Hydrogen Production by Wastewater Alkaline Electro-Oxidation

Abstract

The current work presents the electro-oxidation of olive mill and biodiesel wastewaters in an alkaline medium with the aim of hydrogen production and simultaneous reduction in the organic pollution content. The process is performed, at laboratory scale, in an own-design single cavity electrolyzer with graphite electrodes and no membrane. The system and the procedures to generate hydrogen under ambient conditions are described. The gas flow generated is analyzed through gas chromatography. The wastewater balance in the liquid electrolyte shows a reduction in the chemical oxygen demand (COD) pointing to a decrease in the organic content. The experimental results confirm the production of hydrogen with different purity levels and the simultaneous reduction in organic contaminants. This wastewater treatment appears as a feasible process to obtain hydrogen at ambient conditions powered with renewable energy sources, resulting in a more competitive hydrogen cost.

1. Introduction

At present, a considerable rise in world energy demand is under way due to population growth and the expansion of economies, mainly emerging ones. This implies a large growth in pollution, resulting in ever-larger green house gas emissions and global warming of the planet, with a consequent increase in the Earth’s average temperature.

The need for a change in the world energy system and for an improvement in energy efficiency is obvious. Thus, the demands for efficient use of limited resources and the development of environmentally acceptable technologies are increasing, due to growing social awareness of reusing and recycling wastes.

Every day tons of wastewater are generated all over the world, for which no single treatment is able to reduce the water’s contaminants to the permitted limits. Wastewater management is a challenge for many countries, as it is essential for human health, the environment, and sustainable economic development. The idea of using organic wastewater as a hydrogen source has arisen due to the difficulties in treatment of organic wastes. In addition, hydrogen generation is increasingly relevant due to its multiple applications and CO₂ zero emissions.

1.1. Hydrogen Economy

Hydrogen is the most abundant element in the universe, but is rarely found in a free form on Earth because of its chemical affinity. Usually, it appears combined with other elements, giving rise to compounds such as water, hydrides, or organic materials. It is an element having a higher energy yield, of 122 KJ/g. Hydrogen is not strictly an energy source, but a flexible energy carrier, which can be produced both from any primary energy source and locally available materials, such as biomass, wind, solar, geothermal, and organic waste [1,2].

For several years, the hydrogen vector concept has been developed as a complement to the electricity vector. Its main advantage over electricity is that it can be stored as a chemical energy support in gaseous, liquid, and solid (metal hydrides) phases. Stored hydrogen significantly benefits the reduction in gas emissions in the automotive sector, and aids in the decarbonization of industrial processes. In addition, it represents a high enthalpy storage mechanism for non-controllable renewable energy sources for real-time coupling of power flow generation and demand. Furthermore, fuel cells can transform hydrogen into electricity, as an energy source for applications that currently use fossil fuels.

Hydrogen production processes have their origin in a wide variety of raw materials.

Table 1. Primary energy sources and more-developed techniques of hydrogen production.

Energy Primary Sources	Hydrogen Production Processes
Marine Solar photovoltaic Wind Hydraulics Geothermal	Electrolysis
Solar thermal	Thermo-chemistry
Nuclear
Algae	Photosynthesis
Petroleum Natural gas Coal	Gasification
Petroleum Natural gas	Steam reformed or partial oxidation
Biogas Bio-fuel
Biomass	Pyrolysis

Green means that it has no CO₂ emissions, grey means that it produces CO₂ emissions, and nuclear energy is represented by the colour violet.

shows the main energy sources, renewable or otherwise, and some processes used for hydrogen production. Each of these technologies is at a different stage of development and offers different benefits, opportunities, and challenges. Most of these technologies are already commercially available for large-scale hydrogen industrial production [1,2]. Other hydrogen production processes are still under development, such as the method here presented.

As there is not yet a wide market for hydrogen energy applications, more than 90% of the hydrogen generated is used on-site in the refinery industry, for desulphurization, in the chemical industry, as a chemical reagent for ammonia and fertilizer production, and in methanol production, with only 10% being used for other purposes [1,2]. Hydrogen demand in 2020 was about 90 Mt, with more than 70 Mt used as pure hydrogen and less than 20 Mt blended with carbon-containing gases in both methanol production and steel manufacturing.

In 2020, the majority of hydrogen produced was obtained from fossil fuels (68.64%; 60.53 Mt), by-products in refineries (21.09%; 18.6 Mt) or fossil with carbon capture and utilization (9.24%; 8.15 Mt); in contrast, the hydrogen production by fossil fuels with carbon capture and storage (0.8%–0.71 Mt) and electrolysis (0.56%–0.49 Mt) was much lower [3]. The latter method, although less developed, holds great promise since, combined with renewable energy sources, it will considerably reduce the carbon footprint. Hydrogen produced from fossil fuels results in close to 900 Mt of CO₂ emissions per year [3].

Electrolysis is being extensively studied and used for “green H₂” production, despite its negative energy balance [4], since the energy requirement for water electrolysis with an alkaline electrolyzer is 51 kWh/kg H₂ [5], whereas the energy content of hydrogen gas is 33.2 kWh/kg H₂, having adensity of 0.08988 kg/Nm³ [6].

The electricity generation mix of peninsular Spain during 2019 and 2020 is displayed in Figure 1 [7]. Renewable energy may increase considerably with the introduction of the hydrogen vector into the system. Coal and gas still supply around 20% of the energy, and their substitution is essential to avoid CO₂ emissions.

The EU Member States have signed and ratified the Conference of the Parties (COP21) Paris agreement to keep global warming “well below 2 degrees Celsius above preindustrial levels, and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius”. This requires virtually carbon-free power generation, increased energy efficiency, and the deep decarbonization of main consumers: transport, buildings, and industry. An ambitious scenario for hydrogen deployment in the EU is the achievement of approximately 2250 terawatt hours (TWh) of hydrogen by 2050, representing roughly one-quarter of the EU’s total energy demand. Concerning power generation, buildings, and industry, this scenario would result in a mix of about 70% centralized water electrolysis, approximately 20% decentralized water electrolysis, and 5% steam methane reforming in 2050. For transportation, 95% of hydrogen is expected to be generated with water electrolysis, half of which will use decentralized water electrolysis. Biogas technology will complete this scenario by providing the remaining 5% [8].

The hydrogen economy aims to produce green hydrogen through the electrolysis process, but the cost is still higher than that through the gasification process. As hydrogen production from wastewater electrolysis is still one of the most expensive methods, at 7.59 USD/kg H₂ [9], a great challenge remains to investigate more efficient and simpler methods to achieve competitive prices.

1.2. Wastewater: Impact and Types

Among the multiple organic residues, olive mill and biodiesel wastewaters are part of the residual waters group within residual organic liquids.

Most urban, farming, and industrial activities require a large amount of water, which leads to the generation of a large quantity of wastewater. Most wastewaters are spilled onto the environment without any treatment due to weak regulations. This has a detrimental impact on human health, economic productivity, freshwater resource quality, and the ecosystem.

Sources of generated wastewater include domestic residences, commercial properties, industrial operations, cattle, and agriculture. Globally, about 80% of wastewater receives no treatment. Pollutants present in wastewater include pathogens (as bacteria, virus, and microscopic parasites), organic particles, soluble organic material, inorganic particles, toxins (e.g., herbicides, pesticides, dyes, or poisons), pharmaceuticals, household wastes, and even dissolved gases [10].

These wastewaters are a complex mixture of different organic and inorganic compounds, some of which are toxic and difficult to degrade, and are mainly treated by conventional technologies such as aerobic and anaerobic processes and chemical coagulation. As these techniques are not able to remove all of the harmful compounds, electrochemical treatments are being developed to complement them [11].

Production processes give rise to large amounts of residual organic liquids such as beet root pulp, palm oil mill effluent, and wastewater from dairies, rice mills, chemicals, sago (large leaf palm), biomethanated distillate, paper and pulp, cheese whey, textile desizing, cassava, olive mills, protein, breweries, and biodiesel [10].

Several research groups have used similar techniques to treat wastewater. Lv et al. reported on a NiFeMo hybrid film that acts as cathode and anode materials in an electrolytic cell for hydrogen generation from urea electrolysis and the purification of urea-containing wastewater. They provide an easy method for synthesizing low-cost high-performance electro-catalysts [12].

Nitrate contamination from industrial wastewater has become a common environmental problem. Yao et al. analyzed the electrochemical reduction in $N{O}_3^{-}$ from wastewater, aiming to enhance the efficiency of $N{O}_3^{-}$ elimination and the selectivity of $N{O}_3^{-}$ to N₂. Results indicated that acidic conditions were suitable for direct and indirect nitrate reduction, while $N{O}_3^{-}$ was mainly reduced via direct reduction under alkaline conditions [13].

Marks et al. used a laboratory-scale reactor to study the feasibility of the energetic valorization of winery effluents into hydrogen by means of dark fermentation and its subsequent conversion into electrical energy using fuel cells. The acidogenic fermentation generated a gas effluent composed of CO₂ and H₂ (55%), resulting in a hydrogen yield about 1.5 L per liter of fermented wastewater at standard conditions [14].

Louhasakul et al. presented an integrated bio-refinery concept combining two biological platforms for the valorization of palm oil mill effluents and for simultaneous production of high market value products, such as microbial oils and bio-energy. Palm oil mill effluents were aerobically fermented to produce lipids, and subsequently the effluent from the fermentation was used as influent feedstock in an anaerobic digester for biogas production. A maximum of 74% of the theoretical methane yield was achieved during continuous reactor operation [15].

Other applications use microbial electrochemical cells (MECs) to produce renewable hydrogen and, simultaneously, wastewater treatment. Krishnan et al. indicated that MEC have the potential to convert organic wastewater into hydrogen and value-added chemicals such as methane, ethanol, and hydrogen peroxide. Compared to other conventional methods, MECs offer a high H₂ yield with a small energy input of 0.4–0.5 V. The principal components of the MEC are similar to those of the electrolysis process, i.e., the anode and cathode electrodes, semi-permeable membrane, electrochemically active microbes, and the power supply unit. Several types of wastewater, such as agricultural, domestic, and industrial wastewater, were analyzed [16].

Kadier et al. showed that microbial electrolysis cells of palm oil mill effluents for biological H₂ production can be successfully derived using modeling processes. These findings indicate that the maximum H₂ production rate can be affected by the incubation temperature, the initial pH, or the influent dilution rate. Experimental analysis revealed that under the optimum conditions of T = 30.23 °C, pH = 6.63, and a 50.71% dilution rate of the influent, a maximum hydrogen production rate of 1.1747 m³ H₂/m³ of solution was achieved [17].

1.2.1. Olive Mill Wastewater

Every year, around 15 million tons of olive mill wastewater are generated all over the world [18]. Olive mill wastewater has a high organic load, measured by its Biological Oxygen Demand (BOD) or its Chemical Oxygen Demand (COD), which is much higher than that of the effluents from other agri-food industries [19]. Organic matter consumes oxygen from the water and prevents fish life, so must be treated before being spilled into rivers or used for other purposes. Furthermore, it also shows a high content of inhibitory compounds such as phenolic compounds, whose characterization and treatment have not been sufficiently addressed [20].

1.2.2. Biodiesel Wastewater

Biodiesel wastewater is a residue from the washing processes used to remove excess contaminants and impurities to obtain high quality biodiesel that meets international standard specifications. The characteristics of this wastewater include a high pH, high level of hexane-extracted oil, and low concentrations of nitrogen and phosphorus, which hinder its natural degradation, because these values are unfavorable for the growth of microorganisms. As the main component of biodiesel wastewater is residual oil, discharges into public drainage may block drains and disturb the biological activity in sewage treatment.

The large amount of biodiesel wastewater generated by the commonly used wet-washing process is drawing the attention of researchers. The washing process, repeated two to five times, depends on the impurity level of methyl ester. Biodiesel production of 100 L yields about 20–120 L of wastewater. In Thailand, biodiesel production of more than 350,000 L/day generates more than 70,000 L/day of wastewater [21].

Previous studies have found that biodiesel from transesterification contains glycerol, soap, methanol, free fatty acids (FFAs), catalysts, and glycerides. These contaminants contribute to its high contents of COD, suspended solids (SS), and oil and grease (O&G), and depend on the process type [21].

1.2.3. Recycling and Disposal

When pouring processed wastewater into a sewage system, the same regulations as those for urban wastewater are applied. The wastewater requirements after treatment, according to Spanish regulations, are shown in

Table 2. Maximum concentrations and minimum percentage reductions for contaminant parameters [22].

Parameter	Concentration	Minimum % of Reduction
Biochemical Oxygen Demand (DOB 5)	25 mL/L O2	70–90
Chemical Oxygen Demand (COD)	125 mL/L O2	75
Total suspended solids (TSS)	35 mL/L	90

. The concentration value or the percentage reduction is applied.

2. Electrolytic Process

In electrolytic processes, a compound is split into elements in the presence of an electric field. Among these processes are:

2.1. Electrolysis

Electrolysis consists of the dissociation or breakdown of the water molecule by the action of an electric field. Liquid water is split into its elemental components, molecular hydrogen and oxygen, according to:

$$H_{2}O\to H_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)$$

(1)

This is a non-spontaneous reaction that requires an external energy input. The cell voltage required for water dissociation is 1.23 V under standard conditions. However, the main drawback, which limits the efficiency of the process, is the sluggish kinetics of the water–oxygen couple (both the oxidation of water into oxygen and the reduction of oxygen into water) under near-room temperature conditions. When the electrolysis cell is operated with current densities of at least 1 A/cm², to obtain a high hydrogen production rate at the cathode, a high overvoltage occurs. As a result, cell voltages of 1.8–2.0 V are generally required to reach significant hydrogen production rates, leading to energy conversion efficiencies of less than unity. This overvoltage is responsible for the energy cost of hydrogen production by water electrolysis [23]. Cell voltages in the range of 2.5–4.5 V were applied in the tests carried out for this research.

The process takes place in an electrolyzer, which consists of two electrodes, i.e., an anode and a cathode, facing each other and separated by a thin layer of ionic conductor, called the electrolyte. Electrodes plugged in to an external power supply provide the energy required to generate free hydrogen at the cathode.

To maintain the charge balance, when a voltage is applied to the electrodes, electrons flow from the negative terminal (anode) to the cathode, where they are consumed (reduced) by protons to form hydrogen. At the same time, ions travel through the electrolyte to the corresponding electrode, where a reaction occurs, giving up electrons that return to the power supply terminal. The charges carried by the ionic compounds, such as OH¯, H⁺, or O²¯, move through the electrolyte to maintain the system electrical neutrality. According to the ionic conducting electrolyte, there are three types of electrolyzers: alkaline, proton exchange membranes (PEMs), and high temperature solid oxides (SOEs) [24], Figure 2.

2.2. Electro-Oxidation

The biological processes of wastewater primary treatment are not capable of degrading colored compounds, and are only suitable for wastewater having a low content of biodegradable materials. Wastewater treatment by electrochemical processes gives rise to chemical reactions aimed at the elimination of these contaminants.

Electrochemical wastewater treatment is carried out using different techniques: electrocoagulation, electrofloculation, electrodeposition, and electro-oxidation, among others [25].

In the electro-oxidation process, oxidants are produced during in situ treatment, either directly on the surface of the electrodes or indirectly from chemical compounds in the treated water [11]. The aim of the electro-oxidation process is either the total oxidation (or mineralization) or partial oxidation (conversion of organic matter to simpler, more easily degradable and less polluting compounds) of organic matter.

An electro-oxidation process can also be defined as an electrolysis process with the presence of an organic compound as an energy source. The electrolysis process has a variable cost depending on the energy consumption per kilogram of hydrogen produced. Therefore, it is essential to reduce the quantity of energy used to be competitive with conventional industrial hydrogen production processes. The presence of organic matter having a low oxidation potential implies a reduction in the cost of the hydrogen production [26].

The current work presents an electrolysis system based on an own-design reactor, and the process to obtain hydrogen by alkaline electrolysis assisted by two types of wastewater, i.e., olive mill and biodiesel wastewaters, and two organic compounds, i.e., glycerol and methanol for comparative purposes. Hydrogen was generated with different purity levels, a low production of undesired gases, such as O₂, CO, CO₂, N₂, and CH₄, and simultaneous reduction in the organic contaminants.

3. Materials and Methods

This section presents the self-designed single cavity electrolyzer without a membrane for hydrogen production under ambient conditions.

3.1. Materials

This section describes the components of the experimental system (Figure 3) and the hydrogen generation process. An own-design single-cavity reactor without a membrane (1) is used for the alkaline electro-oxidation of wastewater or simpler organic compound, as a better alternative to the existing patented approach [27].

The reactor comprises a hollow cylindrical body and two hermetically joined covers on both sides. On top of one of the covers, two flat-faced electrodes are located (2) at a 1 mm distance, one acting as an anode and the other as a cathode.

The electrolyzer envelope is made of methacrylate due to both its high tolerance to alkaline media and transparency. Regarding the electrodes, gold, platinum, and carbon-based catalysts are most commonly used, but are expensive for industrial application. In previous studies [26], different materials, such as nickel, graphite, and stainless steel, have been analyzed in self-design reactors.

Graphite, without a catalyst coating, was selected as the most suitable material for both electrodes, taking into account variables such as its low cost, hydrogen flow generation and absence of other gases, resistance to corrosion in an alkaline environment, electrical conductivity, and porous structure [28]. The electrodes have a surface area of 100 cm² and thickness of 8 mm.

The reactor has four connections: two for the entry and exit of the solution (3), one to remove the generated gas effluent (4), and the fourth to inject argon to remove the air remaining before starting the electro-oxidation process, because argon is the chromatograph carrier gas.

The electrolytic solution (5) is composed of the liquid residue, demineralized water, and the alkalinizing agent, which increases the electrical conductivity, resulting in solutions with a high pH, of between 12 and 14.

The alkali concentration in relation to that of the organic compound plays a fundamental role in the reactions, achieving a higher organic conversion the higher the base concentration [29], since there is a strong dependence between the electro-oxidation ratio and the electrolyte pH.

3.2. Methods

An analysis of the time evolution of the generated gases (%) was performed to determine the optimal ratio between the concentrations of the organic compound and alkaline agent, aiming to reach the highest percentage of hydrogen in the gas flow.

The recirculation pump (6) is in continuous operation. At the laboratory, the wastewater and the alkalizing agent are located in two tanks (7) connected to the circuit by on/off valves (8). The tanks permit the manual addition of either the alkaline agent or the wastewater to increase the pH or the aqueous organic load, respectively, to analyze their effect on the gas flow. Electrodes are connected to an external programmable power source (9).

Once the electro-oxidation process starts, the gas flow (4) is analyzed by gas chromatography (10). A gas chromatography system (CP-4900-MicroGC, VARIAN BV, Houten, The Netherlands, EU) with a sampling rate of 400 s, measures the composition of the gas flow generated in the electro-oxidation process. It is based on the area corresponding to the electrical signals provided by the chromatograph as a function of the gas measured. In addition, the amount of flow is measured at the beginning and at the end of the test.

Figure 4 shows the single cavity electrolyzer without a membrane at the start of an electro-oxidation process, at a constant electrode voltage of 3.5 V, generating a current of 7.5 A. The electric current varies over time due to the electrolytic reactions.

4. Results

The composition of the gas effluent, produced in the alkaline electro-oxidation of four organic compounds, i.e., olive mill wastewater, glycerine, biodiesel wastewater, and methanol, was analyzed to determine the hydrogen generation rate over time. Whenever hydrogen generation decreased or oxygen and carbon dioxide increased, the test was stopped.

The organic compound loads in the liquid electrolyte phase of the electro-oxidation process of olive mill wastewater and biodiesel wastewater (crude glycerol), at the start and at the end of the process, were analyzed in an external laboratory, “Laboratorio Medioambiental S.L, Murcia, Spain”.

The temporal evolution of the gas flow components, generated in the electro-oxidation process of an alkaline solution without and with an organic compound, is shown in Figure 5. The electrolysis process, with graphite electrodes at a constant voltage of 3 V in an aqueous electrolyte of 1 M KOH is shown in Figure 5a, whereas Figure 5b shows the same process in an aqueous solution of 1 M KOH and 1 M glycerol. The absence of undesired gases, such as carbon dioxide and oxygen, only happens when the organic compound, glycerol, is present.

This result is relevant despite the fact that the average current value, close to 1.5 A, is lower than that of the solution without organic compound, close to 2 A, indicating a lower flow volume. Additionally, the hydrogen purity is much higher when the organic compound and unwanted gases, such as O₂ or CO₂, are absent. It should be noted that the oxidation potential of glycerol (C₃H₈O₃) is lower than that of water, which favors the extraction of hydrogen from its molecules, which have lower energy requirements than those of water molecules.

4.1. Hydrogen Generation

In this section, the percentage of the produced gases and the current, over a period of about 1400 min, is displayed in Figure 6, Figure 7, Figure 8 and Figure 9.

4.1.1. Olive Mill Wastewater

In the electrolytic process of olive mill wastewater for hydrogen production, some samples required a pre-filtering before their introduction into the electrolyzer.

The temporal evolution of the percentages of generated gases in an electro-oxidation process over time is displayed in Figure 6. Hydrogen production increases over time, with a minimal generation of other gases throughout the 24 h period of the alkaline electrolytic process. The hydrogen percentage is below 76% of the global flow, while the total percentage of gas generated and measured by the chromatograph is less than 80%. The remaining 20% of the flow gas is argon, which is not measured by the chromatograph, as it is the carrier gas. Thus, it can be considered that the purity of the generated gases is high, since the argon is not generated by the electro-oxidation process. The low values displayed by the electric current of 1–2 A indicates a low gas flow. The flow of generated gases at the beginning was 1.54 L/h and at the end of the process was 0.52 L/h.

4.1.2. Glycerol

Several electro-oxidation tests [26] were undertaken with pure glycerol, before those performed with biodiesel wastewater, to compare hydrogen production rates.

The glycerol electro-oxidation test, Figure 7, showed significant hydrogen generation (%) since the first hour. The highest hydrogen purity, 99.68%, appeared in minute 134 of the electro-oxidation process, and purity remained above 97% until the end of the test. The gas flow was 3.81 L/h at the beginning and 1.32 L/h at the end of the test, much greater than that obtained with olive mill wastewater according to the electric current values. It can be appreciated that the production of others gases is minimal. Concerning hydrogen production in terms of purity (%) and flow, the results obtained with pure glycerol were the best of all the organic compounds analyzed. The electric current shows a high value, of 10 A, during the first minutes, decreasing considerably to 3 A, from 1100 min to the end.

4.1.3. Biodiesel Wastewater

Hydrogen production from the alkaline electro-oxidation of biodiesel wastewater exhibited higher hydrogen production rates (%) than those generated from the olive mill wastewater. Hydrogen production reached a percentage of 98%, close to that obtained in the alkaline electrolysis assisted by pure glycerol. The electrolytic process was performed with a 100 cm² surface area and electrodes having a thickness of 8 and 16 mm, at a constant voltage of 3 V (Figure 8a,b, respectively), to compare results. The results displayed in Figure 8b show that the generation of other gases was always less than 2%. At the beginning of the electrolysis, from minute 1 to 105, there were traces of air in the reactor, which were expelled by the pressure of the generated gas flow.

The period of the alkaline electro-oxidation process of biodiesel wastewater (Figure 8a) was longer than the previous ones, as the hydrogen generation was maintained at high flow rates after 24 h, with minimal production of other gases. Unlike electro-oxidation with pure glycerine, biodiesel wastewater electro-oxidation took longer to reach high percentages of hydrogen production.

In terms of gas flow generation, 1.54 L/h and 0.55 L/h were measured at the start and end of the process (Figure 8a), while Figure 8b shows a higher flow rate was produced during the whole process, with a flow of 2.83 L/h at the beginning and 1.40 L/h at the end. This behavior is reflected in the swiftness with which high hydrogen production (%) is reached. The low production of other gases points to a total oxidation of the biodiesel wastewater.

4.1.4. Methanol Assistant

The alkaline electrolysis assisted by methanol lasted 96 h, to demonstrate the effect of the addition of methanol and alkali agent solutions during the process. The electro-oxidation was performed with 100 cm² graphite electrodes having a thickness of 8 mm at 4.5 V. At minute 1354, the organic compound was added by pouring in 400 mL of methanol, due to the observed increase in oxygen and decrease in the electric current, Figure 9.

The second addition was an alkaline solution 20 M KOH, at minute 4094, as hydrogen production was decreasing and carbon dioxide increasing, indicating partial oxidation of methanol. Carbon dioxide increased from minute 3024, reaching a maximum of 12.56% at minute 4500, and then dropped slightly. A high percentage of hydrogen was produced throughout the process, with a minimum generation of other undesired gases. The most interesting aspect of this alkaline electro-oxidation process is the high flow of gases generated, which was 5.94 L/h at the beginning and 5.27 L/h at the end.

In an electro-oxidation process, the flow of generated gases is proportional to the variations in the electric current, which in turn are proportional to the number of reactions in the electrolyte. The more alkaline the solution, the greater the number of reactions and their intensity. Accordingly, with the addition of the alkaline solution, a large increase in the current was observed; Figure 9.

The pH decreased over time in all the electrolytic processes carried out as the alkalinity of the solution decreased with the reactions.

4.1.5. Efficiency

The efficiency of a water electrolysis cell measures the ratio of the theoretical energy E_t to the real energy E_r required to break down 1 mole of water. Due to heat degradation, E_r > E_t. Therefore, cell efficiency is:

$$\mathsf{ղ}=\frac{{\mathrm{E}}_{\mathrm{t}}}{{\mathrm{E}}_{\mathrm{r}}}$$

(2)

where ${\mathrm{E}}_{\mathrm{r}}=\left({\mathrm{U}}_{\mathrm{cell}}·\mathrm{I}·\mathrm{t}\right)$, ${\mathrm{U}}_{\mathrm{cell}}$ is the real voltage (V), I is the current (A), and t is the duration (s) [30]; and where $E_t$ is the theoretical amount of energy contained in the hydrogen produced, calculated as:

$${\mathrm{E}}_{\mathrm{t}}={\mathrm{E}}_{{\mathrm{H}}_2}=\mathrm{m}·(122\mathrm{KJ}/\mathrm{g})$$

(3)

where m is the mass of accumulated hydrogen produced in a specific time period [4]. Then, efficiency can be expressed as:

$$\mathsf{ղ}=\frac{\mathrm{m}·\left(122\mathrm{KJ}/\mathrm{g}\right)}{{\mathrm{U}}_{\mathrm{cell}}·\mathrm{I}·\mathrm{t}}$$

(4)

Hydrogen mass can be calculated from the ideal gas equation:

$$\mathrm{P}·{\mathrm{V}}_{{\mathrm{H}}_2}=\frac{\mathrm{m}}{\mathrm{M}}·\mathrm{R}·\mathrm{T}$$

(5)

where P is the pressure (1 atm); ${\mathrm{V}}_{{\mathrm{H}}_2}$ is the volumen of hydrogen generated; m is the mass of hydrogen (g); M is the molar mass of hydrogen (2 g/mol); R is the ideal gas constant (0.082 L atm/mol K); and T is the absolute temperature (K).

Finally, efficiency is reformulated as:

$$\mathsf{ղ}=\frac{\mathrm{P}·{\mathrm{V}}_{{\mathrm{H}}_2}·\mathrm{M}·\left(122\mathrm{KJ}/\mathrm{g}\right)}{{\mathrm{U}}_{\mathrm{cell}}·\mathrm{I}·\mathrm{t}·\mathrm{R}·\mathrm{T}}$$

(6)

Efficiencies were calculated at the beginning and at the end of all the tests presented, as shown in

Table 3. Efficiency of the alkaline electro-oxidation process for different organic compounds: olive mill and biodiesel wastewaters, glycerol, and methanol.

Organic Compound	Volume (L/h)	Ucell (V)	I (A)	Temperature (K)	Efficiency (%)
Olive mill wastewater (start)	1.54	3.5	1.68	300.7	71.99
Olive mill wastewater (end)	0.52	3.5	1.18	297.2	35.02
Glycerol (start)	3.81	3	8.17	308.7	41.62
Glycerol (start)	1.32	3	3.07	301.0	39.36
Biodiesel wastewater (start), 8 mm electrodes thickness	1.54	3	3.13	307.7	44.06
Biodiesel wastewater (end), 8 mm electrodes thickness	0.55	3	1.12	293.8	46.05
Biodiesel wastewater (start), 16 mm electrodes thickness	2.83	3	6.55	310.1	38.39
Biodiesel wastewater (end), 16 mm electrodes thickness	1.4	3	3.23	298.4	40.02
Glycerol (start)	5.94	4.5	12.03	311.4	29.12
Glycerol (end)	5.27	4.5	12.03	315.8	25.48

.

In electro-oxidation processes, the efficiency increases when a high hydrogen flow is generated with a minimum energy input (V·I). The highest efficiency corresponded to olive oil mill wastewater, followed by biodiesel wastewater (8 mm electrode thickness), and, finally, the lowest values corresponded to the mono-organic compounds, methanol and glycerol.

4.2. Organic Pollutants Reduction

The organic load reduction is a key aspect in the alkaline electro-oxidation. A decrease in the pollutants implies an extra benefit in the proposed electrochemical process with the single cavity reactor without a membrane.

Two parameters of the liquid phase, related to the reduction in the electrolyte organic load, chemical oxygen demand and carbonates, were measured at an external laboratory (Laboratorio Medioambiental S.L., Murcia, Spain). Chemical oxygen demand reduction was measured by COD digestion and carbonate reduction by acidimetry. The decrease in COD values implies a reduction in organic load content.

In acid electrolysis, carbon dioxide appears in the stream of generated gases; however, in current strongly alkaline electrolysis, the generated carbon dioxide is dissociated, giving rise to carbonates dissolved in the liquid medium.

The formation of carbonates during the electro-oxidation process, which were detected in the residual liquid at the end of the process, only occurred with graphite electrodes, and not with nickel or stainless steel electrodes. In a neutral medium, carbon reacts with oxygen forming carbon dioxide but, being a basic medium, it reacts to form carbonates according to the equilibrium of the carbonate species remaining dissolved in the aqueous medium [26]. Therefore, an increase in carbonates implies a reduction in carbon dioxide.

4.2.1. Olive Mill Wastewater

The characteristics of each alkaline electrolytic process and the parameters measured in the liquid phase are shown in

Table 4. Chemical oxygen demand and carbonates at the start and end of several olive mill wastewater electro-oxidation processes.

Test	Voltage (V)	KOH Molarity	% Volume of Olive Mill Wastewater	pH	Sample	COD (mg/L O2)	COD Reduction (%)	$\mathbf{Carbonates}(\mathbf{mg}\mathbf{C}{\mathbf{O}}_3^{-2}/\mathbf{L})$	Carbonates Reduction (%)
1	3.5	1.51	23.4	13.9 13.9	t = 0 h t = 24 h	40,350 31,000	23.17	36,905 33,185	10.08
2	3.5	3.02	23.4	14.0 13.7	t = 0 h t = 24 h	49,100 25,000	49.08	65,110 58,209	10.60
3	3	1.12	25.0	13.7 9.2	t = 0 h t = 24 h	70,800 47,300	29.93	25,204 27,004	−7.14
4	3.5	0.66	33.3	13.6 9	t = 0 h t = 24 h	67,500 43,700	35.26	16,502 13,202	20.00
5	3.5	0.5	50	13.3 8.2	t = 0 h t = 24 h	102,400 85,700	16.31	11,402 4501	60.52
6	3.5	0.5	75	12.9 8.5	t = 0 h t = 24 h	163,150 148,700	8.85	7201 12,002	−66.67
7	3.5	1	75	13.3 10.3	t = 0 h t = 24 h	145,300 134,200	7.64	18,603 16,202	12.9
8	3.5	1	85	13.4 10.2	t = 0 h t = 24 h	151,850 143,500	5.45	14,202 11,402	19.7
9	5	0.5	85	10.7 8.1	t = 0 h t = 24 h	154,800 151,000	2.45	6601 9001	−36.39

.

The chemical oxygen demand shows a reduction in the liquid phase of all olive mill wastewater electro-oxidation tests performed (Table 4) ranging from 2.45% in test 9 to 49% in test 2. This COD decrement is not enough to allow the wastewater discharge, according to Spanish regulations, but aids as an initial treatment.

As exhibited in Table 4, there was no significant growth of carbonates, indicating that either the pH of the solution was not high enough or the voltage was too low to generate enough recombination to increase the generation of carbonates.

In test 6 (Figure 6 and Table 4) and in test 9, there was a substantial increase in the content of carbonates in the electrolyte, probably because the pH was not high enough throughout the electro-oxidation to aid recombination processes.

4.2.2. Biodiesel Wastewater

The main characteristics of the electro-oxidation process and the parameters measured in the crude glycerol liquid phase are exhibited in

Table 5. COD and carbonate values at the start and end of the alkaline electro-oxidation tests of biodiesel wastewater.

Test	Voltage (V)	KOH Molarity	Glycerine Molarity	Electrode Thickness (mm)	pH	Sample	COD (mg/L O2)	COD Reduction (%)	$\mathbf{Carbonates}(\mathbf{mg}\mathbf{C}{\mathbf{O}}_3^{-2}/\mathbf{L})$	Carbonates Reduction (%)
1	2.5	1	3	8	12.7 12.6	t = 0 h t = 24 h	271,500 214,900	20.85	28,504 9601	66.31
2	3	1.5	3	16	13 12.8	t = 0 h t = 24 h	238,800 220,800 25,000	7.54	42,606 17,201	59.86
3	3	1.5	3	32	13 12	t = 0 h t = 48 h	238,800 212,100 43,700	11.18	42,606 15,602	63.38
4	3	1.5	3	8	13 10.6	t = 0 h t = 48 h	238,800 217,000 85,700	9.13	42,606 20,403	52.11
5	4	1.5	3	8	13 8.1	t = 0 h t = 48 h	238,800 208,600	12.65	42,606 20,403	52.11

. From an initial solution, several samples were extracted to perform the electro-oxidation process, consequently, the initial COD and carbonates in samples 2, 3, 4, and 5 showed the same values. All electrolytic processes carried out showed a decrease in COD and carbonates.

Concerning the chemical oxygen demand, test 1 displayed the greatest decrease, of 20%. In the 2nd test, the opposite occurred, with a decrease of only 7.5%. Regarding carbonates, there was a decrease in the range of 50–66% in all tests. Although no carbonate recombination occurred, the tests show a reduction in COD and high hydrogen production values (%), with very low generation of other gases.

It should be emphasized that, in tests 3 to 5, the duration was twice as long as the previous tests, since at 24 h the generation of hydrogen was still increasing.

5. Conclusions

The electro-oxidation process of organic wastewater in an alkaline medium, with a single cavity reactor, produces hydrogen, with different degrees of purity and flow rates.

Hydrogen generation rates of more than 70% were obtained in the gas flow at electrode voltages of 3–4.5 V. Other gases, such as O₂, CO, CO₂, and CH₄, were present at low levels. Thus, the non-production of molecular oxygen in the alkaline electro-oxidation process allows the use of a single cavity reactor without a membrane, avoiding the expense of the membrane required in two-cavity reactors.

Higher efficiencies can be reached by increasing the electrode surface area, but with a consequent increase in the electric current and in the generated flow rate of hydrogen, which implies a decrease in the efficiency. Hence, the system will be optimized according to the needs of the end-users, rather than just the efficiency value.

The electro-oxidation process results in an additional recovery of the wastewater by reducing its organic content. By providing an alkaline solution with the appropriate molarity, total oxidation is achieved, as carbon dioxide reacts to form carbonates. The alkaline electrolysis assisted by organic wastewater appears to be a promising technique for hydrogen production, with a single cavity electrolyzer under environmental conditions, due to the low generation of additional gases.

Variables that affect this alkaline electro-oxidation process, such as temperature, pH, molarities, or wastewater type, require a deeper analysis to optimize the volume of hydrogen production and to scale up the proposed process. Consequently, a multivariate approach based on artificial intelligence algorithms will be required.

The wastewater electro-oxidation process is still at the laboratory stage, but offers significant potential for slowing climate change without generating CO₂ emissions.