Feasibility Study of a Reverse Osmosis Desalination Unit Powered by Photovoltaic Panels for a Sustainable Water Supply in Algeria

Abstract

In recent years, reverse osmosis water desalination has developed rapidly and has become the most competitive and widely used technology in the world. The number of desalination plants is increasing rapidly as freshwater needs increase. Various membrane technologies have been developed and improved, including nanofiltration (NF) and reverse osmosis (RO), whose desalination costs have been relatively reduced. Therefore, this work proposes an experimental study for a small desalination unit based on RO generated by renewable energy, which is mainly suitable for arid regions or desert areas that do not have electricity and water and can be applied for emergency treatment to meet strong freshwater resource needs. In this study, to meet the drinking water demand, a reverse osmosis desalination system is designed and evaluated in order to improve and optimize its operation. This system has a daily capacity of 2 m³. We used brackish groundwater, which has been characterized as reference water, to produce synthetic water for different salinities until seawater. The analysis is based on data obtained from experiments carried out in the standalone RO pilot designed for the production of fresh water. For this purpose, we conducted relevant experiments to examine the influence of applied pressure, salt concentration and temperature on the RO membrane performance. The effects of different factors that affect the energy consumption in the RO desalination process were analyzed, and those with significant influence were explored. The effectiveness of RO desalination coupled with a photovoltaic (PV) energy system is shown. We found the recovery rate for system operation to be 32%. An optimization study is presented for the operation of an autonomous RO desalination system powered by photovoltaic panels. The energy produced by the PV system was used to feed two pumps forthe production of drinking waterwithanRO membrane, under the conditions of the town of Bou-Ismail. As results, a 3 kWp PV system was installed based on the energy demand. The design data have shown that a 3 kWp PV system can power a 1.8 W RO load given the Bou-Ismail climate. Energy consumption in the case study under Bou-Ismail weather conditions were analyzed. The desalination of brackish water at a TDS value of 5 g/L requires an energy of about 1.5 kWh/m³. Using seawater at a TDS value of 35 g/L, this value increases to 5.6 kWh/m³. The results showed that the optimal recovery rate for system operation was determined to be 32% for a feedwater salinity of 35 g/L, and 80% for a feedwater salinity of 1 g/L.

1. Introduction

Fresh water is an essential element for any form of life. Unfortunately, it is not available in infinite quantities on our planet, and it is not always available where it is needed most. Population growth and urbanization are increasing the demand for water in industrial and domestic urban areas. Problems arise in terms of quality for northern countries and in terms of quantity for southern countries. Algeria unfortunately does not escape this situation (water stress), given the scarcity of this precious liquid. This situation is due to a very significant demographic boom towards the coastal zone (which shelters the big cities),an elevation in the standard of living, increasing agriculture that requires large quantities of water, and increasing industrial activity. In addition, climate change, which causes long periods of drought, increases the pressure due to the lack of water resources. By definition, a region is considered to be under water supply stress when annual water supplies fall below 1700 m³ per capita. A region is faced with water scarcity if water supply drops below 1000 m³ per capita per year [1]. There are societal needs and personal needs for water. For example, personal use is in the range of 180–200 L per day for consumption, cooking, washing, and flushing toilets. Much of the water consumed by society is for non-personal use, including agriculture, industry, healthcare, commercial services, etc. Unfortunately, today, more than 1.4 billion people live on less than 1000 m³ of water per year. Developments in reverse osmosis (RO) membrane technology allow the desalination industry to make the largest desalination facilities in the world to produce drinking water. Nowadays, drinking water production has become a global concern for many regions, especially in the Middle East and North Africa (MENA), North Africa, and Asia. Developing countries suffer from rapid population growth with a scarcity of clean water, a lack of sanitation, an unreliable electricity supply, and off-grid electricity. In fact, more than 1 billion people are without clean drinking water, and approximately 2.3 billion people live in regions with water shortages [2]. According to Nafiseh Aghababaei [3], freshwater contains less than 1000 mg/L of salts or total dissolved solids (TDS). At more than 1000 mg/L, properties such as taste, color, corrosion propensity, and odor may be affected. Saltwater desalination is a rapidly growing sector that satisfies the rapid increase in water demand in the sectors of agriculture and industry. Around 107.95 million m³/d of installed desalination capacity was present globally at the end of 2022, and the market continues to grow [3]. The installed capacity indicates that the annual rate increases by about 10%. Sustainable development in the field of desalination consists of ensuring the efficient treatment andthe availability and sustainable management of water. Indeed, all solar desalination systems integrate solar technologies that can provide a sustainable solution to meet drinking water needs in areas with clean water scarcity and water consumption in the industry [4]. Water desalination processes are classified into two categories, namely thermal processes (distillation: multiple-effect MSF (Multi-Stage Flash) [5,6,7,8] or MED (Multi-Effect distillation)) [9] and membrane processes (RO, electrodialysis) [10,11,12].

RO is a mature membrane method that is technologically well controlled with recovery rates of more than 60%. The big drawback is that it still consumes energy, despite the integration of energy recovery systems. This depends on the water to be desalinated; a lower specific energy consumption is found in brackish water (BWRO) due to its lower osmotic pressure compared to seawater [13,14]. The energy requirements for BWRO and SWRO are approximately 1–1.5 kWh/m³ and 3–4 kWh/m³, respectively. Consequently, the costs of desalination are mainly linked to the costs of the electrical energy consumed. To this end, it is important to take up the challenge of reducing the energy cost of desalination in the future. Desalination of seawater and brackish water is the best way to meet the growing demand for drinking water. However, desalination remains relatively expensive today, despite the drop in the cost of RO [15]. Actually, the majority of conventional water resources are very limited in several regions in the world, according to the climate changes that are explained by changes in global temperature and weather patterns observed today, which have sounded an alarm. Therefore, to solve this problem, the desalination process is presented as a solution by providing high-quality drinking water [16]. Renewable energy RE has many advantages over traditional energy sources. It is clean, sustainable, and has a low environmental impact. RE sources, such as solar, wind, and geothermal energy sources, are becoming more affordable and reliable. Additionally, they can reduce air and water pollution. They can also help reduce reliance on fossil fuels (electricity), which are a major contributor to climate change and which provide energy security [17]. The desalination of brackish water or seawater systems powered by renewable energy sources offers a promising and sustainable solution that resolves water scarcity in many regions of the world [18]. Indeed, solar energy systems coupled with desalination have contributed to sustainable development that can take into account the environment. Different solar energy-driven desalination technologies are mainly based on the capacity of the system, the type of energy source, and the raw water to be purified. Desalination using renewable energies is mainly based on the RO process, followed by the MSF and MED thermal processes [19]. The MENA region’s groundwater resources will decline gradually, and there will be a water demand exceeding 13 million m³/d by 2030. Therefore, desalination capacity is expected to increase speedily by around 110 million m³/d, and the total electricity demand for desalination is also expected to reach 122 TWh [20].

The main source of renewable energy is the sun or solar power, which is converted into electrical or thermal energy. The revolution in renewable energy technologies is resulting in the speedy development of mainly solar photovoltaic (PV) systems, followed by wind energy and solar thermal energy. In fact, the solar PV sector is one of the main generators of energy (electricity generation) that produces economical energy, generating a significant percentage of electricity needs. Actually, the integration of renewable energies in the desalination process has been largely successful, as demonstrated by the RO membrane (RO) process, followed by thermal desalination processes such as MSF and MED, which consume a large amount of energy compared to RO [21].

In MENA countries, there is an increasing need for drinking water supplies because their available resources are decreasing due to climate change and the pollution of surface water or groundwater. The majority of available water resources have largely been exploited. Consequently, the increased demand for water increases the cost of developing non-conventional water sources. RO is a more efficient and economically viable process. Pressure is used to reverse the osmotic flow of water through a semipermeable membrane [22]. These regions cannot sustainably meet their actual water demand without using a supply from non-conventional resources, especially desalination and treated wastewater reuse. Salt groundwater is always abundant and available in semi-arid regions whose salinities can be very high, and it particularly changes depending on the land’s topography and the environment. Different existing experiments show the technical and significant viability of coupling RO water desalination with PV. Various existing experiments demonstrate the technical and significant viability of coupling desalination with an RO plant using PV technology, as installed in Riyadh, Saudi Arabia; Morocco and Tunisia [23]; Doha-Qatar [24]; Egypt [25]; Spain [26]; Tanzania [27,28]; etc.

For the performance evaluation of the desalination system, A.M. Helal [29] conducted an economic feasibility analysis of alternative designs of a PV-RO desalination unit for remote areas in the United Arab Emirates. He used three different energy systems: a diesel generator, PV-diesel, and an off-grid PV system without battery backup to operate the RO system. It was shown that a permeate productivity of 20 m³/d was produced using the assisted PV-ROdiesel, but a freshwater volume of 44 m³/d was obtained using the solar RO on a clear and sunny day. In fact, because desalination is an energy-intensive process, a sustainable energy source is needed [30]. H. Bilal et al. [31] carried out a study onaportable RO (PVRO) system with a battery and without a battery, running the RO system for 5 h. Without energy storage, the analysis showed that permeate productivity was 3.8 L/h, while it increased to 5.9 L/h with the battery. The freshwater productivity was 9.8%. In addition, they found that the batteryless PV system was more economically suitable than the battery-powered system.

Using HOMER (https://www.homerenergy.com/, accessed on 9 August 2023) and Excel software, a techno-economic study of an off-grid PV-RO system in nine districts of Iran was conducted by A. Mostafaeipour et al. [32]. The study concluded that solar desalination is an economically viable technology. Ghafoor, A et al. [33] performed an experimental study of an RO plant (500 L/h) coupled with a solar PV system (2 kWp) to analyze techno-economic feasibility. The performance of the PV-RO system in terms of power produced and membrane productivity, along with an economic analysis, was evaluated. They found that 15–20% of the PV energy is increased by tracking the PV system, while 5–10% is due to cooling the PV panels. The cost per liter of desalinated water was calculated to be USD 0.002592, with a total monthly profit of USD 194.4. The payback period of the PV-RO system was calculated to be 1.83 years. Shalaby, S.M et al. [34] focused on solar-based RO plants, which have been established to decrease the specific energy consumption by using PV or solar thermal power plants—in particular, the organic Rankine cycle. In addition, various preheating techniques performed by recovered heat from other systems, such as a PV cooling unit, a humidification–dehumidification process, and hybrid systems used for brine disposal challenges were presented and discussed. Maftouh et al. [35] presented a comparative and systematic review of the economic feasibility of using solar PV-RO for desalination in the MENA region. They highlighted the importance of RO technology powered by renewable energy resources, in which the detailed challenges associated with the solar RO technique were elucidated. They concluded that RO systems are more cost-effective in MENA countries where water salinity is lower, which explains why North African countries use RO systems. On the other hand, the Gulf Cooperation Council countries choose thermal processes because of the higher salinity of the water. It was found that the use of renewable energy in desalination is economically feasible, depending on the specific needs and water conditions of each country. To maximize the rate of freshwater production, Monjezi, Alireza Abbassi et al. [36] developed an off-grid solar energy-powered RO desalination system with integrated PV thermal (PVT) cooling. They evaluated and compared the required solar panel area with and without a cooling system. The results showed that a reduction of 0.12 kWh/m³ in the specific energy consumption (SEC) of the RO unit can be achieved by using PVT cooling, resulting in a 6% reduction in solar panel surface.

Various factors need to be taken into account when designing brackish water reverse osmosis (BWRO) desalination plants with short operating times, such as those powered by renewable energy sources [37,38]. Many published works have studied the impact of operating parameters and SEC in the intermittent mode (during solar irradiance fluctuation) on the performance of a BWRO desalination plant during operation and, consequently, on osmotic backwash and membrane fouling [39,40,41,42]. The evolution of operating parameters and the ideal SEC over a long period of intermittent operation with constant water production has been carried out and investigated in the relevant work of A. Ruiz-García and I. Nuez [38,40]. The objectives were to provide a long-term performance analysis of a large-scale BWRO desalination plant operating intermittently without membrane replacement. Other variable renewable energy sources for powering RO desalination have been investigated in a case study of wave-powered desalination in Kilifi, Kenya and experimental data on a commercial RO system presented by J.Leijon et al. [39]. They discuss the possibility of using wave energy converters (WECs) to power RO systems. It was found that the power output of the WEC studied in this work is estimated to be 7 kW, and it would be sufficient to power a suitably sized RO system.

Algeria is facing a severe water crisis due to the increasing demand for water, limited water resources, and climate change [43]. The latter has led to decreased rainfall and increased temperatures, resulting in increased water evaporation. In addition, the lack of water has caused severe damage to the environment, leading to desertification and soil erosion. This has had a negative impact on the country’s agricultural sector, leading to reduced crop yields and increased food insecurity. In order to address the water crisis in Algeria, the Algerian government has taken measures to increase the water supply, improve water management and reduce water wastage. Desalination is seen as a viable solution to this problem, as it would provide a reliable source of freshwater that can be used for drinking, agriculture, and industry [44,45,46,47,48].

For the past fifteen years, Algeria, like many other developing countries in the MENA region (Middle East and North Africa), has launched a national program to implement the strategies developed to address the water shortage through the use of unconventional resources—in particular, the desalination of water and the reuse of treated wastewater. The most important method on which the policy of our Ministry of Water Resources (MRE) was based to end this crisis is the desalination of seawater and brackish water. Among these many proven desalination processes, Algeria has opted for the RO technique in the vast majority of cases for its effectiveness. In fact, RO is the most widely used, most profitable process and consumes less energy compared to the distillation process.

Desalination is an alternative option to Algeria’s water crisis. The cost of desalination is a major challenge. In fact, the plants are expensive to construct and operate, and consequently, the water produced is often too high. In addition, the process of desalination is energy-intensive and expensive. The plants require a large amount of energy to operate, which can be difficult to obtain in some Algerian regions. Additionally, the plants require regular maintenance and repairs. The plants use RO technology, which is the most efficient and cost-effective method of desalination.

The number of desalination plants reached 21 in 2019 in Algeria, distributed along the 14 coastal wilayas, providing 17% of the water consumed in the country and supplying 6 million people [43,48]; two more plants are in the pipeline [49]. The country plans to increase the number of desalination plants to 43. Indeed, the El-Magtaâ plant is the largest seawater desalination plant in Algeria and Africa and uses the RO method;it has a capacity of 500,000 m³/day, and recently, an even larger one was built and has been operating since 2014 [47,48]. It covers the needs of more than 5 million people at a rate of 100 L of water per day/person. The plant was built by the Singaporean group Hyflux on behalf of the Algerian Energy Company (AEC), a subsidiary of Sonatrach [45,49].

Algeria has one of the highest solar deposits in the world. The large Sahara Desert covers most of Algeria’s south, where solar energy is the most abundant natural resource. The overall installed PV power is about 1.2 MW [50]. The sunshine duration over almost the entire national territory exceeds 2500 h annually and can reach 3900 h (Hauts-Plateaux and Sahara). The energy received daily on a horizontal surface of 1 m² is of the order of 5 kWh over most of the national territory, i.e., nearly 1700 kWh/m²/year in the north and 2263 kWh/m²/year in the south of the country. This solar deposit exceeds 5 billion GWh/year. For example, 270 MWp was installed in 2015, bringing the cumulative power to 300 MWp; only 50 MWp was installed in 2017, but a call for tenders for 4 GWp was announced for 2018. The National Renewable Energy Program (2015–2030) now has a target of 22 GW of renewable power with a share of 13.5 GW of PV power by 2030 [51]. The biggest constraints of the desalination system are its energy consumption per cubic meter produced and its environmental impacts due to discharges of brine in the natural environment. Despite these constraints, desalination plants are increasingly widespread around the world and include desalination processes to deal with the increasing water demands. Resources are limited in quality and quantity, resulting in the establishment of treatment solutions for brackish water and seawater.

Several types of research have been reported in the literature [52,53,54,55,56] to find ways of integrating renewable energy into the field of membrane desalination in order to increase the daily yield of small solar desalination systems. Algeria has a population of around 44 million. If each person requires 100 L/d (the minimum requirement in many western countries; in practice, most consumers use between 100 and 200 L/d when the water is not metered), then the required water supply is 4.4 million m³/d. The plants currently provide 2.12 million m³/d, equivalent to 780 million cubic meters of water per year, which is not enough to meet the needs of over 10 million people. The required shortfall is 2.28 million m³/d. Algeria is currently building several desalination plants to meet the growing demand by reaching a capacity of about 45% of the fresh water demand. Two seawater desalination plants (SWDP) are under construction that will have a total capacity of 600,000 m³/d and 80,000 m³/d. In addition, at present, five SWDPs are under construction, each having a capacity of 300,000 m³/d. Others have been approved but not built; there is a program to complete six SWDPs by 2024, each having a capacity of 300,000 m³/d. For sustainable desalination, renewable energies could be a good solution, especially since Algeria is one of the sunniest countries in the world, thus favoring the production of solar energy without neglecting the important wind and geothermal deposits located essentially in the south of the country. Despite the desalination plants being located in the north all along the coast, they can obviously be powered by renewable energies produced in the south and injected into the national grid.

Population growth and economic development around the worldhave led to and will continue to lead to a considerable increase in energy consumption. At the present rate of exploitation, the known fossil fuel reserves (coal, shale, oil, gas, and hydrates) could last between one and two centuries. However, as part of the United Nations Net Zero (UNNZ) framework, Western national governments have collectively decided to phase out fossil fuels by 2050–2070 and are attempting to impose this Net Zero Program (Agenda) on the rest of the world. It is important to note that governments have consistently proclaimed since the 1950s that the world will run out of fossil fuels within 20 years of that time. Instead, since the 1950s, oil production has multiplied tenfold, and coal production is currently at a world record level. Today, PV solar energy has experienced strong exponential development in recent years. It is a continuously growing renewable energy whose electricity is produced by transforming part of the solar radiation with the help of a PV cell. PV is currently becoming a source of energy that can compete with conventional sources (with or without connection to the grid) and can meet different requirements in order to ensure sustainable development. For example, in Algeria, the government has put in place several measures to develop the PV and wind industry. A program was adopted that made it possible to strengthen the PV industry and make it competitive with fossil fuels from an economic point of view. The use of this once non-polluting operation and sustainable energy is an undeniable renovation in terms of the impact on man and the environment [55].

To this end, the present work is an experimental study on solar-driven stand-alone RO membranes. The RO desalination system is supplied by a stand-alone PV system of 3 kWp with energy storage. It produces an average daily drinking water amount of 2 m³ per day. The RO membrane’s performance and energy consumption are presented for different water salinities. An autonomous desalination system could serve as a basis for the future development and deployment of small-scale water treatment solutions in water-stressed regions.

In the present study, we propose the development of an RO membrane desalination process coupled with solar PV energy. It is a very promising technology that combines the two systems (RO and PV), allowing for a sustainable process of water desalination. To do this, it consists of coupling a water desalination process with a PV electricity production with a storage system. The experimental system was designed to conduct relevant experiments to evaluate the effect of salt concentration, pressure, and temperature on the operating performance and energy consumption of ROPV.

A study of the application of different synthetic waters of variable salinity concentrations (1–35 g/L) was carried out. Using solar energy for PV power generation with a storage system can power small RO facilities in remote, arid, and even coastal areas where fresh water is scarce and solar radiation is very important and not connected to the network, and where water resources are limited.

The RO desalination system is supplied by a stand-alone PV system of 3 kWp with energy storage. It produces an average daily drinking water amount of 2 m³ per day. The RO membrane performance and energy consumption are presented and discussed.

2. Materials and Methods

The RO membrane is a filter that allows only water molecules through and leaves ions and impurities in the brine. RO is a process that occurs whenan amount of pressure greater than the osmotic pressure is applied to the concentrated solution, as shown in Figure 1. The semipermeable membrane can remove many types of dissolved and suspended chemical species, as well as biological ones (bacteria), from water, and it is used in both industrial processes and theproduction of potable water.

The study of saltwater desalination by membrane separation was carried out on the pilot RO installed at Solar Equipment Development Unit (UDES). Indeed, this filtration system, which treats a volume of 200 L, is illustrated in Figure 2. In this study, RO membrane element “SWC5-LD-4040” with low fouling technology designed to reduce biological growth was used to investigate its performance. It is made of polyamide and has an active surface area of 7.43 m². The experimental setup used is a stand-alone installation made up of two systems that are combined for solar desalination in order to evaluate the functioning of the RO process on a laboratory scale. The first is a pilot membrane filtration and the second one is a solar system for electricity generation based on renewable energy with energy storage. The characteristics and technical details for the PV panels, as well as the features of the solar inverter, battery, and controller used in the PV system for this study, are displayed in

Table 1. Characteristics of the PV modules used.

Electrical and Mechanical Characteristics
Cell Type	Monocrystalline Cell with Anti-Reflection	Junction Box and Connector	IP65, 3 Bypass Diodes and Compatible Cables with MC4 Connector
Number of cells per module	36 cells (4 × 9)	Operating temperature	40 bar at 85 °C
Cell sizes	156 × 156 mm	Nominal power (W)	150
Module dimensions	1482 × 674 × 45 mm	Open circuit voltage Vco (V)	22.35
Module weight	12 kg	Short-circuit current ISC (A)	9.005
Panel frame	Anodisedaluminum alloy	Tension mppVmmp (V)	18.54
Type of glass	Tempered glass, 3.2 mm thick	Curent mppImmp (I)	8.29

and

Table 2. Features of the solar inverter, battery and controller in the PV system.

Characteristics of the Batteries (Condor)	Charge Controller (Condor)	Single-Phase Stand-Alone PV Inverter (Victron)
Manufacturing technology: AGM-Gel battery. Nominal voltage of a battery: 12 V. Storage capacity: 250 Ah. Nominal voltage of the battery pack: 48 V. Autonomy day(s): 2 days.	Charge controller with MPPT. Nominal voltage: 24/48 V. Nominal current: 50 A.	Inverter with MPPT. Nominal apparent output power: 5 Va. Nominal input DC voltage: 48 V dc. Nominal AC output voltage: 230 Vac single-phase.

.

2.1. Solar Desalination System PV-RO

The experimental study involved the evaluation of a stand-alone pilot solar desalination unit in terms of membrane productivity and energy consumption under climatic conditions of the Bou-Ismail region (36°38′33″ N, 2°41′24″ E).

This pilot unit is designed to desalinate the seawater, which is characterized by an electric conductivity level of 56 mS/cm. The RO pilot unit was constructed within the framework of the internal project UDES/EPST-CDER. The system is designed to produce 2.4 m³/d of permeate water. The solar system for electricity generation is based on renewable energy with energy storage. It is equipped with an array of PV modules with a total capacity of 3 kWp. The system consists of a pressure casing containing a spiral membrane module, a high-pressure (HP) and low-pressure (LP) pump, a flow meter, a pressure gauge, two cartridge filters, and storage tanks. The RO unit is attached to a stainless-steel skid intended for the demineralization of brackish water and seawater desalination. A three-phase high-pressure (HP) stainless steel motor pump is connected behind the filters and directly feeds the RO membrane. The manometer is placed at the inlet of the membrane module to measure the 0–80 bar pressure that is regulated by a valve installed on the stainless-steel chassis. We have taps for taking samples of the permeate, concentrate and feed water. The daily power consumption is 2 kWh, the daily charge to the batteries is 952 Ah and the power recovered from the PV is 3000 W.

As shown in Figure 3, using a low-pressure pump, the feed water is pumped into sand as a pre-filter, which is a preliminary filtration stage to remove larger particles in the water; then, commercial activated carbon was used toremove odors and other compounds followed by two microfiltration MF filters (1–5 μm). The HP pump is then connected to the RO unit membrane to provide the pressure driving force needed to facilitate desalination through the RO semi-permeable membrane. The low and high-pressure pumps are powered by a solar PV system during the daytime. The surplus electricity provided by the PV panels during the day is stored in batteries that can power the pumps at night.

To optimize the membrane performance, water with different salt concentrations was desalinated through an osmosis unit powered by renewable energy. This involves evaluating the critical pressure values characterizing the brackish water and seawater on which the membrane treatment operates. In the present study, well water of the Bou-Ismail region (salty borehole water) was taken as benchmark water. Then, different synthetic samples were prepared and analyzed in order to produce saltwater with the same salinity as Mediterranean seawater and Algerian brackish water,

Table 3. Initial analytical characteristics of the different saline waters.

Physico-Chemical Parameters	Unity	Water 1	Water 2	Water 3	Water 4	Water 5
pH	-	7.40	7.10	7.56	7.40	7.60
Conductivity	µS/cm	2231.00	10,600.00	21,200.00	33,700.00	50,000.00
Turbidity	NTU	0.30	0.37	0.49	0.40	0.61
TDS	mg/L	1115.00	5700.00	11,700.00	20,100.00	35,200.00
Salinity	g/L	1.00	5.00	12.00	20.00	35.00
Durety	mg/L CaCO3	894.00	910.00	930.00	960.00	1010.00
Alcalinity	mg/L CaCO3	171.00	170.00	167.00	175.00	162.00
Magnesium	mg/L	675.56	695.17	697.54	705.10	748.68
Calcium	mg/L	218.44	214.83	232.46	254.10	261.32
Chloride	mg/L	165.92	2268.99	5530.67	7941.47	18,337.64

.

The RO system was equipped with an energy recovery system from the brine. A spiral wound Hydranautics membrane producing water at a flow rate of 84 L/h with a recovery rate of 98% was used.

For all the treatment tests with filtration on RO membranes, different samples were analyzed in order to follow the parameters indicated in Table 3. We observe that the feed water is very rich in salts with higher mineral concentrations than well water, and the analytical characteristics of these waters are quite similar. However, we notice that the more the salt concentration increases, the more the conductivity and TDS values increase. Water sample 5 shows salt concentrations close to that of Mediterranean seawater for possible characterization.

Indeed, it is necessary to assess the initial physicochemical characteristics of the feedwater before treatment, which is an essential indication in order to control the quality of the water after membrane treatment. The conductivity analysis has shown that the feed waters have conductivity and TDS values that are well above the standards of the World Health Organization (WHO), which are, respectively, 2700 (μs/cm) and 1000 (mg/L).

2.2. Photovoltaic Power System

2.2.1. Design and Sizing of a PV System

The design of an autonomous PV installation dedicated to a specific application as solar desalination by RO is based on several stages, the required characteristics of which must be precisely chosen to obtain optimal operation. To be able to size this installation, it is essential to make a load estimate and to know the amount of energy available on an inclined plane.

The sizing study was based on the electrical and physical characteristics of the HP pump and the operating mode of the system, namely, the number of operating hours (4 h/d), the number of operating days (5 days/week), and battery operating time (2 days). All features of the HP pump are pump output (11.5 L per minute (1420 TPM)), voltage and current (400 v-3.7 Amp), pump speed (1700 t/m), rated mechanical power (1 KW), power factor (Cos) (0.77) and electric power (1.8 KW).

For the sizing of the annual PV part recorded on the Bou Ismail site, the monthly solar irradiation received on a plane inclined to the latitude of the site’s location is estimated. To do this, we processed the lighting data recorded and provided every 10 min by the Bou Ismail meteorological station. A program on Matlab (R2021a)was developed for reading, processing, and calculating the estimated monthly irradiation on the Bou-Ismail site. The geographical coordinates of the Bou-Ismail region are the following:

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Latitude of the Bou-Ismail site in degrees: 36.64°;

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Longitude of the Bou-Ismail site in degrees: 2.69007°.

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PV generator tilt angle in degrees: 36°;

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The average global solar irradiation incident on the inclined plane of the PV generator of the worst month of the Bou-Ismail site is of the order of Gdg = 2800 Wh/m²/day; (measurement made at the UDES weather station).

Figure 4 illustrates the variation in the monthly energy production calculated during one year in the Bou-Ismail location. The results showed that the maximum energy production generated reached 310 kWh in the month of July, whereas the lowest power generated in the months of November and December is 159.363 kWh and 159.025 kWh, respectively (the most unfavorable months). This total annual production in kWh per year is the sum of the simulation values in kWh. During this period, the total power production from this plant was 6005 kWh. Figure 5 illustrates the evolution of the ambient temperature and solar intensity with operating time for one experiment day. The irradiation is the integral of theirradiance arriving on a plane characterized by its orientation and inclination. Illumination data recorded by the Bou-Ismailmeteorological station is provided for each time interval t = 5 min.

2.2.2. PV System Operation

A PV generator (48 V) drives the input of a PV regulator, which helps protect the electrochemical storage device from overload. The electric accumulator system supplies the input of a stand-alone 48 V-5 KVA PV inverter, which provides at its output a regulated alternating voltage of 230 V. An electronic starter made up of the association (single-phase rectifierthree-phase variable speed drive) ensures the progressive starting of a three-phase asynchronous motor driving a centrifugal pump, which has the effect of considerably reducing overruns of the motor load starting current.

A second single-phase asynchronous motor with a starting capacitor is connected in parallel with the three-phase asynchronous starter-motor cascade in order to extract energy from the PV inverter. The advantages of using a stand-alone single-phase PV inverter reside in the fact that the storage of electrical energy is carried out at low voltage. This avoids the need to put a large number of batteries in series, and the transformer integrated in the inverter makes it possible to increase the output voltage to 230 V-AC. The synoptic diagram of the installation is given in Figure 6.

The result of the sizing study shows that the stand-alone PV conversion chain should include 20 monocrystalline-type 150 Wp PV modules, 02 charge regulators, an inverter and solar batteries. Therefore, the 20 modules of 150 Wp were connected to have a voltage at the output of the 48 V generator. Every four panels are connected in series, and therefore, we obtain this connection five times, and they are then connected in parallel. All of this connection was carried out in collaboration with the UDES electronics workshop team. The modules of this solar generator were placed on aluminum supports fixed on the roof of the Water Treatment Division of UDES. To provide water supply, we have set up a small-decentralized energy-water system PV-RO, which will have the potential to profitably supply an isolated region or small village where energy and drinking water are not available or difficult to produce due totheir high cost. This most promising solar PV desalination prototype demonstration has been tested under specific irradiation in the town of Bou-Ismail by treating saltwater for different salinities.

The solar panels were arranged in five modules in parallel and four modules in series with a nominal power of 3000 W from batteries, supplying sufficient energy for two days.

In this case, we are interested in the autonomous configuration for an isolated site with energy storage. Autonomous solar PV systems are very suitable when it comes to supplying, for example, small water treatment and desalination units that are not connected to the electricity grid. The production of electricity by PV panels remains the most profitable technique and claims to have much more in the future. The sunniest regions ensure better production. Indeed, these installations are more widespread in arid regions and remote sites that are not connected to the electricity network.

3. Results and Discussion

The experimental results obtained during this study are represented in order to show the efficiency of the RO process operating with solar PV energy. The influence of some operating conditions on the performance of the membrane is studied. The main operating parameters affecting the performance of membranes are pressure, temperature, flowrate, recovery rate, pH… etc. In order to test the operation of the RO desalination process as well as solar coupling and the feasibility of an autonomous PV system associated with a reverse osmosis system, the effect of parameters such as pressure, salinity, and temperature on recovery rates and the energy consumption is studied.

3.1. RO Membrane Performance

The effect of the pressure variations on the recovery rate and the energy consumption of the desalination pilot was analyzed. The results are presented for different salinities and treatments. Different synthetic salt waters simulating those existing in Algeria were prepared and used. In this study, the pressure that varies in the range of 5–65 bar with a step of 5 bar is considered

Figure 7 depicts the effect of pressure increase on the recovery rate of feed water through the membrane surface for brackish water and seawater. Plots show that the recovery rate tends to increase with pressure for different salinities, resulting in increased energy consumption. The results showed that the recovery rate increases with increasing inlet pressure, indicating that high operating pressures are needed to achieve fairly high recovery rates, and this strongly depends on the characteristics of the membrane used. In general, the effect of pressure increase on recovery rate is more significant in the case of the high salinity seawater than in brackish water, as shown in Figure 7. For example, the RO plant achieves an optimal recovery rate of 85% for a feedwater salinity of 1 g/L and a rate of 32% for seawater. Certain stability in the recovery rate is observed above 55 bar due to the start of stability of the permeate flow. Following the phenomenon of concentration polarization, it is imperative to apply a pressure below this threshold limit in order to avoid the compaction of membranes and modules.

For the concentration tests, the inlet pressure to the membrane module, the permeate flow rate and the retentate flow rate were evaluated. For comparison, the permeation and the flow rate of concentrate for different salinities ranging from 1 g/L up to 35 g/L and the membrane inlet pressure varyingfrom 5 to 65 barare given below to examine the membrane performance. Figure 8 illustrates the change in the permeate flow rate for different salinity values and as a function of pressure. The curves show that the behavior of filtration or the demineralization of saline water follows the same pattern. For low and high salinities, it can be seen that as the inlet pressure increases, the permeate flow rate increases relatively. For low and high salinities, it is found that as the inlet pressure increases, the permeate flow rate increases strongly in a linear manner. The results showed that for low salinities, the minimum critical pressure for initial permeate production is around 5 bar. However, for high salinities of 20 g/L and 35 g/L, it is 20 bar and 35 bar, respectively, Figure 8. On the other hand, the flow rate of the concentrate decreases rapidly with the increase in pressure, as shown in Figure 9. This evolution shows that if a membrane has a transfer mechanism by diffusion, the flow rate of the permeate is directly proportional to the pressure applied, while the flow rate of the concentrate is independent.

It should be noted that the flow rate curves coincide with a threshold pressure value. The points of intersection for 1 g/L, 5 g/L and 12 g/L between the permeate and concentrate flow rates indicate a constant recovery rate of 45% and 52%, respectively, whereas for high salinity, these curves no longer intersect, as shown in the case of 35 g/L (Figure 10).

According to WHO standards, the maximum allowable limit for total dissolved solids (TDSs) and conductivity in treated water is 1000 mg/L and 2800 µS/cm, respectively [57,58]. The water after treatment by RO exhibits TDSs between 34.5 and 197 mg/L (Figure 11). Thus, the conductivity values are between 69.2 and 387 µS/cm. These values comply with the standards, which indicates that this treated water is then suitable for consumption.

The analytical results of the 12 g/L and 35 g/L feedwater and permeatesamples taken at a pressure of 55 bar are shown in Figure 12 and Figure 13. It can be seen that the ion concentration in the raw water exceeds the WHO standard. However, after treatment of the water by RO, the ion content reaches values within the WHO standard.

Water intended for human consumption must not contain any pathogenic microbes. Fecal contamination is detected by the presence of Escherichia coli or fecal streptococci. The results of the search for and enumeration of pathogenic bacteria in the well water (1 g/L) and in the water treated with the SWC5-LD-4040 membrane at 55 bar are presented in

Table 4. Results of the bacteriological analysis.

Parameters Sought (UFC)	Maximum Allowable Concentration	Well Water	RO Water at 55 Bars
Total coliforms/100 mL	0	10	0
Faecal coliforms/100 mL	0	10	0
Sulphite-reducing anaerobes/20 mL	0	5	0
Fecalenterococcus/Intestinal enterococcus/100 mL	0	9	0

CFU = colony forming unit.

. The membrane filtration technique is used to detect, identify, and count indicator microorganisms in the water sample. An appropriate volume of the sample is filtered through a Millipore membrane with a pore size of 0.45 mm. The membrane is incubated on an agar plate. Bacterial cells trapped on the membrane will grow into colonies that can be counted, and a bacterial density can be calculated. The number of visible colonies (CFU) present on a plate can be multiplied by the dilution factor to provide the CFU/mL value. The feedwater is found to contain fecal contamination germs, which allows us to consider that the well water is bacteriologically impure and threatened by pollution. Nevertheless, the analyzes obtained after an RO membrane treatment show a total absence of fecal contamination germs, and therefore, the water has good bacteriological qualities that are in accordance with the standards for drinking water intended for human consumption.

Table 5. Physicochemical and microbiological analysis of permeate and retentate water, feed water salinity 35 g/L, feed water flow rate 200 L/h.

Parameter	Permeate Water	Retentate Water
Salinity (g/L)	0	65
M.E.S	4	79.5
Ammonium (mg/L)	0.0555	0.0635
Nitrites (mg/L)	0.1017	0.1585
Nitrates (mg/L)	1.29	11.0184
Phosphates (mg/L)	0.3416	0.3271
Chlorides (mg/L)	0	36,675.200
Carbonates (mg/L)	0.0097	0.0675
Calcium (mg/L)	7.711	275.49
Sulphates (mg/L)	346.90	2.884
Total coliforms	Absence	Absence
Fecalcoliform	Absence	Absence
Escherichia coli	Absence	Absence
Salmonella	Absence	Absence

CFU = colony forming unit.

illustrates the results of the physicochemical and microbiological analysis of permeate and retentate water for a high salinity of 35 g/L. These analyses found that the water after RO treatment at a pressure of 55 bar has TDSs between 29 and 255 mg/L. Thus, the conductivity values are between 59 and 250 µS/cm, as shown in Figure 6. These values comply with WHO standards, which indicates that this treated water is suitable for consumption.

3.2. Renewable Energy Supply and Consumption

The electrical power supplied by the HP pump of an RO membrane at different salinities has been measured using an energy meter and a Score smart plug. The different results of the power supplied by the pump are summarized in

Table 6. Results of power supplied by the HP pump.

Pressure (Bar)	Power kW (Salinity 5 g/L)	Power kW (Salinity 12 g/L)	Power kW (Salinity 20 g/L)	Power kW (Salinity 35 g/L)
5	0.391	0.374	0.391	0.366
10	0.479	0.465	0.384	0.458
15	0.568	0.560	0.479	0.574
20	0.667	0.676	0.564	0.654
25	0.777	0.779	0595	0.779
30	0.884	0.884	0.698	0.896
35	1.020	1.000	0.867	1.029
40	1.161	1.144	0.988	1.123
45	1.268	1.266	1.153	1.254
50	1.379	1.399	1.295	1.394
55	1.592	1.553	1.396	1.592
60	1.739	1.667	1.543	1.680
65	1.907	1.855	1.855	1.859

. For each given pressure and salinity, the electrical power generated by the system was calculated. According to the results, the power of the various salinities coincided with powers ranging from 0.36 to 1.9 kW for inlet pressures of 5 to 65 bar for a given salinity. It is noticed that when the pressure increases, the electrical power also increases.

Figure 14 represents the law of variation of the electric power supplied (Pe) by the HP pump as a function of the permeate flow rate (Qp) obtained for different salt concentrations (5 g/L, 12 g/L and 35 g/L). Qualitatively, we obtain a linear law for the four studied salinity values, which is of type y = ax + b, where a and b are two constants. From this constitutive law associated with each feedwater salinity, the energy consumed to produce 1 m³ of water iscalculated. The equation of the linear regression line is in the form Pe = aQp + b, and the point cloud fits correctly to a line and asserts the correlation between the flow rates produced by RO (Qp) and the electrical power supplied by the HP pump (Pe). The correlation coefficients have mean values of R² = 0.98, which is due to measurement uncertainties. For both low and high salinities, it can be seen that as permeate production rates increase, the electrical power supplied by the HP pump increases linearly.

The results from the correlationshow us that the more the salt concentration increases, the more power is required to produce a constant volume, which leads to an increase in energy consumption (Figure 14). The energy consumption required to produce 1 m³ of permeate water at different feedwater salinities is presented in Figure 15. It is recorded that the values go from 2.28 kWh/m³ for saline water with 1 g/L to 5.74 kWh/m³ for saline water with 35 g/L. This causes a gradual increase in specific energy consumption as the salt concentration increases.

The operating temperature is one of the most important parameters that can directly affect the RO membrane permeability because it varies inversely and proportional to the viscosity of the water to be desalinated. In this study, the electrical power supplied by the HP pump during desalination by RO membrane SWC5-LD-4040 is measured for a feedwater salt concentration of 8.6 g/L at two different feed water temperature values, T_w = 19 °C and T_w = 23 °C. The various results of the power supplied by the pump are grouped together in

Table 7. The HP pump operating power results obtainedat a feed water temperature of 19 °C and 23 °C (salinity 8.6 g/L).

Pressure (Bar)	kW Ta = 19 °C	kW Ta = 23 °C
10	0.408	0.478
20	0.652	0.679
30	0.876	0.906
40	1.160	1.151
50	1.388	1.351
55	1.543	1.554
60	1.968	1.675

. We have measured the electrical power supplied by the HP pump during the SWC5-LD-4040 RO desalination process for a feed water salt concentration of 8.6 g/L at two different temperature values, T_w = 19 °C and T_w = 23 °C. The different results of the power supplied by the pump are grouped in Table 7.

Furthermore, the effect of temperature on the power supplied by the HP pump as a function of the permeate water production flow rate is illustrated in Figure 16.

According to the curves, the results from these linear correlations show that the effect of temperature significantly affects the energy consumption, of which the system consumes 2.45 kWh/m³ at a temperature of T_w = 23 °C, and 3.13 kWh/m³ when operation at T_w = 19 °C. When the temperature of the desalination water drops under these circumstances, a relative rise is seen, which causes a strong force to be applied to the membrane, causing its pores to open.

In terms of permeation, it is found that operating at lower temperatures has lower productivity than working at higher temperatures. As a result, it can be seen that there is a systematic loss of energy as the temperature increases due to the increase in permeate water production.

For further comparison and validation, IMSDesign software (version 18.5) (Integrated Membrane Solutions Design) is used to size a RO installation. Using this program has enabled us to calculate and control the values of the main operating parameters and also to assess the quality of the water produced. The experimental results obtained are compared with those of the numerical simulation for the desalination of synthetic seawater.

Table 8. Results of operations carried out experimentally and theoretically; feed water salinity 35 g/L.

Parameter	Unit	RO Water Measured by Experimental	RO Water Predicted by Theory
Recovery rate Y	(%)	31.920	31.920
Permeate flow QP	(m3/h)	0.230	0.230
Supply pressure PA	(bar)	65	64.800
Feed rate QA	(m3/h)	0.711	0.7400
Retentate flow QR	(m3/h)	0.512	0.5100
Electrical power Pe	(kW)	1.800	1.700
Electrical conductivity σ	(μs/cm)	387	300
TDS	(mg/L)	197	161

summarizes the experimental and theoretical results in terms of flow rates, recovery rate, power, TDS and conductivity. It should be noted that the quality of the water obtained by the experiment is slightly higher in comparison with the calculated results. For a recovery rate of 31.92%, the water treated by this RO pilot has conductivity and TDS values of 387 (μs/cm) and 197 (mg/L), respectively. These values are quite close to the values obtained by the simulation—300 (μs/cm) and 161 (mg/L). It also noted that the maximum supply pressures necessary to obtain a permeate flow rate of 0.32 m³/h of water are identical. Therefore, the electric power supplied by the HP pump is also similar. With the help of this simulation, we confirm that treatment on the membrane used at the laboratory level is very efficient and achieved optimum performance, and it practically conforms to theoretical results.

4. Conclusions

This work seeks to improve our understanding of potential sustainable and cost-effective solutions to water scarcity in arid regions or areas with increasing pressures on water supply due to emergency, drought, or saltwater intrusion.

A small decentralized solar energy-water system has the potential to provide a cost-effective supply to a region where energy and drinking water are not available or difficult to produce for its high cost. An experimental study was carried out to assess the effectiveness of an RO desalination system with a salinity from 1000 mg/L (low salinity) up to 35,000 mg/L (high salinity). In this case, the pretreatment of feedwater is provided by the sand filtration-activated carbon adsorption system and two microfiltration MF (1–5 μm) filters. The performance of the tested membrane SWC5-LD-4040 is measured in terms of permeate and retentate flow under different operating conditions, such as feed water salinity, pressure, temperature and recovery rate. We find that the salt rejection increased when feed pressure increases and decreases with salt concentration and recovery rate. For each salinity, the energy consumption of the RO pilot was measured for different flow rate and pressure measurements in order to examine the membrane performance. We note that higher salinity increases energy consumption, whereas the energy increases when feed water temperature is decreased. To monitor the quality of the permeate, the water parameters were followed and the physicochemical and bacteriological results of the desalinated water showed good quality. The PV-RO system demonstrated durability, which is determined in terms of the amount of permeate and its quality, which depended strongly on the membrane used. Desalination of brackish water at a TDS value of 5 g/L requires an energy level of about 1.5 kWh/m³. Using seawater at a TDS value of 35 g/L, this value increases to 5.6 kWh/m³. The difference between the experimental and the numerical results shows good agreement.