Management of reverse osmosis concentrate by solar distillation Open Access

Disposal of rejected brine is a fundamental part of reverse osmosis (RO) desalination technology, it presents negative influences on the marine ecosystem due to the high salt composition and the presence of other harmful chemicals in the concentrate (Heck et al. 2018; Cipolletta et al. 2021). Given the necessity of zero liquid discharge (ZLD) and the increasing amounts of brine rejected, there is a requirement to find alternative technologies for brine management that maximizes water recovery and minimizes the volume of concentrated brine (Chen et al. 2021).

Due to water scarcity and its growing demand, the number of desalination plants in Algeria was increased significantly in the last two decades. The country has developed an ambitious program of seawater desalination through the installation of large stations with a total capacity of 2 million m³ per day (MRE 2020). The Algerian desalination units use RO with 95% and multi-stage flash (MSF) with 5% of the total capacity installed. The optimum recovery ratio for RO plants in the country is estimated at around 45%, while the rest is discharged as brine at about 85 g/L (Mozas et Ghosn 2013). The desalinated water production in the world is around 95.37 million m³/day, while the rejected brine is 141.5 million m³/day (Jones et al. 2019). Currently, this brine causes substantial environmental problems because it has to be disposed of, which usually means it gets dumped back into the sea resulting in a region of hypersaline water that causes damage to fish and sea vegetation (Pérez-González et al. 2012; Morillo et al. 2014; Tong & Elimelech 2016; Heck et al. 2018). The problem is even worse in applications far from the marine environment where the brine may be disposed of in deep wells or other sites, and again is a cause of environmental stress (Belhout et al. 2018).

Several management approaches such as membrane-based, thermal-based, and emerging technologies point out the possibility to treat brine rejected from conventional electric-driven RO desalination plants (Liu et al. 2016; Giwa et al. 2017; Jones et al. 2019). Ahmed et al. (2003) presented an integrated technology (SAL-PROC) for salts extraction from brine by multiple evaporations and/or cooling techniques using mineral and chemical processing, the yearly profit was estimated at $895,000 by recovering 405,000 m³/y of reject brine. In another study (Almasri et al. 2015), brackish water RO brine was treated using a two-stage nano-filtration process. The hypersolution of RO brine can be concentrated by greater than four times using direct contact membrane distillation as highlighted by Bouchrit et al. (2015), while distillation via evaporation and condensation processes separates pure water from a hypersaline solution of brackish or seawater (Kim 2011). Nonetheless, the selection of a suitable brine recovery process is based on several factors, such as feed solution composition, target recovered minerals, land use, meteorological conditions, and others (Giwa et al. 2017).

Solar energy is one of the renewable energy sources required for water desalination that can play a strategic role in the energy transition due to its low carbon footprint. The advantages of this clean energy are its simplicity, low manufacturing cost, including being a sustainable source, and economic. It replaces the negative effects of fossil fuels with more environmentally friendly alternatives.

It is important to provide safe water to communities via reliable technologies using sustainable and renewable energies to save fossil fuels. In fact, desalination systems can provide potable water using solar energy. Indeed, there are different solar desalination techniques used to purify saltwater through thermal and membrane processes. Among the thermal processes, solar distillation is an economically viable technique for treating seawater and brackish water due to its simplicity and low manufacturing cost. The solar still is a system that applies direct or indirect solar energy for freshwater production. It could be designed and built-in in different sizes depending on the capacities needed. Its mechanism is very simple and based on a thermal process (Liquid–vapor phase-change processes) in which the brine heats up in an insulated pan and then evaporates and condenses on a cold wall. However, compared with the membrane desalination processes, the efficiency and yield of solar stills are always low. Recently, researchers have tried to improve the performance of different solar stills. The main parameters affecting solar still productivity are climatic conditions, design, and operational conditions. Several experimental and numerical works on solar stills have been reported in previous studies (Sheeba et al. 2015; Tigrine et al. 2015; Diaf et al. 2016; Kumar et al. 2016; Samuel & Kalidasa Murugavel 2017). Tigrine et al. (2021) realized a new solar still with energy storage coupled with two solar collectors that are used for desalination of groundwater in the Bou-Ismail region. It was found that the average daily distillate output of a concrete solar still with energy storage is more significant than that of a solar still without energy storage. They showed that the use of energy storage increases productivity by 50% in the nocturnal period. In recent years, many researchers have investigated the effects of using various types of nano-fluid on the performance of water purification solar still (Sahota & Tiwari 2016; Mutlq et al. 2019; Parsa et al. 2020; Singh et al. 2020). Ajdari & Ameri (2022) evaluated the performance of an inclined stepped solar still integrated with phase-change materials PCM and a CuO/GO nanocomposite as a nanofluid. They analyzed the effects of various factors like nanocomposite concentration in the base fluid, the volume ratio of CuO and GO nanoparticles in the hybrid nanocomposite, and the flow rate of brine into the solar still. Their results showed that 0.03 wt% CuO and GO boosted the freshwater yield by 48.12 and 81.59%, respectively. Additionally, it has been found that decreasing the brine flow rate from 30 to 8 L h⁻¹ increased freshwater production, and when conversely increasing to 68 L h⁻¹ caused the reduction of the distillate yield.

Given the necessity of ZLD and the increasing amounts of brine rejected, there is a need to find alternative technology for brine management that maximizes water recovery and minimizes the volume of concentrated brine. Solar distillation may be a solution to realize ‘zero discharge’ desalination in which only dry minerals remain after the evaporation process, which can then be used for their mineral value in different industrial applications rather than having to be disposed of as a (nearly) hazardous waste (Ahmed et al. 2001; Saleh et al. 2011; Tong & Elimelech 2016). This technology is significant because it requires relatively modest operational conditions, such as low-temperature heat that can be generated by the greenhouse phenomenon and an inexpensive solar thermal collector (Mickley 2008; Ossandón et al. 2010).

The purpose of the present study is to reuse seawater RO brine to produce freshwater, and solid salts, and minimize the environmental impact of the seawater RO process using an alternative approach of solar distillation. The novelty in the work is that we used a new design of a solar still coupled with solar collectors and an energy storage system carried out by the desalination team of the UDES for water treatment and desalination where all contaminants are removed. The paper is organized into four sections leaving the Introduction as section 1. The methodology of the work adopted by the study is discussed in section 2 under Materials and Methods. This section presents pilots' study of RO desalination and solar distiller. Production and valorization of the brine by solar distillation by providing an evaporation system to recover the resulting hypersaline solution to remove all salt crystals was carried out by the study. The results of the investigation of the chemical composition of produced water and brine and the distillation process have been displayed and discussed in the third section under Results and Discussion. The final conclusion is mentioned in the fourth section of the paper under the heading Conclusion.

The work proposes one of the sustainable technologies in which solar energy is used for brine disposal from the RO desalination technique in Algeria (reject brine) as well as to explore solar evaporation as an alternative method for the removal of a high salt content as a low-cost strategy. Indeed, brine rejection from desalination manufactories can have useful applications in industry and agriculture.

Our particular unconventional approach is to combine two solar desalination technologies namely reverse osmosis driven photovoltaic (ROPV) systems and solar stills as the best way or option of brine disposal methods to minimize desalination waste in order to produce more freshwater volume and less concentration while respecting the environment.

A study of the composition of the Algerian Mediterranean coastal water at the Bou-Ismail region was carried out. Thus, this paper describes an investigation of seawater and the rejected RO brine by analyzing its physicochemical and microbial composition as well as the recovery process using solar distillation aimed at treating the hypersaline concentrate and reducing its environmental impact.

Actually, the RO desalination technique is the most common desalination process for seawater and brackish water, due to its low energy consumption. Reverse osmosis combined with solar energy is one of the most economical alternative solutions as a climate change adaptation option (an adaptation option to climate change), compared to thermal desalination methods that consume more energy and increase air pollution (Do Thi et al. 2021; Shalaby et al. 2022). However, the desalination process for removing salt and other minerals from seawater is frequently requested to solve water supply problems.

In the RO system, seawater is pressurized and passed via a specific membrane module that allows the permeation of water while eliminating all existent solutes (Tigrine et al. 2021). In general, RO desalination consists of four main parts, namely pretreatment, a high-pressure pump, membrane modules, and post-treatment (Tigrine et al. 2016). In fact, the water which passes through this membrane is called permeate and that which remains behind it with the dissolved and suspended solids is known as the brine (retentate).

This solar desalination (RO) pilot developed at UDES was carried out to research improvements in technology and also to carry out a feasibility study in the local Algerian regions. This solar desalination system can use seawater or brackish water as a water supply depending on the characteristics of the membrane.

Brine generated by the RO desalination pilot was recovered using a solar distillation system designed and developed at UDES.

The solar system contains an evaporation pond filled with brine, it represents a volumetric holding capacity of 3 liters and an evaporator surface of around 0.19 m². The evaporator system is coupled directly with a solar heat collector as a heat source of saltwater by the natural phenomenon of the thermosiphon, providing the circulation of water which is selected as the calorific fluid according to its physicochemical properties.

The different temperatures of the distiller were measured using type K thermocouples (Chromel-Alumel). Meteorological parameters such as global irradiation and ambient temperature were measured by a meteorological station installed at UDES. The day July 21st, 2019 was chosen as the experimental day.

The RO unit (Figure 2) was investigated and used for water production. This brine was used as the input of the solar distillation evaporator (Figure 3). Water samples were collected from concentrate water (brine), permeate water and other samples were also collected from the solar still in an evaporation pan and salt in a solid state. All collected samples were analyzed by physicochemical, microbial, and scanning electron microscope techniques.

In this part of the study, the results and discussion of the experimental study on the desalination of seawater by RO, as well as the recovery of brines generated by a solar distillation system are presented.

The first step is the investigation of the chemical and bacteriological composition in order to estimate the quality of the water (seawater to be desalinated, desalinated water, and brine discharges).

In the second step, and with the aim of purifying the brines generated, we present the experimental results obtained during solar distillation, by studying the influence of meteorological parameters and the different temperatures of the solar system distiller, in order to master the technology and recover salt crystals.

Available seawater and reject brine quality in Bou-Ismail region in comparison with other desalination plants from different regions around the world are given in Tables 1 and 2, respectively.

As shown in Table 1, seawater of the Bou-Ismail region has a quite different composition than that of Cap Djenet (Algeria), they belong to the same coastal region with a separation distance of around 120 km, but the salt content, such as calcium (Ca²⁺), magnesium (Mg²⁺), chlorides (Cl⁻) and potassium (K⁺), is somewhat lower than that of other regions from Mediterranean areas such as Italy and Egypt.

The average chemical characteristics of reject brine of the study RO system presented in Table 2, are generally in the same order of similarity as the results of the Cap Djenet (Algeria) desalination plant. For the other treatment stations of other countries like Cyprus, Tunisia, and Italy, the comparison is more or less dissimilar due probably to the difference in seawater composition or to the membrane characteristics used for the desalination process.

Generally, for each parameter, there is a significant decrease in its initial recorded value for seawater compared to reject brine for the samples of the Bou-Ismail region, this explains that the RO desalination system has given very good results in removing salts from seawater. Conversely, reject brine has the highest values, this confirms the effectiveness of the RO membrane in removing salts and various undesirable materials found in seawater.

Bacteriological quality assessment associated with the marine environment (seawater and brine) is a very important parameter to analyze (Mok et al. 2016; Frank et al. 2017; Karbasdehi et al. 2017; Arfaeinia et al. 2019) because the use of RO membranes when the risk of biological eutrophication is present, chances pathogenic behavior in which microorganisms can be ingested, having a negative impact on the marine environment life.

The enumeration of pathogenic bacteria in seawater, and brine is presented in Table 3, seawater contains some microorganisms that remain within the required standards (Karbasdehi et al. 2017), however, the results show a complete absence of microbial organisms for brine, due to the high salinity of brine which is an unfavorable condition for the development of pathogens in the marine environment.

Salts are chemical compounds that contain anions such as NO₂⁻, NO₃⁻, HCO₃⁻, Cl⁻, SO₄²⁻ and cations such as Ca²⁺_, Mg²⁺, K⁺, and Fe³⁺. Total dissolved solids (TDS) data give a value of 37.9 g/l for seawater and 59.47 g/l for reject brine, which explains that rejects brine is 1.5 times more concentrated than the seawater.

As shown by Figure 4, chlorides are the most dominant constituents in the composition of brine, they present significantly higher values in RO brine from the desalination system with an average of 21,433 mg/L than in seawater with an average of 638 mg/L.

Regarding sulfates, their concentration levels in brine contents are 1.5 times higher than the seawater, with an average value of 5,430 mg/L and 3,458 mg/L for brine and seawater, respectively.

Contrary to the two previous salts (chlorides and sulfates), potassium and magnesium exhibited lower levels in reject brine, their averages ranged from 700 mg/L to 840 mg, similar results were found by Amitouche & Remini (2014) and Macedonio et al. (2011), but it still somewhat higher levels than those of seawater. Conversely, calcium level shows relatively low averages in brine and seawater being 380 mg/L and 359 mg/L, on average, respectively, compared to other salts.

According to Figure 5, the ambient temperature ranges between 24.5 and 33.9 °C, it increases as the solar radiation increases, which reaches its maximum of 850 w/m² between 12:00 h and 14:00 h PM for the study day (July 21st, 2019). The solar system reflects 11% of solar radiation considered the most influencing parameter of the functioning of the system as mentioned by several studies (Diaf et al. 2016; Mohsenzadeh et al. 2021; Tigrine et al. 2021). During the off-shine hours from 00:00 AM to 7:00 AM and from 21:00 PM to 00:00 AM, the solar irradiation, as well as the ambient temperature, is zero.

Temperatures of the water and the air inside the distiller increase over time and reach a maximum value of 70 °C and 75.6 °C for the air inside and the water, respectively.

During the sunshine time from 9 AM to 4 PM, the solar radiation that passes through the solar still's glass cover and is captured by the thermal sensor, increases gradually for higher operating temperature (air and water) and higher productivity, until reaches the maximum at 2 PM, this temperature has a direct effect on the temperature of the saline water (brine), it leads to increased water productivity rate to form salt crystals by improving the evaporation process.

Indeed, we indicate that when the radiation varies, the quantity of distilled water varies. For example, if the solar radiation decreases, the productivity (performance) of the solar still decreases. There is no relation between solar radiation and the disposal of salts,

Radiation heat transfer is the process by which thermal energy is exchanged between two surfaces. Radiation is responsible for most of the heat transferred into the system by conduction and convection that increases thermal energy and its intensity increases dramatically with temperature.

The solar energy received by the surface of the water provides the energy necessary for the evaporation of the water, while the salts and other minerals and impurities remain at the bottom; this water vapor then rises to condense on the inner surface of the tilted transparent cover of the still where it is collected.

The results of the daily and nocturnal production of the distilled water show that the solar system has the highest water productivity during the daytime when compared to night-time, because the daytime productivity increases considerably with the increase of temperature, whereas a significant increase in productivity is caused by the heat storage during overnight. Generally, the daily productivity contributed to around 80% of the total productivity of the solar system.

Salts crystallize by evaporation of brine quantity (3 liters) until reaching the saturation thresholds. In solution chemistry, saturation is the concentration at which a substance can no longer dissolve in a solvent (water in this case), the amount of salt recovered by the solar distillation system is 105 grams of salt per 3 liters of brine (this is the equivalent of 35 grams per liter). In countries with sufficient sunshine and temperature, such as Algeria, salt of rejected RO brine can be extracted by evaporation using a salt evaporation pond, and valuable salts could be obtained from brine for which there is great demand for several sectors such as agriculture, industry, environmental and others (Belhout et al. 2018).

The mass percentage results in Figure 8 indicate the SEM analysis of solid crystals that were formed during the solar distillation experiment. The Cl and Na ions were the most dominant constituents in the mass composition of salt crystals with values of 36,87% and 17,13% respectively. These results of Cl value are in the same accordance with the results found in Table 1 and Figure 3 about RO brine composition.

Based on the results of Figure 9, the SEM images of salt crystals reveal the formation of NaCl crystals with different sizes ranging from 50 μm to 1 mm. The SEM method was used to evaluate the composition and the morphology of salt crystals and to confirm the obtained chemical content analysis, in order estimate the quality of brine and its final utilization.

The superiority of this method given as an effective solution for freshwater production is very cheap process, economical, simple, and a low maintenance process compared to other available methods such as membrane distillation (MD), membrane crystallization (Mcr), and forward osmosis (FO) that are water separation processes using a semi-permeable membrane to separate water from dissolved solutes. They are generated by electrical or thermal energy and consume the chemical products used in membrane cleaning and extensive pretreatment. Conventional brine treatment systems like natural evaporation ponds have a significant impact on soil and groundwater due to the disposal of brine discharge from desalination plants. However, low productivity and large land areas are the major limitations of the solar distillation compared to the thermal methods. (Diaf et al. 2016).

For that, in our unit center, the main focus of our research was on increasing the capacity of production by enhancing the thermal efficiency of the system.

Based on the results of this study, the following conclusions were obtained:

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  The reject brine is very concentrated with the different constituents (TDS = 59.45 g/L) compared to seawater (TDS = 37.9 g/L) because of the effectiveness of the RO membrane in removing salts from the seawater.

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  Salt crystal separation from brine can be accomplished by distillation, in which water is vaporized from the salt solution and subsequently recovered by condensation in the solar still.

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  The performance of solar distillation is directly affected by operational and meteorological parameters such as the solar radiation that considered the most influencing parameter of the functioning of the solar system, by affecting the rate of evaporation and condensation processes.

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  The SEM analysis revealed the formation of NaCl crystals recovered by solar distillation.

Management of the generated brine will likely become a serious challenge in the future, brine reject is usually drained back into the sea generating an environmental impact at the reception point. In this context, the application of solar distillation for brine management, can recover water and salts, and reduce its impact on the receiving environment and marine pollution compared with the traditional technologies that have been used to treat brine as a waste disposal problem.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.