m 3 /d by 2020, while only ~ 20% treated water is reused 16 . Notably, reclaimed water production is less influenced by seasonal and regional variations compared to other natural freshwater resources because its quantity is dependent on population and industrial structure 17 , which maximally guarantees a stable and long-term supply for various uses. Reclaimed water has long been used in China for low-value-added purposes, such as municipal irrigation, due to its low water quality 18 . However, the quality of China’s reclaimed water has continuously improved with the introduction of more rigorous water discharge standards and advancements in wastewater treatment technology. Reclaimed water is becoming a significant unconventional water resource to support more uses. For instance, the potential of reclaimed water as a source of rinsing water for electronic chip wafer production has been exploited 19 . Given the stable and extensive supply of reclaimed water, and the spatial co-distribution of WWTPs and power plants (PPs) in practice (Fig. 1 c), the direct or indirect (after purification) utilization of reclaimed water as electrolytic feedstock holds considerable potential to meet the nationwide demand of electrolytic H 2 production in the future. This study, grounded in current alkaline water electrolysis technology, industrial wastewater/water/seawater treatment models, and experimental data, comprehensively assessed the feasibility and potential benefits of reclaimed water electrolysis for H 2 production. The reclaimed water is assumed to be directly fed into electrolyzer or be purified by reverse osmosis (RO) before electrolysis. The spatial distributions of water, electricity, and H 2 market as well as their environmental and economic impacts on H 2 production were taken into consideration during analyses. This approach is compared with existing electrolytic H 2 production routes using pure water, desalinated seawater, and raw seawater (Fig. 2 a). The results show that electrolysis of reclaimed water leads to comparable performance with other routes in terms of resource consumption (electricity and water), carbon emissions, and economic cost, but demonstrates the highest capacity of H 2 production (Fig. 2 b) due to the abundant reclaimed water obtained across most China’s provinces. Moreover, the high spatial distribution of WWTPs enables electrolysis of reclaimed water to be conducted in a decentralized way with easy access to nearby water and electricity, which reduces distances of H 2 and electricity delivery as much as possible. The predictable increase in reclaimed water production with population growth as well as future advances in electrolytic technology and electricity decarbonization demonstrate a greater potential of green H 2 production with higher efficiency. Although more large-scale applications are needed to verify the feasibility of the proposed H 2 production from reclaimed water, the findings herein provide a new strategy to advance China’s H 2 economy and may substantially contribute to China’s future energy transition in conjunction with other H 2 production routes. Cradle-to-gate assessment of electrolytic H production from reclaimed water and other water resources Assessment aims and boundary Figure 2 demonstrates different scenarios of nationwide H 2 production by electrolysis of reclaimed water and other water resources. This study is defined as a cradle-to-gate assessment, targeting a life-cycle from the production of electrolysis-grade water to the H 2 delivery to end-users (Fig. 3 and Supplementary Section 1.1). The assessment addresses key indicators including H 2 production capacity, carbon emissions, and cost for each route employing different water resources. This study excludes Hong Kong, Macau, and Taiwan due to the lack of regional data. Given that China has rigorously restricted the exploitation in nature reserves to protect the fragile ecosystems 20 , this study also excludes provinces of Qinghai and Tibet that are home to numerous water resources reserves, like the Three River Source National Nature Reserve 21 . China has released its plan of H 2 industry development from 2021 to 2035 22 and highlighted the significance of electrolytic green H 2 toward carbon neutrality by 2060 23 . This study evaluates H 2 production using the currently available statistical data of 2021 and predicts the future changes by 2060. The H 2 production capacity in each province was calculated based on one of available resources (electricity or water amounts) or both of them. The H 2 equilibrium for each province is determined by deducting H 2 demand from H 2 production, which indicates the H 2 self-sufficiency/surplus or shortage in each province. For calculating the environmental footprint, the life cycle inventory (LCI) including materials, chemicals, energy, and infrastructure consumables is initially established from published references (Supplementary Section 1.2). The industrial models in terms of wastewater/water treatment, reverse osmosis (RO), and alkaline electrolysis are applied to the LCI for the calibrations (Supplementary Section 1.3). These calibrations corresponding to different water qualities (Supplementary Section 1.4) are conducted in same scale to better align with the assessments. Total carbon emissions and cost generated from life-cycle H 2 production are determined based on the carbon emission intensities of specific activities and rational economic assumptions (Supplementary Section 1.5). Determination of available water resources and electricity The amount of reclaimed water that can serve as an electrolytic feedstock was determined by deducting those primarily used for municipal, industrial, and agricultural reuse as well as ecological replenishment from a total amount of reclaimed water discharged from municipal WWTPs (Supplementary Section 2.1 and 2.2). If the reclaimed water needs to be further purified before electrolysis, its available amount is determined by the capacity of installed RO facilities and their annual operation factors. Similarly, the tap water produced from water treatment plants (WTPs) should also firstly meet the demand of basic industrial and domestic activities before being used as an electrolytic feedstock, which can be provided by the unoccupied capacity of WTPs 24 . The available RO capacity in local regions finally determine the amount of pure water derived from tap water for H 2 production. However, this route is recommended to carry out when freshwater exploitation is above the threshold of moderate water scarcity set by the United Nations 25 , that is, the available per capita water resource after deducting the demand of basic activities and H 2 production, should be maintained at > 3000 m 3 ·capita − 1 ·year − 1 (Supplementary Section 2.1 and 2.3). This will ensure that these regions do not experience risk of water shortage when electrolytic H 2 production is implemented by exploiting additional freshwater resources. For use of desalinated seawater to produce H 2 , the available water amount is also determined by the available RO capacity in coastal regions despite the supply of seawater is infinite (Supplementary Section 2.1 and 2.4). Onshore electric grids are assumed to power electrolysis of reclaimed water, purified tap water and desalinated seawater, while offshore wind power is proposed for driving direct electrolysis of raw seawater. Therefore, direct seawater electrolysis only depends on its available offshore wind power, which is determined by the capacity of installed offshore wind engines and effective operation time (Supplementary Section 2.1 and 2.4). Both routes for seawater electrolysis are only viable in coastal provinces. The study will consider the future growth in electricity supply and demand by water electrolysis when forecasting the H 2 production potential. This ensures that the electricity supply for other essential industrial and domestic purposes remains unaffected, since water electrolysis is an energy-intensive industry. Determination of electrolysis efficiency Herein, the pure water produced from reclaimed water, tap water and seawater by RO modules (Fig. 2 ) is assumed to meet the quality of electrolysis-grade water with 99.7~% salt rejection (Supplementary Section 3.1) 26 . The technical parameters for pure water electrolysis are obtained from the cases of large-scale commercial alkaline electrolyzer (Supplementary Section 3.2), which indicates that production of 1 kg H 2 needs 10–15 kg pure water and 44.0–49.3 kWh electricity 27 . Actual use of water and electricity will increase if water loss and electricity consumption by RO are included. For direct electrolysis of reclaimed water or raw seawater, competitive reactions resulted from pollutants can randomly influence the electrolyzer performance 28 , 29 , therefore, a multi-parameter regression model based on data from references and laboratory measurements was employed to predict the electrolysis efficiencies (Supplementary Section 3.3). These procedures evaluate the specific water and electricity consumption as well as the H 2 production capacity of each route based on the available resources (water and electricity). The H 2 equilibrium of different regions is established by subtracting H 2 demand from H 2 production in these regions, revealing the nationwide H 2 supply-demand relationship (Supplementary Section 3.4). Determination of carbon emissions and cost Carbon emissions of different H 2 production routes depend on the calibrated LCI with categorization of wastewater and water treatment, electrolysis, and H 2 transport. The LCI of materials, chemicals, and energy is determined from industrial processes (Supplementary Section 1.2–1.4). The LCI of alkaline electrolyzer is calibrated by its installed capacity. The LCI of H 2 transport encompasses 30 MPa compression and construction of H 2 -embrittlement-resistant pipes and ships. The carbon intensity of electricity is determined by the renewable weighting of current electric grids and carbon emission factors of different power types (Supplementary Section 1.5). The infrastructures’ carbon emissions are distributed evenly over their lifespan. All carbon emissions above contribute to the levelized carbon emissions of H 2 (LEOH). The levelized cost of H 2 (LCOH) includes fees for wastewater and water treatment, electrolysis, H 2 transport and a carbon tax (Supplementary Section 1.6) derived from an average level of current pilot carbon-trading market in China 30 . Spatial distribution of electrolytic H production from reclaimed water and other water resources Nationwide H 2 supply and demand For direct electrolysis of reclaimed water, the capacity of H 2 production calculated using a total amount of available reclaimed water is 993.2 ± 149.1 million tons in 2021 (5–95% confidence interval based on 100,000 trials in a Monte Carlo simulation, same uncertainty is hereinafter used) (Fig. 4 a), which is much higher than that of 104.8 ± 10.8 million tons based on available electricity. This indicates that reclaimed water reserves are not a bottleneck for electrolytic H 2 production, which has a huge growth potential in the future with increase of electric power. In practice, if the availability of both water and electricity as well as the spatial distribution of these resources are taken into consideration, H 2 production capacity is 95.7 ± 9.8 million tons, which can essentially meet the national demand of 33.4 ± 1.7 million tons in 2021 (Fig. 4 a). If the reclaimed water is pre-purified by RO before being fed into electrolyzer, as practiced in the conventional electrolysis 31 , it exhibits a H 2 potential of 96.0 ± 9.4 million tons determined by both water and electricity resources. This capacity is slightly larger than that by direct electrolysis of reclaimed water, as a higher electrolytic efficiency can be obtained using pure water. Given the membrane technologies for water treatment are gradually matured recently 32 , electrolysis of purified reclaimed water might be a transitional solution at present, despite an additional expense for pre-purification generated. In contrast, the current electrolysis of pure water produced from tap water by RO only shows a H 2 production capacity of 32.1 ± 2.6 million tons based on all resources. Moreover, analyses of H 2 equilibrium across different provinces in China indicate that the reclaimed water routes enable the more provinces to locally produce H 2 and satisfy their H 2 demand compared to other routes (Fig. 4 d), thereby diminishing extensive inter-regional resource disparity, and reducing the distances of H 2 transport and associated cost. These results highlight the superior decentralization and flexibility of reclaimed water as a feedstock for H 2 production. However, the direct electrolysis of reclaimed water, despite avoiding purification expenses, will be challenged by the presence of interferential pollutants in the water. For example, the precipitation of calcium or magnesium hydroxide under alkaline environment may lead to a decline in electrodes performance. The residual chloridion generated from reclaimed water chlorination will accumulate in electrolyzer during continuous exhaustion of water by electrolysis 33 , which may impact electrolysis efficiency, as chloridion oxidation competes with oxygen evolution reaction (Supplementary Section 3.5) 28 . The presence of other micropollutants, such as organics and emerging pollutants, in the reclaimed water has recently shown negative or positive impacts on the electrolytic H 2 production 29 . In order to achieve an optimal balance between electrolysis performance and cost, some practical processes, such as exempting chlorination, deionization by ion exchange resin and physical absorption etc., can be applied for the pretreatment of reclaimed water instead of RO process. The recent advances in direct electrolysis of seawater 34 will also encourage to electrolyze raw reclaimed water. The H 2 production through electrolysis of desalinated or raw seawater is geographically confined to the coastal provinces. For electrolyzing desalinated seawater, the desalination capacity constrains the maximum H 2 production capacity to 21.6 ± 5.6 million tons. This hinders most coastal provinces achieving H 2 self-sufficiency, though their electricity supply is sufficient for producing 72.1 ± 3.7 million tons H 2 that can meet the national demand (Fig. 4 a). In practice, the provinces capable of seawater desalination (e.g., Shandong, Hebei, and Jiangsu) often suffer from a shortage of freshwater resources 35 , the desalination would first meet the basic demand of domestic and industrial sectors. Therefore, the available desalinated seawater for electrolysis might be less than that based on desalination capacity, resulting in less H 2 production (19.8 ± 4.7 million tons). Moreover, some coastal regions with developed industries, such as Shanghai, Tianjin, and Zhejiang, don’t have easy access to high-quality seawater, leading to a decreased desalination efficiency and H 2 potential (Supplementary Section 2.4). Offshore wind power-driven direct seawater electrolysis recently shows a promise for large-scale green H 2 production due to the advances in electrolytic materials and configuration 12 . However, the wind resources in the most offshore areas of Chinese mainland are scarce and unstable 36 (Supplementary Section 2.4). Access to the far offshore wind with strong and stable output for seawater electrolysis will be difficult 37 , since the infrastructures are challenged by the complex wind platform and long-distance offshore H 2 transport 38 . This results in a H 2 potential of merely 2.2 ± 0.2 million tons, which is much lower than the national H 2 demand (Fig. 4 a). Moreover, current direct electrolysis of raw seawater remains in an immature stage with high electricity consumption (43.0–67.3 kWh/kg H 2 ) (Supplementary Section 3.5). All challenges mentioned above demonstrate that the electrolytic H 2 production from seawater might need further engineering efforts. Nationwide cost and carbon emissions of H production Nationwide, the levelized cost of H 2 (LCOH) by electrolyzing reclaimed water (raw or purified) is $ 8.8 ± 0.6/kg H 2 , which is slightly higher than those using conventional purified tap water and desalinated seawater ( $ 8.7 ± 0.3/kg H 2 ) (Fig. 4 b), except direct electrolysis of raw seawater with a relatively high cost of $ 10.1 ± 0.07/kg H 2 . Herein, the LCOH among several routes are similar, this is because the expense of electrolysis in terms of electricity consumption and electrolyzer construction dominates the LCOH (85.81–95.38% of the LCOH) and has a small difference among different technologies 31 (Fig. 4 b). However, the other cost components of different routes vary significantly. Specifically, the wastewater treatment for producing reclaimed water contributes ~ 0.032% to the LCOH, which is lower than those of tap water treatment (~ 0.066%, costs of both tap water production and its further purification) and seawater desalination (~ 0.24%), as the latter treatments often require more expensive chemicals and membranes. If reclaimed water is pre-purified by RO before electrolysis, this pretreatment cost will increase to ~ 0.092% of the LCOH (Supplementary Section 3.1). The WWTPs/WTPs are usually located in regions with intensive human activities, which allow H 2 produced from reclaimed water or tap water to be closer to the end-users. This results in lower H 2 transport cost, accounting for ~ 0.49% of the LCOH (Fig. 4 b). In contrast, the electrolysis of desalinated seawater leads to a higher cost (~ 5.38% of LCOH) of transporting H₂ from coastal areas to inland end-users. If H 2 is produced from direct electrolysis of raw seawater on offshore platforms, the H 2 transport cost will be up to ~ 14.02% of LCOH due to the additional shipping or pipeline transport needed between platforms and coast. Since Chinese national carbon trading scheme has been implemented in power sector 39 , the carbon tax of electricity consumption should be included in the cost of electrolytic H 2 production. The current electric grids in China comprise ~ 72.3% of fossil power 40 , which leads to a carbon tax of ~ 4.11% of the LCOH for electrolysis using onshore electricity, except utilization of offshore wind power for direct seawater electrolysis with a lower carbon footprint (a tax of ~ 0.17% of LCOH). The costs of H 2 produced in different provinces are varied (Fig. 4 e). The provinces rich in hydropower (e.g., Sichuan, Yunnan, and Hubei) typically exhibit lower H 2 cost, as hydropower is generally more cost-effective than other power sources 41 . All findings above underscore the cost advantages of using reclaimed water for electrolytic H 2 production in terms of feedstock production and H₂ transport. However, these routes currently remain less competitive than H₂ production using fossil-fuels, such as natural gas reforming with a cost of ~ $ 4/kg H₂ 42 . This is due to the substantial electricity consumption by current electrolyzers, which necessitate ongoing advancements in electrolytic efficiency and materials to reduce costs in the future. Same as LCOH, the levelized carbon emissions of H 2 (LEOH) by electrolyzing reclaimed water (raw or purified) (40.4 ± 0.7 kg CO 2 -eq/kg H 2 ) is very similar to that using purified tap water and desalinated water (40.2 ± 0.3 kg CO 2 -eq/kg H 2 ) (Fig. 4 c). Because the carbon emissions dominantly stem from the electricity consumption and electrolyzer fabrication, which account for 99.68–99.95% of LEOH in these H 2 routes (Fig. 4 c). This means that endeavors of carbon mitigation should focus on cleaner production of power and materials. Although the direct electrolysis of seawater using green offshore wind has the lowest LEOH of 1.30 ± 0.01 kg CO 2 -eq/kg H 2 (Fig. 4 c) that is well below the LEOH benchmark of green H 2 (< 4.9 kg CO 2 -eq/kg H 2 ) 43 , its carbon emissions derived from H 2 transport (~ 0.35% of LEOH) is the highest among all routes. In contrast, transporting H 2 produced from reclaimed water, purified tap water, and desalinated seawater only generates negligible carbon emissions (0.0013–0.0067% of LEOH). The life-cycle assessment indicates that wastewater treatment for reclaimed water production contributes only ~ 0.047% to LEOH, as it typically employs biological processes that are more environmentally friendly than chemical or membrane treatment used for producing purified tap water (~ 0.073% of LEOH) and desalinated seawater (~ 0.32% of LEOH). In the same way, purification of reclaimed water by RO will increase the carbon emissions to ~ 0.11% of LEOH. Generally, the carbon emissions associated with wastewater/water treatment and H 2 transport remains negligible in the overall carbon emissions. The spatial carbon emissions of H 2 production (Fig. 4 f) also reveals that provinces with abundant renewables (e.g., hydro-, wind-, and photovoltaic power) typically exhibit lower LEOH, underscoring the pivotal role of renewable energy in green H 2 production. Compared to other electrolytic H 2 routes, electrolysis of reclaimed water in WWTPs can obtain additional carbon and economic retributions. For example, the electrolytic byproduct of O 2 can be used in situ for aerobic biological treatment, which may decrease aerator energy consumption by 76.4 ± 4.9% compared to conventional air aeration (Supplementary Section 3.6). Another potential byproduct of chlorine can serve as a disinfectant of wastewater treatment (Supplementary Section 3.6). These results highlight the comprehensive benefits of electrolyzing reclaimed water. Notably, the current suboptimal electrolysis efficiency merely shifts carbon emissions from power generation to H 2 production at this early stage of energy transition in China. This emphasizes the need of advancements in electrolytic technology and structural transformation of electric power sector to achieve cleaner H 2 production in the future. Predictions of electrolytic H production from reclaimed water and other water resources in the future Potential H 2 production capacity In the future, the electrolytic H 2 production is significantly influenced by China’s commitment to carbon peaking and neutrality, changes in energy structure and population, available water resources, and advancements in electrolytic technology (Supplementary Section 4.1). The amount of municipal wastewater production is projected to reach a plateau with China’s population achieving peak by approximately 2030. This indicates a stable supply of reclaimed water in the future. Consequently, electrolysis of reclaimed water is projected to produce H 2 of 160.9 ± 13.7 million tons by 2040 and 213.1 ± 21.6 million tons around 2060 based on both water and electricity resources (Fig. 5 a), which is far higher than the national H 2 demand of 130.9 ± 6.5 million tons by 2060. In contrast, use of pure water generated from tap water for H 2 production shows a less H 2 potential in the future (55.9 ± 4.0 million tons by 2040 and 78.1 ± 6.3 million tons by 2060), indicating that it might not meet the rapid growth of H 2 demand alone after 2040 (Supplementary Section 4.2 − 4.4). Given that exploiting new freshwater would exacerbate current water scarcity in China 44 , relying on tap water for H 2 production might not be a sustainable option in terms of security of both water resources and H 2 supply. The increase of seawater desalination capacity in China is expected to be modest with an estimated growth of 0.050 − 0.30 million tons/day according to current development 45 and future plans 46 , which suggests that desalinated seawater could supply 79.1 ± 17.5 million tons of H 2 by 2060 (Fig. 5 a). Similarly, regression analyses of data from the national statistics of China indicate that the actual growth of offshore wind power has been slower than other renewable energy 47 , leading to a predicted H 2 potential of 21.8 ± 0.1 million tons by 2060 through direct electrolysis of raw seawater (Fig. 5 a). However, it is noteworthy that recent technical breakthroughs in GW-level offshore wind turbines in China 48 might significantly enhance the H 2 potential of this route in the future. Future cost and carbon emissions of H 2 production The LCOH of electrolyzing reclaimed water and purified seawater/tap water are projected to increase to $ 11.5 ± 0.6– $ 11.7 ± 1.1/kg H 2 by 2040 (Fig. 5 b), which would mainly result from the cost increase in the electricity supply during this period (Supplementary Section 4.3). This fluctuation of electricity price could be attributed to the massive investments of renewable facilities, renewable intermittency and electric grids expansion, despite a significant decrease in the cost of renewable technologies themselves 49 . Then, the LCOH would gradually decrease to $ 4.0 ± 0.5 kg H 2 by 2050 and $ 3.5 ± 0.4/kg H 2 by 2060 (Fig. 5 b) when the more efficient electrolytic technologies are used, and the electric grids and H 2 transport networks become more economical. Similarly, the LCOH of raw seawater electrolysis would first increase to $ 14.8 ± 0.2/kg H 2 by 2030 (Fig. 5 b) due to the external costs derived from offshore wind electricity 48 and H 2 transport infrastructures 38 , and then it would drop to $ 4.4 ± 0.1/kg H 2 by 2060, but it is still higher than the other routes due to lower electrolytic efficiency. The current price of fossil H 2 is ~ $ 4.0/kg H 2 (without carbon taxes) 42 or ~ $ 4.3/kg H 2 (with carbon taxes) 43 , indicating that the electrolytic H 2 produced from reclaimed water would not be competitive with fossil H 2 until 2050, if there is no a breakthrough in electrolytic technologies and the changes in environmental policies. Therefore, China is encouraged to provide more economic subsidies to the electrolytic H 2 production and associated research and development, or impose more carbon taxes on the petrochemical H 2 industry, ensuring that the electrolytic H 2 routes are more commercially competitive. The LEOH of electrolyzing reclaimed water and purified seawater/tap water are expected to significantly decline from the current high levels to 4.99 ± 0.05 kg CO 2 -eq/kg H 2 by 2050 and 2.60 ± 0.04 kg CO 2 -eq/kg H 2 by 2060 with the progressive energy transition nationwide and advancements in electrolytic technologies (Fig. 5 c), while LEOH of direct seawater electrolysis are projected to slightly decrease (1.1 ± 0.01 kg CO 2 -eq/kg H 2 by 2060). However, the reclaimed water routes are unlikely to achieve the LEOH benchmark of green H 2 before 2050 under the current scenario of energy transition 50 . This suggests that China should take more actions to accelerate electric grids decarbonization, such as separation of renewable electricity used for electrolysis from current grids, or integrating the electrolytic H 2 industry with intelligent grids for the flexible access to sustainable or economical electricity resources 51 , and using more sustainable carbon capture and sequestration technologies in power sector 52 . Discussion This study provides a novel insight into the electrolytic H₂ production from municipal reclaimed water, an abundant unconventional water resource in China, which is compared with the other electrolytic H 2 routes by using pure water, desalinated or raw seawater. The analyses show that reclaimed water emerges as a promising alternative of conventional water resources for water splitting mainly due to its huge H 2 production potentials (95.7–213.1 million tons per year between 2021 and 2060), which far meet the national H 2 demand of 33.4–130.9 million tons per year during this period. In contrast, the pure water or seawater routes can only provide 19.8–79.1 million tons H 2 per year. Although there is a small difference in cost and carbon emissions of H 2 production using different water resources under current technical conditions, reclaimed water highlights its strengths of widespread availability and stable supply, since it is closely associated with human activities and less influenced by seasonal or regional variations compared to other water resources. This spatiotemporal advantage of reclaimed water supports a decentralized H₂ production aligning with the distribution of WWTPs, enabling localized access to water feedstock, electricity, and H₂ markets, thereby reducing the burdens of H₂ transport and resources allocation. However, this new route remains challenged. The direct electrolysis of raw reclaimed water with more simplified operation and lower cost is preferred, however, the micropollutants in the reclaimed water may reduce the electrolytic efficiency 29 . The impacts of pollutants on the H 2 production and their degradation by electrolytic process need to be further examined. This requires to develop specialized electrolyzers with new catalysts and configuration to alleviate the interferences of pollutants, such as these advancements in direct electrolysis of seawater 12 . Moreover, China is still in the early stage of electrolytic H 2 development and lacks requisite infrastructures, such as H 2 transmission facilities and refueling stations 7 , which are also crucial to support the widespread deployment of electrolyzing H₂ production from the reclaimed water. While the existing natural gas networks can serve as a transitional solution by mixing H 2 with methane 53 , their limited capacity and the security risk 54 necessitate substantial construction of specialized H₂ infrastructure in the future. The study also emphasizes the importance of green H₂ production to China’s commitment of carbon neutrality. The integration of reclaimed water electrolysis with the renewable electricity and state-of-the-art electrolytic technology, therefore, is essential for promoting electrolytic H₂ industry. The latest policy released by the National Energy Administration of China on October 9th, 2024 requires self-consumption of electricity generated by decentralized commercial photovoltaic (PV) with installation capacity of 6–50 MW rather than trade of it to the electrical grid, while PV with capacity of < 6 MW is only allowed to sell residual power on the electricity markets 55 . This shows a great promise to directly use surplus PV electricity to support decentralized H 2 production from reclaimed water. For example, it is foreseeable that the PV deployed in the WWTPs 56 and the roofs of urban buildings 57 can facilitate it since their power generation often exceeds demand. China has installed ~ 254.4 GW of decentralized PV in commercial sectors by the end of 2023 58 . If a half of PV power is used for electrolysis (PV effective operation duration of 2400–3200 h/year 59 ), ~ 8.4 ± 2.6 million tons H 2 can be produced per year from reclaimed water, which nearly meets 25% of China’s current H 2 demand. Consequently, this policy is expected to significantly reduce the carbon emissions and costs of H 2 production, as off-grid PV power is widely recognized as lower carbon and cheaper than that of on-grid PV 60 . Methods Statistical data and models The amount of provincial municipal wastewater production, ratios of wastewater collection and advanced wastewater treatment, ratio of reclaimed water reuse, and treatment capacity of water treatment plants are provided by the 2021 Urban Constructure Statistical Yearbook 61 , issued by the Ministry of Housing and Urban-Rural Development of China. The water demand for ecological replenishment and provincial amount of freshwater are obtained from the 2021 Water Resources Bulletin 62 , published by the Ministry of Water Resources of China. The solar and wind power potential are obtained from the 2021 Wind and solar Resources Bulletin, published by the China Meteorological Administration 63 . The 2021 Statistical Yearbook 64 , released by the State Statistics Bureau of China, provides provincial population, primary energy consumption, and the electric power structure. The 2021 National Seawater Utilization Report from the Ministry of Natural Resources of China 65 , outlines provincial capacity of seawater desalination facilities. The classification of seawater quality is obtained from the 2021 Bulletin of Marine Ecology and Environment Status of China 66 , provided by the Ministry of Ecology and Environment of China. A government report from National Energy Administration of China provides the installed capacity and effective operation time of offshore wind power 47 . Costs of provincial power generation and electricity transmission are referenced from reports released by the National Development and Reform Commission of China 67 . Standards of water quality is obtained from the open data provided by the Standardization Administration of China 68 . The parameters of industrial alkaline electrolyzer refers to a commercial system (model A3880) 69 . An industrial 3-stage reverse-osmosis system from Dupont 70 is utilized as a model in analysis of water purification for electrolysis. The models for reclaimed water production in WWTPs and tap water production in WTPs were constructed based on China’s extensive analyses 71 . Geographical information system Geographical data of provinces is sourced from the Resource and Environment Science and Data Center of China 72 . The wind atlas including wind speed and wind power potential are provided by the World Bank's Data Catalog 73 . The list of WWTPs nationwide is obtained from the Ministry of Ecology and Environment of China 74 . The latitude and longitude of WWTPs are retrieved via the Baidu-Map open-access application programming interface 75 . The distribution of power plants in China is obtained from Google's Global Power Plant Database 76 . Above data is plotted using Python with Geopandas and Matplotlib packages. Parameters normalization All values with different unit and order of magnitude in Fig. 1 b are normalized into 0–100 score using the following Eq. 7 7 to ensure the comparability among them. $$\:Score={\left[AF\times\:\frac{\text{l}\text{n}\left({P}_{i,j}\right)}{\text{l}\text{n}\left({Max}_{i,j}\right)}\right]}^{2}$$ Where the subscript i represents the evaluated items, the subscript j represents the H 2 routes using different water resources, AF (dimensionless, 10) is the amplification factor, \(\:{\varvec{P}}_{\varvec{i},\varvec{j}}\) is the value of the evaluated items, and \(\:{\varvec{M}\varvec{a}\varvec{x}}_{\varvec{i},\varvec{j}}\) is the maximum value among a set of the evaluated items. Prediction data smoothing The data of predicted H 2 production, cost and carbon emissions with a ten-year interval undergo a smoothing processing by using a polynomial fitting to illustrate the trend of continuous change. Uncertainty analyses In this study, the parameters supporting the evaluations of H 2 production derive from the statistical data and the calibrated industry models with uncertainties. Thus, Monte Carlo simulations with Latin hypercube sampling are employed to quantify the influence of these uncertainties on the evaluation results. All results are then displayed as the mean ± standard deviation within a 5–95% confidence interval based on 100,000 Monte Carlo simulation trials. Declarations Data availability All data presented in this manuscript are available in the paper and its Supplementary Information. Source data are provided with this paper. Acknowledgements This work was supported by the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (52321005), and Shenzhen Science and Technology Program (KQTD20190929172630447). Author contributions L.L. conceived the research conceptualization, manuscript editing and supervision. 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Pi, S. et al. Solar-driven waste-to-chemical conversion by wastewater-derived semiconductor biohybrids. Nature Sustainability 6 , 1673-1684 (2023). Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformation.docx Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5380448","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Analysis","associatedPublications":[],"authors":[{"id":380429191,"identity":"e006affc-b1b3-4795-ae07-d8820d5b9f51","order_by":0,"name":"Lu Lu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAoElEQVRIiWNgGAWjYBACPgbmBoaPDRIgtgFxWtgYGBsYZzZISJCmhZm3gYEULRKJbdK2OyzqGNibt0kw1NwhUkvuGaDDeI6VSTAce0asljagFokcMwnGhsNEarEEaZF/Q4oWRrAtPMRq4XnYbNnbJiHZxpNWbJFwjAgt/OzJB2/8bKvj52c/vPHGhxoitCCsAxEJJGgYBaNgFIyCUYAHAAC/FSzhJsXZvAAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-8290-8834","institution":"Harbin Institute of Technology, Shenzhen","correspondingAuthor":true,"prefix":"","firstName":"Lu","middleName":"","lastName":"Lu","suffix":""},{"id":380429192,"identity":"f3c01778-1e60-461c-bab4-f83d10e85548","order_by":1,"name":"Weixiang Chao","email":"","orcid":"","institution":"Harbin Institute of Technology, Shenzhen","correspondingAuthor":false,"prefix":"","firstName":"Weixiang","middleName":"","lastName":"Chao","suffix":""},{"id":380429193,"identity":"0f402d05-547f-4002-9708-d289fbb4a248","order_by":2,"name":"Xiuling Yan","email":"","orcid":"https://orcid.org/0009-0002-4519-872X","institution":"Harbin Institute of Technology, Shenzhen","correspondingAuthor":false,"prefix":"","firstName":"Xiuling","middleName":"","lastName":"Yan","suffix":""}],"badges":[],"createdAt":"2024-11-03 04:20:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5380448/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5380448/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":69803653,"identity":"10f3b0c4-4b4f-413b-9a97-9697080e7a68","added_by":"auto","created_at":"2024-11-25 11:35:47","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":772629,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpatial distribution of renewable energy, water resources, municipal wastewater treatment plants (WWTPs), and power plants (PPs) in China. a\u003c/strong\u003e, Solar and wind power potential as well as water resources amount in different regions of China. The color shades across China’s provinces indicate the numerical order of values from minimum to maximum level. \u003cstrong\u003eb\u003c/strong\u003e, Treatment capacity of WWTPs is indicated by hierarchical sizes of triangle. \u003cstrong\u003ec\u003c/strong\u003e, Installed capacity of PPs is indicated by hierarchical sizes of circle. Different electrical power types, including hydro-, solar-, wind-, gas-, oil-, coal-, nuclear, and geothermal power, are indicated by different colors. The proximity of most WWTPs to PPs suggests the feasibility of co-utilizing reclaimed water and nearby electricity for electrolytic H\u003csub\u003e2\u003c/sub\u003e production. The provinces of Qinghai and Tibet that are home to numerous water resources reserves are excluded from assessment. This study also excludes Hong Kong, Macau, and Taiwan due to the lack of regional data.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5380448/v1/baec29b9fc7e99ae783fce33.png"},{"id":69803654,"identity":"1b30a0e2-5437-41cd-a90b-bf0e5c1b0df5","added_by":"auto","created_at":"2024-11-25 11:35:47","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":528394,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScenarios of electrolytic H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e production from reclaimed water and other water resources. a\u003c/strong\u003e, Proposed and current routes for electrolytic H\u003csub\u003e2\u003c/sub\u003e production. Electrolysis of reclaimed water (purified or raw) or pure water is conducted near wastewater or water treatment facilitates using onshore electric grids. Desalinated seawater produced by multi-stage reverse osmosis is electrolyzed along the coast using onshore electricity, while direct seawater electrolysis uses wind power from offshore platforms. H\u003csub\u003e2\u003c/sub\u003e is transported to the end-users by pipeline or vehicles. \u003cstrong\u003eb\u003c/strong\u003e, Evaluation of these technical routes in terms of potential amount of H\u003csub\u003e2\u003c/sub\u003e production, water and electricity consumption, carbon emissions, and H\u003csub\u003e2\u003c/sub\u003e cost by comprehensively considering resource availability, process operation, facilities maintenance, and H\u003csub\u003e2\u003c/sub\u003e transport. The evaluation parameters are normalized to scores ranging from 0 to 100, corresponding to classes 1 to 4 with increasing score. A higher class indicates more H\u003csub\u003e2\u003c/sub\u003e production, but more water and electricity consumption, and higher H\u003csub\u003e2\u003c/sub\u003e carbon emissions and cost.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5380448/v1/4160fdd9862e72321aa6b980.png"},{"id":69805221,"identity":"13fccc38-abb9-4c6e-b756-42f1bf90e475","added_by":"auto","created_at":"2024-11-25 11:43:47","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":576318,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eProcedures used for evaluating electrolytic H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e production and relevant environmental \u0026amp; economic impacts.\u003c/strong\u003e The assessments are based on water and electricity availability as well as H\u003csub\u003e2\u003c/sub\u003e demand and transport. (\u003cem\u003ei\u003c/em\u003e) Amount of H\u003csub\u003e2\u003c/sub\u003e produced is calculated based on the spatiotemporal availability of both water and electricity resources, current mature alkaline electrolysis and wastewater/water/seawater treatment, and calibrated industrial models. (\u003cem\u003eii\u003c/em\u003e) A H\u003csub\u003e2\u003c/sub\u003e equilibrium is obtained by deducting H\u003csub\u003e2\u003c/sub\u003e demand from H\u003csub\u003e2\u003c/sub\u003e production. (\u003cem\u003eiii\u003c/em\u003e) Carbon emissions and cost of H\u003csub\u003e2\u003c/sub\u003e are determined by life cycle inventories, carbon emission factors, and economic assumptions of the listed processes. (\u003cem\u003eiv\u003c/em\u003e) Scenarios of electrolytic H\u003csub\u003e2\u003c/sub\u003e production in the future are predicted by considering changes in national population, power sector decarbonization, government policies, economic structure, advances in electrolysis technologies, and H\u003csub\u003e2\u003c/sub\u003e markets.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5380448/v1/6021e16c41453faf179f9ae6.png"},{"id":69803657,"identity":"9da43f76-2136-4b37-84ea-46a091f7532c","added_by":"auto","created_at":"2024-11-25 11:35:47","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1179941,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpatial distribution of electrolytic H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e production from reclaimed water and other water resources in China. a\u003c/strong\u003e, A total capacity of H\u003csub\u003e2\u003c/sub\u003e production based on available resources (water and electricity). Scenario of direct seawater electrolysis based on available water is not included since the raw seawater is infinitely available. A current national H\u003csub\u003e2\u003c/sub\u003e demand by 2021 (33.4 ± 1.7 million tons) is demonstrated by a blue dotted line. \u003cstrong\u003eb\u003c/strong\u003e, The major cost components of per kg H\u003csub\u003e2\u003c/sub\u003e production. \u003cstrong\u003ec\u003c/strong\u003e, The contributions of electrolysis, wastewater \u0026amp; water treatment, and H\u003csub\u003e2\u003c/sub\u003e transport to the carbon emissions per kg H\u003csub\u003e2\u003c/sub\u003e production. \u003cstrong\u003ed\u003c/strong\u003e, H\u003csub\u003e2\u003c/sub\u003e equilibrium as well as \u003cstrong\u003ee\u003c/strong\u003e, cost and \u003cstrong\u003ef\u003c/strong\u003e, carbon emissions per kg H\u003csub\u003e2\u003c/sub\u003e production in different regions of China. All calculations are based on 2021 statistical data in China. The scales of the y-axis in \u003cstrong\u003ea\u003c/strong\u003e,\u003cstrong\u003e b\u003c/strong\u003e,\u003cstrong\u003e c\u003c/strong\u003e are represented in the base 10 exponent. The color shades across China’s provinces indicate the numerical order of normalized values from minimum to maximum level. The provinces of Qinghai and Tibet that are home to numerous water resources reserves are excluded from assessment. This study also excludes Hong Kong, Macau, and Taiwan due to the lack of regional data.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5380448/v1/6ce958a64d77cfc68e497dc3.png"},{"id":69803656,"identity":"4934f402-840a-44c1-97c3-ad5b3014610b","added_by":"auto","created_at":"2024-11-25 11:35:47","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":187547,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePredictions of potential electrolytic H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e production from reclaimed water\u003c/strong\u003e a\u003cstrong\u003end other water resources\u003c/strong\u003e \u003cstrong\u003efrom 2021 to 2060. a\u003c/strong\u003e, H\u003csub\u003e2\u003c/sub\u003e production potential determined by both available water and electricity as well as national H\u003csub\u003e2\u003c/sub\u003e demand. The scales of the y-axis are represented in the base 10 exponent. \u003cstrong\u003eb\u003c/strong\u003e, Estimated cost and \u003cstrong\u003ec\u003c/strong\u003e, carbon emissions per kg electrolytic H\u003csub\u003e2\u003c/sub\u003e production. The solid lines represent the mean values, while the expanded shades around these lines represent their uncertainty range.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5380448/v1/637f951e8b36136cebdc5afe.png"},{"id":69805633,"identity":"497b6942-b2ea-4e6d-9374-f4cab521b5eb","added_by":"auto","created_at":"2024-11-25 11:52:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3997281,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5380448/v1/c0d679af-c246-4767-b10d-db4869fdad42.pdf"},{"id":69803658,"identity":"c6c8d511-f394-4c6a-b071-96e1ff4a5949","added_by":"auto","created_at":"2024-11-25 11:35:47","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":7903291,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5380448/v1/1437d0cf6c3d56e3f3cecfd2.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"High Spatiotemporal Availability of Hydrogen by Electrolysis of Municipal Reclaimed Water in China","fulltext":[{"header":"Introduction","content":"\u003cp\u003eChina is aggressively advancing hydrogen (H\u003csub\u003e2\u003c/sub\u003e) energy development to meet its pledge of achieving carbon neutrality by 2060\u003csup\u003e1\u003c/sup\u003e, and the nation\u0026rsquo;s H\u003csub\u003e2\u003c/sub\u003e demand is expected to rise from the current\u0026thinsp;~\u0026thinsp;33\u0026nbsp;million tons\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e to \u0026gt;\u0026thinsp;130\u0026nbsp;million tons by 2060\u003csup\u003e3\u003c/sup\u003e. Electrochemical splitting of pure water is one of the important approaches to produce H\u003csub\u003e2\u003c/sub\u003e, as it is able to produce green H\u003csub\u003e2\u003c/sub\u003e by using renewable electricity. Although several analyses have demonstrated the sustainability of electrolytic H\u003csub\u003e2\u003c/sub\u003e production\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, steady access to both low-cost or renewable electricity and pure water remains challenged for large-scale electrolysis. For instance, China\u0026rsquo;s water electrolysis facilities are concentrated in the eastern provinces and rely on relatively abundant freshwater or desalinated seawater\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e, but the extensive energy stress caused by dense population and industry in this region may restrict further scale-up\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. In contrast, northwest provinces with abundant renewable solar and wind electricity, such as Xinjiang, Gansu, and Ningxia, are usually suffer from water deficiency\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Additionally, the challenges of transporting hydrogen\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e will generally require local H\u003csub\u003e2\u003c/sub\u003e production in the future to avoid current mismatches between H\u003csub\u003e2\u003c/sub\u003e demand and production capability among regions.\u003c/p\u003e \u003cp\u003eTo meet these challenges, co-location of electricity, electrolysis-grade water and H\u003csub\u003e2\u003c/sub\u003e markets is crucial to minimize the cost of H\u003csub\u003e2\u003c/sub\u003e production and its environmental footprint\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. This necessitates a reconfiguration of current electrolysis industries to better utilize spatially uneven electricity and water resources. In China, substantial infrastructure investment in the energy sector, especially renewable energies, has significantly improved access to electricity\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. However, the widespread water scarcity per capita severely restricts exploitation of current freshwater resources for other purposes\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. The surplus freshwater is only available in areas with moderate populations and abundant water resources, such as Southwest China\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. The utilization of unconventional water resources, such as seawater and brine, to generate pure water is also constrained by the capacity of installed desalination facilities\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. Emerging H\u003csub\u003e2\u003c/sub\u003e production via direct electrolysis of seawater is restricted to coastal areas and remains technologically immature, although it allows easy access to nearby offshore wind power and unlimited seawater\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eReclaimed water, treated municipal or industrial wastewater that is produced from wastewater treatment plants (WWTPs) and can be reused\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, might be a promising feedstock of water splitting, as it is intrinsically linked to domestic and industrial activities with widespread spatial distribution\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. For example, \u0026gt;\u0026thinsp;4800 municipal WWTPs have been constructed in China\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) with a total wastewater treatment capacity of 2.3\u0026times;10\u003csup\u003e8\u003c/sup\u003e m\u003csup\u003e3\u003c/sup\u003e/d by 2020, while only\u0026thinsp;~\u0026thinsp;20% treated water is reused\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Notably, reclaimed water production is less influenced by seasonal and regional variations compared to other natural freshwater resources because its quantity is dependent on population and industrial structure\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, which maximally guarantees a stable and long-term supply for various uses. Reclaimed water has long been used in China for low-value-added purposes, such as municipal irrigation, due to its low water quality\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. However, the quality of China\u0026rsquo;s reclaimed water has continuously improved with the introduction of more rigorous water discharge standards and advancements in wastewater treatment technology. Reclaimed water is becoming a significant unconventional water resource to support more uses. For instance, the potential of reclaimed water as a source of rinsing water for electronic chip wafer production has been exploited\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Given the stable and extensive supply of reclaimed water, and the spatial co-distribution of WWTPs and power plants (PPs) in practice (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec), the direct or indirect (after purification) utilization of reclaimed water as electrolytic feedstock holds considerable potential to meet the nationwide demand of electrolytic H\u003csub\u003e2\u003c/sub\u003e production in the future.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis study, grounded in current alkaline water electrolysis technology, industrial wastewater/water/seawater treatment models, and experimental data, comprehensively assessed the feasibility and potential benefits of reclaimed water electrolysis for H\u003csub\u003e2\u003c/sub\u003e production. The reclaimed water is assumed to be directly fed into electrolyzer or be purified by reverse osmosis (RO) before electrolysis. The spatial distributions of water, electricity, and H\u003csub\u003e2\u003c/sub\u003e market as well as their environmental and economic impacts on H\u003csub\u003e2\u003c/sub\u003e production were taken into consideration during analyses. This approach is compared with existing electrolytic H\u003csub\u003e2\u003c/sub\u003e production routes using pure water, desalinated seawater, and raw seawater (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). The results show that electrolysis of reclaimed water leads to comparable performance with other routes in terms of resource consumption (electricity and water), carbon emissions, and economic cost, but demonstrates the highest capacity of H\u003csub\u003e2\u003c/sub\u003e production (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) due to the abundant reclaimed water obtained across most China\u0026rsquo;s provinces. Moreover, the high spatial distribution of WWTPs enables electrolysis of reclaimed water to be conducted in a decentralized way with easy access to nearby water and electricity, which reduces distances of H\u003csub\u003e2\u003c/sub\u003e and electricity delivery as much as possible. The predictable increase in reclaimed water production with population growth as well as future advances in electrolytic technology and electricity decarbonization demonstrate a greater potential of green H\u003csub\u003e2\u003c/sub\u003e production with higher efficiency. Although more large-scale applications are needed to verify the feasibility of the proposed H\u003csub\u003e2\u003c/sub\u003e production from reclaimed water, the findings herein provide a new strategy to advance China\u0026rsquo;s H\u003csub\u003e2\u003c/sub\u003e economy and may substantially contribute to China\u0026rsquo;s future energy transition in conjunction with other H\u003csub\u003e2\u003c/sub\u003e production routes.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eCradle-to-gate assessment of electrolytic H production from reclaimed water and other water resources\u003c/h3\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eAssessment aims and boundary\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e demonstrates different scenarios of nationwide H\u003csub\u003e2\u003c/sub\u003e production by electrolysis of reclaimed water and other water resources. This study is defined as a cradle-to-gate assessment, targeting a life-cycle from the production of electrolysis-grade water to the H\u003csub\u003e2\u003c/sub\u003e delivery to end-users (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Supplementary Section 1.1). The assessment addresses key indicators including H\u003csub\u003e2\u003c/sub\u003e production capacity, carbon emissions, and cost for each route employing different water resources. This study excludes Hong Kong, Macau, and Taiwan due to the lack of regional data. Given that China has rigorously restricted the exploitation in nature reserves to protect the fragile ecosystems\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e, this study also excludes provinces of Qinghai and Tibet that are home to numerous water resources reserves, like the Three River Source National Nature Reserve\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. China has released its plan of H\u003csub\u003e2\u003c/sub\u003e industry development from 2021 to 2035\u003csup\u003e22\u003c/sup\u003e and highlighted the significance of electrolytic green H\u003csub\u003e2\u003c/sub\u003e toward carbon neutrality by 2060\u003csup\u003e23\u003c/sup\u003e. This study evaluates H\u003csub\u003e2\u003c/sub\u003e production using the currently available statistical data of 2021 and predicts the future changes by 2060. The H\u003csub\u003e2\u003c/sub\u003e production capacity in each province was calculated based on one of available resources (electricity or water amounts) or both of them. The H\u003csub\u003e2\u003c/sub\u003e equilibrium for each province is determined by deducting H\u003csub\u003e2\u003c/sub\u003e demand from H\u003csub\u003e2\u003c/sub\u003e production, which indicates the H\u003csub\u003e2\u003c/sub\u003e self-sufficiency/surplus or shortage in each province. For calculating the environmental footprint, the life cycle inventory (LCI) including materials, chemicals, energy, and infrastructure consumables is initially established from published references (Supplementary Section 1.2). The industrial models in terms of wastewater/water treatment, reverse osmosis (RO), and alkaline electrolysis are applied to the LCI for the calibrations (Supplementary Section 1.3). These calibrations corresponding to different water qualities (Supplementary Section 1.4) are conducted in same scale to better align with the assessments. Total carbon emissions and cost generated from life-cycle H\u003csub\u003e2\u003c/sub\u003e production are determined based on the carbon emission intensities of specific activities and rational economic assumptions (Supplementary Section 1.5).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eDetermination of available water resources and electricity\u003c/h3\u003e\n\u003cp\u003eThe amount of reclaimed water that can serve as an electrolytic feedstock was determined by deducting those primarily used for municipal, industrial, and agricultural reuse as well as ecological replenishment from a total amount of reclaimed water discharged from municipal WWTPs (Supplementary Section 2.1 and 2.2). If the reclaimed water needs to be further purified before electrolysis, its available amount is determined by the capacity of installed RO facilities and their annual operation factors. Similarly, the tap water produced from water treatment plants (WTPs) should also firstly meet the demand of basic industrial and domestic activities before being used as an electrolytic feedstock, which can be provided by the unoccupied capacity of WTPs\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. The available RO capacity in local regions finally determine the amount of pure water derived from tap water for H\u003csub\u003e2\u003c/sub\u003e production. However, this route is recommended to carry out when freshwater exploitation is above the threshold of moderate water scarcity set by the United Nations\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e, that is, the available per capita water resource after deducting the demand of basic activities and H\u003csub\u003e2\u003c/sub\u003e production, should be maintained at \u0026gt;\u0026thinsp;3000 m\u003csup\u003e3\u003c/sup\u003e\u0026middot;capita\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Supplementary Section 2.1 and 2.3). This will ensure that these regions do not experience risk of water shortage when electrolytic H\u003csub\u003e2\u003c/sub\u003e production is implemented by exploiting additional freshwater resources. For use of desalinated seawater to produce H\u003csub\u003e2\u003c/sub\u003e, the available water amount is also determined by the available RO capacity in coastal regions despite the supply of seawater is infinite (Supplementary Section 2.1 and 2.4). Onshore electric grids are assumed to power electrolysis of reclaimed water, purified tap water and desalinated seawater, while offshore wind power is proposed for driving direct electrolysis of raw seawater. Therefore, direct seawater electrolysis only depends on its available offshore wind power, which is determined by the capacity of installed offshore wind engines and effective operation time (Supplementary Section 2.1 and 2.4). Both routes for seawater electrolysis are only viable in coastal provinces. The study will consider the future growth in electricity supply and demand by water electrolysis when forecasting the H\u003csub\u003e2\u003c/sub\u003e production potential. This ensures that the electricity supply for other essential industrial and domestic purposes remains unaffected, since water electrolysis is an energy-intensive industry.\u003c/p\u003e\n\u003ch3\u003eDetermination of electrolysis efficiency\u003c/h3\u003e\n\u003cp\u003eHerein, the pure water produced from reclaimed water, tap water and seawater by RO modules (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) is assumed to meet the quality of electrolysis-grade water with 99.7~% salt rejection (Supplementary Section 3.1)\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. The technical parameters for pure water electrolysis are obtained from the cases of large-scale commercial alkaline electrolyzer (Supplementary Section 3.2), which indicates that production of 1 kg H\u003csub\u003e2\u003c/sub\u003e needs 10\u0026ndash;15 kg pure water and 44.0\u0026ndash;49.3 kWh electricity\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Actual use of water and electricity will increase if water loss and electricity consumption by RO are included. For direct electrolysis of reclaimed water or raw seawater, competitive reactions resulted from pollutants can randomly influence the electrolyzer performance\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, therefore, a multi-parameter regression model based on data from references and laboratory measurements was employed to predict the electrolysis efficiencies (Supplementary Section 3.3). These procedures evaluate the specific water and electricity consumption as well as the H\u003csub\u003e2\u003c/sub\u003e production capacity of each route based on the available resources (water and electricity). The H\u003csub\u003e2\u003c/sub\u003e equilibrium of different regions is established by subtracting H\u003csub\u003e2\u003c/sub\u003e demand from H\u003csub\u003e2\u003c/sub\u003e production in these regions, revealing the nationwide H\u003csub\u003e2\u003c/sub\u003e supply-demand relationship (Supplementary Section 3.4).\u003c/p\u003e\n\u003ch3\u003eDetermination of carbon emissions and cost\u003c/h3\u003e\n\u003cp\u003eCarbon emissions of different H\u003csub\u003e2\u003c/sub\u003e production routes depend on the calibrated LCI with categorization of wastewater and water treatment, electrolysis, and H\u003csub\u003e2\u003c/sub\u003e transport. The LCI of materials, chemicals, and energy is determined from industrial processes (Supplementary Section 1.2\u0026ndash;1.4). The LCI of alkaline electrolyzer is calibrated by its installed capacity. The LCI of H\u003csub\u003e2\u003c/sub\u003e transport encompasses 30 MPa compression and construction of H\u003csub\u003e2\u003c/sub\u003e-embrittlement-resistant pipes and ships. The carbon intensity of electricity is determined by the renewable weighting of current electric grids and carbon emission factors of different power types (Supplementary Section 1.5). The infrastructures\u0026rsquo; carbon emissions are distributed evenly over their lifespan. All carbon emissions above contribute to the levelized carbon emissions of H\u003csub\u003e2\u003c/sub\u003e (LEOH). The levelized cost of H\u003csub\u003e2\u003c/sub\u003e (LCOH) includes fees for wastewater and water treatment, electrolysis, H\u003csub\u003e2\u003c/sub\u003e transport and a carbon tax (Supplementary Section 1.6) derived from an average level of current pilot carbon-trading market in China\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eSpatial distribution of electrolytic H production from reclaimed water and other water resources\u003c/h3\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eNationwide H\u003csub\u003e2\u003c/sub\u003e supply and demand\u003c/h2\u003e \u003cp\u003eFor direct electrolysis of reclaimed water, the capacity of H\u003csub\u003e2\u003c/sub\u003e production calculated using a total amount of available reclaimed water is 993.2\u0026thinsp;\u0026plusmn;\u0026thinsp;149.1\u0026nbsp;million tons in 2021 (5\u0026ndash;95% confidence interval based on 100,000 trials in a Monte Carlo simulation, same uncertainty is hereinafter used) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), which is much higher than that of 104.8\u0026thinsp;\u0026plusmn;\u0026thinsp;10.8\u0026nbsp;million tons based on available electricity. This indicates that reclaimed water reserves are not a bottleneck for electrolytic H\u003csub\u003e2\u003c/sub\u003e production, which has a huge growth potential in the future with increase of electric power. In practice, if the availability of both water and electricity as well as the spatial distribution of these resources are taken into consideration, H\u003csub\u003e2\u003c/sub\u003e production capacity is 95.7\u0026thinsp;\u0026plusmn;\u0026thinsp;9.8\u0026nbsp;million tons, which can essentially meet the national demand of 33.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7\u0026nbsp;million tons in 2021 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). If the reclaimed water is pre-purified by RO before being fed into electrolyzer, as practiced in the conventional electrolysis\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, it exhibits a H\u003csub\u003e2\u003c/sub\u003e potential of 96.0\u0026thinsp;\u0026plusmn;\u0026thinsp;9.4\u0026nbsp;million tons determined by both water and electricity resources. This capacity is slightly larger than that by direct electrolysis of reclaimed water, as a higher electrolytic efficiency can be obtained using pure water. Given the membrane technologies for water treatment are gradually matured recently\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e, electrolysis of purified reclaimed water might be a transitional solution at present, despite an additional expense for pre-purification generated. In contrast, the current electrolysis of pure water produced from tap water by RO only shows a H\u003csub\u003e2\u003c/sub\u003e production capacity of 32.1\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6\u0026nbsp;million tons based on all resources. Moreover, analyses of H\u003csub\u003e2\u003c/sub\u003e equilibrium across different provinces in China indicate that the reclaimed water routes enable the more provinces to locally produce H\u003csub\u003e2\u003c/sub\u003e and satisfy their H\u003csub\u003e2\u003c/sub\u003e demand compared to other routes (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), thereby diminishing extensive inter-regional resource disparity, and reducing the distances of H\u003csub\u003e2\u003c/sub\u003e transport and associated cost. These results highlight the superior decentralization and flexibility of reclaimed water as a feedstock for H\u003csub\u003e2\u003c/sub\u003e production. However, the direct electrolysis of reclaimed water, despite avoiding purification expenses, will be challenged by the presence of interferential pollutants in the water. For example, the precipitation of calcium or magnesium hydroxide under alkaline environment may lead to a decline in electrodes performance. The residual chloridion generated from reclaimed water chlorination will accumulate in electrolyzer during continuous exhaustion of water by electrolysis\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e, which may impact electrolysis efficiency, as chloridion oxidation competes with oxygen evolution reaction (Supplementary Section 3.5)\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. The presence of other micropollutants, such as organics and emerging pollutants, in the reclaimed water has recently shown negative or positive impacts on the electrolytic H\u003csub\u003e2\u003c/sub\u003e production\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. In order to achieve an optimal balance between electrolysis performance and cost, some practical processes, such as exempting chlorination, deionization by ion exchange resin and physical absorption etc., can be applied for the pretreatment of reclaimed water instead of RO process. The recent advances in direct electrolysis of seawater\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e will also encourage to electrolyze raw reclaimed water.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe H\u003csub\u003e2\u003c/sub\u003e production through electrolysis of desalinated or raw seawater is geographically confined to the coastal provinces. For electrolyzing desalinated seawater, the desalination capacity constrains the maximum H\u003csub\u003e2\u003c/sub\u003e production capacity to 21.6\u0026thinsp;\u0026plusmn;\u0026thinsp;5.6\u0026nbsp;million tons. This hinders most coastal provinces achieving H\u003csub\u003e2\u003c/sub\u003e self-sufficiency, though their electricity supply is sufficient for producing 72.1\u0026thinsp;\u0026plusmn;\u0026thinsp;3.7\u0026nbsp;million tons H\u003csub\u003e2\u003c/sub\u003e that can meet the national demand (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). In practice, the provinces capable of seawater desalination (e.g., Shandong, Hebei, and Jiangsu) often suffer from a shortage of freshwater resources\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e, the desalination would first meet the basic demand of domestic and industrial sectors. Therefore, the available desalinated seawater for electrolysis might be less than that based on desalination capacity, resulting in less H\u003csub\u003e2\u003c/sub\u003e production (19.8\u0026thinsp;\u0026plusmn;\u0026thinsp;4.7\u0026nbsp;million tons). Moreover, some coastal regions with developed industries, such as Shanghai, Tianjin, and Zhejiang, don\u0026rsquo;t have easy access to high-quality seawater, leading to a decreased desalination efficiency and H\u003csub\u003e2\u003c/sub\u003e potential (Supplementary Section 2.4). Offshore wind power-driven direct seawater electrolysis recently shows a promise for large-scale green H\u003csub\u003e2\u003c/sub\u003e production due to the advances in electrolytic materials and configuration\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. However, the wind resources in the most offshore areas of Chinese mainland are scarce and unstable \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e (Supplementary Section 2.4). Access to the far offshore wind with strong and stable output for seawater electrolysis will be difficult \u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e, since the infrastructures are challenged by the complex wind platform and long-distance offshore H\u003csub\u003e2\u003c/sub\u003e transport\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. This results in a H\u003csub\u003e2\u003c/sub\u003e potential of merely 2.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u0026nbsp;million tons, which is much lower than the national H\u003csub\u003e2\u003c/sub\u003e demand (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). Moreover, current direct electrolysis of raw seawater remains in an immature stage with high electricity consumption (43.0\u0026ndash;67.3 kWh/kg H\u003csub\u003e2\u003c/sub\u003e) (Supplementary Section 3.5). All challenges mentioned above demonstrate that the electrolytic H\u003csub\u003e2\u003c/sub\u003e production from seawater might need further engineering efforts.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eNationwide cost and carbon emissions of H production\u003c/h3\u003e\n\u003cp\u003eNationwide, the levelized cost of H\u003csub\u003e2\u003c/sub\u003e (LCOH) by electrolyzing reclaimed water (raw or purified) is \u003cspan\u003e$\u003c/span\u003e8.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6/kg H\u003csub\u003e2\u003c/sub\u003e, which is slightly higher than those using conventional purified tap water and desalinated seawater (\u003cspan\u003e$\u003c/span\u003e8.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3/kg H\u003csub\u003e2\u003c/sub\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), except direct electrolysis of raw seawater with a relatively high cost of \u003cspan\u003e$\u003c/span\u003e10.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07/kg H\u003csub\u003e2\u003c/sub\u003e. Herein, the LCOH among several routes are similar, this is because the expense of electrolysis in terms of electricity consumption and electrolyzer construction dominates the LCOH (85.81\u0026ndash;95.38% of the LCOH) and has a small difference among different technologies\u003csup\u003e \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e \u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eHowever, the other cost components of different routes vary significantly. Specifically, the wastewater treatment for producing reclaimed water contributes\u0026thinsp;~\u0026thinsp;0.032% to the LCOH, which is lower than those of tap water treatment (~\u0026thinsp;0.066%, costs of both tap water production and its further purification) and seawater desalination (~\u0026thinsp;0.24%), as the latter treatments often require more expensive chemicals and membranes. If reclaimed water is pre-purified by RO before electrolysis, this pretreatment cost will increase to ~\u0026thinsp;0.092% of the LCOH (Supplementary Section 3.1). The WWTPs/WTPs are usually located in regions with intensive human activities, which allow H\u003csub\u003e2\u003c/sub\u003e produced from reclaimed water or tap water to be closer to the end-users. This results in lower H\u003csub\u003e2\u003c/sub\u003e transport cost, accounting for ~\u0026thinsp;0.49% of the LCOH (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). In contrast, the electrolysis of desalinated seawater leads to a higher cost (~\u0026thinsp;5.38% of LCOH) of transporting H₂ from coastal areas to inland end-users. If H\u003csub\u003e2\u003c/sub\u003e is produced from direct electrolysis of raw seawater on offshore platforms, the H\u003csub\u003e2\u003c/sub\u003e transport cost will be up to ~\u0026thinsp;14.02% of LCOH due to the additional shipping or pipeline transport needed between platforms and coast. Since Chinese national carbon trading scheme has been implemented in power sector\u003csup\u003e \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e \u003c/sup\u003e, the carbon tax of electricity consumption should be included in the cost of electrolytic H\u003csub\u003e2\u003c/sub\u003e production. The current electric grids in China comprise\u0026thinsp;~\u0026thinsp;72.3% of fossil power\u003csup\u003e \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e \u003c/sup\u003e, which leads to a carbon tax of ~\u0026thinsp;4.11% of the LCOH for electrolysis using onshore electricity, except utilization of offshore wind power for direct seawater electrolysis with a lower carbon footprint (a tax of ~\u0026thinsp;0.17% of LCOH). The costs of H\u003csub\u003e2\u003c/sub\u003e produced in different provinces are varied (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). The provinces rich in hydropower (e.g., Sichuan, Yunnan, and Hubei) typically exhibit lower H\u003csub\u003e2\u003c/sub\u003e cost, as hydropower is generally more cost-effective than other power sources\u003csup\u003e \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e \u003c/sup\u003e. All findings above underscore the cost advantages of using reclaimed water for electrolytic H\u003csub\u003e2\u003c/sub\u003e production in terms of feedstock production and H₂ transport. However, these routes currently remain less competitive than H₂ production using fossil-fuels, such as natural gas reforming with a cost of ~\u003cspan\u003e$\u003c/span\u003e4/kg H₂\u003csup\u003e \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e \u003c/sup\u003e. This is due to the substantial electricity consumption by current electrolyzers, which necessitate ongoing advancements in electrolytic efficiency and materials to reduce costs in the future.\u003c/p\u003e \u003cp\u003eSame as LCOH, the levelized carbon emissions of H\u003csub\u003e2\u003c/sub\u003e (LEOH) by electrolyzing reclaimed water (raw or purified) (40.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e) is very similar to that using purified tap water and desalinated water (40.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). Because the carbon emissions dominantly stem from the electricity consumption and electrolyzer fabrication, which account for 99.68\u0026ndash;99.95% of LEOH in these H\u003csub\u003e2\u003c/sub\u003e routes (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). This means that endeavors of carbon mitigation should focus on cleaner production of power and materials.\u003c/p\u003e \u003cp\u003eAlthough the direct electrolysis of seawater using green offshore wind has the lowest LEOH of 1.30\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec) that is well below the LEOH benchmark of green H\u003csub\u003e2\u003c/sub\u003e (\u0026lt;\u0026thinsp;4.9 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e)\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e, its carbon emissions derived from H\u003csub\u003e2\u003c/sub\u003e transport (~\u0026thinsp;0.35% of LEOH) is the highest among all routes. In contrast, transporting H\u003csub\u003e2\u003c/sub\u003e produced from reclaimed water, purified tap water, and desalinated seawater only generates negligible carbon emissions (0.0013\u0026ndash;0.0067% of LEOH). The life-cycle assessment indicates that wastewater treatment for reclaimed water production contributes only\u0026thinsp;~\u0026thinsp;0.047% to LEOH, as it typically employs biological processes that are more environmentally friendly than chemical or membrane treatment used for producing purified tap water (~\u0026thinsp;0.073% of LEOH) and desalinated seawater (~\u0026thinsp;0.32% of LEOH). In the same way, purification of reclaimed water by RO will increase the carbon emissions to ~\u0026thinsp;0.11% of LEOH. Generally, the carbon emissions associated with wastewater/water treatment and H\u003csub\u003e2\u003c/sub\u003e transport remains negligible in the overall carbon emissions. The spatial carbon emissions of H\u003csub\u003e2\u003c/sub\u003e production (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef) also reveals that provinces with abundant renewables (e.g., hydro-, wind-, and photovoltaic power) typically exhibit lower LEOH, underscoring the pivotal role of renewable energy in green H\u003csub\u003e2\u003c/sub\u003e production.\u003c/p\u003e \u003cp\u003eCompared to other electrolytic H\u003csub\u003e2\u003c/sub\u003e routes, electrolysis of reclaimed water in WWTPs can obtain additional carbon and economic retributions. For example, the electrolytic byproduct of O\u003csub\u003e2\u003c/sub\u003e can be used \u003cem\u003ein situ\u003c/em\u003e for aerobic biological treatment, which may decrease aerator energy consumption by 76.4\u0026thinsp;\u0026plusmn;\u0026thinsp;4.9% compared to conventional air aeration (Supplementary Section 3.6). Another potential byproduct of chlorine can serve as a disinfectant of wastewater treatment (Supplementary Section 3.6). These results highlight the comprehensive benefits of electrolyzing reclaimed water. Notably, the current suboptimal electrolysis efficiency merely shifts carbon emissions from power generation to H\u003csub\u003e2\u003c/sub\u003e production at this early stage of energy transition in China. This emphasizes the need of advancements in electrolytic technology and structural transformation of electric power sector to achieve cleaner H\u003csub\u003e2\u003c/sub\u003e production in the future.\u003c/p\u003e\n\u003ch3\u003ePredictions of electrolytic H production from reclaimed water and other water resources in the future\u003c/h3\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e\u003cem\u003ePotential H\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e \u003cem\u003eproduction capacity\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eIn the future, the electrolytic H\u003csub\u003e2\u003c/sub\u003e production is significantly influenced by China\u0026rsquo;s commitment to carbon peaking and neutrality, changes in energy structure and population, available water resources, and advancements in electrolytic technology (Supplementary Section 4.1). The amount of municipal wastewater production is projected to reach a plateau with China\u0026rsquo;s population achieving peak by approximately 2030. This indicates a stable supply of reclaimed water in the future. Consequently, electrolysis of reclaimed water is projected to produce H\u003csub\u003e2\u003c/sub\u003e of 160.9\u0026thinsp;\u0026plusmn;\u0026thinsp;13.7\u0026nbsp;million tons by 2040 and 213.1\u0026thinsp;\u0026plusmn;\u0026thinsp;21.6\u0026nbsp;million tons around 2060 based on both water and electricity resources (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea), which is far higher than the national H\u003csub\u003e2\u003c/sub\u003e demand of 130.9\u0026thinsp;\u0026plusmn;\u0026thinsp;6.5\u0026nbsp;million tons by 2060. In contrast, use of pure water generated from tap water for H\u003csub\u003e2\u003c/sub\u003e production shows a less H\u003csub\u003e2\u003c/sub\u003e potential in the future (55.9\u0026thinsp;\u0026plusmn;\u0026thinsp;4.0\u0026nbsp;million tons by 2040 and 78.1\u0026thinsp;\u0026plusmn;\u0026thinsp;6.3\u0026nbsp;million tons by 2060), indicating that it might not meet the rapid growth of H\u003csub\u003e2\u003c/sub\u003e demand alone after 2040 (Supplementary Section 4.2\u0026thinsp;\u0026minus;\u0026thinsp;4.4). Given that exploiting new freshwater would exacerbate current water scarcity in China\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e, relying on tap water for H\u003csub\u003e2\u003c/sub\u003e production might not be a sustainable option in terms of security of both water resources and H\u003csub\u003e2\u003c/sub\u003e supply. The increase of seawater desalination capacity in China is expected to be modest with an estimated growth of 0.050\u0026thinsp;\u0026minus;\u0026thinsp;0.30\u0026nbsp;million tons/day according to current development\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e and future plans\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e, which suggests that desalinated seawater could supply 79.1\u0026thinsp;\u0026plusmn;\u0026thinsp;17.5\u0026nbsp;million tons of H\u003csub\u003e2\u003c/sub\u003e by 2060 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Similarly, regression analyses of data from the national statistics of China indicate that the actual growth of offshore wind power has been slower than other renewable energy\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e, leading to a predicted H\u003csub\u003e2\u003c/sub\u003e potential of 21.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u0026nbsp;million tons by 2060 through direct electrolysis of raw seawater (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). However, it is noteworthy that recent technical breakthroughs in GW-level offshore wind turbines in China\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e might significantly enhance the H\u003csub\u003e2\u003c/sub\u003e potential of this route in the future.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eFuture cost and carbon emissions of H\u003csub\u003e2\u003c/sub\u003e production\u003c/h2\u003e \u003cp\u003eThe LCOH of electrolyzing reclaimed water and purified seawater/tap water are projected to increase to \u003cspan\u003e$\u003c/span\u003e11.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u0026ndash;\u003cspan\u003e$\u003c/span\u003e11.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1/kg H\u003csub\u003e2\u003c/sub\u003e by 2040 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), which would mainly result from the cost increase in the electricity supply during this period (Supplementary Section 4.3). This fluctuation of electricity price could be attributed to the massive investments of renewable facilities, renewable intermittency and electric grids expansion, despite a significant decrease in the cost of renewable technologies themselves\u003csup\u003e \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e \u003c/sup\u003e. Then, the LCOH would gradually decrease to \u003cspan\u003e$\u003c/span\u003e4.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 kg H\u003csub\u003e2\u003c/sub\u003e by 2050 and \u003cspan\u003e$\u003c/span\u003e3.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4/kg H\u003csub\u003e2\u003c/sub\u003e by 2060 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) when the more efficient electrolytic technologies are used, and the electric grids and H\u003csub\u003e2\u003c/sub\u003e transport networks become more economical. Similarly, the LCOH of raw seawater electrolysis would first increase to \u003cspan\u003e$\u003c/span\u003e14.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2/kg H\u003csub\u003e2\u003c/sub\u003e by 2030 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) due to the external costs derived from offshore wind electricity\u003csup\u003e \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e \u003c/sup\u003e and H\u003csub\u003e2\u003c/sub\u003e transport infrastructures\u003csup\u003e \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e \u003c/sup\u003e, and then it would drop to \u003cspan\u003e$\u003c/span\u003e4.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1/kg H\u003csub\u003e2\u003c/sub\u003e by 2060, but it is still higher than the other routes due to lower electrolytic efficiency. The current price of fossil H\u003csub\u003e2\u003c/sub\u003e is ~\u003cspan\u003e$\u003c/span\u003e4.0/kg H\u003csub\u003e2\u003c/sub\u003e (without carbon taxes)\u003csup\u003e \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e \u003c/sup\u003e or ~\u003cspan\u003e$\u003c/span\u003e4.3/kg H\u003csub\u003e2\u003c/sub\u003e (with carbon taxes)\u003csup\u003e \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e \u003c/sup\u003e, indicating that the electrolytic H\u003csub\u003e2\u003c/sub\u003e produced from reclaimed water would not be competitive with fossil H\u003csub\u003e2\u003c/sub\u003e until 2050, if there is no a breakthrough in electrolytic technologies and the changes in environmental policies. Therefore, China is encouraged to provide more economic subsidies to the electrolytic H\u003csub\u003e2\u003c/sub\u003e production and associated research and development, or impose more carbon taxes on the petrochemical H\u003csub\u003e2\u003c/sub\u003e industry, ensuring that the electrolytic H\u003csub\u003e2\u003c/sub\u003e routes are more commercially competitive.\u003c/p\u003e \u003cp\u003eThe LEOH of electrolyzing reclaimed water and purified seawater/tap water are expected to significantly decline from the current high levels to 4.99\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e by 2050 and 2.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e by 2060 with the progressive energy transition nationwide and advancements in electrolytic technologies (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec), while LEOH of direct seawater electrolysis are projected to slightly decrease (1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e by 2060). However, the reclaimed water routes are unlikely to achieve the LEOH benchmark of green H\u003csub\u003e2\u003c/sub\u003e before 2050 under the current scenario of energy transition\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. This suggests that China should take more actions to accelerate electric grids decarbonization, such as separation of renewable electricity used for electrolysis from current grids, or integrating the electrolytic H\u003csub\u003e2\u003c/sub\u003e industry with intelligent grids for the flexible access to sustainable or economical electricity resources\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e, and using more sustainable carbon capture and sequestration technologies in power sector\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis study provides a novel insight into the electrolytic H₂ production from municipal reclaimed water, an abundant unconventional water resource in China, which is compared with the other electrolytic H\u003csub\u003e2\u003c/sub\u003e routes by using pure water, desalinated or raw seawater. The analyses show that reclaimed water emerges as a promising alternative of conventional water resources for water splitting mainly due to its huge H\u003csub\u003e2\u003c/sub\u003e production potentials (95.7\u0026ndash;213.1\u0026nbsp;million tons per year between 2021 and 2060), which far meet the national H\u003csub\u003e2\u003c/sub\u003e demand of 33.4\u0026ndash;130.9\u0026nbsp;million tons per year during this period. In contrast, the pure water or seawater routes can only provide 19.8\u0026ndash;79.1\u0026nbsp;million tons H\u003csub\u003e2\u003c/sub\u003e per year. Although there is a small difference in cost and carbon emissions of H\u003csub\u003e2\u003c/sub\u003e production using different water resources under current technical conditions, reclaimed water highlights its strengths of widespread availability and stable supply, since it is closely associated with human activities and less influenced by seasonal or regional variations compared to other water resources. This spatiotemporal advantage of reclaimed water supports a decentralized H₂ production aligning with the distribution of WWTPs, enabling localized access to water feedstock, electricity, and H₂ markets, thereby reducing the burdens of H₂ transport and resources allocation.\u003c/p\u003e\n\u003cp\u003eHowever, this new route remains challenged. The direct electrolysis of raw reclaimed water with more simplified operation and lower cost is preferred, however, the micropollutants in the reclaimed water may reduce the electrolytic efficiency\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. The impacts of pollutants on the H\u003csub\u003e2\u003c/sub\u003e production and their degradation by electrolytic process need to be further examined. This requires to develop specialized electrolyzers with new catalysts and configuration to alleviate the interferences of pollutants, such as these advancements in direct electrolysis of seawater\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Moreover, China is still in the early stage of electrolytic H\u003csub\u003e2\u003c/sub\u003e development and lacks requisite infrastructures, such as H\u003csub\u003e2\u003c/sub\u003e transmission facilities and refueling stations\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, which are also crucial to support the widespread deployment of electrolyzing H₂ production from the reclaimed water. While the existing natural gas networks can serve as a transitional solution by mixing H\u003csub\u003e2\u003c/sub\u003e with methane\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e, their limited capacity and the security risk\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e necessitate substantial construction of specialized H₂ infrastructure in the future.\u003c/p\u003e\n\u003cp\u003eThe study also emphasizes the importance of green H₂ production to China\u0026rsquo;s commitment of carbon neutrality. The integration of reclaimed water electrolysis with the renewable electricity and state-of-the-art electrolytic technology, therefore, is essential for promoting electrolytic H₂ industry. The latest policy released by the National Energy Administration of China on October 9th, 2024 requires self-consumption of electricity generated by decentralized commercial photovoltaic (PV) with installation capacity of 6\u0026ndash;50 MW rather than trade of it to the electrical grid, while PV with capacity of \u0026lt;\u0026thinsp;6 MW is only allowed to sell residual power on the electricity markets\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e. This shows a great promise to directly use surplus PV electricity to support decentralized H\u003csub\u003e2\u003c/sub\u003e production from reclaimed water. For example, it is foreseeable that the PV deployed in the WWTPs\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e and the roofs of urban buildings\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e can facilitate it since their power generation often exceeds demand. China has installed\u0026thinsp;~\u0026thinsp;254.4 GW of decentralized PV in commercial sectors by the end of 2023\u003csup\u003e58\u003c/sup\u003e. If a half of PV power is used for electrolysis (PV effective operation duration of 2400\u0026ndash;3200 h/year\u003csup\u003e59\u003c/sup\u003e), ~\u0026thinsp;8.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6\u0026nbsp;million tons H\u003csub\u003e2\u003c/sub\u003e can be produced per year from reclaimed water, which nearly meets 25% of China\u0026rsquo;s current H\u003csub\u003e2\u003c/sub\u003e demand. Consequently, this policy is expected to significantly reduce the carbon emissions and costs of H\u003csub\u003e2\u003c/sub\u003e production, as off-grid PV power is widely recognized as lower carbon and cheaper than that of on-grid PV\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Methods","content":"\u003ch2\u003eStatistical data and models\u003c/h2\u003e\n\u003cp\u003eThe amount of provincial municipal wastewater production, ratios of wastewater collection and advanced wastewater treatment, ratio of reclaimed water reuse, and treatment capacity of water treatment plants are provided by the 2021 Urban Constructure Statistical Yearbook\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e, issued by the Ministry of Housing and Urban-Rural Development of China. The water demand for ecological replenishment and provincial amount of freshwater are obtained from the 2021 Water Resources Bulletin\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e, published by the Ministry of Water Resources of China. The solar and wind power potential are obtained from the 2021 Wind and solar Resources Bulletin, published by the China Meteorological Administration\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e. The 2021 Statistical Yearbook\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e64\u003c/span\u003e\u003c/sup\u003e, released by the State Statistics Bureau of China, provides provincial population, primary energy consumption, and the electric power structure. The 2021 National Seawater Utilization Report from the Ministry of Natural Resources of China\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e65\u003c/span\u003e\u003c/sup\u003e, outlines provincial capacity of seawater desalination facilities. The classification of seawater quality is obtained from the 2021 Bulletin of Marine Ecology and Environment Status of China\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e, provided by the Ministry of Ecology and Environment of China. A government report from National Energy Administration of China provides the installed capacity and effective operation time of offshore wind power\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Costs of provincial power generation and electricity transmission are referenced from reports released by the National Development and Reform Commission of China\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e. Standards of water quality is obtained from the open data provided by the Standardization Administration of China\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e68\u003c/span\u003e\u003c/sup\u003e. The parameters of industrial alkaline electrolyzer refers to a commercial system (model A3880)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e. An industrial 3-stage reverse-osmosis system from Dupont \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e is utilized as a model in analysis of water purification for electrolysis. The models for reclaimed water production in WWTPs and tap water production in WTPs were constructed based on China\u0026rsquo;s extensive analyses\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e71\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003ch2\u003eGeographical information system\u003c/h2\u003e\n\u003cp\u003eGeographical data of provinces is sourced from the Resource and Environment Science and Data Center of China\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e72\u003c/span\u003e\u003c/sup\u003e. The wind atlas including wind speed and wind power potential are provided by the World Bank\u0026apos;s Data Catalog\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e73\u003c/span\u003e\u003c/sup\u003e. The list of WWTPs nationwide is obtained from the Ministry of Ecology and Environment of China \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e74\u003c/span\u003e\u003c/sup\u003e. The latitude and longitude of WWTPs are retrieved via the Baidu-Map open-access application programming interface\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e75\u003c/span\u003e\u003c/sup\u003e. The distribution of power plants in China is obtained from Google\u0026apos;s Global Power Plant Database\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e76\u003c/span\u003e\u003c/sup\u003e. Above data is plotted using Python with Geopandas and Matplotlib packages.\u003c/p\u003e\n\u003ch2\u003eParameters normalization\u003c/h2\u003e\n\u003cp\u003eAll values with different unit and order of magnitude in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb are normalized into 0\u0026ndash;100 score using the following Eq.\u0026nbsp;7\u003csup\u003e7\u003c/sup\u003e to ensure the comparability among them.\u003c/p\u003e\n\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e$$\\:Score={\\left[AF\\times\\:\\frac{\\text{l}\\text{n}\\left({P}_{i,j}\\right)}{\\text{l}\\text{n}\\left({Max}_{i,j}\\right)}\\right]}^{2}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eWhere the subscript \u003cstrong\u003ei\u003c/strong\u003e represents the evaluated items, the subscript \u003cstrong\u003ej\u003c/strong\u003e represents the H\u003csub\u003e2\u003c/sub\u003e routes using different water resources, \u003cstrong\u003eAF\u003c/strong\u003e (dimensionless, 10) is the amplification factor, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varvec{P}}_{\\varvec{i},\\varvec{j}}\\)\u003c/span\u003e\u003c/span\u003e is the value of the evaluated items, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varvec{M}\\varvec{a}\\varvec{x}}_{\\varvec{i},\\varvec{j}}\\)\u003c/span\u003e\u003c/span\u003e is the maximum value among a set of the evaluated items.\u003c/p\u003e\n\u003ch2\u003ePrediction data smoothing\u003c/h2\u003e\n\u003cp\u003eThe data of predicted H\u003csub\u003e2\u003c/sub\u003e production, cost and carbon emissions with a ten-year interval undergo a smoothing processing by using a polynomial fitting to illustrate the trend of continuous change.\u003c/p\u003e\n\u003ch2\u003eUncertainty analyses\u003c/h2\u003e\n\u003cp\u003eIn this study, the parameters supporting the evaluations of H\u003csub\u003e2\u003c/sub\u003e production derive from the statistical data and the calibrated industry models with uncertainties. Thus, Monte Carlo simulations with Latin hypercube sampling are employed to quantify the influence of these uncertainties on the evaluation results. All results are then displayed as the mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation within a 5\u0026ndash;95% confidence interval based on 100,000 Monte Carlo simulation trials.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data presented in this manuscript are available in the paper and its Supplementary Information. Source data are provided with this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (52321005), and Shenzhen Science and Technology Program (KQTD20190929172630447).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eL.L. conceived the research\u0026nbsp;conceptualization, manuscript editing and\u0026nbsp;supervision.\u0026nbsp;W.C.\u0026nbsp;conducted the data analysis and wrote the manuscript. X.Y. contributed to the data collection.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eGuo, Y., Peng, L., Tian, J. \u0026amp; Mauzerall, D. L. 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(Earth Engine Data Catalog, 2018); https://developers.google.cn/earth-engine/datasets/catalog/WRI_GPPD_power_plants#terms-of-use.\u003c/li\u003e\n \u003cli\u003ePi, S.\u003cem\u003e\u0026nbsp;et al.\u003c/em\u003e Solar-driven waste-to-chemical conversion by wastewater-derived semiconductor biohybrids. \u003cem\u003eNature Sustainability\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 1673-1684 (2023).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5380448/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5380448/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe large-scale electrolytic H\u003csub\u003e2\u003c/sub\u003e production requires a steady supply of both electricity and water as well as nearby H\u003csub\u003e2\u003c/sub\u003e demands, which is usually challenged in China due to a spatiotemporal mismatch of these resources. This necessitates reconfiguring them or finding alternatives. Reclaimed water produced from municipal wastewater treatment plants shows widespread distribution with human activities and huge reserves. Here, we conducted a life-cycle assessment of H\u003csub\u003e2\u003c/sub\u003e production from reclaimed water in terms of potential capacity, cost and carbon emissions based on available water and electricity and H\u003csub\u003e2\u003c/sub\u003e market in each province of China, which was compared to that using pure water and seawater. Reclaimed water with easy access to nearby electricity and H\u003csub\u003e2\u003c/sub\u003e demand shows a great H\u003csub\u003e2\u003c/sub\u003e production potential of 95.7–213.1 million tons per year between 2021 and 2060, which can theoretically meet the national H\u003csub\u003e2\u003c/sub\u003e demand (33.4–130.9 million tons) alone and allows H\u003csub\u003e2\u003c/sub\u003e self-sufficiency in most provinces. In contrast, other electrolytic H\u003csub\u003e2\u003c/sub\u003e routes only have a potential of 19.8–79.1 million tons. The\u0026nbsp;current cost ($8.8 ± 0.6/kg H\u003csub\u003e2\u003c/sub\u003e) and carbon emissions (40.4 ± 0.7 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e) of H\u003csub\u003e2\u003c/sub\u003e produced from reclaimed water are comparable to other electrolytic approaches, but expect to reach $4.0 ± 0.5 kg H\u003csub\u003e2\u003c/sub\u003e and 4.99 ± 0.05 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e by 2050 due to advances in electrolysis technology and electricity decarbonization, indicating a highly competitive performance with those of fossil H\u003csub\u003e2\u003c/sub\u003e (~$4.3/kg H₂) and defined green H\u003csub\u003e2\u003c/sub\u003e (~4.9 kg CO\u003csub\u003e2\u003c/sub\u003e-eq/kg H\u003csub\u003e2\u003c/sub\u003e). This study provides a new insight into sustainable electrolytic H₂ production by using unconventional water resources.\u003c/p\u003e","manuscriptTitle":"High Spatiotemporal Availability of Hydrogen by Electrolysis of Municipal Reclaimed Water in China","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-25 11:35:42","doi":"10.21203/rs.3.rs-5380448/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1fc165d9-f4cd-4120-986f-50f1b6531a57","owner":[],"postedDate":"November 25th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":40515132,"name":"Earth and environmental sciences/Environmental sciences/Environmental impact"},{"id":40515133,"name":"Physical sciences/Energy science and technology/Renewable energy/Hydrogen energy"},{"id":40515134,"name":"Scientific community and society/Water resources"},{"id":40515135,"name":"Scientific community and society/Energy and society/Energy supply and demand"},{"id":40515136,"name":"Physical sciences/Energy science and technology/Energy infrastructure/Energy grids and networks"}],"tags":[],"updatedAt":"2024-11-25T11:35:42+00:00","versionOfRecord":[],"versionCreatedAt":"2024-11-25 11:35:42","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5380448","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5380448","identity":"rs-5380448","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}