1. Introduction
Globally, there are many issues associated with water scarcity that have a negative effect on humans and on the environment [
1]. As a result, it is paramount to find a way of recycling water in order to reduce these impacts. One of the solutions to this is wastewater treatment. Wastewater utilization is one method to decrease the water shortage in many countries [
2]. If water is properly treated, it is important to reuse it for beneficial purposes [
3]. There are several options for reusing treated effluent [
4]. There are several categories for the reusing of treated water, as shown in
Table 1 [
4], and each option can go through several stages in the treatment process, depending on the quality of the water required [
5].
Transferring treated water to the sea is one way of getting rid of water. It is important to know the quality of the treated water so that it does not negatively affect marine life. However, in the presence of water scarcity problems, this method is not considered effective [
4].
Water scarcity can be reduced by methods of irrigation, such as reusing treated water for the irrigation [
6]. Instead of using groundwater or potable water in agriculture, treated water contributes to saving water and reducing the rate of water scarcity in the world, and is suitable for irrigation. Based on this, forty-four countries around the world use approximately 15 million m
3/day of treated water for crop irrigation [
4]. However, there are several parameters that gives brief descriptions of the water quality. These parameters can affect human health and the environment; they include Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD), and others [
7]. In view of this, three guidelines in three different years—1973, 1989, and 2005—were provided by the World Health Organization (WHO), with a focus on correct methods for the safe use of treated water in irrigation [
8]. The Food and Agriculture Organization (FAO) is also interested in using treated effluent for irrigation [
8], as it has issued several guidelines focusing on the water quality suitable for use in irrigation. This is necessary because human health and the soil quality for agriculture in the future could be affected [
4].
The reuse of water for drinking is becoming very important worldwide, as it has environmental, technological, and economic impacts. This is achieved by increasing sewage treatment plants in highly efficient ways to ensure high water quality, such as by using Reverses Osmosis (RO) systems that use membranes for increasing the water quality [
9], as well as advanced oxidation processes (O
3, H
2O
2, etc.), as sewage treatment plants must be highly efficient and advanced to provide water free of any bacteria. However, the Omani culture is somewhat different, as it is difficult to convince the community to drink treated water. From this point of view, there is no great benefit in treating water with high efficiency if the community is not convinced of the idea [
10].
It is possible to transfer treated water to groundwater, thereby increasing irrigation efficiency and improving the quality of agricultural crops. Studies in Oman found that when transferring treated water to groundwater, the area sown with both chickpeas and corn could be significantly increased [
11].
Another method for utilizing treated water is by producing hydrogen gas as a source of energy, since nowadays, sources of fossil fuel are decreasing and the world must focus on alternative sources of renewable energies such as hydrogen gas [
12]. From 2004 to the present, there have been many studies on the issue of hydrogen gas production, and the Sultanate of Oman has recently begun researching and addressing the idea of hydrogen gas production. Hydrogen gas is considered as a suitable gas in several fields of industry; for example, by 2050, it is expected that hydrogen gas will constitute about 10% of the global demand for heating buildings [
12]. The production of hydrogen gas from wastewater is one of the latest methods in the field of energy production, and will contribute significantly to reducing greenhouse gas emissions by 57–73% [
13]. Hydrogen can also be produced from wastewater, as this water contains a high percentage of organic matter, which, in turn, is the ideal substrate for microbes to produce bio-hydrogen [
14]. This is a suitable way to reduce the pollution resulting from sewage water and also to generate clean energy that contributes to reducing the emissions of greenhouse gases. Wastewater from food processing, dairy products, olive mills, and other sources helps in the production of hydrogen gas [
15]. Studies have shown that the production of hydrogen gas depends on several factors, including the type and source of wastewater, treatment technology, operating methods, and production methods [
16].
In this paper, hydrogen gas was produced using a reactor with a volume of 0.09 H2 L/L media, considering several topologies. The sludge and treated-water ratio was varied to generate hydrogen gas considering temperature and hydraulic retention time. Efforts were made to carry out the Life Cycle Impact Assessment (LCIA) in order to ascertain the depletion of the resources involved and find strategies for mitigating impacts on the ecosystem. A further analysis was carried in this study by comparing the LCIA with and without the use of solar energy. The LCIA can be improved by changing the type of the energy used, since it is clear from this study that electricity is one of the greatest negative impacts of the bio-hydrogen gas production process. The study was carried out in the hybrid OpenLCA and ecoinvent database environment.
1.1. Bio-Hydrogen Production
The increase in the demand for energy and the decrease in the production of fossil fuels has led to the search for other sources of energy that are renewable in nature [
17]. Fossil fuels have significant impacts on the environment, leading to an increase in global warming and an increase in temperatures [
18]. Therefore, the focus on renewable energy sources has become increasingly important in recent decades, and hydrogen energy has become an interest of many researchers and engineers. Since hydrogen energy does not significantly affect global warming, it is considered to be a renewable energy that relies heavily on organic materials [
19].
Bio-hydrogen is produced by specialized microbes used in the reactor. There are several ways in which bio-hydrogen is produced, as shown in
Figure 1 [
19]. Photosynthesis is the basic function of green plants, as they convert light energy into chemical energy that can be used in all cell activities. In the photo-hydrogen production process, the presence of microorganisms such as diatoms, cyanobacteria, and green algae is necessary, because these microorganisms help to convert light energy into H–H bonds [
19].
In direct bio-photolysis, light is absorbed by cyanobacteria called phycobiliproteins. These bacteria contain hydrogen and nitrogen enzymes [
20]. The absorbed energy is used to split water molecules into hydrogen and hydroxy; then, the hydrogen atoms combine with each other to produce hydrogen gas [
19].
In indirect bio-photolysis, photosynthetic bacteria are used to produce carbohydrates. These bacteria are fermented in the presence of light and then analyzed to produce hydrogen gas [
20]. The photosynthesis processes are very cheap and simple. However, the production of hydrogen can have low efficiency [
21].
In photo-fermentation, hydrogen gas is produced by anaerobic bacteria; Volatile Fatty Acids (VFA) such as acetic acid, and lactic acid are used as a source of carbon in the presence of light to produce hydrogen gas. There are several advantages of the photo-fermentation process; one is that organic acids have a high conversion rate. On the other hand, using light can make the cost very high, since this will lead to an increase in energy consumption [
22]. Because of this, dark fermentation is another way of producing bio-hydrogen. Dark fermentation is used in the absence of light to produce bio-hydrogen from organic matter, considering organic carbon as the energy source. One of the advantages of this process is that dark fermentation has high productivity, requires only a simple reactor, and uses cheaper substrates. The differences between the dark fermentation and phot- fermentation methods in bio-hydrogen gas production are summarized in the
Table 2 [
23].
1.2. Life-Cycle Assessment (LCA) and OpenLCA Evaluation Software
The International Standard Organization (ISO) provides a series of standards related to the evaluation of life cycle, its definition, and principles in ISO 14040. Life-cycle assessment is a widely used method to assess the environmental impacts of product life cycles and their technological processes, as well as waste management systems and waste disposal processes [
24]. Life-cycle assessment is defined as the set of inputs, outputs, and environmental impacts of a particular product throughout its life cycle [
24]. The LCA is used as a tool to study and analyze the environmental consequences of a product at all stages of its life cycle, and to try to reduce these consequences [
25]. The role of the LCA is to determine the likelihood of environmental consequences occurring in any life cycle and to attempt to reduce these consequences in various ways.
In recent decades, LCA has become an important tool for studying the life-cycle sustainability of any product, especially in research and industry, due to its quantitative nature. It can also help in studying interactions between inputs and outputs and environmental influences [
26]. There are many aspects of the ISO 14040 standards for the design and implementation of life-cycle assessment [
27]. According to the 14040, 2006, LCA can be divided into four basic stages, as shown in the
Figure 2 [
26].
The OpenLCA application allows the identification of the flow of materials and energy for the manufacture of a product, and facilitates the achievement of the desired goal of the LCA study. OpenLCA is freely available and used for life-cycle assessment and sustainability assessment [
28]. It was developed by Green Delta in 2006. Because it is a program available to everyone, it is suitable for sensitive data [
29].
One of the main strengths of LCA is its comprehensiveness in the study of environmental impacts, which allows the comparison of many systems, the use of many resources, and the determination of emissions in each system. It helps to find ways to improve the environmental performance of the system, helps decision makers to know the environmental impact of any process, and helps in obtaining benefits from all inputs and outputs [
30].
However, there are some challenges facing the use of LCA [
31]. LCA is only an estimate of the environmental effects; that is, it is not a real result [
32]. Furthermore, the time aspect has another impact on LCA, as the data is taken depending on the steady state. Therefore, the system of evaluation can be affected during the course of the evaluation period, which could greatly affect the LCA scheme employed.
1.3. Solar Energy System
In the last decade, the renewable energy sector has developed immensely in northern countries such as Sweden and Norway, which will be nearly 100% dependent on electricity from renewable sources in the next 5–10 years [
33]. Renewable energy is one of the important sources of energy that can help solve many environmental issues such as global warming, greenhouse gases (GHGs), and others. Solar energy is one form of renewable that is converted to electrical power in a clean manner by using photovoltaic systems. India has plans to employ solar energy for around 40% of its total energy, since it is the third biggest contributor of ozone-depleting substances on the planet [
33].
2. Material and Methods
2.1. Experimental Set-Up
This study is divided into two main aspects of hydrogen production. The first aspect is about hydrogen production from sludge and treated water. The second aspect is to study the life cycle and find the environmental effects of hydrogen production for use as energy in the future.
The salient part of the hydrogen production experiment used in this study is finding out the effect of an absorber and batch reactor on hydrogen gas production. In this experiment, a reactor was used to produce bio-hydrogen gas from a sample containing 50% sludge and 50% treated water. The reactor was covered for dark fermentation conditions, without using any additional sources of energy such as light or glucose.
The selected Sewage Treatment Plant (STP) in Oman was used as a case study in this paper. In the STP, water can be tested in many locations: before treatment, after the Membrane Bioreactor (MBR) treatment, and after the chlorination disinfection. Therefore, samples can be taken from these three locations and, after the MBR, the selected option can be utilized based on the sludge collected from the sludge collection tank.
Before starting the experiments, the connection between the bioreactor and other parts should be made by connecting the pipes correctly based on
Figure 3, as shown in model of the study [
34]. In this model, the connections are very important, and they should be made in a proper way to prevent any leakage during the experiment in order to obtain the correct results [
35].
Figure 4 shows how the connections are made correctly in the experimental set-up in the Germany University laboratory (GUtech lab, Oman) [
36].
The methodology for producing bio-hydrogen in this study is based on the following process. In this experiment, 500 mL of sludge is mixed with 500 mL of treated effluent in bioreactor. The bioreactor is a BioFlo/CelliGen115 from New Brunswick Company, Oman. Before staring the experiment, it is important to check the connection between the main parts, such as the absorber, gas bag, and the others, with the bioreactor. The check step can be performed using a leak detector. The bioreactor helps in carrying out many functions at the same time. These functions are measuring the value of pH, heating the sample, and bringing the temperature to 37 degrees Celsius. Using the motor for agitation during the fermentation process helps nitrogen gas to flow easily into the sample over about 15 min [
37]. The last step is covering the reactor for dark fermentation conditions. The complete details of all these steps are highlighted in
Table 3.
Gas Chromatography (GC) is a form of analysis that is used to determine a specific chemical component [
38]. This component can be gas or organic molecules. In the bio-hydrogen study, after collecting the gases from the experiments, hydrogen gas is separated and analyzed using a GC system. The GC device is a 6890N at Sultan Qaboos University in Oman [
39].
2.2. Goal and Scope of LCA
The goal and scope of the first stage in LCA is implemented with the objective of bio-hydrogen production from treated water and sludge [
40]. The goal of the LCA is to study the impacts of treating wastewater in STP and to know how the production of bio-hydrogen can affect these impacts, which will help in determining the best practices for reducing environmental impacts and producing hydrogen gas for many fields, especially for industrial use.
The system boundary and the activities of producing hydrogen from STP are carried out in the scope of the LCA. The overall flow diagram of the general system boundary is shown in the
Figure 5. From
Figure 5, it is clear from the boundary that there are several processes that were not considered, for example, transport of wastewater to the STP, the collection of influents, or disposal of the waste from the grit chamber.
Figure 6 shows the mass- and energy-flow model of the study. To focus on hydrogen production, the process starts with treated wastewater and removal of the waste from the first clarifier by using the blowers and pumps. Then, it moves to the MBR treatment, which uses pumps, blowers, and a stirrer to complete the treatment. After, the bio-hydrogen production process starts in parallel with the chlorination stage. Finally, the produced products are the bio-hydrogen gas and treated water for irrigation and cooling.
2.3. Life-Cycle Inventory Analysis
Gathering all the important information is considered in the inventory analysis stage [
41]. The data considered in the inventory analysis focus on the energy and mass flows of the STP, which were collected from Oman water authority in (26 May 2022). These flows are actually the outputs and inputs of mass and energy, for example, the energy consumption for blowers and pumps, the amount of chlorine, and others, as shown in
Table 4 and
Table 5.
2.4. Life-Cycle Impact Assessment (LCIA)
This study was carried out using the OpenLCA 1.11.0 software to find the impact assessment of the conventional process and hydrogen production process. This was achieved by using the ecoinvent 3.6 database from OpenLCA Nexus, with allocation cut-off classification as a system model, since this process focuses on producing a product from waste treatment (i.e., producing hydrogen from wastewater treatment as a source of energy and treated water for cooling and irrigation). The evaluation steps are the last part of the OpenLCA environment, and are important because they help select the best method in terms of the impacts of producing hydrogen from wastewater. There are several methods that can be used to present the data or the results in OpenLCA software. One of them is ReCiPe, which is based on the ecoinvent database. The selected version used in this study was the hierarchist midpoint, which has strong level coherence with a high level of obligatory prescriptions for proving the effects.
The last step in OpenLCA is to obtain the results of each impact category for all the mass and energy flows. Each column shows how each input or output affects the environment. There are many impact categories that OpenLCA computes (about 18 impacts). Each impact has a specific unit, for example, for climate change, the unit is kilograms of carbon dioxide, thus reflecting that carbon dioxide significantly affects global warming processes. All the impacts are gathered and compared before and after implementing the bio-hydrogen gas production. Three of the 18 impact categories were determined in the study. The selection of impacts for interpretation by LCIA was based on studying one ecosystem impact, one human impact, and one resource depletion impacts: climate change, ionizing radiation, and water depletion. The results of the LCIA process help to estimate the future impacts of both processes.
4. Conclusions
Wastewater treatment can help to reduce the impact of water shortages globally. Oman is one of the countries working on wastewater treatment to reuse treated water for many purposes, due to its location in a desert region. Bio-hydrogen gas is one sustainable way to reuse the treated water and produce green energy at the same time. Bio-hydrogen production can be achieved through different methods; one is dark fermentation, where the production of hydrogen depends just on the bacteria in the wastewater. However, for any process or any system, it is important to study the impacts and try to form a good recommendation to reduce negative effects.
The Life-Cycle Assessment (LCA) is one of the methods used to study the environmental impacts for any process. This study used OpenLCA software to study the LCA process, using ecoinvent as a database to complete the study and find the results. Studying the impacts of the life cycle of any process is important to be able to know whether the process negatively affects the environment and human life. In this study, the effects were analyzed and ways to reduce them were proposed by calculating the impacts based on the various category indicators using OpenLCA software and the ecoinvent database. As shown in the result, the scenario of hydrogen production with solar power is the best option, since this process will help to reduce water consumption by 2.83 × 104 m3 and the carbon footprint by 1.12 × 104 kg CO2-Eq. Thus, this study is in line with Oman’s Vision 2040, considering the fact that hydrogen will be a major transformational step in the field of energy in the Sultanate of Oman.