A Review of Renewable Energy Powered Seawater Desalination Treatment Process for Zero Waste

Abstract

Freshwater resources have faced serious threats in recent decades, primarily due to rapid population growth and climate change. Seawater desalination has emerged as an essential process to ensure a sustainable supply of freshwater to meet the global demand for freshwater. However, this approach has some shortcomings, such as the disposal of brines containing high levels of contaminants creating environmental problems, and the energy-intensive nature of desalination, primarily powered by fossil fuels, which contribute to greenhouse gas emissions. Consequently, as a solution, the zero liquid discharge approach has been identified by the body of research to be one of the viable methods to solve these problems. Over 90% of freshwater and reusable salts could be recovered through this approach. Adopting renewable energy-powered systems could make zero-liquid discharge desalination plants operate in an entirely environmentally friendly and sustainable manner. This review explores the integration of renewable energy-powered systems for the optimisation of seawater desalination treatment processes for zero-waste and improved productivity. The review also examines technologies and strategies that improve the efficiency and sustainability of desalination systems. By analysing recent research, we provide insights into the advancements, challenges, and prospects for optimizing renewable energy-powered seawater desalination processes aimed at achieving zero waste.

1. Introduction

In the modern world, water is essential for preserving a healthy environment, with the sea providing about 97% of the Earth’s water [1]. Surface and subsurface water are the two types of natural water sources. Most of the world’s fresh water supply is found in surface water, such as lakes and rivers. However, recent below-average rainfall has severely reduced this water supply. The scarcity of or progressive decline in the annual worldwide rainfall has also caused subsurface water sources, which are meant to be more dependable, to fail [2]. Globally there are an estimated one billion people who lack access to potable water, which has led to more than five million water-borne disease fatalities annually—four million of these being children [2]. Population growth and rising water demands have made the availability of water a major issue [3,4]. Although freshwater is thought to be widely available, it is difficult to access in its unlimited cradle. There is a severe shortage of potable water in several nations across the world. However, seawater must undergo extensive processing before it can be consumed by humans. Khawaji, et al. [5] estimated that desalinating brackish water and seawater could provide access to drinkable water for over 75 million people worldwide. According to Inventory [6], at the end of 2002, there were 17,348 seawater and brackish water desalination plants worldwide, generating 37.8 million cubic meters of drinking water daily. According to Assad, et al. [7], the World Health Organization (WHO) reported that 25% of people on Earth reside in areas without access to potable water because these areas lack the basic infrastructure required to extract and purify water from subterranean natural reservoirs and watercourses. Today, the world is plagued by widespread water scarcity, with Africa and the Arab world being the most afflicted. Although 6.3% of the world’s population lives in these places, only 1.4% of the globe’s freshwater resources are renewable [5,7]. Desalination techniques for seawater and brackish water need to deal with the persistent issue of the growing need for potable water and limited availability. Karagiannis and Soldatos [8] predict that by 2025, if water demand is not reduced or substitute/additional water sources are not significantly established, over 67% of the world’s population—including those in both developed and developing nations—may be at risk of a water shortage. This paper identifies available technologies to address the global water scarcity problems with specific focus on optimizing a renewable energy-powered seawater reverse osmosis system for zero waste. The operational conditions and variable parameters of an effective full-sized seawater reverse osmosis (RO) desalination system can be challenging to model because they vary constantly due to seasonal variations and progressive membrane fouling over long-term filtration. Similarly, investors and consulting engineers must carefully assess the design of the RO plant, as well as the estimated capital and operational costs for large projects. These factors play a significant role in the pre-construction planning and evaluation processes and by thoroughly analyzing these factors, stakeholders can be able to make informed decisions and ensure the success of such projects. Brine waste or concentrate from the desalination process, which mainly contains organic matter, metals, surfactants and toxic substances, is another key problem that impacts the RO system’s rate of recovery, thereby causing an increase in cost per unit of potable water production. The concentrate is typically rich in colour, chemical oxygen demand (COD), total dissolved solids (TDS), and salinity. Brine concentrate management requires a well-operated, low-cost technology to moderate the environmental impact of the brine discharge on the community. Significant economic advantages can be achieved from the brine concentrate through innovative processing technology to recover the constituent salts for commercial purposes. Seawater’s desalination treatment process is usually characterised by the generation of highly contaminated rejected brine streams. This rejected brine concentrate requires well-operated, low-cost technology to reduce the environmental impact for safe disposal. Significant economic improvements can be realized by recovering high purity industrial grade salt, bromine, boron, potassium, lithium, rubidium, calcium sulphate and magnesium from brine concentrate.

The next section of the paper discusses water scarcity and the need for further research on this topic. Section 5 elaborates on desalination as one of the solutions to global water scarcity, while Section 6 highlights the reverse osmosis process as the most efficient method of desalination. Section 9 and Section 10 delve into the key issues associated with the reverse osmosis process, including high operational power demand and significant waste volumes generated from the system, and explore the available solutions to these problems. Section 11 identifies the challenges and issues of the SWRO process, while Section 12 concludes the review paper.

The objective of this review paper is to evaluate the current state of renewable energy-powered desalination technology and strategies for achieving a zero-waste paradigm, focusing on waste management and energy efficiency.

2. Methodology

The planning stage began by defining the objective of the literature review, focusing on themes like water scarcity, seawater availability, desalination challenges, and potential solutions. After establishing the primary research objective, we formulated sub-topics for qualitative reviews.

A search and evaluation procedure for information was then created to ensure article quality, selecting content from peer-reviewed journals indexed by DHET and Scopus [9].

The literature search in the selected databases initiated the implementation phase. Duplicate articles identified during the review were consolidated and analysed. Each article was assessed as relevant or irrelevant based on its abstract and title. Following this, the “Quality assessment” was completed for the relevant articles. The authors then chose the publications most closely related to the topic after a careful review of the literature. Relevant data were cross-checked, as per the previous phase [10].

The analytical process began with “data extraction”, which involved gathering information relevant to the study’s objectives. Methodological guidelines were adhered to while systematically identifying and evaluating the data and evidence from the articles. Evidence was collected, coded and assessed through comparisons to establish linkages between the articles, ultimately providing conclusive support for the proposed issues and emerging answers [11].

The reporting stage commenced with the integration of the study findings, which were methodically presented through qualitative summaries, figures, and tables. The analysed data were then published.

3. Water Scarcity

The availability of water resources, population expansion, climate change, pollution, poor resource management, the speed of water consumption and water withdrawal globally are some of the variables that contribute to water scarcity. Additionally, the unequal distribution of water resources both geographically and socially can also contribute to water scarcity. Furthermore, the ever-growing demand for water due to urbanization, industrialization, and (increased) agricultural activities puts pressure on already scarce water resources. Water scarcity will remain a major problem impacting countries and communities globally in the absence of effective water management and conservation initiatives. Azevedo [12] observed that local needs, which vary globally based on location, are connected to water scarcity. According to Ceribasi, et al. [1], approximately 80% of people globally may be exposed to water scarcity. Karagiannis and Soldatos [8] estimate that roughly 25% of the world’s population is experiencing severe water scarcity, which is predicted to leave many people without access to potable water. Drought, desertification, and global warming are predicted to exacerbate the issue to the point where even nations without current water shortages may face them in the near future. Pangarkar, et al. [2] claim that the gap between the world’s demand and supply of water has grown to the point where, in some regions, it poses a serious threat to human survival. According to Pangarkar, et al. [3], the issue of fresh water scarcity is becoming more and more of a global concern because so little of the water on Earth is fit for human use and this percentage comes from non-saline sources. Similarly, Greenlee, et al. [4] reported findings from a geological survey conducted in the United States, which revealed that approximately 96.5% of the world’s (potential) water resources exist in the oceans and seas. Only around 1.7% of the global water supply consists of ice. The remaining percentage consists of groundwater located in salty aquifers, as well as brackish water and weakly saline water. This distribution of water resources highlights the dominance of oceans and seas as the primary repositories of Earth’s water, with a relatively small portion held in ice and other sources. Ceribasi, et al. [1] concluded that saltwater desalination has become increasingly popular since saline water makes up more than 97% of the world’s water, found in oceans, seas, and other saline water sources. This process has attracted significant recognition as a practical alternative water supply, especially in countries where freshwater resources are depleted or misused. Being one of the most important issues facing the world, the urgent demand for freshwater supplies has been elevated to the top priority of the international agenda.

Water withdrawal is the quantity of water extracted for any purpose from a river, lake, or aquifer, whereas water consumption is the portion of the extracted water that evaporates as a result of vaporization, absorption, chemical conversion, or transmission, or is rendered unavailable for further use as a result of human use or consumption [12]. Recent statistics have shown that agricultural sector activities account for approximately 70% of freshwater withdrawals worldwide. This staggering figure highlights the heavy reliance of the agricultural sector on water resources. In comparison, commercial activities utilize around 20% of potable water, while the domestic sector accounts for the remaining 10% [13]. Over 90% of freshwater withdrawals in less-developed countries are attributed to agriculture, whereas in wealthier nations, industry accounts for a considerably higher share of freshwater withdrawals. The majority of freshwater extraction and consumption figures are based on estimates rather than on accurate calculations. The OECD predicts that the increase in domestic consumption (130%), industrial demand (400%), and thermal electricity generation (140%) will result in a 55% increase in worldwide water withdrawals [13]. Approximately 2.8 billion people on Earth currently live in regions vulnerable to water scarcity and of this number, 1.2 billion live in locations where water scarcity is already a problem, and half a billion people are rapidly approaching this status. Commercial water scarcity affects the remaining 1.1 billion individuals who experience water scarcity. This population resides in regions of the world where water is readily supplied by nature, but their access to it is restricted due to institutional, financial, or distribution infrastructure problems, even though the amount of water that is available is adequate to cater for their needs. This is the situation in the sub-Saharan region of Africa. Shortage of water in the physical sense is when a community lacks an adequate water supply to satisfy its needs. This kind of scarcity is prevalent in arid areas. Shortage in other regions experiencing artificially induced water shortage is triggered by excessive water withdrawal, resulting in environmental damage to groundwater tables and river systems

4. Seawater Availability

A community’s economy and quality of life both improve when enough water is available. Even though over half of the surface of the earth is covered in water, only around 3% of that water is freshwater [8]. Seawater, covering about 71% of the Earth’s surface and making up 97% of its water, is one of the planet’s most abundant resources [14,15]. However, due to its high salinity, it is unsuitable for direct consumption or use, posing significant challenges in water-scarce regions [16]. To address this, desalination techniques are essential for converting the oceans’ plentiful saline water into clean drinking water [17]. According to Ceribasi, et al. [1], finding an effective desalination technique and the high expense of desalinating saltwater have put pressure on scientists to arrive at a solution. Because of this, a large number of current seawater desalination research projects focus on developing affordable techniques for creating freshwater suitable for human use. Despite the ease of access to data on water precipitation, Azevedo [12] noted that monitoring groundwater levels and river runoff is very expensive and challenging in many places. Groundwater and surface water are examples of regenerative water, often referred to as renewable water. On the other hand, deep aquifers that recharge at a negligible rate during a period of time based on the human time scale are considered non-regenerative water sources.

5. Desalination Technologies

Various methods of desalinating seawater are being identified and investigated to increase the amount of available potable water. The desalination process is classified into two based on the process’s physical characteristics [18]. The two categories are membrane technology and thermal technology. Thermal technology uses the concept of vaporization/evaporation to separate salinity from water, whereas membrane technology uses a filtering device to produce potable water from saltwater. Thermal technology is subdivided into multistage flash distillation, freeze separation techniques, multiple effect distillation, solar still distillation, and vapour compression. Membrane technology is divided into reverse osmosis (RO) and electro dialysis procedures (Figure 1).

A new desalination method, known as forward osmosis desalination, is poised to transform the concept of generating freshwater from brackish water or saltwater. This innovative method enhances desalination’s cost-effectiveness and energy efficiency by extracting water from dissolved salts through a semi-permeable membrane. By harnessing osmotic pressure differences, forward osmosis desalination can revolutionise water treatment and provide sustainable solutions to resolve freshwater scarcity worldwide [1]. It will take the development of dependable new forward osmosis membranes and the extraction of solutes for this technology to progress from the laboratory to practical applications. Flux behaviour across a variety of osmotic membranes is presently being studied. There is also research being carried out on novel draw solutes that can readily regenerate and do not require energy for water recovery. The study conducted by Kamble and Pitale [20] investigated various solar-powered desalination systems, including but not limited to MSF, MED, humidification–dehumidification, electrodialysis, solar still, and adsorption systems. It concluded that, of all the desalination methods previously discussed, solar-powered RO desalination systems based on solar photovoltaic technology are the most widely utilized and embraced since the RO and the PV are readily available and adaptable. The global saltwater desalination plants installed and categorized by technology are roughly 49% for thermal processes and 35% for membrane systems [2]. According to Ceribasi, et al. [1], the principles of membrane separation, thermal vaporization, electro-dialysis, etc. have served as the foundation for the development of desalination technologies. The authors proceeded to further classify desalination processes into two main groups: membrane and thermal desalination technologies. The least expensive and most practical method of desalination is the solar still [21]. A solar still is a compact device that utilizes solar energy to extract potable water from contaminated water. It operates on the basic principle that water vaporized from an exposed container in an open space will recondense into water on a chilled surface.

5.1. Thermal Desalination Process

The thermal desalination process involves heating seawater as a source of saline water or other saline sources to create steam, which will require cooling to generate condensed water with less salt [1]. In thermal desalination, pressure is lowered to reduce the amount of heat needed for the saline water to evaporate. According to Research and Clayton [22], thermal desalination systems can reduce saline water’s salt concentration to as little as 10 mg/L or even less for TDS, between 60,000 and 70,000 mg/L.

5.2. Membrane Desalination Process

The most common method for desalinating seawater is membrane desalination [1]. There are diverse applications for membrane technology when brackish water and seawater desalination challenges are encountered. Seawater desalination technology can be classified according to the range of involved components and the prime mover input used. The membrane desalination process relies on the semi-permeable membrane’s ability to selectively allow water molecules to pass through. Forward and Reverse Osmosis are the two fundamental desalination techniques used in membrane-type desalination, and they can be used to categorize the membrane desalination process. According to Lattemann, et al. [23], Forward and Reverse Osmosis were concepts that scientists discovered many years ago. However, the concept of utilizing RO in the desalination process is somewhat novel. Osmosis is the movement of water from a low-concentration solution to a high-concentration solution across a semi-permeable membrane. The reverse osmosis process happens when external pressure to the membrane’s higher-concentration side is applied causing the higher-concentration solution to diffuse into the lower-concentration solution. According to Ceribasi, et al. [1], RO desalination is the process by means of which a semi-permeable membrane rejects salt and only permits pure water to flow through. When the feed water is pushed to one side of a semi-permeable membrane, the hydrodynamic pressure needs to be high enough to surpass the osmotic pressure in order to produce a reverse water flow. This is shown in Figure 2. Fikana and Raafi’u [24] reiterated that RO has been established to be the most widely accepted desalination method worldwide.

Ceribasi, et al. [1] investigated the continuous mobility of industrial RO processes, where the use of a high-pressure pump is necessary to apply external force to the systems. This process involves delivering the salt water at high pressure before it is dispensed for membrane separation. When dealing with seawater, the input feed pressure must be increased to between 40 and 82 bars (600 and 1200 psi), and when dealing with brackish water, it must be increased to between 2 and 17 bars (30 and 250 psi).

5.3. Reverse Osmosis Process

Primarily, any floating materials that can give the membrane a foul smell are eliminated from the flow of the sea water or brackish water sources using a hydraulic strainer. Depending on the salinity level, the remaining flow is elevated to the functional pressure of the system before being sent to the desalination unit. During the desalination process, water permeates through the membrane and accumulates as a permeate flux downstream of the membrane. The standard water treatment methodology will be used during the after-treatment phase to treat the permeate flux to the WHO-safe water standard. Strohwald [25] reported that RO systems have been investigated and shown to be successful in desalinating seawater. Choosing an efficient pre-treatment system was stressed in one of the studies. Despite the low quality of the raw water source, the use of a low-cost tubular ultra-filtration system in conjunction with a double media and cartridge filtration resulted in extremely good RO residual water of exceptional quality. Even in situations where ultra-filtration membrane fouling is evident, sponge balls can aid in restoring flow. Using a scale inhibitor allows for reverse osmosis reclamations of 40% without damaging the membranes. The single-stage RO unit’s residual water quality is often well within the approved SABS limits for home supply and free of RO membrane fouling.

Energy and Rate of Water Desalination

Conventional sources and renewable sources are the two primary types of energy sources used in desalination systems [8]. Renewable-powered desalination is the solution to sustainability, reducing energy usage and CO₂ emissions while also having a positive climate impact. Renewable-sourced energy can be in three forms for desalination systems. Wind, solar (photovoltaics or solar collectors), and geothermal energy are the three renewable energy sources. Renewable energy systems can also be adapted to a conventional energy source (e.g., local power grid) as a backup. The most commonly used membrane technique is RO. When it comes to effectively desalinating saline seawater, thermal methods appear to be more efficient than the membrane approach. However, critical research has revealed that thermal methods are costlier since an enormous quantity of fuel is needed to cause the saline water to evaporate. The use of membrane techniques, namely RO, has replaced thermal technologies in favour of a more cost-effective way to desalinate brackish water. However, membrane technologies are not commonly utilized for desalination due to the exorbitant price of replacing the membranes. Nthunya, et al. [26] reaffirmed that advancements in technology have contributed to a reduction in the overall cost of desalination by optimizing energy efficiency (via hybrid systems or multi-flash distillation), enhancing energy recycling through cogeneration, and enhancing transfer procedures.

6. Seawater Reverse Osmosis

Large volumes of standard potable water can be produced using desalination system technologies at a cost that is competitively low, but the system’s high energy consumption is still a significant drawback [2]. The most current advancements in membrane technology, such as Reverse Osmosis (RO), Nano-Filtration (NF), and Electro-Dialysis (ED), have garnered recognition recently due to their dependable capabilities for separation. Since RO membrane technology is appropriate for applications involving both seawater and brackish water, it has been largely regarded as the best option for desalination systems. Higher permeate flux, lower salt rejection and lower osmotic pressure are common characteristics of brackish water desalination (RO) membranes. This desalination process also requires lower operational pressures [27]. However, this approach is usually identified to have inherent challenges as a result of polarization films and byproducts, which can lead to the growth of bacteria and pollutants. According to Pangarkar, et al. [2], problems like this are addressed by employing alternative membrane technology like membrane distillation for desalination of subsurface water. Typically, RO membrane desalination methods are tailored to adopt either pressure or traditional electrical-driven technologies. There are four groups under which the pressure-driven membrane process can be classified: Reverse Osmosis (RO), Ultra-Filtration (UF), Micro-Filtration (MF), and Nano-Filtration (NF). Nano-filtration processes are recognized for their effectiveness in salt desalination. Four main sub-systems make up a typical RO system, according to Poullikkas [28]: the membrane module, the high-pressure pump, the pre-treatment system, and the post-treatment system. When a high-pressure pump is activated, the pre-treated feed water is directed to pass through the surface of the membrane. For brackish water, the working pressure of RO ranges from 17 to 27 bars, while for seawater, it varies from 55 to 82 bars. According to Strohwald [25], seawater desalination has been a commercial application for RO membranes ever since Loeb and Sourirajan developed asymmetric cellulose acetate membranes in the early 1960s. According to the paper, the majority of prominent membrane manufacturers, such as DuPont (USA), Filmtec (USA), and Toyobo (Japan), developed membranes using synthetic polymers designed especially for seawater desalination. In the middle of the 1970s, desalination plants began to appear all over the world as the RO method of producing water from seawater became increasingly popular. The author notes that because there is no phase shift involved, RO desalination has lower running costs than MSF evaporation. Due to economic factors, excessive energy usage and advancement in RO technology, the market share of MSF evaporation plummeted from 67% in the early 1980s to 3% in 1989, and in the same timeframe, RO increased from 23% to 85% [29]. The desalination of seawater and brackish water is receiving increased attention due to the rapid depletion of water resources [30]. Nowadays, desalination requires a substantial amount of energy, which makes it less economical. According to Tzen and Morris [31], the most popular and cost-effective way to desalinate brackish water is by using RO. Other approaches do exist; however, they are not that common. However, one instance may be found on the Greek island of Kimolos, where the MED process uses the island’s plentiful geothermal energy to produce 80 m³/day of potable water at a rate of 2.00 h/m³. The total dissolved solids (TDS) in brackish water impacts the daily cost of potable water production, which ranges from 2000 ppm to 10,000 ppm.

Raju and Ravinder [30] compared the expenses linked to brackish water desalination. The desalination plant for 230 ppm brackish water in Jordan costs a low 0.26 USD/m³, whereas the plant for 5000 ppm brackish water in Florida costs a low 0.27 USD/m³. Their study revealed that comparable systems employing varying total dissolved solids (TDS) levels generally exhibit notable cost variations. According to Tzen and Morris [31], desalinating 10,000 parts per million brackish water with conventional energy sources costs 0.43 USD/m³, but in a similar scenario, employing renewable energy sources can cost as much as 10.32 USD/m³. At the initial stage, the desalination cost using traditional energy sources like gas, oil, or electricity is initially cheaper than using renewable energy. However, renewable energy proves to be more cost-effective in the long run. The RO desalination process, as seen in Figure 3, has gained popularity in recent years due to the decreasing cost of membranes. RO was mostly employed for brackish water desalination a few years ago but because of its reduced energy requirements, it has recently emerged as the most widely used technique for desalinating varied types of water. Consequently, larger facilities that can produce in excess of 320,000 m³ per day are now using RO technology.

6.1. Pretreatment and Post-Treatment

In reverse osmosis seawater desalination processes, pretreatment is a critical factor in achieving and maintaining the performance of RO desalination systems. The pretreatment system removes suspended solids from the feed stream and prevents the formation of salt precipitates or microbial growth on the membrane surface. Membrane surfaces must be kept clean and in good condition at all times [32]. There are two types of pretreatment methods for an RO desalination system: the conventional pretreatment system and the membrane pretreatment system, which includes microfiltration (MF) and ultrafiltration (UF). Conventional pretreatment methods include chemical feed/injection, clotting, flocculation, sedimentation, media filtration, and/or dissolved air flotation. The selection of a pretreatment system for any reverse osmosis operation depends on the number of particles, silt, algae and organic substances present in the raw RO water, which, by extension, is extremely reliant on the raw seawater quality. Chemical treatment coupled with media filtration are both conventional pretreatment processes, used in refining seawater to the best quality suitable for RO feedwater, thereby preventing biofouling of membrane elements. In contrast, an ultrafiltration (UF) membrane removes floating solids by visible filtration through a superficial elimination mechanism similar to a fine screen. Suspended elements that are bigger than the biggest size of the UF membrane pore are disallowed and retained in the concentrate, while liquid and other floating particles that are smaller than the largest pore size of UF membranes are able to penetrate to the permeate side of the membrane. The presence of flocs, or suspended solids in raw seawater, if not properly removed, can cause serious damage to the downstream process equipment such as pumps and membrane elements. Therefore, pretreatment procedures are strongly recommended to avoid sudden increases in silt and other particulates that can have devastating consequences on downstream processes. Organic compounds, which are also present in raw seawater in addition to suspended matter, are harmful, and foul membrane elements from downstream processes, thereby increasing the frequency of membrane element cleaning, reducing production capacity, reducing the durability of the membrane and increasing operating costs [33]. The accumulation of external matter from feed water on the exterior of the dynamic membrane and/or in the feed spacer leading to operational problems is known as fouling. Membrane fouling, in turn, can be defined as the accumulation of several layers on the membrane surface and feed spacer which includes scaling. Raju and Ravinder [30] classified membrane fouling into three common types, namely colloidal, biological and organic fouling. Colloidal fouling describes the entrapment of colloidal substances, or particles such as iron flocs or silt on the surface of the membrane, while biological fouling (biofouling) describes the accumulation of biofilm substances on the membrane. Biological polluting is the adsorption of certain types of organic compounds like humic materials and oil on the exterior of the membrane. Scaling is the precipitation and deposition of poorly soluble salts within the system, which includes calcium sulfate, calcium fluoride, barium sulfate, strontium sulfate, and calcium carbonate. A well-designed and maintained pretreatment system will extend the life of a membrane and offer a solution for impurities that cause deposits and fouling of the membranes. Major RO membrane failures are generally due to deficiencies in pretreatment systems. The silt density index of the feed stream must be monitored frequently to avoid the extreme presence of colloidal material, while excess colloidal material can be removed by proper coagulation and filtration. The right configuration and selection of the pretreatment system, which depends on the properties of raw saltwater, favour the useful life and efficiency of the membrane by reducing scaling, fouling and membrane degradation. RO feed water pretreatment should include a whole systematic approach for a continuous and reliable operation, because the cost of scrubbing, interruption, and the loss of system routine can be very high. The type of pretreatment system to be adopted at any time is highly dependent on the source of feed water (i.e., groundwater or surface water). Groundwater is generally a constant source of feed water with little potential for fouling. Acidification and/or anti-sealant dosing plus the use of a cartridge filter are simple pretreatments for this water. On the other hand, surface water varies according to the season with high potential for microbiological and colloidal fouling. Surface water pretreatment is usually more intensive than groundwater pretreatment [30]. Most surface water and groundwater in the natural state are practically saturated with CaCO₃ according to [34]. The solubility of the water sources depends on the pH value, as shown in Equation (1):

Ca²⁺ + HCO₃⁻ ↔ H⁺ + CaCO₃

(1)

The addition of H⁺ as an acid shifts the equilibrium of the equation to the left keeping the calcium carbonate dissolved. The introduction of sulfate into the feed stream, on the other hand, has the potential to cause sulfate deposits. The CaCO₃ is inclined to liquefy in the concentrate stream rather than precipitate. The tendency of CaCO₃ dissolution can be expressed by the Langelier saturation index (LSI) for salty water and the Stiff and Davis stability index (S&DSI) for sea water. At saturation pH (pHs), water is in equilibrium with CaCO₃. The definitions of LSI and S&DSI are:

LSI = pH − pHs (TDS < 10,000 mg/L)

(2)

S&DSI = pH − pHs (TDS > 10,000 mg/L)

(3)

It should be noted that the methods used for predicting the PHs for LSI and S&DSI are different in regulating calcium carbonate scaling by acid addition. The LSI or S&DSI must be negative in the concentration stream. Acid addition is the only method used in the control of carbonate scale [34]. Scale inhibitors, also known as anti-sealants, slow down the precipitation process of soluble salts as they are absorbed by salt crystals (after formation) to prevent the attraction of supersaturated salts to the crystal surfaces. Under these circumstances, crystals will never develop to a size or concentration sufficient enough to come out of suspension. In addition, many scale inhibitors have various dispersive properties that envelop suspended salt particles or organic solids with an anionically charged scale inhibitor. These anionically charged particles repel each other to prevent these particles from accumulating into larger particles that can precipitate. Scale inhibitors are effective in controlling carbonate, sulfate and fluoride scale [33]. Ultrafiltration (UF) membranes have smaller sizes compared to microfiltration (MF) membranes with approximately 0.01 µm to 0.02 µm, while MF membranes can contain particles with about 0.1 µm to 0.2 µm size. Consequently, the UF membrane has a finer sieve capacity compared to the MF membrane, which justifies its frequent use. The UF membrane pretreatment benefits include reduced overall desalination costs and improved water safety. The UF membrane pretreatment process capital cost has been reported to be 20% to 50% higher compared to conventional pretreatment processes [19]. However, improved quality permeate flow from a UF membrane pretreatment procedure can bring down the overall cost (initial and subsequent) of maintaining more recent RO operation with regard to plant configuration size cutting and increasing manufacturing dimensions, but it depends on the saltiness of the spring water. Improved permeate flux from the UF pretreatment will also help bring down the cleaning frequency and element replacement of the RO membrane, thus cutting down on the cost of the operation. While this estimated cost reduction from UF is desirable, the actual benefit of UF pretreatment over conventional pretreatment is that it provides more stable and reliable treated water to the RO system, thereby reducing unplanned system downtime. However, the UF process has the limitation that organic substances easily penetrate the pretreatment process, which causes soiling of the membrane component which renders the pretreatment process ineffective. To counteract this scenario and reduce the number of organic substances that penetrate the RO process downstream, a coagulant pretreatment must be considered to reduce the natural burden on the UF compartment [19].

Post-treatment is the process of stabilizing the water and preparing it for distribution. Since most alkaline mineral components in water appear to be bigger than the pores of normal RO and cannot pass through the membrane, permeate flux assumes an acidic status. Acidic water is not only harmful to humans if ingested; acidity can destroy the components of the RO system downstream of the membrane. In the case of such problems, naturally occurring alkaline minerals like lime and caustic soda are added to the permeate to restore its pH to an acceptable level. Post-treatment, which includes pH adjustment, also consists of disinfection to combat microorganisms during distribution and eliminate pathogens from the mixing process [19]. In some cases, desalinated water can be mixed with water from other sources to enhance its taste, expand the supply, and also improve its quality for safe water consumption. Disinfection with ultraviolet radiation or chemical agents (chlorine) to ensure the elimination of bacteria or viruses that have passed through the membrane is an essential part of the follow-up treatment [32]. Also, if acid is added to the membrane feed water to control calcium carbonate scaling, the associated decrease in pH value will lead to the conversion of bicarbonate to carbonic acid in the stream (feeding the membrane). Decarbonization by means of an aeration process is regularly combined so as to eliminate surplus carbonic acid from membrane permeate. By so doing, the pH of the finished water will increase. Decarbonization is regularly used with chemical pH adjustment to convert carbonic acid to bicarbonate alkalinity. The alkalinity can be offset with chemical treatment by adding sodium carbonate or sodium bicarbonate. Adding hardness to control corrosion is another important aspect of the post-treatment. This can be achieved by mixing raw water, adding lime, and using limestone. Although an RO system is capable of removing 99% of dissolved salts, protozoa, bacteria, viruses, and many chemical contaminants, post-treatment is proposed to guarantee that desalinated water is potable and completely free of contaminants that may have been present in the feed water. In order to reduce corrosivity, corrosion inhibitors can be added to the membrane permeates. Corrosion inhibitors work by forming protective films on pipe walls (phosphate and silicate inhibitors) or reacting with metal ions to form a passivation layer (orthophosphates). Various computerized corrosion protection models are available to assess the total impact of any proposed model [19,35,36].

6.2. Basic Principles of the Reverse Osmosis Process

According to Greenlee, et al. [4], Osmosis is the process by which water moves from a less concentrated saline solution across a semi-permeable membrane to a region with a higher concentration of saline solution. A semi-permeable material will allow only pure water to pass through and disallow saline liquid. A semi-permeable membrane allows water to pass through when there is pure water on one side and a salt solution on the other while preventing the passage of salt. Greenlee, et al. [4] in their study further elucidated the concept of osmotic pressure within the realm of reverse osmosis (RO) by characterizing it as the utmost vertical disparity in water levels as water migrates from a region of lower solute concentration through a semi-permeable barrier to an area of higher concentration. To ensure that the solution strength is balanced, reverse osmosis changes the direction of water flow when the pressure applied is higher than the osmotic pressure. Desalination is accomplished by the system’s attempt to keep the equilibrium in position by letting water move from the clear water compartment to the salt solution compartment. This dilutes and equalizes the concentration on both sides of the membrane. Osmosis may produce a rise in the salt solution level, which will keep rising until the salt solution pressure in the water column rises to the point where the force of the water column stops the inflow. Osmotic pressure is the water level equilibrium point measured in terms of the water pressure exerted on the membrane (Figure 4).

Reverse osmosis is a method in which pressure is used to change the direction of the water flow across a membrane, leading to the creation of clear water from a saline solution because the membrane does not allow salt to pass through [4]. Manolakos, et al. [37] described reverse osmosis (RO) as a physical process that uses the osmotic differential pressure between saline and clear water to separate salt from water. Raju and Ravinder [30] also described reverse osmosis (RO) as a process where an input stream is forced through a semi-permeable membrane under high pressure, resulting in two separate aqueous streams, with one containing high salt concentration and the other containing low salt concentration. According to Tewari [38], when the applied pressure exceeds the osmotic pressure, clear water flows through the membrane, thereby gathering brine as a byproduct. This process leads to concentrated brine remaining in the feed section of the system, while low-salt water flows through the membrane. According to Raju and Ravinder [30], The osmotic pressure of brackish groundwater is usually much lower than that of seawater, requiring less energy for desalination. This reduced pressure allows for the use of inexpensive plastic components in RO systems. Tzen and Morris [31] accounted that a polymer substance is used to create RO membranes that produce a coated, web-like structure. Feed water is intended to pass through a convoluted pathway under high pressure to access the membrane and then penetrate the permeate side of the system. According to Maalouf [34], high-salt rejection membranes are commonly used in RO plants. These membranes are designed to have a lifespan of about seven years with proper pretreatment. Factors such as target recovery, temperature, salinity, and cleaning methods can affect the membrane’s efficiency for salt passage. To convert seawater into potable water (as shown in

Table 1. Potable water organoleptic properties [39].

Concentration (mg/L)	Classification
TDS ≤ 300	Excellent
300 ≤ TDS ≤ 600	Good
600 ≤ TDS ≤ 900	Fair
900 ≤ TDS ≤ 1200	Poor
TDS ˃ 1200	Unacceptable

), it is necessary to reduce TDS levels. This can be achieved through various desalination methods including traditional techniques or more advanced ones such as electrodeionization (EDI), multi-effect distillation (MED), electrodialysis (ED), and multi-stage flash (MSF) distillation. In some instances, a hybrid configuration combining multiple desalination approaches can yield optimal outcomes while saving energy.

6.3. Reverse Osmosis Membrane Fouling

Raju and Ravinder [30] pointed out that the use of RO in potable water production has become problematic due to RO membrane fouling, aggravated by high concentrations of inorganic and organic elements. The authors highlighted that suspended particles, bacteria, floating solids, and melting inorganic substances (BaSO₄, CaSO₄, and CaCO₃) are the usual causes of membrane fouling. Organic compounds such as humic acid can also exacerbate the problem. Xiao, et al. [40] classified pollution as organic, inorganic, or biofouling. The main elements in the contaminating deposits include organic matter, iron, phosphorus, and microorganisms, and these are mixed with chemical components present in saltwater or surface water. Pangarkar, et al. [2] concluded that salt precipitation and membrane scaling are typically the cause of the acute fouling problem in brackish RO systems. The author also mentioned that it is crucial to consider concentration polarization when inorganic pollutants contaminate membranes. Fouling typically raises the resistance, thereby reducing the permeated flux. The resistances responsible for reducing the flux are membrane resistance (Rm), concentration polarization resistance (Rcp), cake resistance (Rc), and pore blocking resistance (Rp). The total resistance during membrane filtration can be taken as:

RT = Rm + Rcp + Rc + Rp.

(4)

The type of membrane—porous or nonporous—determines the flux resistance caused by inorganic fouling. The non-porous membranes are not subject to Rp. Feed pretreatment and membrane cleaning are the primary technologies/techniques always employed to control fouling. The main objective of any RO pretreatment system for brackish water or seawater is to reduce the fouling tendency of the system.

7. Properties of Saline and Product Water Produced with Brine

7.1. Physicochemical Properties

The quality and volume of RO waste brine are influenced by the feed water quality, pre-treatment approach, desalination process, water recovery rate, and waste disposal method [41]. Omerspahic, et al. [42] reported in their study that during the feedwater pretreatment phase of membrane desalination, chemicals such as acids, biocides, antiscalants, antifoams, and corrosion inhibitors are commonly used, impacting the physicochemical composition of the resulting brine. Additionally, environmental factors like temperature, pH, and ionic strength can influence the concentration of pollutants in desalination brine. The quality of the brine is also affected by the membrane pore size used in the process [43,44]. According to Jones, et al. [45], the desalination plant’s capacity and the water recovery rate, which is the proportion of freshwater generated relative to the total volume of feed water used, determine the quantity of brine produced. Better quality feed water leads to a greater recovery rate, as higher salinity levels in the feed water will result in more concentrated brine if the water recovery rate stays the same. As the water recovery rate increases, the amount of brine produced is reduced and is more concentrated. Even though RO is dependent on hydraulic pressure and does not alter the temperature of the seawater it processes, it requires a number of extremely intricate pre-treatment steps, such as the addition of coagulants and antiscalants, which have the potential to alter the pH of the water and produce brine with a salinity that is significantly higher than ambient water [46]. According to the WHO—Geneva [47], the allowable salt content in water is 500 ppm and 1000 ppm in special cases, but most water available on earth has a maximum salt content of 10,000 ppm. Seawater usually contains salts (about 35,000 ppm to 45,000 ppm of total dissolved salts). Extreme brackishness will lead to taste and stomach problems. Desalination systems are designed to solve these problems by purifying seawater or brackish water and providing clean water with allowable limits of 500 ppm or less [35]. The density of freshwater is 1.00 (grams/mL or kg/L), which can be increased by adding salt. The more salty the water is, the higher its density. Water will expand and become less dense when it becomes hot. The colder the water, the higher the density. Figure 5 shows the relationship between temperature, salinity and density. The Pacific Ocean has the lightest water with a density of less than 26.0; the Atlantic has the densest water, between 27.5 and 28.0. Antarctic bottom water is densest in the Pacific and Indian Oceans, but not the Atlantic [48]. Dissolved gases in seawater are always in equilibrium with the atmosphere, but their relative concentration depends on the solubility of each gas, as well as on salt content and temperature. As the salt content increases, more water molecules are immobilized by salt ions, which reduces the amount of dissolved gas. As the water temperature rises, the mobility of the gas molecules increases, and facilitates their discharge from the water, which reduces the quantity of gas that dissolves. According to Mustafa Omerspahic et al., desalination brine effluents include significant levels of Cl⁻ and Na⁺, as well as other ions such as Ca²⁺, Mg²⁺, and SO²⁻ [41]. Several studies have been published on the presence of heavy metals such as copper (Cu) in desalination brine effluents and seawater. Brine discharges from RO often include metals in trace quantities, such as iron (Fe), nickel (Ni), chromium (Cr), and molybdenum (Mo). Furthermore, chemicals such as biocides, surface-active agents, anti-scale additives, and solid residues from filter backflushing may be present in the effluent discharge on a continuous or periodic basis, posing a risk to the environment. Brine effluents from RO desalination plants not only have a high salt content but typically also contain compounds from the desalination process, such as phosphonate-based antiscalants and ferric sulphate (or alum)-based coagulants [49].

Table 2. Ion composition of Seawater [47].

Constituent	Normal Seawater	Eastern Mediterranean	Arabian Gulf at Kuwait	Red Sea at Jeddah
Chloride (${C1}^{-1}$)	18,890	21,200	23,000	22,219
Sodium (${Na}^{+1}$)	10,556	11,800	15,850	14,225
Sulfate (${{SO}_4}^{-2}$)	2649	2950	3200	3078
Magnesium (${Mg}^{+2}$)	1262	1403	1765	742
Calcium (${Ca}^{+2}$)	400	423	500	225
Potassium ($K^{+1}$)	380	463	460	210
Bicarbonate (${{HCO}_3}^{-1}$)	140	-	142	146
Strontium (${Sr}^{+2}$)	13	-	-	-
Bromide (${Br}^{-1})$	65	155	80	72
Boric Acid ($H_3{BO}_3$)	26	72	-	-
Flouride ($F^{-1})$	1	-	-	-
Silicate (${{SiO}_3}^{-2})$	1	-	1.5	-
Iodidie ($I^{-1}$)	<1	2	-	-
Other	1	-	-	-
Total dissolves solids	34,483	38,600	45,000	41,000

shows the salt ion content of different seawater sources.

Shomar and Hawari [50] highlighted the differences between natural and desalinated waters, noting that natural waters exhibit a wide range of physical, chemical, and biological characteristics influenced by climatic and biogeochemical factors, while desalinated water has a controlled chemical profile. The study also indicated that the quality and appearance of desalinated water depend on the chemicals and materials used in the desalination process. Although groundwater is sometimes mixed with desalinated water to reduce corrosion and stabilize quality, post-treatment procedures often result in inconsistent quality [51]. Water with a Total Dissolved Solids (TDS) level of 25–50 mg/L is reported to be less thirst-quenching and may have undesirable flavours (tasteless, metallic) [52].

7.2. Thermodynamics Property of Saline Water

In his research, Kalogirou [35] examined seawater salinity, defining saline water as a blend of salt and pure water. The desalination process yields two output streams: brine, which contains a high concentration of dissolved salts, and desalinated water, which has a low concentration. Thus, the properties of pure water and salt are essential considerations in desalination operations. Parts per million (ppm) is the standard unit for salinity, defined as ppm = mf_s × 10⁶. A salinity of 1000 ppm is equivalent to 0.1% salinity, or mf_s = 0.001 salt mass fraction. Consequently, the mole fraction of salt, x_s, is determined by Equation (5) [35,53]:

$${\mathit{m}\mathit{f}}_{\mathit{s}}={\displaystyle \frac{{\mathit{m}}_{\mathit{s}}}{{\mathit{m}}_{\mathit{s}\mathit{w}}}}={\displaystyle \frac{{\mathit{N}}_{\mathit{s}}{\mathit{M}}_{\mathit{s}}}{{\mathit{N}}_{\mathit{s}\mathit{w}}{\mathit{M}}_{\mathit{s}\mathit{w}}}}={\mathit{x}}_{\mathit{s}}{\displaystyle \frac{{\mathit{M}}_{\mathit{s}}}{{\mathit{M}}_{\mathit{S}\mathit{w}}}}\mathit{a}\mathit{n}\mathit{d}{\mathit{m}\mathit{f}}_{\mathit{w}}={\mathit{x}}_{\mathit{w}}$$

(5)

where N is the number of moles, x is the mole fraction, m is mass, and M is the molar mass. Salt, water, and saline water are represented by the subscripts s, w, and sw, respectively. The apparent molar mass of saline water is determined by Equation (6) below. NaCl and water molar masses are 58.5 and 18.0 kg/kmol, respectively. Salinity is typically expressed in terms of mass fractions, but the minimum work calculations require mole fractions. By combining Equations (5) and (6) and taking into account that x_s + x_w = 1, the following relations in Equation (7) are obtained for converting mass fractions to mole fractions [54]:

$${\mathit{M}}_{\mathit{s}\mathit{w}}={\displaystyle \frac{{\mathit{m}}_{\mathit{s}\mathit{w}}}{{\mathit{N}}_{\mathit{s}\mathit{w}}}}={\displaystyle \frac{{\mathit{N}}_{\mathit{s}}{\mathit{M}}_{\mathit{s}}+{\mathit{N}}_{\mathit{w}}{\mathit{M}}_{\mathit{w}}}{{\mathit{N}}_{\mathit{s}\mathit{w}}}}={\mathit{x}}_{\mathit{s}}{\mathit{M}}_{\mathit{s}}+{\mathit{x}}_{\mathit{w}}{\mathit{M}}_{\mathit{w}}$$

(6)

$${\mathit{x}}_{\mathit{s}}={\displaystyle \frac{{\mathit{M}}_{\mathit{w}}}{{\mathit{M}}_{\mathit{w}}\left(1/{\mathit{m}\mathit{f}}_{\mathit{s}}-1\right)+{\mathit{M}}_{\mathit{w}}}}\mathit{a}\mathit{n}\mathit{d}{\mathit{x}}_{\mathit{w}}={\displaystyle \frac{{\mathit{M}}_{\mathit{s}}}{{\mathit{M}}_{\mathit{w}}\left(1/{\mathit{m}\mathit{f}}_{\mathit{w}}-1\right)+{\mathit{M}}_{\mathit{s}}}}$$

(7)

Dilute solutions have a concentration of less than 5% and behave similarly to ideal solutions, resulting in minimal interaction between different molecules [54]. The extensive properties of a mixture equal the sum of its individual components’ properties. Consequently, Equation (8) can be used to determine the mixture’s enthalpy and entropy, and Equation (9) can be used to calculate the quantities per unit mass of the mixture. In an ideal gas mixture, no heat is generated or absorbed during mixing, resulting in a zero enthalpy of mixing. This implies that neither the mixture’s enthalpy nor that of its components changes. Therefore, the total enthalpy of the individual components at a given temperature and pressure equals the enthalpy of the mixture at those conditions [55]. This principle also applies to saline solutions.

$$\begin{gathered} \mathit{H}=\sum {\mathit{m}}_{\mathit{i}}{\mathit{h}}_{\mathit{i}}={\mathit{m}}_{\mathit{s}}{\mathit{h}}_{\mathit{s}}+{\mathit{m}}_{\mathit{w}}{\mathit{h}}_{\mathit{w}}\mathit{a}\mathit{n}\mathit{d} \\ \mathit{S}=\sum {\mathit{m}}_{\mathit{i}}{\mathit{s}}_{\mathit{i}}={\mathit{m}}_{\mathit{s}}{\mathit{s}}_{\mathit{s}}+{\mathit{m}}_{\mathit{w}}{\mathit{s}}_{\mathit{w}} \end{gathered}$$

(8)

$$\begin{gathered} \mathit{h}=\sum {\mathit{m}\mathit{f}}_{\mathit{i}}{\mathit{h}}_{\mathit{i}}={\mathit{m}\mathit{f}}_{\mathit{s}}{\mathit{h}}_{\mathit{s}}+{\mathit{m}\mathit{f}}_{\mathit{w}}{\mathit{h}}_{\mathit{w}}\mathit{a}\mathit{n}\mathit{d} \\ \mathit{S}=\sum {\mathit{m}\mathit{f}}_{\mathit{i}}{\mathit{s}}_{\mathit{i}}={\mathit{m}\mathit{f}}_{\mathit{s}}{\mathit{s}}_{\mathit{s}}+{\mathit{m}\mathit{f}}_{\mathit{w}}{\mathit{s}}_{\mathit{w}} \end{gathered}$$

(9)

The seawater used for desalination is at a temperature of about 15 °C (288.15 K), and a pressure of 1 The seawater used in desalination typically has a salinity ranging from 1500 to 35,000 parts per million (ppm), a pressure of 1 atmosphere, and a temperature of approximately 15 °C (288.15 K). These conditions are considered representative of the surrounding environment. Thermodynamic principles for solids are applied to determine the properties of salt. The enthalpy and entropy values of salt are defined as zero at the reference state, which is the state of salt at 0 °C. Equation (10) can therefore be used to calculate the enthalpy and entropy of salt at temperature T, as shown below. Using a specific heat of salt of C_ps = 0.8368 kJ/kg K, the salt’s entropy and enthalpy at T_o = 288.15 K are calculated as h_so = 12.552 kJ/kg and S_so = 0.04473 kJ/kg K, respectively. It is important to note that the enthalpy and entropy of incompressible substances are independent of pressure [54]. Since mixing is irreversible, the entropy of the mixture at a given temperature and pressure must exceed the sum of the entropies of its components at the same conditions prior to mixing. Equation (11) below shows the entropy of a component per mole in an ideal solution at temperature T and pressure P [56]:

$${\mathit{h}}_{\mathit{s}}={\mathit{h}}_{\mathit{s}\mathit{o}}+{\mathit{c}}_{\mathit{p}\mathit{s}}\left(\mathit{T}-{\mathit{T}}_{\mathit{o}}\right)\mathit{a}\mathit{n}\mathit{d}{\mathit{S}}_{\mathit{s}}={\mathit{s}}_{\mathit{s}\mathit{o}}+{\mathit{c}}_{\mathit{p}\mathit{s}}\mathit{I}\mathit{n}(\mathit{T}/{\mathit{T}}_{\mathit{o}})$$

(10)

$${\mathit{s}}_{\mathit{i}}={\mathit{s}}_{\mathit{i},\mathit{p}\mathit{u}\mathit{r}\mathit{e}}\left(\mathit{T},\mathit{P}\right)-\mathit{R}\mathit{I}\mathit{n}\left({\mathit{x}}_{\mathit{i}}\right)$$

(11)

Since x_i is less than 1, ln(x_i) is a negative value; thus, −Rln(x_i) is always a positive value. As a result, at the temperature and pressure of the mixture, the entropy of a component in a mixture is always greater than the entropy of that component when it exists alone. Therefore, as shown in Equation (12) below, the entropy of a saline solution is equal to the sum of the entropies of salt and water in the saline solution [54].

$$\begin{gathered} \mathit{s}={\mathit{x}}_{\mathit{s}}{\mathit{s}}_{\mathit{s}}+{\mathit{x}}_{\mathit{w}}{\mathit{s}}_{\mathit{w}}={\mathit{x}}_{\mathit{s}}[{\mathit{s}}_{\mathit{w},\mathit{p}\mathit{u}\mathit{r}\mathit{e}}\left(\mathit{T},\mathit{P}\right)-\mathit{R}\mathit{I}\mathit{n}({\mathit{x}}_{\mathit{s}}\left)\right] \\ +{\mathit{x}}_{\mathit{w}}\left[{\mathit{s}}_{\mathit{w},\mathit{p}\mathit{u}\mathit{r}\mathit{e}}\left(\mathit{T},\mathit{P}\right)-\mathit{R}\mathit{I}\mathit{n}\left({\mathit{x}}_{\mathit{w}}\right)\right]={\mathit{x}}_{\mathit{X}}{\mathit{s}}_{\mathit{s},\mathit{p}\mathit{u}\mathit{r}\mathit{e}}(\mathit{T},\mathit{P}) \\ -\mathit{R}[{\mathit{x}}_{\mathit{s}}\mathit{I}\mathit{n}\left({\mathit{x}}_{\mathit{s}}\right)+{\mathit{x}}_{\mathit{w}}\mathit{I}\mathit{n}\left({\mathit{x}}_{\mathit{w}}\right)] \end{gathered}$$

(12)

By dividing the given quantity (per unit mole) by the molar mass of salty water, we can determine the entropy of saline water per unit mass as shown in Equation (13):

$$\begin{gathered} \mathit{s}={\mathit{m}\mathit{f}}_{\mathit{s}}{\mathit{s}}_{\mathit{s},\mathit{p}\mathit{u}\mathit{r}\mathit{e}}\left(\mathit{T},\mathit{P}\right)+{\mathit{m}\mathit{f}}_{\mathit{w}}{\mathit{s}}_{\mathit{w},\mathit{p}\mathit{u}\mathit{r}\mathit{e}}\left(\mathit{T},\mathit{P}\right)-\mathit{R}\left[{\mathit{x}}_{\mathit{s}}\mathit{I}\mathit{n}\right({\mathit{x}}_{\mathit{s}}) \\ +{\mathit{x}}_{\mathit{w}}\mathit{I}\mathit{n}\left({\mathit{x}}_{\mathit{w}}\right)\left]\right(\mathit{k}\mathit{J}/\mathit{K}\mathit{g}\mathit{K}) \end{gathered}$$

(13)

8. Desalination Energy Requirements and Recovery

Azevedo [12] established that compared to other water treatment methods, desalination is considered energy-intensive due to its higher energy consumption per litre. The amount of energy used in the desalination process can vary depending on factors like facility design, technology, temperature, feed water quality, energy recovery equipment, and the desired water quality. Technological advancements in the last forty years have reduced the energy needed for desalination, with this trend expected to continue as technology evolves. Areas for improving desalination energy consumption include: (a) enhanced system design; (b) efficient pumping methods; (c) energy recovery systems; (d) innovative membrane materials; and (e) advanced technology. The primary determinant of energy consumption in membrane desalination systems is the feed water’s salinity and the recovery rate. For Reverse Osmosis, energy is primarily used for pumping the feed water. The consumption of energy in RO processes typically varies from 3.7 kWh/m³ to 8 kWh/m³ for seawater, depending on the facility size. With energy recovery systems, a seawater RO unit with a daily capacity of 24,000 m³ consumes about 4 to 6 kWh/m³. Conversely, a brackish water RO system’s energy consumption ranges from 1.5 to 2.5 kWh/m³ [57].

According to Raju and Ravinder [30], brackish water or seawater desalination technology is the most efficient way to generate high amounts of potable water at a competitive cost; nevertheless, the considerable energy usage of the system has continued to pose a significant challenge. According to Muthumariappan and Engineer [58]. The feed pressure required to pump water determines the flow rate of the desalination system, and this pressure serves as the main power source in the RO system. To achieve the desired permeate flux, the high concentration of brine in seawater requires a hydrostatic pressure of up to 7000 kPa, and the higher the salt content, the higher the pressure and pumping power that the system will require. The high-pressure pump unit, responsible for about 70% of the total energy required for the RO process, represents the primary energy cost. The hydrostatic pressure applied must exceed the osmotic pressure of the membrane’s feed water portion. The osmotic pressure on the feed compartment of the membrane increases as the (RO) unit recovery rate increases, leading to a rise in the required feed pressure. The brine waste produced by the system has high pressure having shared a significant portion of the feed pressure. This resultant brine waste pressure can be used effectively to boost the raw water feed pressure through the use of suitable technology, known as the energy recovery system. The total energy consumed by the RO unit can be reduced with the help of this energy recovery technology by utilizing the pressure from the brine waste to assist in boosting the feed pressure of the raw water, thus increasing the efficiency of the system. By implementing this technology, not only can the feed pressure requirement be met more effectively, but also the operating costs of the RO unit can be significantly reduced.

9. Renewable Energy Power Solution

Desalinating brackish water and sea water has become a widely accepted method for producing potable water globally. Due to their isolation from larger cities, some communities in third-world nations are frequently cut off from the national energy grid. This has left a significant gap in the availability of potable water supplies, which are typically fueled by conventional electricity. Desalinating seawater to remove salt is highly energy-intensive and costly. This process presents challenges such as high energy consumption and environmental pollution from fossil fuel combustion. According to [35], producing 22 million m³ per day requires 203 million tons of oil annually, equating to about 8.5 EJ/yr or 2.36 × 10¹⁰ kWh/yr of fuel. Consequently, the operating costs of running a desalination plant with fossil fuel-based energy are significantly high. Similarly, using fossil fuels to supply fresh water is counterproductive and harmful to the climate, particularly given rising concerns about fossil fuel-related environmental issues and carbon emissions. Thus, along with its high energy demand, water desalination poses a significant environmental contamination risk.

To address this and minimize environmental impact, desalination plants need an energy source with low emissions and must be cost-effective. These challenges can be tackled by leveraging renewable sources such as solar photovoltaic, wind, thermal, or geothermal energy. Since the capacity of desalination currently exceeds 70 million m³/day worldwide, Lotfy, et al. [59] justified desalination using renewable energy to reduce greenhouse gas emissions. It is anticipated that the cost of renewable energy solutions will mean that these solutions will continue to be unattractive to isolated communities and areas with sparse populations and inadequate infrastructure. Less than 1% of the world’s desalination capacity is now accounted for by renewable-powered plants and facilities. The majority of these desalination plants use RO technology, which accounts for 62% of global capacity, followed by MSF and MED. The primary renewable energy source used for water desalination facilities is solar photovoltaic (PV), which powers 43% of all desalination plants. Thermal and wind energy are the next most common renewable energy sources [12]. The feasibility of establishing a renewable energy plant is contingent upon several criteria, such as the salinity of the feed water, the plant’s location, the accessibility of renewable energy sources, and the availability of the national electricity grid.

Solar PV System

PV (photovoltaic) systems use semi-conductors and PV cells to convert solar radiation into direct current. The PV modules consist of photovoltaic cells that generate direct current, which can be collected and stored in a battery or sent straight into an inverter for conversion into alternating power [12]. The RO desalination pumping system is directly coupled to the solar PV system. The system consists of a collection of batteries for energy storage and a charge controller that prevents deep discharges and overcharging while monitoring and controlling the charging process. The membrane system produces a set amount of high-quality water while maintaining consistent pressure and flow. Research on battery-less membrane systems is crucial due to the impacts of sun irradiation fluctuations. Real-world tests have shown these systems can generate high-quality water, although the outcome may vary based on the membrane used. An advantage of RO desalination is its ability to adjust capacity according to the energy supply. The novel RO technique by ENERCON GmbH Germany, allowing variable energy input degrees and utilizing a piston system for energy recovery, was discussed by Ghafoor, et al. [29]. Utilization of this innovative technology in reverse osmosis systems enables adjustment of production output from 12.5% to 100% of standard capacity through piston speed modifications and control of energy input or water demand. Other companies are also exploring and creating similar demand–response systems. Ghafoor, et al. [29] conducted a study on a 500 L/h solar-powered RO desalination system and it was revealed that for every 1 °C rise in feed water temperature, membrane output increased, resulting in 60% permeate flux and 40% brine production. Elmaadawy, et al. [60] conducted an evaluation of the economics and efficiency of an RO system integrated with a PV system and diesel generator, only a diesel generator, and a PV system without battery storage. Research findings indicated that 20 m³/d capacity was attained through the combination of PV-RO with the diesel generator, 20 m³/d capacity using solely the diesel generator, and 44 m³/d water yield was accomplished using solar-powered RO specifically on sunny days. Figure 6 illustrates a diagram outlining a PV-RO system design. Bilal, et al. [61] conducted an investigation on PV systems—both with and without batteries—for use in running RO systems. According to the findings, 5.9 L/h of permeate flux was generated for five hours while the battery was used, and 3.8 L/h when it was not. Compared to a PV system without a battery, the freshwater throughput with a battery-dependent system was 9.8% greater. The authors came to the conclusion that while both kinds of PV systems had advantages, battery-less systems were more financially viable than battery-based systems.

Azevedo [12] concluded that the PV-RO system as shown in Figure 5 has been regarded as one of the most reliable solutions for desalination driven by renewable energy, particularly for rural places given that both PV and RO are modular and easily accessible. The modularity of the system, which has reduced the cost of production through economies of scale, has allowed small-scale systems to be created by directly connecting the DC output of PV modules to DC pumps and electronics, which in turn increases the system’s overall efficiency by 5% to 10% by preventing losses in DC-AC power conversion and AC-DC rectification [61].

10. Zero Waste Seawater Desalination

The seawater desalination technology has been classified as a low-profit investment due to its high operating costs, low recovery of permeate flux, and high system-generated brine concentrate [19]. Separating salt concentration and useful metals from brine before disposal. greatly lowers the cost of producing clean water from seawater reverse osmosis (SWRO) while also removing the harmful environmental effects of discarding brine concentrate without metals and salt into the sea. The elevated levels of salt, organic debris, and other additional materials in brine can pose serious environmental risks if released into the environment. According to Cipolletta, et al. [62], the present global brine production stands at about 141.5 million m³/day, accumulating to 51.7 billion m³/year. This number encompasses a high dissolved salt concentration (up to 400,000 mg/L total dissolved solids (TDS)) with organic matter, metals, nutrients and a low percentage of pathogenic substances. Rejected brine negatively impacts the ecosystem by causing eutrophication, pH fluctuations, and an accumulation of heavy metals. Consequently, the concept of treating brine rather than discarding it is highly favoured and promoted. Brine treatment not only reduces negative environmental effects but also makes it possible to recover excess freshwater and/or valuable substances (like salts) that would otherwise have been lost. Additionally, the operating cost of a desalination system is very high as a result of the high volume of brine waste generated and the high power requirement. This high cost of production can be alleviated with the envisaged income from brine waste management thereby positioning the system for attractive investment. The management and disposal of brine have been among the top research works worldwide [63]. Historically, brine has been managed using different methods such as evaporation ponds, deep well injection, ocean/sea discharge, and sewage outflow. These practices are no longer feasible due to rising management costs, tighter discharge regulations, and environmental risks [64]. To lessen the effects of brine discharge on the environment, brine concentrate management requires using economically viable, accurately operated technologies. Finding a technically, environmentally, and financially feasible system of managing brine concentration is getting more and more difficult, thus making the process very complex and economically unviable. To achieve maximum production of freshwater with minimal output waste, the concepts of Minimum Liquid Discharge (MLD) or Zero Liquid Discharge (ZLD) have to be implemented in brine waste management [62]. About 95% and 100% of the freshwater in MLD and ZLD, respectively, is reclaimed while valuable metals, salts and energy can also be recovered [65]. Creating practical selections for salt reutilization and definitive methods for salt extraction is also crucial for lowering the overall total cost of the process [66]. de Nicolás, et al. [67] describe MLD as a brine management model that recovers over 95% of freshwater while minimizing brine discharge with the concentrated effluent constituting just 5% of the total volume which can be managed more sustainably. The level of resource and water recovery, as well as the volume and quality of effluent produced, differentiates MLD from ZLD. The MLD process is designed for a high but incomplete recovery of water and resources, resulting in reduced liquid effluent, while ZLD targets 100% recovery, eliminating any liquid effluent [68,69,70]. The following phases are typically involved in the ZLD and MLD stages: firstly, a pre-treatment step that eliminates suspended solids, organic matter, and other contaminants from wastewater through physical, chemical, or biological methods (e.g., filtration, coagulation, flocculation, sedimentation, flotation, oxidation, or biodegradation). This step is crucial to avoid fouling, scaling, or wetting of membranes and evaporators in subsequent processes. Secondly, a concentration step separates water from dissolved salts and solutes using thermal or membrane technologies, resulting in a permeate stream of potable water and a high salinity concentrate for further treatment. The final phase involves evaporation and crystallization, employing thermally based technologies to further concentrate the concentrate stream until the solutes saturate and crystallize. This process produces a solid stream of recoverable or disposable salt crystals, along with a high-quality distillate stream of water that can be recycled or reused [41,68]. Based on our review, not very many research studies have been carried out on brine management considering the methods for metal recovery, life cycle assessment (LCA), and the development of desalination technologies. The emphasis of previous research has been more on identifying methods and techniques of recovering either one or two metallic salts from the brine concentrate, with no work completed on determining or identifying the metallic salt constituent of the brine waste.

Both the natural environment and humanity depend on water as a resource. However, in many regions of the world, rising urbanisation has increased the demand for freshwater. In light of this, desalination is regarded as a dependable and practical solution to the growing need for freshwater. Desalination technology is being explored in more than 150 countries, and globally, approximately 21,000 desalination facilities produce more than 120 million cubic meters of freshwater per day in countries like Kuwait, France, the United States of America, Saudi Arabia, and others [65]. Since desalination is being used in a growing number of both coastal and inland locations, the subject of brine discharge has emerged as a significant concern, and it has been shown in numerous studies to have detrimental effects on the environment. Apart from high salinity, brine may also comprise chemical deposits like antiscalants, flocculants and coagulants from pre- and post-treatment procedures. As of 2019, the estimated amount of desalination brine produced worldwide was 128,652,000 m³/day [71]. Various practices for the disposal of brine have been employed, including land application, sewage disposal, deep-well injection, and surface water disposal and numerous studies have shown that some of these disposal practices have negative ecological effects. Specifically, deep-well injection is inappropriate in regions with potential for seismic and/or volcanic activity, such as Greece, as well as inland locations due to possible pollution of the water sources and sewage discharge [41]. In summary, zero liquid discharge (ZLD), a substitute and more environmentally friendly method, has been taken into consideration to mitigate the effects of brine discharge. Farizoglu and Uzuner [72] investigated the suitability of using a ceramic membrane filtration unit, an aerobic jet loop reactor, and a high-performance bioreactor for the treatment of wastewater from dairy processing.

The research findings of Ji, et al. [73] indicated that fully functional reverse osmosis (RO), forward osmosis (FO), and membrane distillation (MD) technologies have been adopted to treat the concentrate brine solution to lower the amount of brine that enters into a commercial crystallizer. It further highlighted the fact that multiple processes (such as brine concentration and salt crystallization) are required to recover water and salts in the abovementioned membrane-based technologies thereby increasing their complexity and making the process economically unviable. Unlike the conventional minimal liquid discharge (MLD) and zero liquid discharge (ZLD) processes that demand high energy to completely extract and harvest valuable solid salts and freshwater from brine, it is considered to be more desirable to advance simple ZLD or MLD procedures that consume less energy to sustainably manage brine. Consequently, the study of Ji, et al. [73] demonstrated a novel and efficient membrane-promoted crystallization (MPC) method for effectively recovering water and KCl from KCl concentrate under mild circumstances without the need for anticorrosive tools and which simplifies the production of KCl by combining salt crystallization and brine concentration into a single unit through the use of a tubular hierarchical ceramic membrane (macroporous a-Al₂O₃ hollow substrate, γ-Al₂O₃ interlayer and mesoporous ZrO₂ top layer). Another research study conducted by Li, et al. [74] methodically assessed the viability of the concentration process of a prototype textile wastewater system using three distinct kinds of FO membranes under three different modes of operation and suggested a hybrid FO-MD process using a bespoke self-standing and symmetrical membrane, as well as a hydrophobic polytetrafluoroethylene membrane in the FO and MD units, respectively. The research study by Panagopoulos, et al. [41,71] offered a financial and environmental evaluation of the ZLD seawater desalination systems and ZLD brackish water desalination systems in the Eastern Mediterranean region. The findings highlight the promising performance of ZLD systems in treating and optimizing the value of desalination brine. Furthermore, the report recommends further research in vital areas like integrating ZLD systems with Renewable Energy Sources (RES) and utilizing advanced ZLD technologies to minimize significant energy usage and accompanying greenhouse gas emissions, thereby enhancing the environmental sustainability of ZLD systems. Cipolletta, et al. [62] in their study examined the technologies for brine treatment aimed at Minimum Liquid Discharge (MLD)/Zero Liquid Discharge (ZLD) and resource recovery and demonstrated their benefits and drawbacks. The report suggested that the correct combination of different treatment methods can greatly improve brine management and shift the emphasis to recovery and reuse rather than removal. The author concluded that ZLD technologies aim to recover 100% of the water using both membrane and thermal-based technologies, but the processes are frequently hampered by their high cost and high energy needs. MLD is adjudged to be a viable solution that can extract up to 95% water primarily through membrane-based technologies. The study highlighted that resource recovery from brine and potential valorization pathways for the reclaimed materials are factors that contributed to the reduction in the general plant costs and assisted them in achieving the goals of a circular economy. Another research study by Lugo, et al. [75] produced an extensive framework for techno-economic assessments (TEAs) to evaluate an innovative algae resource recovery and near-zero-liquid discharge potable reuse system versus a traditional potable water reuse system. The levelized costs of water from individual units are estimated by the TEA study, and the integrated processes include sludge treatment, enhanced water purification for drinking water reuse, and secondary wastewater treatment. Based on the study’s results, the existing efforts to treat wastewater and recycle drinking water are deemed unsustainable from both environmental and economic standpoints. This is attributed to their costly nature, substantial energy requirements, resource depletion, and notable greenhouse gas emissions. The study by Panagopoulos and Haralambous [65] examined the minimum energy consumption (MEC) compared to the popular research on minimum energy consumption in seawater desalination. The research presented the actual energy consumption (AEC) of desalination technologies and introduced a mathematical model for the calculation of the minimum energy consumption (MEC) in desalination brine treatment. The study showed that MEC increases with a higher recovery rate, feed brine salinity, feed brine temperature, and freshwater purity, while it decreases with an increased molar mass of dissolved salt. The model considered various parameters such as type of dissolved salt, salinity, and temperature. The research study by Mansour, et al. [76] examined brine concentrate which characteristically comprises dissolved salts and residual chemicals, and which can include a variety of agents, such as coagulants, antiscalants, biocide/antifouling agents, and anti-foaming agents, as well as potentially heavy metals. It went further to verify that the particular breakdown of brine components varies depending on the industry and the brine source. A demonstration of brine concentrate produced by desalination technologies was provided, which typically includes minerals like calcium, barium, silica, and sulfate that contribute to hardness. Conversely, a municipal wastewater treatment plant has a high level of organic carbon that must be reduced when it exceeds a certain threshold before further treatment. The study concluded that the range of technologies compatible with the system depends heavily on the elements of the brine feed. This is because some components and/or compounds are too strong for some technologies to cope with them beyond a certain level or threshold. In different circumstances, the total quantity of dissolved solids present in the feed stream determines which technology can best be used. Certain technologies, for instance, can only be used in brackish water and cannot withstand water that contains more solids than a predetermined level, highlighting the role of filters that can be placed in the feed stream in each technology, whether it be the presence of a specific substance or the total amount of dissolved solids in the stream blocked from entering by the technology.

11. What Are the Future Challenges

According to Strohwald [25], RO units can produce high-quality drinking water, but there are risks involved such as early capacity reduction and inadequate salt rejection. These problems may arise from malfunctioning pretreatment and maintenance systems, potentially exposing the membranes to the following:

➢

Chlorine leakage from the pretreatment area

➢

Bacterial contamination

➢

Residual membrane fouling missed during pretreatment, undetected by the plugging index determination

➢

Pump grease contamination.

The following operational/system design problems were identified in recent studies conducted on seawater desalination for producing drinking water in a naval setting:

i.

High-pressure pump vibration and noise. High-pressure positive displacement pumps often encounter problems like cracked plungers, failed bearings, valve pitting, and water leaks past the pump packing stage. Some DuPont preemptors may also suffer damage due to incorrect intake plumbing sizing and routing, along with intense pump vibration causing high mechanical noise and cavitation issues. These challenges underscore the importance of correctly sizing suction and discharge pipework and strategically positioning pulsation dampeners in the design phase [25].

ii.

High-pressure pumps. Choosing the wrong pump for the specific function and operation can result in excessive vibration and eventual failure. Difficulties may also arise in sourcing replacement parts for pumps sourced from foreign countries. It is essential to select pumps that are tailor-made for RO applications.

iii.

Pretreatment. Premature fouling of RO membranes due to insufficient and unsuitable equipment has led to reduced capacity and product quality. Despite the system’s proven ability to generate drinking water from seawater and brackish water, these particular challenges have fostered the misconception that RO is unreliable and excessively costly. The best approach to debunk this fallacy is to showcase the effectiveness of well-designed plants in the industry [25].

iv.

Management of bacteria. When using chlorine as a bactericide, caution is vital to prevent a chlorine leak from the pretreatment area. Although chlorine is effective in halting germ growth, it can be harmful to the sensitive RO membranes, causing a breakdown in their function if chlorine breakthrough occurs.

12. Conclusions

This review paper concludes that reverse osmosis (RO) technology is an excellent desalination process to produce zero liquid discharge (ZLD) if properly integrated. It can produce large quantities of clean drinking water cost-effectively and is the most commonly used membrane technology for treating brackish water and seawater. Nonetheless, the primary disadvantage of an RO system is its high energy usage. This study also evaluated the economic viability of RO systems powered by renewable energy sources for achieving Zero Liquid Discharge (ZLD). This review study also came to the conclusion that, in the long term, managing brine is crucial for safeguarding the ecosystem and preserving the environment. The study found that the best way to control brine is by using appropriate technology that enables ZLD to minimize its volume as much as possible. While the resulting salts could be disposed of on land or in water, the environmental impact would be minimal. It is evident that emerging technologies can effectively reduce brine volume based on the study findings. Various metals and salts hold economic value and can be sold for profit. Effective brine management involves integrating multiple processes to recover water and chemicals. Ultimately, brine should be seen as a resource, not waste, to ensure sustainable management practices.

In conclusion, addressing water scarcity requires a multifaceted strategy involving conservation efforts, sustainable water management practices, infrastructure funding, and supportive legislation for water efficiency which can address the root causes of water scarcity and ensure a sustainable future for upcoming generations.