Lower-Carbon Hydrogen Production from Wastewater: A Comprehensive Review

Abstract

Hydrogen has the capability of being a potential energy carrier and providing a long-term solution for sustainable, lower-carbon, and ecologically benign fuel supply. Because lower-carbon hydrogen is widely used in chemical synthesis, it is regarded as a fuel with no emissions for transportation. This review paper offers a novel technique for producing hydrogen using wastewater in a sustainable manner. The many techniques for producing hydrogen with reduced carbon emissions from wastewater are recognized and examined in detail, taking into account the available prospects, significant obstacles, and potential future paths. A comparison of the assessment showed that water electrolysis and dark fermentation technologies are the most effective methods for hydrogen generation from wastewater, with microbial electrolysis and photofermentation. Thus, the incorporation of systems that are simultaneously producing lower-carbon hydrogen and meant for wastewater treatment is important for the minimization of emissions from greenhouse gases and recovering the energy utilized in the treatment of wastewater.

1. Introduction

There has been a significant demand for global energy due to population inflation, hence there is a need for the development of new resources [1]. Global energy consumption reached over 19 TW in 2019, up nearly 1.3% from the previous year’s [2]. The production of energy has relied mostly on fuels based on carbon for hassle-free and efficient operation; an estimate states that about 84% of main power sources consist of coal, natural gas, and oil. Lower-carbon hydrogen is predicted to meet roughly 24% of the world’s energy needs by 2050, which contributes to a significant curtailment of about one-third in the emission of greenhouse gases [3]. Solar energy is the most pertinent source among various renewable energy alternatives for zero-carbon sustainable hydrogen generation [4]. Raw thermal power for thermochemical cycles is one method of producing hydrogen using solar energy [5], electrolytic cells, photobiological processes, and photocatalysis [6,7]. Generally, hydrogen production from electrolysis can be derived from the below-given equation:

H₂O + Electricity (232.2 kJ/mol) = H₂ + 1/2 O₂ + Heat (48.6 kJ/mol)

(1)

Since the intended electrolysis efficiency is directly correlated with the process’s overall cost, the electrolyser efficiency is an essential parameter in the generation of hydrogen [8]. Theoretically, according to Equation (1), nearly 1 kg of H₂ production needs roughly nine kilograms of fresh water by the water hydrolysis method. According to researchers’ estimates, the energy content of wastewater ranges from 17.8 to 28.7 kJ/g chemical oxygen demand, corresponding to around between three and four times that which is needed for waste treatment [9]. The incorporation of systems that are concurrently producing hydrogen and meant for wastewater treatment is important for the minimization of emissions from greenhouse gases and recovering the energy utilized in the treatment of wastewater [10]. Several studies covered the synthesis of hydrogen from wastewater in different contexts, both with and without light. Examples of processes that rely on light include photofermentation, solar energy conversion, and photoelectrochemical systems, while examples of processes that are light-independent include dark fermentation, electrochemical reactions, and Microbial Electrolysis Cells (MECs) [11]. Preeti et al. [12] discussed how different operating settings affect the generation of dark fermentative biological H₂ from industrial wastewater. Banau et al. reported [13] the latest advancements, improved plans, financial considerations, and expansion of dark fermentative hydrogen generation. It was also emphasized that researchers should switch from artificial to actual wastewater substrates and that there is a chance to combine treatments with simultaneous bioremediation. Given its supremacy in generating high yields of lower-carbon hydrogen, microbial electrolysis has been regarded as one of the state-of-the-art methods for producing lower-carbon hydrogen utilizing lower energy in comparison to water electrolysis, thereby reducing chemical oxygen demand [14,15]. Thus, several studies have put forth alternatives for reducing energy demand using potentiate, self-sufficient renewable sources of energy. In addition, wastewater treatment methods and electrolytic hydrogen production are generally classified as energy-intensive techniques. Thus, taking all these things on board, renewable and environmentally friendly energy sources are required that are alternative to conventional sources of energy that are the major pollution sources [16]. Although various reviews have reported work on hydrogen production, there is still a void for a thorough evaluation regarding the production of lower-carbon hydrogen from wastewater. Thus, by undertaking a thorough assessment, this work seeks to examine potential difficulties, advantages, and paths for producing hydrogen with integrated wastewater management.

2. Wastewater Analysis

It must be noted that the wastewater characteristics are directly dependent upon the originating source. The major wastewater sources are storm waters and industrial and domestic wastewater. The understanding of the characteristics of wastewater is one of the crucial factors for the design and management of water treatment systems. The only goal of conventional wastewater treatment systems is getting rid of all the pollutants in the wastewater. However, extra energy resources and chemicals are also needed as the typical techniques of treating wastewater. Furthermore, wastewater also often contains precious minerals, acids, bases, salts, and organic compounds with significant energy potential. Reusing and recovering water sources is crucial for sustainable development when treating those water resources. Wastewater is produced nowadays by a variety of industries and sources. Before safely releasing wastewater into the environment, those compounds must undergo substantial treatment. Global wastewater creation is estimated to be over 380 billion m³ annually, and by 2050, it is predicted to have an increase of 51% [17]. However, because of the laws and rules put in place by governments to safeguard water bodies, treating wastewater without reuse and recovery is becoming a difficult task. The literature takes into consideration a wide range of techniques and strategies for treating various kinds of wastewater. The principal objective of conventional wastewater treatment techniques is to ensure that the wastewater complies with government discharge standards [18,19].

The petrochemical sector also produces hydrocarbons, lead, and other pollutants including oil and grease. Metal processing is the foundation of the mining, iron, and steel industries. Meanwhile, biological treatment is challenging due to the antibiotics present in wastewater. Depending on the dye type and method, the textile and dye industries also have high COD/BOD ratios. The variety of contaminants frequently necessitates the coordinated application of several treatment techniques and thorough plant design.

3. Hydrogen Production Technologies and Simultaneous Wastewater Treatment

There are four primary mechanisms for producing hydrogen: biological, electrical, photonic, and thermal. These primary techniques then split off into several secondary techniques. As there are numerous well-established ways of producing hydrogen, many more are continuously being developed. On the other hand, the scientific field of hydrogen recovery from wastewater is more limited. Thus, to attain sustainability, an integrated strategy proposed by Diner and Accra is taken into consideration, depicted in Figure 1 [20,21]. Providing a lower-carbon and sustainable energy solution requires considering the demands of each individual. When selecting an energy source, further considerations should be made at the beginning of the process, such as cost-effectiveness, sustainability, abundance, accessibility, reliability, and safety [22].

However, for a more accurate comparison and evaluation, a few noteworthy studies from before this period have also been included. The following forms the basis of the primary standards for the present review process:

• Research on treating wastewater by producing hydrogen;
• Research assessing the effectiveness of wastewater treatment;
• Studies assessing the production of hydrogen by wastewater treatment.

3.1. Biological Treatment

3.1.1. Bio Photolysis

By using water-splitting with the help of microorganisms, a process known as bio photolysis can produce hydrogen with a lower carbon content. Under illumination, biophotolysis can take place either directly or indirectly, producing O₂ and H₂ [23,24]. Direct biophotolysis involves the active participation of hydrogenase enzymes and photosystems (PS1 and PS2). Most cyanobacteria can be used for indirect biophotolysis. H₂ and H₂O are produced via dark fermentation, which comes after photosynthesis. Prior to the hydrogen generation when the hydrogenase enzyme is present, CO₂ is transformed into an endogenous reserve carbohydrate [25]. Though biophotolysis is a promising method for producing hydrogen, there are some significant drawbacks to its utilization. Less than 2% of the process’s energy is converted into light by the hydrogenase enzyme, which is highly sensitive to oxygen. To address these issues, recent research on biophotolysis has been conducted [26]. Using sun energy, carbon-based substrates are converted into hydrogen by the photofermentation process, which uses photosynthetic microorganisms.

$$6{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O} \stackrel{PS and hv}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$$

(2)

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{H}}_2\mathrm{O} \stackrel{Hydrogenase}{\to }12{\mathrm{H}}_2\mathrm{O}+6\mathrm{C}{\mathrm{O}}_2$$

(3)

Compared to other hydrogen production techniques, this approach produces less H₂ even if it can treat wastewater. Commercial development is slowed by the process’s strong reliance on microbiological microbial activity. For the process to be optimized or improved, metabolic and genetic engineering are therefore needed. Yongzhen Tao et al. [27] employed Rhodobacter sphaeroides, a purple non-sulphur bacterium, to create hydrogen from different carbon sources. Depending on the carbon source, energy efficiency might range from 52.6% to 89.7%. Purple non-sulphur bacteria have been utilized in treating various types of waste, including vegetable waste, dairy wastewater, tofu wastewater, palm oil mill effluents, soy sauce wastewater, and olive mill wastewater [28,29].

Liu et al. [30] developed the synthesis of H₂ from glucose, fructose, and sucrose and identified a novel strain of Rhodopseudomonas sp. from bioreactor sludge. Using photofermentation, valuable compounds can also be produced. Using photofermentation, Policastro et al. [31] were able to create hydrogen and poly-β-hydroxybutyrate. According to reports, the production of hydrogen was 468 mL/L and poly-β-hydroxybutyrate was 1500 mg/L, respectively.

Biophotolysis can be categorized into two types: indirect biophotolysis and direct biophotolysis [32]. The former involves solar radiation-induced direct breakdown of a substrate, often water, by algae and cyanobacteria in an anaerobic environment to create hydrogen. By reducing a proton with electrons that are liberated by the ferredoxin reduction of the cell’s hydrogenase enzyme, hydrogen is created. The equation below illustrates the procedure [33]:

2H₂O + Solar radiation → 2H₂ + O₂

(4)

In indirect biophotolysis, stored carbohydrates are oxidized to create hydrogen. The stored carbon supply created during photosynthesis when CO₂ is fixed via the Calvin cycle is the source of the electrons required in this process. Cyanobacteria use a process known as photosynthesis to change CO₂ and H₂O into organic molecules and O₂.

3.1.2. Dark Fermentation

In dark fermentation, bacteria harness organic substrates to generate energy and electrons, leading to hydrogen (H₂) generation (Hallenbeck et al. [23]). Since this process does not depend on sunlight, the bioreactor design is less complex compared to other biological methods. Additionally, wastewater can be treated in conjunction with hydrogen production. However, the entire procedure is restricted to anaerobic digesting principles.

Combining dark fermentation and photofermentation offers an effective approach to overcoming the limitations inherent in both processes. Dark fermentation is among the most extensively studied biological methods for hydrogen production. However, because hydrogen is produced alongside volatile fatty acids as a by-product, the overall hydrogen yield is limited [26]. To address this challenge, biological systems can be used sequentially in combination with other strategies. For instance, Laurinavichene et al. [34] applied a sequential process integrating dark- and photofermentation, achieving hydrogen production up to 17.6 L H₂/L.

Furthermore, diverse studies have focused on integrating photo and dark-fermentation processes to enhance H₂ generation efficiency and treatment outcomes. Chookaew et al. [35] examined a two-stage process for biohydrogen production from crude glycerol. The complete system produced 6.45 mmol H₂/g COD of hydrogen in total. To improve the pace at which hydrogen was produced, the photofermentation was fed the effluent from the dark fermentation. Researchers did point out that in order to produce enough H₂, the photofermentation influent needs to be diluted five times. Clostridium butyricum and Rhodopseudomonas palustris were employed by Yilmazer Hitit et al. [36] in an integrated dark- and photofermentation process aimed at reducing COD (chemical oxygen demand) and producing hydrogen. The process achieved 97% COD removal, with a total output of 7.21 mmol H₂/g COD.

3.1.3. Photofermentation

Photofermentation is a process where organic entities are converted into hydrogen and CO₂ by photosynthetic bacteria in the presence of sunlight. This transformation is driven by purple non-sulphur (PNS) bacteria and certain lower-carbon sulphur bacteria, such as Rhodobacter spheroids and Chlorobium vibrioforme. These microorganisms harness sunlight to convert carbon-rich compounds, like fatty and organic acids, into hydrogen. Through the action of nitrogenase enzymes, photosynthetic or anaerobic bacteria decompose organic matter under light, generating hydrogen as a byproduct [32,37]. In photo-heterotrophic conditions, organic compounds and light serve as the sources of carbon and energy, respectively, for developing purple non-sulphur bacteria-based H₂ generation [26]. Below is a display of the overall responses:

2CH₃COOH + 2H₂O → 2CO₂ + 4H₂ (ΔG^o = +104 kJ)

(5)

C₆H₁₂O₆ + 6H₂O → 6CO₂ + 12H₂ (ΔG^o = +3.2 kJ)

(6)

The carbon-to-nitrogen ratio, pH, and light intensity are key factors influencing the photofermentation process. Many studies have investigated the effects of these variables [38]. Al-Mohammedawi et al. [39] demonstrated the photofermentation process of Rhodobacter sphaeroids DSM 158 at a pH of 7.4, a carbon-to-nitrogen ratio of 27.5, and a light intensity of 126 W/m². Under these conditions, a biohydrogen production rate of 41.74 mL/L·h and a light conversion efficiency of 0.31 were achieved. Recently Policastro et al. [40] showed that nine single pure cultures (PCs)—two photofermentative and seven dark fermentative strains—were used in the technique, which was executed in sterile conditions. Further studies were conducted to validate the effectiveness of co-fermentative conditions using phototrophic consortia (PCs) and microbial cultures (MC). The results showed that the initial mixed culture yielded the highest output, reaching 290 NmL H₂ L⁻¹.

3.1.4. Photoelectrochemical Methods

Two accessible sources of solar energy and water are used in photoelectrochemical (PEC) cells to directly convert photonic energy to hydrogen [41]. A semiconductor/electrolyte junction combines photoelectron–chemical water splitting and solar-to-electrical energy conversion in PEC systems. In PEC systems, two electrodes are utilized: the working and the counter electrode. Platinum (Pt) is commonly used as the counter electrode, while the working electrode is made of either n- or p-type semiconductors [42,43]. Most PEC systems incorporate reference electrodes alongside the working and counter electrodes, allowing for a detailed examination of the half-reactions occurring within the cell [44]. At the working electrode, photons with energy levels equal to or exceeding the semiconductor’s band gap (Eg) generate pairs of electrons and holes [45]. When a p-type semiconductor serves as the working electrode, the holes capture electrons transported from the counter electrode due to the oxidation of water into O₂ and H⁺. The resulting electrons are then utilized to reduce H⁺ into H₂ [46].

Thus, an anodic photocurrent is generated by n-type semiconductors as holes migrate toward the electrolyte, while a cathodic photocurrent is produced by p-type semiconductors through the transport of electrons toward the electrolyte, driven by the movement of holes [47]. In p-type and n-type semiconductors, the Fermi energy level is situated above the valence band (VB) and below the conduction band (CB), respectively [48]. An equilibrium is established when the redox potential aligns with the Fermi energy of the semiconductor at the semiconductor/electrolyte interface, halting the transfer of charges. Consequently, even with continued illumination, charge separation and photovoltaic generation come to a stop. In p-type semiconductors, this reaction takes place at the working electrode, while in n-type semiconductors, H₂ is generated at the counter electrode [49]. Important factors in spontaneous water splitting are the placements of the conductor and valance band edges [50]. Under typical working circumstances, such as room temperature and atmospheric pressure, water splitting is a non-spontaneous reaction as a result of a positive Gibbs free energy shift (Equation (7)). In theory, 1.23 eV potential must be applied in order to drive this reaction.

$${\mathrm{H}}_2\mathrm{O}+\mathrm{h}\mathrm{v}\to {\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2 \Delta \mathrm{G}°=238 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(7)

The theoretical energy needed is increased, though, by voltage losses, electrode and component resistances, and recombination of photogenerated e⁻-h⁺ couples. An amount of 1.8 eV is needed to trigger the spontaneous water-splitting process. Consequently, the secret to a successful PEC water splitting is choosing the semiconductor’s (working electrode) ideal band gap. The National Renewable Energy Laboratory (NREL) demonstrated that a solar-to-hydrogen (STH) conversion efficiency of 16.2% was achieved when using GaAs photovoltaic cells as the underneath layer and Ga-InP photoelectrochemical junctions as the upper layer [51].

Additionally, there are more affordable, stable PEC systems based on WO₃, which produce H₂ with STH efficiencies of roughly 3–5% [52]. The STH efficiency refers to the ratio of chemical energy production (H₂) to solar energy input. To calculate the chemical energy output, the molar rate of hydrogen production (ηH₂) is multiplied by the change in Gibbs free energy per mole of H₂ at room temperature (ΔG°). The illumination power density (Pi) and the illuminated semiconductor area (A) are multiplied to determine the solar energy input (Equation (8)):

$$\mathrm{STH} (\%)={\displaystyle \frac{\mathsf{\eta }{\mathrm{H}}_2\times \mathsf{\Delta }\mathrm{G}°}{{\mathrm{P}}_{\mathrm{i}}\times \mathrm{A}}}$$

(8)

3.1.5. Electrolysis

Electrolysis, commonly referred to as “water splitting into hydrogen and oxygen,” is one of the primary methods for hydrogen production. In the framework of a sustainable energy system for the future, water electrolysis is considered a vital element for a range of energy applications, including electricity generation, transportation, heating, and chemical production. Electrons travelling over an external circuit in a continuous cycle power the process. The anode, cathode, electrolyte, and power supply make up an electrolysis unit (Figure 2). There are three primary categories of electrolyser units: solid oxide electrolysers (SOE), alkaline electrolysers, and polymer electrolyte membrane (PEM) electrolysers.

In order to generate hydrogen, Lu et al. [53] investigated the electrolysis of wastewater containing aniline. Yao et al. [54] differently managed an electrolysis cell’s anode/cathode potentials to remove COD and nitrogen. NiCo₂O₄ nanosheets were employed by Wang et al. [55] in an electrolytic cell to generate hydrogen from precursor urea. N-NiZnCu LDH/rGO was synthesized by Hu et al. [56] and investigated with urea, hydrazine, and ammonia for catalytic electrolysis. Wastewater containing these substances can be treated using this catalytic electrolysis. Chen et al. [57] electrolysed wastewater–coal slurry; however, no wastewater characterization was carried out in this investigation. Instead, the wastewater was employed to increase the efficiency of coal electrolysis.

The literature indicates that microbial electrolysis cells (MEC) have been extensively studied. Microbial electrolysis is a novel blueprint for microbial fuel cells (MFCs). In the case of MFC, an alternating current is supplied to generate hydrogen (Figure 3) [58]. Zakaria et al. [59], in their study, utilized a microbial electrolysis cell (MEC) to treat primary sludge from wastewater treatment plants. The study found that the hydrogen production rate was 145 L/m³/day, with a chemical oxygen demand (COD) removal efficiency of 73%. In 2017, Khan et al. [60] explored the application of MEC for hydrogen production from municipal wastewater, discussing plans for a pilot-scale plant in the future. Jayabalan et al. [61] investigated biohydrogen synthesis from sugar industry wastewater using metal oxide/graphene catalysts in MEC. Their findings showed a hydrogen recovery rate of 20.8%. Shen et al. [62] explored MEC technology in hydrothermal liquefaction for wastewater treatment, achieving 90–98% COD removal, 57–93% nitrogen removal efficiency, and a hydrogen production rate of 168.01 ± 7.01 mL/L/day. This method splits water into oxygen and hydrogen using electricity, offering the potential for a highly low-carbon footprint when powered by renewable energy.

3.1.6. Plasmolysis

Plasmolysis is the process of creating plasma by passing water or its vapours through a reactor. In specific, plasma and catalyst-integrated technology (PACT) facilitates plasmolysis, also referred to as plasma reforming [63]. A tubular PACT reactor with two electrodes is enclosed in a quartz tube; the exterior is often made of an aluminium electrode, and the interior is primarily formed of catalytically active metals like Ru, Au, Pd, Ni, and Rh. The plasmolysis process might be carried out using a variety of feed gasses. Corona discharges that generate radicals with high-energy electron streamers are typically employed in chemical synthesis. However, the possibility of low-power input limits corona discharges. Arcing tends to occur when power levels are increased. To prevent this, dielectric barrier discharges are considered crucial for operating a reactor at higher power inputs without causing arcing [64]. According to recent reports, Lozano-Parada et al. have demonstrated the ability to produce ozone at 170 V AC by the use of a microreactor, as opposed to the kilowatts required for a standard ozone generator. The ability to adjust fluid dynamics with extremely short gradients—which are particularly thought to be electronically controlled and precise—is another advantage of microreactors [65]. This could be demonstrated by the fact that the micro-plasma requires less interelectrode distance, which reduces ambipolar diffusion and lowers the energy cost of producing ozone to a reasonable degree of economic viability [66]. By linking power supplies, hydrogen can be produced on-site and distributed directly to consumers. This allows for the immediate production and delivery of hydrogen to combustion plants, reactors, or fuel cells [67].

One of the most recent innovations is the usage of micro-plasmas. Process control is improved by micro-plasma and reduced power consumption compared to conventional plasma reactors. The plasma microreactor can also be ignited using battery sources [68,69]. By integrating power supplies, hydrogen can be generated locally and distributed directly to users. This enables the immediate production and delivery of hydrogen to combustion plants, reactors, and fuel cells. This approach could potentially resolve the “hydrogen storage challenges,” which are regarded as one of the key obstacles to the widespread use of hydrogen in both light and heavy vehicles [70]. There are numerous plasma reactors available that can produce hydrogen with varying power consumption and feedstock. The process of plasmolysis to produce hydrogen is fraught with issues. Dissolution is one of the key factors affecting the efficiency of hydrogen production. To enhance dissolution and overall system performance, several kinetic models have been developed. Chen et al. reported that 0.32 mol% of hydrogen was converted. This conversion was achieved when plasma was ignited at 2.5 kV with 14.2% water content in an argon environment [71].

Another major challenge in hydrogen production through water vapour plasmolysis is the separation process. Various techniques such as pressure swing adsorption (PSA), compression, heat exchange, and cryogenic distillation are employed to separate the hydrogen [72,73]. The hydrogen manufacturing system’s capital costs are significant as a result of these labour-intensive procedures. Another method for separating hydrogen is membrane separation; however, separation is less effective due to issues with ageing, permeability, and selectivity [74]. By adding microbubbles to the system, which raises the mass transfer coefficient, the separation of H₂ could be made more economical and efficient. Because oxygen dissolves in water 25 times more readily than hydrogen does, hydrogen-rich gas would thus emerge from the top side if the product gases could be introduced via a sizable reactor column using the microbubble technique [75]. Broadly speaking, there are three approaches to microbubble generation. First, the compression and release of gas through a nozzle utilizing an air stream. In the second class, local cavities are created in the ultrasonic waves using power ultrasound. The fluids of the third class are oscillated by mechanical vibration or fluidic oscillations. A fluidic oscillator might create the smallest volume of hemispherical-shaped bubbles. It is thought to be the least expensive and requires the least amount of maintenance because it is made entirely of stationary parts. It may create microbubbles between 80 and 120 μm with an observed mass transfer coefficient of roughly 55% when compared to a steady flow that uses the same diffuser to create bubbles of around 1 mm [75]. Hence, raising the mass transfer coefficient aids in solving the dissolving issue. Plasma reactors produce hydrogen and oxygen, which must be separated. Consequently, in cold climates, an ice trap filled with frozen liquid at a temperature of −1.5 to −2 °C is used to separate the exit steam dissociation species [76]. The condensation of uncondensed water could prevent the recombination of H₂ and O₂. After that, it would be possible to separate the water and create dry gas. The flow meter and catalytic hydrogen sensor receive the dry gases as they exit the condenser. As suggested, inert argon gas might be recycled; Figure 4 illustrates this approach.

Another barrier to the large-scale economic viability of plasmolysis is the storage of H₂. A number of storage techniques, such as complex metal hydrides, ammonia liquid carriers, porous H₂ storage materials, relatively high entropy alloys [78], and liquid hydrogen carriers, were proposed as various, secure, and efficient hydrogen energy carriers. The liquid state is regarded by the global hydrogen energy industry as a viable choice for large-scale hydrogen storage and transportation [79]. Recently, there has been a sharp decline in the prices of electricity auctions in regions with favourable solar and wind conditions for lower-carbon energy installations. The advantage of low prices showed the potential for energy supply chains to use a carrier to move lower-carbon energy from wealthy renewable locations. Energy may be transported economically by converting renewable energy sources into hydrogen and transporting that hydrogen across great distances in liquid form. For isolated locations without direct grid connections, it can be a good option because of its great potential for producing renewable energy.

3.1.7. Photoreforming of Lignocellulose into Hydrogen

Photoreforming is a novel method that turns lignocellulose biomass like agricultural waste into hydrogen gas by harnessing light energy. This strategy is a component of a larger initiative to create renewable and sustainable energy sources. Over the years, lignocellulose (a renewable source of carbon feedstock) has been explored for its potential to yield important chemicals and fuels [80]. It is generated yearly by photosynthesis and has the most intricate structure on the planet [81]. Crop leftovers and non-agricultural regions can provide this type of biomass. Theoretically, these residues can generate 50–85 EJ/year of energy, or around 20% of the energy needs of society, with 3–5 Gt/year of productivity globally [82]. The two primary methods for developing lignocellulose have been biological and chemocatalytic procedures. While lignocellulose can be broken down through fermentation and enzymatic catalysis using enzymes, bacteria, and microorganisms in biological processes, this method remains inefficient due to the complex structure of lignocellulose, which hinders enzyme access to the biomass. Lignin, the primary component of lignocellulose, has attracted significant attention for its abundance and renewable nature. Compared to cellulose and hemicellulose, lignin possesses a more diverse, complex, and stable structure. Furthermore, the brown colour of lignin might have an impact on how well the photocatalyst absorbs light. As a result, photoreforming lignin to generate hydrogen is comparatively more challenging. Nevertheless, the lignin photoreforming for hydrogen evolution method has the potential to produce certain aromatic hydrocarbons, which suggests significant industrial importance. There is ongoing debate about whether lignin behaves similarly across different processes. However, due to its complex structure, researchers often begin by studying its hydrogen generation activity in photoreforming using a model lignin compound like sinapyl alcohol. In one study using activated NCNCNx and NiP catalysts, the photoreforming of sinapyl alcohol resulted in hydrogen production at a rate of 0.04 mmol g⁻¹ h⁻¹, whereas lignin under identical conditions produced only 0.002 mmol g⁻¹ h⁻¹ [83]. Li et al. [84] developed a homogeneous one-dimensional NiS/CdS nanocomposite for hydrogen production. When using lactic acid and lignin as h⁺ scavengers, the photoreforming process achieved a maximum hydrogen yield of 1.51 mmol g⁻¹ h⁻¹ with an apparent quantum efficiency (AQE) of 44.9%. The use of lignin and lactic acid as h⁺ scavengers also resulted in the formation of several liquid products, including alcohols and their derivatives. As an alternative, CdS quantum dots can be used to photoreform lignin in an alkaline aqueous solution [85]. Because CdS/CdOX has a lower band gap, it can absorb visible light that is longer than 420 nm. Enhanced photoreforming efficiency is ensured, as lignin displays a strong absorption peak at 300 nm and a shoulder peak at 350 nm, suggesting it does not compete with the photocatalyst for light. Using the CdS/CdOX catalyst, hydrogen was produced at a rate of 0.26 mmol g⁻¹ h⁻¹ with lignin loadings of 0.25 mg mL⁻¹. However, the majority of research on photoreforming lignin is still in its early phases. Lignin’s varied and complicated structural properties continue to make photocatalytic reforming a challenging process.

3.2. Non-Renewable Hydrogen Production Sources

3.2.1. Steam Methane Reforming (SMR)

SMR is a tried-and-true technique for producing hydrogen as it produces more hydrogen than electrolysis, accounting for over 80% of the total [86]. To heat the system and remove sulphur, which can contaminate and hinder hydrogen production, as well as catalyst performance, other hydrocarbons containing methane are initially introduced into the steam system [87]. Sulphur could be eliminated by hydrodesulphurization and adsorption methods utilizing activated carbon. The endothermic reaction depicted in Figure 5 transforms steam and purified methane into hydrogen after passing through a catalyst. The efficiency of steam methane reforming (SMR) is improved by a process called the water–gas shift reaction (WGS), where carbon monoxide (CO) reacts with water in the presence of a catalyst.

To prevent the creation of coke, the endothermic process is modified to 700–1100 K and 3.5 MPa pressure. Pure hydrogen is left behind after pressure swing adsorption removes CO₂ and other contaminants [89]. Because of the potential hydrogen yield, the steam reformation of ethanol (SRE) has focused a great deal of attention on hydrogen (6 mol of H₂ per mol of ethanol feed) [90,91]. The appropriate SRE response is as follows:

C₂H₅OH + 3H₂O → 6H₂ + 2CO₂

(9)

The aforementioned reaction occurs as an endothermic process at operating temperatures above 500 °C, atmospheric pressure, and with an excess of water. This approach allows for the simultaneous enhancement of steam methane reforming efficiency and ethanol production. Below are the breakdown reactions for SMR and the WGS reaction;

C₂H₅OH → CO + CH₄ + H₂ (H_298K = 49.0 kJmol⁻¹)

(10)

CH₄ + H₂O → CO + 3H₂ (H_298K = 206.3 kJmol⁻¹)

(11)

CH₄ + 2H₂O→CO₂ + 4H₂ (H_298K = 165.1 kJmol⁻¹)

(12)

CO + H₂O→CO₂ + H₂ (H_298K = −41.2 kJmol⁻¹)

(13)

The reaction can be enhanced using noble metals like platinum (Pt) and rhodium (Rh) or by incorporating non-precious metal catalysts such as nickel (Ni) alongside these noble metals. The most effective catalysts are those based on nickel, which are also less expensive and do not impose any restrictions on the process [92]. The greatest concentration of hydrogen using Ni-Co/CaO-Ca₁₂Al₁₄O₃₃ (SESR) is 82.23%, yielding 2.31 L/g at 650 °C [93]. High-temperature CO₂ adsorbent is catalysed by lithium zirconate, and the gas–water shift reaction and equilibrium limits allow for a 97% product concentration. Reports indicate that MgO-modified Ni/CaO can achieve 96% purity through the process of sorption-enhanced steam reforming [94]. From 1975 to 2018, annual hydrogen production reached 115 million tons. Currently, nearly 90% of the H₂ generated from fossil fuels is captured, leading to the annual emission of over 830 million tonnes of CO₂ [77]. Hydrogen might be produced by SMR, oil fraction, coal gasification, electrolysis, and SMR at rates of 48–30%, 18%, and 4%, respectively [95]. Because of the potential hydrogen yield, the steam reforming of ethanol (SRE) focused a great deal of attention on hydrogen (6 mol of H₂ per mole of ethanol feed). It is difficult to make an SRE system economically feasible for hydrogen production and purification. For an efficiency of 94.95%, the 2013 SRE cost was estimated to be 0.27 US dollars per kWh [96]. In 2008, the estimated cost of producing hydrogen via steam methane reforming (SMR) was $1.28 per kilogram, with an additional 3% allocated for operating and maintenance expenses. The system efficiency is 80%, hydrogen purity is 97%, and the total capital cost is 150 million (Rs.) [97]. At the moment, the market value of hydrogen generated with SMR is roughly 2.9 $/kg [98]. With an annual hydrogen production of 65 million tons, steam methane reforming (SMR) emerged as the leading source of CO2 emissions, contributing nearly 2% of total global CO₂ emissions in 2015. A proposal has been made to expand renewable energy capacity from 1.5 GW in 2015 to over 15 GW by 2050. About 80% of the hydrogen produced globally is produced using SMR, a well-established and mature technology. Owing to extensive commercial experience and advancements in this technology, production costs have been decreased to $2.9 $/kg and yield efficiency has increased to 74–55% [98]. However, SMR necessitates both high temperatures (700–1000 °C) and intricate catalytic reactions. SMR uses non-renewable feedstocks, which are progressively running out due to global energy demand, to produce hydrogen. These feedstocks include biodiesel, liquefied petroleum gas, naphtha, methanol, and jet fuel.

SMR generates CO₂, a primary contributor to the possibility of global warming [99]. The greenhouse gas footprint per kilogram (GWP) of non-lower-carbon fuels was determined to be 9.46 kg CO₂/kg H₂ in a study on hydrogen generation using life-cycle analysis (LCA) and SMR. It takes more energy and equipment to capture 100% of the CO₂ in exhaust gases, and this is not possible [100]. Currently, lower-carbon energy sources are of great interest. By 2060, renewable or lower-carbon energy sources will be a major supplier of electricity. As a result, lower-carbon hydrogen production sources have great positive environmental impacts.

3.2.2. Gasification

An outdated method of producing hydrogen is gasification. A gaseous end product with a favourable chemical heating value is generated from a carbon-rich source. Three types of reactors are commonly employed for biomass gasification: fluidized bed reactors, entrained flow, and fixed bed, with the fluidized bed being the most effective. Fluidized bed reactors (FBRs) are characterized by great stability, improved temperature controllability, and significant mass and heat transfer between the solid and liquid phases [101]. Coal and biomass are two of the commercially viable feedstocks for the efficient generation of hydrogen in gasification systems [102,103].

The gasification of coal makes for economically and physically viable large-scale hydrogen production. Figure 6 demonstrates how coal undergoes partial oxidation during gasification using steam and O₂, primarily producing carbon monoxide (CO) and hydrogen (H₂). These components are then combined with carbon dioxide (CO₂) and steam in a high-pressure and high-temperature reactor to form syngas. To boost the yield of H₂, the syngas undergoes a water–gas shift process. If elemental sulphur or sulphuric acid extraction is appropriate, the gas product may be cleaned. Another major source of CO₂ is the gasification of coal. Furthermore, a coal gasification system operating at 75% efficiency recorded an output of 29.33 kg of CO₂ per kilogram of hydrogen produced by Kothari et al. [97]. Although CO₂ is captured using Carbon Capture and Storage (CCS) technology, this approach is not thought to be a practical one on a wide scale. Furthermore, given the present rates of production, it is estimated that the world’s coal reserves will endure for 150 years [104]. Future hydrogen production from renewable feedstocks is becoming more and more important due to the depletion of reserves and environmental effects.

Biomass gasification has been a popular method for producing carbon monoxide, hydrogen, methane, carbon dioxide, and other hydrocarbons in recent decades, as illustrated in Figure 7. The process involves heating biomass to 700–1200 °C and producing high-value nitrogen-free gas through steam processing. Catalysts play a crucial role in increasing hydrogen yield during biomass gasification, with nickel-based catalysts being particularly beneficial due to their cost-effectiveness and widespread industrial use in hydrogen production from biomass gasification [105,106]. Biomass gasification is more cost-effective than coal gasification; a 1 MW biomass gasification system is estimated to cost approximately $367 per kW, compared to around $720 per kW for a small-scale coal gasification plant [107]. But coal also contributes significantly to greenhouse gas emissions, which is what produces acid rain. Coal and biomass co-gasification is seen as a promising area of study. The co-gasification of two fuels minimizes overall emissions while maintaining the energy content of the produced gas [108]. It is important to note that increasing the biomass content in the gasification process—up to 10 per cent weight—enhances the production of CO and H₂.

The gasification of biomass powered by the sun is considered a very popular and advantageous process of producing hydrogen. The system of solar-integrated biomass gasification is delicate and intricate. Slow initialization of the operation is a major problem when employing a lower-carbon energy source, even though it improves continuous reliability more successfully. Storage solutions must therefore address the energy source’s unpredictability and cyclical existence. Large-scale research into gasification presents certain obstacles, yet it has the capacity to be a valuable renewable energy source. An important issue in the gasifier is caused by aggressive circumstances in the gasification process. The life of feed injectors, protective coatings, and thermocouples that are used in gasifiers varies according to how harsh the circumstances are inside the gasifier. Due to these gasifier issues, the average operating time is lowered, necessitating the installation of a second gasifier in series for continuous operation. The plant cost may be significantly impacted by these problems and obstacles. Another drawback associated with this is that using this process, hydrogen is produced from coal or biomass. It can make use of waste materials, but it also releases carbon dioxide, therefore long-term viability of the source is essential.

3.2.3. Nuclear Energy

Generating hydrogen from nuclear sources, energy must be supplied by nuclear reactors to break down water molecules. One promising large-scale hydrogen production method is thermochemical water splitting. Another fascinating process, known as radiolysis, involves the degradation of water molecules through high-energy radiation such as gamma rays, X-rays, or ionizing particles, resulting in hydrogen gas as a by-product [110,111]. This technology is, however, still in its early stages of development and needs to overcome its financial and technological obstacles before it can be widely adopted. The possibility of producing thermochemical hydrogen from nuclear power plants has been studied for many years [112]. Nuclear energy can be harnessed through various methods to produce hydrogen, including nuclear thermal conversion of water and nuclear power electrolysis, as illustrated in Figure 8. One type of H₂ generated using nuclear energy and electrolysis is termed yellow hydrogen (hydrogen devoid of carbon).

It may be possible to enhance the capacity of hydrogen generation by more than 95% by combining nuclear power with hydrogen production. While producing hydrogen through nuclear energy is a desirable way to cut carbon emissions, there are also serious environmental and health risks associated with handling and processing radioactive waste, the possibility of accidents, and uranium mining [114]. A Lucid Catalyst analysis (2020) states that the cost per kilogram of hydrogen produced with nuclear power is $2. This cost is similar to that of grey hydrogen, which may be produced for between $0.7 and $1.6 per kilogram without emitting any carbon dioxide. Nuclear energy based on hydrogen could be a way to lower the price of electricity production. At the moment, conventional methods are thought to be more competitive than thermochemical cycles. However, if they can be successfully combined with nuclear reactors, there is room for development. Integrating solar and nuclear energy presents several challenges for achieving sustainable hydrogen production. As the nuclear coupling cycle is still in the R&D stage, it is not yet ready for commercial implementation.

Table 1. A Comparison of hydrogen production from various techniques and the statistical analysis of various production techniques.

S.No.	Production Technique	Cost	Environmental Impact	Efficiency
1	Biomass Gasification	Average	Low	65–69%
2	Gasification	Moderate	Optimum	60–80%
3	Electrolysis	Depends on energy source used	Optimum to high	56–80%
4	Photoelectrochemical Water Splitting	High	Low	Less than 25%
5	Thermochemical Water Splitting	High	Low	40–50%
6	Steam Methane Reforming	Low-cost	Higher emissions of CO2	65–75%
7	Coal Gasification	Low	-	60–70%
8	Hydrogen From Wind	Optimum	Less	5–70%

summarizes a comparative analysis of hydrogen production from various techniques, along with a statistical evaluation of these different production methods.

4. Future Prospects/Challenges

Water electrolysis has become the most viable technology for H₂ production today, even as other systems using renewable energy sources are under development. This electrochemical process is capable of generating high-quality hydrogen. To reduce greenhouse gas emissions and support the energy transition, integrating the electrochemical route with renewable electricity sources presents a promising opportunity [115]. The primary challenges related to capital and operational costs, as well as process efficiency, must be addressed before solar hybrid H₂ generation and concurrent wastewater treatment can be scaled up and implemented sustainably over the long term [21]. Aydin et al. [21] identified several key factors that must be considered to evaluate these methods. This study aims to assess the following aspects: (a) the effectiveness of wastewater management, (b) H₂ production, and (c) the utility of the approaches. The primary benefit of the procedures is that they are environmentally benign, resulting in zero or very low carbon emissions related to material transportation or construction.

L. Du et al. [116] reported alkaline hydrogen oxidation and development through the bifunctional multi-element nonprecious alloy synthesized by electroshock. According to calculations using density functional theory, alloying W, Mo, Co, Cu, and Ni can adjust each metal’s electronic structure and give a variety of active sites to maximize the adsorption of hydroxyl-based intermediates.

The low hydrogen yields are the fundamental drawbacks of most treatments, including biological treatment, Anaerobic Membrane Bioreactors (AnMBRs), and Advanced Oxidation Processes (AOPs). As a result, the optimization of these two parameters serves as the foundation for several studies [16,117]. Specifically, when combined with bio-derived processes, as well as traditional methods or other Advanced Oxidation Processes (AOPs), these techniques can enhance process efficiency, wastewater treatment, and hydrogen generation [118]. Though this combination will use more energy, it can be avoided by combining the best approaches to maximize technological efficiency while consuming the least amount of energy possible. On the other hand, most research is conducted at the laboratory scale, and there are not many upgraded systems. MEC has achieved Technology Readiness Level (TRL) 6, with several prototypes utilizing household and urban wastewater [119,120,121]. The treatment plant size:type ratio significantly influences the economic assessment of wastewater treatment processes, leading to uncertainties in cost evaluations due to a lack of data. Additionally, the solar:hydrogen process may attain a 10% efficiency with photovoltaic solar cells possessing efficiencies up to 83%, respectively [7]. The cost of the technology is heavily affected by the expenses associated with the catalysts and electrodes utilized in photocatalysis [122,123], photoelectrocatalysis, and electrolysis [124].

It is also critical to note that the literature is deficient in research studies that address the efficiency of the combined process, life-cycle assessment, and techno-economic perspectives. Therefore, in order to scale up hybrid technology, specific research must be intensified.

Lastly, it is crucial to note that, depending on the final use of the hydrogen, the purity of the gas generated from wastewater is still a crucial factor to consider. Applications requiring a high purity of hydrogen may necessitate greater investment in the purification stage, which could have an impact on the applied technology’s economics. Only laboratory-scale tests have been conducted in this context, such as the Orosa et al. study [125], where hydrogen has been analysed. Thus, in order to scale up the technique, more extensive pilot studies are required.

More recently Li et al. [126] have reported platinum-based nanoclusters for their use in alkaline hydrogen generation. Calculations utilizing density functional theory (DFT) indicate a strong Pt–nitrogen-doped coupling. Additionally, this coupling positions the d-band centre optimally for the appropriate desorption–adsorption of intermediates during the alkaline hydrogen evolution reaction (HER). These effects considerably enhance the rate of hydrogen production.

5. Conclusions

Hydrogen production from wastewater is still a relatively new technique. However, it is critical to promote practices that will lessen carbon footprints because of the expenses associated with conventional treatment, energy consumption, and chemical requirements in the operations. This study evaluates and compares nine different approaches for H₂ generation from wastewater.

• While considerable investigations have been conducted on H₂ generation from wastewater, numerous works have taken place in laboratory settings.
• Although there are commercially established techniques (electrodialysis and electrolysis), research and development are still ongoing about their potential applications in treating wastewater to produce hydrogen.
• The working circumstances of supercritical water gasification result in a significant energy need. It can produce much more hydrogen, though, if it is backed by waste heat from other businesses. The purity of the generated gas is also crucial as it may need further separation procedures because it contains CO₂ and CH₄.
• The price of lower-carbon hydrogen per kilogram should be about $1–1.5 for hydrogen generation to be profitable and comparable with other sources of hydrogen production. Considering the ways that were previously evaluated in the study, further technical optimization and system integrations need to be taken into consideration.

On comparing H₂ production by taking on board the variations from a regional point of view and the treatment technologies utilized, it was found that higher concentrations of organic content are present in wastewater from urban sources due to industrial and domestic effluents in comparison to rural wastewater. The rate at which organic matter decomposes might vary throughout climatically distinct regions, which can impact wastewater’s overall composition. While underdeveloped countries might rely on more straightforward, economical techniques, developed regions might have the resources and cutting-edge technologies to put in place complex treatment systems. More sophisticated treatment techniques may be used in areas with strict environmental requirements in order to produce hydrogen while reducing pollution. It was found that investments in wastewater-derived hydrogen production may be more common in regions that provide funding for renewable energy projects.

A comparison of case studies from countries like Europe has revealed that anaerobic digestion and biohydrogen production are being investigated as components of their circular economy plans; these systems are frequently integrated into already-existing wastewater treatment facilities. As long as they have significant energy demands and are committed to lowering greenhouse gas emissions, nations like South Korea and Japan are concentrating on developing novel electrohydrogenesis technologies.

Consequently, it is possible to implement approaches wherein some of the costs associated with conventional methods can be recovered. Although producing hydrogen from wastewater is not a widely used technique yet, it is crucial to disseminate methods that will minimize the carbon footprint. The current findings show that water electrolysis and dark fermentation are the most viable and effective methods. The eco-friendly approach with the lowest greenhouse gas pollution is microbial electrolysis, followed by photosynthesis and dark fermentation.